A predictive method for pipe in pipe, cylindrical modules and spherical packed bed latent thermal energy storage systems

Abstract

Due to the transient operation of latent thermal energy storage (LTES) heat exchangers, methods from classical heat exchanger theory such as the effectiveness number of transfer unit method are not applicable. As a result, the study of these systems lacks a general theoretical framework. Recently, an approximate analytical solution method was developed which integrates phase fraction under a constant boundary condition to an LTES heat exchanger with a varying heat transfer fluid temperature. In this paper, the method is applied to three geometries: pipe in pipe, cylindrical modules and spherical packed bed geometries. The resulting model predicts the phase change fraction and effectiveness as a function of time, based on one-dimensional solutions for the movement of the phase change front. The predictions of the analytical model are compared to results of a simplified numerical method of an LTES heat exchanger. The average deviation of the predicted phase change fraction is 3.3 10−2 %, 4.6 10−1 % and 3.7 10−1 % for the pipe in pipe, cylindrical and spherical module respectively. The difference in model performance stems from the description of the fundamental heat transfer behavior. In a second validation step, the outlet temperature predicted by the model is compared to an experimental dataset for a LTES shell and tube heat exchanger. The predictions of the model can be improved further by including sensible heat, heat losses and allowing time-varying inlet conditions.