[NOTE: since I wrote the post below I have received email from David Rabkin, the project leader at the MOS. David has commented on the post (and I will continue the conversation in the comments below) but he would like it made clear that the sensors have not been calibrated and the data is not to be considered entirely reliable. And I want to make it clear that I have no official connection or financial interest in the MOS or any of the turbine manufacturers. In the weeks prior to my visit I attempted to contact Mr. Rabkin by phone and email to get a tour, but as I understand he has a lot on his plate, and I didn't hear back from him - and so what I write below are my personal observations and conjectures, with no inside information. Now that we are in touch I hope to get the real scoop on what's up with the turbines.]
This morning I had a holiday-related package to deliver to my aunt Lucy, who works at the Boston Museum of Science, and being a wind turbine enthusiast I took advantage of the opportunity to check out the exhibit and turbine installation that the museum has installed. They have a cool display that shows what each turbine is doing in real time, and an interface where you can access cumulative data for each machine as well.
First, here’s a view of the installation from the north bank of the Charles river:
From left to right, the turbines are 1. Mariah’s 1.2kW Windspire, 2. Southwest Windpower’s 2.4kW Skystream, 3. Swift’s 1kW 5 blade turbine from the UK, 4. five Aerovironment 1kW “Architectural Wind” turbines, and 5. Proven’s 6kW unit.
Here’s an image of the view to the west from the function room (row of big windows just under the museum tower) – there’s at least a two mile fetch upriver, past the Longfellow bridge and MIT:
I was most interested in the energy production data for the various turbines. Here’s a snapshot of the top level display, taken at random while I was there. (As in the previous post, in the interest of full disclosure I should note that some years ago I worked for Southwest Windpower, where I did a substantial chunk of the mechanical design for the Skystream turbine.)
The display shows a current windspeed of 17 mph, out of the south. I think the direction sensor must be miscalibrated, since the wind was out of the northwest. The displayed windspeed ranged from around 10-20mph (updating roughly every second) while I was watching it. Qualitatively it was a very windy day; in riding my bike back to Central Sq there were times when the headwind on Broadway just about brought me to a standstill. If you look closely, you see that the Windspire, the Swift, and the Aerovironment turbines are listed as “spinning, but not enough wind”, and the Skystream and Proven turbines are listed as producing 575 and 685W respectively, at the instant I happened to snap the picture. The story was the same for the entire time I was watching, The Skystream and Proven machines were producing power, and the others were listed as spinning but not producing.
Then I clicked through for the cumulative data for each turbine, which I tabulated as follows – yellow numbers I took from the MOS display, green is my additions (you can click the image to get readable-size text):
The project apparently got started on October 9, or at least the display indicated that the cumulative production was taken starting from that date. Since inception, the Proven has produced 538 kWh, the Skystream 344 kWh, the Aerovironment machines 67 kWh, the Windspire 57 kWh, and the Swift brings up the rear at 2.4kWh. The MOS display also calculates capacity factor, which ranges from a high of 9.2% for the Skystream to 4.6% for the Proven, to 2.4% for the Windspire, 0.68% for the Aerovironment machines, and 0.08% for the Swift. Capacity factor is the amount of energy actually produced in a given site, as compared to what the turbine would produce if it operated continuously at its nameplate power rating – values of 20-40% are the norm for commercial installations, with many homeowner installations lower than that. The MOS also calculates a “relative production” for each turbine, which I gather to be the actual production as a percentage of manufacturer’s claimed energy production for a site with the average windspeed at the Boston MOS. This ranges from 61% for the Skystream down to 0.5% for the Swift.
The Swift shows zero production for the last month, and I have never seen it spin more than 1 rev per second, so I suspect it has either burned out (alternator shorted, which applies a continuous high torque to the rotor) or been shut down for some reason. The Windspire (which appeared to be running when I last walked past the museum a few weeks ago) also appeared to be shut down or shorted out today (more on that later). No such excuses can be made for the low productivity of the five Aerovironment machines, which had produced 2 orders of magnitude less energy on the day than the Skystream, despite having over 2X the aggregate nameplate rating.
Just for fun I looked up the swept area of each machine, so I could calculate energy production on a per area basis (also in the table above) and scrounged up very rough cost estimates for each machine (I cannot vouch for these numbers, they are just what I found in a few minutes poking around the web) and used these to estimate a best-case cost of energy assuming a 20 year life (which is way longer than any manufacturer is willing to warrant a turbine). These COE numbers range from 35 cents per kWh for the Skystream to $50 per kWh for the Swift.
My aunt kindly showed me around and took me up to the top floor for a closer look at the machines. Here’s a view of the Proven:
I also noted from this vantage point that the anemometers for each of the large turbines were at about 10′ off the roof, well below hub height in each case. This should be kept in mind if power curve data is ever reported from this installation. I could not get close-up pictures of the Swift and Aerovironment turbines, because they were mounted just above my head, and we did not have access to the roof. (On the plus side, I did not notice any noise from these machines being transmitted into the building – on the minus side, they were not generating any significant energy.) I did get a clue as to what might be going on with the Mariah turbine, which continued to rotate slowly in stall. Check out the following video at
http://www.youtube.com/watch?v=QLoVdzdq80s (apparently I have to pay wordpress to get it to embed.)
What I observed is that the tip of the tower was rotating eccentrically by close to the diameter of the tower tip – maybe 4″ as best I could estimate from that distance. That much eccentricity would cause a hell of a vibration at full RPMs – enough to freak someone out and cause them to shut it off perhaps, and maybe enough to cause the rotor in the alternator to crash against the stator and short it out, leading to a permanent braked condition.
In conclusion, based on the data that the MOS has collected, the Skystream appears to come out on top in terms of capacity factor and energy capture per swept area, with the Proven machine a respectable second. The Skystream has the best inferred cost of energy by a factor of 2. The Swift has been out of commission for at least a month, and the Windspire seems to have suffered some kind of failure since I last inspected on December 6. These apparent issues make it difficult to compare performance of these machines, but it is not a particularly good sign to have two turbines down for the count after less than 3 months. It would seem advisable for the manufacturers of these machines to be closely monitoring an installation in such a prominent location and conducting prompt repairs, given the potential damage to their reputations (thousands of people tour the Boston MOS every day). All 5 Aerovironment machines seem to be operating normally, which makes the extremely low production numbers and capacity factor troubling. An inferred cost of energy of $9 per kWh is nothing to be proud of, and probably indicates that these machines will never pay back their embedded energy (that would be an interesting exercise for somebody to do). Given the competent reputation that Aerovironment seems to have as an engineering company (they made working human powered aircraft etc.) and the fact that the turbines don’t seem grossly broken, it seems that the constrained-yaw, parapet-mounted turbine concept should be considered suspect until proven otherwise by published energy production data (which the manufacturer does not appear to offer.) More generally, the “unconventional” designs do not seem to fare very well so far in the Boston MOS experiment, as compared to the 3 blade HAWT standard. And it appears that the production data support my initial conjecture of December 9th – namely that putting wind turbines on buildings does not obviate the need for a tower of significant height.