An investigation of an oil/packed bed thermal energy storage system using phase change material for domestic cooking

Abstract

Feeding is pivotal to human existence and a large amount of energy is utilized globally, each day, for the cooking of food. Sadly, most of the current sources of energy used for most homes have varying degrees of negative impacts on human and environmental health. In developing countries, particularly, the combustion of biomass still forms a major source of energy for domestic cooking applications. About 1.5 million deaths, yearly, are attributed to indoor pollution from to the combustion of biomass that is used for cooking foods in the developing countries. Solar thermal energy is a free, safe and renewable energy resource which may be harnessed into meeting cooking energy needs. However, due to the time-dependency of the availability of solar energy which might not match the periods of demand, it becomes necessary to store the thermal energy during the hours of sunshine for use during periods of insufficient or no solar radiation. Latent heat thermal storage systems provide large thermal energy storage densities by utilizing phase change materials (PCMs). A packed bed of spherically encapsulated erythritol was considered in this research, using sunflower oil as the heat transfer fluid (HTF). A preliminary numerical study indicated that the proposed design will provide good heat storage performance. The experimental study revealed that an aluminum alloy 1050-H14 was chemically compatible with both the PCM and HTF. A separate numerical study indicated that a wall thickness of 1 mm for an aluminum spherical capsule with a diameter of 50 mm will provide mechanical stability when filled with meso-erythritol and heated for thermal energy storage. Experimental tests of thermal stability revealed that meso-erythritol should not be used above 177.0 $^o$C as it will begin to degrade. The tests of cycling stability revealed meso-erythritol to be chemically stable after several heating and cooling cycles with the solidification enthalpy remaining almost constant. However, the melting temperature is seen to change from about 119 $^o$C to about 105 $^o$C due to the fast rate of energy withdrawal from the meso-erythritol sample, forcing it into a metastable state. Very little weight degradation was observed after several heating and cooling cycles. On comparing the thermal stability and thermal cycling results obtained for meso-erythritol with those of acetanilide and an Indium-Tin alloy, meso-erythritol showed comparable performances with the alloy and better performance compared to acetanilide during the heating cycles. However, the performances of meso-erythritol during the cooling cycles were marred by severe supercooling. The lower level of health hazard presented by meso-erythritol as compared to acetanilide and the Indium-Tin alloy still made it attractive to be investigated as the PCM of choice. The charging and discharging performances of meso-erythritol, acetanilide and the Indium-Tin alloy were investigated simultaneously inside separate spherical aluminum capsules that were fabricated. Meso-erythritol had the highest thermal energy storage density, while acetanilide due to its low density, had the least. However, the large degrees of supercooling exhibited by meso-erythritol impacted negatively on the quality of thermal energy discharged by the PCM as most of the energy was discharged at temperatures below 100 $^o$C desired for cooking. The Indium-Tin alloy showed the best performance but it was deemed too expensive for use in the proposed packed bed TES system. An oil/packed bed thermal energy storage system was designed and fabricated, with meso-erythritol filled inside 50 mm spherical aluminum spheres as the packed bed while sunflower oil was used as the heat transfer fluid. A secondary storage was included in the system for immediate utilization of thermal energy simultaneously as the packed bed was being charged. Good heat transfer was obtained between the HTF and the encapsulated PCM. High HTF flow rates and high HTF temperatures were observed to result in high rates of thermal energy storage. The rate of thermal energy discharged by the packed bed also increased with an increase in the HTF flow rate. Severe supercooling was exhibited by the encapsulated meso-erythritol which negatively impacted upon the temperature at which the latent heat was discharged. The fabricated thermal storage system can provide hot water for domestic use while storing enough thermal energy for cooking vegetables later.