An expert in rotor structural dynamics, the author writes that he sees no inherent technical problems in designing large wind turbines and that they need be no more complex than smaller turbines. He refers to an article written by David Milborrow in Windpower Monthly's April issue.

From Glidden S Doman,Rotor Technology Research, USA

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The otherwise timely and excellent article "Size and the Economic Limit" (Windpower Monthly, April 1995) leaves an impression that I wish to correct. It implies there were unsolved and mysterious structural problems with all of the first generation large wind turbines. This tends to combine with recent published opinions that blades are failing on 500-600 kW machines because there are "vibration problems" that come with large scale and that are not understood.

I speak from the perspective of 52 successful years in rotor structural dynamics, as principal investigator for ERDA in 1976-77 on large wind turbine structural dynamics, consultant on problems with Mods 0, 1, and 2, system design manager on the 3 MW and 4 MW WTS-3 and WTS-4 turbines, and now system design manager on the Italian GAMMA 2 megawatt programme.

It is ironic that the Nasüdden in Sweden and Aeolus in Germany are cited as the lone survivors of the early large machine efforts, since they are still fighting the difficult rotor load problems that defeated the Mod 1 turbine of the same configuration.

Of all the large machine projects that have been constructed, only the WTS-3 and WTS-4 and the GAMMA have had the benefit of truly complete and successful aeroelastic and control analysis. In all three it was known before construction that structural dynamics and control problems would not appear. In all three it has been shown by test measurement that stress, vibration and control posed no problems. It is firmly established that, with a good design configuration, there are no problems that are exacerbated by large size, at least up to 80 metres in diameter. In fact, some aspects of load attenuation and control become easier at large scale. There is absolutely no reason that a large turbine should be in any way more complex than a small one. The article is correct in citing the square cube law as the prime unanswered question.

I am very happy with our Italian wisdom at WEST in coming down to 60 metres in diameter and with the present effort to reduce cost for the serial production of GAMMA machines. With the simplicity of its fixed pitch rotor and the performance of broad-range variable speed, I feel that the GAMMA configuration would deliver optimum economics at any diameter.

On whether 60 metres is too large a wind turbine for optimum cost of energy performance, the jury is still out. But it is interesting that several others are moving up to challenge us at 60 metres.

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