30

CHAPTER 2

LITERATURE OVERVIEW

Apart from power generation, the application of the concentrating

collectors along with a thermal energy storage system in industrial process

heating and institutional cooking, has been gaining momentum recently. In

the present work, a detailed survey has been made on the various aspects of

this field of research, which includes parabolic trough collectors, thermal

energy storage materials, system and applications, solar cookers and also

experimental and computational analyses of various types of cooking units.

2.1 PARABOLIC TROUGH COLLECTOR

Among the solar concentrating collectors, the parabolic trough

collector (PTC) is a well proven technology designed to reach temperatures

above 100 C and up to 450 C. Parabolic trough collectors are employed in a

variety of applications, including industrial steam production for electricity,

for process heat application and hot-water production. PTCs are mostly

preferred for solar steam-generation, because of the ability to obtain high

temperatures without any serious degradation of the collector efficiency. The

main attracting feature of the PTC is its high collector efficiency even at

higher temperatures.

Several developments have been carried out to improve the

performance of the PTC. The design and performance characteristics of a

parabolic trough solar collector system were thoroughly studied by Kalogirou

31

et al (1994a). The optimization of the collector aperture and the rim angle as

well as the selection of a receiver diameter was addressed in this study. In a

later study, the authors Kalogirou et al (1994b) described a low cost method

for the mass-production of high accuracy parabolic surfaces with fiberglass.

According to the ASHRAE 93-1986 standard, the performance test of a

parabolic trough collector was conducted by Kalogirou (1996). Almanza et al

(1997) investigated the receiver behavior of PTCs in direct steam generation

under different experimental conditions, and they concluded that by replacing

the steel absorber tube with copper, it is almost possible to eliminate the

thermal stress in the wall of the pipe, due to the smaller circumferential

temperature differences.

Kalogirou (1998) used parabolic trough solar energy collectors for

sea-water desalination, and studied the performance of the PTC desalination

system. Odeh et al (1998) developed the thermal model of a trough collector

to find its thermal loss. Since the model was developed considering the

absorber tube temperature rather than the fluid bulk temperature, the authors

suggested that the thermal model can be used for any working fluid, to predict

the performance of the PTC.

Lokurlu et al (2005) developed a new kind of solar air-conditioning

unit through parabolic trough collectors, combined with a double effect

absorption chiller for the air-conditioning of buildings. The performance and

detection of the optical losses of the PTC are very important issues, in order

to improve the optical efficiency to ensure the desired output energy quality.

Riffelmann et al (2006) developed and established the PARASCAN and the

CAMERA-TARGET methods, to assess the flux distribution in the focal

region of PTCs. The evaluation of the geometric properties of the

concentrating collectors becomes important, when the thermal output is lesser

than expected. The use of the PTC on a small scale model for hot water

32

generation was performed by Valan Arasu and Sornakumar (2006).

Brooks et al (2006) conducted the baseline performance study of a parabolic

trough solar collector, using the ASHRAE 93-1986 standard. Lupfert et al

(2007) summarized the various techniques available for analyzing the optical

performance of the PTC.

The thermal analysis of a PTC porous disc receiver was

numerically investigated by Ravi Kumar and Reddy (2009). The investigation

revealed that the use of a porous medium in a tubular solar receiver enhances

the system performance significantly. PTC applications can be divided into

two main groups. The first is applications where the temperature requirement

is in the range of 300-400 C, especially in concentrated solar power plants.

The second group of applications requires temperatures in the range of

100-250 C like various industrial process heat applications. A comprehensive

review of the use of the PTC for various applications of thermal energy up to

400 C was presented by García et al (2010). Tao and He (2010) developed a

unified two-dimensional numerical model for the coupled heat transfer

process in a parabolic solar collector tube, which includes natural convection,

forced convection, heat conduction, and the fluid-solid conjugate problem.

The vacuum solar receiver is the key component of a parabolic trough solar

collector, which plays a prominent role in the system efficiency. Gong et al

(2010) established and optimized a 1-D theoretical model of China’s first high

temperature parabolic trough solar receiver, to compute the receiver’s major

heat loss through a glass envelope, and then they systematically analyzed the

major influential factors of heat loss. Wang et al (2010) introduced an

eccentric type tube receiver, to minimize the thermal stresses of the tube

receiver. The authors suggested that employing an eccentric tube receiver

with optimum eccentricity and oriented angle for the PTC system, can reduce

the thermal stress and enhance the reliability of the tube receiver effectively.

33

He et al (2011) established a coupled simulation method, based on

the Monte Carlo Ray Trace (MCRT) and the Finite Volume Method (FVM),

to solve the complex coupled heat transfer problem of radiation, heat

conduction and convection in a parabolic trough solar collector system. The

numerical study of conduction and convection heat losses from a half-

insulated air-filled annulus of the receiver of a parabolic trough collector, was

performed by Ansary and Zeitoun (2011). When a new design for a solar

collector is developed, it is necessary to guarantee that its intercept factor is

good enough to produce the expected thermal jump. García-Cortés et al

(2011) carried out experiments to determine the real shape and the intercept

factor of a new prototype of a parabolic solar trough collector. The

interception coefficient of the collector was calculated by the author and from

the results he concluded that 10% of the incident rays do not reach the

absorber. The author finally suggested that the mounting of some facets

should be revised in order to reduce the energy efficiency degradation. The

heat transfer and optical analysis of the PTC is essential to optimize and

understand its performance under different operating conditions. A detailed

one dimensional numerical heat transfer analysis of a PTC was carried out by

Padilla et al (2011). They validated the numerical results with the

experimental data and concluded that the results showed a better agreement

with the experimental data.

