department of chemical & biological...
TRANSCRIPT
Department of
Chemical &
Biological Engineering.
Programme:
EPSRC Thermal Management of Industrial
Processes
Report:
Literature Review
Title:
Thermal Energy Storage Technologies
Authors:
Dr Chian Wen Chan
Dr Nigel Russell
Investigators:
Professor Jim Swithenbank
Professor Vida Sharifi
Date:
June 2011
SUWIC
Department of Chemical and Biological
Engineering
The University of Sheffield
www.suwic.group.shef.ac.uk
2
3
4
Executive summary
The energy industry is increasingly interested in new technologies or innovative use of existing technologies in thermal energy storage (TES) to reduce energy wastage and save on carbon emissions. This is a result of the growing concerns about environmental and economic conundrums facing industry such as:
(i) the potential imminent energy shortage with the demise of fossil fuels;
(ii) increasing complexity in extracting less accessible fossil fuel sites;
(iii) climate change as a result of greenhouse gas emissions; and
(iv) future price volatility of fuel source (especially fossil fuel).
This objective of this report is to identify TES technologies and their associated variables. The thermal reservoir may be maintained at a temperature above (hotter) or below (colder) that of the ambient environment. TES benefits are:
• to reduce CO2 emissions through efficient energy management by minimising energy waste;
• to reduce dependency on fossil fuels;
• less exposed to fuel price volatility hence provides a less risky investment option;
• no hazardous by-products to the environment or people; and
• making intermittent renewable sources such as solar more attractive and reliable for power generation.
By identifying the available TES technologies, their possible applications and all the associated variables such as heat transfer efficiency, thermal conductivity, safety, and reliability, there will be a future report that will deal with feasibility studies through case and/or modelling studies of:
• the economic costs;
• mass and energy balances; and
• technological maturity.
In the UK there are a number of heat sources that have been identified as TES potentials which are described towards the end of this report. The TES technologies investigated here are (i) sensible heat storage, (ii) phase change materials (latent heat storage), (iii) chemical sorbent (bond storage), (iv) thermoelectric materials, and (v) magneto-caloric materials. A hybrid of TES technologies is also suggested in this report.
5
6
Table of contents
1. Introduction to Thermal Energy Storage ...................................................................8
2. Waste Heat Sources for TES....................................................................................10
2.1 Incineration ........................................................................................................10
2.2 Power plant ........................................................................................................11
2.3 Passivhaus ..........................................................................................................11
2.4 Passive Solar Heating ........................................................................................12
2.5 Vehicle Engines .................................................................................................13
2.6 Industrial sources ...............................................................................................14
3. Important variables ..................................................................................................15
3.1 Heat pumps ........................................................................................................15
3.1.1 Mechanical heat pumps...................................................................................15
3.1.2 Adsorption heat pumps ...................................................................................15
3.1.3 Comparing heat pumps ...................................................................................16
3.2 Coefficient of Performance................................................................................17
3.2.1 Mechanical heat pumps...................................................................................17
3.2.2 Adsorbent heat pumps.....................................................................................19
4. TES Technologies....................................................................................................21
4.1 Phase change materials for latent heat storage ..................................................22
4.1.1 Description..................................................................................................22
4.1.4 Comparing PCMs........................................................................................24
4.1.5 Storage of PCMs.........................................................................................25
4.2 Sensible heat storage..........................................................................................29
4.2.1 Description..................................................................................................29
4.2.3 Liquid..............................................................................................................32
4.2.4 Solids...............................................................................................................33
4.2.7 Storage tanks...................................................................................................36
4.3 Chemical sorbent/ bond storage.........................................................................37
4.3.1 Common storage pairs ....................................................................................37
4.3.2 Metal hydride storage .....................................................................................39
4.3.3 Storage for heat upgrade .................................................................................41
4.3.4 Characteristics of adsorbent............................................................................42
4.3.4 Typical adsorbents ..........................................................................................42
4.3.5 Heat pump cycles............................................................................................44
4.3.5.1 Advanced adsorption heat pump cycles.......................................................44
4.3.5.2 Thermal wave process..................................................................................45
4.3.5.2 Binary working fluid heat pump..................................................................46
4.3.6 Enhancing heat transfer in adsorber................................................................47
4.4 Thermoelectric materials ...................................................................................49
4.5 Magneto-caloric materials .................................................................................50
5. Proposed future work on TES..................................................................................51
6. Conclusions..............................................................................................................53
References....................................................................................................................55
7
Nomenclature
PCM Phase Change Material
TES Thermal Energy Storage
COP Coefficient of Performance
E Energy storage
m Mass, [kg]
T1, T2 Lower and upper temperature levels, [K]
Cp Specific heat capacity at constant temperature, [kJ kg-1K-1]
λ Latent heat of fusion
V volume, [m3]
Ρ density, [kg m-3]
∆T temperature difference between maximum and minimum, [K]
Qs Heat transfer of the medium, [kJ s-1]
8
1. Introduction to Thermal Energy Storage
Developing efficient and inexpensive energy storage devices is seen as important as developing new sources of energy. In the energy industry stakeholders and investors are increasingly interested in new technologies or the innovative use of existing technologies in thermal energy storage (TES) to reduce energy wastage. This is a result of growing concerns about environmental and economic conundrums facing industry such as:
(v) the potential imminent energy shortage through fossil fuel depletion;
(vi) increasing complexity in extracting less accessible fossil fuels;
(vii) climate change as a result of greenhouse gas emissions;
(viii) future price volatility of fuel sources; and
(ix) daily and seasonal energy demand variations require peaking power plants, usually gas turbines, and load following power plant. The former when compared to the latter typically has:
a. lower capital costs;
b. lower efficiency;
c. higher fuel cost;
d. more expensive to operate; and
e. faster start-up and shutdown time.
Peaking power plants and load following power plants are plants that adjust their power output as demand for electricity fluctuates throughout the day. Peaking plants are generally gas turbines that burn natural gas. Base-load power plants operate continuously stopping only for maintenance or unexpected outages.
TES comprises a number of technologies that store thermal energy in energy storage reservoirs for later use. The thermal reservoir may be maintained at a temperature above or below that of the ambient environment. TES is an important part of the overall solution to fight climate change and to achieve sustainable development without sacrificing economic growth because TES has many benefits including:
• reduction or elimination of the use of economically and environmentally unfriendly peaking power plants;
• reduction of CO2 emissions through efficient energy management by minimising energy waste;
• reduction of fossil fuel dependence;
• less exposure to fuel price volatility hence provides a less risky investment option;
• no hazardous by-products to the environment;
• potential to provide desalination, reducing fresh water scarcity problems; and
• making intermittent renewable sources such as solar more attractive and reliable for power generation by:
9
o reducing variability/intermittency, the extent of uncontrolled changes in output (Sinden, 2005);
o increasing maneuverability/dispatchability, the ability of a given power source to increase and/or decrease output quickly on demand (Kuntz and Dawe, 2005);
o increasing capacity factor, the average expected output of a generator, usually over an annual period;
o increasing capacity credit, the amount of output from a power source that may be statistically relied upon, expressed as a percentage (Giebel, 2011); and
o allowing for higher penetration, the amount of energy generated as a percentage of annual consumption (Holttinen et al, 2011).
District heating is expected to become important in the future (Lund, 2007, 2009 and 2010) as the energy industry changes the way district energy is produced and distributed (Froning, 2008; Nilsson et al, 2008), since policy on energy conservation poses stringent requirements in the building energy sector, which will lead to reduced space-heating loads and therefore to a lower required distribution temperature for heating. TES technologies will play an important role in low-energy district heating. The introduction of low-energy district heating networks will enhance energy and exergy efficiencies.
The application of low-energy district heating offers many advantages:
• higher energy output in biomass based and waste-based energy plants;
• higher energy output from available medium temperature (60-100ºC) water
flows;
• higher power-to-heat ratios in combined heat and power (CHP) plants; and
• increased performance of water-based heat storage, amongst others.
Torio and Schmidt (2010) carried out a case study on a small district heating system in Kassel (Germany). They have shown that lowering supply temperatures from 95 to 57.7ºC increases the final exergy efficiency of the systems from 32% to 39.3%. Similarly, reducing return temperatures to the district heating network from 40.8 to 37.7ºC increases the exergy performance in 3.7%.
This report reviews the various types of TES technologies available which can be deployed based on technological limitations of current infrastructures and the future direction of energy systems.
10
2. Waste Heat Sources for TES
2.1 Incineration
Incineration of waste is becoming a more attractive option to dispose of waste instead of landfill since legislation is making landfill increasingly more uneconomical and difficult. Recovery of energy from incineration plants seems to be the next logical step to make the most of optimising the usefulness of waste and to reduce carbon emissions.
The Landfill Directive set down by the European Union led to the United Kingdom imposing waste legislation including the landfill tax and Landfill Allowance Trading Scheme. This legislation is designed to reduce the release of greenhouse gases produced by landfill through the use of alternative methods of waste treatment. The UK will look to incineration to play an increasingly larger role in the treatment of municipal waste and supply of energy in the UK. In 2008, plans for potential incinerator locations were in place for at least 30 sites (Letsrecycle, 2011)
In Europe, with the ban on landfilling untreated waste, many incinerators have been built in the last decade with more under construction. Some of the electricity generated from waste is deemed to be from a Renewable Energy Source (RES) and is thus eligible for tax credits if privately operated. Some incinerators in Europe are equipped with waste recovery, allowing the reuse of ferrous and non-ferrous materials found in landfills. A prominent example is the AEB Waste Fired Power Plant in Amsterdam, The Netherlands (de Jong, 2009).
In the United States of America, incineration was granted qualification for renewable energy production tax credits in 2004 (United States Environmental Protection Agency, 2011). Projects to add capacity to existing plants are underway and municipalities are evaluating the option of building incineration plants over landfill.
Incineration processes featuring energy recovery and aimed at minimising the emitted flue gas flow rate for a given load of waste material have recognized benefits from both economic and environmental points of view (Urban, 1979 and 1982; Liuzzo et
al, 1992, 1995). Sheffield has been using waste heat from its waste incinerator to provide district heating to more than 130 buildings since 1988 (Sheffield City Council, 2011)
The next phase of the improvement of the incinerator’s performance is flue gas recirculation (FGR) which lowers the flow rate of the flue gas and incineration air by 10 – 15% (Liuzzo et al, 2007). The benefits include:
• larger attainable energy recovery;
• lower capital costs associated with the smaller installed capacity of the downstream processing equipment;
• decrease in atmospheric pollution by reducing nitrogen oxides of thermal origin entrained by the lowered flame temperature (Tillman et al, 1989) in the range of 50 – 80% (Agrawal and Wood, 2002); and
11
• boosting the thermal efficiency of the plant by approximately 2 – 3%.
Although not frequently implemented in the thermal treatment of wastes FGR can be applied to both new and existing incineration plants and is one of the most promising techniques to improve their performances (Brem, 2003).
2.2 Power plant
Thermal energy storage has not been developed for electric storage yet. Desrues et al (2009) researched a thermal energy storage process for large scale electric applications, which does not suffer from geographical constraints such as the need for a large altitude difference between two large water reservoirs for pumped hydroelectric storage, or a large cavern for compressed air energy storage (Denholm and Holloway, 2005; Denholm and Kulcinski, 2004).
Load following power plant is the best type of power plant for potential TES applications. This is due to the fact that energy demand (usually electricity) and production are always in conflict with each other, i.e. production of electricity is high when demand is low; conversely, demand for electricity is high when production is low or non-existent. TES can be applied when energy supply exceeds demand, and then have its energy released in times of peak demands. As stated earlier, this helps to smooth out the production cycle and reducing dependency on inefficient peaking power plants.
In solar thermal power plants, TES provides a very good prospect for research and application (Hoshi et al, 2005; Tyagi and Buddhi, 2007; Zalba et al, 2003) as heat energy comes directly from the sun during daylight hours, without much energy loss since electrical energy is not needed for thermal energy storage. TES energy can be released to operate steam turbines during the hours of darkness to generate electricity.
TES also makes nuclear power generation a more attractive option. Nuclear power plant is almost always base-load power plant as they typically run at all times through the year since it is more economical to operate them at constant production levels, unless repairs or maintenance are scheduled. The reasons being that:
• these base-load generators often have very high capital costs;
• high plant load factor;
• very low marginal costs; and
• may take many hours, if not days, to achieve a steady state power output.
Applying TES to nuclear power plants allow them to also operate as load following power plant, since these plants can still run at full operation where excess capacity can go into TES, therefore allowing policy-makers more options and incentives to quickly wean of fossil fuel power plants.
2.3 Passivhaus
Passivhaus buildings essentially turn a building into a massive TES system. Passivhaus make extensive use of internal waste heat from lighting, electrical appliances (but not confined to heaters alone), body heat from people and other animals within the building. An average human being emits heat equivalent to 100
12
watts each of radiated thermal energy. With the comprehensive energy conservation measures, this means that a conventional central heating system is not necessary, hence why these buildings can be considered as unconventional TES systems.
A dual purpose 800 to 1,500 Watt heating and/or cooling element can be integrated with the air supply duct of the ventilation system for use on cold days. The heating element can be heated by a small heat pump via:
• solar thermal energy;
• annualised geothermal solar;
• natural gas or oil burner; or
• heat from the exhaust ventilation air.
A well designed Passivhaus in the European climate should not need any supplemental heat source if the heating load is kept under 10W/m² (Feist, 2011). Due to their design, passive houses usually have the following traits:
• The air is fresh, and very clean;
• High resistance to heat flow;
• Internal temperature is homogeneous;
• The temperature changes only very slowly - with ventilation and heating systems switched off, a passive house typically loses less than 0.5ºC, stabilising at around 15ºC (59ºF) in the central European climate;
• Briefly opening windows or doors has only a very limited effect; the air very quickly returns to the normal operating temperature after; and
• Passivhaus in the United States can achieve 75 – 95% energy savings for space heating and cooling (Tanner, 2011); while in the UK an average new house built to the Passivhaus standard would use 77% less energy for space heating (Joosten et al, 2005); and in Ireland, energy savings of up to 85% and carbon emissions reduction of 94% (Kondratenko et al, 2006)
2.4 Passive Solar Heating
Passive solar heating (PSH) distributes solar energy in the form of heat in the winter or at night and rejects solar heat in the summer or during the day. It does not involve the use of mechanical or electrical devices. The efficiency of PSV:
• 5-25% for modest systems;
• 40% for “highly optimized” systems; or
• up to 75% for “very intense” systems
There are six primary passive solar energy configurations (Chiras, 2002):
• direct solar gain: attempts to control the amount of direct solar radiation reaching the living space. This direct solar gain is a critical part of passive solar house design;
13
• indirect solar gain: attempts to control solar radiation reaching an area adjacent to, but not part of, the living space where heat is slowly transmitted indirectly via conduction and convection to the building;
• isolated solar gain: utilising solar energy to passively move heat from or to the living space using a fluid such as water or air either by natural or forced convection;
• heat storage: heat storage or thermal mass keeps the building warm when the sun can’t heat it. In diurnal solar houses, the storage is designed for one to a few days. The usual method is a bespoke thermal mass which includes a Trombe wall, a ventilated concrete floor, a cistern, water wall or roof pond. In areas that have long periods without solar gain, a technique is available that uses the ground as a thermal mass large enough for annualised heat storage (Stephens, 2011);
• insulation and glazing: reduces unwanted heat leakage; and
• passive cooling: energy-consuming mechanical components like pumps and fans are not used.
2.5 Vehicle Engines
The average efficiency of gasoline as used by a typical petrol engine is about 18% to 20%. In diesel engines however it is about 35% where 30% of the input energy is wasted in the coolant and exhaust gases (Zhang, 2000). Recovering the waste heat to regenerate the bed of adsorption systems can increase the overall efficiency of the engines.
The mechanical vapour compression system is currently the most widely available technology for refrigeration and/or air-conditioning. Using adsorption refrigeration/cooling systems instead of mechanical compression ones could significantly reduce fuel consumption (Wang and Oliveira, 2006). Some preliminary investigations on vehicle engines such as locomotives (Wang et al, 2006) and fishing boats (Wang et al, 2003) had been conducted.
Excluding other forms of transport, there are approximately 600 million passenger cars worldwide (Worldmapper, 2011; Worldometers, 2011). Around the world, there were about 806 million cars and light trucks on the road in 2007 burning over a billion cubic meters of petrol/gasoline and diesel fuel yearly. The numbers are still increasing rapidly especially in China and India (Plunket Research, 2011). There is a huge untapped market here to increase engine efficiency via TES as transportation accounts for significant CO2 emissions. As shown in Figure 1, in the United States transportation alone accounted for 28% of its total energy consumption in 2004.
14
Figure 1: Percentage of energy consumption by sectors in USA in 2004 (U.S Energy Information Administration, 2011).
2.6 Industrial sources
Manufacturing industries are very diverse by nature. Industries provide a diverse source of waste heat of differing qualities.
In 2006 five industrial sectors accounted for 68% of all energy used by industry, Figure 2 (U.S Energy Information Administration, 2011). Industries usually resort to investments with the shortest possible payback such as heat recovery and reduction of losses. This makes more economic sense to focus on the research and applications of TES. The breakdown of energy consumption by industrial sub-sectors in the US is shown in Figure 3.
China’s GDP has been growing steadily by about 10% every year since 2003. China alone accounts for about 23% of world industrial energy use. China’s industrial sector is extremely energy-intensive and accounted for 60% of the country’s total energy use in 2000 and 70% in 2003. It is still growing at an average rate of 5% per year (United Nations Industrial Development Organization, 2011; Wang, 2006). This growth is five times faster than the average growth that took place in industrial sectors worldwide during the same time period. TES has a huge potential development there.
