Solar Cooking by Using PCM as a Thermal Heat Storage

Abhishek SaxenaDepartment of Mechanical Engineering,

M.I.T., Moradabad, (U.P.) INDIAE-mail: culturebeat94@yahoo.com

Shalini LathDepartment of Mechanical Engineering,

ACME College of Engineering, Ghaziabad, (U.P.) INDIA

Vineet TirthProfessor and Director

Department of Mechanical Engineering,M.I.T., Moradabad, (U.P.) INDIA

ABSTRACTSolar thermal energy refers to the technologies that utilize the sun energy for cooking, for heating water and other heat-transfer fluids for a variety of residential, industrial and utility applications. Simple and widely used applications of solar thermal energy include cooking, water heating, space heating, power generation and agricultural drying. If we talk purposely about the solar cooking, then the available literature shows that cooking in the evening or off sunshine hours in solar devices is possible by operating the cooker on auxiliary power or by using different phase change materials (PCMs) in solar cookers, which are filled beneath the absorbing tray to be worked as thermal heat storage. In the present article some of different phase change materials are studied for solar cooking and among them stearic acid (commercial grade) is found to be a good latent heat storage which is experimentally tested in a simple box type solar cooker and the comparison is made with another similar design solar cooker without PCM.Keywords: Heat Storage, PCM, solar box cooker, stearic acid.

1. INTRODUCTIONSolar energy is an inexhaustible resource. The sun produces vast amounts of renewable solar energy that can be collected and converted into heat and electricity. Humans have harnessed the power of the sun for millennia. In the 5th century B.C., the Greeks took advantage of passive solar energy by designing their homes to capture the sun’s heat during the winter. Later, the Romans improved on solar architecture of buildings by covering south-facing windows with clear materials such as mica or glass, preventing the escape of solar heat captured during the day [1]. In the field of solar cooking the idea was come through the concentrating effect of a parabolic mirror focused on a cooking utensil, a quite satisfactory source of heat for the cooking of food—when the sun is shining. It was reported that a practical device has been made and marketed in India at a cost of about $14, for use in regions where about the only kind of fuel was animal dung. In such areas solar cook-stoves would be of great benefit, permitting the fertilizing value of the animal excrement to be put back on the soil where it really belongs [2]. Cooking by means of solar energy started in the ‘70s in response to the growing shortage of firewood resulting from chronic deforestation. A solar cooker needed solar energy—just a free fuel from the sky for operation. Having the concept to utilize this free fuel from the sun in the mind, many of the research institutes developed solar cookers for some different designs but with certain limitations. Among all those different designs a simple solar box type cooker is used

commonly due to its simplicity. The use of a solar box cooker is limited because cooking of food is not possible due to frequent clouds in the day or in the evening. If storage of solar energy can be provided in a box cooker, then there is a possibility of cooking food in the evening and the storage will increase the effectiveness and reliability of the solar cookers [3, 4].

TES systems have a variety of applications such as thermal protection and control of electronic components, heating and cooling of buildings and hot water preparation. By utilizing waste heat, TES systems can reduce the consumption of limited fossil fuel reserves and restrain output of CO2 to preserve the environment. PCMs are promising candidates for consideration as heat storage media due to their large energy storage capacity. A PCM storage system with small PCM size can be used for rapidly supplying a large amount of heat to the object [5]. Hence, PCM is a good option to store the solar energy during sunshine hours and can be utilized for cooking in late evening or off sunshine hours. The theory behind it; as the source temperature rises, the chemical bonds within the PCM break up as the material changes phase from solid to liquid. The phase change is a heat-seeking (endothermic) process and therefore, the PCM absorbs heat. Upon storing heat in the storage material, the material begins to melt when the phase change temperature is achieved. The temperature then stays constant until the melting process is finished. The heat stored during the phase change process (melting process) of the material is called latent heat. Latent heat storages are defined

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by a high level of energy density. Different type of heat transfer has been discussed during cyclical melting and freezing of a PCM. Energy storage in PCM has a lot of advantages over sensible systems because of the lower mass and volume of the system. The energy is stored at a relatively constant temperature and energy losses to the surroundings are lower than with conventional systems. Paraffin waxes are despicable and have a moderate TES density but low thermal conductivity and therefore demanding a large surface area. Hydrated salts have a superior energy storage density and a higher thermal conductivity [6, 7]. The storage of thermal energy in the form of sensible heat and latent heat has become a significant aspect of energy management with the prominence on efficient use and conservation of the waste heat and solar energy not only in industry and buildings but for solar cooking also [8].

