In the near future, renewable energy sources will take up a large share of our energy mix, therefore energy storage systems will play an important role in our energy system. Latent thermal energy storage systems using phase change material heat exchangers can increase the storage density compared to sensible thermal energy storage systems. However, conventional heat exchanger design methods cannot not be applied for the design of latent thermal energy storage systems due to the transient operation of these systems. It is proposed that the behavior of such an LTES system can be characterized by the overall liquid fraction of the heat exchanger as a function of time. This can be determined by integrating the local liquid fraction as a function of time, which is linked to the position of the phase change front as a function of time. The phase change front position can be approximated based on an analytical solution to the Stefan problem.
In the present paper, the proposed method to predict the phase change front position based on this solution, is validated for a tube in tube PCM heat exchanger, by visually tracking the position of the phase change front at the outer edge during experiments. It is concluded that the analytical model underestimates the time required for the phase change front to reach the outside of the PCM tube for the first time but overestimates the propagation speed of the front through the heat exchanger. Additional experiments are required to further develop and validate the proposed analytical model.