Optimization of a solar cascaded phase change slab-plate heat exchanger thermal storage system

Abstract

Thermal storage is a key element to stable usage of globally distributed solar energy. Phase charge materials (PCMs) are the most effective materials for high efficiency thermal energy storage. Slab plate heat exchangers are technical and cost beneficial tanks in order to practically store solar thermal energy, using PCMs. In this study, configuration of a slab plate heat exchanger, was analyzed by two dimensional numerical model using effective heat capacity method. Air and commercial paraffin based materials RT50, RT65 and RT80 were used as heat transfer fluid and PCM respectively. The model was verified with the experimental data. In order to take the real ambient situations into account, a solar collector was modeled for hot air inlet to the energy storage unit during winter (coldest day of winter selected for study). The effect of HTF inlet temperature, flow rate and density of PCM on the thermal behavior of the energy storage unit was analyzed. Multiple-PCM (cascaded) unit with variable slab plate thicknesses was studied as optimization case with charging and charging-discharging cycles as two different approaches. Optimal order of PCMs inside the energy storage unit and the thickness distribution of slab plates were reported as the optimization results. Multiple-PCM energy storage unit has improved the average thermal performance of the energy storage by 12% compared to the single-PCM energy storage unit in the winter.