Keeping the balance: How flexible nuclear operation can help add more wind and solar to the grid

Optimization model shows that operating nuclear plants flexibly can reduce electricity costs, increase revenue for nuclear plants, and cut CO2 emissions in electric power systems.

Ivy Pepin MITEI

In the Southwestern United States, the country’s sunniest region, sunlight can shine down for up to 14 hours a day. This makes the location ideal for implementing solar energy—and the perfect test-bed for MIT Energy Initiative (MITEI) researcher Jesse Jenkins and his colleagues at Argonne National Laboratory to model the benefits of pairing renewable resources with more flexible operation of nuclear power plants. They report their findings in a recent paper in Applied Energy.

During summer 2015, Jenkins worked as a research fellow with Argonne National Laboratory on two power systems projects: one on the role of energy storage in a low-carbon electricity grid, and the other on the role of nuclear plants. Linking the two projects, he says, is the goal of using new sources of operating flexibility to integrate more renewable resources into the grid.

In power grids, supply and demand hang in a delicate balance on a second-to-second timeframe. Flexible backup energy sources must stay online at all times to maintain this equilibrium by meeting small variations in demand throughout the day or stepping in quickly if a power plant should suddenly go offline. If supply ever gets too far out of step with demand, devices designed to protect transmission lines and sensitive electronics from damage will quickly trip into action, causing blackouts as they work to shed demand or generation and restore the balance. Currently, certain coal, oil, natural gas, and hydro plants take on the important role of providing these standby capacity services, known as frequency regulation and operating reserves.

Nuclear power plants generally operate at full capacity, but they are also technically capable of more flexible operation. This capability lets them respond dynamically to seasonal changes in demand or hourly changes in market prices. Reactors could also provide the standby backup regulation and reserve services needed to balance supply and demand. According to Jenkins, all reactor designs now being licensed or built in the U.S., Canada, and Europe are capable of flexible operation, as are many older reactors now in service.

“We primarily rely on gas and coal plants to meet all those flexibility needs today, while we operate our nuclear plants fixed, or ‘must-run,’ 24/7,” says Jenkins. “The question here is, what would the benefits be if we stopped operating them so inflexibly, if we started using more of their technical capabilities to ‘ramp’ output up and down on different time scales from seconds to hours to seasons?” The answer, he says, is less reliance on the gas and coal plants—and more renewable energy integration.

Modeling for the energy transition

As markets increasingly incorporate variable renewables like wind and solar, maintaining the supply-demand balance becomes more complicated. Energy demand changes over the course of the day, usually staying low overnight, spiking briefly in the morning, and then peaking in the evening when people come home from work.

“Throughout these daily and seasonal changes in electricity use, there is a constant level of demand, known as the ‘base load,’ which is invariant,” says Jenkins. “Since nuclear plants have very low operating costs and cost a lot up-front to build, they are economically well-suited to operating all the time to meet this base load.” He adds, “That’s why when nuclear plants were originally licensed in the U.S., it wasn’t really necessary for them to play a role in following demand patterns throughout the day, and so nuclear plants in the U.S. weren’t licensed to operate that way.”

However, nuclear power plants were designed for flexibility “because the engineers who designed them envisioned a world in which nuclear took over the whole system,” Jenkins explains. This never really happened, except in France, which gets over 70% of its electricity from nuclear and has accordingly operated some of its nuclear plants to follow changing demand for years.

Now, as power grids around the world incorporate more and more variable renewable resources like wind and solar, the value of flexibility is increasing. Nuclear plants in places with increasing renewable energy penetration, like Germany, are therefore also moving toward flexible operation.

Because power systems today have very little energy storage capability, there are a growing number of places, from California and Iowa to Germany and China, where excess renewable energy might be produced on a sunny or windy day and must simply be wasted. Rather than disabling a solar panel or wind turbine, Jenkins points out, it makes more sense to operate the nuclear plant at a lower output and to absorb as much free wind or sun as possible. And operating nuclear plants flexibly has benefits beyond integrating renewable energy and reducing carbon dioxide emissions: By cutting the amount of wasted fuel, flexible operation can increase revenue for reactor owners, enhance system reliability, and reduce electricity costs for consumers.

