MYTH: WIND CAPACITY FACTOR DETERMINES THE NUMBER OF HOURS WIND TURBINES STAND STILL

it is commonly heard that wind turbines stand still about 60 to 70 percent of the time; .this is a myth that is often used to discredit wind energy and it is therefore useful to debunk it.  

This myth refers to the fact that most wind farms have a capacity factor between about 30 and 40 percent, implying that the rest of the time the wind turbines stand still. This is not the case. The capacity factor is defined as the energy produced over a given period of time divided by the energy produced IF the generator would have run at full rated capacity during that whole same period of time. This formula is true for a wind turbine, but also for a hydro, fossil fueled or nuclear power plant (more on that later).   Here is an example for a wind turbine. Let's say the turbine has a capacity of 1,500 kW and that power output is reached when the wind is blowing at 25 mph at the hub height. For wind speeds less than that, the power output is less. Let's say that the power output at 15 mph is 500 kW, one third of the fully rated capacity.   Now, let's say that for a full year the wind blows constantly at 15 mph, that is, all 8760 hours of the year at this wind speed. So the power output throughout the whole year would also be constant, and in this example 500 kW.   So, what is the capacity factor? The actual energy produced is 500 kW x 8760 hours = 4,380,000 kWh = 4,380 MWh. The energy produced IF the generator would have run all hours at the fully rated capacity is 1,500 x 8760 hours = 13,140,000 kWh =13,140 MWh. Divide the two numbers and one gets 0.30 as the capacity factor (also expressed in percentage as 30%).   Thus, while the wind is blowing 100 percent of the time, the capacity factor is 0.3 or 30 percent.  

To read up on this a bit more, please see the tutorial: http://www.windpower.org/en/tour.htm  

The other implied assumption is that the utilities' other generators are working close to full time, which is not the case either.  Utilities use baseload units (running almost all of the time, except for maintenance and forced outages, and refueling for nukes; they may have a capacity factor of 85% or so) , intermediate and peak load units which run significantly less hours in the year (and have thus a significantly lower capacity factor possibly equal to or lower than a wind farm). But those units are nevertheless needed to rapidly increase their output to meet peak load (base load units can not "ramp up" like that).  For a useful discussion of the variability of wind farms and integration in the utility system, see chapter two in the attachment which starts on page 19 of the PDF file.

Carel C. DeWinkel, Ph.D.
Senior Policy Analyst
Renewable Energy Division
Oregon Department of Energy