A simplified numerical model of PCM water energy storage

A generic numerical model of PCM water energy storage is developed and validated by experiments. The numerical model consists of a water region and a PCM region. Models of the two regions are derived by the energy differential equations and solved by the implicit method. The solid-liquid PCM behavior is modeled based on its enthalpy-temperature relation, in which the melting/solidification phase is linearized.

Special treatment is developed for the time steps with a phase change, which significantly improves the prediction accuracies of PCM temperatures and the melting fractions. An iteration method is applied to the two regions for coupling calculation. The energy balance of the model is examined in each time step.

Extended functionalities of the numerical model are further developed, including separate heat loss coefficients, auxiliary heaters, flexible inlet and outlet layout, the mixing effect in the water tank, and three PCM supercooling-activation modes. The experimental verification for a PCM water energy storage was carried out at Technical University of Denmark.

The PCM and water layer temperatures are calculated. The simulated and measured outlet temperatures and heat content are compared. The results show that the simulated outlet temperatures are maximum of 4.6 K deviation from the measured, with a relative error of 10.4 %. The relative error of the heat content is within 1 %, and the energy balance errors are within 5 %.

The extended application and limitation of the numerical model are discussed, and the possible error sources are analyzed.