THERMODYNAMIC ANALYSIS OF A NOVEL CRYOGENIC RANKINE CYCLE FOR WIND ENERGY STORAGE

A novel electrical energy storage system based on cryogenic liquid nitrogen as storage medium was developed and investigated in order to integrate fluctuating wind energy into the electrical grid. In times of surplus electric power from wind turbines the electrical energy is used to generate very cold liquid nitrogen with an air separation unit which will be stored in cryogenic tanks. In times of electricity demand the energy which is stored in the coldness of the liquid nitrogen will be transferred into electrical energy by a Rankine cycle. The external heat input is solely supplied from the ambience because all changes of state of this cryogenic Rankine cycle are below the ambient temperature level. The cycle drives an expansion turbine for power generation with a power of 10 MW. In this work two variants of the cryogenic Rankine cycle are presented. The thermodynamic analyses show that the volumetric energy density of this liquid nitrogen energy storage system (LINESS) amounts > 50 kWh/m(3), which is much higher than of many alternative energy storage systems. But the overall efficiency of this storage system is moderate and amounts 13%. The investigations also show that the technical feasibility of the turbine is given, but a standard steam turbine cannot be adopted for this cycle. The main advantage of this novel storage system compared to compressed air or hydrogen power storage systems is that it can be built independent of geological premises due to the high volumetric energy density.