Concentrated solar plants have been designed to store thermal energy so as to produce power after sundown, but heat storage should also be of interest to operators of nuclear power plants. Adding heat storage to light-water reactors is the enabling technology for a carbon-free electricity industry based on solar, wind, and nuclear power. And it can accomplish this with little disruption to the operations of existing nuclear plants. This article delves into the current heat storage technologies that are at various states of readiness to be deployed.

## Article

The story of human civilization is very much intertwined with the control of heat. There is evidence that humans started using fire at least 300,000 years ago, and since then we have burned carbon fuels such as wood, coal, oil, and natural gas.

While the cooking fire and the gas turbine are different technologies, their economics are similar. The manpower, resources, and money are in collecting the fuel and bringing it to the fire. The cost of the cooking fire or gas turbine is small relative to the cost of fuel. It is economic to operate a cooking fire or gas turbine at low capacity factors, turning these machines off when energy demand is low.

Low-carbon electricity changes the economics of energy. Nuclear, wind, and solar have high capital costs and low operating costs; thus, energy becomes very expensive if these technologies are operated at part load. If one operates these machines at half capacity, the cost of energy production approximately doubles. For non-dispatchable wind and solar facilities, it means that whenever the wind blows or the sun shines, operators will use those plants to produce as much electricity as they can, even if it commands a low price. That sort of revenue collapse has become a growing reality in the last several years in Europe, the United States, Japan, and China and could produce a real drag on the expansion of those sources in the near future.

The reshaping of the power industry has hit nuclear especially hard. Most existing light-water reactors were built to provide consistent baseload power at dependable prices rather than accommodate the volatility and variability inherent in solar and wind power. During certain times of the year in some power markets, the net load—the difference between electricity demand and the power supplied by non- dispatchable wind and solar facilities—can surge by many gigawatts between the early afternoon and early evening. Nuclear power plants were never designed to handle such swings.

The solution that is usually proposed to address such challenges is energy storage. Wind and photovoltaic (PV) systems produce electricity and thus couple well to technologies that store work, such as batteries or pumped hydropower. The work is stored in times of excess electricity production and converted to electricity for sale at times of high prices.

But energy can also be stored as heat. Concentrated solar plants have been designed to store thermal energy so as to produce power after sundown, but heat storage should also be of interest to operators of nuclear power plants. With policy goals to reduce greenhouse gas emissions creating incentives to bring online as much variable solar and wind power as possible, coupling nuclear power and heat storage is a way for nuclear plants to change from primarily baseload providers of electricity to providing variable electricity for the grid and ultimately becoming buyers and sellers of electricity.

Adding heat storage to light-water reactors is the enabling technology for a carbon-free electricity industry based on solar, wind, and nuclear power. And it can accomplish this with little disruption to the operations of existing nuclear plants.

## Grid-Scale Potential

Heat storage for variable electricity production is an old technology. The first large-scale heat storage system for electricity production was the Charlottenburg Power Station steam accumulators built in Berlin in 1929. The accumulator system was comprised of 16 tanks, each 4.3 meters in diameter and 20 meters high, that were charged with steam from a coal boiler at times of low electricity demand. During high demand, the steam released by the accumulators drove a turbine with a peak electricity output of 50 MWe.

From that beginning, heat storage capacity has grown to grid scale. Today heat storage systems coupled to concentrated solar power systems have capacities in excess of a GWh(t) to enable electricity production after sunset at times of higher electricity prices.

In contrast, battery storage facilities are limited to the megawatt-hour scale. And potential for each technology is reflected in the long-term cost goals set by the U.S. Department of Energy. The goal for battery storage is to reach $150 per kWh of electricity storage capacity, plus about the same amount for the installation of power conversion and other required systems to couple to the grid; if a battery storage system can reach that goal, it will more than double electricity costs. By comparison, the DOE goal for thermal energy storage for use at a concentrated solar power plant is$15 per kWh of heat storage capacity. Heat storage is much less expensive than work storage.

