Tech Breakdown: What is Electricity Curtailment?

By Kristen Abels

Managing the supply and demand of electricity is a tricky process. As mentioned in March’s Employer Spotlight article, the IESO is responsible for forecasting electricity demands for the entire province of Ontario in five-minute intervals to coordinate supply and demand. This role is vital to avoid power outages (brownouts and blackouts) resulting from a generation shortage, and also crucial to avoid excessive power generation.  

This coordination is challenging given that:

  1. Electricity demands fluctuate throughout the day, week, and year, as shown in Figure 1.

  2. Certain electricity generation sources – namely intermittent renewable sources like solar and wind – are less reliable for consistent electricity generation.

  3. Other electricity generation sources such as nuclear and natural gas combined cycle power plants are less amenable to power output fluctuations (see our article on Electricity Generation to understand why!); instead, they operate at a fairly constant baseload.

Figure 1. Example of Ontario hourly and weekly electricity demand fluctuations throughout a mild spring week (April 2017) and a warmer summer week (July-August 2017). Surplus baseload generation is shown in grey. Original image can be found here.

Figure 1. Example of Ontario hourly and weekly electricity demand fluctuations throughout a mild spring week (April 2017) and a warmer summer week (July-August 2017). Surplus baseload generation is shown in grey. Original image can be found here.

These are just some of the factors that make it challenging to exactly coordinate electricity supply and demand at all times. In cases where supply exceeds demand, this can lead to curtailment. 

“What is curtailment?”, you may be wondering. Consider again the wide fluctuations in electricity demand in Ontario throughout the day, the week, and the year (Figure 1). Typically, generating stations like nuclear power plants are able to supply a fairly constant baseload, whereas natural gas peaking plants and even renewables like hydro are able to supply the extra electricity during peak hours. Unfortunately, this means that during periods of low demand, the generating capacity from renewables may not be needed beyond the baseload, leading to electricity exports or curtailment. Physically, curtailment is spilling water at hydro stations, turning down or turning off renewables like wind or solar, or, in the worst case, bypassing steam around turbines in nuclear generating stations. As the Ontario Energy Commissioner says, these are cases where ‘potential electricity production with zero marginal cost goes unused or is “wasted”’ [1]. Figure 2 shows the trends in curtailed electricity in Ontario from 2011-2017, with the majority coming from hydro, followed by wind and solar.

Figure 2. Ontario electricity curtailment trends and breakdown from 2011-2017. Original image can be found here.

Figure 2. Ontario electricity curtailment trends and breakdown from 2011-2017. Original image can be found here.

The Ontario Society of Professional Engineers recently reported that Ontario’s 2019 curtailed energy amounted to 6.5 TWh of clean electricity, bringing the six-year total to 38.5 TWh [2]. The 2019 curtailment alone represents enough energy to power more that 650,000 homes over an entire year and amounts to a $1 billion loss in electricity [3]. With intermittent renewable source capacities continuing to grow in Ontario and around the world, the issue of excess electricity and curtailment - as well as the significant opportunity for improvement that these issues create - will no doubt grow in the years to come. How might we take advantage of this opportunity and harness this excess electricity? Energy storage? Electricity to fuels? Only time will tell!

References

[1] Environmental Commissioner of Ontario, "Why does Ontario export and curtail so much electricity?," 2018. [Online]. Available: http://docs.assets.eco.on.ca/reports/energy/2018/Making-Connections-07.pdf. [Accessed 14 February 2021].

[2] Ontario Society of Professional Engineers, "Ontario Wasted Enough Clean Electricity to Power 720,000 Homes in 2019: OSPE Data," 3 September 2020. [Online]. Available: https://ospe.on.ca/advocacy/ontario-wasted-enough-clean-electricity-to-power-720000-homes-in-2019-ospe-data/. [Accessed 14 February 2021].

[3] Ontario Society of Professional Engineers, "Ontario Wasted More Than $1 Billion Worth of Clean Energy in 2016," 29 June 2017. [Online]. Available: https://ospe.on.ca/advocacy/ontario-wasted-more-than-1-billion-worth-of-clean-energy-in-2016-enough-to-power-760000-homes/. [Accessed 14 February 2021].

Kristen Abels