Thermodynamic optimization of solar aided liquid air energy storage systems

Liquid air energy storage is a promising large-scale energy storage technology with high energy density for increasingly weather-dependent power grids, with no geographical constraints. The round-trip efficiency of a standalone liquid air energy storage system is predicted to be between 40 % and 67 %. An attractive way to increase the economic viability of the liquid air energy storage system is to couple the system with additional heat sources. Incorporating concentrated solar power has recently been proposed to increase the temperature at the inlet of the air turbines, and thus boosting the discharging power output and round-trip efficiency. This paper aims to find the optimal system design based on concentrated solar power temperature, considering both energy storage and power production metrics. The analyzed liquid air energy storage system is based on the Linde cycle, two tank thermal oil system for compression heat storage, and a two tank, two stage cold recovery system using methanol and liquid propane. A heliostat system with two-tank direct molten salt thermal energy storage is used as an additional heat source. A supercritical organic Rankine cycle system is used for excess heat recovery with R32 as working fluid. Novel ways of integrating the organic Rankine cycle system for multiple-source excess heat utilization were analyzed. Effects of charging and discharging pressures, number of air compressors and turbines and solar heat transfer fluid temperatures on the optimal organic Rankine cycle design and round-trip exergy ratio were studied. The results suggest that a maximum round-trip exergy ratio of 62.2 % can be achieved. The paper provides a basis for further optimization of design and operation of the solar aided liquid air energy storage systems, especially in off-design conditions for low sun availability.