2.2 LATENT HEAT THERMAL STORAGE

Latent heat thermal storage has been a major topic in research for

the last two decades. In the present section, a review has been carried out on

the storage material, heat transfer, and other studies of the storage system and

its various applications.

34

2.2.1 Phase Change Materials

A detailed review of low-temperature phase change materials has

been done by Abhat (1983). A broad review of research in the field of phase

change heat storage, especially on salt hydrates, has been done by Lane

(1983). This book gives a detailed account of the development of phase

change materials, the criteria for their selection and the chemical aspects of

the phase change phenomena. Fouda et al (1984) studied the characteristics of

Glauber’s salt as a heat storage medium in a pilot heat storage system. The

effect of several variables was studied over many complete cycles of the unit,

and the quantitative results were presented in terms of thermal recovery

efficiencies and the volumetric heat transfer co-efficient in the direct contact

storage unit. Vaccarino et al (1985) studied a low–temperature heat storage

system utilizing mixtures of Magnesium salt hydrates and Ammonium nitrate

as the PCM suitable for practical exploitation in connection with commercial

flat plate solar collectors.

Ghoneim et al (1991) studied the behaviour of three phase change

materials including sodium sulphate decahydrate, medicinal paraffin and

P116 wax for the use of thermal storage walls in solar passive systems. The

melting and freezing characteristics of the various organic and inorganic heat

storage materials, classified as paraffin, fatty acids, inorganic salt hydrates

and eutectic compounds were investigated, using the techniques of Thermal

Analysis, and Differential Scanning Calorimetry (DSC). A study was made by

Hoogendoorn and Bart (1992) on organic phase change materials for thermal

storage in solar systems. The latent heat effects of these materials are obtained

from the Differential Thermal Analyser (DTA) measurements. It was

concluded that paraffin based phase change materials are attractive for use in

solar heat storage systems for the temperature range of 25-150oC. Sharma et

al (1999) conducted experiments to study the change in the latent heat of

35

fusion, melting temperature and specific heat of commercial grade stearic

acid, acetamide and paraffin wax subjected to repeated melt/ freeze cycles.

The study showed that acetamide and paraffin wax were found to be more

suitable phase change materials.

Dimaano and Watanabe (2001) investigated an LHS system with a

capric and lauric acid mixture. The thermal performance and phase change

stability of stearic acid as a PCM were studied experimentally by Sari and

Kaygusuz (2001), and they compared the heat transfer characteristics of the

stearic acid with other studies given in the literature. Py et al (2001) presented

a new supported PCM made of paraffin impregnated by capillary forces in a

compressed natural graphite matrix. Saito et al (2001) performed an analytical

and experimental investigation on the heat removal process of a thermal

energy storage capsule, using gelled Glauber’s salt. Dincer and Rosen (2002)

and Farid et al (2004) presented a detailed review of thermal energy storage

with phase change materials, heat transfer analysis and applications. Cabeza

et al (2003) studied the suitability and thermal performance of sodium acetate

trihydrate thickened with benotine and starch as phase change energy storage

material. The addition of gellants and thickeners avoided the segregation of

these materials.

A review was carried out by Zalba et al (2003) that focused on the

materials, the heat transfer analysis and applications of PCM based TES

systems. They listed over 150 materials used in research as PCMs, and about

45 commercially available PCMs. Nagano et al (2004) studied the feasibility

of a mixture of magnesium nitrate hexahydrate as a base material, and

magnesium chloride hexahydrate as an additive, to store and utilize urban

waste heat from emerged co-generation systems. He et al (2004) used the

liquid-solid phase diagram of the binary system of tetradecane and

hexadecane to obtain information of the phase transition processes for cool

36

storage applications. The analysis of the phase diagram indicates that, except

for the minimum melting point mixture, all mixtures melt and freeze in a

temperature range and not at a constant temperature. Shiina and Inagaki

(2005) studied the enhancement of effective thermal conductivity of phase

change materials by saturating it with porous metals. The authors concluded

that considerable reduction in melting time was obtained, especially for low

conductivity PCMs and for a high heat transfer coefficient. Zukowski (2007)

experimentally studied paraffin wax (RII-56) as a PCM enclosed in a

polyethylene film bag, for a short term thermal energy storage unit. Kenisarin

and Mahkamov (2007) analysed the publications of the last 15 years on the

properties and applications of PCMs, and methods of enhancing the heat and

mass transfer in storage devices.

Every latent heat thermal energy storage system requires a suitable

PCM for use in a particular kind of thermal energy storage application. One of

the important factors to be considered when choosing an appropriate PCM is

the life of the PCM, i.e., its ability to resist change in the melting temperature,

and the latent heat of fusion with time due to thermal cycling. Shukla et al

(2008) performed a thermal cycling test of selected inorganic and organic

PCMs. They concluded that paraffin waxes show reasonably good thermal

reliability, and Erythritol, a sugar alcohol, is a promising PCM for high

temperature thermal energy storage, as its latent heat began to show only

gradual degradation after 500 thermal cycles. According to Kaizawa et al

(2008), Erythritol, Xylitol and D-Mannitol appear to be reliable PCMs for

high temperature applications, due to their large latent heat and good

operational safety. The potential use of D-Mannitol as a phase change

material is supported by its thermal properties. The melting temperature and

highest available temperature of D-Mannitol in the given experimental

conditions were reported by Bruni et al (2009).

