Increasing efficiency and power density of a liquid piston air compressor / expander with porous media heat transfer elements

Abstract

In this thesis, a power dense and efficient air compressor/ expander is investigated experimentally. High power density and high efficiency are realized with a quasi-isothermal process, made possible by a liquid piston compressor/ expander and the addition of porous media heat transfer elements. Uniform and non-uniform distributions of porous media are considered and compared with a baseline case.Experiments are conducted using a 2.2 L displacement compressor/ expander. Air is compressed from 7 bar to 210 bar in compression tests and expanded from 210 bar to 7 bar in expansion tests. Baseline compression times vary from 2s to 400s and compression power density varies from 4 kW/m3 to 600 kW/m3. Baseline expansion times vary from 1s to 400s and expansion power density varies from 4 kW/m3 to 2 MW/m3. The baseline compression experiments covered. This study finds that as power density increases, efficiency decreases. At 90% efficiency, a moderate amount of porous media (uniform distribution of 76% porosity) improves compression power density by a factor of 10 and expansion efficiency by a factor of 17. Further improvements are possible with an optimized porous medium geometry.These results have implications for many applications where efficient gas compression/expansion is required including: compressed air for energy storage at scales that range from residential-scale to grid-scale, pneumatics, compressed industrial gasses, and compressed gaseous fuels like hydrogen and natural gas. Quasi-isothermal compression and expansion also enables the realization of thermodynamic cycles that require isothermal compression or expansion.