Optimal trajectories for a liquid piston compressor/expander in a Compressed Air Energy Storage system with consideration of heat transfer and friction

For a Compressed Air Energy Storage (CAES) approach to be viable, the air compressor/expander must be sufficiently powerful and efficient. Since efficiency is governed by heat transfer, there is a generally a tradeoff between efficiency and compression/expansion time (or power). In this paper, we determine Pareto optimal compression/expansion profiles for a liquid piston air compressor/expander that maximizes efficiency (power) for a given power (efficiency). Compared to previous works, a numerical optimization approach is proposed that allows for more general heat transfer model, the consideration of the viscous friction, and system limitations in the optimization. The resulting optimal profiles are compared to other trajectories. At compression ratio of 10, the optimal profile results in 10%-40% increase in power density relative to other methods. Optimal geometries that trades off friction and heat transfer improvement can also be determined using this method.