Rooftop wind turbines offer a relatively inexpensive way to participate in the rush to renewable energy. The principle allure is low installation cost. Typically, tower installations for larger turbines add 50-80% to the cost of the system. With a rooftop turbine, an $800 tower is replaced by a $39 roof mounted tripod, or a length of pipe attached to the side of the house.
According to Ron Stimmel of the American Wind Energy Association, “…rooftop turbines still account for only 1 percent or so of the 10,000 small wind turbines that are sold each year in the country.” For this reason, few performance statistics are available from actual owners.
It’s possible, however, to get an idea of the estimated power output of a rooftop turbine from the manufacturer’s website. For example, the Energy Performance chart for Southwest Wind power’s 200W Air Breeze predicts 20-30 kWh (kilowatt hours) per month from 10-12 mph winds occurring 6 hours per day.
To put this in perspective, 30 kWh is less than 5% of the power consumed each month by the average household. Overall, the payback from a single, roof mounted turbine in tangible financial terms leaves a lot to be desired; especially in light of the fact that unlike solar, there is no Federal tax credit for residential wind power.
But anyone thinking about installing a rooftop turbine(s) is probably aware of this already. The rooftop wind enthusiast is not motivated by financial relief, but rather the intangible rewards of pride of ownership, walking the walk, or simple satisfaction as a hobbyist.
Motivation aside, it doesn’t make sense on any level to proceed with a wind powered project unless the three following requirements are met:
1. Higher than average cost per kWh. The more you pay per kWh for electricity, the more economical wind power becomes. A rule of thumb in the wind turbine industry states a residential wind powered system shouldn’t be considered unless you pay $0.10 or more per kWh for electricity.
The easiest way to determine your cost per kWh is divide the current monthly balance on your electric bill by kWh usage (also plainly stated on your bill).
2. Building codes. Visit your building department to find out if there are any zoning restrictions for roof top turbines. Don’t build your rooftop system first and ask questions later – only to be told it violates zoning laws and must come down.
3. Minimum wind speed. Wind speed of 7 mph, or 3 m/s (meters per second) is the start up speed for just about every wind turbine. However, in order to get any “juice” out of the turbine, minimum wind speeds of 10-12 mph for 6 hours per day are necessary.
Although wind speed charts for the United States and Canada are available to help site larger turbines, they’re of little use for rooftop systems because the data is collected at a minimum altitude of 30 meters (98.4 feet).
However, you can get a good idea of the wind speed at proposed turbine locations by purchasing an anemometer (wind speed meter) for around $150. The kit comes with a rooftop sensor, 60′ of cable and mounting hardware for the read out display. These are not data logging meters, so you’ll have to keep notes on your observations.
If you’re really serious about wind speed data collection, you can purchase a data logging anemometer kit with tracking software for $400.
Depending on where you live, wind speeds generally fluctuate with the seasons. Readings taken during the hot summer months of July and August, when you’re begging for a breeze, are not usually a reliable indicator of average yearly wind speed.
Also, rooftop turbines located at the edge of a long flat roof experience much more wind than those located on roof peaks.
If, after a month or two of collecting data you determine there just isn’t enough wind for a rooftop wind turbine, you can always resell the anemometer and turn your attention towards solar.
Some people enjoy taking matters into their own hands and instead of paying for a wind generator, they prefer to create a project of their own and build one. Building a wind generator from nothing is actually not as difficult as one may think. In order to build a simple model of a wind turbine, you will need to go to a shop with basic power and hand tools, and have some degree of dedication.
Large wind generators of 2000 Watts and up are a major project needing very strong construction so prepare yourself in advance for the time and effort if that‘s what you want to do. But the smaller ones in the 700 to 1000 Watt, 8 to 11 foot range can be built with not too much difficulty.
In fact, it is highly recommend that you tackle a smaller wind turbine before even thinking about building a large one. You will need to be able to cut and weld steel, and a metal lathe can be handy although to make life easier on yourself you could take into consideration hiring a machine shop that turns brake rotors to do some small steps for you.
In most locations, gentle winds (5-15 mph) are the most common, and strong winds are very rare. You should try to make a sturdy machine that will produce good power in low wind speeds, and that will be able to survive high wind events while still producing maximum power. Remember, the power available in the wind goes up by a factor of 8 as the wind speed doubles.
Other critical factors are rotor size and tower height. The power a wind turbine can harvest goes up by at least a factor of 4 as you double the rotor size. And making a tower higher gets you above turbulence for better performance and increased power output. Putting a wind turbine on a short tower is like mounting solar panels in the shade it simply just does not make any sense, doing so, does it now.
Before you jump into something serious like building your own wind turbine, do your homework. Some things work and other things will not work. You can therefore save hours and dollars by learning from other people’s successes and failures.