Balancing the grid with nuclear and renewables
“Toto, I've a feeling we're not in Kansas anymore.” A familiar catchphrase in American culture from the classic “The Wizard of Oz” where Dorothy, the heroine of the story, finds herself in an unfamiliar land---taken from the monotone farm where she lives in Kansas and placed in the vibrant Land of Oz. As we survey the future of the utility industry, we can say with certainty that we are no longer in Kansas either.
The changes in the industry over the past several years have been staggering. From the plunge in oil and gas prices to the massive growth of renewable power across the country, the utility industry is experiencing changes unlike any seen since electricity was introduced in the 1920s. The use of coal is decreasing---a combination of low natural gas prices and U.S. Environmental Protection Agency regulations are making its power too expensive to produce. At the same time, the use of renewables is increasing, which can be attributed to a variety of factors ranging from production and investment tax credits and state renewable mandates in the Renewable Portfolio Standards to falling material costs and environmentally conscience consumers.
While natural gas and renewables are driving changes to the supply landscape, consumers are writing a story of their own. Whether it is programmable, self-learning, sensor-driven thermostats; apps that learn consumer habits and adjust thermostats automatically; or home-based solar energy storage batteries bringing homeowners one step closer to self-sufficiency, the world is changing rapidly.
Demands of the future grid
California’s electricity market provides an excellent case study as to what the grid of the future will demand of generators, namely that they provide flexible operations. The concept of flexible operations entails meeting the demands of the grid in conjunction with the given generation mix. For instance, if the sun is shining, conventional generation reduces power to make way for solar power.
The California Independent System Operator (CAISO) has identified the changes expected over the coming years and developed what they refer to as the “Duck Chart” (see CAISO, Flexible Resources Help Renewables - Fast Facts). The chart shows the ramping capabilities of the grid necessary to maintain stability in light of the renewable resources being brought online.
According to CAISO’s Duck Chart, on a day in 2020, the net load, or the need for conventional generation, is only estimated to be 12,000 megawatts (MW) during the peak of the day when solar is most prevalent and 60 percent of generation comes from renewables. However, only five hours later, 13,000 MW of conventional generation will need to be brought online. This highlights the need for flexible operation from conventional generators. It is highly likely that similar scenarios will play out across the country within the next decade. In fact, in some unregulated markets, it already is.
In markets such as PJM and ERCOT, over-generation of renewable energy occurs when renewable generators keep producing even when utilities have no demand for the energy. This drives electricity prices near or below $0 for parts of the day, radically altering pricing structures for utilities. Some companies now use a time-dependent pricing model, where electricity prices change depending on the time of day and electricity demand. For example, as reported in The New York Times in November 2015, TXU Energy gives electricity away for free between 9 p.m. and 6 a.m. due to the prevalence of wind in Texas. While this is an extreme example, the time-of-use model is being implemented across the nation.
In the PJM, renewable growth is just beginning. Over the past several years, power prices have routinely been driven into negative territory during the night and renewables only make up a small fraction of the overall installed capacity. A study carried out in 2014 for PJM points out that the PJM territory could handle up to 30 percent renewables penetration (see PJM Renewable Integration Study, Prepared by: General Electric International, Inc.). However, if all of the states in the independent system operator (ISO) meet their Renewable Portfolio Standards, the generation mix would be 14 percent renewables or 40,190 MW. The study showed that flexibility is a must and recommended that the PJM “investigate possible methods for improving ramp rate performance.”
In the midst of this revolution, the nuclear energy industry continues to offer one of the cleanest and most reliable forms of energy available. Nuclear power has long been the workhorse of the grid, reliably churning out megawatt after megawatt, day after day and year after year. However, beginning with the Kewaunee Power Station shut down in 2012, the nuclear industry has suffered the closure of several facilities mostly due to economic unbalances in the energy market. The industry is working to tackle this challenge, but we cannot continue business as usual. We must meet the demands of tomorrow’s electricity grid.
Considering that variable output is a necessity for the future, we need to determine how to transition a baseload plant to operate flexibly. We need to take multi-billion dollar assets like our nuclear power plants and move them to the grid of tomorrow. That grid will offer a cleaner, more reliable, more efficient and more flexible way of delivering power. However, in order to remain relevant, nuclear power plants must incorporate flexible operations. To date, this is territory where few plants in the United States have ventured.
Perspectives on flexible operations
Flexible operations can be thought of from four different perspectives: daily load following, frequency control, spinning reserves and extended low-power operation (ELPO). While frequency control and spinning reserves are technically feasible for a nuclear power plant, they are operations that need to be controlled automatically by someone other than a licensed reactor operator, which are not allowed to conduct these actions per the U.S. Nuclear Regulatory Commission’s (NRC) conditions of licenses, 10CFR 50.54.
Daily load following provides power adjustments to match daily fluctuations in load and generation. This is the most traditional form of flexible operations since initial plant designs intended for some amount of load following to occur.
