AbstractHeat pumps for domestic heating and hot water supply are currently a niche technology in many EU countries, but they are increasingly expected to form an important role in a low carbon future. This is largely because a future of rapidly decar-bonisedelectricity supply is imagined, in which using electric-ity via heat pumps is one of the lowest carbon heating options. However, heat pumps are not necessarily a low carbon option at present. Inthe UK, with fairly carbon-intensive electricity and where natural gas is available for heating, heat pumps do not make significant carbon savings for most householders.

For heat pumps to become a credible low carbon solution in the UK, three transitions unrelated to heat pumps have to occur: (1) transition to low carbon electricity supply; (2) transition to well-insulated housing stock via retrofit; (3) transition to low temperature household heat distribution systems. The most difficult condition for the success of a transition to heat pumpsmight be entirely unrelated to the technology itself. The risks of heat pumps not delivering expected carbon savings, therefore, are much greater than the risks inherent in this technology. The

future role of heat pumps links demand and supply side issues. The benefits of heat pumps vary between countries, regions, individual households and also over time. This makes a policy promoting heat pumps different, and more complex, than policy supporting energy efficiency, which offers universal benefits.

This paper explores the technological, economic, social and energy supply factors which determine the benefits heat pumps could deliver in the UK and other EU countries. It looks at potential mechanisms for moving heat pumps from a niche product to the mainstream. It focuses on the wider is-sues around the parallel transitions required and debates how, and whether, energy and carbon policy can deal with the complexities involved.

Introduction

Heat pumps, which take low temperature heat from the environment and turn it into higher temperature heat by using electrical energy, currently occupy a small niche within the European residential heating market. They have been more widely adopted insome countries than others, adoption rates being influenced by a wide variety of economic, social, energy-related and technological factors. However, European policy is poised to encourage the wider uptake of heat pumps by including them in a list of renewable energy technologies which can be used to meet national obligations to increase the percent-age of heat generated from renewable sources. The UK and many other EU countries have adopted very ambitious long term carbon reduction targets. In order to reach these targets, many future projectionsof carbon emissions from the housing stock, rely on using low- orzero-carbon electricity for heat-ing. For example, in most scenarios explored by Skea, Ekins et al. (2011) heat pumps becomethe dominant technology sup-plying UK residential heating and hotwater from 2030-2035 onwards. Heat pumps are widely seen as a key

technology for efficiently delivering low-carbon heating (Spiers,Gross et al. 2010).

The significant future for heat pumps which is envisaged stands in sharp contrast to their current position in the UK, where theyare installed in low numbers, generally in new build housing which is remote from the natural gas network. Only in homes off the gas network, might the capital and run-ning costs of heat pumps be competitive with alternatives: they cannot compete on costs with natural gas systems. In addi-tion, an efficient gas condensing boiler will generally produce lower carbon emissions than electrically-powered heat pumps, given the current UK electricity mix. Heat pump installation in existing homes faces two important additional challenges. Firstly, to work at their most efficient, heat pumps require the use of a low temperature heat distribution system, e.g. using underfloor heating or largerradiators, which differs from the traditional UK high temperaturewater-based radiator sys-tems. Secondly, much existing UK housingis simply too inef-ficient for low temperature heat to keep it warm, so additional insulation would be required. Given these multiple and com-plex ‘barriers’ to the adoption of heat pumps, amongst others, there is absolutely no reason to expect that heatpumps will break out from their niche and become a mainstream heating option. Yet, that is just what low carbon future scenarios seem to demand. Can such a difficult transition be achieved, and if so, how?

In order to explore this question, and the future of heat pumps in the UK and European residential sectors, firstly the technology behind heat pumps is briefly explained. The char-acteristics of the technology determine how it can be best used. Then the market for heat pumps in Europe and the UK is exam-ined,with a discussion about the factors influencing the very different national preferences for heat pumps. The following section looks at the benefits of heat pumps in the UK at present,with reference to recent field trials and calculation of carbon

savings compared with alternative heat sources. Then the future carbon emissions of heat pumps under low carbon electricity scenarios are examined. Current EU and UK policy is summa-rised and its likely future effects are debated. The routes for heat pumps to emerge from their current small niche are illustrated. Finally, the evidence presented in this paper is discussed and conclusions are drawn.

