Exergetic Analysis of a Steam-flashing Thermal Storage System

Thermal energy storage is attractive in the design of concentrator solar thermal systems because of its ability to increase turbine capacity factor and to facilitate dispatchable, if not continuous, power output from a solar field. At the right cost, a storage system can improve overall economics of a solar energy system. Presented here is a simulation study of the performance of a cycle that uses large-scale thermal energy storage via hot compressed liquid water. Such a cycle is potentially interesting because of its ability to allow collector field, thermal storage, and power cycle to all work with the same fluid, thereby eliminating losses associated with heat exchangers working between different fluids. An exergetic analysis demonstrates that most of the exergy destruction that results from integrating this type of storage is due to the steam flashing process. Analysis identifies that a variable-pressure accumulator sized with a volumetric capacity for peak-load is the most effective method of implementing a steam flashing storage system. Although high efficiencies are possible it seems likely that the high cost of high pressure storage ultimately makes the concept nonviable.