Oil and Nitrate-Salt Coolant Trade-Offs With Crushed-Rock Heat Storage and CSP

Abstract

The large-scale use of wind or solar results in collapse of electricity prices at times of large wind or solar production.  Addition of heat storage to Concentrated Solar Power (CSP) plants enables (1) storing heat at times of high solar input, excess electricity production and low prices and (2) producing electricity when needed at times of high prices. We are developing a Crushed Rock Ultra-large Stored Heat (CRUSH) system with capital cost goals of $2-4/kWh that enables hourly to multi-week storage with very large CSP systems. Heat is stored in crushed rock piles up to 40 meters high in an insulated building. Hot oil or nitrate salt is sprinkled on the rock, trickles through the rock, heats the rock and the resultant cold fluid is recovered by drain pans. Heat is recovered by sprinkling cold oil or salt on hot rock, trickling through the rock, and heating the fluid with oil or nitrate salt recovered by drain pans. There are different constraints for oil versus salt systems. Given the high-cost of heat-transfer oils, rock sizes and types are chosen to minimize residual oil in the crushed rock. Nitrate salts are less expensive; however, nitrate salts will interact with many rock types placing constraints on acceptable rock types. Rock impurities in the oil or nitrate salt can impose constraints on the CSP system and define fluid-system cleanup requirements.