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effects of atmospheric turbulence on unsteady local lift forces, then on loads and stresses,
and inally upon fatigue life were not adequately represented in design models. Based on
the Mod-2 test results, major improvements in these models have been made. However,
achieving long fatigue life in the presence of small-scale turbulence remains a principal tech-
nical challenge to the development of advanced wind turbines that are still reliable and cost-
effective.
After modiications and repairs associated with initial mechanical problems, the Mod-2
test program and the turbines themselves went on to accomplish major test and perfor-
mance objectives. The three Mod-2 HAWTs at Goodnoe Hills together accumulated over
16,000 hours of operating time and supplied over 10 million kWh to the local grid, over 60
percent of that amount in the inal year of testing. A major test objective that was achieved
was proving conclusively that groups of modern wind turbines could operate in a totally
automatic, unattended mode. Another was establishing a data base on rotor wake structures
and wake effects on downwind turbines that has led to the development of improved wake
models being applied now to wind power stations (see Chapter 6).
Two additional Mod-2 turbines were built by Boeing for utility companies who wished
to examine the performance of large-scale wind turbines directly from their perspective.
One, purchased by the Paciic Gas and Electric Company, was installed on a test site in
Solano County, northeast of San Francisco. A second, for the Bureau of Reclamation, was in-
stalled near the WTS-4 HAWT at Medicine Bow, Wyoming, for comparative testing. While
all ive of these machines achieved signiicant (and sometimes spectacular) test and operating
results, large wind turbines at that time were not yet at the stage where they could compete
successfully for utility company operating funds, nor was there an industry willing to take
the inancial risks associated with their commercial development. All Mod-2 turbines were
later dismantled, while the technology continued to be developed and commercial use
awaited a change in energy economics.
The Mod-5 Program
Plans had already been laid in the late l970s for the development of third-generation
wind turbines. First, conceptual studies of advanced large- and medium-scale turbines
(designated as Mod-3 and Mod-4) were conducted, but these were never carried beyond the
preparatory stage. Later, consideration was given to two hardware-development programs:
A large-scale
Mod-5 HAWT
program and a medium-scale
Mod-6 HAWT and VAWT
pro-
gram. Two contractors were chosen to design and develop what became known as the
General Electric Mod-5A
and the
Boeing Mod-5B
HAWTs. At that point energy and fuel
costs peaked and then turned downward, and energy was no longer a major national
priority. As a result, the Mod-6 program (whose contractors had not yet been selected) was
canceled, and General Electric chose not to proceed past the design stage. Thus, only one
third-generation turbine, the Mod-5B, was completed under the Federal Wind Energy
Program. NASA-Lewis engineers managed the project and
Hawaiian Electric Industries
became Boeing's utility partner.
The Mod-5B (Fig. 2-1 and 4-25), installed at the Kahuku wind power station on Oahu,
has an overall coniguration similar to that of the Mod-2, with a two-bladed, partial-span
controlled, teetered, and upwind rotor atop a steel shell tower [Boeing 1988]. The pitchable
tip sections were extended 3 m longer than those on the Mod-2 rotor, leading to a rotor
diameter of 97.5 m. This made the Mod-5B HAWT the largest wind turbine in the world of
its day. Its rated power was 3.2 MW, up 28 percent from the Mod-2 rating.
The major advancement in technology achieved by the Mod-5B is that it was the irst
large-scale wind turbine to operate successfully at
variable speed
. The speed varied from
13 to 17.3 rpm depending on wind speed, thus improving energy capture as well as
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