Swift Turbine Specs Look Dubious

“Oh no, not again!” I’m saying to myself, and really hoping that — just once — I’ve done my sums wrong.

You’ve seen my rant about how the initial public specification of the WindSave rooftop wind turbine was an impossibility (and, in fairness to them, how the corrected specs are much more like the thing). And you may have seen that I’ve written about the Swift before. But the Renewable Devices Swift was all over the blogosphere (a hateful word, I must say) today; both Treehugger and sustainablog were on it.

So I download the very pretty PDF spec sheet. Here’s the technical table from the document, which is dated 19 November 2004:
swift wind turbine spec, from document dated 2004/11/19
So that’s a diameter of 2m, rated wind speed of 10.5ms-1, and a rated power of 1.5kW. Plugging that into my simplified Cp equation

Cp = P / ( 0.48106 d2 v3 )

which gives:

Cp = 1500 / ( 0.48106 × 22 × 10.53 ) = 0.67

As this is higher than the Betz Limit of 0.59, the claimed power output of the Swift wind turbine is theoretically impossible.

I don’t know how to put this, but rooftops are sheltered places; if they get any wind at all, it tends to be turbulent and highly directional. You get huge updrafts, none of which help generate power. I know of some very open sites that struggle to get the 3.5ms-1 cut-in speed of the Swift, and that speed is at 50m+, not on a rooftop.

Renewable energy, for me, is about using the appropriate technology for the right location. Devices like the Swift are a distraction from the whole conservation/renewables agenda.


  1. I am please to see that people are out there questioning the performance of this machine. I have been in the small wind business for 18 years and our company has produced more than 80,000 machines. I am not excactly sure who is behind this product but it certainly does appear to be well funded. The problem is they have stolen a number of ideas from the competitors of the world and incorporated them into this product. Their stall regulation is something we invented and patented in the U.S. We were the first company to promote the use of “roof top” mounting. Many of the coponents we see are of our designs. The ring is from the Marlec company in the UK.

    You are absolutely correct in your energy calcualtions. This machine does not have a chance of producing the claimed output. Furthermore, roof top mounting is not all that they claim. While it can be done, understanding prevailing wind resources are critical to roof top mounting. Furthermore, mounting it next to a chimney is just plain stupid. THe soot from the smoke will wreck the efficiency in the first month.

    The machine is certainly interesting. It does have a lot going for it and will be a good competitor in the market place. However, if they take the positon of mounting and energy production as they have in their brochures, this product will certainly be another scar in the long line of small wind turbines that have come and gone.

    This machine should never be allowed to go to market unless they can offer independent performance data from a reputable testing facility.

    Andy Kruse

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