Figure 2: World industrial sector energy consumption by major energy-intensive industry in 2007 (U.S Energy Information Administration, 2011).
15
Figure 3: Primary industrial energy use by its sub-sectors in United States in 1997 (Worrell and Price, 2001).
3. Important variables
3.1 Heat pumps
Heating and cooling systems are widely used in industrial and comfort applications. In recent years the share of the energy for heating and cooling purposes in total energy consumption has increased. A heat pump moves heat from one location at a lower temperature, the heat source, to another location at a higher temperature, the heat sink, using mechanical work or a high-temperature heat source. A heat pump can be used to provide heating or cooling. Even though the heat pump can heat, it still uses the same basic refrigeration cycle to do this. There are typically three types of heat pumps: (i) mechanical, (ii) absorption and (iii) adsorption. The latter two are more environmentally friendly than the former.
3.1.1 Mechanical heat pumps
Mechanical heat pumps typically use vapour-compression refrigeration. Due to the economic benefits resulting from high coefficient of performance (COP) values, mechanical heat pump systems become convenient devices for heating and cooling purposes (Ülkü, 1986 and 1987). However, traditional heat pumps play an important role in the depletion of the ozone layer and global warming. Vapour compression heat pumps operate using electrical power, usually generated by fossil fuels. The primary energy efficiency of mechanical heat pumps will therefore be less than their COP values due to energy losses occur in power plants and compressors, and during transfer of electrical power. Mechanical heat pumps have been used in a few European countries to thermally upgrade low grade heat to high grade heat (Eriksson and Vamling, 2007)
3.1.2 Adsorption heat pumps
Adsorption heat pumps provide heating and cooling by employing thermal energy sources such as solar and geothermal energies or waste heat from industrial processes. The heat pumps operate by cycling adsorbate between the adsorber (containing adsorbent such as zeolite, active carbon, or silica gel), condenser and evaporator
16
(Ülkü, 1986 and 1987; Meunier, 2002). Their ability for thermal energy storage is highly desirable (Demir et al, 2008). Adsorption heat pumps are typically more complicated to design because of their:
• intermittent working principles;
• technology for working under high vacuum; and
• design of adsorbent bed that is coupled with heat and mass transfers.
General areas of research for the improvement of heat pumps include (Demir et al, 2008):
• advanced adsorption cycles in order to increase COP;
• operate with lower temperature;
• drive heat source to provide continuous cooling or heating process;
• new adsorbent–adsorbate pairs or promoting the existence pairs in order to increase adsorption rate, enhance COP and decrease the temperature of driving heat source; and
• design of an adsorbent bed for appropriate heat and mass transfer.
Table 1: The advantages and disadvantages of adsorption heat pumps.
Advantages Disadvantages
• Can work with low temperature;
• Do not require moving parts for circulation of heat transfer fluid;
• Long life time;
• Operate without noise and vibration;
• Simple to operate;
• Do not require frequent maintenance;
• No hazardous materials; and
• Can be employed as thermal energy storage device.
• Low COP values;
• Intermittently working principles;
• Require high technology and special designs to maintain high vacuum;
• Large volume and weight relative to traditional mechanical heat pump systems (Meunier, 2002; Ülkü and Mobedi, 1989;
3.1.3 Comparing heat pumps
The major advantage of adsorption heat pumps is that they do not need maintenance for long periods since they do not contain few, if any, moving parts. The life-time of an absorption heat pump is shorter than the adsorption heat pump due to salt corrosion. The absorbent used in the absorption system should be changed every 4–5 years. In the adsorption heat pump, the system does not require changing adsorbent–adsorbate pairs for a long period of time. No corrosive chemical materials are employed in adsorption heat pump systems (Demir et al, 2008).
17
Table 2: A comparison between heat pump types.
Type of heat pumps and working pairs COP
Adsorption • Carbon - methanol
• Zeolite – water
• Silica gel -water
• 0.12 - 1.06
• 0.28 – 1.4
• 0.25 - 0.65
Absorption • Methanol – water • 0.7 – 1.1
Vapour compression • 3 - 4
3.2 Coefficient of Performance
3.2.1 Mechanical heat pumps
The COP increases as the temperature difference decreases between heat source and destination. The COP can be maximised by choosing a heating system requiring only a low final water temperature (e.g. underfloor heating) and a heat source with a high average temperature (e.g. the ground). Domestic hot water (DHW) and radiators require high water temperatures, affecting the choice of heat pump technology.
The equation for COP is:
W
QCOP H= Equation 1.1
Where,
o W is the energy consumed by the heat pump
o QH is the heat supplied to the hot reservoir
The COP for heating and cooling are different, because the heat reservoir for each function is different, with WQQ CH += where QC is the heat supplied to the cold
reservoir.
• For cooling, the COP is the ratio of the heat removed from the cold reservoir to input work (air conditioners or refrigerators):
W
QCOP
C
cooling = Equation 1.2
• For heating, the COP is the ratio of the heat removed from the cold reservoir plus the heat added to the hot reservoir by the input work to input work (heat pumps):
W
WQCOP
C
heating
+= Equation 1.3
According to the first law of thermodynamics, in a reversible system we can show that QH = QC + W and W = QH − QC, where QH is the heat given off by the hot heat
18
reservoir and Qcold is the heat taken in by the cold heat reservoir. Therefore Equation 1.2 becomes Equation 1.4 while Equation 1.3 becomes Equation 1.5;
o For cooling, CH
C
coolingQQ
QCOP
−= Equation 1.4
o For heating, CH
H
heatingQQ
QCOP
−= Equation 1.5
For a heat pump operating at maximum theoretical efficiency (i.e. Carnot efficiency):
c
c
H
H
T
Q
T
Q= Equation 1.6
Where,
o TH is the temperature of the hot reservoir, K
o TC is the temperature of the cold reservoir, K
At maximum theoretical efficiency,
For cooling, Equation 4 becomes:CH
C
coolingTT
TCOP
−= Equation 1.7
For heating, Equation 5 becomes:CH
H
coolingTT
TCOP
−= Equation 1.8
It can also be shown that COPcooling = COPheating − 1.
For the sake of comparing heat pump appliances to each other, independently from other system components, the American Refrigerant Institute (AHRI, 2011) and
International Organization for Standardization, assume that TH is 298 K (25ºC) and
TC is 273 K (0ºC). According to above formula, the maximum achievable COP would be 8.8. Test results of the best systems are around 4.5. When accounting for energy needed to pump water through the piping systems, the COP could be 3.5 or less. This shows that there is enormous room for improvement.
As the formula shows, to improve the COP of a heat pump system, one needs to
reduce the temperature gap Thot minus Tcold at which the system works.
Also the heat pump itself can be greatly improved. The two simplest ways to improve heat pump units are:
• to double the size of the internal heat exchangers relative to the power of the compressor;
• to reduce the system internal temperature gap over the compressor.
This last measure however makes such heat pumps unsuitable to produce output above roughly 40ºC which means that a separate machine is needed for producing hot tap water.
19
3.2.2 Adsorbent heat pumps
Figure 4: Heat transfer configuration of ideal adsorption heat pump cycle (Ülkü. 1986)
The adsorption heat pump cycle works between three temperature levels whereas a vapour compression cycle works between two temperature levels and also requires mechanical power. The adsorption heat pump cycle can be considered as two separate cycles as seen in Figure 4:
• The first cycle is a heat pump (adsorption) in which the working fluid is vaporised in evaporator by taking heat (QL) from the low-level temperature source and releasing (Qa) heat to the first intermediate temperature source.
• The second cycle is a heat engine (desorption), which receives heat (Qz) from the high-temperature source and releases heat (Qc) to the second intermediate temperature source. The transfer of heat (Qc) to the second intermediate temperature source occurs during the condensation of working fluid in condenser.
• It is assumed that the work obtained in the heat engine is employed to run the heat pump. The temperatures of intermediate sources (Tc and Ta) are generally close to each other. Thus, three temperature levels can be considered for an adsorption heat pump and the ideal coefficient of performance of an adsorption heat pump cycle can be obtained as (Pons and Kodoma, 2000; Kodoma et al, 2000; Spinner et al, 2001):
1
1
−
−
==
L
C
Z
C
Z
L
cooling
TT
TT
Q
QCOP
1
11
−
−
+==
L
C
Z
C
Z
C
heating
TT
TT
Q
QCOP
20
Figure 5: Thermodynamic cycle of a basic adsorption pump (Demir et al., 2008)
An adsorption heat pump cycle consists of four steps (Figure 5):
• Isosteric heating (a–b): The valves between the adsorbent bed and the condenser and evaporator are closed. The temperature of adsorbent bed is increased from Ta to Tb by heating the adsorbent bed without desorption.
• Isobaric desorption (b–c): After the isosteric heating of adsorbent bed, the heating process is continued. The valve between the adsorbent bed and condenser is opened. Desorption process is started and water vapour is condensed in the condenser. The pressure of the cycle remains constant and part of the heat which is transferred to the adsorbent bed increases the temperature of adsorbate–adsorbent pair and adsorbent bed while the other part causes the desorption process;
• Isosteric cooling (c– d): The valve between the condenser and adsorbent bed is closed and the temperature of adsorbent bed (Tc), which is the maximum temperature of the cycle, is decreased to Td. During this process, both the pressure and temperature of the adsorbent bed are decreased to the evaporator values;
• Isobaric adsorption (d– a): The valve between the adsorbent bed and evaporator is opened and vaporization of the adsorbate in the evaporator is started. During adsorbing of the adsorbate in the adsorbent, heat is released due to heat of adsorption. This generated heat should be removed from the adsorbent bed and the temperature of adsorbate–adsorbent pair and container should be decreased to Ta.
The cooling effect in the cycle occurs during the isobaric adsorption process (d–a) when the adsorbate is evaporated by gaining heat from environment. The heating effect appears during the isobaric desorption process (b–c) when the adsorbate is condensed by releasing heat to surroundings. In addition to the isobaric desorption process, the adsorbent bed is also cooled during c–d and d–a processes. The heat released during these processes can also be utilised for heating purposes in any process (Gui et al, 2002). Hence, the cooling and heating COP of a basic adsorption heat pump can be determined as:
bcab
e
coolingQQ
QCOP
+=
21
bcab
dacdc
heatingQQ
QQQCOP
+
++=
Specific cooling (SCP) or heating power (SHP) are expressions that describe the effectiveness of the system. The SCP/SHP is the ratio of cooling/heating power per mass of adsorbent per cycle time. In some studies, SCP/SHP is determined according to cooling/heating power per cycle time and per mass of reactor which consists of mass of adsorbent, mass of heat exchanger in the adsorbent bed, mass of heat transfer fluid (HTF) and mass of container. There are also studies in which SCP/SHP is calculated based on the ratio of reactor volume and cycle time to cooling/heating power. According to the authors, the meaningful definition for SCP/SHP is the ratio of cooling/heating power to the mass of adsorbent and cycle time. The definition of SCP/SHP involves the period of cycle and contributes to the comparison of various adsorption heat pump designs (Pons et al, 1999; Chahbani et al, 2002; Poyelle et al, 1999)
4. TES Technologies
This section identifies technologies suitable for low-grade heat storage and high-grade heat storage. These distinct classifications are required since the obstacles to low-grade heat storage are different from high-grade heat storage, such as potential for certain phase change materials (PCMs) to degrade at higher temperatures. Low-grade heat storage is also thermodynamically more challenging since the temperature difference is smaller than for high-grade heat storage, resulting in much less efficient heat transfer. Low-grade heat is usually defined as effluents having temperatures less than 150ºC. Low-grade heat such as geothermal, waste heat and heat from low- to mid-temperature solar collectors accounts for 50% or more of the total heat generated worldwide as these sources cannot be converted efficiently to electrical power by conventional power generation methods (Hung et al, 2011). There have been many studies to utilise this low-grade heat efficiently and economically using a wide variety of methods such as TES and organic Rankine cycle with or without supercritical applications (Chen et al, 2011 and 2010).
When attempting to choose and/or design TES systems the important attributes to consider are (Ataer, 2011):
• the temperature range over which the storage has to operate;
• the capacity of the storage has a significant effect on the operation of the rest of the system, because a smaller storage unit operates at a higher mean temperature. This results in a reduced heat transfer equipment output as compared to a system having a larger storage unit;
• the optimum capacity (“short-term” storage units) is a TES system which can meet fluctuations over a period of two or three days, as it is the most economical for building applications;
• the heat losses from the storage have to be kept to a minimum, especially important for long-term storage;
• the rate of charging and discharging; and
22
• the cost of the storage unit (storage medium, the containers and insulation, and the operating cost).
This section examines the types of TES technologies available and whether the applications are for high-grade or low-grade heat. TES can be in the form of stored cold as well as heat. The types of TES methods being investigated discussed in detail in this report are:
a) Phase Change Material (PCM)/ latent heat storage;
b) Thermoelectric materials;
c) Magnetic refrigeration;
d) Chemical sorbent/ bond storage; and
e) Sensible heat
4.1 Phase change materials for latent heat storage
4.1.1 Description
Latent heat thermal energy storage is attractive as it provides a high-energy storage density. The high-energy storage density is due to phase change either by freezing/melting or boiling/condensing, and this material is described as a phase change material (PCM). The study of PCMs was pioneered by Telkes and Raymond (1949). PCM was only extensively researched during the energy crisis of late 1970s and early 1980s for use in different applications especially for solar heating systems (Telkes, 1980; Pillai and Brinkworth, 1976; Feldman et al, 1984; Bell and Smith, 1980; Kimura and Kai, 1984; Solomon, 1979; Garg and Nasim, 1981; Knowles, 1983; Miller, 1983). The intensity of the research tailed off after the energy crisis until recently.
Relative to sensible heat energy storage systems, PCM requires a smaller weight and volume of material for a given amount of energy, and has the capacity to store heat of fusion at a constant or near constant temperature. Morrison and Abdel-Khalik (1978) and Ghoneim (1989) demonstrated that to store the same amount of energy in paraffin for a given mass:
• a rock (sensible heat storage material) requires more than seven times, and;
• Na2SO4.10H2O requires more eight times the storage mass of paraffin.
The criteria that govern the selection of PCM are (Abhat, 1983; Regin et al, 2008):
• Possess a melting point in the desired operating temperature range;
• Possess high latent heat of fusion per unit mass;
• High specific heat to provide additional significant sensible heat storage;
• High thermal conductivity, for ease of for charging and discharging;
• Small volumetric changes during phase transition, hence a simple container and heat exchanger geometry can be used;
• Exhibit little or no sub-cooling during freezing;
23
• Possess chemical stability;
• Non-poisonous, non-flammable and non-explosive;
• Abundant and cheap;
• Storage unit pressure drop; and
• Pumping power.
The principle characteristics of PCMs are highlighted in Table 3.
Table 3: Main desirable characteristics of PCMs (Regin et al, 2008).
Thermal
properties
Physical
properties
Chemical
properties
Economic factors
• Phase change temperature in suitable operating range
• High latent heat per unit mass
• High specific heat
• High density
• Low density variation during phase change
• Little or no super- cooling
• Chemical stability
• No chemical decomposition
• Compatibility with container materials
• Non-poisonous, non-flammable and non-explosive
• Available in large quantities
• Inexpensive
Investigations have been carried out into a wide range of PCMs, generally subdividing them into organic and inorganic (Sharma et al, 2009).
4.1.2 Organic PCMs
Organic PCMs are typically paraffins (alkanes). Paraffins have high heat of fusion, negligible super-cooling, low vapour pressure in the melt, chemically inert and stable, self-nucleating, no phase segregation and commercial availability at reasonable cost (Pillai and Brinkworth, 1976; Lane, 1983; Abhat, 1983; Hasnain et al, 1989; Hasnain, 1990). Pure paraffin waxes are very expensive; therefore only technical grade paraffins (which are mixtures of many hydrocarbons) are used and therefore have a melting temperature range (eutectic behaviour) rather than a sharp melting point (Hasnain, 1998). Non-paraffin organics include fatty acids, esters, alcohols and glycols. Fatty acids have melting points suitable for heating applications and have heat of fusion values comparable to those of paraffins and salt hydrates (Markley,
24
1967; Perry, 1984; Weast and Astle, 1985) without exhibiting super-cooling, although their cost is about three times greater than paraffins. Feldman et al (1986 and 1989), Galen and Brink (1986), Hasan and Sayigh (1994) suggested that fatty acid esters such as butyl stearate, vinyl stearate and methyl-12 hydroxystearate, can be considered suitable for passive heat storage.
4.1.3 Inorganic PCMs
The high storage density of salt hydrates is difficult to maintain as it decreases with cycling, leading to the continuous decline in their storage efficiency. Sub-cooling is another serious problem associated with all hydrated salts. The solution to both problems is to have used hydrated salts in direct contact with an immiscible heat transfer fluid to improve heat transfer (Farid and Khalaf, 1994; Farid and Yacoub, 1989; Fouda et al, 1984; Edie and Melsheimer, 1976; Costello et al, 1978). The agitation caused by the heat transfer fluid has minimised the sub-cooling and prevented phase segregation. The hydrated salts studied were CaCl2.6H2O, Na2SO4.10H2O, Na2HPO4.12H2O, NaCO3.10H2O, and Na2S2O4.5H2O. The water was required to prevent clogging of the fluidised bed and formation of anhydrous salts, which would reduce the storage density of the hydrated salts. The crystallisation temperature of these hydrated salts is between 30ºC and 50ºC. The heat discharge process employing these salts occurs with continuously decreasing crystallisation temperature due to the dilution of the liquid phase. This is not desirable in most applications.
