Heat transfer enhancement techniques used in liquid piston gas compression can contribute to improving the efficiency of compressed air energy storage systems by achieving a near-isothermal compression process. This work examines the effectiveness of a simultaneous use of two proven heat transfer enhancement techniques, metal wire mesh inserts and spray injection methods, in liquid piston gas compression. By varying the dimension of the inserts and the pressure of the spray, a comparative study was performed to explore the plausibility of additional improvement. The addition of an insert can help abating the temperature rise when the insert does not take much space or when the spray flowrate is low. At higher pressure, however, the addition of spacious inserts can lead to less efficient temperature abatement. This is because inserts can distract the free-fall of droplets and hinder their speed. In order to analytically account for the compromised cooling effects of droplets, Reynolds number, Nusselt number, and heat transfer coefficients of droplets are estimated under the test conditions. Reynolds number of a free-falling droplet can be more than 1000 times that of a stationary droplet, which results in 3.95 to 4.22 times differences in heat transfer coefficients.

Understanding how the pressure level affects the efficiency of liquid piston gas compression is essential for a greater applicability of the technology in compressed air energy storage. To explore the impacts, compression starting at three different initial pressure levels (1, 2, 3 bar) with a pressure ratio of 2 is performed, and how isothermal compression efficiencies are affected depending on the initial pressures is analyzed. Under the experimental conditions, higher initial pressure leads to lower isothermal efficiency. Air dissolution during the compression is also investigated because the chamber is a pressure-varying and a liquid-containing environment, where the gas solubility changes during the process. Evaluating the dissolution is critical as it affects the energy output when the compressed air is expanded to regenerate the energy. The changes in the air mass and the retrievable volume of the air after expansion are quantified based on Henry’s law. For a compression at higher pressure, because the air solubility is proportional to pressure, a greater reduction in the air mass and volume percentages is expected. This trend of the mass decreasing with the pressure level leads to less energy output than the originally intended output when the stored energy is retrieved in a discharging process.