Heat pump technology and performanceHow heat pumps workElectric compression heat pumps take low temperature heat energy from the environment and turn it into higher tempera-ture heat byusing electrical energy. Heat pumps make use of the fact that when liquids evaporate they absorb a large amount of energy, their specific latent heat of vaporisation, and this en-ergy is released when the vapour condenses back to a liquid. By using this property, large amounts of energy can be absorbed by and released from the heat pump.

Figure 1: Representation of a heat pump

Figure 1 shows how heat pumps work in more detail. Low temperature heat is absorbed from the environment by the working fluid in the evaporator. This energy turns the low temperature, low pressure liquidinto a vapour. This vapour reaches the compressor, which increases thepressure of the vapour, thereby increasing its temperature. In the condenser energy moves from this high temperature, high pressure vapour to the (lower-temperature) environment, and the vapour condenses to a liquid. When this high temperature, high pressure liquid passes through the expansion valve, it is transformed into a low pressure, and therefore low temperature liquid. The cycle starts again.

The maximum theoretical coefficient of performance (COP) of a heat pump in terms of the Kelvin temperatures1 of the warm condenser (T1) and the cool evaporator (T2) is

COPmax = T1/(T1-T2)

The COP is the efficiency of the heat pump – it indicates how many units of heat can be delivered per unit of energy (usually electricity) input. The theoretical maximum COP which can be achieved to provide heat at 35 ºC when the outside temperature is 2 ºC is 9.3. However, in real life, such high efficiencies are not achieved. The COP equation shows that a heat pump operates most efficiently when thetemperature gap between the heat source and the heat demand is minimized. In practice, this means that heat pumps work better where lower rather than higher temperature heat is required. The equation also shows that the higher T2, the input heat temperature, the higher COP can be achieved. For an example of how these factors affect the performance of real heat pumps, see Table 1, which shows how COP varies with input and output temperatures for two Worcester Bosch air source heat pumps.

Table 1: Variation of COP with inlet and delivery temperatures for twoASHPs.

Temparature

Heat pump COP

Inlet Delivery 7kW 9.5kW

-7°C 35°C 2.3 2.5

2°C 35°C 3.0 3.3

7°C 35°C 3.4 3.8

7°C 45°C 2.8 3.0

As output temperature strongly influences COP, this makes new build properties which can be designed with low tem-perature heat distribution systems more suited to heat pumps than properties with existing high temperature systems. An underfloor heating system in a new home would typically use heat at 30–35 ºC, compared with traditional UK radiator sys-tems which use heat at 60–75 ºC. ‘Oversized’ radiators can use water at moderate temperatures, say, 40–55 ºC. Hot water in stored systems is typically heated to 55–60 ºC. HPsystem de-sign needs to balance the heat requirements of users, the COP which can be achieved at different heat delivery temperatures (andtherefore running costs), and capital costs. It is common for HP

installations to include supplementary direct electric heating, for use in extremely cold weather and/or to top up the temperature of hot water. This reduces the capital cost of HP systems. In theory another heating system, such as a gas boiler, could be used for topping-up HP,creating a ‘bivalent’ system, but this is not an option used in the UK.

In addition to COP, it is important to look at the system ef-ficiency of a heat pump installation. The system efficiency is the amount of heat the heat pump produces compared to the amount of electricity needed to run the entire heating system (including domestic hot water,supplementary heating and pumps). For a given installation the system efficiency will be lower than the COP. For many purposes, including estimating householder costs and benefits, system efficiency figures are more relevant than COPs.

Heat pumps are a long-established technology. While there areexpectations that the technology will continue to gradually improve,and higher COP figures can be achieved (IEA 2010) and that bettersystem design and installation can improve performance (EST 2010),there is no expectation of a dramatic improvement to heat pumpperformance.

Types of heat pumpThere are various types of heat pump available. The most common areair source heat pumps (ASHP), which absorb energy from the air, andground source heat pumps (GSHP), which absorb energy from the ground.Heat pumps can supply energy for space heating, for space and waterheating or for water heating alone. Presently, those which provideboth space and water heating are most popular in the UK housingsector (Roy, Caird et al. 2008). Heat pumps can be reversible, so thatthey provide cooling in summer, and these are popular in warmer EUcountries.

Ground source and air source heat pumps each have some advantages and disadvantages compared with the other technology, which are summarizedin Table 2.

