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[VAWT] Plausible improvements of the Savonius windmill


Cut a barrel in half, mount it on a shaft and you have a windturbine. If the barrel is used, then what else probably would end up on a scrap heap is instead used for converting renewable energy.

Simplicity and low cost makes this an attractive option especially for societies with limited economy and a malfunctioning or non existing electric grid. Small off-grid electrical networks can be built and people who perhaps most needs electricity get that. Isn't that neat?


Savonius (from http://solarvan.co.uk/savonius)

An overlooked potential?
The Savonius though has a widespread reputation of having low effiency and is often dismissed as a credible option around forums and in formal litterature. However, when looking at the graph below from a publication Wortman did 1983, the effiency can be realtively high provided that the TSR(Tip-to-Wind-Speed-Ratio) is held at a correct value and the windmill should work quite nicely. In practice this could probably be done by controlling the generators braking torque by controlling it's output current.
 



As far as I could see nobody else had published info regarding how the effiency of the Savonius could be improved by controlling the TSR. I therefore sent this in as a proposal for a Bachelors Thesis at Chalmers and we're now a group working on how or if this could be achieved.

Preliminary calculations
My part in the project so far has mostly been to determine analytical expressions and doing calculations. The preliminary results were most interesting. They are not yet mature for publicing, but anyway showed that if the generator isn't carefully matched just rotor effiency only easily can drop to on average 5-10%  in the operating wind range.

Today I derived a preliminary analytic expression, with some simplifications, for average diode losses in a step-up converter connected to the generator. These also grow significantly high if the all system parameters aren't carefully choosed.

A Savonius mill that is built without considering theoretical implications could thus get a total effiency of just a few percents. And even if these parameters are carefully choosen, parameter variations still could have a huge impact on the total effiency. These could be reasons why the Savonius concept as a whole perhaps has got a worse reputation than it deserves.

First prototype
Rapid prototyping is great whenever possible I believe. The other day we created this quick-and-dirty model of a Savonius. It's made of used cable drums and melting glue and hooked up to a small BLDC motor.


This was spinned by bursting some pressurized air on it.
We already learned some from this first prototype, among other things that the generator picks up quite a lot of electrical noise from the sorroundings.

[VAWT]?
From now on I choose to tag my posts so that they can be separated more easily. This project is tagged [VAWT], standing for Vertical Axis Wind Turbine.






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