Contribution to the Technique of Compressed Air Energy Storage: The Concept of Finned Piston

Development of intermittent solar and wind energies and the demand peak increase has made energy storage very important in energy policy. Thus, energy storage will play a key role in enabling the world to develop a low-carbon electricity system. An interesting approach to do so can be storing energy in form of the compressed air. Recently Compressed Air Energy Storage system (CAES) has attracted attentions as a promising technology for energy storage. In the general frame of CAES and after presenting liquid piston, the LEI Laboratory of EPFL has introduced the concept of finned piston. The main goal is to achieve energy storage by means of compressed air thanks to high isothermal efficiency compression/expansion processes. As mentioned, compressed air with a liquid piston and with a performance of nearly 65% has been realized already as a promising solution for the cost-effective small/medium electricity storage applications. As a post-development, and to improve the problems related to the complexity of a liquid piston, a so called "dry finned piston" system was proposed. The effective work for this new system at LEI was be to realize an analytical and a finite element model for such a system. The key development of this task is to solve the problem of non-isothermal behavior of the dry piston systems utilizing a directly integrated exchanger, inside of the compression / expansion chambers. In this regard, theoretical modeling and simulation has been started with a classic reciprocating piston based on principles of mass and energy conservation. The model consists of different subsystems like driver mechanism, cylinder head, valves, heat transfer, etc. This will serve as a basis for developing the more complicated finned piston compressor. Besides, the Finite Element Method (FEM) has been used to account for the detailed local thermal characterization in 3D geometry. Next, analytical modeling has been extended to finned piston. The heat and mass transfer has been accounted for using thermoelectric and pneumatic-electric analogy. Also FEM modeling has been carried out for the finned piston. The analytical model was verified using experimentation. In order to propose an experimental validation, a test bench has been developed for both of the pistons. The experimental results confirms a higher exergetic efficiency for the finned piston due to its close to isothermal behavior thanks to increased heat transfer surface. The last chapter is dedicated to a parametric study to investigate the effect of change of the design parameters on system performance. This thesis open doors toward more investigation to improve the design. The effective tool here is the comprehensive model developed in an innovative way.

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