Characterisation and Design of Latent Thermal Energy Storage Systems

Latent thermal energy storage (LTES) is considered a crucial energy technology for further decarbonization of our energy system by creating compact and efficient thermal energy storage units. Despite the potential and the long research history, LTES systems are not very widespread in the energy market, nor in residential applications, nor in industrial processes. One of the reasons why (L)TES is not an established technology is attributed to the difficulty in designing LTES units and predicting their performance because of their inherent transient nature and behavior. Characteristic correlations predicting the transient behavior of LTES systems without too many computational efforts are valuable for engineering practices. This characterization of LTES systems can be done with the recently developed charging time energy fraction (CTEF) method. This method allows fitting a predictive model for the outlet heat transfer fluid temperature of a LTES unit as a function of the input conditions. The validity of this CTEF method has been proven for a few different LTES systems, working at low temperatures (-5 - +20 °C) as well as above 220 °C where heat losses have to be accounted for. Although useful for predicting the behavior of LTES units in ideal charging and discharging cycles the predictive power is only valid for a specific geometry and it can thus not be used to predict the behavior of other geometries. As there is no general design methodology for LTES systems, the community would greatly benefit from similar easy to use models in the design stage. Therefore, future work is dedicated to further developing the CTEF method and trying to extend it to a design model.