37

A detailed review on the use of a latent heat storage material in

solar cookers was addressed by Sharma at al (2009). A wide range of PCMs

with their properties, advantages and limitations was comprehensively

reported by Agyenim et al (2010). The selection of a phase change material

for any latent heat thermal storage system requires good thermophysical

properties. The possibility of D-Mannitol as a phase change material was

examined by Kumaresan et al (2011) through DSC and TG-DTG/DTA

analysis.

2.2.2 Configurations and Heat Transfer Enhancement in Thermal

Storage Systems

Understanding thermal behaviour during phase change in a storage

system is extremely important for the design of an efficient storage system. A

number of researchers studied the heat transfer performance and various

configurations of latent heat TES systems for energy storage, focusing on the

efficiency of the storage system.

Saitoh (1983) reported that a spherical shape gives the best

performance among the various existing LHS units including flat plate,

helical coil, and cylindrical capsule types. Abe et al (1984) developed a direct

contact LHS unit using form-stable HDPE rods as the PCM, and performed a

series of experiments for different flow rates, PCM initial temperatures and

HTF (Ethylene glycol) inlet temperatures to study the charge and discharge

characteristics of the storage unit on a lab scale. Dietz (1984) experimentally

studied the thermal performance of an LHS unit, consisting of vertically

oriented tubes filled with Calcium Chloride Hexahydrate (CaCl2.H2O) as the

PCM. The variation of the charging and discharging rates was measured for

different flow rates and temperatures of the HTF (air). These rates decreased

with time as a result of the decreasing effective heat transfer area and

increasing thermal resistance of the PCM. Kamimoto et al (1986) extended

38

the work of Abe et al (1984) and developed an LHS unit of 30 kWh capacity,

using form stable HDPE rods for solar thermal applications, and the recovery

of industrial waste heat around 140-150oC. From the heat transfer

experiments and a thermal insulation test, it was confirmed that this storage

unit shows excellent performance due to good direct contact heat transfer and

a formation of thermocline. Saitoh and Hirose (1986) investigated the

transient thermal characteristics of a phase change thermal energy storage unit

using spherical capsules. The effects of the variation in the capsule diameter,

the flow rate of the heat transfer fluid, the inlet temperature difference, the

capsule material and the PCM, on the thermal performance of the storage unit

were classified in detail via a computer simulation. They found that the LHS

unit using spherical capsules had a high heat storage capacity 2.5 times larger

than the usual SHS unit for equal storage volume, when the temperature

swing was 40 K (PCM: Na2HPO4/12H2O).

Sozen et al (1991) investigated the thermal energy storage

characteristics of an SHS and LHS packed bed, consisting of a horizontal

channel filled with randomly packed particles of PCM encapsulated spherical

capsules. The HTF was refrigerant-12, which was modelled as an ideal gas.

The SHS material used was 1% carbon – steel, and the PCM was myristic

acid. The investigations showed distinctly different energy storage

characteristics for these two kinds of packed beds. The dynamic thermal

behaviour of a latent heat thermal energy storage was presented by Silva and

Pires (2002). The authors concluded that the correlations developed can be

used for the rapid estimation of the charging and discharging times and so can

be useful in the design of latent heat thermal energy storage. The performance

of a compact latent heat phase change material integrated in solar collector

system was investigated by Mettawee and Assassa (2006). The propagation of

melting and freezing front was studied during the charging and discharging

process. The experimental results showed that in the charging process, the

39

average heat transfer coefficient increases sharply with increasing the molten

layer thickness, as the natural convection grows strong. In the discharge

process, the useful heat gain was found to increase as the water mass flow rate

increases. The performance of a packed bed latent heat thermal energy storage

unit integrated with solar water heating system was carried out by Nallusamy

et al (2006). The author concluded that the packed bed LHS system reduces

the size of the storage tank appreciably compared to the conventional storage

system and that the LHS system employing batchwise discharging of hot

water from the TES tank is best suited for applications where the requirement

is intermittent.

The improvement in the heat transfer rates in the storage units

employing PCMs with different temperatures were investigated by many

researchers. Farid and Kanzawa (1989) and Farid et al (1990) proposed the

use of PCMs with different melting temperatures in a LHS module with air as

HTF. The PCM was encapsulated in multirows of vertical cylinders. Both

experimental and numerical results showed some improvements in the heat

transfer rates during both heat charge and discharge when three types of

PCMs were used. Watanabe et al (1993) extended the experiments of Farid

et al (1990) by using water as the HTF and proved that there was obvious

enhancement of the charging-discharging rates in the LHS system using three

PCMs. Adebiyi et al (1996) reported that the efficiency of storage system

using five PCM families in a packed bed LHS system exceeded those using

single PCM family by as much as 13-26 percent. Wang et al (2001) studied

the charging process of a cylindrical LHS capsule with stearic acid, sliced

paraffin and lauric acid as PCMs. Experimental results demonstrated that,

compared to the capsule with single PCM, the charging rate of the capsule

employing three PCMs enhanced obviously.

40

Several other heat transfer enhancement techniques such as using

fins, inserting a metal matrix into the PCM, using PCM dispersed with high

conductivity particles and micro-encapsulation of PCM were studied and

reported by various researchers. Since shell and tube heat exchangers are

commonly used in LHS systems, phase change around tubes with fin

configurations has attracted many researchers. Most of the studies are

concerned with PCM on the shell side and HTF on the tube side. The use of

finned tubes with different configurations has been proposed by various

researchers such as Eftekhar et al (1984), Padmanabhan and Krishna Murthy

(1986), Marcos (1990), Sadusuke and Naokatsu (1991), Lacroix (1993), Costa

et al (1998), Velraj et al (1997, 1999), Ismail et al (2001), Lamberg and Si en

(2003), Stritih (2004) and Castell (2008).