Fig. 1: Classification of solar cookerswith thermal heat storage

2. THE USE OF PCM’s IN SOLAR COOKERSLHS is one of the most efficient ways of storing thermal energy. Unlike the SHS method, the LHS method provides much higher storage density, with a smaller temperature difference between storing and releasing heat. PCM can be used for energy storage and temperature control. Among them, the solid-solid PCMs are the focus of attention [9].

They can be applied in many fields such as solar energy utilization, waste heat recovery, electric appliances with thermostatic regulators and so on. Uses of various PCM and TES in solar cookers have been as shown in Table 1, while the classification of solar cookers with thermal heat storage is shown in Fig. 1.

3. PERFORMANCE TESTING OF STEARIC ACID AS LHS INSIDE SOLAR COOKER

For the performance evaluation of the thermal heat storage for solar cooking purposely two similar design solar cookers were considered (cookers A & B). Both the cookers A & B consist of a double walled hot box and made of 22 SWG Al sheet. The specific dimensions of the outer and inner box were

Table 1: Different type of PCM used as latentheat storage in solar cooking

Reference PCM tested Mode of cooking

Maximum Tstag

Ramdan et al. [10]Domanski et al. [11]

Haraksingh et al. [12]

Buddhi and Sahoo [13]Nandwani et al. [14]Sharma et al. [15]Oturanc et al. [16]Buddhi et al. [17]Sharma et al. [18]

Hussein et al. [19]

Chen et al. [20]

Muthusivagami et al. [21]EI-Sebaii et al. [22]

Saxena et al. [23]

Salt Hydrate

Magnesium Nitrate Hexa-Hydrate

Coconut oil

Stearic acid

Polyethylene

AcetamideEngine oilAcetanilideErythritol

Magnesium NitrateHexa-HydrateStearic acid, AcetamidePCM-A-164

Acetanilide, MgCL-Hexa-hydrateGranular media

FP- solar cookerSBC with multi-step inner reflectors FP solar cooker(Integrated)SBC

SBC

SBCSBCSBCSolar cooker integrated with ETCFP solar cooker(Integrated)SBC

Concentrating cookerFP solar cooker(Integrated)SBC

120°C

95°C

150°C

122°C

132°C

127°C105°C135°C

138°C

140°C

–

140°C

134°C

145°C

approximately 540 × 540 ×160 mm3 and 455 × 435 × 65 mm3. The space between the outer tray and outer box was filled of glass wool. The inner tray and outer boxes were painted dull black to absorb maximum solar energy. The leakage from the box to the surroundings was minimized by having a rubber gasket (1.5 mm thick) in between the double glass cover (10 mm glazing) and the box. One 4 mm thick plane mirror reflector was fixed over the box to get additional heat energy. The absorber trays of both cookers were painted black on both sides and designed to be kept at least two cooking vessels of Al alloy of 16 cm diameter (with a height of 9 cm & 0.5 mm thickness) inside the cookers for cooking. The cooking vessels were round shaped & painted black from outside.

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A double glazing with 25 mm gap was used to transfer the direct radiation and reflected radiation via a mirror booster to the absorber trays. Both the cookers were tested on load and no load condition to evaluate performance of PCM (stearic acid) used for late hours cooking by placing towards south direction. It is notable that in the case of solar cooker A, 1500 grams stearic acid was filled between the lower (bottom side) insulated base (Tin made) of box cooker and the Al base absorber tray (Fig. 2). The glass wool was filled beneath the LHS carrying tray and the outer cabinet of the box cooker A.