Heat pumps can be categorized on the basis of the cold source and hot source that they use.

According to the fluid used for the transfer of heat from the cold source to the heat pump, and from the heat pump to the hot source, there may be four types:

I) AIR-WATER the heat pump draws heat from the cold source, whichconsists of air (external), and transfers it to the hot source, which consists of a water circuit (for the heating of the rooms).

II) AIR-AIR the heat pump draws heat from the cold source, which consists of air (external), and transfers it to the hot source, which likewise consists of air (that of the heated environment).

III) WATER-WATER the heat pump draws heat from the cold source, which consists of water (from lakes, rivers or the water table) and gives it off to the hot source, which consists of awater circuit (for the heating of the rooms).

IV) WATER-AIR the heat pump draws heat from the cold source, whichconsists of water (from lakes, rivers or the water table) and gives it off to the hot source, which consists of air (that ofthe heated environment).

AIR as a cold source possesses the advantage of being available everywhere; in any case, as the temperature of the cold source falls, so does the output supplied by the heat pump.

WATER as a cold source ensures optimal performance of the heat pump without it suffering from the effects of external climatic conditions;however it entails additional cost, due to the system of water adduction.

The GROUND as a cold source possesses the advantage of undergoing lesstemperature changes than the air.

Compression vs absorption:The two main types of heat pumps are compression and absorption. Compression heat pumps operate on mechanical energy (typically driven by electricity), while absorption heat pumps may also run on heat as an energy source (from electricity or burnable fuels).[8] An absorption heat pump may be fueled by natural gas or LP gas, for example. While the Gas Utilization Efficiency in such a device, which is the ratio of the energy supplied to the energy consumed, may average only 1.5; that is better than a natural gas or LP gas furnace,which can only approach 1.

Electrical Heat Pump:If external air is used, a defrosting system is needed at temperatures of around 0ºC, which entails additional energy consumption.During this phase the heat pump uses the heat of the hot source in order to defrost the battery and so heating ceases for some minutes.

Electric heat pumps require an area of land which is 2 to 3 timeslarger than that of the surface to be heated. They are therefore a costly solution, both in terms of the land required and the complexity of the system.

Gas Absorption Heat Pump:

If external air is used, a defrosting system is needed, which is automatically activated at temperatures of around 0ºC. Heat output supplied during the defrosting phase (which lasts a few minutes) falls, but does not cease completely.Gas absorption heat pumps require an area of land which is 1.5 to 2 times larger than that of the surface to be heated.

Heat pump installations

Electric Heat Pump:- In residential buildings the heat pump may be installed in thebasement or in the boiler room. In this case, noise and condensationdo not cause problems and the vicinity of a traditional boiler canmake bivalent use of the heat pump possible.

- In tertiary and industrial buildings the outputs required usuallymake it necessary to install heat pumps on the outside of thebuilding, usually with air as a cold source and a supplementaryboiler.

Gas Absorption Heat Pump- In residential buildings the heat pump may be installed in the open,without any additional protection. External installation avoids theuse of internal space and the risk of transmitting noise to rooms.

- In tertiary and industrial buildings the outputs required usuallymake it necessary to install heat pumps on the outside of thebuilding, usually with air as a cold source. Usually boilers or othersupplementary heating systems are not required.

In commercial environments such as hairdressing studios, restaurantkitchens, etc, the installation of a heat pump can be a veryconvenient choice, as the processes of cooling and de-humidificationmake the working environment more comfortable.

CONDITIONS OF USAGE:Usage conditions, too, influence the choice of heat pump. As hasalready been mentioned, the heating power of heat pumps varies withchanges in the temperature of the cold source and of the hot source.The greater the difference between these two values, the greater isthe reduction in heating output, and vice versa. The choice of asuitable heat pump therefore depends also on the usage of both thecold source (i.e. air or water and its temperature) and the hot source(water for heating at a low or medium temperature).

Sectors using Heat Pump:

Industrial Heat Pumps:Vilter single screw compressors are revolutionizing the foodprocessing industry by renewing the heat absorbed fromrefrigeration into hot water, reducing or eliminating the needfor fossil fuels.

Over the next few years, Industrial Heat Pumps technology willfundamentally change the utilization of energy in the foodprocessing industry. Join the growing list of major foodprocessors that are using this revolutionary breakthrough ofsingle screw technology for industrial heat pump and industrialrefrigeration applications.

