Conceptual Design of Nuclear-Geothermal Energy Storage Systems for Variable Electricity Production

Abstract

Nuclear plants have high capital costs and low operating costs that favor base-load operation. This characteristic of nuclear power has been a critical constraint that limits the portion of nuclear power plants in a grid to stay below the base-load demand. A novel gigawatt-year thermal-energy storage technology is proposed to enable base load nuclear plants to produce variable electricity to meet seasonal variations in electricity demand. A large volume of underground rock is heated with hot water (or steam or carbon dioxide) from a nuclear power plant during periods of low electricity demand, and the heat is extracted during times of high demand and converted to electricity using a standard geothermal plant (Figure 1). Among various technical options, technically mature ones were selected for the reference design; a Pressurized Water Reactor (PWR) injects hot fluid into an underground reservoir through an intermediate heat exchanger and bypass flow lines on either the primary or secondary side. The reservoir size of 500 m in each dimension at 1.5 km underneath the surface is engineered to have permeability of 2 Darcy using commercial hydraulic fracture methods, and is cyclically heated up and cooled down between the temperatures of 50°C and 250°C. Peak power electricity is produced by exploiting the stored thermal energy via an Enhanced Geothermal System (EGS) that employs a binary flash cycle. Models of a nuclear-EGS system performance, taking into account heat transfer in the reservoir, thermal front velocity in the reservoir, conductive heat & water losses, geothermal power plant electricity production performance, operating conditions and system interfaces were developed and independently compared with Computational Fluid Dynamics (CFD) simulations using FLUENT 6.3 to confirm the validity of the models. The design study with the validated models reveals that the reference nuclear-EGS system based on 2.8~6.0 GW(th) of nuclear power would have a thermal storage size of 0.7~1.5 GW(th)-year, which corresponds to 0.08~0.2 GW(e)-year with electricity round trip efficiency of 0.34~0.46. Reservoir permeability and geofluid temperature are found to be the most important design parameters that affect performance of nuclear-EGS storage systems. A grid that deploys a nuclear-EGS system will have three distinct electricity sectors: nuclear base load, EGS intermediate load, and gas turbine peak power. The nuclear-EGS storage system introduces economic benefits to a grid by leveraging economic gains arising from replacing expensive intermediate and peak electricity with cheap base-load electricity. A nuclear-EGS system has a higher capital cost than natural gas turbines; consequently, it replaces intermediate-load power plants but not all the gas turbines that operate for a small number of hours per year. It was found that the deployment of a operate for a small number of hours per year. It was found that the deployment of a Nuclear-EGS could cut the electricity production cost of the New England Independent Systems Operator (NE-ISO) by as much as 14% of the storage-free cost (Fig. 2). Economic competitiveness of nuclear power plants is the most decisive factor for the deployment of the system in a grid. Because this was the first analysis of a nuclear EGS system, we used off-the-shelf technology wherever possible to reduce uncertainties and have confidence that the system will work. Significant improvements in roundtrip efficiency and economics may be possible by development of more advanced systems. For example, existing geothermal power plants are small (megawatts) versus several hundred megawatts for a nuclear EGS system. They use double flash power systems. The larger scale may enable the use of triple-flash and other more efficient power cycles. Reservoir development methods designed explicitly for nuclear EGS systems may significantly lower the costs of reservoir development. Like any other system dependent upon geology, costs and performance will depend upon the local geology.