The spinning brush moves up and down the tower so that it washes the entire length of each blade. The strands of the brush are thick, and they are made of soft material such as cotton. The spinning brush is similar to a spinning brush you’d expect to see in an automatic car wash. The strands are not stiff, but they are held in an extended radial position by centrifugal force. Soapy water is continuously applied to the brush as the blades are washed so that the strands stay wet and soapy.
The turbine blades rotate at a slow to moderate rpm while they are being cleaned. The strands of the spinning brush slap against the sides of the blades that face the tower, and then drag across these surfaces. This cleans the tower sides of the blades. But what about the sides that face away from the tower? The length of each strand is equal to the distance required to reach the blades, plus the length of the chord near the root (where the chord is a maximum). Whenever a blade collides with the spinning brush, some of the strands will wrap around the side of the blade that faces away from the tower. These strands will subsequently drag across the far surface of the blade as it continues to move through the spinning brush. Thus, both sides of the blade are thoroughly cleaned.
An Alternative Method
The spinning brush idea is simple. I see no reason why it shouldn’t work. But in case it doesn’t, here’s an alternative approach.
Imagine a tube that carries soapy water through the inside of a blade, exiting the trailing edge of the airfoil at a point exactly halfway between the root rib and the tip. The diagram above depicts only the water ejected from the top blade as it travels from the 6 o’clock position to the 12 o’clock position. The water pressure and rotor rpm are adjusted so that the water that the top blade ejected as it passed through 6 o’clock has just enough time to fall half a blade length before the next blade passes through 6 o’clock. You can see the result in the diagram – the water hits the tip of the bottom blade. Given this state of affairs, it follows that the water that the top blade is ejecting in the 12 o’clock position depicted in the diagram will also fall half a blade length before the bottom blade passes through 12 o’clock, and that water will hit the blade at its root. Since the stream of water is continuous, and since it hits the tip of a 6 o’clock blade and the root of a 12 o’clock blade, then it must hit every other point on a blade as it passes from 6 o’clock to 12 o’clock. So the entire length of each blade is washed.
Of course, either of these blade washing techniques will work on upwind or downwind turbines, regardless of how may blades they have.