Heat storage technologies that can be coupled to nuclear power plants can be divided into six classes, each at different states of readiness to be deployed. It is not likely that there will be a single technology because of differences in the market.

A market with large-scale solar has a daily cycle of low-and high-priced electricity whereas a market with large-scale wind sees a multiday cycle of low-price electricity.

Of those six classes, two can be considered to be viable in the near term: Steam accumulation and liquid sensible heat storage.

A steam accumulator is a pressure vessel nearly full of water that is heated to its saturation temperature by steam injection. The heat is stored as high-temperature high-pressure water; when steam is needed, valves open and some of the water is flashed to steam that is sent to a turbine or feedwater heaters generating electricity while the remainder of the water decreases in temperature.

The technology is commercial—remember, it was deployed at Charlottenburg in 1929—and has been deployed at several concentrated solar thermal power stations. It has the advantage of having a faster response than any other heat storage system, making it well suited for the kind of quick ramp up needed to accommodate highly variable renewable energy.

Sensible heat storage is in some ways similar to a steam accumulator. It involves heating a second fluid (rather than water) with steam, storing that hot fluid at atmospheric pressure, and using that fluid at a later time to provide the heat to produce steam which is then sent to a turbine. Many concentrated solar power systems store heat in tanks of hot salt or hot oil to enable electricity output from the steam cycle after the sun sets.

Westinghouse has begun development of a sensible heat storage system for light water reactors. In that system, steam heats a low-pressure heat-transfer oil which then transfers its heat to a heat storage module in which vertical concrete plates serve as the primary heat storage medium rather than the heat-transfer oil. Concrete is to be used because it is a less expensive heat storage medium than oil. The hot oil flows through narrow channels between slabs of concrete. To recover the heat, the direction of oil flow is reversed. The hot oil would be used to generate steam that can be used in either the main reactor turbine, a separate power system, or as a partial replacement for steam to feedwater heaters.

## Liquid Air and Solid Rock

Four other concepts show a great deal of potential for storing heat from light water reactors, but none are ready for deployment. For instance, cryogenic air energy storage systems hold heat in the form of liquefied air that can be stored in facilities similar to those that store liquefied natural gas. The air would be liquefied when electricity prices are low; when electricity demand called for it, the liquid air would be compressed, heated using low-temperature heat from a nuclear power plant’s cooling water, then further heated with steam from the reactor and sent through an air turbine before being exhausted to the atmosphere. The round-trip efficiency for this technology coupled to a LWR is estimated to be over 70 percent.

The distinguishing feature of cryogenic air energy storage is that the peak-to-baseload electricity output is higher than for other heat storage systems. A pilot facility coupled to a biofuels power plant is now operating in the United Kingdom, but it will be some time before a system is optimized for nuclear power plants.

Another concept—storing heat in a volume of crushed rock—is simple and features the lowest incremental heat storage costs. To charge the system, steam from the reactor heats air via a heat exchanger, and then that hot air circulates through crushed rock, which absorbs the heat and can be raised to arbitrarily high temperatures. To extract the heat, the flow of air reverses, carrying energy out of the rock to a heat exchanger to generate steam for a turbine.

This storage technology is being developed for concentrated solar thermal power systems with several pilot plants worldwide where the heat input is hot air from the solar power tower. In addition, Siemens is developing a variant for use with wind systems that relies on electric resistance heaters to produce the hot air.

Instead of using steam to create superheated air, the steam can be used directly to heat pebbles or some other solid material. This packed-bed thermal energy storage system consists of a pressure vessel filled with solid pebbles with a steam valve at the top and water outlet at the bottom. Heat is stored as sensible heat in the pebbles. To charge the system, steam is injected into the pebble bed. It condenses as the cold pebbles are heated and exits as water from the bottom of the vessel. At the end of the charging cycle all pebbles are hot and hot water fills the voids at the bottom of the vessel. To discharge the system, water is injected into the bottom of the vessel and steam produced by the hot pebbles travels up and out the top.

