Enhancing pumped thermal energy storage efficiency: A comparative study of flash vapor and ejector-subcooler techniques

The pumped thermal energy storage technique is an effective means of energy storage. This study compares flash vapor injection compression with ejector-subcooler vapor injection compression using energy, exergy, and exergoeconomic evaluations. These systems are integrated with a geothermal field and a customized transcritical carbon dioxide cycle for power and cooling generation. The findings demonstrate that increasing condensation temperature amplifies the power consumption of vapor compression, whereas an increase in evaporation temperature reduces it in both configurations. The ejector subcooler exhibits superior round-trip and exergetic efficiency compared to the flash configuration, with values of 65.28% versus 64.04% and 34.77% versus 37.79%, respectively. The transcritical carbon dioxide cycle generates a greater net power in the ejector subcooler configuration, yielding 421.80 kW compared to 347.80 kW. Although the flash setup incurs a lower fixed capital investment of 1.19 $M compared to 1.75 $M, and a product cost rate of 35.97 $/h versus 74.68 $/h, the ejector subcooler setup offers a shorter payback period of 4.78 years compared to 5.68 years, along with a greater income, amounting to 1.75 $M versus 1.33 $M. In optimal conditions, the levelized cost of storage and the coefficient of performance of the ejector subcooler attain 0.18 $/kWh and 7.31.