Computer simulation of intermittent supply of compressed air mass and heat into piston expander in micro compressed air energy storage

The development of highly efficient and long-term electricity storage technology at micro- and small-scale is currently of greatest interest to scientists and engineers [1]. This is related to high-level integration of inflexible renewable energy sources in terms of maintaining key parameters in the local energy system. One of the technologies is micro-Compressed Air Energy Storage (micro-CAES). The biggest barrier to the development of micro-CAES is the low Roundtrip Efficiency (RE) and high investment costs [2]. The low RE value results from storage pressure decreasing during discharge and the endothermic process of air expansion. Due to the pressure drop in the Mechanical Energy Storage (MES) during discharge, it forces the use of pressure reducers (which causes energy losses) or the implementation of complex isobaric storage systems to stabilise inlet pressure into expander. Additionally, due to the temperature drop of air during expansion, it is necessary to supply heat either before (adiabatic CAES approach) or within (isothermal CAES approach) of the expander [3]. In this study, we verify a developed control strategy for high-efficient intermittent MES discharge to stabilise output electrical power [4]. The proposed algorithm is based on the relationship between the pulsed air volume and the storage tank pressure, while maintaining constant output power from the expander. To validate the algorithm, experimental test was conducted using a test rig comprising a three-piston air expander with an electric power output of 2 kW, a storage pressure of 10 bar, and a storage volume of 2 m³. The developed algorithm was implemented on a PLC controller. Additionally, a heat exchanger is installed before the piston expander to heat the air. The influence of the control algorithm and air temperature on the piston expander's dynamics, output power and exergy discharge efficiency was investigated. We estimate that the exergy discharge efficiency of converting the exergy of compressed air into electrical energy in piston expander may be as high as over 70%.