Gas compression and expansion for the purposes of energy storage must necessarily be very high efficiency. In the case of compressed air energy storage, the round-trip efficiency of the storage cycle is limited to c times e. If both compression and expansion machines had efficiencies of 90%, then the round-trip efficiency would definitely be below 81%. For energy storage involving pumped heat, the round-trip efficiency is much more sensitive to losses in the compression and expansion processes. Typically for pumped-heat applications, the ratio of "hot" to "cold" temperatures is 2.5 (i.e. 5/2). In that case, for every 3J of net exergy passed into storage 5J of work is done in a compressor and 2J of work is recovered from an expander. The converse is true during energy recovery from storage: 5J of work is recovered from the expander while 2J of work is put into the compressor. Overall, for a pumped thermal energy storage system like this, if the machinery is 99% efficient (i.e. losses are 1%), then the losses during charging are 2.333% and losses during discharging are also 2.333% so total round-trip losses are around 4.666%.

This project will explore the development of an ultra-high efficiency compressor exploiting liquid-piston technology. The most important loss mechanisms present in an existing design will be identified and a revised design will be put forward that retains the attractions of the original design but achieves higher performance. Sources of loss include: (a) heat transfer between gas and compressor components, (b) pressure-drops across valves and pipe-work, (c) heat transfer within the body of the machine itself, (d) friction losses in the displacer part of the machine, (e) oil leakage losses past the pistons of the displacer and between the different body chambers of the compressor and (f) viscous losses associated with oil motions.