I am looking for a low-cost batteryless grid-tied wind-electric system for my home in southern California. I already spoke with the city building planning department and the utility, and they both support wind-electric systems.
I consume an average of 192 kWh per month (6.4 kWh per day). I would like to generate at least some of my energy, but not at the level or cost of the systems mentioned in your articles and wind buyer’s guide. It is often windy here at night and our electrical baseline is very low, so I thought that it would be cool to get net metering and sell a little back to the utility. Any suggestions?
Rainer Boelzle • San Diego, California
Your request is common, though problematic. While it’s quite possible to start with a small solar-electric system and add onto it later, wind electricity is generally impractical to approach on a modular basis, and cost effectiveness decreases the smaller you get. The fundamental reason for this is the nature of the wind energy resource.
The power available in the wind increases cubically with increasing wind speed. When comparing a 6 mph wind to a 12 mph wind, the difference in energy is 63 = 216 vs. 123 = 1,728. That’s eight times as much energy potential with a doubling in wind speed. You can see that it’s very important to expose the generator to higher wind speeds.
The other pertinent fact is that wind speed increases as you move away from ground level and its hills, trees, buildings, and other obstructions. Wind speed increases more quickly as you rise above rough surface terrain than over water and other smooth surfaces.
The clear conclusion is that effective wind-electric systems need to include tall towers to get well above any ground obstructions. The standard rule in the industry is to make sure the lower blade tip is at least 30 feet above anything within 500 feet—and higher is better.
So when you contemplate making “a little” wind electricity, you’re often fighting the reality that tall towers are expensive, and that’s what’s needed to get into the viable wind resource. While it’s possible to get lightweight towers (such as anemometer towers) to put very small machines up high, the cost of the tower will be significantly more than the machine. A larger machine with an appropriate tower will get you renewable energy for a lower cost per kWh.
If you opt to put up a very short tower, you will find that there isn’t much wind energy available, regardless of local anecdotal observation or your gut feeling. When there are windy times at those low levels, it’s very turbulent wind, which is hard on wind generators.
Using your 192 kWh per month and a simple formula, we could do some reverse calculations and make some wild guesses about what size of wind generator you might need if you wanted a wind system to provide all of your electricity. Bear in mind that I (and perhaps you, too) have no idea what your resource actually is.
You can multiply the swept area (in square feet) by the average wind speed cubed (in miles per hour), and divide that total by 32,000 to get a rough estimate of average daily kWh production. Dividing by 1,050 instead of 32,000 gives a monthly average. So going the other way, 192 kWh × 1,050 = 201,600. If we stipulate an average wind speed of 10 mph, we see that you’d need a wind generator of about 200 square feet (201,600 ÷ 103) to generate your total kWh requirements in a 10 mph average wind speed resource. This will be a turbine with about a 16-foot-diameter rotor; the machine alone might cost between $15,000 and $20,000 (the full system cost will be several times that).
Bear in mind that to actually get the turbine into a 10 mph resource almost anywhere you might live, you will need a tall tower. If you choose a 30- to 60-foot tower to economize, surprise: You will actually be spending more money per kWh delivered. Looking at our recent wind turbine buyer’s guides, you’ll see that even a 2 mph drop in wind speed will decrease production of a turbine like this by about half. A 50% cut in production means a doubling of the cost per kWh, unless the purchase cost of the system also is cut in half (which it will not be, since shortening the tower is usually only a small reduction in overall system cost). And if your tower is even shorter—closer to the height of surrounding buildings and trees—your turbine won’t even have an 8 mph resource to capture.
The bottom line is that in most cases, trying to go cheaper—by going with a smaller machine or a shorter tower—ends up with more costly electricity. The best way to lower the cost of wind electricity is to choose a large rotor diameter machine and put it on a tall tower. Cutting corners on either swept area (square footage) or wind speed (via shorter towers) will reduce the financial satisfaction. It’s tough news to hear, but perhaps this will save you disappointment later.
Ian Woofenden • Home Power senior editor