V.C. Patil, P. Acharya, P.I. Ro
North Carolina State University,
Keywords: energy storage, compressed air, liquid piston, isothermal efficiency
Summary:Optimal utilization of renewable energy sources needs integration of large-scale energy storage systems with the electricity grid. Ocean compressed air energy storage (OCAES) can act as a promising large-scale energy storage system for electric grids near coastal region. In OCAES, energy is stored in the form of compressed air under the ocean. Underwater energy storage results in a constant-pressure storage system which has the potential to show high efficiency compared to conventional constant-volume compressed air energy storage systems. Development of an efficient compressor and expander has an important role in achieving a high roundtrip efficiency of the OCAES system. The objective of this study is to develop a highly efficient compressor for ocean compressed air energy storage application. Experimental research in the liquid piston compressor has been performed to develop highly efficient compressor for OCAES application. In liquid piston compressor, the gas is compressed by the flow of high-pressure liquid in a compressed chamber. Due to its low frictional losses and high heat transfer capability, the liquid piston can achieve an efficient compression. Further, heat transfer enhancement using aqueous foam and water spray cooling are tested experimentally in the liquid piston compressor for efficiency improvement. Experimental results indicate that liquid piston compressor has an isothermal efficiency of 84-86% for a compression ratio of 2.5 and increasing the stroke time of compression increases the isothermal efficiency of compression. Aqueous foam is found to be very effective in improving the isothermal efficiency of compression. Increasing the volume of aqueous foam in the compression chamber increases the isothermal efficiency of compression, however, with a rise in cyclic variability. A completely filled compression chamber with aqueous foam shows the isothermal efficiency of compression around 89-92%. It is observed that water spray cooling is a highly effective method for improving the efficiency of compression at various injection pressures and spray angles. The increase of injection pressure of spray increases the isothermal efficiency of compression, however, at the expense of more spray generation work. With 70psi (483kPa) spray injection pressure, the liquid piston compressor can achieve an isothermal efficiency of 94-96%. Comparison of results with 60o, 90o and 120o spray angles indicate that variation in spray angle has a marginal effect on isothermal efficiency. Further, roundtrip efficiency of OCAES is evaluated for aqueous foam and spray cooling based liquid piston compressors. Results indicate that aqueous foam and spray cooling based liquid piston compressors are effective in improving the efficiency of OCAES significantly. The use of aqueous foam has the potential to improve the end-to-end efficiency of OCAES by 7-20%. However, the use of aqueous foam in the liquid piston compressor results in a higher uncertainty range because of higher cyclic variability in the compression process. Spray cooling is highly effective with the potential to show an improvement of 15-26% in the roundtrip efficiency of OCAES over the base roundtrip efficiency of 43-47%. Further optimization of aqueous foam and spray configurations in liquid piston compressors can lead to a further improvement in roundtrip efficiency of OCAES.