Pelton wheel

This is a Pelton wheel, the type of water turbine used in the Cobb Power Station. The Pelton wheel is suited for hydro stations that have a high head, which Cobb does. They are very efficient and can generate power from a small volume of water. For instance, the flow of the Cobb River into the reservoir is roughly equivalent to the flow of the Avon River through Christchurch city, and, let’s face it, the Avon is kind of just a big stream. Because there’s such a drop from the reservoir down to the station with the Cobb, a large amount of pressure is built up by the head. This pressure is then focused into nozzles that raise the pressure even higher. The result is that a very small amount of water spurts out the nozzles at high speed into the cups of the Pelton wheel. This spins the wheel to incredibly high velocities and all this energy is converted into power. Most other hydro stations in New Zealand don’t have such a high head; instead of Pelton wheels, they use Francis turbines that rotate at much lower speeds and require much higher volumes of water to generate power.