Worldwide electricity markets are changing due to decreasing prices of fossil fuels and addition of renewable generators (wind and solar). Large scale renewables deployment collapses prices at times of high wind or solar input that limits their deployment and impacts nuclear plant revenue. These changes have reduced the demand for base-load electricity but increased the demand for dispatchable electricity—a market currently served in the United States primarily by natural gas turbines. At the same time there is a longer-term need for dispatchable low-carbon electricity production—a replacement for fossil-fuel electricity production.
The changes may be challenging the economics of nuclear power today but may create new opportunities for existing and new-build nuclear energy systems in the future. Heat storage coupled to LWRs may enable base-load reactor operation with variable electricity to the grid—heat into storage when low electricity prices and production of added electricity using stored heat when prices are high.
To address these nuclear energy challenges the Massachusetts Institute of Technology (MIT), Idaho National Laboratory (INL), and Exelon conducted a workshop on Light Water Reactor (LWR) Heat Storage for Peak Power and Increased Revenue on June 27-28, 2017 at MIT. The workshop goals were to define and understand the market, regulatory, and technical options for coupling heat storage for variable power to existing and future LWRs with recommendations for the path forward to improve LWR economics. Observations and outcomes from the workshop include:
Nuclear reactors generate heat and thus couple efficiently to heat storage that is 10 to 40 times less expensive than electricity storage (pumped hydro, battery, etc.); thus potentially a lower-cost way to meet variable electricity demand. Favorable heat storage economics has resulted in concentrated solar power systems under construction to include heat storage to vary electricity production. Many of these technologies are applicable LWRs and most are applicable to other reactor types.
Six classes of heat storage technologies have been identified that can couple to light-water reactors: steam accumulators, sensible heat storage, cryogenic air storage, packed pebble-bed heat storage, hot-rock storage and geothermal heat storage. Some storage technologies are ready for demonstration, others require significant R&D.
Heat storage systems coupled to LWRs are different from storage technologies such as batteries and pumped hydro. Batteries and pumped hydro storage have electricity input rates to storage that are near electricity output rates; thus the strategy is buy low and sell high. With most heat storage systems, there are separate capital costs associated with heat input, storage, and heat-to-electricity production.
Accumulators and some other heat storage technologies have very low costs for heat addition to storage. The profitable strategy may be to send steam to storage 6 hours per day when prices are the lowest and produce added electricity 18 hours per day to minimize the cost of the more expensive heat-to-electricity component of the storage system. For many existing reactors up to 20% of the steam would go to storage when low prices. The maximum power output would increase by less than 5% to avoid major upgrades of the turbine hall. When viewing the nuclear plant as a black box, the addition of storage would appear to have increased its “base-load” capacity by a few percent and dramatically increased the capability to rapidly go down and back up in power. Inside the plant the reactor is operating at full capacity.
Other technologies such as nuclear geothermal inject hot water underground and use a geothermal power system for electricity production. Because of the extremely low cost of storage, such systems may enable seasonal energy storage, provide assured generating capacity and provide the option for a strategic multi-year heat reserve—the low-carbon equivalent to a strategic oil reserve.
The business case is central. Five years ago coupling heat storage to a LWR reactor would not have been economic. The changes in the electricity markets indicate that such an option may now be economical in some markets. As the markets continue to change, the economic case improves.
There is a need for demonstration projects to address institutional issues, to provide technology demonstrations for the near-term options and collect sufficient information to determine the economics.