Design Optimization through CFD Simulation and Experimental Validation of a Finned Phase Change Material (PCM) Thermal Energy Storage for Solar Cooking Application (A Case Study of Jimma)

Abstract

The interest of exploiting solar energy for electrification and other household applications such as heating and cooking is increasing due to the reason that they are clean and ecofriendly. In doing so, energy storage mediums are integrated to maintain the energy demand with the intermittent nature of solar energy. Phase change materials (PCMs) has shown a great energy storage use in solar thermal applications despite their low thermal conductivity property which makes low heat transfer during charge/discharge or energy store/release process. Studies show embedding extended surfaces (fins) in the PCM is easier and economical among the different heat transfer enhancement methods. But there are limited works on the problem of increasing geometric parameters of fins, as it replaces the mass of the PCM which results in reduced latent heat storage. This work intends to study and optimize the heat transfer rate and latent heat storage of a solar cooking with finned heat storage system. The study was started by collecting the energy demand for household cooking and solar irradiance assessment of the study site. Following this bench mark, a PCM vessel and parabolic trough solar collector were designed. Numerical simulations using CFD tool ANSYS 16.0 and experiments has been conducted to investigate the effect of fin length and thickness on the performance of the system. Design optimization was made using response surface methodology with central composite design for design points. Hitec salt or molten salt with 53% KNO3, 6% NaNO3 and 41% NaNO2 by composition (with 142 °C melting temperature, 110 kJ/kg heat of fusion) is selected as suitable PCM material for the application. The findings show that increasing fin length show better heat transfer rate than increasing fin thickness. The thicker and longer fin with dimensions of 1.5 and 140 mm respectively, gave 65.97% faster rate than the system without fin. In the optimization process it was found that the fin with 0.8 mm thickness and 140 mm length gives optimized design for the heat transfer rate and energy storage capacity. The optimized design takes 10.21 hr. for complete solidification process and can release 2237.91kJ heat.