4.1.4 Comparing PCMs
A comparison of the most commonly used organic and inorganic PCMs for heat storage is shown in Table 4.
Table 4: Comparison of PCMs (Mehling and Cabeza, 2008; Farid et al, 2004; Wang et al, 2009).
Organic Inorganic
Advantages Not corrosive
Low or no sub-cooling
Chemical and thermal stability
Greater phase change
enthalpy
Disadvantages Lower phase change enthalpy
Low thermal conductivity
Flammability
Sub-cooling due to super-saturation
Corrosion
Phase separation
Lack of thermal stability
Methods for improvement
High aliphatic hydrocarbon, acid/esters or salts, alcohols, aromatic hydrocarbons, aromatic ketone, lactam, freon, multi-carbonated category, polymers
Crystalline hydrate, molten salt, metal or alloy
Methods for improvement
High thermal conductivity additives, fire-retardant additives
Mixed with nucleating and thickening agents, thin layer arranged horizontally, mechanical stir
25
The flammability hazard with regards to organic PCM can be overcome using flame retardation techniques, but only in building applications (Cabeza et al, 2011):
� Adding alternate non-flammable surface such as aluminium foil and rigid polyvinyl chloride film;
� Sequential treatment of plasterboard, first in PCM and then in an insoluble liquid fire retardant. The insoluble fire retardant displaces part of the PCM and some remains on the surface providing self extinguishing properties;
� Using brominated hexadecane and octadecane as PCM because when these halogenated PCM compounds are combined with antimony oxide in plasterboard the product will be self extinguishing; and
� Fire retardant surface coatings.
The major disadvantage with regards to PCMs is the low thermal conductivity leading to low charging and discharging rates (especially for the organic based materials). Future development involves improvement in heat transfer/exchange in the PCMs when undergoing phase transition at the required operating temperature range, the design of the container for holding the PCM and formulation of the phase change problem (Agyenim et al, 2010).
4.1.5 Storage of PCMs
Storage types of PCMs (Regin et al, 2008):
• Storage tanks in heat exchangers:
� higher heat storage density compared to other storage;
� smaller heat transfer area;
� inserting fins or using high conductivity particles, metal structures, fibres in the PCM side, direct contact heat exchangers or rolling cylinder method to improve heat transfer; and
� majority of the heat enhancement techniques have been based on the application of fins embedded in the phase change material due to simplicity, ease in fabrication and low cost of construction. This is followed by the impregnation of metal.
• Macro-encapsulation:
� most common type of PCM containment (Lane, 1986);
� PCM is encapsulated in a discrete unit, from rectangular panels to spheres to pouches (larger than 1cm in diameter);
� applicable to both liquid and air as heat transfer fluids;
� can avoid large phase separations;
� increase the rate of heat transfer;
� provide a self-supporting structure for the PCM; and
� potentially corrosive.
26
• Micro-encapsulation (Griffiths and Eames, 2007; Hawlader et al, 2003):
� small PCM particles are contained within a sealed, continuous matrix;
� suffers from low heat transfer rate;
� the rigidity of the matrix prevents convective currents and forces all heat transfer to occur by conduction thereby seriously reducing the heat transfer rates; and
� the cost of the microencapsulation system is high compared to other storage methods, and is used only in thermal control applications.
There are a number of techniques for micro-encapsulation (Hawlader et al, 2003; Boh and Sumiga, 2008):
a) Physical methods:
• pan coating;
• air-suspension coating;
• centrifugal extrusion;
• vibrating nozzle;
• spray drying; and
• coacervation.
b) Chemical methods:
• interfacial polymerisation;
• in-situ polymerisation; and
• matrix polymerisation.
There are a number of benefits from encapsulating PCMs, they (Regin et al, 2008):
(i) meet the requirements of strength, flexibility, corrosion resistance and thermal stability;
(ii) act as barrier to protect the PCM from harmful interaction with the environment;
(iii) provide sufficient surface for heat transfer; and
(iv) provide structural stability and easy handling.
4.1.6 Improving heat transfer
Most PCMs have low thermal conductivity and low thermal diffusivities. During extraction of energy from storage the liquid freezes on the heat transfer surfaces and an immobile layer of solid material continuously grows as it gives up its heat of fusion. These factors lead to slow charging and discharging rates, hence heat transfer enhancement techniques are usually required. The variation of surface heat flux depends on the predominance of the convective resistance (fixed resistance) and the conductive resistance (variable resistance):
(i) when the convective resistance is dominant, nearly uniform surface heat flux with time can be achieved; and
27
(ii) when conductive resistance is dominant, the surface heat flux will decrease with time.
Several studies have been conducted to examine heat transfer enhancement techniques in phase change materials (PCMs) and include:
• finned tubes of different configurations (Abdel-Wahed et al, 1979; Ermis et al, 2007; Ismail et al, 2001; Choi and Kim 1992; Agyenim et al, 2008; Horbaniuc et al, 1999; Sparrow et al, 1981; Sasaguchi et al, 1994; Zhan and Faghri, 1996; Velraj et al, 1997);
• bubble agitation (Velraj et al, 1997);
• insertion of a metal matrix into the PCM (Trelles and Dufly, 2002; Hoogendoorn and Bart, 1992; (Hasnain, 1990);
• using PCM dispersed with high conductivity particles (Mettawee and Assassa, 2007);
• shell and tube (multi-tubes) (Agyenim et al, 2010-b, Hendra et al, 2005); and
• encapsulation to improve heat transfer.
Volume change during phase change however, complicates the system design (Regin et al, 2008).
4.1.7 PCM containment and heat transfer
The geometry of the PCM container, the thermal and geometric parameters of the container required for a given amount of PCM are also important because:
• they directly influence heat transfer, and;
• affect the melt time and the performance of the PCM storage unit.
PCMs are typically placed in:
• long thin heat pipes (Horbaniuc et al, 1999);
• cylindrical containers, Figures 6-7 (Agyenim et al, 2009; Papanicolaou and Belessiontis, 2001), where three modes of cylindrical PCM container configurations are distinguished:
(i) The first is where the PCM fills the shell and the heat transfer fluid flows through a single tube, Figure 6a (Ghoneim, 1989; Esen et al, 1998; Agyenim et al, 2008, 2009 and 2010-b) designated the pipe model;
(ii) The second model the PCM fills the tube and the heat transfer fluid flows parallel to the tube, Figure 6b (Diner and Rosen, 2002; Bansal and Buddhi, 1992; Esen et al, 1998). Esen et al (1998) recommended the pipe model because it recorded a shorter melt time with lower heat loss rate to the environment because most heat supplied from the centre ends up heating the PCM. This was because the thicker the PCM mass, the longer the melt time of the PCM;
(iii) The third cylinder model is the shell and tube system, Figure 6c (Ghoneim, 1989; Agyenmim et al, 2010-b) commonly used to improve heat transfer in PCMs; and
28
(iv) rectangular containers, Figure 6d (Zivkovic and Fujii, 2001; Silva et al, 2002).
PCM containers are usually rectangular and cylindrical of which, the shell and tube system accounting for more than 70%, as most engineering systems employ cylindrical pipes and that heat loss from the shell and tube system is minimal.
Figure 6: Classification of commonly used PCM containers in terms of the geometry and configuration (Agyenim et al, 2010).
Literature reviewed has shown a superior performance using the shell and tube configuration followed by the pipe model with the PCM at the shell side and the heat transfer fluid flowing through the centre.
Figure 7: Various schematics of containment used in latent heat thermal energy storage (LHTS) system (Yanbing et al, 1999): (a) flat-plate; (b) shell and tube with
internal flow; (c) shell and tube with parallel flow; (d) shell and tube with cross flow; (e) sphere packed bed.
29
There are way too many PCMs to completely describe the thermophysical properties here. For more information about the various PCM compounds refer to Agyenim et al (2010), Cabeza et al (2011) and Regin et al (2008).
For a less mobile storage system, nucleating agents and stabilisers (to prevent keep nucleating agents in suspension) can be used for some of the hydrated salts (Ryu et al, 1992; Lane, 1991). Calcium chloride was usually selected in practical applications, even though its latent heat was lower than other hydrated salts because it was easier to stabilize, showing minimum phase segregation. Corrosion associated with salt hydrates can be avoided by using brass, copper and stainless steel for PCM containment instead of aluminium and steel (Cabeza et al, 2011).
4.1.8 Hybrid PCM application
The composite salt/ceramic thermal energy storage media offers the potential of using phase change materials via direct contact heat exchange, eliminating the requirement of a separate heat exchanger. This concept reduces the need for storage of materials and containment vessel size, via micro-encapsulation of a PCM (composite salt) within the submicron pores of a ceramic matrix. The liquid salt is retained within the solid ceramic network by surface tension and capillary forces. PCM has been successfully impregnated in gypsum wall board to enhance the thermal energy storage capacity of buildings (Rudd, 1993).
4.2 Sensible heat storage
4.2.1 Description
Sensible heat storage is when energy is stored or extracted by heating or cooling a material, but does not undergo phase change. Sensible heat storage systems are simpler in design than latent heat (PCN) or bond storage systems (which will be described later), but:
• larger in size; and
• cannot store or deliver energy at a constant temperature.
A variety of substances have been used including liquids such as water, heat transfer oils and certain inorganic molten salts; and solids like rocks, pebbles and refractory. In the case of solids, the material is invariably in porous form and heat is stored or extracted by the flow of a gas or a liquid through the pores or voids.
The choice of the substance used depends on:
• the temperature level of the application (e.g., water being used for temperature below 100°C and refractory bricks being used for temperatures around 1000°C);
• the value of the specific heat capacity (ρ.Cp);
• the volume required due to limitations in transport and/or habitat applications (denser materials have smaller volumes, hence larger energy capacity per unit volume); and
• the cost.
30
Table 5: Typical sensible heat storage media.
Phases Examples
Liquid Hot water, organic liquids, molten salts, liquid metals
Solid Metals, minerals, ceramics
The thermal stratification (or temperature-ordered stratification) is due to the differences in buoyancy the less dense, hotter water at the top and denser, colder water at the bottom. In a tank, perfect stratification is impossible since (Ataer, 2011):
• the water entering the tank will cause a certain amount of agitation and mixing;
• a certain amount of diffusion due to temperature differences;
• natural convection due to heat losses from the surface of the storage tank, resulting in lower temperature of water near the vertical walls. This convection current destroys the temperature layers.
Two fundamentally different ways of looking at stratification are a density approach used by environmental scientists (e.g., Moretti and McLaughlin, 1977; Stefan and Gu, 1992), and a temperature approach used by thermal engineers (e.g., Sliwinski et al, 1978; Kandari, 1990; Davidson et al, 1994). The latter is commonly used in TES applications.
Crandall and Thacher (2004) reported that the packed beds can have high degree of stratification and this was a major advantage. Stratification provides higher temperature at top of the bed and coolest at the bottom. This allowed the warmest air to be delivered from the top of packed bed.
4.2.2 Charging/discharging efficiency
Stratification efficiencies based on the first law of thermodynamics calculate the fraction of energy that is recovered from charging and discharging with fixed inlet temperature and mass flow. Between the charging and discharging, a storing period may, or may not, be included.
Abdoly and Rapp (1982) define a fraction of recoverable heat as a measure of thermocline degradation during storing. In a discharging process, they only consider heat to be useful if it has not been degraded more than 20% of its original temperature value towards the ambient temperature, while at the same time, the initial temperature does not go below the thermocline (Nelson et al 1999; Zurigat and Ghajar, 2002). The charging and discharging efficiencies can be defined as:
iniinlet
iniavg
iniinletstore
iniavgstore
ChTT
TtT
TTcm
TtTcmt
−
−=
−
−=
)(
].[.
])(.[.)(,1η
η1,Ch representing the actual energy change at time t divided by the maximum energy change after ideal plug flow replacement of the entire storage volume (Chan et al, 1983).
31
*
)(
][..
])(.[.)( ,1
,2t
t
TTcmt
TtTcmt
Ch
iniinlet
iniavgstore
Ch
ηη =
−
−=
&
The term η2,Ch represents the actual energy change divided by the energy change that would have occurred in the same experiment assuming an ideal plug flow (Yoo and Pak, 1993; Mavros et al, 1994; Hahne and Chen, 1998; Bouhdjar and Harhad; 2002; Shah et al, 2005).
4.2.2 Figures of Merit
Further development of charging and discharging efficiencies lead to figures of merit (FOMs) derived from cycles of full, or partial, charging and subsequent discharging (Tran et al, 1989; Wildin, 1990; van Berkel et al, 1999; Bahnfleth and Song, 2005).
A TES with a more pronounced temperature gradient and hence better stratification always contains more exergy than a comparative storage with equal energy content but less pronounced stratification (Rosen et al, 2004). Therefore FOMs based on the second law of thermodynamics may be used to give information about the stratification efficiency.
The dimensionless MIX-number (Davidson et al, 1994) expresses the degree of mixing that occurs during a charging process based on moment of energy (ME). The moment of energy of a TES is an integration of the sensible energy content along its vertical axis, weighted with the height of its location along the vertical axis. In practice a summation over i
th storage segments along the vertical axis is used to calculate ME:
∑=
=N
i
iiE EyM1
.
The MIX-number is the difference of moment of energy between a perfectly stratified storage and the experimental storage, divided by the difference of moment of energy between a perfectly stratified storage and a fully mixed storage:
1,0,1,0,
exp,1,0,
hmixEhstrE
EhstrE
DavMM
MMMIX
−
−=
Both “theoretical” storage temperature profiles, stratified and fully mixed, are calculated assuming full stratification and mixing respectively, from the beginning of the experiment, and including heat losses to the surroundings calculated with the heat loss coefficient that was determined for the experimental storage. The “moment of energy” introduced by Davidson was later adapted and used by Andersen et al (2007). In contrast to the method of Davidson, the theoretical storages in this later method are mixed or stratified at each instant and thus have the same energy content as the experimental storage at any time.
1,1,
exp,1,
mixEastrE
EastrE
AndMM
MMMIX
−
−=
The advantage of this latter method is that the determination of the heat loss coefficient of the experimental storage is not necessary.
32
In this case, stratification efficiency based on these methods is defined according to (Haller et al, 2009):
DavDavMIX MIX−= 1,η
AndAndMIX MIX−= 1,η
EGREG R−= 1η
4.2.3 Liquid
Water can be used as a storage and also as a transport medium of energy. It is the most widely used storage medium especially for solar-based warm water and space heating applications (Duffie and Beckman, 1989; Wyman et al, 1980). The sizes of the water storage tanks used vary from a few hundred litres to a few thousand cubic meters. An approximate thumb rule followed for fixing the size is to use about 75 to 100 litres of storage per square meter of collector area (Ataer, 2011).
Solar ponds are a form of thermal stratification of water, can be classified according to four basic factors:
(a) convecting or non-convecting;
(b) partitioned (multi-layered) or non-partitioned;
(c) gelled or non-gelled; and
(d) separate collector and storage or in-pond storage.
However, most of the research efforts are presently concentrated on the non-convecting salt gradient solar pond (Lodhi, 1996). This type has a density gradient which is created by using water containing salt (or sea water). The salt concentration increases with depth from the surface. Sodium chloride (NaCl) and magnesium chloride (MgCl2) are most commonly used. The salt gradient pond has a black or dark bottom where the solar radiation is absorbed, where water temperature can be up to 95ºC. Extraction of the thermal energy stored in the lower layers of the pond can easily be accomplished without disturbing the upper layers.
The most commonly proposed substitutes for water are petroleum based oils and molten salts. The heat capacities are 25-40% of that of water on a weight basis. However, these substitutes have lower vapour pressure than water and are capable of operating at high temperatures exceeding 300ºC. The oils are limited to less than 350ºC due to stability and safety reasons and can be quite expensive (Hasnain, 1998).
A few molten mixtures of inorganic salts have been considered for high temperatures (300ºC and above). Sodium hydroxide has a melting point of 320ºC and could be used for temperatures up to 800ºC (Lodhi, 1996), but is highly corrosive and difficult to contain. Liquid metals are also possible sensible heat storage media. While most of their properties are similar to those of water, they have low specific heats and higher potential for reactivity with the container. However, they have higher thermal conductivity. An oxygen and oxide free environment is important in order to prevent corrosion of liquid metals (Sorour, 1988). The properties of commercially available liquid storage media are shown in Table 6.
33
Table 6: Thermo-physical properties of liquid storage media.
