The Vast Potential of Energy Storage for Nuclear

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In a September 2020 report from the Clean Energy Ministerial's Nuclear Innovation: Clean Energy Future (NICE Future) initiative, experts envisioned an interesting array of nuclear energy prospects that could retain and even expand the sector's relevance in low-carbon power markets. They proposed that along with district heating, and high-temperature heat for industrial processes, new roles for nuclear power and heat could involve hydrogen and synthetic fuel production; seawater desalination and purification; and off-grid power and heat applications in remote regions. But to retain nuclear power plant capacity factors-and stabilize their revenue generation-as well as provide grid flexibility, stability, and that inherent, coveted integration within a renewables-heavy grid, experts argue coupling baseload nuclear power to large-scale energy storage systems (ESS) may be the best option over the near term.

However, not all ESS systems are suited to nuclear, and benefits vary widely depending on their application. ESS can be integrated either to the plant directly known as behind-the-meter [BTM] storage (storage and control is at the power plant-level), or ESS can be integrated to the grid known as front-of-the-meter [FTM] storage (storage and control is at the grid-level)," explained researchers from the Korea Advanced Institute of Science and Technology (KAIST) and the Canadian Nuclear Laboratories (CNL) in a March 2021-published comprehensive evaluation of various large-scale ESS technologies that could be applied to existing pressurized water reactors (PWRs)-which represent about 67% of the world's operating reactors.

One key advantage BTM technologies offer is a direct integration with ESS such that the generated steam from the nuclear reactor can be more efficiently stored and discharged in the form of thermal or mechanical energy, to minimize conversion losses," they wrote. Another advantage of this option, when compared to a direct integration of ESS with renewables, is that direct integration with nuclear power can avoid the off-design efficiency and component degradation issues due to the intermittent nature of the renewable input source."

While each ESS technology may play various functions on the grid, an ideal application for BTM-integrated ESS in a PWR would be for energy management-to essentially provide balancing, such as time-shifting, peak-shaving, and seasonal energy storage. That essentially narrows suitable ESS technologies to three main types: thermal energy storage (TES); tank-based compressed gas energy storage (CGES); and liquid-air energy storage (LAES). The main rationale for the selection is that they can be installed as large-scale ESS without the geological constraints and that they do not suffer from AC-DC [alternating current-direct current] conversion loss and relevant transmission issues," the researchers said.

The researchers nixed pumped hydro storage and cavern-based compressed air energy storage, owing to their locational constraints. Also eliminated was lithium-ion storage, which is gaining prominence as a utility-scale option at other resources, owing to its limitations in terms of capacity and cost." If ESS is to provide load-shifting and energy management for PWRs, energy capacity needs expansion for several hours at the [necessary] power level of several hundred MWs," they said. Meanwhile, though hydrogen energy systems are widely touted as a serious option to address seasonal storage, currently proposed solutions are yet to be demonstrated, and they are currently inefficient, costly, and, according to a 2017 study by the Idaho National Laboratory (INL), may be unable to perform energy arbitrage.

For the KAIST and CNL researchers, CGES and LAES are promising for their high-density and recent development progress. Under their CGES evaluation, the researchers notably highlighted compressed CO ₂ energy storage (CCES), looking separately at compressed supercritical CO ₂ and trans-critical CCES, which involves storing liquid CO ₂ in a low-pressure tank and then reheating it before compression. LAES, which involves liquefaction of air, is also promising, but though Highview Power is slated to begin operating its first 50-MW/250-MWh LAES facility in 2022, the company's initial projects are still smaller-scale facilities designed for grid stability applications.

That leaves TES. In the NICE Future Initiative report, Charles Forsberg, principal research scientist at the Massachusetts Institute of Technology's (MIT's) Department of Nuclear Science and Engineering, highlighted several different variations, though he noted there is no single optimum or best heat storage technology," because different types of nuclear plants deliver heat at different temperatures and use different coolants." And, while the TES must match the reactor type," they must also suit the market in which they are used. In an energy system with a large PV capacity, the TES would be used on a daily basis, for example, requiring a large economic incentive for efficiency. But if used to store excess energy during times of low demand-during weekends for weekday consumption, for example-the economics will prefer a storage system with very low capital costs even if it is somewhat less efficient." Following are some TES technologies being considered for nuclear.

