>>THE HYDROELECTRIC ENERGY

THE HYDROELECTRIC SYSTEMS

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator . In this case the energy extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head . The amount of potential energy in water is proportional to the head. To obtain very high head, water for a hydraulic turbine may be run through a large pipe called a penstock .

Pumped storage hydroelectricity produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped storage schemes currently provide the only commercially important means of large-scale grid energy storage and improve the daily load factor of the generation system. Hydroelectric plants with no reservoir capacity are called run-of-the-river plants, since it is not then possible to store water. A tidal power plant makes use of the daily rise and fall of water due to tides; such sources are highly predictable, and if conditions permit construction of barrages and reservoirs, can also be dispatchable to generate power during high demand periods.

Less common types of hydro schemes use water's kinetic energy or undammed sources such as undershot waterwheels , the relatively recent field of hydrokinetics.

Formula

A simple formula for approximating electric power production at a hydroelectric plant is: P = h r k , where P is Power in watts, h is height in meters, r is flow rate in cubic meters per second, and k is a conversion factor of 7500 watts (assuming an efficiency factor of about 76.5 percent and acceleration due to gravity of 9.81 m/s 2 , and fresh water with a density of 1000 kg per cubic metre. Efficiency is often higher with larger modern turbines and may be lower with very old or small installations due to proportionately higher friction losses).

Annual electric energy production depends on the available water supply. In some installations the water flow rate can vary by a factor of 10:1 over the course of a year.

Industrial hydroelectric plants

While many hydroelectric projects supply public electricity networks, some are created to serve specific industrial enterprises. Dedicated hydroelectric projects are often built to provide the substantial amounts of electricity needed for aluminium electrolytic plants, for example. In the Scottish Highlands there are examples at Kinlochleven and Lochaber , constructed during the early years of the 20th century. In Suriname , the Brokopondo Reservoir was constructed to provide electricity for the Alcoa aluminium industry. New Zealand's Manapouri Power Station was constructed to supply electricity to the aluminium smelter at Tiwai Point . As of 2007 the Kárahnjúkar Hydropower Project in Iceland remains controversial.

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