Analytical modeling and thermal performance analysis of a flat plate latent heat storage unit

The thermal performance of a flat plate latent heat storage unit (LHSU) consisting of parallel flat plate slabs of phase change material (PCM) was investigated with analytical techniques. The approximate analytical expressions of the heat transfer fluid (HTF) transient temperature distribution and the time wise solid-liquid PCM interface location were developed by solving the energy conservation equations and were validated by comparing the present results with fully converged numerical predictions and analytical solutions in published literature. An index of effective latent heat storage ratio, E_r, which was defined as the ratio of the actual amount of available latent thermal energy before HTF outlet temperature reaches a specified value to the total latent heat storage capacity, was proposed to evaluate the thermal performance of a LHSU. The effects of geometric parameters and the thermal conductivity of PCM (k_p) on E_r were investigated. The results show that the analytical model is valid as the Stefan number is less than 0.15. Moreover, E_r increases with the rise of the given dimensionless HTF outlet temperature and the enhancement of k_p, and decreases linearly with the increase of HTF channel height and PCM slab thickness, with an abrupt change in the derivative of E_r at an inflection point. A dimensionless criterion was proposed to help design thermally efficient flat plate LHSU. Additionally, we found that increasing PCM mass by lengthening the PCM slab could significantly improve E_r, while thickening the PCM slab would lead to the opposite trend. This investigation provides guidelines for the optimal design of a flat plate LHSU.