Medium Fluid type T (ºC)
Density,
(kg/m3)
Cp,
(kJ/kg.K) k, (W/m.K)
Water Water 0 to 100 1000 4.19 0.63 at 38ºC
Water-Ethylene Glycol 50/50
Water-alcohol mixture 1050 3.47
Caloria HT43 Oil -10 to 315 2.3
Dowtherms Oil 12 to 260 867 2.2 0.112 at 260ºC
Therminol 55 Oil -18 to 315 2.4
Therminol 66 Oil -9 to 343 750 2.1 0.106 at 343oC
Ethylene glycol 1116 2.382 0.249 at 20ºC
Hitec Molten salt 141 to 540 1680 1.56 0.61
Engine oil Oil up to 160 888 1.88 0.145
Draw salt Molten salt 220 to 540 1733 1.55 0.57
Lithium Liquid metal 180 to 1300 510 4.19 38.1
Sodium Liquid metal 100 to 760 960 1.3 67.5
Ethanol Alcohol up to 78 790 2.4
Propanol Alcohol up to 97 800 2.5
Butanol Alcohol up to 118 809 2.4
Isobutanol Alcohol up to 100 808 3
Isopentanol Alcohol up to 148 831 2.2
Octane Oil up to 126 704 2.4
4.2.4 Solids
Solid materials such as rocks, metals, concrete, sand or bricks, can be used for thermal energy storage as they do not freeze or boil. The difficulties of the high vapour pressure of water and the limitations of other liquids can be avoided by storing thermal energy as sensible heat in solids. Solids do not leak from their container. The highest product in the list of solid materials for sensible heat storage is cast iron, which exceeds the energy density level of water storage. However, cast iron is more expensive than stone or brick and, hence, the payback period is much longer. Pebble beds or rock piles are generally preferred due to their low cost.
Most of the materials proposed for high temperature (120-1400ºC) energy storage are either inorganic salts or metals (Shitzer and Levy, 1983). The use of metal media is advantageous where high thermal conductivity is required and where cost is of secondary importance. Solid industrial wastes like copper slag, iron slag, cast iron slag, aluminium slag and copper chips could be used as storage material for energy storage, Table 7.
34
Table 7: Thermo-physical properties of solid storage media.
Medium
Density,
(kg/m3)
Specific
heat,
(J/kg.K)
Heat
capacity,
(J/m3.K) k, (W/m.K) α,10
6 (m
2/s
1)
Aluminium 2707 896 2.4 204 at 20ºC 84
Aluminium oxide 3900 840 3.3
Aluminium sulphate 2710 750 2
Brick 1698 840 1.4 0.69 at 29ºC 0.48
Brick magnesia 3000 1130 3.4 5.1 1.5
Concrete 2240 1130 2.5 0.9 - 1.3 0.36-0.51
Cast iron 7900 837 6.6 29.3 4.4
Pure iron 7897 452 3.6 73 at 20ºC 20.5
Calcium chloride 2510 670 1.7
Copper 8954 383 3.4 385 at 20ºC 112
Earth (wet) 1700 2093 3.6 2.5 0.7
Earth (dry) 1260 795 1 0.25 0.25
Potassium chloride 1980 670 1.3
Potassium sulphate 2660 920 2.4
Sodium carbonate 2510 1090 2.7
Granite 2640 820 2.2 1.7 to 4 0.8 – 1.8
Limestone 2500 900 2.3 1.3 0.56 – 0.59
Marble 2600 800 2.1 2.1 to 2.9 1 – 1.4
Sandstone 2200 710 1.6 1.83 1 – 1.2
4.2.5 Packed bed
The pebble bed or rock pile consists of a bed of loosely packed rock material through which the heat transport fluid can flow. The thermal energy is stored in the packed bed by forcing heated air into the bed and utilised again by re-circulating ambient air into the heated bed. The energy stored in a packed bed storage system depends on the thermo-physical properties of the material.
Rock used varies from 1 to 5 cm. An approximate thumb rule followed for sizing is to use 300-500 kg of rock per square metre of solar collector area for space heating
35
applications (Ataer, 2011). For a temperature change of 50ºC, rocks and concrete will store around 36 kJ/kg or about 105 kJ/m3.
4.2.6 Fluidised bed
Fluidised beds can be utilised for low, intermediate and high temperature solar applications (Weast et al, 1980). The rate of heat exchange between the heat carrying fluid and the storage medium is much than in rock beds.
A summary of sensible heat storage systems is presented in Table 8.
Table 8: Summary of sensible heat storage systems (Ataer, 2011).
Sources Advantages Disadvantages
Water • Inexpensive, easy to handle, non-toxic, non-combustible and abundant.
• High specific heat and high density;
• Heat exchangers may be avoided if water medium is used for transport;
• Natural convection flows can be utilised when pumping energy is scarce.
• Simultaneous charging and discharging;
• Adjustment and control is variable and flexible.
• Adjustment and control is variable and flexible resulting in pressure build up or volume expansion, destroying tanks.
• Highly corrosive;
• Working temperatures are limited to less than 100ºC;
• Difficult to stratify.
Solid storage media
• Below 100ºC, using water as heat transfer fluid, it is simple in design and relatively inexpensive;
• Rock/pebble-bed storages can also be used for temperatures of up to 1000ºC, and still be inexpensive with high specific heat;
• Non-toxic and non-flammable;
• Act as heat transfer surface and medium
• Large heat transfer area
• Cost rises sharply for temperatures above 100ºC, as storage tanks must be able to contain water at its vapour pressure;
• Organic oils, molten salts and liquid metals use are limited because of their handling, containment, storage capacities and cost.
• Larger amounts of solids are needed than using water, due to lower storing capacity than water.
36
4.2.7 Storage tanks
The storage tanks are suitably insulated with glass wool, mineral wool or polyurethane. The thickness of insulation used is large and ranges from 10 to 20 cm. Because of this the cost of the insulation represents a significant part of the total cost and more research has to be done.
By using thermal stratification in tanks storage efficiency can also be improved. To maintain stratification over long time intervals the tank should be provided with extremely good thermal insulation or with special installations. There is still a possibility of enforced stratification by:
• employing multiple storage tanks at different temperatures is also meaningful, with the liquid in different tanks remaining at different temperatures even when the liquid in each tank is completely mixed, due to the physical separation between tanks;
• using rock beds, since hot air is brought in contact with different parts of the rock bed in the path of its flow and these parts of the rock bed, heated to different temperatures, cannot mix.
The thermal storage via thermal stratification, Figure 8, has higher efficiency due to:
• liquid (or water) of a high temperature than the overall mixing temperature can be extracted at the top of the container;
• liquid (or water) of a lower temperature than the mixing temperature can be drawn off from the bottom.
Figure 8: An example of thermally stratified hot liquid tank.
Alternatively, for an underground tank insulation of the earth surrounding the tank may provide the bulk of the insulation thickness required. However it may take as much as one year for the earth around a large storage tank to reach a steady state by heating and drying, and a considerable amount of energy may be required for this purpose. If the water is at atmospheric pressure, the temperature is limited to 100ºC. It is possible to store water at temperature a little above 100ºC by using pressurised tanks.
In order to reduce the costs an alternative way which is being examined for large-scale storage, is to use naturally occurring confined underground aquifers which already contain water, Figure 9.
37
Figure 9: Thermal storage using natural aquifer.
Hot water is pumped into aquifers to be stored, displacing the existing cold ground water. Since the investment required is a series of openings for injecting and withdrawing water it is expected that storage costs for such systems would be low.
4.3 Chemical sorbent/ bond storage
4.3.1 Common storage pairs
Chemical sorbent or bond storage uses heat to produce a physicochemical reaction and then storing the products. Desorption (endothermic: absorbing heat) and adsorption (exothermic: releasing heat) are two examples for the bond reactions. The heat is released when the adsorption is made to occur. Adsorption and desorption are usually used for adsorbent heat pump. Typically, adsorbates are water and ammonia.
The type of applications determines the kind of method which is to be adopted. Some of the considerations, which determine the selection of the method of storage and its design are shown in Tables 9-12.
Table 9: General bond/chemical storage pairs.
Phases Examples
Solid-gas CaO/H2O, MgO/H2O, FeCl2/NH3
Gas-gas CH4/H2O
Liquid-gas LiBr/H2O, NaOH/H2O, H2SO4/H2O
38
Table 10: Typical solid-gas pairs.
Adsorbent –
adsorbate pair
Literature Tcon, ºC Teva, ºC Treg, ºC COP
Zeolite - water Ülkü, 1986 53 27 123 0.34
Ülkü, 1987 52 25 200 0.4
Poyelle, 1999 40 4 230 0.74
Szarzynski et
al, 1997 35 3 120 – 360 single
0.3–0.38,
double
0.5–0.73
Pons and Szarznyski, 2000
45 3 290
Tather and Erdem- Senatalar, 2004
20 2 150 0.3
Meunier, 1985 40 0 350 0.94-1
Bonarccorsi et
al, 2006 40 7 200 0.46
Silica gel - water
Hamamoto et
al, 30 14 55 0.25
Khan et al, 2005
0.3-0.65
Henning and Wiemken, 2003
35 10 80 0.6
Wang et al, 2005-a, b
30.5 15.1 55-92 0.32-0.4
Nunez et al, 2005
35 – 45 10 – 20 80 – 95 0.5
Activated carbon - methanol
Sumathy, 2002 35 -10 100 0.12
Wu et al, 2002 27 -10 100 0.5
Carbon - ammonia
Telto, 2005 20 9.5 150 0.3
Metcalf, 2005 30 – 50 15 130 – 195 0.6 – 0.62
Lambert, 2007 35 -15 160 1.2
39
Table 11: Main requirements for adsorbent-adsorbate pairs.
Adsorbate Adsorbent
• high latent heat
• non-corrosive
• non-toxicity
• thermal stability
• chemical stability
• High adsorption capacity
• high thermal conductivity
• low cost
• thermal stability
Table 12: Thermo-physical properties of adsorbents (Srivastava and Eames, 1998)
Adsorbent Adsorbate Heat of adsorption,
kJ/kg
Remarks
Silica gel CH3OH
H2O
1000-1500
2800
Not suitable for above 200ºC
Cooling
Activated alumina H2O 3000
Zeolite (various grades)
H2O
NH3
CO2
CH3OH
3300-4200
4000-6000
800-1000
2300-2600
Natural zeolites have lower values than synthetic zeolites
Charcoal C2H4
NH3
H2O
CH3OH
C2H5OH
1000-1200
2000-2700
2300-2600
1800-2000
1200-1400
Reacts at about 100ºC
Zeolite–water, active carbon–methanol, silica gel—water, and carbon–ammonia are some of the common adsorbent–adsorbate pairs used in adsorption heat pump systems (Srivastava and Eames, 1998; Cerkvenik et al, 2001; Wang et al, 2005-c).
4.3.2 Metal hydride storage
Metal hydride heating and cooling systems operate similar to conventional solid sorption systems, employing, for example, activated carbon/ammonia, silica gel/water, as working pairs. The potentially most useful complex metal hydrides are aluminium hydride and composite metal hydrides. Na(AlH4) and Na(BH4) are reported as promising alternatives especially as hydrogen storage materials for mobile applications such as for use in fuel cells. Their use for thermodynamic devices has, however, not been considered so far (Muthukumar and Groll, 2010).
40
Metal hydride systems are expensive due to high material costs and expensive fast reaction beds. The commercial success of such systems in comparison with liquid sorption systems and competing dry sorption systems is not certain (Muthukumar and Groll, 2010).
The heat-driven hydride slurry heat pump (HHSHP) has good heat transfer and relatively fast absorption rates when highly agitated. The HHSHP requires inert liquids that suspend the metal hydride powder and form a hydrogen-liquid hydride slurry (HLHS) with a continuous operation.
The heat exchange between the slurry streams can achieve a high performance of the system. However, the inert liquids bring an additional mass transfer resistance into the system (Kim et al, 1997), the absorption resistance. In general, the absorption resistance is related to the properties of the liquid, the flow conditions and the geometry.
For desorption, since the inert liquids are always saturated with the hydrogen evolved from the metal hydrides, hydrogen gas bypasses the inert liquids via bubbles formed on the surface of the suspended metal hydrides. A limiting mass transfer resistance should be associated with metal hydrides not with inert liquids (McCabe et al, 1985; Snijder et al, 1993).
The desired characteristics of inert liquid for slurry (Kim et al, 1997) can be summarised as follows:
• Large mass diffusivity of hydrogen;
• Large thermal conductivity;
• Small density;
• Small viscosity;
• No chemical reactivity with hydrogen;
• No or very little chemical reactivity with corrosion additives;
• Very small vapour pressure;
• Small specific heat;
• Cheap; and
• Negligible toxicity.
Metal hydride particles can be uniformly dispersed in a liquid when thoroughly mixed. These small particles, however, tend to stick to each other and become larger aggregates. As their size increases, the aggregate will settle out and accumulate. A small amount of a non-ionic surfactant (<0.01 wt%) can be added to the slurry in order to stabilise the suspension to reduce the aggregation, eliminating sedimentation.
The pumping problems associated with slurry flows include wear and erosion of the slurry pump and start-up problems with the slurry where the metal hydride particles have settled out and accumulated in low areas. The centrifugal pump is appropriate for absorption because of the loose fit between the impeller and the pump wall. However, questions still remain with respect to pump life due to wear and erosion (Botsaris and Glazman, 1989).
41
Specially developed porous metal hydrides (PMH) or Misch metal (Mm) matrix alloys, containing Ni, Fe, La, Al, H, have very high rates of sorption and heat of adsorption with hydrogen as adsorbates and have promising uses in development of metal hydride refrigeration and heat pump systems (Nagel et al, 1984; Ron, 1984; Yanoma et al, 1988; Charters et al, 1996; De Beijers and Horsman, 1993).
4.3.3 Storage for heat upgrade
A less conventional form of chemical bond storage dehydrogenation of alcohols and hydrogenation of aldehydes or ketone can be illustrated by:
HydrogenketoneorAldehydeAlcohol +↔ )(
This reaction sequence can upgrade thermal energy with no consumption or production of other chemicals (Lauerhass and Rudd, 1983). The process comprises two main units: an endothermic reactor (low-temperature heat is supplied to a distillation column for separation products and residual reactants) and an exothermic reactor (high-temperature heat is released) (Mooksuwan and Kumar, 2000).
The feasibility is based on dehydrogenation of i-propanol and hydrogenation of acetone has been proven experimentally (Taneda et al, 1995). The dehydrogenation can be of methanol, ethanol or n-butanol and hydrogenation of formaldehyde, acetaldehyde or butyraldehyde, respectively. The dehydrogenation reaction takes place in liquid phase with catalysts in liquid-film state (Saito, 1995) at low-temperature (70–100ºC) and requires thermal energy; while the hydrogenation reaction is carried out in gas-phase at a higher temperature (150–200ºC) as an exothermic reaction.
Dehydrogenation reaction occurs at the boiling point of liquid phase and its temperature level is fixed by the equilibrium conditions at reaction pressure. The alcohol produced by hydrogenation reaction of aldehyde or ketone and hydrogen is recycled for dehydrogenation reaction (KlinSoda and Piumsomboon, 2007).
Since two reverse reactions running at different temperature levels are involved, at least two reactors and one heat exchanger are used in the system cycle (Gastauer and Kameyama, 1995). Part of low-level thermal energy supplied is upgraded to high-level energy and the rest is removed by condenser at ambient temperature (Karaca et
al, 2002)
The rate of dehydrogenation reaction depends on the aldehyde (or ketone) concentration in the liquid reaction mixture, the products (aldehyde or ketone and hydrogen) are to be continuously removed (aldehyde or ketone by vaporisation) from the endothermic reactor. Reaction and vaporisation take place simultaneously. To efficiently convert thermal energy into chemical energy a high aldehyde (or ketone) concentration in liquid mixture is required. However, since the reaction rate rapidly decreases as the content of aldehyde (or ketone) in the reaction mixture increases the operation of dehydrogenation reactor is limited to low aldehyde (or ketone) concentrations (Gastauer and Kameyama, 1995).
Typical hydrogenation and dehydrogenation reactions:
• At 80 – 90ºC
( ) ( ) ( ) ( ) ( )ggl HCOCHCHOHCH 22323 +→
42
∆H = 100.4 kJ/mol
• At 150-210ºC
( ) ( ) ( ) ( ) ( )ggg CHOHCHHCOCH 23223 →+
∆H = -55.0 kJ/mol
The benefit of upgrading heat is turning low value heat into high value heat that better serves economic purposes. Researchers have considered thermal upgrading for district heating, via modelling, concluding that it is more economically feasible and attractive than mechanical vapour compression heat pumps (Ajah et al, 2008).
4.3.4 Characteristics of adsorbent
The characteristics of the adsorbents:
• Generally have low thermal conductivity leading to longer periods of adsorption and desorption;
• Mass transfer depends on adsorbate flowing through the bed (interparticle flow) and through the adsorbent (intra-particle diffusion due to concentration differences, molecular diffusion, Knudsen diffusion, and surface diffusion);
• Performance governed largely by available surface area;
• Distribution of temperature and adsorbate concentration is influenced by the bed porosity;
• Adsorption period increases with the increase of the porosity value;
• Increase of porosity reduces the thermal conductivity, heat transfer rate and the period of the adsorption process;
• Non-polar or hydrophobic adsorbents have more affinity for oils and gases than for water, e.g. activated carbons, polymer adsorbents and silicalites (Srivastava and Eames, 1998); and
• Polar or hydrophilic adsorbents have more affinity for water, e.g. gel, zeolites and porous or active alumina.
4.3.4 Typical adsorbents
Attributes of silica gel:
o Silica gel is chemically bonded traces of water (about 5%) in the form SiO2.xH2O;
o Silica gel is only used at temperature under 200oC because when overheated, this water is lost along with its adsorption capacity;
o Available in various pore sizes, the smaller the pore size the greater is the surface area per unit mass, typically 650 m2g-1;
43
o Large capacity for water adsorption, especially at high vapour pressures; and
o Heat of adsorption of water vapour is predominantly due the heat of condensation of water (Ponec et al, 1974; Oscik, 1982; Ruthven, 1984; Suzuki, 1990).
