Portions of the following have been adapted Wind Power for Home, Farm, & Business published by Chelsea Green (spring 2004). This book includes a chapter on how to site wind turbines to best advantage. I am posting the following in response to several requests by universities to install small wind turbines on the roofs of engineering laboratories. There also continues to be interest in building-integrated wind turbines in Europe, mostly due to EU funding.
Mounting wind turbines–of any kind–on a building is a very bad idea. I’ve yet to see an application where this has worked or will likely work. In short, rooftop turbines will not do what their promoters claim and often will cause their owners no end of grief.
Why I Say This
Much has been written about the small Jacobs windcharger that was installed in the late 1970s on a tenement in the Bronx. True, it was done once, and it can be done again. But what’s the point? The Bronx project was intended as a challenge to Consolidated Edison Company, New York City’s utility. It succeeded, and it proved that electricity could be fed back into the utility’s network without destroying the city. Later, the turbine was removed.
Rooftop mounting of small wind turbines remains controversial. Few topics can stir more heated debate among the small wind community as can rooftop mounting, and notably Southwest Windpower’s aggressive marketing of the concept. The Arizona manufacturer of the popular Air series suggests installing their micro turbine on rooftops as a way to compete with the simplicity of mounting photovoltaic panels. Their advertising generates howls of protest from critics such as Wisconsin’s Mick Sagrillo.
All wind turbines vibrate, and they transmit this vibration to the structure on which they’re mounted. All rooftops create turbulence that interferes with the wind turbine’s operation. Even if Southwest Windpower engineers were able to design a sophisticated dampening system that isolated the wind turbine from the structure, they couldn’t eliminate the power-robbing and damaging turbulence created by the building.
Worse, a rooftop-mounted turbine can provide a nasty surprise, as an owner in upstate New York learned. One stormy night his Air turbine destroyed itself–and then plunged through his roof. That was the end of his experimentation with rooftop mounting.
To avoid rooftop turbulence, the wind turbine must be raised well above the roof line. This often negates any potential savings on the tower, and increases the complexity of mounting the wind turbine and installing it safely.
Few who consider rooftop mounting ask whether the building can support the loads created by both the wind turbine and tower. The wooden roofs of homes in North America can’t support more than a micro turbine at best. A reinforced concrete roof on a commercial or industrial building might be able to withstand a slightly larger turbine. Can the roof, then, handle the dynamic loads–the vibrations–that the tower will transmit to the structure? If the building is an unoccupied warehouse, the vibrations won’t bother anyone, but if it’s an office building, they may prove annoying.
Rooftop mounting has been tried and with the exception of the founder of the company that encourages its use, the technique has been found wanting. It’s simply not worth the trouble. As they might say on Manhattan’s lower east side, “Rooftop mounting? You gotta be kidding me. Forgedda about it.”
University On-Campus Testing
Testing of small wind turbines on campus should be done in open areas without heavy pedestrian traffic, such as near athletic fields, farm test plots, and so on. A good example of such a test site is that operated by the Alternative Energy Institute at West Texas A&M University in Canyon, Texas where the turbines are located away from the main campus buildings.
Several universities have expressed interest in testing micro and mini wind turbines on campus buildings. My concerns about such locations are twofold.
First, typically at such installations the turbines seldom operate. I photographed a Bergey 1.5 kW a top a university-related building in Dublin, Ireland. The turbine was not operating and probably didn’t operate. Wherever wind turbines will be seen by the public, the wind turbines should be in regular operation. This is a fundamental principle for conveying the message that wind turbines work and are useful. Inoperative turbines, for whatever reason, violate this principle.
If the turbines will be used in a testing program, they should be left to run attended when no tests are being performed. If this is not possible, then the turbines should be lowered to the roof after tests are complete.
Second, there is probably too much turbulence over large buildings to test small wind turbines properly. However, roof top testing does make a good exercise in researching and calculating the effects of turbulence on small wind turbines.
If there are no other options for a test site, then here a few suggestions. Use only micro and mini turbines (nothing greater than 2.5 meters in diameter) and use no combination of tower height and location that would permit the turbine to fall off the building. Marlec 910F, Ampair 100, LVM 6F, and AirX would be suitable. Whisper H40 and Bergey XL1 may also be acceptable. Note that for turbines using a friction fit onto the stub tower (pipe section) use a through bolt to ensure that the turbine never leaves the tower unless you intend it to.
Though there are numerous examples of the Air series of micro turbines shedding a blade, the current version of the AirX (built through the first half of 2003) regulates so often and produces so little power in high winds that this hazard no longer exists.
The greatest hazard from rooftop mounting is caused by turbines that have not been thoroughly tested on open windy sites by reputable laboratories or experienced hobbyists, such as Michael Klemen in North Dakota, or by private individuals who test small wind turbines, myself and Hugh Piggott for example. We have no idea how these turbines will perform under load in extremely turbulent conditions.
Comments by Kevin Harrison on Rooftop Mounting
In 1997 a group of undergraduate mechanical engineering students installed a Bergey 850 on the roof of the Upson engineering building at the University of North Dakota (UND). Apparently it was used for a short time and then tilted down on it’s 30 foot tower.
I arrived at UND in 2002 to complete my Master’s degree and found many of the permanent magnets loose inside the machine. It seems logical that water entered the turbine and froze, forcing the magnets off. The turbine was mounted above the machine shop and while I was rebuilding the magnet assembly (rotor) I was also catching wind of all the noise it makes on the roof. Even on the 30 foot tower the wind turbine was obstructed from the wind from many directions. I put the turbine back into service in October 2002 and acquired enough data for my research but was surprised to hear all the noise it made in the machine shop below and subsequently lowered it back to the roof (protected from the weather).
The original students also installed the tower on a spring loaded pad in an attempt to dampen the vibrations. I would not recommend an installation like this unless all other options have been exhausted. Locating the wind turbine free from the turbulance caused by buildings, trees, and other objects will be well worth the extra effort and money. I am confident that you will thank yourselves later for installing them in an open field, even if you have to talk to a local farmer with a couple of acres he/she could loan to you.
University of North Dakota
Engineering Doctoral Program
Comments by by Nick Martin on Rooftop Mounting
The case (or not) for Roof Mounted Wind Turbines by Nick Martin
Comments by Hugh Piggott “Rooftop wind turbines are a load of nonsense” can be found at www.scoraigwind.com. Page down or search for “rooftop”.
Hugh Piggott’s measurement of urban wind speed over a roof top at Wind Speed Measurement in the City.
Andy Mahoney’s Rooftop Mounting Failure at http://navitron.org.uk/forum/index.php?topic=942.0