High Power Density Thermal Energy Storage with Phase Change Material in Enhanced Compact Heat Exchangers

Performance of a novel ultracompact thermal energy storage (TES) heat exchanger, designed as a micro-channel finned-tube exchanger is presented. With water as the heating-cooling fluid in the micro-channels, a salt hydrate phase change material (PCM), lithium nitrate trihydrate (LiNO3∙3H2O), was encased on the fin side. To establish the hypothesis that small-length-scale encasement (< 3 mm) of PCM substantially enhances heat transfer to yield very high power-density energy storage, heat exchanger designs with 10 and 24 fins/inch were considered. They were subjected to thermal cycling, or repeated heating (melting) and cooling (freezing), with inlet fluid flow mimicking diurnal variation between 42? - 25? (representing typical arid-region conditions) over an accelerated time period. By employing salt self-seeding to obviate subcooling during cooling or recrystallization, the TES was found to exhibit stable long-term (100 heating-cooling cycles) operation with very high PCM-side heat transfer coefficients (~ 100-500 W/m2∙K) and storage power density (~ 160-175 kW/m3). In fact, with optimization of heating-cooling fluid flow rate for given charging-discharging time period and exchanger size, power density > 300 kW/m3 can be achieved. The results clearly establish that highly compact heat exchangers used as TES units can provide very high-performance alternatives to conventional ones.