Page 304 - Alternative Energy Systems in Building Design

278 WIND ENERGY TECHNOLOGIES

needed to power a typical household. A 1.8-MW turbine can produce more than
5.2 million kWh in a year, enough to power more than 500 households. The aver-
age U.S. household consumes about 10,000 kWh of electricity each year.
A practical example of a project is a 250-kW turbine installed at the elementary
school in Spirit Lake, Iowa, that provides an average of 350,000 kWh of electricity per
year, more than is necessary for the 53,000-square-foot school. Excess electricity is
fed into the local utility system, which earned the school $25,000 in the turbine’s first
5 years of operation. The school uses electricity from the utility at times when the
wind does not blow. This project has been so successful that the Spirit Lake School
District has since installed a second turbine with a capacity of 750 kW.
Wind speed is a crucial element in projecting turbine performance, and a site’s wind
speed is measured through wind resource assessment prior to a wind system’s construc-
tion. Generally, an annual average wind speed greater than 4 m/s, or 9 mi/h, is required
for small wind electric turbines. Less wind is required for water-pumping operations.
Utility-scale wind power plants require minimum average wind speeds of 6 m/s
(13 mi/h). The power available in the wind is proportional to the cube of its speed,
which means that doubling the wind speed increases the available power by a factor
of 8. Thus a turbine operating at a site with an average wind speed of 12 mi/h could,
in theory, generate about 33 percent more electricity than one at an 11 mi/h site
because the cube of 12 (i.e., 1768) is 33 percent larger than the cube of 11 (i.e., 1331).
In the real world, the turbine will not produce quite that much more electricity, but it
still will generate much more than the 9 percent difference in wind speed.
The important thing to understand is that what seems like a small difference in wind
speed can mean a much larger difference in available energy and in electricity
produced and therefore a larger difference in the cost of the electricity generated. Also,
there is little energy to be harvested at very low wind speeds; 6 mi/h winds contain
less than one-eighth the energy of 12 mi/h winds.

CONSTRUCTION OF WIND TURBINES

Utility-scale wind turbines for land-based wind farms come in various sizes, with rotor
diameters ranging from about 50 m to about 90 m and with towers of roughly the same
size. A 90-m machine, definitely at the large end of the scale, with a 90-m tower would
have a total height, from the tower base to the tip of the rotor, of approximately
135 m (442 ft). Offshore turbine designs now under development have rotors that have
a 110-m rotor diameter. It is easier to transport large rotor blades by ship than by land.
Small wind turbines intended for residential or small-business use are much smaller.
Most have rotor diameters of 8 m or less and would be mounted on towers of 40 m in
height or less.
Most manufacturers of utility-scale turbines offer machines in the 700-kW to 1.8-MW
range. Ten 700-kW units would make a 7-MW wind plant, whereas ten 1.8-MW
machines would make an 18-MW facility. In the future, machines of larger size will
be available, although they probably will be installed offshore, where larger trans-
portation and construction equipment can be used. Units larger than 4 MW in capacity
are now under development.