Hoongendoorn and Bart (1992) reported that the low value of the

thermal conductivity of the PCMs could be greatly improved by embedding a

metal matrix structure in them. Chow et al (1996) evaluated two thermal

conductivity enhancement techniques. The first technique focuses on placing

PCM in capsules of various shapes in a liquid metal medium. The second

technique involves a metal/PCM composite. Bugaje (1997) investigated

methods of enhancing the thermal response of paraffin wax heat storage tubes

by incorporation of aluminium matrices. Velraj et al (1999) studied the

influence of rasching rings dispersed in paraffin on the performance of LHS

unit of 50 litre capacity. Fukai et al (2002) used carbon-fiber brushes to

improve the thermal conductivities of PCMs packed around heat transfer

tubes. The transient thermal responses in the brush/n-octadecane composites

were experimentally measured and the effects of volume fraction of fibers and

diameter of brush on the heat transfer rate discussed. Cabeza et al (2002a)

performed an experiment in a small thermal energy storage device to study

heat transfer improvement in PCM with three different heat transfer

enhancement methods. Koizumi (2004) made an attempt to enhance the LHS

41

rate of solid PCM in a spherical capsule and found that by inserting the

copper plates into the capsules is an effective technique for enhancing the

LHS rate, especially in large spherical capsules.

Progress in LHS systems mainly depends on heat storage material

investigations and on the development of heat exchangers that assure a high

effective heat transfer rate to allow rapid charging and discharging. Latent

heat TES systems are broadly classified into the capsule-type and shell-and-

tube type, according to the mode of exchanging heat energy within the

container. Advantages and disadvantages of different geometries of PCM

encapsulation with different materials and their compatibility were discussed

by Lane (1986). A series of numerical tests were undertaken to asses the

effects of the shell radius, mass flow rates, and inlet temperature of the HTF.

The transient performance of a double pipe heat exchanger as a thermal

energy storage container was investigated both experimentally and

theoretically by Fath (1991). The results indicated that increasing the HTF

inlet temperature and flow rate as well as the heat exchanger length increases

the heat transfer rate and stored energy.

Ryu et al (1991) studied the heat transfer characteristics of cool-

thermal storage units during the charging period using vertical and horizontal

tube systems. The two systems were compared with respect to heat transfer

rate, coefficient of performance and super cooling of the PCM and it was

found that the vertical tube system exhibits better thermal performance than

the horizontal tube system.

One of the most effective and compact latent heat TES system is a

packed spherical capsule bed with different diameters. An experimental and

numerical study was carried out by Ismail and Henriquez (2002) on LHS

system composed of spherical capsules filled with water as PCM placed

inside a cylindrical tank. The authors studied the effect of spherical capsule

42

materials such as copper, PVC and polyethylene and found that the use of

polyethylene or PVC for spherical capsule facilitates the construction of the

storage and reduces its costs. Dincer and Rosen (2002) dealt with the

problems of heat transfer with phase change materials in simple and complex

geometries and around isothermal finned cylinders. The results were

presented and validated with actual and existing data. Barba and Spiga (2003)

analyzed the discharge process of the LHS system, for constant temperature

conditions, in three different geometrical configurations, i.e. PCM

encapsulated in slab, cylindrical or spherical polyethylene containers and

found that the shortest time for complete solidification is matched for small

spherical capsules.

The performance of latent heat thermal storage systems is limited

by the poor thermal conductivity of the PCMs used. Jagadheeswaran and

Pohekar (2009) reviewed the various techniques employed to enhance the

performance of latent heat thermal storage units, which include using

extended surfaces, employing the multiple PCMs method, thermal

conductivity enhancement and micro-encapsulation of the PCM.

2.2.3 Applications of Thermal Energy Storage System

Thermal Energy Storage (TES) is one of the key technologies for

energy conservation, and therefore is of great practical importance. One of its

main advantages is that it is best suited for solar thermal applications. Dincer

(1999) made a detailed study of the evaluation and selection of sensible and

latent heat storage technologies, systems and applications in the field of solar

energy. Another significant advantage of TES is that, although it may have

been designed primarily for the storage of solar energy, it is not restricted to

that. It may be used to store surplus energy from the power plants, usually in

the form of waste water, waste energy from air conditioners, waste energy

from industrial processes, and so on. Zalba et al (2003) presented an excellent

43

review on the various applications using PCM based thermal storage systems.

Table 2.1 gives a literature review of various applications of TES systems.

