Commentary on the Skystream 3.7

By Paul Gipe

May 9, 2009

by Paul Gipe

This article is the fourth revision of that posted on June 30, 2006. In the second revision, Doug Selsam‘s keen eye noted a discrepancy between Southwest Windpower’s advertising brochure and it’s technical specifications. Selsam is a small wind turbine developer and thus could be considered a competitor to Southwest Windpower. Nevertheless, Selsam’s observation was later confirmed by Hugh Piggott. As of the second week in March, 2007, Southwest Windpower had revised its specifications sheet removing the discrepancy noted by Selsam and others. This article has been updated accordingly. This revision revises the recommendation and conclusion. The bottom line? I’d use it with the qualifications noted below.

Southwest Windpower has begun marketing its Skystream 3.7. There will be a lot of hype about this wind turbine–a lot.

The Skystream uses a 3.72-meter diameter, three-blade, downwind rotor. Southwest Windpower claims that this rotor will deliver 1.8 kW at its rated speed of 9 m/s and generate 4,800 kWh per year at an average annual wind speed of 5.5 m/s (12.3 mph).


First, the Good News

SWP has learned some lessons. It’s designating the wind turbine as the “3.7” for its rotor diameter in meters rather than the flawed and misleading power at rated wind speed that has been used in the past. This is a bold step for an American company and they should be complimented on the move.

SWP has also rated the wind turbine at a relatively low wind speed of 9 m/s or 20 mph. Most of its competitors rate their wind turbines at much higher speeds. The rated power at a specific wind speed has very little to do with how much energy the wind turbine will deliver to the load. Much of the time small wind turbines operate at well below their rated speed. By lowering the rated speed, SWP has effectively lowered the kW rating of the wind turbine. This gives consumers a more realistic idea of what they can expect in comparison to past practices.

Unlike SWP’s AirX, the Skystream’s power rating falls slightly below the standard power rating for turbines of this size. The rating for the AirX, for example, is twice the standard rating.

At rated power, SWP’s Skystream has a rotor loading of 166 watts/m² of the rotor swept area. This is half the rotor loading of the AirX of 373 watts/m².

And like SWP’s AirX, the Skystream is sexy and we all know that sex sells–even wind turbines.

That’s the good news.


Now, The Rest of the Story

Skystream’s advertised rated power of 1.8 kW at nearly 9 m/s is extremely high and though reports suggest that the aerodynamic efficiency of the rotor is quite high, this could be optimistic. The Skystream is projected to deliver 37% efficiency in a 9 m/s wind. Big turbines can deliver this, but no commercial small turbine in production has done this. If SWP’s Skystream can deliver this kind of performance it will be precedent setting. Keep in mind though this is the efficiency at only one wind speed. It is not the efficiency across an entire range of wind speeds.

Skystream’s original specification sheet included a performance graph at the bottom of the page. This chart presented real world data collected on SWP’s Storm prototype–the forerunner of the Skystream. It revealed exactly what the prototype turbine delivered during standardized tests at the National Renewable Energy Laboratory. And what the chart showed was that the prototype turbine delivered 1.2 kW at 9 m/s, not 1.8 kW.

SWP’s performance projections for the Skystream remain optimistic–not on another planet out-of-this-world optimistic, but still optimistic.

SWP projects that the Skystream will generate one-quarter more electricity from its wind turbine than the average projected by many other small wind turbines for the same average wind speed. Most small wind turbines of this class project about 3,800 kWh/year at an average wind speed of 5.5 m/s. And most small turbines fail to deliver the energy projected. That should say something.

Can the Skystream meet its projection? Possibly, but the odds are against it. How far off the projection will the bulk of Skystreams fall? That’s a good question and we won’t know until the turbine has been operated by several independent test fields as well as by consumers.

Most alarming though is the rotor itself. There is no aerodynamic means of controlling the rotor or limiting its power in an emergency. SWP says that the rotor will be controlled by a modern form of dynamic braking. That’s all well and good but the wind turbine doesn’t have an aerodynamic means for limiting power should dynamic braking fail.

Why is this important? The answer is simple. Without a reliable means to protect the rotor from overspeed, the wind turbine is at risk of destroying itself. This is fundamental to wind turbine design. Even with mechanical or electrical braking and aerodynamic controls there is still no certainty that a wind turbine will not destroy itself. But without aerodynamic controls, the likelihood increases. This lesson has been repeated over the decades of modern wind power.

The Danes learned 30 years ago–yes thirty years ago–that all wind turbines must have an aerodynamic means for limiting rotor power in an emergency. The Danish wind turbine owners association was formed in part to force Danish manufacturers to abide by this requirement. How did they do this? They simply refused to buy any wind turbine that didn’t have it. They did so because they had already lost too many wind turbines. Their action led to the Danish wind turbine industry dominating the world market for more than two decades.

Danish consumers literally saved Danish wind turbine manufacturers’ bacon by insisting on this provision. This simple requirement is the reason that there are so many 20-year old Danish wind turbines still operating in California and only a few remaining American wind turbines from the period are still in use. Danish wind turbines, while far from perfect, were designed to be fail-safe.

Has the wind turbine industry progressed so far that we can now safely do away with this requirement? I don’t think so. Not even in small wind turbines where the forces are much less.

Nevertheless, I know of no occassions where the Skystream has thrown a blade or destroyed the rotor and nacelle through overspeed. Maybe the folks at Southwest Windpower have made it possible to design a simple rotor that survives.

Having written that paragraph several years ago when Skystream was first introduced, I have yet to personally know of any Skstream overspeed failures. However, Southwest Windpower insiders have since acknowledged that there in fact have been overspeeds and specifically mentioned confirmation of my warning.

For insight into these charges see Truth About Skystream & SWWP. The revelations in this post also confirm my earlier findings on the Air 403 and early versions of the Air X.

What I have not revealed previously is that I had privately and confidentially advised Southwest Windpower’s principals about my concerns during development of Storm, the predecessor of the Skystream. I did so in a personal visit to Southwest Windpower in Flagstaff in the spring of 2005 and in a subsequent email. While Southwest Windpower and I have had a rocky relationship over the years I do want them and small wind in general to be successful. We don’t need more failed turbines and certainly not another failed company.

I made a special effort to alert the company and its principals of my concerns when the easiest course of action would have been to “let them find out on their own.”

I am not pleased that I was correct.

Take Away Message

Should you buy a Skystream? Only you know your risk tolerance.

Would I buy a Skystream as it is today? My original conclusion was no, but I’ve watched it operate in high winds and it seemed to perform well. I’d seriously consider a Skystream, assuming no reports come in from the field of turbines destroying themselves.

Would I use a Skystream as it is today if one was given to me? My original conclusion was no, unless I was going to use it in an area that had no pedestrian traffic. As of the end of 2008, I’d say yes, I’d use it duly noting my reservations on SWP’s design philosophy and I’d keep a close eye on it.