Optimization models are helpful in simulating the potential economic and environmental benefits of incorporating renewables, but current models for electric power systems still represent nuclear units as inflexible, must-run resources. Jenkins and the research team at Argonne are closing this gap by developing a new approach to modeling flexible nuclear operation and employing this novel technique to study the potential benefits in power systems with relatively high shares of variable renewable energy sources. They simulated six cases in the American Southwest, ranging from inflexible nuclear plants, to plants with moderate flexibility, to those with high flexibility.

Modeling flexible nuclear plant operation poses its own challenges. A nuclear reactor has a range of operating constraints that arise from the physics of nuclear reactors and are distinct from the technical constraints on more conventional coal- or gas-fired power plants. For example, the minimum stable output of a nuclear reactor changes over the course of the fuel irradiation cycle, and production can’t be ramped up or down too quickly without causing a strain on the nuclear fuel rods and the reactor itself. “The task was to try to synthesize the main physical engineering constraints limiting the ability of reactors to change their output on different timescales, and then translate that into the mathematical constraints that we use in modeling and optimization for the power system,” says co-author Audun Botterud, a principal research scientist in Argonne’s Energy Systems Division and in MIT’s Laboratory for Information and Decision Systems.

The research team created a “mixed integer linear programming” (MILP) formulation that accounts for the specific operating constraints on ramp maneuvers of nuclear power plants. “It’s a mathematical program that minimizes the cost of operating the power grid over the whole year while respecting the engineering constraints that power system operators and individual power plants have to maintain,” Jenkins explains. The simulation works in two stages, optimizing for demand predicted one day in advance and then in real time—matching the way the electricity markets work in the U.S.

Reducing curtailment of renewables This graph projects the amount of renewable energy that would be curtailed (and therefore wasted) for each level of nuclear flexibility, as simulated by the two-stage unit commitment and economic dispatch model: NoFlex, where nuclear plants are operated at their maximum output; Flex, where plants are somewhat flexible; and FullFlex, where plants are highly flexible. In cases with a production tax credit (PTC) applied to wind power, solar energy would be curtailed before wind, as curtailing wind output means forfeiting the tax credit—but overall, total renewable curtailment rates are nearly identical with the PTC. As shown in the graph, nuclear flexibility significantly reduces renewables curtailment. Modest flexibility in the Flex and FlexPTC cases reduces curtailment of wind and solar by 43%, and high flexibility (FullFlex and FullFlexPTC cases) reduces curtailment by 58%. Without any flexible nuclear operation (NoFlex), 16.7% of available renewable energy output is wasted.

The MILP formulation has applications beyond the specific region studied. “The general findings would hold in other places with similar shares of these two resources [nuclear and renewables],” says Jenkins. And, importantly, the study demonstrates how one of the world’s biggest sources of low-carbon energy (nuclear) and the world’s fastest growing energy source (renewables) can work together rather than replace each other.

“What this study shows is that rather than shut down nuclear plants, you can operate them in a way that makes room for renewables,” says Jenkins. “It shows that flexible nuclear plants can play much better with variable renewables than many people think, which might lead to reevaluations of the role of these two resources together.

Bridging the different knowledge bases, between folks who do power system modeling at the grid level and nuclear engineers and physicists who understand the details of nuclear reactor dynamics, was the most challenging but also the most interesting and productive aspect of this project,” says Jenkins. “These are two communities that don’t always talk to each other, and they speak different languages and have different backgrounds and expertise. This kind of collaboration is an example of the unique interdisciplinary work that can happen at a place like a national laboratory or the MIT Energy Initiative.”

This research was supported by Argonne National Laboratory and the National Science Foundation.

This article appears in the issue of Energy Futures.

Electric powerEnergy storageNuclear energyPower distribution and energy storageRenewable energy

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