Table 2.1 Various applications of TES systems

Sl.No

Type ofapplication References

1 Air heating

Kaygusuz et al (1991), Fath (1995), Mehmet (2000) , Aboul-Enein et al (2000), Enibe (2002), Jain and Jain (2004),Madhlopa and Ngwalo (2007), Mohan raj and Chandrasekar(2008) and Bal et al (2010)

2

Solar waterheating(LHS and SHSsystems)

Bansal and Buddhi (1992), Sodha et al (1997), Reddy et al(1999), Kurklu et al (2002), Mehling et al (2003), Baran andSari (2003) and Lee et al (2006) and Nallusamy et al (2006),(2009)

3 Buildingapplications

Peippo et al (1991), Khalifa et al (1998), Kissock et al (1998),Vakilaltojjar and Saman (2000), Lee et al (2000), Ismail andHenriquez (2001), Mehling et al (2002), Velraj et al (2002),Khudhair and Farid (2004), Lin et al(2005), Pasupathy andVelraj (2006), Tyagi and Buddhi (2007), Arkar and Medved(2007), Shilei et al (2007), Zhang et al (2008) and Zhu et al(2009)

5Cooling(Refrigeration andair conditioning)

Tackett (1989), Ryu et al (1991), Hasnain (1998),Vakilaltojjar and Saman (2001), Cabeza et al (2002b),Cheralathan et al (2006) , Morgan and Krarti (2010), Erekand Dincer (2009), Yau and Lee (2010) and Zhai and Wang(2010)

6Solar powerplants (LHS andSHS systems)

Herrmann and Kearney (2002), Kearney et al (2003) andPacheco et al (2002), Laing et al (2006), Seeniraj andLakshmi Narasimhan (2008) and García et al (2011)

7 Green HouseHeating

Hyun-Kap Song (1997), Kurklu (1998a), Kurklu (1998b), andBascetincelikk et al (1998), Najjar and Hassan (2008), Sethiand Sharma (2008) and Benli and Durmus (2009)

8 Electronic coolingPal and Joshi (1997), Bellettre et al (1997), Laouadi andLacroix (1999), Alawadhi and Amon (2003) , Alawadhi(2005) and Kandasamy et al (2007),(2008)

9 Automobiles

Schatz (1992), Mostafavi and Agnew (1996), Korin et al(1999), Vasiliev et al (2000), Zhang (2000), Desai andBannurm(2001), Talbi et al (2002) Subramanian et al (2004)and Pandiyarajan et al (2011)

44

The storage of thermal energy is very important in many

engineering applications, which are mentioned above. Because of the

discrepancy between the energy supply and demand in solar heating

applications, a thermal energy storage device has to be used.

Since the present work is focused on storage based solar cookers,

the applications of thermal energy storage in solar cookers done by various

researchers are listed separately, and given in the following section under the

sub section of solar cookers with storage.

2.3 SOLAR COOKERS

A large number of solar cookers have been developed in many

countries and these cookers are classified based on the type of the collector,

the place of cooking, and the storage medium employed. The detailed

classification of solar cookers is given in section 1.4.3. Though in the present

research an indirect type solar cooker is used, an overview of all the types of

solar cookers studied is reported under two major groups, of without and with

storage.

2.3.1 Solar Cookers without Storage

The very first design of a box type cooker as shown in Figure 1.7a,

was designed by Nicholas-de-Saussure. It was simply an insulated box with

two glass panes forming the cover. This design forms the basis of all the

present designs of box type cookers. Reflectors were added to increase the

efficiency and reliability of the simple box type cooker. Ashok (1998)

summarized the history of the development of the box type cookers. A

performance study of the box-type solar cooker was made by Gaur et al

(1999), Nahar (2001), Amer (2003), Ekechukwu and Ugwuoke (2003),

Narasimha Rao and Subramanyam (2003, 2005) and Sebaii and Ibrahim

45

(2005). A comparative thermal performance test of a box type solar cooker

using two different cooking vessels, with fin and without fin, was conducted

by Harmim et al (2008). It was experimentally demonstrated that the cooking

time can be reduced by using a finned cooking vessel. Schwarzer et al (2008)

studied the various types of solar cookers, their basic characteristics and

experimental procedures to test the different types of solar cookers. Mirdha

and Dhariwal (2008) theoretically analyzed the various designs of solar

cookers, with respect to north and south facing booster mirrors.

Concentrating type solar cookers are expected to demonstrate high

performance because of the large collection area employed. However, the net

amount of heat used is still low. Habeebullah et al (1995) developed a

numerical model for concentrating type solar cooker. The author introduced a

new concept of oven receiver to boost the overall cooker efficiency. The

analysis showed that the oven type receiving pot has both a higher fluid

temperature and overall receiver efficiency. The performance evaluation of

spiral type and paraboloidal type point focus solar cooker was conducted and

compared by Taha et al (1988). The experimental results showed that the

utilization efficiency of spiral type solar cooker is higher than paraboloidal

type. Abou-Ziyan (1998) designed, constructed and tested a two different

tracking solar cookers namely paraboloid dish solar cooker (PDSC) and a

booster mirror solar box cooker (BMSBC). The study showed that PDSC had

higher rates of cooking (2 to 6 times) and temperature than the BMSBC

because of the high concentration ratio. The effect of wind speed on

efficiency and temperature of the cooker was also reported in this paper.

A new type of conical solar cooker suitable for cooking different

kinds of meat and legumes was designed and tested by Sharaf (2002). The

author reported that the developed conical solar cooker has the advantages of

low price, ease of manufacture, lightweight, high efficiency, small solar area

46

and short maximum time for cooking. Sonune and Philip (2003) developed

and tested a Fresnel type domestic SPRERI concentrating cooker. The

developed cooker has an aperture area of 1.5 m2 and a focal length of 0.75 m

and was found to provide an adequate temperature needed for cooking, frying

and preparation of food stuff for a family of 4 to 5 persons.

The theoretical exergy analysis of a simple solar parabolic cooker

(SPC) and the distribution of the exergy losses in the cooker was first studied

and presented by Petela (2005). The author concluded that the exergy

efficiency of the SPC was found to be relatively very low (~1%), and to be

about 10 times smaller than the respective energy efficiency which is in

agreement with experimental data from the literature.