Industrial heat pumps are mainly used for:1)space heating;2)heating and cooling of process streams;3)water heating for washing, sanitation and cleaning;4)steam production;5)drying/dehumidification;6)evaporation;7)distillation;

8)concentration.

Heat pumps in residential and commercial buildings:Heat pumps for heating and cooling buildings can be divided intofour main categories depending on their operational function:

*Heating-only heat pumps, providing space heating and/or waterheating.

*Heating and cooling heat pumps, providing both space heating andcooling.

The most common type is the reversible air-to-air heat pump,which either operates in heating or cooling mode. Large heatpumps in commercial/institutional buildings use water loops(hydronic ) for heat and cold distribution, so they can provideheating and cooling simultaneously.

*Integrated heat pump systems, providing space heating, cooling,water heating and sometimes exhaust air heat recovery.

Water heating can be by desuperheating only, or by desuperheatingand condenser heating. The latter permits water heating when nospace heating or cooling is required.

*Heat pump water heaters, fully dedicated to water heating.

They often use air from the immediate surroundings as heatsource, but can also be exhaust-air heat pumps, or desuperheaterson air-to-air and water-to-air heat pumps.

Applications:HVAC (Heating , Ventilation and air conditioning):

In HVAC applications, a heat pump is typically a vapor-compression refrigeration device that includes a reversing valve

and optimized heat exchangers so that the direction of heat flow(thermal energy movement) may be reversed. The reversing valveswitches the direction of refrigerant through the cycle andtherefore the heat pump may deliver either heating or cooling toa building. In cooler climates, the default setting of thereversing valve is heating. The default setting in warmerclimates is cooling. Because the two heat exchangers, thecondenser and evaporator, must swap functions, they are optimizedto perform adequately in both modes. Therefore, the efficiency ofa reversible heat pump is typically slightly less than twoseparately optimized machine.

Plumbing:

In plumbing applications, a heat pump is sometimes used to heator preheat water for swimming pools or domestic water heaters;the heat energy removed from an air-conditioned space may berecovered for water heating.

Refrigerants:

Until the 1990s, the refrigerants were often chlorofluorocarbonssuch as R-12 (dichlorodifluoromethane), one in a class of severalrefrigerants using the brand name Freon, a trademark of DuPont.Its manufacture was discontinued in 1995 because of the damagethat CFCs cause to the ozone layer if released into theatmosphere.

One widely adopted replacement refrigerant is thehydrofluorocarbon (HFC) known as R-134a (1,1,1,2-tetrafluoroethane). Heat pumps using R-134a are not as efficientas those using R-12 that they replace (in automotiveapplications) and therefore, more energy is required to operatesystems utilizing R-134a than those using R-12. Other substancessuch as liquid R-717 ammonia are widely used in large-scalesystems, or occasionally the less corrosive but more flammablepropane or butane, can also be used.

Since 2001, carbon dioxide, R-744, has increasingly been used,utilizing the transcritical cycle, although it requires muchhigher working pressures. In residential and commercialapplications, the hydrochlorofluorocarbon (HCFC) R-22 is stillwidely used, however, HFC R-410A does not deplete the ozone layerand is being used more frequently. Hydrogen, helium, nitrogen, orplain air is used in the Stirling cycle, providing the maximumnumber of options in environmentally friendly gases.More recentrefrigerators use R600A which is isobutane, and does not depletethe ozone and is friendly to the environment. Dimethyl ether(DME) is also gaining popularity as a refrigerant.

Efficiency:When comparing the performance of heat pumps, it is best to avoidthe word "efficiency", which has a very specific thermodynamicdefinition. The term coefficient of performance (COP) is used todescribe the ratio of useful heat movement per work input. Most

vapor-compression heat pumps use electrically powered motors fortheir work input. However, in many vehicle applications,mechanical energy from an internal combustion engine provides theneeded work.

Heat pumps are more effective for heating than for cooling aninterior space if the temperature differential is held equal.This is because the compressor's input energy is also convertedto useful heat when in heating mode, and is discharged along withthe transported heat via the condenser to the interior space. Butfor cooling, the condenser is normally outdoors, and thecompressor's dissipated work (waste heat) must also betransported to outdoors using more input energy, rather thanbeing put to a useful purpose. For the same reason, opening afood refrigerator or freezer actually heats up the room ratherthan cooling it because its refrigeration cycle rejects heat tothe indoor air. This heat includes the compressor's dissipatedwork as well as the heat removed from the inside of theappliance.

