Successful development of the Nuclear Geothermal Heat Storage (NGHS) system can extend utilization of nuclear power plants beyond production of base load electricity and accommodate fluctuations in electricity demand. Instead of selling the electricity below its cost during low demand (or even paying to get rid of it) heat can be stored in an underground reservoir and recovered to produce electricity when demand and electricity prices are high.
The NGHS system operates in two modes. Reactor heat at times of low electricity demand is transferred through a heat exchanger to a geofluid (water, air, or carbon dioxide). The geofluid is injected into permeable rock a kilometer or more underground, flows through the zone transferring heat to the rock, is pumped back to the surface, and sent back to the heat exchanger to transfer more heat underground. At times of high energy demand, the geofluid goes in the reverse direction and the heat is used to produce electricity. This paper compares and analyzes the performance of alternative geofluids for different heat storage temperatures.
An NGHS system with storage capacity 1GWth-year and operating at maximum temperatures of 250°C, 500°C and 730°C1 was analyzed. These temperatures correspond to coupling of a geothermal reservoir with today’s light water reactors and future advanced and high temperature reactors. For long and reliable operation of the NGHS a geofluid with very low erosion rates of underground rock has to be used, and, because of the nature of this system, low cost and environmental friendliness are required. Water, CO2 and air were chosen as the candidates to fit this need.
Reservoir size, system pressure losses, pump/compressor power and number of wells were computed for each case and results compared. Many parameters are involved and changing one may change results significantly. For example, increasing well diameter from 0.5 m, which is today the maximum drilling diameter, to 1m reduces the high frictional pressure loss of the well and especially for CO2 and air considerably reduces the number of wells. Another important parameter is reservoir permeability which determines its pressure loss. Hence, a number of calculations with various data sets were performed to study effects of these parameters, and to elect the most feasible geofluid for NGHS.
Pressurized liquid water was found to be a very promising geofluid, however, its tendency to dissolve rock at supercritical state disallows its use at higher temperatures. It is the preferred heat transfer fluid below 300°C reservoir storage temperatures. At temperatures of 500°C and 730°C, the CO2 performs better than air in all examined aspects. Furthermore, CO2 density differences between the injection and production well cause a significant increase of pressure at the outlet of the production well during heat recovery. If this pressure increase could be exploited, the system round trip efficiency will be enhanced.