This story is from Erik Andrus, owner of Good Companion Bakery, and submitted as part of our Wisdom From Our Elders collection of self-sufficient tales from yesteryear.
The idea for the Savonius rotor began with our cheapness. We did not want to pay for the electricity it to run our Austrian flour mill for the long stretches required to mill flour for our bakery. We were not interested in the large cost of a wind turbine. I was surprised to find out that although milling is one of the oldest and most proven applications for wind power (along with pumping water), nobody does this anymore, at least not outside of farming museums.
Why not? Small scale decentralized grain growing and milling are making a comeback, why shouldn’t the same be true for wind-powered mills?
One of the few examples of a wind-powered grain mill I was able to find was the Savonius rotor built by Job Ebenezer. It was part of the University of New Mexico’s project Technology for the Poor in 1978, at the height of the oil embargo. I subsequently made contact with a retired turbine engineer named Victor Gardy, living in Charlotte. Victor has had a long-term side interest in the Savonius rotor, and provided insights into my energy choices.
A little bit about the Savonius. As a design, it doesn’t get a lot of respect in the engineering community, because it’s an inefficient device. Wind devices are all pretty inefficient (a 100 percent efficient wind machine would capture 100 percent of the energy in a breeze, leaving dead-calm air downwind of it, which is impossible). The maximum efficiency theoretically possible for wind devices is around 58 percent. Most devices are in the 20 percent range. The Savonius is around 15 percent, and can’t be improved much higher than that no matter what materials are used or how they are configured into a design.
This is because the Savonius is a vertical axis wind turbine (or VAWT), and as such has inherent loss of efficiency that a horizontal axis wind turbine would not have. This is because the wind is collected in cups, which then fall away from the wind, allowing the next cup to be filled. The cups return with their backsides facing into the wind, which creates drag. The cup catching the wind is enough to overcome the drag and allow the rotor to turn, but energy is still being wasted, no matter what you do. This is very annoying to most professional engineers, and leads most to write off the VAWT concept entirely. So I grant you, the Savonius is not-so-efficient.
So what’s good about it? Well, lots of things. Let’s assume we are going to build our own rotor out of everyday materials, and that the resulting device will be somewhat inefficient. So what? You can compensate for that inefficiency by building two, or by making it bigger, until you reach the production level you want. Let’s compare the Savonius with the typical wind power solutions available on the market today. I’m assuming that in both cases we are aiming to power a home, small business, or a farm, and not a town, city ward, or a factory. The table below is very general, but it should give you the general idea that the efficiency of the device is far from the whole story.
|
Home-built Savonius |
Typical Horizontal-Axis |
|
|
Cost |
A few thousand dollars |
Thirty or fourty thousand dollars |
|
Installation |
Low. Possibly even |
As high as possible. 100 feet or more. |
|
Efficiency |
Lower |
Higher |
|
Average wind |
Low |
High |
|
Weight |
Heavy |
Light |
|
Nature of |
Ordinary materials available |
Highly-engineered materials only produced |
|
Skill level |
Fairly Low |
Very High |
|
Level of Danger |
Fairly Low |
Very High |
As you can see, despite the inefficiency, the home-built Savonius has some pretty big advantages. A typical turbine might require climbing a 100 foot tower or having to winch the whole tower up and down. The cost of purchase might feel more like a second mortgage than an energy investment, and you would have to pay a specialist to service your windmill (the warranty of which would be nullified if you worked on it yourself). Luckily, if you’re a handy person, with a moderately windy spot, who is up for an alternative energy investment of a few thousand dollars, than the Savonius might be the alternative for you. At lease it is a serious option for a small farm like ours.
At first I was interested in a Savonius that would transmit power directly to a flour mill without being converted to electricity. Victor Gardy convinced me that electricity is a sensible approach which would avoid some of the pitfalls of mechanical transmission. I would not be limited to milling only when the wind was available, and within a limited range of wind speeds. A net-metered or battery-bank system allows for more flexibility, and would allow the device to contribute towards all farm energy usage, not just the flour mill.
It seemed like we had a good approach that was innovative enough to be of real interest to like-minded folks but not so innovative as to be a total shot in the dark. We applied for a SARE (Sustainable Agriculture Research and Education) Farmer Grant to build a prototype device and subject it to extensive testing.
We received funding in 2010 and are still in the midst of the project. By late winter or early spring 2011 we will have the device collecting data. Once we have collected three months of voltage-production data we will be ready to publish a complete report, including a construction manual, and to go on tour presenting our findings. Exactly how well a low-cost low-height Savonius will compare to commercially available wind and solar alternatives is yet to be seen. Once we have the production data, it should speak for itself. However if I may hazard a guess (with the support of Victor Gardy, who is serving as a consultant and advisor) I would estimate that our prototype will produce enough electricity to offset the cost of its materials as well as a living wage for the builder in about three or four years. Compare this to a 15-year payback for a typical commercial option. One Bergey 10kw windmill we looked at had a payback period of about 23 years at our current utility rates. Investing in a Savonius rotor is an valuable alternative to the high cost of a wind turbine.
Photo courtesy of Erik Andrus, Good Companion Bakery
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