Combined Optimal Design and Control of a near Isothermal Liquid Piston Air Compressor/expander for a Compressed Air Energy Storage (caes) System for Wind Turbines

The key component of Compressed Air Energy Storage (CAES) system is an air compressor/expander. The roundtrip efficiency of this energy storage technology depends greatly on the efficiency of the air compressor/expander. There is a trade off between the thermal efficiency and power density of this component. Different ideas and approaches were introduced and studied in the previous works to improve this trade off by enhancing the heat transfer between air and its environment. In the present work, a combination of optimal compression/expansion rate, optimal chamber shape and optimal heat exchanger material distribution in the chamber is considered to maximize the power density of a compression/expansion chamber for a given desired efficiency. Results show that the power density can be improved by more than 20 folds if the optimal combination of flow rate, shape and porosity are used together. INTRODUCTION The availability of a cost-effective, scalable and dispatchable energy storage system is the key to eliminating the most pressing integration issue of renewable energy and the grid: integrating unpredictable and intermittent green energy, such as wind and solar energy, into the electrical grid. In our previous work [1–4], we have proposed a novel Compressed Air Energy Storage system (Fig. 1) for wind turbines that can store energy prior to electricity generation. With the use of a novel open accumulator system architecture and a near isothermal liquid piston air compressor/expander, this system can be efficient, power dense and costeffective. A critical component of this system is the high pressure (200 bar) air compressor/expander which is responsible for the conversion between mechanical work and compressed air in storage. To be useful, the compressor/expander must be efficient and power dense. For a fixed compression/expansion ratio, increasing power density means that the compressor/expander will operate in shorter cycle time. A smaller component can therefore be used to achieve the same power capability, reducing capital expense (CAPEX).