Innovative cryogenic Phase Change Material (PCM) based cold thermal energy storage for Liquid Air Energy Storage (LAES) – Numerical dynamic modelling and experimental study of a packed bed unit

Electrical energy storage represents a necessary link between sustainability goals and the enhancement of intermittent renewable energy sources penetration in electricity grids. Liquid air energy storage (LAES) is a promising large scale thermo-mechanical energy storage system whose round trip efficiency is largely affected by the performance of the sub-thermal energy storages. The high grade cold storage (HGCS) is by far the most important due to the crucial thermodynamic recovery of the waste cold stream released by the liquid air regasification process. LAES pilot plant and pre-commercial demonstrator, as well as the vast majority of the theoretical and experimental analysis found in literature studies, currently design to store that exergetically valuable cold source in sensible heat (SH) thermal energy storage, economically efficient but low energy density solution. Conversely, phase change material (PCM) has the potential to store a larger amount of energy using the same amount of storage volume. The objective of the present work is to numerically and experimentally investigate the thermal behaviour of a novel cryogenic HGCS packed bed filled by PCM and determine how the novelty introduced affects the LAES thermodynamic and economic performance compared to the SH configuration. To this end, a simplified transient one-dimensional numerical model to simulate the charging and discharging phase of the HGCS system has been developed and successfully validated against experimental results provided by literature for SH medium and an experimental campaign carried out on a novel lab scale HGCS at TESLAB@NTU for PCM, representing a unicum in literature for both PCM and LAES applications. The numerical results have shown that the introduction of a PCM in the HGCS mitigates the thermocline effect shown in SH configuration ensuring: a) longer discharge phase by means of the thermal buffer phenomena triggered by the phase change process and b) lower specific consumption compared to SH configuration (0.272 vs 0.330 kWhe/kgLA) due to a lower time average outlet temperature of the heat transfer fluid during the HGCS discharge, corresponding to LAES charge phase. From an economic perspective, the decrease of the time average specific consumptions results in a notable payback period inferior to 3 years, making the economic investment considerably attractive.