Thermodynamic investigation of quasi-isothermal air compression/expansion for energy storage

Compressed air energy storage system is a promising solution in the energy storage field: it is characterized by a high reliability, low environmental impact and a remarkable energy density. This study focuses on two of the advanced technologies in the isothermal compressed air energy storage systems developed by LightSail Energy (mechanical piston with water injection) and Enairys Powertech (liquid piston with integrated heat exchanger). A first numerical model based on thermodynamic and energy analysis of the concerned technologies is presented in this work. These mathematical models enabled the comprehension of the main operation of these technologies and therefore having many orders of magnitudes of different physical parameters such as air/water temperature, air pressure, compensated/recovered work, heat absorbed/released, droplet diameter and heat transfer coefficient. Mechanical piston with water injection technology showed to be favorable to the production of hot water (15 degrees C water gradient) during air compression and cold water during air expansion, where liquid piston with integrated heat exchanger represented a strong isothermal compression and expansion with a minimal air/water temperature gradient (1 degrees C / 0.2 degrees C).