Energy Bites: What's the Deal with Capacity Factors?
I have an array of solar panels in my backyard. The nameplate capacity of the array is 3.6 kilowatts. That means that if the sun is shining on the array at just the right angle, the solar panels can put out 3,600 watts of electricity.
If the sun shone on the array at just the right angle for an hour, the array would generate 3.6 kilowatt-hours of electricity over the course of that hour.
If the sun were to shine on the panels 24 hours a day for 365 days, they would produce 31,536-kilowatt hours. Of course, that’s not what happens in my corner of Vermont. In a typical year, my panels produce about 3,400-kilowatt hours of electricity. The ratio of those two numbers 3,400/31,526 = 11% is the “capacity factor” of my system.
My capacity factor would be larger if the panels weren’t shaded in the morning by a hill to the east. It would be larger if my panels tracked the sun across the sky. It would be larger if the weather around here were sunnier. It would be larger if I were more prompt about clearing March snows off the panels. (The lengthening days and light-reflecting snow cover make March a good month for solar.)
Each electricity generation technology—wind, solar, hydro, nuclear, etc.—has its own characteristics that influence capacity factor. And each individual installation has its own unique influences.
For example, the turbines in Lowell, Vermont rely on winds that are unique to that location. Those winds are not alone in determining the turbines’ capacity factor.
You might remember a time when the Lowell turbines were “curtailed” (i.e., shut down) because they endangered the stability of the grid. Curtailment lowered the output (and the capacity factor). Green Mountain Power responded by installing a $10.5M synchronous condenser that enabled the turbines to play nicer on the grid.
But, even something as glamorous as a synchronous condenser cannot eliminate the transmission system limitations that still cause Lowell’s production to be curtailed occasionally.
Equipment breakdowns and maintenance also reduce output and lower the capacity factor. As the Lowell turbines age, these interruptions will become more frequent. They are already quite frequent among the turbines in nearby Sheffield. The relative unreliability of the Sheffield turbines may explain why they struggle with a capacity factor near 20% while the Lowell turbines operate closer to 30%.
Here are capacity factors for the 12-month period ending on October 31, 2022 for some of Vermont’s wind complexes. I’ve listed them in order of age, oldest first.
Our experience watching the output of Vermont’s turbines suggests that operators need a few months to work the bugs out of a new facility. Then there is a period during which the equipment performs well. Then the equipment starts to age, break down, and require more frequent repairs.
An additional impediment to turbine operations is noise. Anybody that lives near a wind turbine can tell you that they are noisy. In Vermont, turbines operate under noise rules that are intended to protect neighbors. (Turbine neighbors tell us that the rules are too loose, unenforced, and definitely not protective.)
When conditions (like ice on the blades) create a noise hazard, the operators are supposed to shut the turbines down. This lowers output, income, and capacity factors.
Some operators haven’t wanted to comply with the noise rules. This has resulted in lengthy and expensive efforts by neighbors to get regulators to enforce the rules.
If you don’t live near turbines, be grateful—they aren’t good neighbors (check out the Vermont Neighbors Project).
Stay tuned for more on a variety of environmental and energy-related topics. And as the legislative session picks up, Becca will tell us what’s going on in Montpelier.
Mark Whitworth, President of Energize Vermont