Thursday, November 13, 2008

Better Wind Turbines

A more efficient generator could convert more of the wind's energy into electricity.

By Kevin Bullis http://www.technologyreview.com

Power housing: Copper wiring stretches from a prototype of a new kind of generator, which was designed with the variability of wind speed in mind. Credit: Exro Technologies

ExRo Technologies, a startup based in Vancouver, BC, has developed a new kind of generator that's well suited to harvesting energy from wind. It could lower the cost of wind turbines while increasing their power output by 50 percent.

The new generator runs efficiently over a wider range of conditions than conventional generators do. When the shaft running through an ordinary generator is turning at the optimal rate, more than 90 percent of its energy can be converted into electricity. But if it speeds up or slows down, the generator's efficiency drops dramatically. This isn't a problem in conventional power plants, where the turbines turn at a steady rate, fed by a constant supply of energy from coal or some other fuel. But wind speed can vary wildly. Turbine blades that change pitch to catch more or less wind can help, as can transmissions that mediate between the spinning blades and the generator shaft. But transmissions add both manufacturing and maintenance costs, and there's a limit to how much changing the blade angle can compensate for changing winds.

ExRo's new design replaces a mechanical transmission with what amounts to an electronic one. That increases the range of wind speeds at which it can operate efficiently and makes it more responsive to sudden gusts and lulls. While at the highest wind speeds the blades will still need to be pitched to shed wind, the generator will allow the turbine to capture more of the energy in high-speed winds and gusts. As a result, the turbine could produce 50 percent more power over the course of a year, says Jonathan Ritchey, ExRo's chief technology officer. Indeed, in some locations, the power output could double, says Ed Nowicki, a professor of electrical engineering at the University of Calgary, who has consulted to ExRo.

The generator works on the same principles as many ordinary generators: magnets attached to a rotating shaft create a current as they pass stationary copper coils arrayed around the shaft. In ordinary generators, all of the coils are wired together. In ExRo's generator, in contrast, the individual coils can be turned on and off with electronic switches. At low wind speeds, only a few of the coils will switch on--just enough to efficiently harvest the small amount of energy in low-speed wind. (If more coils were active, they would provide more resistance to the revolving magnets.) At higher wind speeds, more coils will turn on to convert more energy into electricity. The switches can be thrown quickly to adapt to fast-changing wind speeds.

Inside look: Two rings of magnets (blue-gray), one within and the other outside a series of copper coils (red and green), turn to generate electricity. Credit: Exro Technologies

Another part of the design makes the generator more responsive to changing wind speeds. Harvesting large amounts of energy requires many coils. These could be arranged inside a very-large-diameter generator, but then the rotor on which the magnets were mounted would have to be larger, too. That would make it harder to get the rotor moving, or to change its rotation speed. (The greater distance between the center of the generator and the coils increases what's known as the moment of inertia.) The ExRo generator instead distributes the coils among several small-diameter generators--which the researchers call stacks--along the length of the shaft. Smaller diameters make it easier to change rotational speeds. The multiple-stack design also makes customizing the generator for a particular wind site easier. For a site with low-speed winds, few stacks would be needed. For a site with high-speed winds, more could be added, allowing the generator to convert more energy into electricity.

Other companies have developed designs that incorporate multiple generators, which can be activated separately, depending on wind speed. But these have to be engaged and disengaged mechanically, adding weight and complexity to the generator and increasing costs. Reducing maintenance and weight by eliminating the need for mechanical gears and clutches could allow ExRo to keep costs down. And that, says Paul Sclavounos, a professor of mechanical engineering at MIT, is the key consideration in determining whether to try to capture more of the wind's energy. ExRo may have an advantage, he says, because the key to its technology is electronic control, which is inexpensive. Indeed, the company claims that a wind-turbine operator could make 57 percent more money from a turbine over the course of a year by using the new generator.

ExRo has developed and tested a lab-scale prototype. Its estimates of increased power production come from models that use data from existing wind-turbine sites. By the end of this year or early next year, the company will begin field-testing a small, five-kilowatt wind turbine. Ritchey says that the company won't have firm figures for power production until those tests are complete. The next step will be to install larger, megawatt-scale generators in existing wind turbines.