North Carolina State University,
Keywords: renewable energy, energy storage, compressed air, end-to-end efficiency
Summary:Intermittent renewable energy resources need energy storage system for their optimal utilization. Utilization of renewable energy resources can be improved through incorporating utility-scale energy storage in power grid. Ocean compressed air energy storage (OCAES) is a promising way for a utility- scale energy storage. In OCAES, energy is stored in the form of compressed air in an underwater storage device. This storage device can be a receiver vessel, vented to sea water, mounted on the sea floor and attached to the compressed air source by the pipe. OCAES uses hydrostatic pressures in the deep ocean in order to store compressed air at a constant high pressure. Constant air pressure storage in OCAES results in a significant improvement in efficiency over land based constant-volume air storage. A liquid piston based OCAES is analyzed for the end-to-end efficiency in this study. Laboratory scale experimental setup of a liquid piston compressor is used for investigations of the compression process. Specifications of various components in the OCAES were designed for maximum power capacity of 0.5 MW with 2 MWh energy storage using the analytical models of individual components. Storage pressure of 10 bar gauge (100 m of ocean depth) is considered. Numerical analysis based on the analytical models and experimental data is done using Monte Carlo simulations. The end-to-end efficiency of the liquid piston based OCAES is observed to be highly dependent on the polytropic index of compression and expansion. End-to-End efficiency increases from 24% to 72% with a decrease in the polytropic index from 1.4 to 1. Experimentally observed polytropic index shows 45% end-to-end efficiency. An isothermal liquid piston compressor (n=1) shows the end-to-end efficiency of 72%. Existing compressed air energy storage (CAES) plants in Huntorf (Germany) and McIntosh AL (USA) show efficiency of 42% and 54% respectively. The comparison of results with the end-to-end efficiency of existing compressed air energy storage systems indicates that the isothermal liquid piston based OCAES shows significantly higher end-to-end efficiency.