The amount of electrical energy that can be generated from a water source depends primarily on two things: the distance the water has to fall and the quantity of water flowing. Hydro-electric power stations are therefore situated where they can take advantage of the greatest fall of a large quantity of water- at the bottom of a deep and steep-sided valley or gorge, or near the base of a dam (see figure 1).

Water is collected and stored in the dam above the station for use when it is required. Some dams create big reservoirs to store water by raising the levels of rivers to increase their capacity. Other dams simply arrest the flow of rivers and divert the water down to the power station through pipelines. While a water turbine is much more sophisticated than the old water wheels, it is similar in operation. In both cases blades are attached to a shaft and when flowing water presses against the blades, the shaft rotates. (The effect is the same as wind pressing against the blades of a windmill.) After the water has given up its energy to the turbine, it is discharged through drainage pipes or channels called the "tailrace" of the power station for irrigation or water supply purposes or, in some parts of the world, even into the ocean.

In a conventional coal-fired (thermal) power station each "generating unit" consists of a boiler, a steam turbine, and the generator itself. A hydro-electric generating unit is simpler and consists of a water turbine to convert the energy of flowing water into mechanical energy, and an electric generator to convert mechanical energy into electrical energy. The amount of energy available from water depends on both the quantity of water available and its pressure at the turbine. The pressure is referred to the head, and is measured as the height that the surface of the water is above the turbine. The greater the height (or head) of the water above the turbine, the more energy each cubic metre of water can impart to spin a turbine (which in turn drives a generator). The greater the quantity of water, the greater the number and size of turbines that may be spun, and the greater the power output of the generators.

Type of Water Turbines

Water for a hydro-electric power station’s turbines can come from a specially constructed dam set high up in a mountain range, or simply from a river close to ground level. As water sources vary, water turbines have been designed to suit the different locations. The design used is determined largely by head and quantity of water available at a particular site. The three main types are Pelton wheels, Francis turbines, and Kaplan or propeller type turbines (named after their inventors). All can be mounted vertically or horizontally. The Kaplan or propeller type turbines can be mounted at almost any angle, but this is usually vertical or horizontal. The Pelton wheel (see figure 3) is used where a small flow of water is available with a ‘large head’. It resembles the waterwheels used at water mills in the past. The Pelton wheel has small ‘buckets’ all around its rim. Water from the dam is fed through nozzles at very high speed hitting the buckets, pushing the wheel around.

The Francis turbine (see figure 4) is used where a large flow and a high or medium head of water is involved.

The Francis turbine is also similar to a waterwheel in that it looks like a spinning wheel with fixed blades in between two rims. This wheel is called a ‘runner’. A circle of guide vanes surround the runner and control the amount of water driving it. Water is fed to the runner from all sides by these vanes causing it to spin. Propeller type turbines are designed to operate where a small head of water is involved. These turbines resemble ship’s propellers. However, with some of these (Kaplan turbines, see figure 5) the angle (pitch) of the blades can be altered to suit the water flow.

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