Solar Cooker Study under Oman Conditions for Late Evening Cooking Using Stearic Acid and Acetanilide as PCM Materials

Solar energy is an alternative source of nonrenewable energy in Oman. Sultanate of Oman government showed initiation into utilization of solar energy for domestic applications. Conversion of solar radiation into useful heat is the simplest application of solar energy, in which it can be used for late evening cooking. In this context, present work highlighted the design and development of solar cooker for Oman climatic conditions.The current work signifies usage of solar cooker for late evening cooking using stearic acid and acetanilide as phase change materials (PCM). Solar cooker parts are developed in-house and connected to water heating system compounded with evacuated tubes solar collector and storage tank. The circumference of cooker unit is incorporated with spiral stainless steel heat exchanger and annulus area of the pot is filled with PCMmaterial. PCM releases heat at late evening and effective cooking up to 7:30 PM is noticed. The experimental results indicated the cooker efficiency of 30% and collector efficiency of 60–65% during the study. Overall, experiments showed satisfactory performance on the developed cooker.


Introduction
According to statistics, 50% of the commercial sources of energy like petrol, gas, and electricity are used in our daily consumption.Humans should take advantage of other sources of energy; renewable energy is one of the alternative available energy sources.It is also essential to encourage the community to start using alternative energy sources for the sake of cleaner environment.Solar cooker is now an invention that uses solar energy for late evening cooking.
In the late 1950s, the scientist Maria Telkes was the person who worked on box type solar cooker.Later, the mid of the 20th century showed a number of individuals and groups experimenting projects on solar cookers.The technical concept of solar cooker can be evaluated by thermal storage potential and energy is stored by raising the temperature of a storage medium; therefore, phase change materials (PCM) play important role in the present scenario.These materials have the ability to store large amounts of thermal energy under isothermal conditions, which means they can deliver or store energy at constant temperature and excrete the heat whenever there is a difference in the degree of the temperature.Sharma et al. [1] investigated the thermal performance of solar cookers based on an evacuated tube solar collector with PCM storage unit.The experiment results indicated that the noon cooking did not affect the evening cooking and that evening cooking using PCM heat storage unit is faster than noon cooking.Similarly, Agrawal and Yadav [2] carried out experiments with solar cooker with two sensible heat storage materials for evening cooking.They used a parabolic type of solar cooker and sensible heat storage materials like iron grits and iron balls.In their study relationship between the temperature and the solar radiation intensity for both sensible heat materials is noticed; experimental results highlighted the fact that solar cooker is more efficient when filling the iron balls in the inner space and the iron grits in the outer space.In addition, Sharma et al. [3] designed, developed, and performed evaluation of the latent heat storage unit for evening cooking of solar cooker.Cooking experiments are conducted with different loads and loading times during the summer and winter seasons.The experimental results showed that evening cooking is possible with a solar cooker having the PCM storage unit and is not possible in a standard solar cooker.Kassem [4]  setup included solar water heating that contained evacuated tubes solar collectors and storage unit.Paraffin was used as the phase change material in heat storage unit.He found that solar cooker based on evacuated tubes solar collector with PCM unit gave good performance in the conditions of high elevation.In a similar way, Saini et al. [5] studied the concept of solar cooker and designed a solar cooker for late cooking.Their study demonstrated different designs for solar cooker and studied the performance of solar collector.The study results revealed that solar cookers are helpful in minimizing CO 2 emissions and the cylindrical storage unit is preferred over rectangular storage unit.In addition, solar cookers having PCM which melts above 100 ∘ C are more promising for the storage unit of the solar cooker.
With this thought, present work signifies the design, development, and comparison of the performance of the solar cooker using stearic acid and acetanilide as phase change materials for Oman conditions.Thermal calculations are made for evacuated tube collector, PCM storage system, and studying the performance at different loadings.

