Heat-Pump-Based Thermal Storage for Intermittent Electrical and Thermal Sources

Thermal storage is advocated as a means for energy storage in some grid-scale electric powerplants, such as for concentrating solar power (CSP). The efficiency of concentrating solar thermal collectors, however, decreases with increasing output temperature, making it difficult to achieve high thermal storage temperatures. A heat pump, as is well known, can operate in either cooling mode or heating mode, and in either case, the coefficient of performance is generally greater than one, enabling a multiplier effect that can serve to either increase or decrease the temperature of thermal storage. A simple steady-state analysis of the “round-trip” system efficiency for storing energy reveals the potential benefits of utilizing a heat pump or refrigerator in such systems. Provided that an inexpensive heat input source is available, the system storage efficiency can reach or even exceed unity, assuming that the energy supplied to the system as heat is neglected. For ice storage at 0 °C, increasing thermal input temperatures above 209 °C increases the system storage efficiency above unity.