Turbines

Hydro turbines generate power by converting the potential energy of rainfall in a high catchment area into kinetic energy to rotate turbines. With several different turbine types, turbine selection depends on a number of factors, principally the distance which the water descends before reaching the turbine (known as the head) and water flow available. Other aspects to be considered include the desired running speed of the generator and whether the turbine will be expected to operate in reduced flow conditions.

Turbines can be classified into two groups; impulse turbines, which have a rotor that operates in air, driven by one or more jets of water and reaction turbines, which have a rotor that is fully immersed in water and is enclosed in a pressure casing.

There are two main types of reaction turbine:

• The Propeller turbine - similar in principle to the propeller of a ship operating in the reverse. Various configurations of propeller turbine exist; a key feature is that for good efficiency the water needs to be given some swirl before entering the turbine runner. With good design, the swirl is absorbed by the runner and the water that emerges flows straight into the draft tube. In some cases the blades of the runner can be adjusted, in which case the turbine is called a Kaplan. Although the cost of adjusting turbine blades and guide vanes means that it is generally only affordable for larger systems, it greatly improves efficiency over a wide range of flows.
• The Francis turbine - essentially a modified form of propeller turbine in which water flows radially inwards into the runner and is turned to emerge axially. Although an efficient turbine, improvements in technology meant it was eventually superseded by more compact propellers capable of faster-running for the same head and flow conditions. However, many of these 'open-flume' Francis turbines are still in place, and this technology is still relevant for refurbishment schemes.

There are three main types of impulse turbine in use:

• The Pelton turbine - consists of a wheel with a series of split buckets set around its rim; a high velocity jet of water is directed tangentially at the wheel. The jet hits each bucket and is split in half, so that each half is turned and deflected back almost through 180º. Nearly all the energy of the water goes into propelling the bucket and the deflected water falls into a discharge channel below.
• The Turgo turbine - The Turgo turbine is similar to the Pelton but the jet is designed to strike the plane of the runner at an angle (typically 20°) so that the water enters the runner on one side and exits on the other. The flow rate is not limited by the discharged fluid interfering with the incoming jet due to the difference in its shape.
• The Crossflow turbine - the shape of the blade is such that on each passage through the periphery of the rotor (it makes two traverses) the water transfers some of its momentum, before falling away with little residual energy.