Near-isothermal-isobaric compressed gas energy storage

In this paper, the effectiveness of storing energy by compressing and expanding a condensable gas is evaluated. A high efficiency energy storage system, which stores energy by compressing/ expanding gas (air) using a liquid (water) piston has been recently introduced and extensively studied. With the use of the liquid piston, the inefficient gas turbomachines used in conventional gas compression/ expansion systems are replaced with high efficiency hydraulic machines. Utilizing heat transfer techniques and replacing the non-condensable air with a condensable gas (i.e. CO2, synthetic refrigerants, hydrocarbon refrigerants, etc.) have been proposed as methods to improve energy density and roundtrip efficiency of such systems, leading to near isothermal and near isobaric charge/discharge processes. In order to investigate the effectiveness of the proposed concept, a miniature lab-scale experimental setup was designed and built to investigate the compression/expansion characteristics and energy storage efficiency of a system utilizing R134a as the energy storage (primary) working fluid, and mineral refrigerant oil as the liquid piston (secondary) working fluid. Several tests are carried out to quantify energy storage efficiency and energy density. It is found that improving heat transfer rates from/into the storage fluid results in increased efficiency and energy density. A heat-transfer enhancement strategy to achieve near isothermal, isobaric expansion and compression is proposed and investigated experimentally. Some results are generalized and presented in non-dimensional form which can be applied to describe and/or design scaled-up systems. Lastly, a potential working fluid for a scaled- up system is discussed. (C) 2017 Elsevier Ltd. All rights reserved.