Investigation on the thermal performance of a high temperature packed bed thermal energy storage system containing carbonate salt based composite phase change materials

Abstract

This paper concerns the thermal performance of a high temperature packed bed thermal energy storage (TES) system containing carbonate salt based composite phase change materials (CPCMs) that made of a eutectic carbonate salt of NaLiCO3 (phase change material, PCM), MgO (ceramic skeleton material, CSM) and graphite flakes (thermal conductivity enhancement material, TCEM). A rectangular packed bed configuration containing CPCMs bricks is built and a three-dimensional computational model is established to study the thermal performance of the system. The enthalpy-porosity approach and surface-to-surface (S2S) radiation model are respectively adopted to model the phase change process and the radiation heat transfer inside the system. A ferric oxide is also used as the sensible heat storage material to compare with the CPCMs based system. The numerical model is first compared with the published experimental data and reasonably good agreements are obtained, indicating the confidence of the model. Extensive modelling is then performed under different conditions to investigate the effects of various parameters including the radiation heat transfer, TCEM mass loading and heat transfer fluid (HTF) operation conditions on the system performance. The results indicate that the system containing CPCMs shows better charging and discharging performance in comparison with the system containing ferric oxide due to the large energy storage density and high thermal conductivity. The thermal radiation has an important influence on the system performance. The system heat transfer efficiency is apparently enhanced when the radiation heat transfer influence is taken into consideration. When the emissivity is at δ = 1, the total charging period of the system is respectively shortened by 10.6% and 25.7% than that of the emissivities at δ = 0.5 and δ = 0. The use of TCEM in the CPCMs significantly enhances the heat transfer performance of the system. An increase in the TCEM loading from 0% to 30% respectively leads to the reduction in charging and discharging processes by almost 30.3% and 29.2%. The results also indicate that, for a fixed charging/discharging power, both the overall charging and discharging periods of the system decrease with the increase of Re number or decrease of Ste number since an increase in the Re number (decrease in the Ste number) leads to an overall enhancement of the heat transfer between the HTF and the CPCMs bricks and hence an overall improvement in the charging and discharging rates.