About Contact

Skystream Delivers Less Than Advertised at French Test Site

Article by: Paul Gipe

August 3, 2011

By Paul Gipe

The French test site for small wind turbines has published a report on the performance of Southwest Windpower’s Skystream 3.7 wind turbine.

The test site in southern France, SEPEN (Site Expérimental pour le Petit Eolien de Narbonne), has tested a number of small wind turbines, including Mariah’s Windspire. See Mariah: Another Performance Report on Windspire VAWT.

The English report is also available en francais.

The report is dated March 25, 2010 and the tests were conducted from the fall of 2008 to the spring of 2009.


Power Less than Advertised

SEPEN found that “the power generated . . . is lower than that announced by the manufacturer in its manual.”

In the manual referenced by SEPEN, Southwest Windpower (SWP) rated the 2 kW turbine at 10 m/s wind speed. SEPEN found that at 10 m/s the turbine delivered 1.55 kW. Thus, the turbine underperforms SWP’s advertised power rating by 20%.

Since this model was tested at SEPEN, SWP has rerated the turbine to take full advantage of US tax credits. See New Federal Subsidies Distort the US Small Wind Market: Or How to Increase the Power of the Skystream 3.7 with the Stroke of a Pen

SWP currently rates the turbine at 2.4 kW at 13 m/s.

According to SEPEN’s power curve corrected for site turbulence, the Skystream will produce 2.1 kW at 13 m/s. SWP’s revised power rating is only 12% higher than that measured by SEPEN. For small wind turbines, many of which fail to meet their advertised power curves by a wide margin, SWP’s new rating is nearly spot on.


Down Time

SEPEN also commented that the turbine stopped twice during the tests, requiring a manual restart. This necessitated either lowering the turbine to the ground or the use of an aerial lift. This could increase operating costs SEPEN warns.


Energy Mixed Results

At low average annual wind speeds, SEPEN reports that SWP overestimates annual generation. At 5.5 m/s average annual wind speed, SWP estimates that the turbine will generate about 4,500 kWh per year. SEPEN’s results indicate the turbine will produce about 4,000 kWh per year or 12% less than advertised.

At higher average annual wind speeds, SEPEN’s results indicate the turbine will deliver slightly more electricity than SWP advertizes. At 7.5 m/s average annual wind speed, SEPEN’s results indicate the turbine will deliver 7,436 kWh per year while SWP estimates the turbine will deliver 7,400 kWh per year.

Very few small wind turbines will be installed at sites with hub height average annual wind speeds approaching 7.5 m/s. Most small wind turbines will be operating in environments where the average annual wind speed will be less than 5.5 m/s (12.3 mph).


Tariffs Required for Skystream

In calculations to determine what feed-in tariff would be necessary for the profitable operation of a Skystream, I had previously assumed that at 5.5 m/s the turbine would produce 4,800 kWh per year.

The results from SEPEN’s tests for a 5.5 m/s site raises the tariffs necessary for the profitable operation of a Skystream from $0.21/kWh to $0.25/kWh for a project in California that would use the ITC and state subsidies. Without the ITC and state subsidies, the results from SEPEN substantially raise the tariff necessary from $0.45/kWh to $0.54/kWh.