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Load Balancing

The supply and demand of power on a grid must be nearly identical at all times. This is a challenge regardless of the source of electricity, and the challenge compounds with large sources of intermittent energy.

Integration Costs

Major sources of electricity have the following grid integration costs, though they vary widely depending on local grid conditions.

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Sources: Agora Energiewende 1, Change et al. 2, Hirth et al. 3, Nuclear Energy Agency 4.

The main source of integration costs for wind and solar is transmission, as power plant sites tend to be far from demand centers. At current renewable levels, load balancing costs are smaller 1.

As renewable penetration grows, electricity prices are depressed at the times when renewables are most productive, hurting their economics.

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Illustration of value deflation, or the phenomenon by which high renewable penetration hurts renewable economics. Source: MIT 5.

Until better grid management options, such as storage or supergrids are deployed, value deflation is an economic barrier to the growth of renewables.

Dispatchable Power

Dispatchable power plants are those whose production can quickly be ramped up or down as needed. Such flexibility is needed on a power grid, especially in the presence of variable renewable generation 6. Natural gas, especially simple cycle, and hydropower are the most flexible dispatchable power options.

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Ramping rates, reported as the percentage of maximum power that a power plant can increase or decrease production per minute, and cold start time, reported as the number of minutes it takes for a power plant to go from being shut down to full power. Sources: DeHaan 7, Nuclear Energy Agency 4, Pescia 8.

A rule of thumb is that every megawatt of wind capacity requires a megawatt of dispatchable backup 9. Simple cycle gas plants are less efficient than combined cycle plants, and the act of cycling a baseload plant reduces efficiency, and so high renewable penetration balanced by thermal plant cycling may negate some of the expected greenhouse gas savings 10. Nuclear plants are generally not run in a flexible manner, due to high capital costs and low fuel costs, but they can be built or retrofitted for greater flexibility 11.

Curtailment

At times of negative wholesale electricity prices, curtailment of power sources may be necessary. Due to the expense of shutting down and restarting fuel-burning plants, renewables sources are more likely to be curtailed 9. Following are estimates of curtailment rates in leading renewable grids 12, 13, 14.

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Rates of curtailment and share of variable renewable on the power grid for select regions. Sources: California ISO 12, Joos and Staffell 13, and Ye et al. 14. China's high curtailment rate is a result of poor planning, not a fundamental limit on renewable penetration 14.

By overbuilding solar and curtailing 20-40% of generation, it is possible to convert solar power into a firm source, and this may be a cheaper solution to intermittency than storage 15. It is estimated that wind power can grow to as much as 50% of peak grid capacity before curtailment cuts deeply into wind power profitability 16.

Additional Load Balancing Strategies

Demand response is a practice of using real-time price signals or other methods to regulate electricity demand. With achievable participation rates, demand response could reduce peak electricity demand by 138 GW, relative to the 950 GW peak demand with no demand response 17. A demand response program could increase the economic solar carrying capacity in Florida by 0.5-2% of total electricity 18. The usefulness of demand response in renewable integration is limited by the fact that it can shift demand by only a few hours 19.

Geographic dispersion of wind energy can reduce volatility by up to 70% 20. When wind farms in a grid are dispersed over 100 kilometers, volatility on the scale of hours drops off rapidly 16. Combining multiple intermittent sources (solar, wind, wave, etc.) further reduces volatility 16.

Two other major load-balancing strategies are changing the structure of the grid and energy storage.

Problem:
Need for Load Balancing
Solution:
Demand Response

References

  1. Agora Energiewende. "The Integration Cost of Wind and Solar Power. An Overview of the Debate on the Effects of Adding Wind and Solar Photovoltaic into Power Systems". December 2015. ↩ ↩2

  2. Change, J., Madjarov, K., Fox-Penner, P., Hanser, P. "Policy Challenges Associated with Renewable Energy Integration". The Brattle Group. April 2011. ↩

  3. Hirth, L., Ueckerdt, F., Edenhofer, O. "Integration costs revisited – An economic framework for wind and solar variability". Renewable Energy 74, pp. 925-939. February 2015. ↩

  4. OECD Nuclear Energy Agency. "Nuclear Energy and Renewables: System Effects in Low-carbon Electricity Systems". 2012. ↩ ↩2

  5. Massachusetts Institute of Technology. "The Future of Solar Energy". May 2015. ↩

  6. National Renewable Energy Laboratory. "Solar Power and the Electric Grid". March 2010. ↩

  7. DeHaan, J. "Renewable Integration and Small Hydro". U. S. Department of the Interior, Bureau of Reclamation, Research and Development Office. October 2013. ↩

  8. Pescia. D. "Flexibility in thermal power plants: With a focus on existing coal-fired power plants". Agora Energiewende. June 2017. ↩

  9. Union of the Electricity Industry. "Integrating intermittent renewables sources into the EU electricity system by 2020: challenges and solutions". 2010. ↩ ↩2

  10. International Energy Agency Clean Coal Centre. "Integrating intermittent renewable energy technologies with coal-fired power plant". November 2011. ↩

  11. International Atomic Energy Agency. "Non-baseload Operation in Nuclear Power Plants: Load Following and Frequency Control Modes of Flexible Operation". IAEA Nuclear Energy Series No. NP-T-3.23. 2018. ↩

  12. California ISO. "Managing oversupply". Accessed August 20, 2019. ↩ ↩2

  13. Joos, M., Staffell, I. "Short-term integration costs of variable renewable energy: Wind curtailment and balancing in Britain and Germany". Renewable and Sustainabile Energy Systems 86, pp. 45-65. April 2018. ↩ ↩2

  14. Ye, Q., Jiaqi, L., Mengye, Z. "Wind Curtailment in China and Lessons from the United States". Brookings-Tsinghua Center for Public Policy. March 2018. ↩ ↩2 ↩3

  15. Perez, M., Perez, R., Rábago, K., Putnam, M. "Overbuilding & curtailment: The cost-effective enablers of firm PV generation". Solar Energy 180, pp. 412-422. March 2019. ↩

  16. Hart, E., Stoutenburg, E., Jacobson, M. "The Potential of Intermittent Renewables to Meet Electric Power Demand: Current Methods and Emerging Analytical Techniques". Proceedings of the IEEE. February 2012. ↩ ↩2 ↩3

  17. Federal Energy Regulatory Commission. "A National Assessment of Demand Response Potential". Prepared by the Brattle Group; Freeman, Sullivan & Co.; Global Energy Partners, LLC. June 2009. ↩

  18. Hale, E., Stoll, B., Novacheck, J. "Integrating solar into Florida's power system: Potential roles for flexibility". Solar Energy 170, pp. 741-751. August 2018. ↩

  19. Müller, T., Möst, D. "Demand Response Potential: Available when Needed?". Energy Policy 115, pp. 181-198. April 2018. ↩

  20. Sovacool, B. "The intermittency of wind, solar, and renewable electricity generators: Technical barrier or rhetorical excuse?". Utilities Policy 17 pp. 288-296. ↩