It was a beautiful cold and breezy day in Cambridge, after the first ~1″ snowfall of the year, so I took the pups for a walk down by the river. I remembered that the Boston Museum of Science had installed a bunch of small wind turbines on their roof, so I headed downstream to take a look. On the north wing of the building they have installed a Windspire (a 1.2kW vertical axis giromill) and a Skystream (a 2.4 kW horizontal-axis 3 blade downwind turbine). The central tower sports a Swift (a ~1kW 5 blade upwind machine with a hula-hoop at the OD of the rotor and two tails) and a row of ~5 small machines by Aerovironment (1kW, 5 blade machine designed for building parapets). The south end of the building had a big Proven machine (3 blade downwind).

It was interesting to compare the behavior of the various turbines. At this point I should mention that I was part of the team that designed the Skystream turbine, so I should not pretend to be entirely objective in the following discussion. The Windspire was running continuously and cuts a dramatic figure; it’s reasonably attractive but the blades are straight and long, and they had a sort of strobing effect as they passed the central axis and flashed in the sun. As I got closer it appeared as if a short segment of the airfoils was missing – there was an interruption in the airfoil between the bottom and the center, which does not appear in their literature. The ring around the very top of the rotor was not spinning quite true, which made me wonder whether there had been some kind of mishap. Between its size and the linear nature of its rotor, the Windspire is more visually imposing than the Skystream, despite sweeping less area and producing only 170kWh/month in a standard site as compared to the Skystream’s 400kWh/month (mfg specs in both cases). The Aerovironment machines were spinning most of the time, though a good bit of the time it appeared as if they weren’t spinning fast enough to generate power, and they were yawing around more than the larger turbines – even in such an exposed site, the flow directly above a building parapet must be fairly turbulent, and the central tower behind the turbines couldn’t have helped. One gets the feeling from the pricing (over $10/watt) and the product literature (which does not specify energy production or offer a power curve) that these are sculpture, not renewable energy equipment. The Swift was the least impressive; it was turning some of the time but never appeared to attain sufficient speed to come out of stall and begin producing energy. Perhaps it was turned off (or maybe burned out). These machines are also in the sculpural category; online articles put the installed cost at $10-12k, which is about the same as a Skystream, which has 3 times the swept area. Off by itself at the far end of the building, the big Proven ran continuously and smo0thly; of all the machines it nodded the least to artistic notions, but projected a reliable, stoutly engineered aura, with large, dense bolt patterns visible at the blade and hub attachments. These machines are also not cheap, with a 2.5kW model running around $18,000 as best I can tell online.

All in all, my impression was that the Skystream and Proven machines were cranking solidly, the Mariah was big for its output and looked a little beat up, and the other machines were probably not delivering much (or any) energy. Probably not coincidentally, this rank order is the same as the heights of the towers the machines were mounted on – the Skystream and Proven machines were on substantial pole towers, while the bottom of the Mariah machine was only ~10′ off the roof, and the other turbines weren’t even visually delineated from the structure.

Elsewhere in the urban wind department, I’ve noticed that there are two big machines on a parking garage over in Alston across the river from Harvard; they appear to be Bergey 10kW units, though they’ve got an aftermarket Harvard paint job.

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