In the general frame of Compressed Air Energy Storage system (CAES), the LEI Laboratory of EPFL has introduced the concept of dry finned piston. The main goal is to achieve energy storage by means of compressed air thanks to high isothermal efficiency compression/expansion processes. For achieving this goal, a new compression and expansion machine has been defined, using a new piston-cylinder assembly consisting of a series of concentric annuli’s that fit together during compression/expansion. This will increase the heat transfer surface, allowing the heat to be removed from the system during compression and to be absorbed by the system during expansion. This feature together with low speed movement makes the process close to isothermal. First the concept of annular compression chambers is explained, with the goal of increasing the heat exchange surface. Then the new compression system with imbricated annulis is described. A test bench has been developed in order to propose an experimental validation of such a compression process concept. It is also of a great interest to compare the dry finned piston to the classic pistons available in the market. The experimental results show a higher energetic efficiency for the finned piston due to its close to isothermal behavior thanks to increased heat transfer surface.