Back in the brutal days when there were no computers, a machine was defined as a device to change the direction, magnitude and appplication point of a force; and the "simple" machines include the lever, the screw, and the wedge. A cherry pit, for example, is a well lubricated wedge, and the way it shoots out from between the schoolboy's thumb and forefinger shows that it is driven by a force almost perpendicular to the one the boy's fingers applied.
Now consider another simple machine that will change the direction of a force, a railroad truck under sail (shown in bird's eye view below). The truck can move only left or right along the rail track, and the sail is so oriented that it will move to the left, or into the wind.
Suppose now that we are looking not at a railroad truck, but at a sailboat on the water; its rudder is so held that it can move (almost) only in the direction of what were railroad tracks in the preceding example. Then the same thing will happen: The boat will sail "into the wind." at cannot of course, sail directly into the wind, but by maintaining a zig-zag course, it can use this principle to advance even into that direction. One of the early sailors who knew this technique was the Viking chieftain Raud the Strong in the 10th century; this so amazed the Norwegian king Olaf Trygvasson, a forerunner of Dr Helen Caldicott, that he accused him of being in league with the devil and had him executed in a particularly horrible way for his witchcraft.
The principle of the airfoil (airplane wing). As the foil is thrust (or glides) forward, the air is compressed underneath it, and rarified above it, resulting in an upward pressure on the foil.
Now the blade of a Darrieus rotor has the tear-shaped profile of an airfoil, and since its attachment to the shaft leaves it¾only two ways to go, it would move forward, into the wind, just like a boat sailing into the wind - if it were the only blade on the rotor, that is; and even then if Iit would travel only a small arc into equilibrium. But when there are two blades opposite sides, it would seem that any effects will exactly cancel and nothing will happen That is exactly right: The Darrieus rotor is not self-starting, and without being brought up to speed first, it will just sit there, like a glider that has not been given a starting velocity by a catapult or a towing airplane.
But if the rotor is first started by other means, the wind direction, as the blades see it, will change. The reason for that is much the same as vertically falling rain drops leave an oblique trace on the window of a moving car. In part this is due to aerodynamic effects, but even if the rain fell in a vacuum, this phenomenon of "aberration" would set in, for in the time that it takes for a raindrop to fall from top to bottom of the window, the window has moved on in a forward direction. The drops, viewed from the car, slant backward against the direction of travel; and for the same reason, the wind, when viewed from the traveling blades, slants 'backward" as shown by the light arrows. And like Raud the Strong, both blades will move into this effective wind.
Darrieus rotors are constructed with two or three blades (foild) and in phi or delta configurations. Sandia Labs in Albuquerque are experimenting with a 17m (56 ft) diameter Darrieus, rated at 60 kW capacity. It is brought up to speed by its generator run as a motor on power supplied by the local utility.
Smaller Darrieus rotors are often started by self-starting (but inefficient) turbines on the same shaft, such as an S-shaped Savonius rotor or cups on horizontal spokes.
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Vol. 6, No. 6
Newsletter: Access to Energy Newsletter Archive Volume: Volume 6 Issue/No.: Vol. 6, No. 6 Date: February 01, 1979 04:09 PM Title: Energy and dishonor
Copyright © 2004 - Access to Energy Newsletter Archive
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