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.
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.