Attributes of activated alumina:
o Activated alumina is aluminium oxide in a porous form prepared by dehydration of aluminium hydrates (mostly Al2O3.3H2O) to about 6% moisture level;
o Surface area ranging between 150 and 500 m2g-1 with pore sizes ranging from 1.5 to 6 nm; and
o Generally useful as a drying agent and adsorbent for polar organic substances.
Attributes of zeolites:
o All zeolites are aluminosilicate minerals;
o Pore volume cane range from 0.05 to 0.30 cm3g-1; and
o Can be heated to about 500ºC without damaging their adsorption and regeneration properties.
Attributes of calcium chloride (CaCl2):
o Calcium chloride is a very widely available;
o Remains solid until saturated, beyond which it dissolves in water, yet can still be used as a low temperature liquid desiccant; and
o Good as a solid chemical adsorbent for methanol and ethanol vapours.
Attributes of activated carbon:
o Activated carbons are made by pyrolysing and carbonising carbonaceous materials at high temperatures (700 to 800oC);
o Available in powders, micro-porous, granulated, molecular sieves and carbon fibres; Powdered form (15 to 25 mm particles) is used for adsorption of liquids;
o Granulated (sieved granules of 4 to 20 mesh or about 3 mm to 0.8 mm diameter) or pellet (extruded pellets of 4 to 6 mm length) forms for air purification and gas separation;
o Microporous form has molecular sieving ability and is widely used for separation of nitrogen and oxygen in air; and
o Activated carbon fibres (fibre diameter of 7 to 15 mm) made by carbonising synthetic fibres which can be used for air and water
44
purification (Ponec et al, 1974; Oscik, 1982; Ruthven, 1984; Suzuki, 1990).
Metal oxides including titanium oxide, zirconium oxide, and magnesium oxide, have been used as adsorbents for special uses involving chemisorption (Suzuki, 1990). Composite adsorbents made from metallic foams, zeolites and natural graphite have good prospects in improving the heat transfer rates in the adsorbent beds, consequently increasing the refrigeration or heat pump system performance (Pons et
al, 1996; Rockenfeller and Kirol, 1993).
Adsorber which contains the adsorbent typically can be available in uncoated and coated form:
1. Working principles of an uncoated adsorber:
• Adsorbate moves in voids between pellet or granule and then adsorbed in the adsorbent;
• Based on porosity of the bed, convection and diffusion of adsorbate between pellets for heat and mass transfer considerations; and
• Fins can be employed in order to increase heat transfer rate in the bed.
2. Working principles of a coated adsorber:
• Adsorbent is coated around a pipe, fin or in metal foam;
• Generates high speed heat and mass transfer; and
• Diffusion in the adsorbent is the main mechanism of mass transfer.
4.3.5 Heat pump cycles
The adsorption heat pump cycle operates under high vacuum. It is difficult to maintain the operating pressure in a high vacuum for a long time. This requires vacuum technology, special materials and gaskets which increase the cost of adsorption heat pump and cause the use of heavier containers. For continuous cooling and heating processes, the number of adsorbers provided should be increased. This is called advanced adsorption heat pump cycles. The increase of COP is obtained by recovering and utilising heat, which is transferred during isosteric cooling and isobaric adsorption in another adsorption cycle. This increases the COP of cycle since the amount of external heat supplied to the cycle is reduced. The advanced cycles can be categorised into two groups (Dous and Meunier, 1989; Meunier, 2002): advanced adsorption heat pump cycles, and thermal wave processes.
4.3.5.1 Advanced adsorption heat pump cycles
These systems consist of two or more adsorbers, operating with the same refrigerant, a single evaporator and a single condenser. A general view of the uniform temperature adsorber cycle is shown in Figure 10.
The basic principles of this system:
45
• One of the adsorbers is preheated with rejection heat of another adsorber which is under the cooling process, using a heat transfer fluid until both adsorbers are at the same temperature (G and H);
• Then, one adsorber is heated by the external heat source (GD) while the other one is cooled by the external heat sink (HA) (Szarzynski et al, 1997);
• Although each adsorber follows exactly the same cycle as the basic adsorption heat pump cycle, the heat which is supplied to the total system decreases; and
• Enhancement of the COP up to 50% (Dous and Meunier, 1989; Meunier, 2002; Szarzynski et al, 1997), by reducing the rate of decrease of heat supplied to the whole system.
Dous and Meunier (2002) have proposed an alternative with COP for cooling of 1.06 where the adsorption cycle consists of two cycles:
(i) a zeolite–water cycle for high temperature stage; and
(ii) an active carbon–methanol cycle for low-temperature stage.
The heat transferred to the active carbon–methanol cycle for isosteric heating and isobaric desorption processes is entirely obtained from the zeolite–water cycle. The driving energy for zeolite–water cycle is supplied from an external heat source.
Figure 10: Working principles of an adsorption cycle with uniform temperature absorbers (Szarzynski et al, 1997).
4.3.5.2 Thermal wave process
The system is also composed of two or more adsorbers, a condenser and an evaporator. The working principle of a thermal wave process is shown in Figure 11 where:
• the cycle consists of two adsorbers (1 and 2) where heat is circulated using a heat transfer fluid;
• when adsorber 1 is under cooling, the adsorber 2 is under heating process and vice versa;
• the heat which is recovered from the adsorbent 1 is transferred to the heat transfer fluid;
46
• the heating of fluid is continued to desorption temperature by a heating system and then it is fed to adsorber 2 for the isobaric desorption process;
• Then, the heat transfer fluid leaving from adsorber 2 is cooled by a cooler to be fed into the adsorber 1; and
• a reversible pump is used to change flow direction of heat transfer fluid for the reverse process, when the adsorber 2 is under cooling and adsorber 1 is under heating stage (Chahbani et al, 2002; Pons and Szarzynski, 20003, Szarzynski et al, 1997; Dous and Meunier, 1989).
The system of Saha et al (1997) and Hamamoto et al (2005) worked on an advanced two stages adsorption heat pump cycle and improved the thermal wave process by employing two additional adsorbers in cycle, where cooling capacity can be improved by allocating required adsorbent mass between adsorbers. The main advantage of the improved two stages adsorption heat pump is being capable to utilise low temperature solar/waste heat (40–95ºC) as driven heat source (Saha et al, 1997 and 2006; Hamamoto et al, 2005; Khan et al, 2005).
Figure 11: Working principles of an adsorption heat pump cycle with thermal wave process (Pons and Szarzynski, 2000).
4.3.5.2 Binary working fluid heat pump
To operate at moderate evaporation or condensation pressure, Wang and Zhu (2002) have proposed an innovative adsorption heat pump cycle which operates with binary working fluid NH3 and H2O. The differences between the cyclic behaviours of the single and binary working fluid systems are shown in Figure 12. The operation pressure of the water–zeolite system is low and requires high vacuum in the cycle, Figure 12A. However, the NH3–zeolite cycle operates at higher pressure which is 4–11 times higher than the ambient pressure, Figure 12B. With an appropriate mixing of ammonia and water, a cycle which operates with a pressure close to ambient pressure can be obtained, Figure 12C. This improvement may solve one of the problems of the adsorption heat pump which is working under high vacuum.
47
Figure 12: Ideal cycle of the system on isosteric graphs for single and binary working fluid (Wang and Zhu, 2002).
4.3.6 Enhancing heat transfer in adsorber
Heat transfer in a reactor bed used for gas–solid reaction is poor and its effective thermal conductivity is generally in the range of 0.1–0.2 W/(m K). Therefore, enhancement of heat transfer in the reactor bed is necessary heat pumps with high performance, Table 13. Heat transfer improvement techniques can be classified into three categories:
(a) preparation of composite reactant/adsorbent combined with heat transfer promoter with high thermal conductivity;
(b) addition of metals or carbon fibres into a bed; and
(c) integration of reactant into a heat exchanger.
In method (a), expanded graphite is normally used as a material for promoting heat transfer. It is usually prepared by impregnating an aqueous solution of inorganic salt into expanded graphite matrices, dried and calcined to deposit the salt inside the pores of the expanded graphite (Hirata et al, 2003). This can improve thermal conductivity up to 10 times larger than that of the bed packed with untreated salt particles (Fujioka et al, 2003). Expanded graphite is a very bulky material, and its effective thermal conductivity can be improved remarkably by compression (Mauran et al, 1993; Han and Lee, 1999). If adsorbents are insoluble in water, resin is used as a binder to bond the adsorbent with expanded graphite (Goetz and Guillot, 2001).
In method (b), insertion of fins with a small volume fraction of 0.01 into a packed bed increased the effective thermal conductivity by six times (Ogura et al, 1991) and insertion of carbon fibre brush with volume fraction of 0.05 increased it by four times (Nakaso et al, 2004).
In the study of method (c), integrated reactors of adsorbent layers packed between fins of heat exchanger were developed and the reaction cycle time was reduced to almost one tenth of that without integration (Fujisawa et al, 2002; Freni et al, 2004). By using monolithic carbon discs, the heat transfer coefficient between the carbon bed and the fins was increased four times, and it was estimated that the cooling power was enhanced to be 90% higher than granular carbon (Tamainot-Telto and Critoph, 1997).
48
Table 13: Techniques for enhancing heat transfer in reactor bed for chemical heat pumps (Fujioka et al, 2008).
Heat transfer
promoter
Preparation technique Reaction couple
Expanded graphite
Simple mixing
Impregnation of aqueous solution of salt using EG, dehydration and calcination
Impregnation of aqueous solution of salt into compressed EG, dehydration and calcination
Mixing activated carbon with compressed EG using resin as a binder
CaCl2/NH3 (Valkov et al, 2002)
CaCl2/CH3OH (Hirata et al, 2003)
CaCl2/CH3NH2 (Fujioka et al, 2003)
CaCl2/CH3NH2 (Mauran et al, 2003)
MnCl2/NH3 (Han and Lee, 1999; Mauran et al, 2003)
CaCl2/NH3 (Han and Lee, 199)
BaCl2/NH3 (Han and Lee, 199)
Activated Carbon/CO2 (Goetz, and Guillot, 2001)
Carbon fibre Impregnation of aqueous solution of salt into fibre and dehydration
Insertion of carbon fibre brush into bed
Formation of an intercalation compound
CoCl2/NH3 (Aidom and Ternan, 2002)
MgO/H2O (Nakaso et al, 2004)
MnCl2/HN3 (Dellero et al, 1999)
Metal foam (Cu, Ni)
Impregnation of a suspension salt, compression and calcination
Zeolite/H2O
Activated carbon/CH3OH (Guilleminot et al, 1993)
Resin (polyanilin)
Coating particles with resin network Zeolite/H2O (Hu et al, 1997)
Aluminium hydroxide
Mixing, compression and calcination
Insertion of fins into bed
Zeolite/H2O (Pino et al, 1996)
CaO/H2O (Ogura et al, 1991)
Metal fin or tube
Integration of reactant with heat exchanger by coating fin tubes with an adsorbent layer
Forming monolithic carbon discs and insertion into aluminium fins
Silica gel/H2O (Fujisawa et al, 2002), Zeolite/ H2O (Freni et al, 2004)
Activated carbon/NH3
(Tamainot-Telto and Critoph, 1997)
49
4.4 Thermoelectric materials
Thermoelectric materials are usually used for small applications where engines cannot be used due to their relatively larger size. The use of waste heat in combustion engines promises to be a high-volume field of application (Niu et al, 2009). Thermoelectric materials generate power based on the heat flux through thermoelectric elements. The heat flux is driven by temperature difference across the elements where voltages are produced based on actual temperature differences. For the opposite effect, it is also known as the Peltier–Seebeck effect, when a voltage is applied a temperature difference is created (Peltier effect).
At the atomic scale an applied temperature gradient results in charged carriers in the material, either electrons or holes, to diffuse from the hot side to the cold side, hence the thermally induced current. A low thermal conductivity is desirable attribute (Agbossou et al, 2010).
Most work on typical efficiencies of around 5–10% (Penella and Gusulla, 2007; Cook-Chennault et al, 2008). Commercial thermoelectric generators range from µW to kW in electrical output. Material properties are the key parameter for improving both output power (increase of thermal heat flow) and efficiency (improvement of Seebeck coefficient). The main problem is maintaining a high temperature gradient, for electrical energy conversion especially when the loading is time-variable, like solar heating. The harvested energy is directly proportional to the temperature gradient, where the proportionality coefficient depends the thermoelectric materials used.
To maintain a relatively low temperature on the cold face of the thermoelectric generator, it is always connected to a metallic heat sink exposed to the air. There are two problems with this widely used system especially in solar applications:
• The temperature on the cold face of the thermoelectric generator rises rapidly when the air around the heat sink is heated by solar radiation and the convection between them decreases; and
• The system can only work during the day, when solar radiation acts directly as the heat source.
A solution is to use phase change materials as a source of constant temperature heat sink. If a PCM has sufficient latent heat, a suitable fusion temperature and sufficient volume, the temperature on the cold face will remain relatively stable over the whole day, as the PCM’s temperature does not change enormously when it becomes liquid. When external heat source is removed, the role is reversed; PCM becomes the heat source while the external environment becomes heat sink.
For solar-driven thermoelectric systems, the efficiency of whole system (ηsystem) can be described as:
ηsystem = COP x ηpv
where COP is the coefficient of performance for the thermoelectric refrigeration at usually less than 0.6 and ηpv is the efficiency of the photovoltaic cell with an average of 0.1. Hence, the average efficiency of a solar-driven thermoelectric system is usually less than 0.06 (Li et al, 2007).
NASA and other space organisations started a dual-use technology project to develop solar-driven refrigeration technology. But due to the low COP or low energy
50
conversion efficiency of the solar-driven thermoelectric devices, currently the solar-driven thermoelectric devices can only be used in limited applications, such as aerospace, military or cases in which the cost is not the main consideration (Li et al, 2007).
4.5 Magneto-caloric materials
Magnetic refrigeration is based on the magnetocaloric effect where ferromagnetic materials is a warming as the magnetic moments of the atom are aligned by the application of a magnetic field, and the corresponding cooling upon removal of the magnetic field (Hull and Uherka, 1989).
Two major difficulties for magnetic refrigeration are:
• the magnetocaloric effect is fairly small in room temperature magnetocaloric materials, e.g. in gadolinium (Gd), the application of a 5 T magnetic field produces a maximum adiabatic temperature change of 11 K (Zimm et al, 1998); and
• the refrigerant is solid, and thus, is not easily pumped through heat exchangers, as in the case of gas and vapour cycle refrigerants.
The problem of heat transfer and temperature span can be overcome with the introduction of a heat transfer fluid and use of regeneration. Regeneration can be accomplished by blowing fluid in reciprocating fashion through a porous bed of magnetocaloric material that is alternately magnetised and demagnetised (Shir et al, 2005).
Magnetic materials available for room temperature magnetic refrigeration are mainly Gd, GdSiGe (Pecharsky and Gschneidner, 1997) alloys, MnAs-like (Wada et al, 2002) materials, and Perovskite (Bohigas, 1988) materials. Shir et al, (2003) demonstrated that basing on the refrigerant capacity calculations (Wood and Potter, 1985), nanocomposite clusters could yield large magnetocaloric effect in a wide temperature region and have several advantages compared to the other refrigerants for a room temperature magnetic refrigeration system, including the ability to closely follow the desired operating line and enhanced temperature change at high temperature and low field (Bennett et al, 1995).
Although most literature focuses on making more efficient magneto-calorific heat pumps than that of the typical vapour-compression heat pumps (i.e. using electricity to generate desired work), there is not much research focus on utilising waste heat via magneto-calorific effect to do the reverse (i.e. generating electricity) even though it is possible by principles. Zou et al, (2009) was one of the first to generate electricity using magnetocaloric compound (Tb5Si2.2Ge1.8) in a laboratory scale experiment.
Current research is focused using these special materials for more efficient heat pumps as they are more environmentally friendly than a typical vapour compression heat pump. This novel technique is largely laboratory based with no significant commercial exploit, although research is intensifying significantly in the last couple of years, Figure 13. The research of using waste heat on magneto-caloric materials lag even further behind, hence it has no foreseeable applications in the immediate future although research-wise.
51
Figure 13: The number of patents per year for magnetic refrigerators and heat pumps operating at or near ambient room temperature (Yu et al, 2010).
5. Proposed future work on TES
5.1 District heating
The problem with distribution of low grade heat traditional heat transfer media such as water is the difficulty in providing constant heating temperature. The end of the heat distribution line where temperature of the water is the lowest will end up providing the least heat. Micro-encapsulation of PCM suspended in water will allow temperature of the liquid heat transfer media, e.g. water or ammonia, to be constant, so long as PCM has not completely released all its latent heat content. This will allow for more or less uniform heating through out the heat distribution line. Cost could also potentially be saved via reduced requirement for insulation for the distribution pipeline, by reducing the heating temperature, since higher temperature might not be necessary in making sure that the end of the distribution pipeline provides sufficient heat (hence reducing the rate of heat loss). The size of the heat exchangers can also potentially be reduced if the low-grade heat can be maintained at a sufficiently high temperature.
A slightly more technologically complicated approach will be using meta-stable surfactant to encapsulate hydroscopic PCMs such as calcium chloride hexahydrate. These PCMs will first solidify when releasing latent heat. Once PCM solidifies, surfactant will gradually allow adsorbate, in this case, water, to enter into the PCM. The exothermic hydration of calcium chloride hexahydrate further releases heat for district heating. Theoretically speaking, this method should incur even less heat loss due to the fact less sensible heat storage and latent heat storage media being used and pumped.
