Long-term thermal energy storage prototype of cold-crystallizing erythritol-polyelectrolyte

Abstract

Phase change materials can improve the energy density of thermal energy storages (TES) by using the latent heat of melting, but they cannot typically operate efficiently in long-term TES. A unique way to store the latent heat for long-term can be achieved with erythritol-polyelectrolyte mixture, which exploits supercooling, glass transition and cold-crystallization in the storing process. However, the main challenge of supercooling TES is to prevent premature crystallization during supercooling while maintaining adequate heat release rate at large scale. This work determines, for the first time, the key operational parameters of using ∼ 6.5 kg of an erythritol-polyelectrolyte mixture in a long-term TES prototype. The prototype yielded an average melting enthalpy of ∼ 166 J/g for the mixture which was corroborated with calorimetric measurements. Results confirmed operational feasibility of the prototype TES with an overall storage efficiency of 0.50–0.80 when the latent heat of melting is used for long-term storage and the sensible heat of supercooling for short-term storage. The prototype enabled determination of previously undefined critical cooling rate, above which premature crystallization during supercooling was not observed despite the stochastic nature of supercooling and crystallization. Furthermore, momentary heat release rate of 1000 W was achieved by allowing complete cold-crystallization before discharging the heat. The operational parameters defined in this work confirm reliable use of cold-crystallization in long-term TES at kilogram scale with realistic operational values which forwards the implementation of advanced cold-crystallizing materials in long-term TES applications.