Analysis of Solar Cooker System
2.1.Intensity of Solar Radiation.The placement of earth's orbit around the sun is such that sun-earth distance varies by 1.7 percent and solar radiation outside the earth's atmosphere is nearly of fixed intensity; the radiant energy flux received per second by a surface of unit area held normal to the direction of sun's rays at the mean earth-sun distance is constant throughout the year.This is called as solar constant,  SC , and its value is 1367 W/m 2 .However, this space radiation suffers variation due to the fact that earth rotates around the sun not in a circuital orbit but follows an elliptic path.The intensity of extraterrestrial radiation  ext measured on a plane normal to the radiation on th day of the year is given in terms of solar constant ( sc ) [7]: In addition, on the basis of transmission coefficients, latitude, declination, and hour angle, the total heat received on horizontal surface at ground is calculated from the relation where  is transmission coefficient,  is latitude angle,  is declination, and  is hour angle.

Evacuated Tube
Collector with Heat Pipe.Normally, evacuated tube collector with heat pipe consists of a metal fin and is used for maximum heat transfer from absorbed solar energy at the absorber.The evaporator is exposed to solar radiation and condenser is attached with tube in configuration plate.The working fluid flows through tube flat plate of condenser.
A schematic view of an evacuated tube collector with heat pipe and the condenser is shown in Figure 1.The condenser of heat pipe transfers the heat to the fluid through a manifold.Each fin plate with heat pipe absorber is enclosed in a separate evacuated cylindrical tube.
Under steady condition, the rate of useful energy available with evacuated tube solar collector can be represented by the equation where  ab is absorbed solar radiation in W/m Similarly, acetanilide is a solid chemical used to store the thermal heat energy in the form of latent heat.It is also known as N-phenylacetamide, acetanil, and acetanilide.Table 1 highlights the thermophysical properties of phase change materials and Figure 2 indicates enthalpy curves of phase change materials.The study gave the confidence in the calculation of solar intensity of radiation in Oman.

Thermal Analysis on Evacuated Tube Solar Collector.
Similarly, from theoretical relations, the parameters like useful energy, final temperature of water, and efficiency of  2 for the values calculated from thermal analysis.

Experimental Validation
Experiment Setup.The solar cooker setup consists of an evacuated tube solar collector, closed-loop water line, PCM storage unit, stainless steel heat exchanger, and cooking vessel.The specifications of the evacuated tube solar collector are highlighted in Table 3. Figure 4 indicates the assembly of evacuated tube solar water heater used for the solar cooker.
In the present work, a storage unit is provided with two hollow concentric aluminum cylinders; its inner and outer diameters are 250 mm and 350 mm, respectively, and 300 mm deep.The space between the inner diameter and outer diameter is filled with PCM materials; see Figure 5(a).Two separate storage units' setup is made to study the performance with acetanilide and stearic acid as PCM materials.From thermal analysis calculations, it is confirmed that 2.5 kg stearic acid and 3 kg acetanilide will be sufficient to achieve     hours, the heated water transfers heat to the PCM and the heat is stored in the form of latent heat.This stored heat is utilized to cook the food in the evening when the sun intensity is not sufficient to cook food.See Figure 7 for cooking of the rice in the late evening.In the current study, acetanilide and stearic acid are used as phase change materials.The PCM storage unit and piping system are insulated to prevent the heat losses.A valve between the evacuated tube solar collector and storage unit controls the water flow.
In addition, other parameters like water outlet and PCM storage temperatures are recorded at regular intervals.In  addition, solar cooker is studied for different loadings of water in the vessel.Figure 8 explains the variation of PCM temperature at different loadings in the vessel.This reading corresponds to thermometer mounted at centerline (right side) of annulus area of PCM.From the experimental study it is confirmed that acetanilide temperature is higher than the stearic acid.Also, in late evening, that is, at 7 PM, for all loadings acetanilide showed the temperature of 78-88 ∘ C.This temperature enables cooking the food items effectively in late evening.In a similar way, water outlet temperature is recorded during the testing; see Figure 9 and it is observed that maximum water outlet temperature of 130 ∘ C at 3 PM on May 9, 2016, is noticed.It is due to higher solar intensity on that day.
From the test results, it is observed that, until 3 PM, the temperature of PCM material is lower than water outlet temperature.However, in late evening, water temperature drops substantially as compared to PCM temperature.It is due to the fact that heat content of PCM is high, so fewer drops are noticed in the evening and this helped to cook the rice effectively.
Heat Storage in PCM and Collector Performance.The measured experiment data is used to evaluate the parameters like energy storage capacity of PCM material and water.The total heat stored in the PCM is calculated by analyzing the enthalpy inside PCM storage unit from the measured PCM temperatures.Similarly, total heat gain by water is evaluated from measured outlet water temperatures.From the study, it is confirmed that cooker performance is directly related to energy storage capacity of PCM and solar intensity of radiation.In addition, collector performance is evaluated from heat energy gained by water from solar radiation.Experimental results of energy content of PCM and heat gain by water are tabulated in Table 4. From the study, it is confirmed that cooker showed better performance for all the loadings with acetanilide compared to the stearic acid.A 60∼ 65% collector efficiency is noticed from experiment results and is almost nearer to the value predicted from thermal analysis.In addition, a maximum cooker efficiency of 30% is noticed, and this value is very low and could be improved in the future by increasing heat storage capacity of PCM material and providing fins around copper coil and also by minimizing leakages in the system.

