Thursday, November 13, 2008
Better Wind Turbines
A more efficient generator could convert more of the wind's energy
By Kevin Bullis
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.
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
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
The article doesnt make a lot of sense.
if the generator is going to spin over a wide RPM range as the wind
speed varies, then it cant be directly connected to the grid as it
wont be synchronous with the grid frequency.
This means the output has to be rectified to DC , and then re inverted
back to AC .
Some generator designs do this anyway, and simply by controlling the
Inverter you can control the generator loading to maximise power
output at any wind speed anyway.
You dont need a fancy generator to do this.
You cant get more power out of a windmill by playing around with the
generator as the output is solely determined by the rotors swept area
and the Betz limit.
This article didn't make a lot of sense. It talks about reducing the number
of coils in order to get more power out of a wind generator.
It talks about a lower efficiency in light winds and of some magic to
increase the efficiency in those conditions. At lower speeds you would have
a problem with voltage being low. If you're using simple field regulation
to maintain voltage, you *would* have more losses as you have to increase
field power to maintain a given terminal voltage. But cutting out more
coils would exacerbate the problem. If anything, you would want to switch
in *more* coils, so that at the lower RPM you can still generate a
reasonable voltage without excessive field power requirements.
It is true that many generators 'cut out' at low wind speeds because it just
gets too difficult to keep the generator functioning (low voltage problems).
And almost any generator tied to the grid deals with syncrhonization issues
(either gearing, inverter, or even the wound-rotor re-injection design).
But there is very little *energy* available in these light winds, so getting
a generator to be able to stay on-line at the lower speeds usually isn't