5.2 Power plants
Metal hydride could potentially be used to store energy from renewable power plants (or nuclear) such as wind power and wave power. The energy produced during period of low demand could be converted to chemical energy by electrolysing water into hydrogen and oxygen. Hydrogen and oxygen can then be combusted together in gas turbine to generate electricity during times of peak demand. Hydrogen can be stored in metal complexes in the form of hydrides as previously mentioned. Although, metals have the ability to store a lot of hydrogen, they are pretty bulky and heavy to handle or transport. However, power plants are stationary hence the storage of
52
hydrogen in the form of metal hydrides is preferable over gas, compressed gas and liquid storage, as metal hydrides have higher energy density. The advantage of metal hydrides is that they also have the ability to function as chemical heat pumps as mentioned earlier, i.e. the metal hydride heating and cooling systems operate similar to conventional solid sorption systems. The metal complexes are also cheap to obtain. The combine benefits of chemical sorption and combustion of hydrogen should increase the overall efficiency and reliability of these power plants.
5.3 Manufacturing industries
Industries generally produce a lot of waste heat. Certain industries such as steel and glass production do not have infrastructure to distribute its waste process heat for district heating. It does not have the economies of scale and in direction competition with energy industries, hence making it difficult for non-energy industries to make a profitable investment in district heating infrastructures. A good alternative will be to upgrade its waste process heat to be return into their processes again. The exothermic hydrogenation of ketone and/or aldehyde to alcohol at high temperature serves as a mechanism for heat upgrade. Regeneration occurs when alcohol is subjected to lower temperatures where it experiences endothermic dehydrogenation. The medium can be reused and regenerated without incurring loss. Some authors propose using mechanical heat pump although Ajah et al, (2008) show that it is not as efficient.
5.4 Automobiles
For automobiles, a good solution will be the application of PCM with thermoelectric materials. Thermoelectric materials need to run at large temperature difference to produce electricity. The charging up phase occurs when the internal engine is running, where PCM melts as it meets the hot exhaust gas. The discharging phase occurs when the internal combustion engine is shut off. The temperature difference between the liquid PCM and lower ambient air drives electricity production. The electricity produced from waste heat can be used to operate electrical and electronic equipments within the automobiles instead of using mechanical energy directly from combustion that was meant for locomotion. A good area of research will be using chemical sorption pumps to cool down automobiles for comfort of passengers especially in warm countries. The current chemical heat pumps are too heavy and bulky to be used efficiently and properly as an air conditioner. Most automobile air-conditioners run on mechanical vapour compression heat pumps that divert part of the mechanical energy away from locomotion.
5.5 Localised heating
The ground is used a sensible heat storage. Heat charging occurs in warmer weather (e.g. summer). The heat stored will then be discharged during winter to maintain warmth within the buildings. The ground does also act as ‘cold’ storage (i.e. heat is discharged during winter, storing ‘cold’ in the process). This ‘cold’ storage can be use to cool the buildings at times of warm weather (e.g. summer). To maintain, rather constant cooling or heating rate in the buildings (i.e. rather constant ground temperature), PCMs can be used. This energy stored in the ground is used as ground-sourced heat pumps (GSHP). They are more efficient than conventional heating and air-conditioning technologies and typically have lower maintenance costs. Energy consumption is reduced by 30–70% in the heating mode and 20–50% in the cooling mode can be obtained (Benli and Durmuş, 2009). GSHP used in conjunction with PCM could potentially yield even higher savings. Using the ground as sensible heat
53
storage in certain countries where permafrost is significant may not go down well environmentally, as many climate scientists are worried that the melting of permafrost as a result of global warming, could raise sea level.
6. Conclusions This report identifies the various technologies available for thermal energy storage. The most promising routes are usually a combination of different technologies and vary by their intended applications.
When considering thermal storage technologies, this report also attempts to cover a few other variables, such as heat pumps to enable transfer of heat for heat storage, heat transfer fluid since various fluids have various heat transfer properties such as thermal conductivity, energy storage density, and design of storage tanks. All these factors determine the feasibility and the appropriate applications of certain thermal storage technologies. The variables considered in this report are only a general overview, not intended as a detailed study of each thermal storage technology.
Although there are applications where thermal energy storages are in the form of stored ‘cold’, for example using excess electricity to make and store ice to be used in cooling applications later on, this report does not attempt to cover the ‘cold’ storage as this review is based on the UK scenario where cooling applications are rarely needed.
A few technologies have been identified to be of great potential:
• Phase change materials (latent heat storage);
• Sensible heat storage;
• Chemical/ bond storage; and
• Metal hydride storage.
Applications of these TES technologies largely depend on type of heat source, for example:
(i) industrial heat source has better economy of scale than smaller more localised heat sources such as vehicle engines. The weight of the heat storage medium is of less importance in industry since the burden of transportation is less than in, for example, vehicle engines.
(ii) chemical/bond storage and metal hydride storage are particularly interesting because they have the potential to upgrade widely available low grade heat source to high grade heat as high grade heat source has more practical applications, especially in heavy industries.
a. Chemical/ bond storage also works as an adsorption heat pump with same function as mechanical vapour compression heat pump. Although adsorption heat pumps have generally lower COP < 1 while mechanical heat pumps have COP > 3, the former is more energy efficient than the latter. Adsorption heat pumps though are impeded by their bulkiness.
54
b. For metal hydrides, apart from storing energy in the form of metal-hydrogen bonds, metal hydrides also store energy in the form of readily combustible hydrogen gas. Metal hydride technologies are still at the stages of infancy.
Potential applications of TES technologies in the UK have been outlined to be in district heating, power plants, manufacturing/ heavy industry and localised heating. Future work will be the feasibility studies of these reviewed technologies in various applications through case and/or modelling studies of:
• the economic costs;
• mass and energy balances; and
• technological impediments.
55
References
Abdel-Wahed, R.M., Ramsey, J.W. and Sparrow, E.M., Photographic study of melting about an embedded horizontal heating cylinder. International Journal of Heat and Mass Transfer, 22 (1979), p. 171–173 Abdoly, M.A. and Rapp, D., Theoretical and experimental studies of stratified thermocline storage of hot water. Energy Conversion and Management, 22 (1982), p. 275–285 Abhat, A., Low temperature latent heat thermal energy storage: heat storage materials. Solar Energy, 30 (1983), p. 313–332. Aidom, Z. and Ternan, M., Salt impregnated carbon fibres as the reactive medium in a chemical heat pump: the NH3-CoCl2 system, Application of Thermal Engineering, 22 (2002), p. 1163–1173.
Air-Conditioning, Heating, Refrigeration Institute, http://www.ahrinet.org/standards.aspx, accessed on the 23rd of February (2011)
Agbossou, A., Zhang, Q., Sebald, G., and Guyomar, D., Solar micro-energy harvesting based on thermoelectric and latent heat effects. Part I: Theoretical analysis, Sensor and Actuarors A: Physical, 163 (2010), p. 277-283
Agrawal, R.K. and Wood, S.C., Innovative solutions for cost-effective NOx control. Pollution Engineering, 34 (2002), 18–25
Agyenim, F., Eames, P. and Smyth , M., Experimental study on the melting and solidification behaviour of a medium temperature phase change storage material (Erythritol) system augmented with fins to power a LiBr/H2O absorption cooling system. Renewable Energy ,36 (2011), p. 108-117 Agyenim, F., Smyth, M. and Eames, P.C., A review of phase change material energy storage; selection of materials suitable for energy storage in the 100-130oC temperature range. Aberdeen, UK: World Renewable Energy Congress (2005), p. 384-389. Agyenim, F., Hewitt, N., Eames, P. and Smyth, M., Numerical and experimental development of medium temperature thermal energy storage (Erythritol) system for the hot side of LiBr/H2O air conditioning applications. In: World Renewable Energy Congress (2008) Agyenim, F., Eames, P., Smyth, M., A comparison of heat transfer enhancement in a medium temperature thermal energy storage heat exchanger using fins. Solar Energy 83 (2009), p. 509–520
Agyenim, F., Hewitt, N., Eames, P. and Smyth, M., A review of materials, heat transfer and phase change problem formulation for latent thermal energy storage
56
problems (LHTESS). Renewable and Sustainable Energy Reviews, 14 (2010), p. 615-628
Agyenim, F., Eames, P. and Smyth, M., Heat transfer enhancement in medium temperature thermal energy storage system using a multi-tube heat transfer array. Renewable Energy, 35 (2010-b), p. 198–207 Ajah, A.N., Mesbah, A., Grievink, J., Herder, P.M., Falcao, P.W., and Wennekes, S., On the robustness, effectiveness and reliability of chemical and mechanical heat pumps for low-temperature heat source district heating: A comparative simulation-based analysis and evaluation. Energy, 33 (2008), p. 908–929 Andersen, E., Furbo, S. and Fan, J., Multilayer fabric stratification pipes for solar tanks. Solar Energy, 81 (2007), p. 1219–1226.
Ataer, O.E., Storage of thermal energy. http://www.eolss.net/ebooks/Sample%20Chapters/C08/E3-14-02-00.pdf, accessed on the 24th of February (2011)
Bahnfleth, W.P. and Song, J., Constant flow rate charging characteristics of a full-scale stratified chilled water storage tank with double ring slotted pipe diffusers. Applied Thermal Engineering, 25 (2005), p. 3067–3082
Bansal, N.K. and Buddhi, D., An analytical study of a latent heat storage system in a cylinder. Energy Conversion Management, 33 (1992), p. 235–242 Bell, M.A. and Smith I.E., Thermal energy storage using saturated salt solutions. Energy, 5(1980), p. 1085–1090 Benli, H. and Durmus, A., Evaluation of ground-source heat pump combined latent heat storage system performance in greenhouse heating. Energy and Buildings, 41 (2009), p. 220–228 Bennett, L.H., Shull, R.D. and Swartzendruber, L.J., Nano-composite material for magnetic refrigeration and super paramagnetic systems using the same. US Patent No. 5381664; January (1995) Boh, B, and Sumiga, B., Microencapsulation technology and its applications in building construction materials. RMZ—Materials and Geo-environment, 55 (2008), p. 329–344 Bohigas, X., Tunable magnetocaloric effect in ceramic Perovskites. Applied Physis Letters, 73 (1988), p. 390–392 Bonaccorsi, L., Freni, A., Proverbio, E., Restuccia, G. and Russo, F., Zeolite coated copper foams for heat pumping applications. Microporous and Mesoporous Materials, 91 (2006), p.7–14
57
Botsaris, G. D. and Glazman, Y. M. Interfacial Phenomena. In: Coal Technology, Marcel Dekker, New York (1989)
Bouhdjar, A. and Harhad, A., Numerical analysis of transient mixed convection flow in storage tank: influence of fluid properties and aspect ratios on stratification. Renewable Energy, 25 (2002), p. 555–567 Bradford, T., Natural gas: This house believes that natural gas will do more than renewables to limit the world’s carbon emissions. Economist debates, The Economist, 16th of February, (2011)
Brem, G., 2003. Advanced grate furnaces for future waste incineration. ISWA Beacon Conference Waste to Energy: State of the Art and Latest News. Malmo (2003) Available from: www.iswa.ch/Info/Documents/ WastetoEnergy03/Brem.pdf
Cabeza, L.F., Castell, A., Barreneche, C., de Gracia, A. and Fernández, A.I., Materials used as PCM in thermal energy storage in buildings: A review. Renewable and Sustainable Energy Reviews, 15 (2011), p.1675–1695 Carlsson, B. and Wettermark, G., Heat-transfer properties of a heat-of-fusion store based on CaCl2·6H2O. Solar Energy, 24 (1980), p. 239 Cerkvenik, B., Poredos, A. and Ziegler, F., Influence of adsorption cycle limitations on the system performance. International Journal of Refrigeration, 24 (2001), p. 475–485 Chahbani, M.H., Labidi, J. and Paris, J., Effect of mass transfer kinetics on the performance of adsorptive heat pump system. Applied Thermal Engineering, 22 (2002), p. 23–40 Charters, W.W.S., Brey, S. and Aye, L. Metal hydride heat pumps, refrigeration, climate control, and energy conversion. Proceedings of Meetings of IIR Commissions, 11th to 14th of February, Melbourne, (1996), p. 53-62 Chen, H., Goswami, D.Y., Rahman, M.M. and Stefanakos, E.K., A supercritical Rankine cycle using zeotropic mixture working fluids for the conversion of low-grade heat into power. Energy, 36 (2011), p. 549-555
Chen, H., Goswami, D.Y. and Stefanakos, E.K., A review of thermodynamic cycles and working fluids for the conversion of low-grade heat. Renewable and Sustainable Energy Reviews, 14 (2010) p. 3059–3067
Chiras, D. The Solar House: Passive Heating and Cooling. Chelsea Green Publishing Company; (2002)
Choi, J.C. and Kim, S.D., Heat transfer characteristics of a latent heat storage system using MgCl2.6H2O. Energy, 17(1992), p. 1153–1164
58
Cook-Chennault, K.A., Thambi, N. and Sastry, A.M., Powering MEMS portable devices—a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems, Smart Materials and Structures, 17 (2008), p. 1–33 Costello, Y.A., Melsheimer, S.S. and Edie, D.D., Heat transfer and calorimetric studies of a direct contact-latent heat energy storage system; thermal storage and heat transfer in solar energy system. ASME Meeting, San Francisco, USA, (1978), p. 10–5 Crandall, D.M. and Thacher, E.F., Segmented thermal storage. Solar Energy, 77 (2004), p. 435–440 Davidson, J.H., Adams, D.A. and Miller, J.A., A coefficient to characterize mixing in solar water storage tanks. Transactions of the ASME. Journal of Solar Energy Engineering, 116 (1994), p. 94–99 De Beijer, H.A. and J.W.K. Horsman, J.W.K., SWEAT Thermo-mechanical heat pump storage system. International Absorption Heat Pump Conference, ASME-AES, 31 (1993), p. 457-462 De Jong, M., Lamers, F., Lamain, J. and Veldman, H., High efficiency waste fired power plant of AEB – experiences with materials, http://www.beacon-wte.org/fileadmin/avfallsverige/Documentation_2009/F_Lamers.pdf, accessed on the 18th of February (2011)
Dellero, T., Sarmeo, T.D., Touzain, Ph., A chemical heat pump using carbon fibers as additive. Part I: enhancements of thermal conduction. Applied Thermal Engineering, 19 (1999), p. 991–1000
Demir, H., Mobedi, M. and Ülkü, S., A review on adsorption heat pump: Problems and solutions. Renewable and Sustainable Energy Reviews, 12 (2008), p. 2381–2403
Demir, H., Mobedi, M. and Ülkü, S., Effects of porosity on heat and mass transfer in a granular adsorbent bed. International Communications in Heat and Mass Transfer, 36 (2009), p. 372–377
Denholm, P. and Holloway, T., Improved accounting of emissions from utility energy storage system operation, Environmental Science and Technology, 39 (2005) p. 9016 – 9022
Denholm, P. and Kulcinski, G.L., Life cycle energy requirements and greenhouse gas emissions from large scale energy storage systems, Energy Conversion and Management, 45 (2004), p. 2153 – 2172.
59
Dincer, I. and Rosen, M.A., Thermal energy storage, systems and applications. Chichester (England): John Wiley & Sons (2002)
Dous, N. and Meunier, F., Experimental study of cascading adsorption cycles. Chemical Engineering Science, 44 (1989), p. 225–235
Duffie, J. A. and Beckman, W. A., Solar Energy of Thermal Processes, John Wiley & Sons, (1989) Edie, D.D. and Melsheimer, S.S., An immiscible fluid-heat of fusion energy storage system. In: Proceedings of Sharing the Sun: Solar Technology in the Seventies. A Joint Conference of the American Section of the International Solar Energy Society and the Solar Energy Society of Canada Inc., Winnipeg. The American Section of the International Solar Energy Society, (1976), p. 227–237 Eriksson, M. and Vamling, L., Future use of heat pumps in Swedish district heating systems: Short- and long-term impact of policy instruments and planned investments. Applied Energy, 84 (2007), p. 1240-1257
Ermis, K., Erek, A. and Dincer, I., Heat transfer analysis of phase change process in a finned-tube thermal energy storage system using artificial neural network. International Journal of Heat and Mass Transfer, 50 (2007), p. 3163–3175.
