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Large Wind Turbines

February 8, 2018
Paul Gipe

Performance of Swiss Darrieus Wind Turbine Alpha Real Mirrors that in California of the 1980s


Erik Möllerström, a Swedish academic, sent me a copy of a report on a long-forgotten Swiss project intended to demonstrate a wind-biogas system designed for remote locations using a Darrieus wind turbine.

The 1992 report on the project by the Centre de Recherche Energétiques et Municpales reads like so many of the others from the period. In short, the four-year project was a failure for a host of reasons that the report highlights.

This was a project that wasn't meant to be.

The project was bedeviled by technical problems and glitches, most notably with the biogas engine fed from sewage treatment plant digesters serving the Swiss city of Martigny. The use of digester gas was well known at the time, but certainly not as well known as today. They encountered all the problems you would expect. The gas was corrosive and its energy content varied with the operation of the plant. They had trouble keeping the gas engine running and it didn't perform as well as expected.

The wind turbine didn't perform any better.

The wind turbine used was a prototype phi-configuration Darrieus developed by Alpha Real, a Swiss engineering firm. Little information is available today on the company and its wind turbine that looked strikingly similar to Sandia National Laboratory's 19-meter test mule in New Mexico.

Alpha Real claimed that at the time it was Europe's largest Darrieus wind turbine. It could have been. They also claimed it had been "rigorously" tested. It had not. If it had been many of the subsequent problems with the wind turbine would have been avoided.

To test the wind-biogas system, they needed to put the wind turbine near a sewage treatment plant. For those who don't know, sewage treatment plants are typically down river so at the lowest place in the community. Sewage treatment plants are not located on hill tops or well-exposed ridges where we want to put wind turbines--if we can.

Worse, Martigny is in Switzerland's Valais canton southwest of Montreaux in a steep-sided mountain valley surrounded by the Alps.

Thus, the location along the La Drance River was a very poor site for a wind turbine--of any kind--but especially one on a short tower. To make a bad situation worse, one photo shows the site surrounded by shrubs and trees.

One of the great drawbacks of Darrieus turbines of the period was their use with very short towers. This was exacerbated at Martigny by the obstructions.

Embarrassingly, all this was visible from the Autoroute du Rhône that followed La Drance through the valley.

The 19.2-meter diameter two-blade rotor used aluminum extrusions at which the Swiss were skilled. The torque tube stood 28.4 meters tall and drove a two-speed asynchronous, doubly-wound, generator rated at 160 kW.

Alpha Real's promotional materials were unusual for Darrieus developers of the day. All-in-all their claims were quite tempered. They prominently reported the rotor's swept area of 320 meters². They also reported the turbine's specific power of 500 W/m². Both parameters are fundamental to understanding a wind turbine and yet many developers of prototype Vertical-Axis Wind Turbines (VAWTs) still don't report such details--or don't even know why these measures are important.

The Swiss company made the usual claims about Darrieus VAWTs being omni-directional and that the turbines were easier to integrate into the landscape, but it certainly wasn't over-the-top.

Like most phi-configuration Darrieus, the turbine wasn't self-starting. However, the 30 kW necessary to drive the rotor up to speed consumed little overall energy. In 1988 the rotor consumed 564 kWh for starting and 336 kWh in 1989.

The tested power at 10 minute average wind speed of 150 kW was close to Alpha Real's nominal rating.

The turbine performed best in 1991 when it generated 84,500 kWh or only 52% of its projected production of 164,000 kWh per year for the later variant at an average wind speed of 5.5 m/s.

The lost production was attributed to the turbine's poor reliability and less wind at the site than expected. Most of the lost production was simply due to the poor site in a region of low wind.

None of this should have been a surprise to anyone in Switzerland. Data on the poor performance of Darrieus turbines in California was readily available even if there was no internet in 1988. Data on wind turbine performance in California was published--in hard copy--beginning in 1985. There was ample time for Swiss engineers and researchers to hear about the problems in California at professional conferences and among their peers.

Similarly, the mechanical and electrical problems the turbine encountered were not unknown at the time.

Alpha Real's Darrieus turbine suffered a sufficiently serious enough hydraulic leak that required redesign of the gearbox. One of the struts sheared in two at the torque tube. The disk brake warped, requiring replacement.

If that wasn't enough, the "finger" attachments of the blades to the torque tube seemed destined for failure. Sure enough, the report notes that fatigue cracks in the blade attachments required a complete rethink.

The report concludes that even if the turbine worked reliably, it would never meet its performance estimates.

In summary, the experience of Alpha Real's Darrieus turbine at Martigny from 1988 to 1991 mirrors that of similar wind turbines operating in California at the time.

 


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