The current prototype the Compressed Air Energy Storage (CAES) installations achieve the efficiency of converting the electricity consumed in the compression process into electricity supplied to the grid by not more than 50%, with an efficiency record of 61%. The efficiency of the gas compression stage along with heat recovery and storage is well recognized in the literature and ranges from 85 to over 90%. However, there is a gap in research on the gas expansion process, with an efficiency range of 40-70%. According to the compressed gas injection into the expander should be treated as an impinging wave upon a piston (plate of finite mass) which transfers energy and momentum. The paper presents development of mathematical model and preliminary study of the expansion process in the cylinder-piston subsystem of CAES system. Developed model covers all modes of heat transfer and dynamics of piston movement. This approach allows to analyse details of the injection process and mechanical interaction with moving piston. The developed model was used to analyse influence of the crucial geometrical parameters of the microCAES on the performance of compression process, showing a space for optimisation of the process in terms of torque availability at the shaft.