Heat-Transfer Mechanisms in a Solar Cooking Pot with Thermal Energy Storage

This paper presents a detailed analysis of the heat-transfer mechanisms in a solar cooking pot with thermal energy storage using computational fluid dynamics (CFD). The vast majority of studies on solar cookers have been experimentally performed using local temperature measurements with thermocouples. Therefore, the heat-transfer mechanisms can only be studied using lumped capacitance models as the detailed profiles of temperature and heat fluxes inside the cooker are missing. CFD is an alternative modelling technique to obtain this detailed information. In this study, sunflower oil is used as both cooking fluid and energy storage medium. Comparison of the model with the available experimental data shows that the deviation is within the measurement accuracy of the latter. Hence, despite some assumptions, such as axisymmetry and an estimation of the heat transfer parameters to the ambient, the model is able to describe the involved physical processes accurately. It is shown that, initially, the main heat-transfer mechanism is conduction from the cooker's bottom towards the thermal energy storage (TES). This heats up the oil near the bottom of the TES, creating convective plumes, which significantly enhance the heat transfer. In equilibrium, about 79% of the incoming solar flux goes towards heating up the TES. The heat is further transferred to the pot, where convective plumes also appear much later in time. However, the heat transfer to the pot is much smaller, with an average heat-transfer coefficient of 1.6 Wm(-2)K(-1) compared to 7.5 Wm(-2)K(-1) for the TES. After two hours of charging, the oil reaches a temperature of 397 K in the TES and 396 K in the cooking pot. Moreover, the temperature distribution in the cooker is quasi-uniform. During the charging period, the storage efficiency of the TES is about 29%. With the results in this study, solar cooking pots with TES can be further optimized towards efficiently transmitting the heat form the solar radiation to the food to be cooked.