The utility of a parabolic solar cooker on techno-economic, social,

behavioral and common criteria in the present Indian context in comparison

with other contemporary cooking energy devices was studied by Pohekar and

Ramachandran (2006). The study showed that Liquefied Petroleum Gas

(LPG) stove is the most preferred cooking device, followed by microwave

ovens and kerosene stoves. PSC has occupied fifth rank amongst the devices.

In the conventional solar cooker (box type), it is expected that the

food stuff is put once in the box and is taken out at an appropriate time, not

allowing intervention. This is because; whenever the lid, which is also the

energy collecting surface, is opened a large amount of trapped heat goes out.

Though concentrating type cookers deliver high temperature for cooking,

alike in the box type the user to go out in the sun, during its use and for its

hourly tracking. An indirect type solar cooker is highly regarded for indoor

cooking, where cooking is carried out in the shade or inside a building.

Schwarzer and silva (2003) developed a indirect type flat-plate solar cooker

and it is shown in Figure 1.10a. The cookers demonstrated by them can be

incorporated into the construction of kitchen. Peanut or sunflower oil was

47

used as heat transfer medium and the cooker was designed with two non-

removing pots. Advantages of this cooker are possibility of fast cooking, large

pot volumes and the possibility of indoor cooking and heat flow control in the

pots. Solar cookers based on conventional flat plate solar collectors suffer

from the drawbacks such as the performance deterioration due to the reversed

cycle during night and cloudy periods of the day and high heat capacity.

Further the disadvantages are non-removable pots, which make cleaning and

dishing food difficult.

Balzar et al (1996) developed a solar cooking system which

consists of a vacuum-tube collector with integrated long heat pipes directly

leading to the oven plate. Solar cookers using vacuum tube collectors have

several advantages. They do not need tracking. They can reach high

temperatures and cooking can take place in the shade or inside a building

because of the spatial separation of collecting part and oven unit. They require

an effective heat transfer system in order to transfer the heat from the

collector to the hot plate without a marked decrease of temperature. Heat

pipes are very appropriate for this purpose. Their thermal conductance is

extremely high and the heat transfer between the evaporator and the

condenser section is nearly isothermal.

Kumar et al (2001) designed the community type solar pressure

cooker based on evacuated tube solar collector (ETSC). It consists of an

evacuated tubular solar collector and a pressure cooker and both units are

coupled together by heat exchanger. The incident solar irradiance falls onto

the collector and heats up the working fluid inside the tubes. The vaporized

fluid rises upwards to the heat exchanger and transfers energy by

condensation to the water flowing in the secondary loop of the heat

exchanger. The condensed fluid return back to the collector tubes and the

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process of heat transfer continues. It was reported that the system based on

ETSC supplied heat at higher temperature (120 C) than normal flat plate

collector.

Oommen and Jayaraman (2001) developed a compound parabolic

concentrator (CPC) based solar cooker. The CPC module was attached with 5

litre capacity of pressure cooker. Cooking experiments were conducted by

using a rice water mixture as the load and water as heat transfer fluid (HTF).

The efficiency of the system was reported for different inlet temperatures of

HTF. The author concluded that the developed steam solar cooker is a

relatively sophisticated device that unites some of the characteristics of

reflector cookers, steam cookers, pressure cookers and heat accumulating

cookers.

2.3.2 Solar Cookers with Storage

The use of solar cookers without storage is limited because these

cookers cannot be used on partially cloudy days and /or in the late evening

hours. The storage option (sensible / latent) in the cooker will increase its

utility and reliability.

Figure 2.1 Sensible heat storage type cookers (a) using engine oil

(b) using sand and (c) using vegetable oil

b ca b c

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Figure 2.1a shows the schematic of a hot box cooker with used

engine oil as a storage material, developed by Nahar (2003). The device

consists of a double walled hot box. The maximum stagnation temperature

attained inside the cooking chambers of the hot box solar cooker with storage

material was the same, as that of the hot box solar cooker without storage

during the daytime, but it was 23 C more in the storage solar cooker from

17.00 to 24.00 hrs.

Figure 2.1b shows the schematic of the flat plate solar cooker,

using sand as the storage medium, developed by Ramadan et al (1998). The

cheapest storage material and the best performance were achieved by making

a jacket of sensible heat storage material such as sand, 0.5 cm thick. Six

hours / day of cooking time was recorded, and approximately 3 hours / day of

indoor cooking was achieved.

Figure 2.1c shows the schematic of the flat plate solar cooker

developed by Schwarzer and Silva (2003), using vegetable oil as the storage

medium. The system consisted of one or more flat plate collectors with a

coated absorber and double glazed covering, cooking pots and a storage tank

to store thermal energy. Vegetable oil was used as the heat transfer fluid. The

oil was heated up in the collectors and circulated by natural flow to the

cooking unit, where it transferred part of its sensible energy to the double

walled cooking pots. The major advantages are the possibility of indoor

cooking, the use of a thermal storage tank to keep the food warm for longer

periods of time or night cooking, and the reach of high temperatures of the

working fluid in a short period of time.

The major limitations of sensible heat storage materials include low

specific heat capacity and a decrease in the effectiveness of cooking, as the

temperature of the storage material decreases during discharging.

