Performance improvement of liquid air energy storage: Introducing Stirling engine and solar energy

Abstract

Liquid air energy storage (LAES) is a promising energy storage system with the main advantage of being geographically unconstrained. The efficiency of LAES could be improved by utilizing compression heat and integration with other systems. As an effective heat recovery process, the Stirling engine (SE) is introduced to the LAES system. The Pseudo Stirling model and Malmo relation are used to obtain practical results. Firstly, the Stirling engine is used to utilize compression heat, in which the Round Trip Efficiency (RTE) could be improved by 3.16% points and 8.96% points in LAES-SE and Liquified Natural Gas (LNG)-LAES-SE systems. Then, with solar energy introduced as heat source for the Stirling engine and air before the expander as heat sink, a more flexible Solar-LAES-SE system with decoupled charging and discharging sections is proposed. The RTE and exergy efficiency of the Solar-LAES-SE system are studied and compared with those in a Solar-LAES-Organic Rankine Cycle (ORC) and an LAES system directly heated by Solar energy. The exergy analysis shows that it is more effective to directly heat the air before expanders by solar energy than to integrate an ORC or a Stirling engine if the solar energy is insufficient. The Solar-LAES-SE systems show the best performance in terms of RTE when the air temperature before expanders is low and the molten salt (solar energy carrier) outlet temperature is high. The cost evaluation of the solar energy storage section shows that when the molten salt outlet temperature is lower than 370 °C, the cost of the molten salt-based storage system for the Solar-LAES-SE will be lower than the cost of the thermal oil-based storage system for Solar-LAES-ORC. The reason is a reduced flowrate of molten salt for decreased outlet temperature, thereby increasing temperature change for the molten salt. With a fixed duty of the heat transfer, this reduces the flowrate of molten salt. As a result, the cost of molten salt and its storage is reduced.