The COP for a heat pump in a heating or cooling application, withsteady-state operation, is:

Summary on situation in participating countries :

The table below shows roughly the penetration of heat pumps atthe moment and in 2020 and/or 2030 in participating countries.

The shares of different types of heat pumps are different indifferent countries:

- In Finland roughly 20 % were ground source based in 2010, butthe air/air heat pumps have been very popular during the recentyears

- In France the share of geothermal (ground source) is recentlyabout 40 % .

- In Spain in practice all heat pumps are air-based.

The Benefits of the Heat Pump SystemBefore I can give you the features and benefits of any Heat PumpSystem, I need to explain how a Heat Pump System functions. Inresidential use there are two types commonly used.

Geo-Thermal known as water to air by most technicians. The nexttype would be a Standard Split System. Outside and inside units,

I will explain both types. First the Geo-Thermal. There are twotypes of Geo units, a closed loop and an open loop.

It is one of the most efficient heating and cooling systems youcan install for your home. It does not convert electricity intoheat. Rather, it absorbs warmth from the atmosphere or ground,multiplies it and then transfers this heat to the home. This isused for household warming.

During warmer weather, the pump functions as an air conditioner.It transfers heat from the house thus creating a cooling effect.This saves on energy utilized for air conditioning. Electric heatpumps are a useful device to help control heating and coolingcosts. They are a much cheaper system to use than those fuelledby natural gas. Electric heating systems do not generate dry airwhen heating. This spares you from having to use a humidifier tocombat the dry air as is the case with furnaces.

The pump also provides uniform heating for the home. You will notexperience the frequent temperature fluctuations associated withother systems. Electric pumps are quiet and comfortable to use.The air compressor is usually placed outside the building. This

helps to keep noise levels low. You are not subjected to a noisysession when the pump is operational. Some other heat pumpvarieties produce much noise when in use.

Electrically fuelled pumps also have green benefits. They do notpollute the environment. Other systems such as furnaces causeconsiderable pollution during combustion. The convenience,cleanliness and efficiency of an electric heating pump add to thevalue of your home.

Heat pumps in the building sector significantly reduce globalcarbon dioxide emissions.A new generation of CO2-based heat pumpscould avoid the high global warming potential of standardrefrigerants and generate much higher temperatures.

Disadvantages of an Electric Heat Pump

Heat pumps have very high startup costs. While a heat pump willprobably save money in the long run, the installation costs mayprevent many homeowners from choosing one. Secondly, heat pumpshave trouble operating in cold areas. Prolonged exposure tosubfreezing temperatures will damage the system and prevent itfrom operating at full efficiency. Many homeowners find that theheat generated by a heat pump created in their home during thewinter months feels "cold." However, this problem can usually befixed by changing the air direction.

Installation involves considerable technical expertise andspecialized tools. Proper wiring must be done to ensure thesystem works right. It is best that a qualified electricianinstall the pump. This pushes up overall costs.

Electric heat pumps are high maintenance heating systems. Regularmaintenance is required to ensure the pump operates efficientlythroughout. This adds to overall costs.

In areas where temperatures frequently fall below freezing inwinter, efficiency of the pump is hampered. The system iscompelled to spend more time and energy to keep warm before itcan transmit heat to the home. This would compel you to backupyour system if you live in a cold climatic zone.

I'm pretty sure that one of the reasons more people don't useheat pumps for private use (apart from installation and initialoutlay) is because of the noise these things create (which doesseem to have improved lately).

Discussion:

For climates with moderate heating and cooling needs, heat pumps offeran energy-efficient alternative to furnaces and air conditioners. Likeyour refrigerator, heat pumps use electricity to move heat from a coolspace to a warm space, making the cool space cooler and the warm spacewarmer. During the heating season, heat pumps move heat from the cooloutdoors into your warm house and during the cooling season, heatpumps move heat from your cool house into the warm outdoors. Becausethey move heat rather than generate heat, heat pumps can provideequivalent space conditioning at as little as one quarter of the costof operating conventional heating or cooling appliances.