Development and assessment of a cooking unit integrated with solar thermal energy storage system

Abstract

The thermal performance of a newly developed cooking unit integrated with a thermal energy storage (TES) system suitable for solar thermal applications has been tested and analysed. The experimental set-up consisted of a TES tank, connecting pipes, a manual control valve and a cooking unit. Sun flower oil was used as both the heat storage material and heat transfer fluid. The heat transfer was such that hot oil owed by gravity from the TES tank through a pipe to the bottom of the cooking unit, which was in contact with the oil. The results showed that the heating rate increased with increasing ow rates, and the efficiency of the cooking unit was between 40% to 50%, increasing with the ow rate. The rate of heat loss in the cooking unit was determined, and the overall heat loss coefficient was found to be about 0.54 W K-1. From the energy balances the heat transfer coefficient was found to range from 110 to 150 W m-2K-1. In order to safeguard oil coming into contact with water during cooking, a cooking test with a pot inserted into another pot was done to investigate the heat transfer for pot-in pot using a standard cooker. The results revealed that having a small quantity of oil between pots enhances the heat transfer coefficient as compared to having air between the pots. The energy needs for cooking local food items in Uganda has been determined from laboratory cooking tests and from a survey on institutional cooking practices at selected schools in Uganda. Results showed that for high moisture content food items like matooke, more energy is required to boil than to simmer. On the other hand for dry food items (beans), more energy is required after boiling to fully cook the food items. In addition, soaking of dry beans prior to cooking led to about 33% energy savings. The school survey done revealed that about 2.6 kg of firewood is used to cook 1 kg of dry beans and about 1 kg of fi rewood used for 1 kg of maize our. Therefore, a transfer to storage based cooker for institutional use could lead to signi ficant reduction in terms of firewood fuel consumption hence mitigating the adverse effects associated with its use. For an oil based thermal energy storage solution, supplying hot oil to a dedicated cooker, the results indicate that 14 litres hot oil at temperatures of about 200 oC can be sufficient for the daily cooking energy needs for a household of 5 persons. Reducing the quantity of oil with use of rock bed solutions can therefore enable up-scaling the system for institutional use.