Selecting rock types for very-low-cost crushed rock heat storage systems with nitrate salt heat transfer

Today the demand for variable electricity is met by fossil-fuel power plants because the capital costs and the cost of storing fossil fuels are very low. A low-carbon grid requires replacements for stored fossil fuels to provide variable heat and electricity as needed. The thermal energy replacement option is to send heat from nuclear and concentrated solar power (CSP) plants operating at full capacity to heat storage with variable heat output to the power block to provide variable electricity to the grid. Very low-priced electricity from wind and solar can also be converted into stored heat. Today the lowest-cost commercial heat storage systems are in CSP plants and use nitrate salt stored in hot and cold storage tanks. Advanced heat storage systems use nitrate salt for heat transfer but replace some or all the nitrate salt with lower cost crushed rock for heat storage. These systems may lower the capital cost of heat storage by a factor of five or more to several dollars per kWh of stored heat. Locally acquired crushed rock is the lowest-cost heat-storage material but not all rock is compatible with heat storage requirements including chemical compatibility, mechanical properties, and durability under thermal cycling. Rocks have been sorted into categories likely, possibly, and unsuitable and the basis for these conclusions has been documented. Potentially suitable rock types include basalt, peridotite, taconite, quartzite, quartzitic sandstone and serpentinite. Except for taconite, sedimentary rocks are unlikely to be suitable for service.