1. Field
The present invention relates to a blade with a turbine operating in water, such as a device driven by tidal or river flow. It is particularly designed for a turbine arranged to rotate about a horizontal axis. However, the blade is suitable for use in any orientation.
2. Description of the Related Art
For a turbine arranged to rotate about a horizontal axis in water, the blades are conventionally hollow. However, this leads to a problem in that as the blade rotates about the horizontal axis, its depth changes. At the tip of the blade, there is a variation in depth in a single rotation equal to the diameter of the blade. This subjects the blade to significant variations in static pressure, which, on a hollow blade translates to repeat expansive and compressive forces giving rise to potentially severe cyclical stress problems.
One attempt to solve this problem is disclosed in WO 2004/029448.
This raises the possibility of filling the void with a light-weight material such as foamed plastic, but observes that the cyclic loading will cause the filler material to become detached from the outer shell thereby creating a void. WO 2004/029448 rejects this approach in favour of filling the blade with liquid. The pressure of this liquid is then allowed to fluctuate either by allowing liquid into and out of the blade, or by providing a pressure compensating device such as a diaphragm or piston to equalise the internal and external pressures.
With a liquid filled device, the centrifugal forces which are experienced in use will cause an increase in pressure at the tip of the blade. If this is vented, the internal space of the blade is not sealed and can be subject to fouling. On the other hand, if the space is sealed then stress at the tip can cause damage. Also, the variations in internal pressure caused by the centrifugal forces are unpredictable and can damage the internal structure.