Esen M, Durmus A, Durmus A. Geometric design of solar-aided latent heat store depending on various parameters and phase change materials. Solar Energy, 62(198), p.19–28
Farid, M.M., Khudhair, A.M., Razack, S.A.K. and Al-Hallaj, S., A review on phase change energy storage: materials and applications. Energy Conversion and Management, 45 (2004), p. 1597-1615
Farid, M.M. and Yacoub, K., Performance of direct contact latent heat storage unit. Solar Energy, 43 (1989), p. 237–252 Farid, M.M. and Khalaf AN. Performance of direct contact latent heat storage units with two hydrated salts. Solar Energy, 52 (1994), p. 179–189
Feist, W., Climate neutral passive house estate in Hannover-Kronsberg: Construction and measurement results, PEP Project Information No.1, accessed on the 16th of February (2011), p. 72
60
Feldman, D., Shapiro, M.M. and Fazio, P and Sayegh, S., The compressive strength of cement blocks permeated with an organic phase change material. Energy and
Buildings, 6 (1984), p. 85–92
Feldman, D., Shapiro, M.M. and Banu, D., Organic phase change materials for thermal energy storage, Solar Energy Materials, 13 (1986), p. 1-10 Feldman, D., Shapiro, M.M., Banu, D. and Fuks, C. J., Fatty acids and their mixtures as phase-change materials for thermal energy storage. Solar Energy Materials, 18 (1989), p. 201-216 Fouda, A.E., Despault, G.J., Taylor, J.B. and Capes, C.E., Solar storage system using salt hydrate latent heat and direct contact heat exchange––II, characteristics of pilot operating with sodium sulfate solution. Solar Energy, 32 (1984), p. 57–65
Freni, A., Russo, F., Vasta, S., Tokarev, M., Aristov, Yu.I. and Restuccia, G., An advanced solid sorption chiller using sws-1l, Proceedings of the 3rd International Conference Heat Powered Cycles, Larunaka, Cyprus (2004), p. 1214
Froning, S., Low energy communities with district heating and cooling. 25th Conference on passive and low energy architecture. Dublin (2008)
Fujioka, K., Hatanaka, K. and Hirata, Y., Composite reactants of calcium chloride combined with functional carbon materials for chemical heat pumps. Applied Thermal Engineering, 28 (2008), p. 304–310 Fujioka, K., Sasaki, K. Oido, K. and Hirata, Y., Enhancement of heat transfer in calcium chloride reactor bed for chemical heat pumps by use of graphite composite particles, Proceedings of 5th Minsk International Seminar ‘‘Heat Pipes, Heat Pumps, Refrigerators’’, Minsk, Belarus (2003) Fujisawa, R., Watanabe, F., Kobayashi, N. and Hasatani, M., Operating properties of adsorption heat pump driven by a fuel cell. Kagaku kogaku Ronbunshu, 28 (2002), p. 247–254 Galen, E. and Van den Brink, G.J., Energy storage in phase change materials for solar applications. International Journal of Ambient Energy, 7 (1986), p. 31–46
Garg, H.P. and Nasim, M., Studies on low-temperature salt-hydrate for thermal
storage applications. Energy Conversion and Management, 21 (1981), p. 125–130
Gastauer, P. and Kameyama, H., The thermal efficiency of the isopropanol/acetone/hydrogen chemical heat pump: analysis and improvement. In: Proceedings of International Hydrogen and Clean Energy Symposium, IHCE’95, February 6–8, Keidanren Hall, Tokyo, Japan, (1995), p. 317–320
61
Ghoneim, A.A., Comparison of theoretical models of phase-change and sensible heat storage for air and water-based solar heating systems. Solar Energy, 42(1989), p. 209–20.
Giebel, G., Wind power has a capacity credit Risø National Laboratory. Accessed on the 15th of February, (2011)
Goetz, V. and Guillot, A., An open activated carbon/CO2 sorption cooling system. Industrial and Engineering Chemistry Research, 40 (2001), p. 2904–2913
Griffiths, P.W. and Eames, P.C., Performance of chilled ceiling panels using phase change material slurries as the heat transport medium. Applied Thermal Engineering, 27 (2007), p. 1756–1760
Gui, Y.B., Wang, R.Z., Wang, W., Wu, J.Y., Xu, Y.X., Performance modeling and testing on a heat-regenerative adsorptive reversible heat pump. Applied Thermal Engineering, 22 (2002), p. 309–320
Guilleminot, J.J., Choisier, A., Chalefen, J.B., Nicolas, S. and Reymoney, J.L., Heat transfer intensification in fixed bed adsorbers. Heat Recovery System CHP 13 (1993), p. 297–300. Hahne, E. and Chen, Y., Numerical study of flow and heat transfer characteristics in hot water stores. Solar Energy, 64 (1998), p. 9–18 Haller, M.Y., Cruikshank, C.A., Streicher, W., Harrison, S.J., Andersen, E. and Furbo, S., Methods to determine stratification efficiency of thermal energy storage processes – Review and theoretical comparison. Solar Energy, 83 (2009), p. 1847–1860
Hamada, Y., Ohtsu, W. and Fukai, J.. Thermal response in thermal energy storage material around heat transfer tubes: effect of additives on heat transfer rates. Solar Energy, 75 (2003), p. 317–328
Hamamoto, Y., Alam, K.C.A., Akisawa, A. and Kashiwagi, T., Performance evaluation of a two-stage adsorption refrigeration cycle with different mass ratio. International Journal of Refrigeration, 28 (2005), p. 344–352
Han, J.H. and Lee, K.H., Effective thermal conductivity of graphite–metallic salt complex for chemical heat pumps, Journal Thermophysics and. Heat Transfer, 13 (1999), p. 481–488
62
Hasan, A. and Sayigh, A. A., Some fatty acids as phase-change thermal energy storage materials. Renewable Energy, 4 (1994), p. 69-76
Hasnain, S. M., Ph.D Dissertation, Department of Fuel and Energy, University of Leeds, UK, 1990
Hasnain, S.M., Review on sustainable thermal energy storage technologies, part 1: Heat storage materials and techniques. Energy Conversion and Management, 39 (1998), p. 1127-1138
Hawlader, M.N.A., Uddin, M.S. and Khin, M.M., Microencapsulated PCM thermal energy storage system. Applied Energy 74 (2003), p. 195–202.
Hendra, R., Hamdani, Mahlia, T.M.I. and Masjuki, H.H., Thermal and melting heat transfer characteristics in a latent heat storage system using mikro. Applied Thermal Engineering, 25 (2005), p. 1503–1515.
Henning, H.M. and Wiemken, E., Solar assisted air conditioning of buildings overview on technologies and state of the art. In: Proceedings of ISES Solar World Congress. Gothenburg, Sweden, 2003
Hirata, Y., Fujioka, K., and Fujiki, S., Preparation of fine particles of calcium chloride with expanded graphite for enhancement of diving reaction for chemical heat pumps, Journal of Chemical Engineering Japan, 37 (2003), p. 827–832
Holttinen, H., Meibom, P., Orths, A., Van Hulle, F., Ensslin, C., Hofmann, L., McCann, J., Pierik, J., Tande, J.O., Estanqueiro, A., Söder, L., Strbac, G., Parsons, B., Smith, J.C. and Lemström, B., International Energy Agency Wind Task Force, "Design and Operation of Power Systems with Large Amounts of Wind Power", Oklahoma Conference Presentation, October (2006)
Hoogendoorn, C.J. and Bart, G.C.J., Performance and modelling of latent heat stores. Solar Energy, 48(1992):53–58
Horbaniuc, B., Dumitrascua, G. and Popescub, A., Mathematical models for the study of solidification within a longitudinally finned heat pipe latent heat thermal storage system. Energy Conversion & Management, 40(1999), p. 1765–1774.
Hoshi, A., Mills, D.R., Bittar, A. and Saitoh, T.S., Screening of high melting point phase change materials (PCM) in solar thermal concentrating technology based on CLFR. Solar Energy, 79 (2005) p. 332–339
63
Hu, E.J., Zhu, D.S., Sang, X.Y., Wang, L. and Tan, Y.K., Enhancement of thermal conductivity by using polymer–zeolite in solid adsorption heat pumps, Trans ASME, Journal of Heat Transfer, 119 (1997), p. 627–629
Hull, J.R. and Uherka, K.L., Magnetic heat pumps for near room temperature applications, Energy 14 (1989), p. 177–185 Hung, T.C., Shai, T.Y., Wang, S.K., A review of organic Rankine cycles (ORCs) for the recovery of low-grade waste heat. Energy 22 (1997), p. 661-667
International Organization for Standardization (ISO), http://www.iso.org/iso/standards_development.htm, accessed on the 23rd of February (2011)
Ismail, K.A.R., Alves, C.L.F., Modesto, M.S., Numerical and experimental study on the solidification of PCM around a vertical axially finned isothermal cylinder. Applied Thermal Engineering, 21 (2001), p. 53–77
Joosten, L., Strom, I., and Boonstra, C., Energy saving potential. PEP: Promotion of European Passive Houses, 23rd of May (2005)
Kamimoto, M., Abe, Y., Kanari, K., Swata, S. and Ozawa, T., Latent thermal storage unit using form-stable high density polyethylene; Part I: Performance of the storage unit Trans ASME, Journal of Solar Energy Engineering, 108 (1986), p. 282
Kandari, A.M., Thermal stratification in hot storage-tanks. Applied Energy, 35 (1990), p. 299–315
Karaca, F., Kincay, O. and Bolat, E., Economic analysis and comparison of chemical heat pump systems. Applied Thermal Engineering, 22 (2002), p. 1789-1799
Khan, M.Z.I., Alam, K.C.A., Saha, B.B., Hamamoto, Y., Akisawa, A. and Kashiwagi, T., Parametric study of a two-stage adsorption chiller using re-heat the effect of overall thermal conductance and adsorbent mass on system performance. International Journal of Thermal Sciences, 45 (2005), p. 511–519
Khudhair, A.M. and Farid, M.M. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Conversion and Management, 45 (2004), p. 263–275 Kim, K.J., Feldman, K.T. Jr. and Razani, A., Heat-driven hydride heat pumps. International Journal of Refrigeration, 20 (1997), p. 339-351
64
Kimura, H. and Kai, J., Phase change stability of CaCl2.6H2O. Solar Energy, 33 (1984), p. 49–55
KlinSoda, I. and Piumsomboon, P., Isopropanol–acetone–hydrogen chemical heat pump: A demonstration unit. Energy Conversion and Management, 48 (2007), p. 1200–1207
Knowles, T.R., Proportioning composites for efficient thermal storage walls. Solar Energy, 31 (1983), p. 319–326
Kodama, A., Jin, W., Goto, M., Hirose, T. and Pons, M., Entropic analysis of adsorption open cycles for air conditioning. Part 2: interpretation of experimental data. International Journal of Energy Research, 24 (2000), p. 263–278
Kondratenko, I., Brophy, V., Hernandez, P. and Burke, K., Potential for energy and CO2 emission savings through the application of the Passive house standard in Ireland. 10th International Passive House Conference (2006)
Kuntz, M.T. and Dawe, J., Renewable, rechargeable, remarkable. VRB Power Systems. Mechanical Engineering, accessed on the 15th of February, (2011)
Lambert, M.A., Design of solar powered adsorption heat pump with ice storage. Applied Thermal Engineering, 27 (2007), p. 219-233
Lane, G.A, Solar Heat Storage; Latent Heat Materials. Vol. 1. CRC Press Inc., USA., (1983)
Lane, G.A., Phase change materials for energy storage nucleation to prevent subcooling. Solar Energy Materials and Solar Cells, 27 (1991), p. 135–160
Lauerhass, L.N. and Rudd, D.F., On the thermodynamics of the chemical heat pump, Chemical Engineering Science, 36 (1981), p. 803–807
Li, H., Luo, L. and Fraisse, G., Development and applications of solar-based thermoelectric technologies. Renewable and Sustainable Energy Reviews, 11 (2007), p. 923–936
Liuzzo, G., Verdone, N. and Bravi, M., The benefits of flue gas recirculation in waste incineration. Waste Management, 27 (2007), p. 106 - 116
65
Liuzzo, G., Bemporad, E., Lacquaniti, L., Palitto, M., I prodotti di combustione negli impianti termici e negli inceneritori di rifiuti (Combustion products in thermal power plants and waste incinerators). Ingegneria Sanitaria-Ambientale, 39 (1992), p. 21–44.
Liuzzo, G., Verdone, N., Bemporad, E., Lacquaniti, L., Palitto, M., Rendimenti di conversione energetica mediante il trattamento dei fumi ad alta temperatura negli inceneritori di rifiuti (Energy conversion efficiencies through high temperature flue gas cleaning in waste incinerators). GEA 8 (1995), 44–52
Lodhi, M. A. K., Solar ponds in alkaline lake and oil well regions. Energy Conversion and Management, 37 (1996), p. 1677-1694
Loh, W.S., El-Sharkawy, I.I., Ng, K.C. and Saha, B.B., Adsorption cooling cycles for alternative adsorbent/adsorbate pairs working at partial vacuum and pressurized conditions, Applied Thermal Engineering, 29 (2009), p. 793–798 Lund, H. and Mathiesen, B.V., Energy system analysis of 100% renewable energy systems - the case of Denmark in years 2030 and 2050. Energy, 34 (2009), p. 524-531 Lund, H., Renewable energy strategies for sustainable development. Energy, 32 (2007), p. 912-919. Lund, H., Möller, B., Mathiesen, B.V. and Dyrelund, A., The role of district heating in future renewable energy systems. Energy, 35 (2010), p. 1381-1390 Markley, K. S., Fatty acids, Part 4, John Wiley and Sons, UK, (1967) Mauran, S, Parades, P., L’Haridon, F., Heat and mass transfer in consolidated reacting beds for thermochemical systems. Heat Recovery System, CHP 13 (1993), p. 315–319 Mavros, P., Belessiotis, V. and Haralambopoulos, D., Stratified energy storage vessels: characterization of performance and modeling of mixing behavior. Solar Energy, 52 (1994), p. 327–336 McCabe, W.L., Smith, J.C. and Hardott, P. Unit Operations of Chemical Engineering. McGraw-Hill, New York (1985) Mehling, H. and Cabeza, L.F., Heat and cold storage with PCM. An up to date introduction into basics and applications. Springer; (2008) Metcalf, J.S., Simulation of the effect of generator heat transfer parameters on power density and efficiency in multiple-bed regenerative carbon–ammonia sorption heat pumps. In: Proceedings of the international sorption heat pump conference, Denver, USA, (2005)
66
Meunier, F., Adsorptive cooling: a clean technology. Clean Products and Processes, 3 (2002), p. 8–20
Meunier F. Second law analysis of a solid adsorption heat pump operating on reversible cascade cycles: application to the zeolite–water pair. Journal of Heat Recovery Systems, 5 (1985), p. 133–141.
Mettawee, E.S. and Assassa, G.M.R., Thermal conductivity enhancement in a latent heat storage system. Solar Energy, 81(2007), p. 839–845.
Miller, W.M., Energy storage via desiccants for food/agricultural applications.
Energy in Agriculture, 2 (1983), p. 341–54.
Mooksuwan, W. and Kumar, S., Study on 2-propanol/acetone/hydrogen chemical heat pump: endothermic dehydrogenation of 2-propanol, International Journal Energy Research, 24 (2000) p. 1109–1122
Moretti, P.M. and McLaughlin, D.K., Hydraulic modeling of mixing in stratified lakes. Journal of the Hydraulics Division, 103 (1977), p. 367–380
Morrison, D.J. and Abdel-Khalik, S.I., Effects of phase-change energy storage on the performance of air-based and liquid-based solar heating systems. Solar Energy, 20 (1978), p. 57–67
Muthukumar, P. and Groll, M., Metal hydride based heating and cooling systems: A review. International Journal of Hydrogen Energy, 35 (2010), p. 3817-3813
Nagel, M., Komasaki, Y., Uchida, M., Suda, S. and Matsubara, Y., Operating characteristics of a metal hydride heat pump for generating cool air, Journal of Less Common Metals, 104 (1984), p. 307-318 Nakaso, K., Anai, M., Sasaki, Y., Hamada, Y. and Fukai, J., Improvement of heat transfer characteristics in a solid–gas thermochemical reactor. 10th APCChE Congress, Kitakyushu, Japan (2004) 2G-07. Nelson, J.E.B., Balakrishnan, A.R. and Murthy, S.S., Experiments on stratified chilled water tanks. International Journal of Refrigeration, 22 (1999), p. 216–234.
Nilsson, S.F., Reidhav, C., Lygnerud, K. and Werner, S., Sparse district heating in Sweden. Applied Energy, 85 (2008), p. 555-564
Niu, X., Yu, J. and Wang, S., Experimental study on low temperature waste heat thermoelectric generator, Journal of Power Sources, 188 (2009), p. 621–626.
