|
|
| Fig. 1: The load and generation characteristics of the Bonneville Power Administration Grid, showing variations in the load and power supplied. (Courtesy of the Bonneville Power Authority) |
Power required from electrical grids is not constant with time, and all commercial scale grids regulate the power generated with the power demanded to maintain stable grid frequency and voltage. The power generating abilities of a grid can be roughly divided into the general categories (1) base-loading power plants (e.g. coal fired power plants) operate close to full power output constantly to maintain the minimum power demands of a grid and (2) peaking power plants (e.g. natural gas-burning plants) are brought on and off-line to accommodate changing power demands above the demand minimum and operate with "load-following" capabilities. [1,2]
Nuclear power plants (NPPs) have historically been used as base-loading power plants for both technical and economic reasons. Technical issues of load-following NPPs stem from the complex requirements of balancing neutron populations in the reactor core, maintaining heat removal from the core, and designing structural components that can survive temperature variations. [3,4] NPPs also have relatively high initial capital costs (U.S. NPPs are estimated to cost $6041/kW), but have low fuel costs, so NPPs can deliver less expensive electricity when their load factors, the average load divided by the maximum load, are kept high. [5-7] Despite these potential issues, interest and experience in operating NPPs with load-following capabilities has grown. [2]
Load-following NPPs are desirable for electric grids where NPPs make up a large share of the power generation, such as France where 70.6% of domestic power generation is from NPPs. [8] Additionally, load-following is useful in grids where intermittent power generation makes up a large component of the grid. An example of this comes from Germany. Germany operated NPPs with load-following capabilities to balance power production from the large but intermittent wind production of the German grid. [9,10] Though the German example of load-following is becoming outdated with the phasing out of NPPs in Germany, operating NPPs in other grids today have various degrees of load-following capabilities and requirements. [11,12]
NPPs in the United States operate almost exclusively in base-load operation. A large exception is the Columbia Generation Station (CGS, 1170 MWe, Richland, Washington). This plant will perform relatively large power shifts to adjust for variations in hydroelectric output during seasons of high or low water levels. CGS load-following is controlled manually, meaning changes in power output must be requested by the grid operator. CGS capabilities require at least 12 hours notice to reduce power output to 85% of full power, and at least 48 hours notice to reduce power to 65% of capacity. [2,13] These abilities are termed "load-shaping" by CGS to differentiate with the more rapid power output shifts associated with load-following peaking plants. [13] CGS is connected to the Bonneville Power Administration grid, and an example of load changes in this grid is shown in Fig. 1. Fig. 1 shows that rapid load following is primarily handled with variations in hydroelectric power output, with NPPs providing a component of the base-load power. In events where hydroelectric power output is limited however, load-following capabilities can be picked up by NPPs.
France has arguably the most experience of any nation with load-following nuclear plants. With NPPs making up 70.6% of French domestic power generation, the state grid operator, Électricité de France, has been pushing development of relatively rapid load- following since the 1970s. Load-following NPPs in France claim power output ramps as much as 5%/min if necessary, though typical ramps are kept below 1.5%/min. [14]
Certain French NPPs routinely decrease power output 50% at night. Despite these impressive abilities, France still imports a significant amount of power during periods of high demand, such as weekends. [15]
The EU has general requirements on NPP load-following. The European Utilities Requirements, an agreement between the grid operators of NPPs in England, France, and Germany, set requirements for modern NPPs as the following: [2]
NPPs should have continuous operating capabilities between 50% and 100% rated power.
Power ramps should be able to reach 3%/min in the operating range.
NPPs can go through full load variations (full power to minimum continuous power back to full power) at a maximum frequency of 2× per day, 5× per week, and 200× per year.
NPPs can perform primary frequency control of electrical grids. This means that NPPs can adjust power rapidly to keep the grid frequency of 50 Hz stable. Rapid here means power output can change within ± 2% of current operating power within 30 seconds of a detected frequency variation.
Modern NPP load-following capabilities are actually similar to the rated abilities of coal fired power plants. Both have power ramp rates on the order of 1-5%/min and start up times to stable full power on the order of a day to multiple days. These load-following capabilities are much slower than that of gas turbine generators that have start up times less than 1 hour and can ramp power at 10-20%/min. [10]
NPPs operating with some ability to load-follow will almost certainly remain a relevant topic of research and policy discussion as renewable power sources penetrate electric grids, reducing the need for fossil fuels but potentially making power generation more intermittent. [2] Though great operational and technical advancements have been made on the load-following capabilities of NPPs, the tricky issue of the economic benefits of load-following remain. There is near universal agreement that the most cost effective way to generate power from a NPP, with it's high capital investment costs and low fuel costs, is to operate the NPP at near full power output constantly. [2]
© Garrett LeCroy. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "United States Electricity Industry Primer," U.S. Office of Electricity Delivery and Energy Reliability, DOE/OE-0017, July 2015.
[2] "Technical and Economic Aspects of Load Following with Nuclear Power Plants," Nuclear Energy Agency, June 2011.
[3] J. H. Bickel, "Grid Stability and Safety Issues Associated with Nuclear Power Plants," Evergreen Safety and Reliability Technologies, 14 May 01.
[4] L. Yang, "Interfacing Nuclear Power Plants With Electric Grids," Physics 241, Stanford University, Winter 2016.
[5] G. P. Watkins, "A Third Factor in the Variation of Productivity: The Load Factor," Am. Econ. Rev. 5, 753 (1915).
[6] "Capital Cost and Performance Characteristic Estimates for Utility Scale Electric Power Generating Technologies," U.S. Energy Information Administration, February 2020.
[7] K. Wang, "Nuclear Power Economic Cost," Physics 241, Stanford University, Winter 2018.
[8] M. Schneider and A. Froggatt, "The World Nuclear Industry Status Report 2020," Mycle Schneider Consulting, September 2020.
[9] J. D. Jenkins et al., "The Benefits of Nuclear Flexibility in Power System Operations With Renewable Energy." Appl. Energy 222, 872 (2018).
[10] "Nuclear Energy and Renewables: System Effects in Low-carbon Electricity Systems," Nuclear Energy Agency, NEA No. 7056, 2012.
[11] L. Kramm, "The German Nuclear Phase-Out After Fukushima: A Peculiar Path or an Example for Others?" Renew. Energy L. Policy Rev. 3, 251 (2012).
[12] M. Mitchell, Germany's Nuclear Power Phase-Out Post-Fukushima," Physics 241, Stanford University, Fall 2018.
[13] D. T. Ingersoll et al., "Can Nuclear Power and Renewables be Friends?" 2015 International Conference on Advances in Nuclear Power Plants (ICAPP), Paper 15555, 3 May 15.
[14] J. Persson, et al., "Additional Costs for Load-Following Nuclear Power Plants," Elforsk, Report 12:71, December 2012.
[15] R. Iskhakov, "French Nuclear Energy," Physics 241, Stanford University, Winter 2014.