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A system for reversibly storing electrical energy as thermal energy. The system can include a reversible subcritical vapor-liquid cycle energy storage system with a single hot storage fluid tank and cold storage fluid tank that are inter connected by an inter storage tank flow path. The inter storag

A system for reversibly storing electrical energy as thermal energy. The system can include a reversible subcritical vapor-liquid cycle energy storage system with a single hot storage fluid tank and cold storage fluid tank that are inter connected by an inter storage tank flow path. The inter storage tank flow path includes an inter storage heat exchanger in the vapor-liquid cycle that enables sensible heat transfer between the working fluid and storage fluid as the storage fluid passes between the hot storage fluid tank and the cold storage fluid tank. This supplements latent heat transfer between the working fluid and the hot storage fluid tank and the cold storage fluid tank.

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1. A system for reversibly storing electrical energy as thermal energy comprising: a reversible subcritical vapour-liquid cycle having a working fluid flow path cycling througha hot storage fluid heat exchanger configured and arranged to exchange thermal energy between a working fluid, as the workin

1. A system for reversibly storing electrical energy as thermal energy comprising: a reversible subcritical vapour-liquid cycle having a working fluid flow path cycling througha hot storage fluid heat exchanger configured and arranged to exchange thermal energy between a working fluid, as the working fluid changes phase, and a hot storage fluid,a vapour pressure changing apparatus configured to change pressure of the working fluid in a vapour phase and arranged between and fluidly adjacent the hot storage fluid heat exchanger and a cold storage fluid heat exchanger,the cold storage fluid heat exchanger arranged fluidly adjacent the vapour pressure changing apparatus, configured and arranged to exchange thermal energy between the working fluid, as the working fluid changes phase, and a cold storage fluid,a liquid pressure changing apparatus arranged fluidly adjacent the cold storage fluid heat exchanger opposite the vapour pressure changing apparatus, configured to change pressure of the working fluid in a liquid phase, andan inter storage heat exchanger arranged fluidly between the liquid pressure changing apparatus and the hot storage fluid heat exchanger, configured and arranged to exchange sensible heat of the working fluid with a storage fluid;a hot storage fluid flow path cycle passing through the hot storage fluid heat exchanger and a hot storage fluid tank storing the hot storage fluid;a cold storage fluid flow path cycle passing through the cold storage fluid heat exchanger and a cold storage fluid tank storing the cold storage fluid; andan inter storage tank flow path cycle fluidly connecting the hot storage fluid tank to the cold storage fluid tank via the inter storage heat exchanger. 2. A method for generating electrical energy from thermal storage comprising the steps of: evaporating a working fluid of a vapour-liquid cycle in a hot storage fluid heat exchanger against a hot storage fluid circulating through a cycle of the hot storage fluid heat exchanger and a hot storage fluid tank;expanding the evaporated working fluid in a turbine of the vapour-liquid cycle to drive a generator to generate electricity;condensing the expanded working fluid in a cold storage fluid heat exchanger against a cold storage fluid circulating through a cycle of the cold storage fluid heat exchanger and a cold storage fluid tank;pressuring the condensed working fluid in a pump; andheating the pressured working fluid in an inter storage heat exchanger against a storage fluid passing from the hot storage fluid tank to the cold storage fluid tank. 3. A method for storing electrical energy as thermal energy comprising the steps of: isenthalpically throttling a working fluid of a vapour-liquid cycle using a throttle valve;evaporating the throttled working fluid in a cold storage fluid heat exchanger against a cold storage fluid circulating through the cold storage fluid heat exchanger and a cold storage fluid tank;compressing the evaporated working fluid in a compressor driven by a motor thereby inputting electrical energy in the vapour-liquid cycle;condensing the compressed working fluid in a hot storage fluid heat exchanger against a hot storage fluid circulating through the hot storage fluid heat exchanger and a hot storage fluid tank; andcooling the condensed working fluid in an inter storage heat exchanger against a storage fluid passing from the cold storage fluid tank to the hot storage fluid tank.