A concept for storing utility-scale electrical energy in the form of latent heat

A concept is introduced here for storing utility-scale electrical energy in the form of latent heat. The storage process utilizes a boiling refrigerant at sub-ambient temperatures to freeze a latent heat storage material using electrically driven compressors. Recovery of the latent heat for electrical generation then uses vapor expansion and condensation which essentially reverses the storage process. Sensible heat storage is incorporated into the cycle to efficiently implement the concept. Both energy storage and generation are carried out under steady flow closed-loop conditions where the T-s diagram is similar to a Rankine cycle. From a thermodynamic perspective, work is supplied to the system while heat is transferred to the surroundings from the latent heat store. The reverse process generates work while using heat supplied by the surroundings. An analysis with expander/compressor isentropic efficiencies and small temperature differentials for the heat transfer processes can give projected round trip efficiencies in the 50–60% range using a common refrigerant. One of the attractive features of this approach is the ability to use different ambient temperatures for storage and generation. Exploiting diurnal temperature differences or sources of low grade heat (50–90 °C) significantly increases the apparent round trip storage efficiency.

► Conventional thinking has thermal energy storage as impractical.
►  An approach using internal cycle heat storage gives roundtrip efficiency > 50%.
► Low grade waste heat can be utilized to boost round trip efficiencies further.
► No extreme conditions are present in the cycle and common technology can be used.