ASK THE EXPERTS: Low-Cost Wind Electricity

Beginner
A wind turbine on a tower
This turbine is mounted on a tower with a height well above the minimum of 30 feet above anything within 500 feet.

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 WoofendenHome Power senior editor

Comments (6)

molsze's picture

I came across a new / interesting wind turbine design concept on the internet, that I believe has the ability to change all of the current perceived, conventional small wind turbine, rationality. The wind turbine is called SheerWind. In my opinion it is basically a wind turbine situated inside of a hybrid wind tunnel. You all should investigate their concept / design, as in my opinion it is far more than novel or unique. I first experienced this very same concept, when I was a teenager, as the home that my family built, in Michigan, incorporated an enclosed breezeway, situated between the detached three car garage & the house. The breezeway was approximately 20 ft. in length & was venturied. The front yard was in total sunlight / no shade & the back yard was almost entirely shaded, via 80 ft. oak & maple trees. As such the temperature difference between the front & back yards on a hot sunny day was typically 15 - 25 degrees. As such, even on days when there was no wind / airflow to be felt in the front yard, there was "always" @ least a 4-6 mph. breeze / airflow through the breezeway, that was solely attributable to thermal convection. The average annual wind speed through the breezeway was conservatively, approx. 10 - 15 mph. (not exaggerated) As such, my idea has always been to expand upon / improve the design of my fathers first home / breezeway & subsequently situate a wind turbine inside of it. If the breezeway was venturied both horizontally & vertically & if spring actuated dampers / louvers were installed, you would be able to greatly increase / regulate the wind speed, through the breezeway. Just as the SheerWind wind turbine does / accomplishes. I believe if a breezeway is oriented to collect & intensify the prevailing predominant wind direction & if modest thermal convection is also created / generated / designed into it, you can easily, viably generate @ least 40-60 percent of your home electricity needs / requirements. My brother & I would get into heated arguments as to whose turn it was to go get more firewood that was stored out in my fathers breezeway in the winter time, as it could be 10 degrees below zero & the wind would be blowing through the breezeway @ 40 mph. so the wind chill factor was colddddd. I would like to here your thoughts on the SheerWind wind turbine, as in my mind it is primarily the same concept as a venturied breezeway, that I intend to build in the future.

Michael Welch's picture

Hi there Molsze. It is my understanding that this turbine is not actually available, though I have been hearing about it for a long time.

"600% more energy" is very hard to believe. I hope it is not one of those investment scams where they get folks to put their funds into projects that will never actually work and be produced.

Once (if) it becomes available, hopefully they will get it independently certified under the Small Wind Certification program so we can find out if it actually does work.

http://www.smallwindcertification.org/

sudhakar chaphalkar's picture

How about the Honeywell wind turbine? Is that worth the money?
Thank you. Bob

Michael Welch's picture

I have not heard good things about that turbine. But that is probably because they are proponents of putting turbines on buildings, which just does not work very well. Turbines belong on tall towers away from turbulence-causing buildings. So as long as you are going to the expense of putting it on a tall tower, you might as well put up a turbine which is tried and true.

Again, let's see if they can get a certification -- that should tell us what we need to know.

molsze's picture

I can understand your doubts, as to SheerWinds claim of 600% increased efficiency, as opposed to a conventional pole mounted wind turbine & the fact that it has not been independently tested, as yet. Try to realize that a pole mounted wind turbine can not have the air flow to the wind turbine, directed, compressed, intensified, ultimately sped up & then regulated / modulated to maintain an optimum wind speed / airflow, for substantially greater durations, thus greatly increasing the overall operating efficiency / electricity generating capacity of the wind turbine. It also can not realize man made / designed / enhanced thermal convection. Also envision this if you will, & we all have experienced it & that is what happens when you open a door on a house only to have a open door on the opposite side of the house, slam shut. In some aspects this is the same principal. In my previous e-mail I conveyed aspects of the breezeway on fathers first home. They were not exaggerations in the least bit. That is why I believe the SheerWind wind turbine to be truly viable. I believe the reason why we have yet to see more of it is because ultimately it is a relatively simple concept / design & they (SheerWind) are currently at a loss of how to market it, as once it gets out what it is. Anybody will be able to build one. I would also suspect that some other entity will potentially figure out the same / similar design / concept, in the very near future. We all shall see. I on the other hand, have already seen / experienced this concept / design in my fathers breezeway. Re-guardless of what transpires in the future with corporate wind turbine technology, I believe a small wind turbine/s situated inside of a properly oriented (predominant prevailing wind direction) venturied breezeway, with passive actuated dampers / louvers & with a greatly accentuated micro-climate / thermal convection can & will generate 40 - 60 percent of the annual electricity demands, for a residential home, especially if the home / structure is also highly energy efficient, i.e. poured concrete. Only time will tell.

Michael Welch's picture

Sorry, but their claims are very hard to imagine. Let me know when (if) they ever get going, and when they get the Certification mentioned above. Then we will have something we can trust.

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