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Latent heat storage makes use of the energy stored when a

substance changes from one phase to another. The use of PCMs for storing

heat in the form of latent heat was recognized as one of the areas to provide a

compact and efficient storage system, due to their high storage density and

constant operating temperature. Few studies have been conducted with latent

heat storage materials in a box type solar cooker to cook food late in the

evening. Domanski et al (1995) studied the use of a phase change material as

a storage medium for a box type solar cooker designed to cook food in the

late evening hours and/or during the non-sunshine hours. They used

magnesium nitrate hexahydrate (Mg(NO3)2.6H2O) as a PCM for the heat

storage. Buddhi and Sahoo (1997) designed and tested a solar cooker with

latent heat storage for cooking food late in the evening. In this design, the

phase change material (PCM) was filled below the absorbing plate.

Commercial grade stearic acid (melting point 55 C, latent heat of fusion

161 kJ / kg ) was used as a latent heat storage material. In such type of design,

the authors observed that the rate of heat transfer from the PCM to the

cooking pot during the discharging mode of the PCM was slow, and more

time was required for cooking food in the evening.

Bushnell (1998) presented a prototype for solar ovens, which

employs pentacrythritol as a solid–solid PCM. The author described the

performance from the efficiency measurement, and determination of the

figure of merit. Sharma et al (2000) developed a PCM based storage unit with

acetamide (melting point 82oC, latent heat of fusion 263 kJ/kg) for a box type

solar cooker to cook the food in the late evening. They recommended that the

melting temperature of a PCM should be between 105 and 110oC for evening

cooking. Buddhi et al (2003) later developed a latent heat storage unit for a

box type solar cooker with three reflectors to store a larger quantity of heat

through a PCM. They used acetanilide (melting point 118oC, latent heat of

fusion 222 kJ / kg) as a PCM for night cooking.

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In a later study, Sharma et al (2005) developed a solar cooker based

on evacuated tube solar collector (ETSC) with PCM storage. The PCM

storage unit used two hollow concentric aluminum cylinders and the space

between the cylinders was filled with 45 kg erythritol (melting point 118oC,

latent heat of fusion 339.8 kJ / kg) used as the PCM. A pump circulates the

heated water (HTF) from the ETSC through the insulated pipes to the PCM

storage unit by using stainless steel tubular heat exchanger that wraps around

the cooking unit by closed loop. During sunshine hours, heated water transfer

its heat to the PCM and stored in the form of latent heat, through a stainless

steel tubular heat exchanger. This stored heat was utilized to cook the food in

the evening time or when sun intensity was not sufficient to cook the food.

They concluded that two times cooking (noon & evening) is possible in a day.

Noon cooking did not affect the cooking in the evening and evening cooking

using PCM storage was found faster than noon cooking. Experiments and

analysis indicated that the prototype solar cooker yielded satisfactory

performance in spite of low heat transfer.

Hussein et al (2008) developed a novel indirect solar cooker with

outdoor elliptical cross section wickless heat pipes, flat-plate solar collector

with an integrated indoor PCM thermal storage and cooking unit. Two plane

reflectors were used to enhance the insolation falling on the collector, while

magnesium nitrate hexa-hydrate (melting temperature 89oC, latent heat of

fusion 134 kJ/kg) was used as the PCM inside the indoor cooking unit of the

cooker. It was found that the average daily enhancement in the solar radiation

incident on the collector surface by the south and north facing reflectors is

about 24%. Different experiments were performed on the solar cooker

without load and with different loads at different loading times to study the

possibility of benefit from the virtues of the elliptical cross section wickless

heat pipes and PCMs in indirect solar cookers to cook food at noon and

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evening and to keep food warm at night and in early morning. The results

indicated that the present solar cooker can be used successfully for cooking

different kinds of meals at noon, afternoon and evening times, while it can be

used for heating or keeping meals hot at night and early morning. A detailed

review on the research and development of various types of solar cookers was

carried out by Muthusivagami et al (2010).

2.4 INVESTIGATIONS ON THE COOKING UNIT

Owing to the importance of solar cooking, several types of solar

cookers have been developed over the years. One of the major requirements

in using solar energy for cooking applications is the development of a cooking

unit, which should be fast and energy efficient. In the present investigation, a

cooking unit which is an integral part of the storage type indirect solar cooker,

is analysed for its suitability and performance, and hence, a review has been

carried out on the various studies made on the conventional type of cooking

units and its effect on the preparation of food.

A detailed survey on the various designs of cooking equipments,

their behaviour and energy efficiencies were carried out by Probert and

Newborough (1985). Improvements regarding the equipment design, cooking

techniques and consumer education were suggested by the authors from an

energy-thrift perspective. Geller (1982) carried out a detailed study at Ungara

area, in India, to analyse the type of cooking units used there, and the

efficiencies of these cooking units based on the energy balance study. He

found that the cookers have an efficiency of only 6%, and recommended the

use of aluminium pots rather than clay pots for an improvement in efficiency.

All the energy losses incurred during the cooking process are presented in

Figure 2.2.

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Figure 2.2 Energy balance for cooking

Newborough and Probert (1988a) found that, in Britain, in many of

the canteens, cafés, restaurants or hotels, much of the energy expended

during storing, preparing, cooking and serving food was wasted without

achieving any useful purpose. The authors found three interrelated factors

which lead to energy profligacy in catering, analyzed each of these

malpractices from an energy-thrift perspective, and recommended methods to

reduce energy misuse in those areas. In a separate study (Newborough and

Probert, 1988b), the authors developed a test rig to simulate the thermal

behavior of a conventionally-heated electric toaster to measure the steady

state heat transfer. The authors also modified the conventional electric toaster,

and demonstrated a substantial improvement in thermal efficiency.

