Commercial Wind Turbine Systems and Applications - Wind Turbine Technology: Fundamental Concepts of Wind Turbine Engineering - page 245

Distributed Wind Turbines on Utility Grids

Distributed wind turbines are dispersed across a utility's service area, as opposed to
turbines in wind power stations that are bulk electricity producers concentrated in one loca-
tion. Distributed turbines generally serve residential, farm, business, or light commercial
loads with only excess power sold to a utility. Most of these turbines are small machines,
with ratings ranging from a few hundred watts to 00 kW. They are usually connected to
the distribution grid at low voltage (typically 220 to 4,60 VAC). At the beginning of 2008,
there were about 25 companies manufacturing and selling small-scale wind turbines in the
United States, and an estimated 85 similar companies in other countries, including American
companies [Cohen and Wind 200].
In Europe, most wind power installations are distributed applications. Denmark had
3,25 MW installed at the end of 2007, with nearly all in small clusters of to 3 turbines.
These units were supplying about 20 percent of that country's electricity.

U.S. Experience with Grid-Connected Distributed Turbines

The U.S. market for small-scale grid-connected wind turbines, designed to serve single-
family homes and small businesses, has been hampered from the start by several factors:

- relatively high cost of small-scale turbines

- low power purchase prices and other financial incentives

- uncertainty about wind resources

- unproven turbine performance and reliability

- lack of distribution and service systems

- environmental concerns vs. potential benefits

In the early 970s, many small-turbine manufacturers in the United States identified a
market for machines in the to 5 kW range. They believed this size was ample to meet most
of the electrical needs of homes without electric heat, throughout most of the country. Prior
to the passage of the Public Utility Regulatory Policies Act (PURPA) , and in areas where
power purchase rates were low, it was desirable to use as much electricity on-site as possible
and to minimize the excess that was sold to the utility. Moreover, the companies entering
the wind turbine manufacturing business often did not have the capital or the engineering
expertise to build machines much larger than 5 kW.
Largely as a result of commercial initiatives using venture capital, and with some support
from the U.S. Federal Wind Energy Program , performance and reliability of small machines
continued to improve and unit costs declined. As listed in Table 4-9, AWEA reported that
sales of distributed wind turbines in 2007 in the United States totaled nearly 0 MW, with a
slight majority used in grid-connected applications. Approximately 90 percent of this new

Table 4-9. Annual Sales of Small-Scale Wind Turbines in U.S. During 2007

Application Number of Units Capacity added

(MW)

Sales
(millions $ )

Off Grid 7,800 4.0 4

On Grid ,292 5.7 28

Total 9,092 9.7 42