The last form of flexible operations is ELPO, which is intended to prevent oversupply during seasonal power imbalances. ELPO may be used during the spring in regions where the spring melt brings plentiful hydro power or during spring and fall when winds are plentiful and electricity demand is low.
Flexible operations implementation
With a firm understanding of the kind of performance necessary, a whole plant evaluation is required---assessing everything from fuel pellet to switchyard and programs to operating procedures. Implementing flexible operations for a nuclear power plant is carried out in much the same way as a power uprate, only going in the opposite direction. What happens when components are run at 60 percent of their capacity? What happens when components are cooler? How does the fuel respond? Do inspections need to be carried out more or less frequently? What does this do to the schedule for refueling outages? These are just a few questions when an operator begins considering flexible operations.
Starting from the fuel and working out, there are a variety of savings that operators can realize and impacts that they need to address. First of all, fuel savings benefits can be in the form of reduced enrichments and fuel assembly quantity or the ability to extend the length of the cycle. In the case of AREVA’s GAIA fuel design for pressurized water reactors (PWRs), additional fuel can be loaded in each assembly, thereby offering the flexibility to extend the cycle even further.
Another technological advancement that benefits the implementation of flexible operations is doping pellets with chromium. Pellet-clad interaction (PCI) is an area that must be evaluated for flexible operations; however, with chromium-doped pellets, the PCI behavior is significantly improved, reducing the risk of fuel rod failures. So, whether it allows for a longer cycle and fewer outages or less fuel, operators could save millions of dollars in fuel investments by implementing flexible operations.
Taking a step outside the core to the rest of the nuclear steam supply system (NSSS), the operator will need to complete re-evaluations of the NSSS and safety analysis. New plant conditions to meet grid demand must be evaluated in compliance with the NRC Standard Review Plan, Chapter 15: Safety and Transient Analysis and NSSS Loading Analysis. Other impacts specific to PWRs relate to reactor coolant system dilution capabilities and volumetric radwaste throughput due to reactivity changes, particularly at the end of the cycle.
Reactivity control in PWRs, as accomplished through control rods, also can be modified. The current design across the U.S. fleet uses a combination of materials that have a large effect on reactivity; these are referred to as black rods. As a result of inserting black rods, the power distribution of the core is changed dramatically, which is ideal for baseload operation. However, another type of control rod is available, a grey rod, which allows for faster changes in power without causing large power distribution changes across the core.
In the case of boiling water reactors, reactivity is controlled through the use of recirculation pumps, which offer a means of rapid power change without a significant effect on the power distribution of the core. The use of recirculation pumps is a suitable means of power regulation between 65 and 100 percent. Control rods can be moved for maneuvering operations, but as with PWRs, they impart a large change in the power distribution of the core. They are typically only moved to compensate for slower changes, such as xenon transients, which impact reactivity.
Moving into the turbine island and balance of plant, the greatest areas for improvement become apparent. Valves and pumps that are designed to be operated at 100 percent most of the time may cause issues at reduced flows. In the case of pumps, as flow is reduced below 80 percent of the best efficiency point, increased pump vibration issues may be experienced. A similar situation can be observed with check valves, which are set to be operated at 100 percent. As flow decreases, excessive chatter may be observed. In both cases, modifications are available that would alleviate these problems. In the case of secondary side pumps, variable frequency drives adjust flow to demand and can easily be installed to reduce wear on the system.
Each plant program must also be evaluated for impact as well. This includes impacts to chemistry, life cycle management, plant life extension and flow accelerated corrosion, just to name a few. In the case of the Electric Power Research Institute’s (EPRI) Water Chemistry Guidelines, it is assumed that the plant is in baseload operation. EPRI formed a technical advisory group to begin evaluating potential flexible operations issues in recognition of the needs of the future. A thorough investigation of each program is necessary, noting that flexible operations is a whole plant change.
Due to the breadth and depth of implementing flexible operations, this process offers a great opportunity to add handling ability and power under the hood. Implementing power uprates and flexible operations both require the same evaluations and considerations. Incorporating both simultaneously allows the plant to increase income during the dog days of summer, but hedge losses during the cool autumn nights when the load is low and wind power may ramp up.
There is ample evidence that the flexible operations strategy, as applied to nuclear power facilities, works. The French and German fleets have been operating flexibly since the 1980s. In the case of France, where nearly 80 percent of the country’s power comes from nuclear, the concept of flexible operations is a necessity to meet the daily changes in demand. Over the course of time, the French nuclear industry compiled a variety of lessons learned from monitoring and programs to fuel improvements. In a fleet where plants operate daily between 30 and 100 percent, such experience is invaluable.
As we come to terms with the new landscape of the U.S. grid of tomorrow, we have to realize we are no longer in Kansas, but put on our ruby slippers and head toward Oz. The nuclear energy industry is capable of rising to the occasion and continuing to serve as the backbone of the grid of tomorrow with flexible operations.