Sheridan and Shilton (1999) evaluated the efficacy of cooking

hamburger patties by an infra-red source at two different wavelengths. The

authors reported that the gas consumption when using the longer wavelength

infra-red source, was reduced by 55% over that for the shorter wavelength, a

higher energy source. Bizzo et al (2004), discussed several aspects of cooking

54

fuel safety, considering traditional clean fuels such as the LPG, natural gas

and kerosene, and non-traditional and/or renewable fuels such as the DME,

producer gases, biosyngas, ethanol and ethanol-gel. In addition to the safety

aspects, the authors also suggested the type of equipment used for the LPG

and kerosene mode of cooking. A realistic, integrated and comprehensive

software was developed by Halder et al (2011), that can simulate a food

process and its safety by combining a fundamental, physics-based model of

the process, with the kinetics of the microbiological and chemical changes in

the food during processing, to provide the needed information at any time and

at any location. The heat utilisation efficiency of all commonly used

cookstoves in Bangladesh was thoroughly discussed by Lucky and Hossain

(2001), with the emphasis placed on natural gas cookstoves. The authors also

presented the cost of cooking for different fuel–cookstove combinations.

A mathematical framework to model the heat transfer efficiency of

cooking pots was proposed by Hannani et al (2006). The model consists of

combining the experimental results and the statistical data of the Residential

Energy Consumption Survey (RECS) of Iran, with a soft-computing concept

such as the neural network. The results showed that the efficiency increases

with an increasing diameter-to-flame ratio, bottom wall curvature, pot wall

slope, and overall conductivity. With an increasing edge radius and pot

height-to-pot diameter ratio, the efficiency decreases. An attempt to use waste

vegetable oil as a fuel for a cooking stove was made by Natarajan et al (2008).

The authors reported that the efficiency of the stove using vegetable oil as

fuel was 48.9% as compared to 34.9% with that of a conventional stove. The

Water Boiling Test (WBT), the Controlled Cooking Test (CCT) and the

Kitchen Performance Test (KPT) are the different protocols available to

measure and compare the performance and pollutant emissions of various

55

cooking stove. Jetter and Kariher (2009) studied the performance and

pollutant emissions of 14 solid-fuel household cook stove by using WBT

protocol.

Improved cooking stove projects in the developing world have the

potential to reduce deforestation, improve health, and slow down climate

change. To meet these requirements, stoves must be carefully designed

through testing and verification of performance. To compare the fuel use,

carbon monoxide (CO) and particulate matter emissions produced during

cooking, the performance of 50 different stove designs was investigated by

MacCarty et al (2010), following the 2003 University of California-Berkeley

(UCB) revised Water Boiling Test procedure. The role of donor organizations

in promoting energy efficient cook stoves was presented by Kees and

Feldmann (2011). In this paper, some basic facts on cooking energy, clean

and efficient technologies for cooking, role of public sector in dissemination

of clean cooking energy technology and the experience of GTZ’s in this

regard stoves were addressed by the author.

Bottani and Volpi (2009) developed a mathematical model for the

prediction of the cooking time of meat products in industrial steam ovens. The

analytical model was validated by comparing the predicted cooking time with

experimental cooking data related to the time–temperature curves of seven

meat samples and the model had a maximum deviation of 4.6%. The authors

suggested this model for direct implementation as a tool to monitor and

automate the industrial meat cooking treatments by means of computer

control. Wählby et al (2000), carried out an experimental study, to find out if

and how the food quality changes, when the airflow is changed from

traditional hot air (forced convection) to impingement and concluded that the

effect is the biggest at the beginning of the cooling process. Alvis et al (2009)

presented a paper that has been devoted to explore the main effects of

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deep-fat frying on the fried material, and to review the most important

methods used to measure the convective heat transfer coefficient. They

suggested that the coefficient value defines to a great extent the equipment

size, and also establishes the contacts between phases and processing time.

Farinu and Baik (2008) carried out a numerical simulation to determine the

convective mass transfer coefficient (hm) using the finite element method

during the frying of sweetpotato. They established a correlation between the

maximum heat transfer coefficient (h) and the maximum mass transfer

coefficient, and also between the maximum h and the corresponding hm at that

time.

2.5 SPECIFIC OBJECTIVES OF THE PRESENT WORK

In the present research, a detailed survey has been made on solar

collectors, storage systems, solar cookers and cooking units. It is observed

from the literature survey that the parabolic dish type collectors have been

successfully implemented in large scale community level cooking

requirements. In these systems solar energy is directly used for cooking

without storage, as the cooking is required only in the afternoon hours.

However, the success of household applications is possible, only if the

cooking is made available even during the non sunshine hours. Hence, it

requires better storage systems. Moreover a review of the literature infers that

considerable efforts have been invested in research and development of direct

type solar cookers with and without storage. However, limited research works

were reported on indirect types, particularly, parabolic trough type solar

cookers with a storage system.

In order to utilize the solar energy by using a parabolic trough

collector for residential cooking application, it was proposed to redesign the

conventional type of cooking unit, so as to circulate the heat transfer fluid

through the cooking unit for efficient heat exchange. Further, from the

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literature, it is found that there is no theoretically established procedure for

the design of such cooking units.

Considering the above, the specific objectives of the present

research work are formulated as below.

i) to develop a solar system integrated with a cooking unit

through a PCM based thermal storage tank, and to analyze the

charging / discharging performance of the system.

ii) to analyze the performance and optimize certain parameters

for the newly developed cooking units using the CFD

simulation.