67
Nunez, T., Mittelbach, W., Henning, M.H., Development of a small capacity adsorption system for heating and cooling applications. In: Proceedings of the international sorption heat pump conference, Denver, USA, (2005)
Ogura, H., Miyazaki, M., Matsuda, H., Hasatani, M., Yanadori, M., Hiramatsu, M., Experimental study on heat transfer enhancement of the solid reactant particle bed in a chemical heat pump using Ca(OH)2/ CaO reaction. Kagaku kogaku Ronbunshu 17 (1991), p. 916–923
Oscik, J., Adsorption. Ellis Horwood Ltd. John Wiley and Sons, (1982)
Papanicolaou, E. and Belessiontis, V., Transient natural convection in a cylindrical enclosure at high Rayleigh numbers. Heat and Mass Transfer, 45 (2001), p. 1425–1444
Paris, J., Falardeau, M. and Houle, J. F., In: Proceedings of World Renewable Energy Congress, Reading, UK, (1990), p. 2397 Paris J., Falardeau M. and Villeneuve C., Thermal Storage by Latent Heat: A Viable Option for Energy Conservation in Buildings. Energy Sources, 15 (1993), p. 85-93
Pecharsky, V.K. and Gschneidner Jr, K.A., Effect of alloying on the giant magnetocaloric effect of Gd5(Si2Ge2), Journal of Magnetism and Magnetic Mater, 167 (1997), p. 179–184
Penella, M.T. and Gasulla, M., A review of commercial energy harvesters for autonomous sensors, IEEE Instrumentation and Measurement Technology Conference, Poland, 1st to 3rd of May, (2007) 1807–1811
Perry, R., In: Chemical Engineering Handbook, Sixth Edition. McGraw Hill, New York, (1984)
Pillai, K.K. and Brinkworth, B.J., The storage of low grade thermal energy using phase change materials. Applied Energy, 2 (1976), p. 205–216
Pino, L., Aristov, Y., Cacciola, G., Restuccia, G., Composite materials based on zeolite 4A for adsorption heat pumps. Adsorption, 3 (1996), p. 33–40
Ponec, V., Knor, Z. and Cerny, S., Adsorption on Solids. Butterworths and Co., London, (1974)
68
Pons, M., and Kodama, A., Entropic analysis of adsorption open cycles for air conditioning. Part 1: first and second law analyses. International Journal of Energy Research, 24 (2000), p. 251–262
Pons, M., Laurent, D., Meunier, F., Experimental temperature fronts for adsorptive heat pump applications, Applied Thermal Engineering, 16 (1996), p. 395-404
Pons, M., Meunier, F., Cacciola, G., Critoph, R.E., Groll, M., Puigjaner, L., Spinner, B. and Ziegler, F., Thermodynamic based comparison of sorption systems for cooling and heat pumping. International Journal of Refrigeration, 22 (1999) p. 5–17
Pons, M. and Szarzynski, S., Accounting for the real properties of the heat transfer fluid in heat-regenerative adsorption cycles for refrigeration. International Journal of Refrigeration, 23 (2000), p. 284 – 291
Poyelle, F., Guilleminot, J.J., Meunier, F., Experimental tests and predictive model of an adsorptive air conditioning unit. Industrial Engineering and Chemistry Research, 38 (1999), p. 298–309
Regin, A.F., Solanki, S.C. and Saini, J.S., Heat transfer characteristics of thermal energy storage system using PCM capsules: A review. Renewable and Sustainable Energy Reviews, 12 (2008), p. 2438–2458
Rockenfeller, U., and Kirol, L.D., HVAC and heat pump development employing complex compound working media, International Absorption Heat Transfer Conference, 19th to 21 Jan., New Orleans, U.S.A., ASME-AES, 31 (1993), p. 433-437
Ron, M., A hydrogen heat pump as a bus air conditioner, Journal of Less Common Metals, 104 (1984), p. 259-278 Rosen, M.A., Tang, R. and Dincer, I., Effect of stratification on energy and exergy capacities in thermal storage systems. International Journal of Energy Research, 28 (2004), p. 177–193. Rudd, A., Phase Change Material Wallboard for Distributed Thermal Storage in Buildings. ASHRAE Transactions: Research, Volume 99, Part 2. Atlanta, GA (1993)
Ruthven, D.M., Principles of Adsorption and Adsorption Processes. John Wiley and Sons, 1984 Ryu, H.W., Woo, S.W., Shin, B.C. aqnd Kim, S.D., Prevention of subcooling and stabilization of inorganic salt hydrates as latent heat storage materials. Solar Energy Materials and Solar Cells, 27 (1992), p. 161–172
69
Saha, B.B., Akisawa, A., Kashiwagi, T., Silica gel water advanced adsorption refrigeration cycle. Energy, 22 (1997), p. 437–447
Saha, B.B., Koyama, S., Ng, C.K., Hamamoto, Y., Akisawa, A., Kashiwagi, T., Study on a dual-mode, multistage, multi-bed regenerative adsorption chiller. Renewable Energy, 31 (2006), p. 2076–2090
Saito, Y., Chemical heat pump and hydrogen transportation. In: Proceedings of International Hydrogen and Clean Energy Symposium, IHCE’95, February 6–8, Keidanren Hall, Tokyo, Japan, (1995), p. 123–130
Salyer, I.O. and Botham, R.A., in Proc. 13th IECEC, Vol. 948, (1978)
Sasaguchi, K. and Takeo, H., Effect of the orientation of a finned surface on the melting of frozen porous media. Heat and Mass Transfer, 37(1994), p.13–26
Schmitt F, Hoffman HW, Göhler T. Strategies to manage heat losses e technique and economy, IEA-DHC ANNEX VII; (2005)
Shah, L., Andersen, E. and Furbo, S., Theoretical and experimental investigations of inlet stratifiers for solar storage tanks. Applied Thermal Engineering, 25 (2005), p. 2086–2099
Sharma, A., Tyagi, V.V., Chen, C.R. and Buddhi, D., Review on thermal energy storage with phase change materials and applications. Renewable and Sustainable Energy Reviews, 13 (2009), p. 318–45
Shir, F., Mavriplis, C., Bennett, L.H. and Torre, E.D., Analysis of room temperature magnetic regenerative refrigeration, International Journal of Refrigeration, 28 (2005), p. 616-627
Shir, F., Yanik, L., Della Torre, E., Bennett, L.H. and Shull, R.D., Room temperature active magnetic refrigeration: magnetic nanocomposites, Journal of Applied Physics, 93 (2003), p. 8295–8297 Shitzer, A. and Levy, M., Transient Behavior of a Rock-Bed Thermal Storage System Subjected to Variable Inlet Air Temperatures: Analysis and Experimentation. Solar Energy Engineering, 105 (1983), p. 200
Silva, P.D., Goncalves, L.C. and Pires, L., Transient behaviour of a latent-heat thermal energy store: numerical and experimental studies. Applied Energy, 73 (2002) p. 83–98
70
Sinden, G., Assessing the Costs of Intermittent Power Generation. UK Energy Research Centre, 5th of July, (2005)
Sliwinski, B.J., Mech, A.R. and Shih, T.S., Stratification in thermal storage during charging. In: Proceedings of the 6th International Heat Transfer Conference, Toronto, vol. 4, (1978), p. 149–154 Snijder, E. D., Versteeg, G. F., van Swaaij, W. P. M., Kinetics of hydrogen absorption and desorption in LaNi5-xAlx slurries. AICHE Journal, 39 (1993) p. 1444-1454
Solomon, A.D., Design criteria in PCM wall thermal storage. Energy, 4 (1979), p. 701–709
Sorour, M. M., Performance of a small sensible heat energy storage unit. Energy Conversion and Management, 28 (1988), p. 211
Sparrow, E.M., Larson, E.D. and Ramsey, J.W., Freezing on a finned tube for either
conduction-controlled or natural convection-controlled heat transfer. International
Journal of Heat and Mass Transfer, 24 (1981), p. 273–284
Spinner, B., Sorin, M. and Stitou, D., Internal energy flow analysis within a single effect sorption heat pump. International Journal of Refrigeration, 24 (2001), p. 185–191
Srivastava, N.C. and Eames, I.W., A review of adsorbents and adsorbates in solid–vapour adsorption heat pump systems. Applied Thermal Engineering, 18 (1998), p. 707–714
Stefan, H.G. and Gu, R., Efficiency of jet-mixing of temperature stratified water. Journal of Environmental Engineering, ASCE 118 (1992), p. 363–379
Stephens, D., “Annualized geo-solar heating” as a sustainable residential-sale solution for temperate climates with less than ideal daily heating season on solar availability, http://greenershelter.org/TokyoPaper.pdf, accessed on the 18th of February (2011)
Sumathy, K., An energy efficient solar ice-maker. In: Proceedings of the 8th international symposium on renewable energy education (ISREE8), August 4–8, (2002)
Suzuki, M., Adsorption Engineering. Elsevier Science Publishers, (1990)
71
Szarzynski, S., Feng, Y. and Pons, M., Study of different internal vapour transports for adsorption cycles with heat regeneration. International Journal of Refrigeration, 20 (1997), p. 390–401
Tamainot-Telto, Z, and Critoph, R.E., Adsorption refrigerator using monolithic carbon–ammonia pair, International Journal of Refrigeration, 20 (1997), p. 146– 155
Taneda, D., Yasutomi, I. and Shibata, S., Studies of 2-propanol/acetone/hydrogen chemical heat pump. In: Proceedings of International Hydrogen and Clean Energy Symposium, IHCE’95, February 6–8, Keidanren Hall, Tokyo, Japan, (1995), p. 325–328
Tather, M. and Erdem-Senatalar, A., Polymeric heat exchangers to increase the COP values of adsorption heat pumps utilizing zeolite coatings. Applied Thermal Engineering, 24 (2004), p. 69–78
Telkes, M., Thermal energy storage in salt hydrates. Solar Energy Materials, 2(1980), p. 381–393.
Telkes, M. and Raymond, E., Storing solar heat in chemicals — a report on the Dover house. Heat Vent, 46(1949), p. 80–86
Telto, T.Z., Compact sorption generator prototype. In: Proceedings of the international sorption heat pump conference, Denver, USA, (2005)
Tillman, D.A., Rossi, A.J., Vick, K.M., Incineration of Municipal and Hazardous Solid Wastes. Academic Press, Inc. (1989)
Torio, H. and Schmidt, D., Development of system concepts for improving the performance of a waste heat district heating network with exergy analysis. Energy and Buildings 42 (2010) 1601–1609
Tran, N., Kreider, J.F. and Brothers, P., Field measurements of chilled water storage thermal performance. ASHRAE Transactions, 95 (1989), p. 1106–1112 Trelles, J.P. and Dufly, J.J., Numerical simulation of porous latent heat thermal energy storage for thermoelectric cooling. Applied Thermal Engineering, 23 (2003), p. 1647–1664
Tyagi, V.V. and Buddhi, D., PCM thermal storage in buildings: a state of art, Renewable and Sustainable Energy Reviews, 11 (2007), p. 1146–1166
72
Ülkü, S., Adsorption heat pumps. Heat Recovery System, (1986), p. 277–284
Ülkü, S., Solar adsorption heat pumps, solar energy utilization: fundamentals and applications. Yüncü, H., Paykoç, E. and Yener, Y. (Eds). The Netherlands: Martinus Nijkoff Publishers, (1987)
Ülkü, S. and Mobedi, M., Adsorption in energy storage, (Second Preliminary Edition). In: S. Kakac, S and Kilkiş, B., (Eds), NATO ASI on Energy Storage 167, Kluwer (1989), p. 487–507
United Nations Industrial Development Organization (UNIDO). Policies for promoting industrial energy efficiency in developing countries and transition economies, http://www.unido.org/fileadmin/media/documents/pdf/ Energy_Environment/ind_energy_efficiencyEbookv2.pdf, accessed on the 18th of February (2011)
United States Environmental Protection Agency, http://www.epa.gov/osw/hazard/wastemin/minimize/energyrec/rpsinc.htm, accessed on the 18th of February (2011)
Urban, A.I., Experiments pertaining to the recirculation of flue gas in waste incinerator system. In: International Recycling Congress, Berlin (1979), p. 405–410
Urban, A.I., Optimization of combustion by recirculation of flue gas. In: Recycling International: Recovery Energy Mater. Residues Waste, International Recycling Congress, Berlin (1982), p. 259–264
U.S Energy Information Administration. International Energy Outlook: World Energy and Economic Outlook, at: http://www.eia.doe.gov/oiaf/ieo, accessed on the 18th of of February (2011)
Valkov, V., Dodelet, J.P., Dllero, T., Nikanpour, D., Roy, J.-Fr. and Rondequ, L.C., A new thermochemical material and reactor for space systems, Proceedings of the International Sorption Heat Pump Conference, Shanghai, (2002), p. 438–443 van Berkel, J., Thermocline entrainment in stratified energy stores. Ph.D. Thesis, Technische Universiteit Eindhoven, The Netherlands, (1997), p. 32
Velraj, R., Seeniraj, R.V., Hafner, B., Faber, C. and Schwarzer, K., Experimental analysis and numerical modelling of inward solidification on a finned vertical tube for a latent heat storage unit. Solar Energy, 60(1997), p. 281–290
73
Wada, H., Taniguchi, K. and Tanabe, Y. Extremely large magnetic entropy change of MnAs-1Sbx near room temperature, Mater. Trans. JIM, 43 (2002), p. 73–77
Wang, C.D., Wu, Y.J., Wang, Z.R. and Xia, Z.Z., Research on a novel adsorption chiller driven by low grade heat source. In: Proceedings of the international sorption heat pump conference, Denver, USA, (2005-a)
Wang, Z.R, Kong, Q.X., Wu, Y.J., Huangfu, Y and Wu, W.D., Performance research of a micro cchp system with adsorption chiller. In: Proceedings of the international sorption heat pump conference, Denver, USA, (2005-b)
Wang, S.G., Wang, R.Z. and Li, X.R., Research and development of consolidated adsorbent for adsorption systems. Renew Energy, 30 (2005-c), p. 1425–1441
Wang, Y., Energy efficiency policy and CO2 in China’s industry: tapping the
potential. Beijing, China: Tsinghua University; (2006)
Wang, D.C., Xia, Z.Z. and Wu, J.Y., Design and performance prediction of a novel zeolite-water adsorption air conditioner. Energy Conversion and Management, 47 (2006), p. 590 – 610
Wang, L.W., Wu, J.Y., Wang, R.Z., Xu, Y.X. and Wang, S.G., Experimental study of a solidified activated carbon-methanol adsorption ice maker. Applied Thermal Engineering, 23 (2003), p. 1453 – 1462
Wang, R.Z. and Oliveira, R.G., Adsorption refrigeration – An efficient way to make good use of waste heat and solar heat. Progress in Energy and Combustion Science, 32 (2006), p. 424 – 458
Wang, X., Zhang, Y., Xiao, W., Zeng, R., Zhang, Q, and Di, H., Review on thermal performance of phase change energy storage building envelope. Chinese Science Bulletin, 54 (2009), p. 920–928. Weast, R. C. and Astle, M. J., CRC Handbook of Data on Organic Compounds, CRC Press Inc., (1985)
Weast, T. E., Shannon, L. J. and Ananth, K. P., Study of thermal energy storage using fluidized bed heat exchangers, in Proceedings of the 15th International Energy Conversion Engineering Conference, (1980)
74
Wildin, M.W., Diffuser design for naturally stratified thermal storage. ASHRAE Transactions, 96 (1990), p. 1094–1102
Wood, M.E. and Potter, W.H., General analysis of magnetic refrigeration and its optimization using a new concept: maximizing of refrigerant capacity. Cryogenics, 25 (1985), p. 666–683
Wang, S. and Zhu, D., A novel type of coupling cycle for adsorption heat pumps. Applied Thermal Engineering, 22 (2002), p. 1083–1086.
Wongsuwan, W., Kumar, S., Neveu, P. and Meunier, F., A review of chemical heat pump technology and applications. Applied Thermal Engineering, 21 (2001), p. 1489–1519
Worrell, E. and Price, L., Policy scenarios for energy efficiency improvement in industry. Energy Policy, 29 (2001), p.1223–41
Wu, J.Y., Wang, R.Z. and Xu, Y.X., Dynamic analysis of heat recovery process for a continuous heat recovery adsorption heat pump. Energy Conversion and Management 43, (2002), p. 2201–2211
Wyman, C., Cestle, J. and Kreith, F., A review of collector and energy storage technology for intermediate temperature applications. Solar Energy, 24 (1980), p. 517
Yanbing, K., Yinping, Z., Yi, J. and Xin, Z.Y., A general model for analyzing the thermal characteristics of a class of Latent Heat Thermal Energy Storage Systems. ASME Journal of Solar Energy Engineering, 121 (1999), p. 185–193
Yanoma, A., Yoneta, M., Nitta, T. and Okuda, A. The performance of a large scale metal hydride heat pump, JSME International Journal, series II, 31 (1988) p. 741-747 Yoo, H. and Pak, E., Theoretical model of the charging process for stratified thermal storage tank. Solar Energy, 51 (1993), p. 513–519.
Yu, B., Liu, M., Egolf, P.W. and Kitanovski, A., A review of magnetic refrigerator and heat pump prototypes built before 2010. International Journal of Refrigeration, 33 (2010), p. 1029-1060
Zalba, B., Marin, J.M., Cabeza, L.F. and Mehling, H., Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering, 23 (2003), p. 251 – 283
Zhang, L.Z., Design and testing of an automobile waste heat adsorption cooling system. Applied Thermal Engineering, 20 (2000), p. 103 – 114.
75
Zhang, Y. and Faghri, A., Heat transfer enhancement in latent heat thermal energy storage system by using the internally finned tube. International Journal of Heat Mass Transfer, 39 (1996), p. 3165–3173
Zimm, C., Jastrab, A., Sternberg, A., Pecharsky, V., Gschneidner, K., Osborne, M. and Anderson, I., Description and performance of a near room temperature magnetic refrigerator. Advanced Cryogenic Engineering, 43 (1998), p. 1759–1766
Zivkovic. B. and Fujii, I., An analysis of isothermal phase change material within rectangular and cylindrical containers. Solar Energy, 70(2001), p. 51–56
Zou, M., Pecharsky, V.K., Gschneidner, K.W. Jr., Schlagel, D.L. and Lograsso, T.A., Spontaneous generation of voltage in the magnetocaloric compound Tb5Si2.2Ge1.8 and elemental Gd. Journal of Alloys and Compounds,, 488 (2009), p. 550–553 Zurigat, Y.H., Ghajar, A.J., 2002. Chapter 6: heat transfer and stratification in sensible heat storage. In: Dincer, I. and Rosen, M. (Eds.), Thermal Energy Storage – Systems and Applications. John Wiley & Sons, New York, (2002), p. 264–270. http://waldseebiohaus.typepad.com/biohaus/design.html, accessed on the 16th of February (2011)
http://www.sasi.group.shef.ac.uk/worldmapper/display.php?selected=31, accessed on the 17th of February (2011)
http://www.worldometers.info/cars/, accessed on the 17th of February (2011)
http://www.plunkettresearch.com/automobiles%20trucks%20market%20research/industry%20overview, accessed on the 17th of February (2011)
http://www.letsrecycle.com/do/ecco.py/view_item?listid=37&listcatid=217&listitemid=10222, accessed on the 18th of February (2011)
http://www.sheffield.gov.uk/?pgid=134715&fs=b, access on the 21st of March (2011)