Improving Round Trip Efficiency (RTE) in liquid air energy storage by integration with external thermal energy sources

Abstract

As a promising large-scale electricity storage system, Liquid Air Energy Storage (LAES) has the advantage of being geographically unconstrained, with a considerable potential to improve energy efficiency by integrating with external sources of thermal energy. In this work, the Stirling Engine (SE) and using LNG and solar energy are introduced to improve the energy efficiency. Since the traditional Round Trip Efficiency (RTE) is defined for stand-alone systems, an important contribution of this work is a revised definition of RTE that accounts for the input of external sources of thermal energy. Three types of LAES systems are modelled and optimized: (i) using an SE to recover surplus compression heat, (ii) a redesigned LNG integrated system using cold energy from LNG regasification for cooling in the compression section and air liquefaction part, and (iii) integration with solar energy. The optimization results show that the RTE of the LAES-SE system with cooling water as cold sink is 68.20%, 3.20% points higher than a base case using ORC. For LNG integrated LAES systems, it is significantly better to use the cold energy in LNG to cool air in the compression section and contribute to air liquefaction in the cold box than to drive a Stirling Engine. The traditional RTEs for the two alternatives are 102.80% and 73.79%, respectively. Two methods were tested to account for the value of external energy input to the system in the RTE. Solar-LAES-SE is better in terms of energy and economy than Solar-LAES-ORC and Solar energy directly heated LAES.