What we are trying to capture, the bottom line, is energy. Energy is a
commodity you can buy at so many cents per kilowatt-hour (kWh) from a
utility.

Energy is captured/produced/used at a certain rate, known as Power.
If you multiply the rate by the time, you get the energy -
kW of power, times hours = kWh.

Bits of wind, floating along with the breeze, carry kinetic energy in
proportion to the weight of the wind, and the square of its speed. If you
get more wind, you get more bits of wind every second, carrying much more
energy per bit, so you get much higher power. Wind power increases with
the cube of the speed. Wind power is measured in kilowatts.

Wind power is converted to shaft power. (Some is lost in the process.)
Shaft power is measured in kilowatts, same as wind power and electrical
power. (Or you can measure it in horsepower if you want to make life
harder for yourself.) Shaft power is the product of two factors, torque
and rotational speed (rpm). Some machines have high torque and low rpm
(pumps etc). Other machines have low torque and high rpm (generators).
Both machine process power.

A generator converts shaft power into electrical power. Some power is lost
with all such conversions. Electrical power is the product of voltage and
current (assuming unity power factor). Rather like speed and torque.

If the wires become disconnected, and the current stops, you break the
chain of power conversion. The rotor does not have any torque slowing it
down. Torque from the wind cranks it faster and faster until it is so
inefficient it produces no torque - just a lot of noise and vibration. If
the wires are short circuited, you may get too much current, too much
torque, and the rotor will stall, and run very slowly, again becoming very
inefficient.

For any rotor there is a best load, which absorbs the right amount of power
at any given speed. A rotor with many wide blades will work best when
delivering high torque at low speed, whereas a slender 2-blader will work
best at higher rpm, giving lower torque. Same power from both. A very
important part of the machine design is to match the rotor to a load which
runs at the right speed.

To capture plenty of energy we need a large rotor. The is especially true
in low winds. If we design a machine for high winds, we may capture high
power from those few hours of winds, but we suffer from poor overall energy
production. This type of wind generator may be very suitable for sites
with a large photovoltaic input, but where we place reliance on wind, we
need to be able to deliver a useful energy output on low wind days. This
places an emphasis on maximising the rotor size.

As the rotor is made bigger, it is also made slower, because the blade tip
has further to travel. matching the rotor to a generator becomes harder.
Most generators need to run fast. Use of gearboxes should be avoided for a
number of reasons. When you start to design a low speed generator, you
find that it is bulky, heavy and expensive.

Wind power is not an easy option, but it is interesting. The wind teaches
lots of lessons. If you want to have reliable power, you need a large,
simple, solidly built machine, with a reasonably low rpm and a large
generator. It's not rocket science. Magnetohydrodynamics is unlikely to
yield a useful result in the near future. Even vertical axis wind turbines
are a bit too far outside of the box to work properly. Being conservative
at least gives you half a chance of success. But it's definitely not
boring.

Fair winds to you all :-)

Hugh

Scoraig, Scotland
http://homepages.enterprise.net/hugh0piggott