The improvements in the performance of solar cookers can be achieved with modified energy collection methods. A pyramid type solar cooker as shown in Figure 1a was developed by Kumar et al., ¹⁰ with black painted aluminum absorbing plate. The average temperature of 130 °C was maintained for a long duration in no load condition. The average water temperature attained inside the cooker was 98.6 °C. To achieve higher temperatures for longer duration, a box type solar cooker as shown in Figure 1b was developed and investigated by Mirdha and Dhariwal. ¹¹

A CPC based building integrated cooker was developed and analyzed by Harmim et al. ( Figure 2). ¹² The cooker was fixed on the south wall with an opening in the kitchen. The highest temperature of around 166 °C. This arrangement was found highly useful as it brings solar energy into the kitchen. A funnel based concentrator as shown in Figure 3, was used to focus solar energy on a stove integrated with energy storage medium. Temperature range of 250 °C was achieved with this modification. ¹³

These modification shows that higher temperatures can be achieved with geometrical modification in the solar cookers. Use of reflectors is the most common method to achieve higher temperature limits.

The storage of energy during the daytime for supply during off-sunshine hours is possible if it is stored in one or other form. SHS and LHS mediums are classified under TES. SHS mediums do not have a change in their physical state while LHS mediums absorb energy, converting their physical state from solid to liquid, and from liquid to vapor. The heat and temperature variations in LHS and SHS have been presented in Figure 4. The detailed classification of energy storage mediums is presented in Figure 5.

The heat storage capacity of latent heat thermal energy storage (LHTES) mediums is high as compared to SHTES medium. As the latent heat of fusion (Q _latent) does not have any temperature change, the charging and discharging include both temperature change and phase change in LHTES.

SHS mediums

SHS mediums store energy in the form of heat creating temperature difference. Materials having a high specific heat are more useful as SHS as they can store higher energy for a unit change of temperature difference. They are relatively cheaper and easily available as compared to other materials, which increase their usefulness.

SHS mediums can be in solid or liquid form. In liquid, water is used in many applications as it is cheaply available and has high specific heat. For high temperature applications, various SHS liquid mediums like oils, liquid metals, or salts can be used. In a few articles, ^{15 , 16} working of SHS mediums to up to 180°C has been reported, but the increase in the total weight of the system is a concern with these mediums. Economic analysis for different solar concentrating devices used in cooking applications has been carried out by Widjaja et al., ¹⁶ with SHS medium, the payback period for solar cooking system was half of the payback period without SHS.

Saxena et al. ¹⁷ experimentally investigated the use of grainy carbon powder as an SHS medium in box-type solar cookers (BTSC); the best output was reported with a composite prepared with LHS i.e. paraffin wax. The use of palm oil as an energy storage medium was studied ¹⁸ and it was reported that the heating efficiency was doubled with the use of energy storage mediums. A low cost SHS medium was prepared with sand and carbon for testing with solar cooker. ¹⁹ The cooker was found viable for off-sunshine hours cooking with an efficiency of around 37%.

Use of bayburt stone as an SHS medium in BTSC as shown in Figure 6 resulted in efficient cooking during off-sunshine hours. The efficiency improvement of about 40% was achieved with this SHS medium. ²⁰ The temperature in solar cooker with Lapland granite rocks used as SHS during experimentation was maintained above 40°C for almost 225 minutes, while without SHS material, it was above 40°C for only 45 minutes. The author ²¹ concluded that, when using TES, the cooking capability of the solar cookers can be extended for a longer duration.

An oil-pebble bed-based energy storage medium for solar cookers was studied by Mawire et al. ²² Methods of charging and discharging the energy storage medium also play an important role in its performance. Engine oil as an energy storage medium with BTSC systems was studied by Nahar. ²³ A temperature difference of 23°C was achieved with energy storage compared to a system without energy storage. Food was cooked perfectly during off-sunshine hours where it was not cooked without energy storage medium. The various materials used as SHS medium in solar cooking system along with their thermal and physical properties are presented in Table 1.

LHS mediums

LHS mediums generally have a phase change during energy interactions. During energy exchange, i.e. charging and discharging, the energy can be absorbed in raising the temperature as well as changing the physical state. They are also referred to as PCM. It is desirable to use LHS mediums where uniform temperature needs to be maintained for a longer duration, however, PCM increases the initial cost of the system. ²⁴ Chemically, the PCMs must be stable, compatible with materials, should be non-toxic and non-flammable. ²⁵

Organic PCMs are the most commonly used energy storage medium in solar cooking. Nitrate salts are mostly used in high temperature applications, while for low and moderate temperatures, organic materials are used. The systems with working temperatures lower than 120°C do not effectively utilize the latent heat storage mediums. ²⁶ The various materials used as LHS medium in solar cooking system along with their thermo-physical properties are presented in Table 2.

Use of organic PCM

Organic PCMs are a group of paraffins and non-paraffin materials. Fatty acids, alcohols, glycols and esters are classified under non-paraffin phase change materials. They have low thermal conductivity in solid phase and are relatively cheap. Paraffin wax is the most commonly used organic PCM as it has comparable thermal properties, i.e., high density and high latent heat.

