Investigating the thermal performance of a subsea liquid piston air compressor pipeline operating at very high pressure ratios

Abstract Liquid piston air compression is presently gaining significant interest as a means for long duration energy storage at large scale. However, various aspects attributed to the thermal performance of liquid piston air compressors at high operating pressure ratios remain yet unanswered. This paper presents a transient thermal analysis of a novel, subsea liquid piston air compressor pipeline operating at pressure ratios up to 200. A thermal model implementing different empirical correlations for the convective heat transfer phenomena is presented. The study reveals a major knowledge gap in terms of Nusselt number correlations available to define the internal heat transfer coefficients explicit to liquid piston pipelines. Results from a parametric study suggest that the internal heat transfer inside a liquid piston having a very large length-to-diameter ratio was characterised by natural convection effects. Furthermore, it was found that to attain a thermal efficiency higher than 0.90, the power density had to be limited to not more that 0.22 kW/m 3 , with a compression time required per stroke being at least 25minutes.