While this technology is in the early stages of laboratory development, it should in theory have very high round-trip efficiency since it doesn’t incur losses in the heat exchanger and because it operates in a counter-current mode—the hottest steam sees the hottest pebbles.

Perhaps the most long-term method for storing heat involves geological formations. In geothermal heat storagesystems, steam from the reactor heats water that is then injected into a subsurface reservoir; the water is later pumped out for use in a conventional geothermal plant to generate electricity.

The concept is derived from steam and hot water injection systems used in the oil industry for heavy oil recovery as well as standard geothermal power systems, but it is in the earliest stages of development and only limited studies have been completed. Even so, it is clear that one unique feature of this technology is that it can provide seasonal heat storage at very low cost. Unfortunately, there is no way to insulate rock deep underground, so losses are unavoidable, and the only way to mitigate that is by increasing the storage capacity. In fact, a geothermal heat storage system would have to have a heat capacity greater than 100 megawatt-years in order to retain enough heat over a seasonal timescale.

When coupled to a heat storage system, a light-water reactor would be able to operate at full power—as it was designed-while meeting the needs of an electricity market driven by solar and wind power variability. At times of low electricity prices some steam would be diverted from the reactor to heat storage but the main steam turbine would remain on line at part load to allow rapid return to full power when needed. At times of high prices, all steam from the reactor is sent to the turbine and heat from storage (usually in the form of steam) would be sent back to the turbine hall to either the turbine or the feed-water heaters in the steam system.

Many existing nuclear power plants already have 5 to 10 percent excess capacity in their turbines and so could accommodate a modest amount of additional steam from heat storage. New power plants, on the other hand, could be designed with heat storage in mind. Such plants could be built to enable output swings from 30 to 130 percent of base-load capacity using existing technologies. Adding a second turbine for peak electricity production would be particularly attractive at multi-unit nuclear stations because a relatively large turbine could be built to minimize capital costs.

It may also make sense to add an auxiliary steam boiler fueled by natural gas or biofuels to provide peak steam capability at times when heat storage is depleted.

Such an auxiliary system might not be used more than 100 hours per year, but it would be fairly inexpensive (estimated costs for such a steam boiler are between \$100-300 per kWe) and would enable the plant to provide assured generating capacity. Today some concentrated solar thermal power systems have natural gas-fired steam boilers to provide assured generating capacity when heat storage is depleted after sunset.

Another potential add-on is resistance heaters so that during times of extraordinarily low (or even negative!) electricity prices—such as when wind turbines are generating at full capacity in the middle of the night-electricity could be converted to heat and stored for later reuse. The round-trip efficiency of such storage would be low, since the rate of conversion from heat to electricity is 30 to 35 percent in LWRs, but it would still be a profitable way to make use of the miss-match between wind power production and consumer demand.

## Bridging the Gap

The electric power industry has entered a new era as a result of technological and policy changes. The plummeting price of wind and solar power has made the large-scale addition of those sources almost irresistible, while at the same time there is a growing realization that the grid must add as much low-carbon electricity as quickly as possible. But electric utilities are conservative organizations and they have slow reaction times—partly because of a mandate to keep the lights on. They will demand high confidence that any new technologies will work.

Those various impulses seem irreconcilable, but heat storage is a technology that can bridge the gap. When coupled with a light-water reactor, an appropriately scaled heat storage system can provide hundreds of megawatts or more of dispatchable, low-carbon electricity. Heat storage can also be added incrementally, retrofitted to existing generating assets at a relatively low cost compared to that of a light-water reactor or even a fossil-fuel power plant. As the pilot projects prove their worth, utilities can add heat storage systems to more plants with very low risk.

Perhaps most importantly to utility decision makers, heat storage is in many cases a proven technology. The concept behind it is well understood and while heat storage has never been deployed on the scale I suggest here, the difference is only a matter of magnitude.

Adapting old-fashioned heat storage to help usher in a modern low-carbon power industry seems a bit counterintuitive—like going back to the future. But given the many contradictory pressures being felt by the industry, I think the time has come for this robust idea.

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