RENEWABLE ENERGY:

BECOMING YOUR OWN POWER COMPANY

7.

Wind Generators

When we designed our home we decided to include a small wind generator as a backup to our PV panels. We figured that on days when the sun didn't shine but the wind was blowing we could still charge our batteries using the wind generator.

When we looked into the feasibility of having a wind generator we learned that we live in an area that has a high enough average wind speed (5-10 mph) to justify a wind generator. We also sited our house on the brow of a small hill, which would make it easier to catch the available wind

Because we wouldn't be using the wind generator as our primary power source we wanted to keep the cost down, which eliminated most of the larger units. For a large wind generator it can cost as much for the tower as for the wind generator itself. Of the smaller units we looked at the best one seemed to be the Air 303 by Southwest Windpower. The 303 is rated at 375 watts of output in peak winds (30 mph) and came in both 12 and 24 volt models.

An added advantage to the 303 is that it has internal circuitry that makes it easier to install in tandem with our PV panels. Most wind generators have to have an outlet for their power whenever they are turning. This is because they generate their power by spinning magnets within a coil of wires.

As long as the power produced has somewhere to go (i.e., into the battery bank) there is no problem. But when the batteries are full the wind generator needs another outlet for its power, called a diversion load, or the energy it generates will build up in the coils, overheating the wind generator and causing it to fail. Some wind generator installations will use a low voltage heating element (like a small milkhouse heater) or a 12 or 24 volt DC heating element in a hot water tank.

The Air 303, however, has internal circuitry that senses when the batteries are full and shuts the unit down. Even though the blades will still spin, it will produce only a small amount of power; any heat that is produced gets dissipated through the aluminum housing of the unit. This meant that we would not have to build in a diversion load to our system and could wire up the 303 just as if it were a PV panel.

It sounded perfect for us and we bought and installed one on our system. Alas we don't live in a perfect world and I began to notice that the wind generator didn't seem to be putting out much power, even in relatively high winds. I sent it back to Southwest Windpower and they went through it completely, installed new components and returned it to me.

But it still did not put out much power. Again I called Southwest Windpower and began a series of brainstorming sessions with Eric Stewart, their design engineer. After much back and forth and two more return trips to Flagstaff for the wind generator we determined the problem.

The wind generator was fine, it just had some limitations not explained in the marketing literature. A wind generator depends on the force of the wind to produce energy. At 9000', where we installed ours, the density of the wind is about 70% of what it is at sea level. This reduction in the force the wind could apply to the blade of the turbine meant that it would take a higher wind speed to operate the unit.

This has a significant effect on the performance of a small wind generator like the 303. The low density of the wind at this elevation means that the 303 doesn't put out its rated output until the wind reaches 40 mph rather than the 28 mph wind speed needed at sea level..

A wind generator's power output increases as the cube of the wind speed. Put simply that means the wind generator will put out 8 times as much power at 40 mph than it does as 20 mph. This also means that if you cut the wind speed in half the power output is only 1/8th as much.

If the 303 puts out 400 watts (16 amps) at 40 mph, then it will put out only 50 watts (2 amps) at 20 mph. Since the average wind speed here is well below 20 mph it is doubtful we will ever get much power out of the small wind generator. The most we have gotten out of the 303 on the windiest day of the year so far is about 15 amphours, less than the amount of energy one of our PV panels produces on a sunny day.

Since the Air 303 costs about the same as one of our PV panels it should at least be able to produce the same amount of power. Since it can't, it doesn't seem to provide an effective option for providing power.

This gives us two options. We can upgrade to a larger wind generator, which will mean added costs to upgrade the tower as well, or we can replace the wind generator with more PV panels.

At the moment we are leaning towards the PV panels. They still won't provide much power on cloudy windy days but the upgrade will be simpler and the power more reliable (we still have more sun here than wind). You can be sure I'll let you know which way we decide to go and how that affects our power situation.

One final note on the wind generator, though. Despite our problems with the power output at this altitude, I can't say enough good things about the people at Southwest Windpower. They showed a genuine interest in my problems and struggled through the process right along with me. They always had time to hear me out and offered good suggestions.

Too often even small companies can get "too big for their britches" and act like the problem is all the customer's fault. Not these guys. If we do get another wind generator you can rest assured it will be from Southwest Windpower.


All Contents © 1997
Wagonmaker Press
Thomas W. Elliot

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