Friday, August 27, 2010

How Much Power Does It Take to Run a Wind Turbine?

Jerry Graf is a concerned citizen who happens to have a Master’s degree in mechanical engineering. He recently read an item in his local paper about a school district that contemplated investing over a quarter million dollars in roof-top wind turbines. “There was a quote from someone in charge of school facilities that indicated they had no idea how much electricity the turbines would actually generate. As I got further into the details myself, I realized the turbines would actually produce next to nothing,” he says.

The incident got Graf interested in analyzing other wind installations. He uses the turbine maker’s published power curve to figure out what the electrical output is likely to be. The power curve is just the turbine’s electrical output plotted against wind speed. But these curves are just estimates. They don’t account for one factor that can be important, particularly for megawatt-scale turbines: the amount of electrical power the turbine itself consumes.

Big turbines often incorporate rechargeable batteries or ultracapacitors to power their own electrical systems. When those get depleted, the power must come from the grid. This power goes into running equipment such as yaw mechanisms that keep the blades turned into the wind; blade-pitch controls that meter the spinning rotor; aircraft lights and data-collection electronics; oil heaters, pumps, and coolers for the multi-ton gearbox; and hydraulic brakes for locking blades down in high winds.

Turbines in northern climes also need blade heaters to prevent icing. Reports I’ve seen say these heaters can consume up to 20% of a turbine’s rated power output. Many big turbines also need dehumidifiers and heaters in their nacelles. And until recently, large turbines employed doubly-fed induction generators that bleed power from the grid to create their magnetic fields. (It should be said, though, that designs now on the drawing boards use permanent magnets instead.)

Instances of low or no wind pose another problem. Large turbines may need to use their generators as motors to help get the blades turning. And some wind skeptics have posed a question about the direct-drive turbines now emerging from the labs: Large ships frequently must expend energy to slowly turn their heavy driveshafts when at port to prevent them from sagging. Could the same be said of these superlarge wind turbines?

Wind-farm operators don’t say much about turbine-power demands. Typically, turbine-power consumption is one of the factors that gets lumped into a wind-farm’s operation and maintenance costs. I’ve never found either a wind-farm operator or a wind-turbine maker willing to discuss these costs. It would not be much of an exaggeration to say the wind industry treats such information as a state secret.

That’s unfortunate. Wind boosters open themselves up to skepticism about their industry’s viability when they don’t disclose real figures about the power their turbines generate and consume. Cynics might be tempted to claim we could reach a point where we’ve commissioned so many wind turbines that we’ll need to build new coal-fired power plants to run them. Without hard data, who’s to say they aren’t right?

No comments: