Enthalpy of fusion prediction for the economic optimisation of salt based latent heat thermal energy stores

Abstract

Thermal energy storage represents a cost-effective means of integrating variable, renewable resources into the energy mix. The utilisation of latent heat in addition to sensible can reduce capital costs. To accurately screen the numerous possible ionic salts, accurate enthalpy of fusion predictions are essential. Three possible modelling approaches were considered: enthalpic, regular solution entropic and rigorous entropic. These approaches were combined with different techniques to represent the enthalpy of mixing and the temperature-composition dependence.

Ultimately it was found that disregarding the entropy of mixing and assuming an ideal liquid phase, combined with an immiscible solid phase resulted in adequate predictions of the experimental data. The approach can be used to rapidly screen a wide range of components using only pure component properties. It can be further generalised to require only melting temperatures through the use of a modified “Richard’s Rule”. The method was used to economically compare binary and ternary combinations over the range of 290–565 °C.

The analysis indicated that ternary eutectic mixtures can achieve storage costs using fine chemicals of around 0.18 ± 0.045 $/kJ, with bulk materials conservatively estimated to cost more than 100 times less at ∼1.8 $/MJ. This assumes that storage is achieved through a combination of latent and sensible heat. A set of eight suitable salt candidates were identified that require detailed thermal studies. Lastly it was demonstrated that the use of non-eutectic mixtures, using for example LiNO3 and KCl, may hold the key to bulk storage costs as low as 0.045 $/MJ, if the issues facing practical implementation can be mitigated.