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Compressed air energy storage (CAES) has emerged as an effective large-scale energy storage technology. This storage system can have many urban applications based on the time-dependent price of electricity. In China, the electric power companies employ batteries to store electricity when the price drops and generate electricity when the price increases. When compared with storage batteries, CAES systems have larger storage capacity (100 MWh, battery < 10 MWh), are more environmentally friendly (no heavy metals pollution), and have longer service life. However, the major limitation that restricts the application of such systems is its poor (<60%) turnaround efficiency (electricity to electricity). Most CAES systems are based on adiabatic compression. Approximately half of the electricity is transformed into heat and exhausted due to the poor heat transfer from the air to environment. Most of the previous research focused on enhancing the heat transfer to achieve quasi-isothermal compression. In this study, a contact heat transfer method is used to cool the compressed air by injecting micron-sized (10–100 μm) water spray into the compressed air. A transient temperature measurement method is developed to investigate the heat transfer behaviour between air and water. The measurements showed the spray-air heat transfer rate to lie in the range of 10–120 W·m −2 ·K −1 . When the compression ratio is two in a compression chamber of 0.94 L, the compression power is reduced from 73.8 J/cycle (adiabatic) to 69.0 J/cycle, using water injection of 0.416 g/cycle. The achieved compression efficiency is improved from 86.7% (adiabatic) to 92.4%. [ABSTRACT FROM AUTHOR]

• This study proposes an isothermal piston formed with a gas-solid-liquid three-layer heat transfer structure. • The porous medium works as a medium to enhance the heat transfer from air to environment. • The isothermal piston could improve the compression efficiency. • Dimensionless parameters Ka and Xu is obtained to analyze the system. Currently, Compressed Air Energy Storage systems mainly use adiabatic compression. Compared with isothermal compression, approximately twice the electricity is transformed into heat. Twice the heat passed into heat exchange mediums leads to twice the heat transfer losses. Enhancement of the heat transfer between air and environment to achieve isothermal compression is an effective approach to improve the turnaround efficiency of CAES systems. An isothermal piston structure is proposed to do isothermal compression. One end of the isothermal piston is connected to a traditional piston, and the other end dips into a liquid medium in the bottom of a cylinder. This forms a gas–solid–liquid three-layer heat transfer structure. A porous medium is used to enhance the heat transfer from the air to the liquid. As the heat capacity of the liquid is much greater than that of the air, the temperature of the liquid remains unchanged as well as the compressed air. A new method is proposed to look at the thermodynamics of the compressor with two dimensionless parameters Ka and Xu. Ka describes the extent of the compressor approaching isothermal. When Ka is over 80, the temperature of the air is reduced by 80% compared with adiabatic condition. In the case of using an aluminum porous medium, the compression efficiency increases by 11% at the compression ratio of 7 and the speed of 1200 r/min. [ABSTRACT FROM AUTHOR]