Another wind power question

There were plenty of engineers working on these--Especially the MOD-2s. They didn't go with three or four foils then and I wanted to know why we do now. I have tried to research why this question and the only thing I found published on the subject was that they 1. turn slower--for bird strike problems and 2. three blades are more ascetically pleasing to the eye when they turn. Both hurt efficiency.

Figure 13. The 200kW MOD-0A wind turbine at Clayton, New Mexico was a qualified success for NASA and DOE

Figure 14. In the words of one exasperated federal program manager, the 2-megawatt General Electric MOD-1 machine was built "hell-for-stout." Unpredicted low frequency sound emissions resonating in the many homes scattered through hills and valleys close to the installation killed the project.

"Hell-for-Stout"

Beginning with the 100kW MOD-0 installed at NASA's Plum Brook Ohio facility in 1975, the U.S. program rapidly moved through several generations, including the MOD-1 and the 100-meter diameter MOD-2 wind turbines (see below.)

Unfortunately, the program was burdened by an early error that took four years to overcome. In 1974, perhaps expecting to reproduce the success of U.S. rocketry development by copying advanced German designs, NASA engineers turned to Ulrich Hutter's blueprints for answers. While borrowing Hutter's two-bladed, downwind rotor configuration for their early designs, they failed to note the importance of the fact that Hutter's machines featured teetering hubs--now known to be essential for reducing dynamic loads created by tower shadow in two-bladed machines.

NASA engineers were astounded by the huge dynamic loads the first (MOD-0) machine developed whenever a blade entered the "dead" space behind the tower (which was also much beefier and blocked more wind than Hutter's). And it took several years of engineering studies, responding to outraged Congressional inquiries (from none other than Barry Goldwater), and other diversions to figure out what was going on and switch to an upwind, teetered hub configuration.

The rigid hub NASA turbines (with a probable useful machine life measured in months) none-the-less served as useful stand-ins for demonstration projects until "real" machines arrived in the early 1980's.

The program's biggest early success was the operation of four MOD-OA 200 kW machines by U.S. utility companies (Figure 13.)

The moniker "real machine" did not apply to the MOD-1 (Figure 14, at left), the program's first attempt at a megawatt-scale system. Because it was designed before the problems with the MOD-0 were understood, the design was a lame duck even before acoustic resonance problems (themselves aggravated by the lack of a teetering hub) scuttled the first and only installation at Boone, North Carolina.

Figure 14. The 3-megawatt, 100-meter diameter MOD-2 operated by PG&E in Solano, California was the most successful private operation of a multi-megawatt wind turbine until the MOD-5B in Oahu, Hawaii.

A "Real" Machine?

The first "real" NASA wind turbine was the 100-meter diameter MOD-2. Three of these machines operated for several years at a site overlooking the Columbia River in the 1980's, providing valuable engineering data and helping to pinpoint design weaknesses. Others operated at Solano, California (left) and near Medicine Bow, Wyoming. The MOD-2 was an inevitably flawed experimental machine because of the huge technological leap it represented from the MOD-1. This provided detractors with an easy target for criticism.

By 1981, the detractors of the Federal program had succeeded in getting most of the development activity scuttled, just when things were poised to get better.

Lessons learned on the MOD-2's were incorporated in the 3.2-megawatt MOD-5B, a 100+ meter behemoth that was still operating (not without problems) on the Island of Oahu, Hawaii in 1997.