Isothermal Efficiency of Liquid Piston Compressors Employed in Compressed Air Energy Storage Systems

This paper models the compressor work for a prototype small-scale compressed air energy storage system to predict the heat transfer to the walls and the system pressure, temperature, and work required. The modeled system uses a hydraulically driven compressor to enable variable frequencies of compression and to improve heat transfer. Since the compression is hydraulically driven, speed of compression can be controlled through adjusting hydraulic pump’s speed in order to enhance the efficiency and also being able to couple the system with different sources of power. The energy conservation model considers gas under non-ideal conditions and properties of the gas are predicted using Redlich-Kwong equation of state. For heat transfer calculations, correlations for thermal entry length for turbulent flows are employed. Material properties of air including specific heat capacity, thermal conductivity and viscosity are corrected at different operating temperatures and pressures. The differential equation form of the first law of thermodynamics is then integrated numerically in the time domain in order to find the instantaneous properties of the bulk gas. Isothermal efficiency is then predicted based on the temperature rise in the working fluid during compression. It is determined that at low frequencies of compression, at a compression ratio of 5:1, isothermal efficiencies as high as 93% can be achieved.