Building a Darrieus that will both operate at all and operate safely for years is difficult.
For starters, consider starting: Look at the power curve at low TSR: A Darrieus already has to be spinning faster than the wind to collect power. It has essentially no starting torque except when the blades are at a particular narrow range of angles to the wind, so a straight-bladed Darrieus usually needs something to push it to get it going. (But you can't count on it to safely NOT start from the wind.) (The spiral designs, and perhaps those with three or more blades, may self-start.)
Notice the tip speed ratio in the 4 to 6 region. Imagine that mill spinning in 100 MPH hurricaine winds. The blades will be moving at several hundred MPH, perhaps trying to break the sound barrier. Even at lower winds those suckers are really moving - and presenting a narrow leading edge. Sort of like swinging an oar so it hits edge-on. Not something you want to walk into - or have birds, housepets, or neighbor kids do the same. And you need a strong structure to hold the blades into a circular path against the centrifugal effect.
The wind load on an efficient turbine is nearly that on a solid wall with the same cross-section to the wind as that swept by the blades. A Darrieus is a very spinly thing, with narrow blades. So that force is very concentrated. And (unlike with a horizontal-axis machine) the force reverses direction twice per revolution. Imagine that massive force flexing the blades back-and-forth several times a minute for years. This is a recipe for material fatigue.
Result: Darrieus mills tend to come apart - usually while operating in a high wind - sending chunks flying at several hundred MPH. Not something you want near your house or your neighbor's yard.
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Now consider a Savonius:
The diagram shows a peak efficiency of about .3 - but that's for a classic Savonius: Two half-cylinders overlapping by about 50%. Sandia labs did an optimized blade profile that they patented (now expired), which they claim does about .37 - nearly the .42 or so of a Darrieus.
Notice that the peak is at a TSR of about 1. The edge of the mill is moving at about the speed of the wind, not several times as fast. Much safer. The profile is also curved so it tends to push out things that reach in slowly, as opposed to being hit by a blade edge.
The structure of a Savonius - classic or Sandia - is quite strong. The blades are large so the wind forces are distributed over a lot of area. Spinning slowly means it doesn't need a lot of strength to hang together. And it's self-starting.
The major downside (as with all VAWTs) is the slow rotation speed. The amount of power you can get out of a given arrangement of magnets and coils goes up with the speed. So you need to do one of three things:
- Build an alternator with a LOT of magnets and coils.
- Build an alternator with a large diameter.
- Come up with some kind of speed-changer (gearbox, belts, bicycle chains and sprockets, or fricton-drive tire) to spin the alternator much faster than the rotor.
The minor downside is that with less efficiency than other mills you have to build it bigger to collect a given amount of power. This also means it's hard to raise it up to where the wind is decent. (In your case you're stuck with it being at ground level so your answer is make it - or several of them - large.)
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Ed Lenz (on this board) has come up with another style of VAWT which has characteristics similar to (but apparently somewhat better than) the Savonius. It consists of three curved wing/bucket vanes and one instance had a measured average efficiency of .365 over a range of wind speeds and loadings - as compared with the claimed - and probably narrow - peak of .370 for the Sandia design.