Fig. 2: A Solar box type cooker with latent heat storage

Thermo-physical Properties  of Stearic Acid

Table 2: Properties of PCM (stearic acid) used [19]

Melting temperature 52°CDensity 847-965 (kg/m3)Appearance White solidStorage temperature 2–9°CRefractive index 1.429 Thermal conductivity 0.29 (W/m K) Latent heat of fusion 169.0 (kJ/kg)Specific heat 1.590 (kJ/kg K)

The experimental testing was conducted on a typical sunny day in the month of March 2013, and started at 12:00 Hrs at noon on 20.03.2013 for both the model cookers A & B and ended at 19:00 Hrs in the evening. The entire experimental work was carried out under the climatic conditions of Moradabad (latitude-28o58ϕ/North and Longitude-78o47ϕ/ East), India. On 20.03.2013, in cooker B (with a simple blackened Al absorber tray), the TB-stag was found maximum 124oC at the 15:00 Hrs, while Tamb was noted 32oC. At the 19:00 Hrs, Tamb was noted to be falling down corresponding to the solar radiation. Tamb was measured near about 19oC with 53oC of TB-stag at the end of the day. The readings were taken at the gap of 60 minutes at no load conditions. Both the cookers were operated simultaneously for thermal performance evaluation at the same place. A six wire copper

constantan thermocouple meter (oC) with accuracy of ±1°C was used for temperature variations while a typically designed solarimeter ‘Surya-Mapi’ (W/m2) with an accuracy of + 1 W/m2 was used for solar intensity measurement. On the same day cooker A was successfully reached to a maximum temperature 134oC at 16:00 Hrs at the same ambient conditions and the obtained temperature (134oC) is good enough for cooking of almost edibles. In the evening at 19:00 Hrs, TA-stag was noted 64oC when Tamb was near about 19oC. Figure 3 shows the temperature variations in both solar box cookers A & B correspond to the solar radiation. Besides this, Figure 4 shows the performance of both the cookers on load conditions.

Fig. 3: Heat curves of cooker A and Bduring stagnation testing

Fig. 4: Heat curves of cooker A and Bduring sensible heat testing

4. RESULT AND DISCUSSIONAfter theoretical study of various PCMs used in solar cooking, stearic acid has been found commonly good LHS, and consider as a heat storing media for experimental testing purposely for solar cooking. The thermal performance of the cooker B and cooker A were determined by conducting the stagnation temperature test to obtain the first figure of merit: F1 and by sensibly heating through a known quantity of water to obtain the second figure of merit: F2. The value of F1 for cooker B was found to be 0.12 and 0.14 for cooker A. To determine F2, each cooker was loaded with 1 kg of water (500 gram in two similar cooking vessels), and the value of F2 was 0.70 for cooker B while 0.88 for cooker A. The equations used to calculate the values for both the figures of merit are given in reference [24]. Solar box cooker with stearic acid (A) was observed to achieve a good temperature to cook every kind of

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eatables like rice, beans, pulses and fish with a good possibility. Besides this, in the comparison of the cooker B, cooker A was able provide a hot environment for the cooking substance to remain hot/warm for long hours after completion of cooking. This temperature (near about 64oC) can also be used to get hot/warmer of some of drinking substances such as: milk, soups and water etc., in the cooker A after sunshine hours because of heat storing capacity of the stearic acid. The cooking time for cooker A, was also noticed to be reduced near about 15 minutes in comparison of box cooker without thermal storage.

5. CONCLUSIONCooking energy plays an important role in sustainable energy management in Indian households as well as worldwide. There are various options to meet the end user needs using both commercial and non-commercial energies. Traditional fuels like wood pellets, dung cakes, and kerosene utilization must be minimized with the developed solar cookers. This will lead to a reduction in human drudgery. Such an effort will not only be useful in improving the quality of life but also in environmental protection. This paper focuses on several qualities of solar energy such as; a free fuel from the sky, environment friendly, huge availability of almost places, low or no running cost, good saving, minimization of the monthly electricity bill, accident free, less attention is required etc. Apart this, in the field of solar cooking the available thermal energy storage technology for solar cookers, with the storage unit; food can be cooked at late evening, while late evening cooking was not possible with a simple solar cooker. So that, solar cooker with storage unit is very beneficial for the cooking methodologies and as well as for the energy conservation. Many of PCM’s are under testing for solar cooking but stearic acid (commercial grade) is commonly used due to easy availability and economically suitable till now.

Nomenclature –

SBC Solar box cookerPCM Phase change materialLHS Latent heat storageTtray Stagnation TemperatureTabm Ambient TemperatureTm Melting TemperatureTstorage Temperature of PCM filled in box cookerFPC Flat plate collectorTES Thermal Energy StorageLH Latent heat of fusionSHS Sensible heat storageT3, TA-stag Absorber tray temperature of cooker AT2, TB-stag Absorber tray temperature of cooker BH Solar radiation

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