Liquid Salts. Heat storage systems already widely in use in high-temperature concentrated solar power (CSP) systems are increasingly being adopted in advanced nuclear reactor designs. Natrium, which is set to demonstrate a 345-MWe sodium fast reactor in Wyoming within seven years under the U.S. Department of Energy's (DOE's) Advanced Reactor Demonstration Program, notably uses a nitrate salt molten salt ESS that its developers claim has the potential to boost the system's output to 500 MWe of power for more than five and a half hours when needed." As TerraPower told POWER last September, the system derives its technology from a system of similar scale that is employed at the 280-MW Solana CSP plant in Arizona.

Nitrate-salt storage system designs are also proposed for fluoride-salt-cooled high-temperature reactors with solid fuel and liquid salt coolants, molten salt reactors with fuel dissolved in the salt, as well as in fusion machines with liquid salt blankets. In each of these cases, the nitrate salt replaces the intermediate heat-transfer loop that separates the low-pressure reactor from the high-pressure power cycle," noted Forsberg. Because the nitrate salt replaces other fluids in the intermediate loop with hot salt, there is no efficiency loss by adding storage to these reactors-salt after being heated goes directly to storage, just like in a CSP system," he explained. Forsberg suggests projected costs for commercial nitrate salt storage systems are near $20/kWh.

Heat Transfer Oils. Another innovative medium derived from the CSP sector-specifically from parabolic trough CSP plants-involves using heat transfer oils such as Eastman's Therminol-66. One example is a 16.6-MW CSP project that forms part of the Bronderslev hybrid solar-biomass plant in Denmark. Therminol-66 (along with ethylene glycol and alumina beads) are slated for testing at INL's experimental Thermal Energy Distribution Systems, a project that in December 2020 began evaluating the interoperability of nuclear reactors, energy storage, and ancillary processes in a real-world setting.

Cast Iron with Cladding. A simpler, lower-cost TES technology involves storing heat in a tank filled with cast-iron hexagonal billets that are up to 20 meters high. The tank has a steel cladding that has a composition chosen to be compatible with any coolant. According to INL, if the temperature difference between hot and cold is increased to 300C, heat-storage costs are reduced by a factor of three. Tripling the hot-cold temperature range in storage cuts storage costs by a factor of three or more, with the potential to meet DOE cost goal for heat storage of $15/kWh, excluding other system costs," it says. For sodium-cooled reactors, heat storage would be placed in the intermediate sodium loop.

2. Brenmiller Energy's bGen high-temperature thermal energy storage unit uses crushed rock media as the storage material, but it also integrates heat exchangers and a steam generator. Courtesy: Brenmiller Energy

Crushed Rock Heat Storage. TES systems that use crushed rock are gaining prominence throughout the power space mainly for their low-cost ability to provide large-scale heat storage. Since its 2019 launch of a 30-MW/130-MWh Electric Thermal Energy Storage (ETES) pilot in Hamburg, for example, Siemens Gamesa says it has racked up interest in the system that has a temperature range of 180C and 750C. This June, New York Power Authority and the Electric Power Research Institute, for example, launched a project to explore Israeli firm Brenmiller Energy's high-temperature crushed rock TES system (Figure 2) in a range of fossil generation assets.

Among efforts with specific nuclear applications is Westinghouse's exploration of a system for new-build PWRs where steam is used to heat oil that in turn transfers it to concrete in prefabricated boxes. The solution uses thin plates with narrow gaps" to create huge surface area relative to volume and minimizes oil fraction." In South Korea, researchers have designed a nuclear heat storage and recovery system that interfaces with the APR1400. The system comprises a packed bed of Hornfels rock, with heat supplied by Therminol-66 oil. The process cycle essentially involves diverting steam from the APR1400 steam cycle upstream of the high-pressure turbine, condensing and cooling it in heat exchangers, and then transporting the hot oil offsite to the packed bed configuration for storage. And in Germany, says Forsberg, researchers are examining nitrate salt heat storage in single tanks filled with crushed rock with lower-density hot salt on top of cold salt.

-Sonal Patelis a senior associate editor for POWER.

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