Capacity Factor – How People Misuse a Number

The concept of capacity factor for generators is one of the most misunderstood concepts in both the public, and in the electricity sector itself.

Capacity factor is really quite a simple concept. It is the percentage of maximum output that a generation facility produces on average. The key words there are “on average”. It does not mean the percentage of time that a facility produces.

For nuclear, the capacity factor would be very similar to the percentage of time that it operates. That is because nuclear basically has two states – on, and off. And this lack of intermediate states actually poses challenges for utility planners, because they have to figure out what to do with power when demand is low. Ontario’s nuclear plants have run about 60% of the time since they were built. Today, 12 out of 20 units are operating. Since they are only ever on, or off, their capacity factor is also about 60%. The nuclear industry argues that refurbishment time should be removed from the equation, and that units that are laid up should be removed. If that method is used, then 12 out of 16 of Ontario’s nuclear stations are operating today, or 75%.

There was a letter from an opponent to wind in the Sun Times that said that wind “only operates 27% of the time.” I have heard Murray Elston, the Chair of the Canadian Nuclear Assocition, and former provincial Liberal cabinet minister say the same thing from the podium at the APPRO conference. Fine. These folks have an agenda, and if using misleading information helps their cause, then so be it.

But in the introduction to the Supply Resources paper the Ontario Power Authority – Ontario’s system planners – perpetrated this error again. “Preliminary studies suggest that wind would be available, on average, 17 percent of the time in summer and 41 percent of the time in winter”.

It is simply untrue to state that wind would be available 17% of the time. Rather, it may produce an average of 17% of its maximum capacity. But achieving this average will be spead over time.

Wind turbines do not have two states – maximum and zero, like nuclear does. Rather, the output from a wind turbine increases as the wind increases. Sometimes it will produce nothing, when it is being serviced, or winds are light. Then it will produce at an increasing level as winds pick up. The average production divided by the maximum production possible is the capacity factor.

By way of example, the Ferndale wind farm has produced 543,000 kWh so far this month. It has been very windy. The maximum that could be produced in the first 9 days is 5100 KW X 24 hours X 9 days = 1,101,600 kWh. So we have produced at an average capacity factor of 49.3%.
But the wind farm has produced power 93% of the time. The power produced ranges between 0 and 5100 kW.

In July, the single V80 produced power 76% of the time. And it produced 297,000 kW, which is a capacity factor of 22%. 297,000/(1800 X 24 X 31) = 22%.

Wind industry people, in particular, need to understand that capacity factor is affected by equipment selection. I have heard many people in the industry talk about a “30% capacity factor site”. This doesn’t exist. All that exists is a 30% capacity factor site for specific equipment. I blogged on this in the past.

I can understand those opposed to wind farms, or nuclear proponents, confusing capacity factor with percentage of time a wind farm operates. But it is inexcusable for this to be confused by utility planners.

If a wind farm only produces 17% of the time in the summer, then backing up this power becomes much more costly, as the back up fossil plant will run 83% of the time, with considerable fuel expense. And that costs a lot. But since it produces 76% of the time, and a diversified wind resource would produce an even larger percentage, the cost of backing up wind drops dramatically, and the technology chosen to back up wind changes.

I hope that our utility planners understand this. It would be helpful if their reports would reflect that understanding.

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