Analysis and Proof‐of‐Concept Experiment of Liquid‐Piston Compression for Ocean Compressed Air Energy Storage (OCAES) System

An analysis and a proof‐of‐concept experiment of liquid‐piston compression were conducted for a table‐top Ocean Compressed Air Energy Storage (OCAES) prototype. A singlecylinder‐ type piston surrounded by water was modeled and analyzed based on convection heat transfer with fully developed internal flow, the assumption adopted by earlier liquid piston study in literature. Transient numerical results of this model were calculated for a polytropic compression with different polytropic index values for 2.5‐second stroke. Also, an experimental model of the liquid piston was built with two different materials, polycarbonate and aluminum alloy, for a compression chamber. Temperature data were measured at six different stroke times to examine any difference in heat transfer rates affected by stroke frequency. The temperature within each cycle was measured during compression from 1 bar to 2.2 bars. It was found that longer stroke time induces smaller temperature rise in the air. The local temperature rise was observed to be 80 °C at 2.5‐second stroke and 7 °C at 40‐second stroke. While the simulations predict a temperature rise of 48.6 °C for a compression stroke time of 2.5 seconds, the temperature rise calculated for adiabatic compression was found to be 98.8 °C. This implies that the heat transfer characteristics of a liquid piston compression process are effective in reducing the air temperature. The experimental results with longer stroke times proved a nearisothermal nature of the liquid piston compression system. Overall, the experimental study outlined in this paper not only confirms the near‐isothermal nature of the liquid piston system but also enables further study of the expansion cycle using the liquid‐piston concept. More importantly, the current study paves a way for future work on a larger scale OCAES system demonstration based on liquid‐piston concept.