Enhancement of round trip efficiency of liquid air energy storage through effective utilization of heat of compression

Abstract

Liquid air energy storage (LAES) uses off-peak and/or renewable electricity to liquefy air and stores the electrical energy in the form of liquid air at approximately −196°C. The liquefaction (charging) process involves multi-stage air compression with the heat of compression harvested by a thermal fluid, which is stored for use in the power recovery (discharging) process. When electricity is needed, the stored liquid air is pumped, heated by environmental heat first and then superheated by the heat of compression stored in the thermal fluid, and other heat sources if available, leading to the expansion of the air by over 700 times to produce power. The current LAES technology, denoted as baseline LAES in this paper, only uses the heat of compression to improve the power output in the discharging process. Our analyses show that the discharging process of the baseline LAES system cannot fully use the stored heat of compression in an efficient manner. The excess heat is in the order of ∼20–40%, mainly because the yield of liquid air lies between 0.6 and 0.78, which is significantly lower than 100%. In this paper, we propose a hybrid LAES configuration, whereby the excess heat of compression is used as a heat source to power an Organic Rankine Cycle (ORC), whereas a Vapor Compression Refrigeration Cycle (VCRC) acts as a heat sink, leading to the production of additional electricity. Thermodynamic analyses show that the newly proposed hybrid LAES system has a round-trip efficiency of 9–12% higher than the baseline LAES system. The exergy efficiency of the discharging process of the hybrid LAES system is 9.6% higher on average than that of the baseline LAES system due to the more effective use of the heat of compression. An economic analysis has also been performed using a project life span of 15years. The results suggest that the combination of the ORC and VCRC gives a payback period of 2.7years and a savings to investment ratio of 3.08, which are much better than the use of the single ORC.