Roof-Top Mounting
An alternative wind generation solution?
By Paul Gipe
October 1, 2003
Note: TheSolarPlan.com doesn't necessarily agree or disagree with the
opinions expressed in the following article. They simply brought up
some interesting points about roof-top wind generation, and we thought we
would share those thoughts with you.
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 downtown
Melbourne, Australia. 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 turbine 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 produce so little power in high winds
that this hazard no longer exists.
The greatest hazard from roof-top 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.
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