Building-Mounted Wind Turbines: Structural Challenges
By MICK SAGRILLO
Mick Sagrillo sits on
the NABCEP board of
directors, is co-chair of
the Small Wind Technical
Committee, and is a
NABCEP-certified
small wind installer.
The mass media seem to love stories about building- mounted wind turbines, even though bona fide manufacturers, professional installers and knowledgeable rebate program managers actively discourage
their purchase and installation. While inventors of such
“breakthrough technologies” claim that the mainstay
small wind industry is just trying to squash potential competition, the real reasons are actually quite different.
Rooftop wind turbines will never be a viable technology for several reasons. First, the small collector size of
these devices (a requirement so that
the weight of the turbine does not collapse the roof) means that only small
amounts of fuel — the wind — can be
accessed and converted into electricity.
Second, wind strength is inadequate at
rooftop height to produce significant
power. Rooftop wind is also turbulent,
and therefore quite destructive of wind
turbines. This combination of limitations means that rooftop turbines can
never be cost-effective. Any number
of small wind test sites have confirmed
these points.
Now a new problem is emerging:
Permitting authorities are receiving
applications from homeowners and
businesses that want to greenwash
their images with building-mounted
turbines. Authorities are therefore
wrestling with how to write appropriate ordinances.
Zoning and permitting language for standard tower-mounted wind turbines is quite straightforward. The
common standard is that the setback for the tower should
match the total structure height from property lines unless
an adjoining property owner agrees to a shorter distance.
What do you do, however, with a wind turbine that is
advertised to be mounted on top of a roof in an urban area
where property lots are 25 feet to 50 feet ( 7. 6 meters to 15
meters) wide?
There’s a more serious problem: Small wind systems from mainstream manufacturers are engineered
structures, designed with a built-in safety factor. The
engineering designs cover all components of the wind
systems including the turbine, electronics (inverters
and controllers), tower and foundation. Tower and
foundation are engineered to the specifications of the
International Building Code (IBC), which dictates the
extreme winds, ice loads, peak gusts and allowable loading on the structure.
ANdErS SANdBErg
Any rooftop installation
requires an engineering
review to assure structural
suitability.
What does a permitting authority do with a turbine
that may (or may not) be engineered, but is mounted
on a structure that was clearly not designed to support
a wind turbine, nor the loads developed by that turbine
when it is spinning and generating electricity? This
is a legitimate concern, as there have been reports of
building-mounted turbines seriously damaging the roof
or wall they were mounted on, in some cases endangering nearby properties.
Residential roofs and commercial building structures
were designed for certain loads specific to the building and
location, including —
Unlike engineered tower systems,
a rooftop wind turbine needs
a complete engineering analysis.
• Dead loads: The weight of the structure plus the additional roof layers and HVAC equipment added over time.
• Live loads: Wind, traffic movement and earth tremors,
and occupational loads including workers on the roof.
• Environmental loads: Snow and ice accumulation,
temperature changes and rainfall. Since these loads are difficult to model, the IBC usually specifies 50-year extremes.
• Dynamic loads: Severe storms, impacts, wave action
near large bodies of water and earthquakes. These are
extremely complex loads to model as they are unpredictable as well as varying in frequency, amplitude, duration
— and may or may not have a cyclic nature. The IBC
specifies considerations for some of these loads, but usually defers to a specific engineering analysis to account for
them. The prudent building owner purchases insurance
for such extreme occurrences. A wind turbine on a roof
or building would be categorized as a dynamic load not
accounted for in the IBC or building structural design.
So how does a permitting authority deal with this?
Unlike engineered tower-mounted wind systems, the
structure that a rooftop wind turbine is to be mounted on
needs to undergo a complete engineering analysis for the
specific wind turbine. This must include the loads placed
on the roof, walls, foundation and associated structural support members by the wind turbine; all shear
loads and overturn moments; the centrifugal forces and
resonant frequency of the spinning rotor and governing
mechanisms; the yaw and generator torque developed by
the turbine; and the aerodynamic thrust loads generated
by the wind system. Only such an analysis will guarantee
the same level of safety engineered into tower-mounted
wind systems. ST
