In order for renewable energy to displace 20% or more of the conventional power generating base without depending on significant hot spinning reserves, reliable and cost effective energy storage will be needed at the utility scale. Developing and deploying practical energy storage at this level is a major challenge and no single technology appears to have a dominant position. Storing electrical energy by way of thermal storage at moderate-to-low temperatures has not received much attention in the past. In fact, the conventional thinking is that heat pump/heat engine mediated energy storage is too inefficient (round trip efficiency of 30% or lower) to be practical. However, an innovative and efficient storage approach is proposed in this paper by incorporating sensible heat storage in a Rankine-type heat pump/heat engine cycle to increase the round trip efficiency. Furthermore, by using a source of waste (or otherwise low-grade) heat, round trip efficiencies can be enhanced further. Currently, there appears to be no significant linkage between waste heat recovery and grid-level energy storage, although the market opportunity for each is considerable. Using the thermal approach described here, a system can be created that uses very low-grade heat in the range between 50 to 70 °C. Furthermore, conventional technology can be used to implement the system where no extreme conditions are present anywhere in the cycle. Hence, it is thought to have advantages over other energy storage concepts being developed.

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