Gigawatt-Year Geothermal Energy Storage Coupled to Nuclear Reactors and Large Concentrated Solar Thermal Systems

Abstract

Capital-intensive, low-operating-cost nuclear and solar thermal power plants are most economical when operated under base-load conditions. However, electricity demand varies on a daily, weekly, and seasonal basis. In deregulated utility markets this implies high prices for electricity at times of high electricity demand and low prices for electricity at times of low electricity demand. We examined coupling nuclear heat sources to geothermal heat storage systems to enable these power sources to meet hourly to seasonal variable electricity demand. Because the heat storage system is independent of the source of heat, the results are applicable to other large heat sources such as large-scale centralized thermal solar systems. At times of low electricity demand the reactor or solar plant heats a fluid that is then injected a kilometer or more underground to heat rock to high temperatures. The fluid travels through the permeable-rock heat-storage zone, transfers heat to the rock, is returned to the surface to be reheated, and re-injected underground. At times of high electricity demand the cycle is reversed, heat is extracted, and the heat is used to power a geothermal power plant to produce intermediate or peak power. Below 300°C pressurized water is the preferred heat transfer fluid. These temperatures couple to existing light-water reactors. At higher temperatures supercritical carbon dioxide is the preferred heat transfer fluid. Underground rock can not be insulated, thus small heat storage systems with high surface to volume ratios are not feasible because of excessive heat losses. The minimum heat storage capacity for seasonal storage is ~0.1 Gigawatt-year. Three technologies can create the required permeable rock: (1) hydrofracture, (2) cave-block mining, and (3) selective rock dissolution. The economic assessments indicated a potentially competitive system for production of intermediate load electricity. The basis for heat storage at temperatures