Hydroelectric - How It Works
Hydroelectric power is the energy derived from flowing water in rivers, or from man-made installations where water flows from a high-level reservoir down through a tunnel and away from the dam. Water power was used for centuries to power machinery, for example for grinding corn or in mills and factories, but was largely replaced by steam power in the Industrial Revolution. Water power is now mainly used to generate electrical energy.
How it works
Turbines placed within the flow of water extract its kinetic energy and convert it to mechanical energy. This causes the turbines to rotate at high speed. The turbines drive a generator that converts the mechanical energy into electrical energy. The amount of hydroelectric power that can be generated is related to the water flow and the vertical distance (known as ‘head’) through which the water has fallen.
In the smallest hydroelectric schemes, the head of water can be a few metres; in larger schemes, the power station that houses the turbines is often hundreds of metres below the reservoir.
Hydroelectric systems can be connected to the main electricity grid, or can be part of a stand-alone power system. In a grid-connected system, any electricity generated in excess of consumption on site can be ‘sold’ to electricity companies. In an off-grid hydroelectric system, electricity can be supplied directly to the user or via a battery bank.
There are three main types of hydroelectric schemes:
Storage schemes: In storage schemes, a dam impounds water in a reservoir that feeds the turbine and generator, usually located within the dam itself.
Run-of-river schemes: Run-of-river schemes utilise the natural flow of a river, where the continuity of flow can be enhanced by a weir. Both storage and run-of-river schemes can be diversion schemes where water is channelled from a river, lake or dammed reservoir to a remote powerhouse containing the turbine and generator. A canal or low-pressure tunnel transports the water to this end point and then back to the river or to another watercourse.
Pumped storage: Pumped storage incorporates two reservoirs. At times of low demand, generally at night, electricity is used to pump water from the lower to the upper basin. This water is then released to create power at a time when demand, and therefore price, is high. Pumped storage is not a renewable application as it is reliant upon an electricity supply and energy losses are always involved when pumping the water. However, by providing a rapid supply of electricity in response to sudden changes in demand, it does have value in aiding the overall efficiency of the generation infrastructure.
Tidal barrage systems can, like run-of-river schemes, use the incoming and outgoing tidal flow, or, like pumped storage schemes, store the incoming tidal flow in a reservoir to be released at low tide.