Evaluation and optimisation of hybrid sensible-latent heat thermal energy storage unit with natural stones to enhance heat transfer

Abstract

Latent heat thermal energy storage improves the utilization efficiency of renewable energy. Phase change materials (PCMs) commonly suffer from low thermal conductivity and many heat transfer enhancement methods have been developed. However, conventional methods need additional material preparation and processing, which increases the cost and makes them less environmentally friendly. In the current study, natural stones are used to enhance the heat transfer of the PCM in a shell-and-tube unit, forming a hybrid sensible-latent heat storage configuration. Namely, stones, which are widely accessible, low-cost and environmentally friendly, not only act as sensible heat storage media but as the thermal enhancer of the PCM. Results indicate that the energy storage rate of cases with 25 mm-sized stones increased by 8.3%–92.6%. The case with a filling height of 72.8 mm is superior owing to the high energy storage rate and large total stored energy. The stone size rarely influences the total stored energy that increases almost linearly with the void fraction, while it affects the energy storage rate significantly. Cases with 20 mm and 40 mm-sized stones generally have a higher storage rate. Finally, the mechanism is analysed.