Conclusion
In the present study, thermal performance of a solar cooker coupled with PCM heat storage unit and evacuated tube solar collector is studied for Oman conditions.A simple cylindrical PCM heat storage unit is designed to store solar energy during sunshine hours and to cook food in the late evening.The experiment results from the present setup drives the following conclusions.
(i) The solar cooker system is able to cook successfully in evening time up to 7:30 PM by providing PCM storage unit, and PCM material temperature reached more than 120 ∘ C at 3 PM, which is sufficient to cook the food in the evening time.
(ii) The study results indicated that acetanilide would be better than stearic acid from the performance point of view.
(iii) During the experiment, collector efficiency of 60∼ 65% and cooker efficiency of 30% are noticed with acetanilide as PCM.Overall, the present study showed satisfactory performance under Oman conditions.

Figure 4 :
Figure 4: Assembly of evacuated tube solar water heater.

Figure 5 :
Figure 5: In-house developed parts of solar cooker.

Figure 6 :
Figure 6: Complete setup of solar cooker.
Figure 6 reveals the complete setup used for cooking experiments.Experiment Methodology.Experimental study on solar cooker is performed in the month of May 2016.During sunshine

Figure 7 :
Figure 7: The late evening cooking of rice.

Figure 8 :
Figure 8: Variation of PCM temperature at different loadings in the vessel.

Figure 9 :
Figure 9: Variation of water outlet temperature.
Characteristics of Phase Change Materials.Stearic acid is a saturated fatty acid with an 18-carbon chain.It is a waxy solid and chemical formula is C 17 H 35 CO 2 H. Stearic acid can store the thermal energy and chemical phase will change from solid to liquid and from liquid to solid during thermal process.
2;   is overall heat loss coefficient in W/m 2 K;   is ambient temperature in K; and   is surface area of collector in m 2 .2.3.Phase Change Materials.Phase change materials are latent heat storage materials and energy release occurs when a material changes its phase from solid to liquid or vice versa.In general, they absorb and release heat at constant temperature and can store more heat per unit volume than sensible heat storage materials.Fatty acids have high heat of fusion values comparable to that of paraffin's and their major drawback is 2-2.5 times costlier than paraffin's.In the current work stearic acid and acetanilide is used as PCM.

Table 1 :
Thermophysical properties of the phase change materials.

Table 2 :
Thermal analysis results for evacuated tube solar collector.

Table 3 :
Specification of evacuated tube solar collector.
evacuated tube solar collector are calculated.Refer to Table

Table 4 :
Observed values of heat gained by water and heat stored by PCM.Acetanilide Stearic acid  PCM , MJ  water , MJ  collector , %  cooker , %  PCM , MJ  water , MJ  collector , %  cooker , % * Data recorded in year 2016.