Efficiency of high pressure air compressors/expanders is critically important to the economic viability of Compressed Air Energy storage (CAES) systems, where air is compressed to a high pressure, stored and expanded to output work when needed. Any rise in internal energy of air during compression is wasted as the compressed air cools back to ambient temperature. Similarly, a drop in temperature of the air during the expansion process would reduce the work output. Therefore, the amount of heat transfer between air and surrounding heat sink/source surfaces determines the compression/expansion efficiency. Slowing down the compression/expansion process would give more time for heat transfer, thus increasing the efficiency. However, it reduces the power of the compressor/expander which is undesirable for a CAES system. A porous medium inside the compression chamber and a liquid piston is an ideal candidate for effectively increasing the heat transfer area and, consequently, thermal efficiency of the compression/expansion process, without sacrificing power.The present study focuses on experimentally testing and evaluating the effectiveness of various types of porous media, including two types of metal foam and two types of plastic interrupted plates of different pore size and porosities inside of a liquid-piston compression/expansion chamber. The liquid piston compression system is a cylindrical cavity first filled with air. As water is pumped into the bottom of the cavity, the air inside is compressed. Flow meters and pressure transducers are used to measure the volume and pressure changes during compression. Porous inserts of various designs are placed inside the chamber to reduce the rise in temperature as the air is compressed and to reduce the temperature drop as the air is expanded. Compression and expansion efficiencies are investigated with and without the porous inserts. For the compression experiments, all the experiments are conducted with constant volume trajectories. However, in expansion experiments, the volume trajectories are determined by constant orifice opening area. The study shows that compression efficiency is increased from 77% without a porous insert to 94% with the best-performing, 40ppi metal foam insert at a compression ratio of 10 and compression time of 2s. Due to the significant amount of water trapping inside the metal foam, in the expansion tests, only interrupted plates are tested. The expansion efficiency is increased from 80% to 90% with the 2.5 mm characteristic size interrupted plate insert for expansion process at expansion ratio of 6 and expansion time of 2s. Normalized pressure volume trajectories and dimensionless temperature profiles are calculated and compared for different types of inserts.
It is concluded that adding a porous insert into the air space of a liquid piston compressor/expander is an effective means of boosting heat transfer rate and increase compression/expansion efficiency. It is recommended that future work is needed to optimize the pore size and layout of the porous insert and to couple other heat transfer augmentation schemes, including spray cooling trajectory optimization and control. Meanwhile, in order to investigate the compression/expansion process at a higher pressure and wider pressure ratios, a high compression/expansion setup is designed and being fabricated in order to have a better control the pressure-volume trajectory, and improve ease of operation. Detailed design requirements, specifications, system schematic, 3-D models and drawings are presented and discussed.
University of Minnesota M.S. thesis. August 2013. Major: Mechanical Engineering. Advisors: Terrence W. Simon, Perry Y. Li. 1 computer file (PDF); ix, 130 pages, appendices A-E.
Compression/expansion within a cylindrical chamber: Application of a liquid piston and various porous inserts.
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