Developing experimental control method and simulation model for solar cookers with heat storage

Abstract

The use of wood and fossils for cooking among others has led to global warming and deforestation. This necessitates the use of renewable energy sources. Solar energy is the only renewable energy source that is clean and freely available all over the earth's surface during sunny hours of the day. Due to its intermittent nature, its thermal energy has to be stored in thermal energy in Thermal Energy Storage (TES) systems for indirect solar cookers. The developed TES systems have some shortcomings like thermal stratification and low efficiency that need to be addressed. Thermal stratification is guaranteed in a TES system when thermally charged at a constant temperature. A mechanical thermostat has been developed to charge oil in a TES system at a constant temperature. This thermostat was composed of: a slider-valve, pneumatic cylinder, heating chamber and expansion system. This thermostat was tested by charging a 3-tank TES system. The 3-tank TES system was composed of a cold oil reservoir tank, a hot oil storage tank, a drainage tank and a cooking unit. A simulation model for charging an oil-based TES system with a forced thermal strati-_cation was developed and validated using experimental data. The results showed that the thermostat charging temperatures had oscillations which were reduced by lowering the ow rate by partially opening the valve along the oil pipe from the cold-oil reservoir to the thermostat. The 3-tank TES system was charged at an efficiency of 51.3% using the thermostat and later discharged through a cooking unit by boiling 0.5 liters of water at discharge efficiencies of 14.1%, 32.5% and 19.7% for the ow rates of 2.1, 2.8 and 6.5 g/s, respectively. These results also showed that 1 kg of rice that required 0.032 kWh could be cooked using 0.82-1.92 liters of oil at 200 _C. The developed 1D model had a mean absolute percentage error in the range 4.9%-8.2% and could study the effect of insulation and ow rate on the system thermal profiles.