The cooking efficiency up to 54% can be achieved and cooking time reduced with the use of paraffin as an energy storage medium in BTSC. It was concluded by Saxena et al. ¹⁷ that the composite of SHS and LHS performed better than the SHS and LHS individually. The cooking time was reduced by one hour when paraffin wax used as an energy storage medium. ²⁷ Comparative studies between solar cooker with and without energy storage was reported by Reddy et al. ² Paraffin PCM was used and a temperature difference of 17 °C was maintained during evening time in the cooker with TES compared to the cooker without TES.

Use of erythritol as an energy storage medium was investigated ²⁸ using a box-type solar cooker and it was reported that both the charging and discharging time was increased using PCM. Thermal stability of the system was better with PCM during off-sunshine hours. Use of acetanilide as PCM with box-type solar cooking system was investigated by Buddhi et al., ²⁹ showing high temperature for well-cooked food. Use of benzoic acid and stearic acid as LHS medium was studied by Adetifa et al. ¹⁸ and it was concluded that the high temperature of water was realized with energy storage, i.e., 80°C, and 50°C was maintained without solar radiations.

Solar cooker with PDC was studied by Senthil ³⁰ using paraffin wax as an energy storage medium. Heating time for water was reduced to 90 min compared to 120 min without PCM. Improved productivity and thermal performance of the cooker were observed with PCM. Solar cooking using solar salt as PCM for high temperature application was carried out by Bhave et al. ³¹ An indoor experiment was carried out and 170–180°C was easily achieved with this system. The device was found convenient for cooking inside the kitchen and with minor geometrical modifications it can serve different end user needs.

Galactitol as PCM was utilized for medium temperature solar cooking applications by John et al. ³² The lifespan of Galactitol as PCM was reported to be less than 100 days if utilized every day for cooking application. Stability at high temperatures of 150°C was reported, but due to the short life span, the author concluded that it is unstable for energy storage applications.

Stearic acid as a PCM using a collapsible parabolic solar cooker was studied by Keith et al. ³³ PCM integrated cooking pot is shown in Figure 7 which was used to cook food and subsequently kept it warm for longer duration. The payback period of the developed solar cooking system was found to be less than 53 weeks.

Paraffin and erythritol were used as energy storage mediums in a parabolic concentrating- type solar cooking system. Lecuona et al. ³⁴ concluded that the use of PCM and insulating the utensil after energy storage allows cooking of dinner and next day breakfast. The study also concluded that higher latent heat and conductivity of erythritol are advantageous for faster indoor cooking.

Use of inorganic PCM

Inorganic PCMs are salts, salt hydrates and metal alloys. Salts and salt hydrates have the highest latent heat to volume ratio, thermal conductivity and smooth phase transition. Drawbacks of inorganic salts as reported by Khan et al. ²⁴ are change of volume, low thermal conductivity and high cost.

Solar salt was used as an energy storage medium with a box-type solar cooker by Coccia et al. ³⁵ It was concluded that thermal stability was improved and heat retention was increased 1.86 times than that without a storage medium. Mussard et al. ³⁶ concluded that the use of inorganic PCM like nitrate salt has potential of replacing traditional cooking methods. These systems are slower for traditional cooking but the quality of heat transfer is better in these systems.

Magnesium chloride hexahydrate was used as PCM with parabolic dish solar collector (PDSC) by Bhave et al. ³⁷ The cooker with PCM tubes is presented in Figure 8. The PCM was heated upto 130°C i.e., 12°C above the melting point of the PCM. 32.66% utilization was achieved by the developed system.

Magnesium nitrate hexahydrate was used as PCM for indirect solar cooking unit by Hussein et al. ³⁸ The developed cooker was found suitable for heating or keeping the food hot during off-sunshine hours till the next morning. Box-type solar cooker incorporating acetamide as PCM was experimentally compared with solar cooker without PCM by Sharma et al. ³⁹ The results concluded that cooking during off-sunshine hour is possible with cooker incorporated with PCM. Due to heavy weight, metallic PCMs are rarely considered as an energy storage medium in solar cooking.

Heat transfer fluids (HTFs)

Solar energy systems while having energy concentration can store thermal energy in direct or indirect cooking arrangements. HTFs are used to transfer the heat from the source to the indirect cooking system. The heat transfer fluid, while having a major role in transferring the heat, can also behave as an energy storage medium. Few articles suggested that the storage medium can also be used as an HTF. ⁴⁰ The high specific heat and thermal conductivity appraise it to behave as energy storage and transfer medium. Low viscosity for minimum pumping power is desirable for heat transfer fluids. Table 3 lists HTFs used in solar cooking and their properties.

Use of therminol-55 as an HTF for energy modelling of cooking systems was carried out by Bade et al. ⁴¹ and proposed this for water scarce regions. It also enhances the heat transfer capability and has a very minimal payback period. Therminol-55 (synthetic oil) was used as heat transfer fluid by Singh ⁴² for realizing indoor cooking using solar energy. Vegetable oil was used as HTF in solar cooking system for big families and positive results have been obtained by Schwarzer et al. ¹⁵