Most hydroelectrity comes from the potential energy of dammed water driving a water turbine coupled to a generator. Therefore the energy extracted from the water much depends on the difference in height between the source and the waters outflow. This height difference is called the head. To obtain a very high head, water for a hydraulic turbine may be run through a large pipe called a penstock.
While most of the power stations are built for public utility networks, there are those dedicated to supply large electricity needed for aluminum electrolytic plants and other industrial establishments. Examples include: In the Scottish Highlands of United Kingdom, Kinlochleven and Lochaber were constructed during the early years of the 20th century. In Suriname, the Brokopondo Reservoir was constructed to provide electricity for the Alcoa aluminum industry. New Zealand's Manapouri Power Station was constructed to supply electricity to the aluminum smelter at Tiwai Point
Generation of Power at a H.E.P StationThe dam is constructed across a river or a lake and water collects behind the dam to form a reservoir. A pressure tunnel is taken off the reservoir and water is brought to the valve house at the start of the penstock. The valve house contains the main sluce valve which controls the water flow to the power house and automatic isolating valves which cuts off supply of water when the penstock bursts. From the valve house water is taken to the turbine through the penstocks. The turbine converts hydraulic energy into mechanical energy. The surge tank protects the penstock from bursting in case the turbine gates close due to electrical load being thrown off. When the gates close, there is a sudden stopping water at the lower ends of the penstock. The surge tank absorbs this pressure swing by increase in its level of water. The water turbines are of 2 types namely: Impulse and Reaction turbines.
Advantages of Hydroelectric Power
- Requires no fuel since it uses water as a source of energy.
- Less or no greenhouse gas emissions in modern plants.
- Can be used for many purposes i.e. Control floods, Irrigation.
- Longer operational life i.e. 30years.
Disadvantages of Hydroelectric Power
- Dam Failure Hazard - When the dam cannot hold the massive force of the held water it breaks and sends water at speeds of 50kph downstream. Death, Instant Power failure and destruction of infrastructure will follow. Dam failures have been some of the largest man-made disasters in history. Also, good design and construction are not an adequate guarantee of safety. Dams are tempting industrial targets for wartime attack, sabotage and terrorism. The Banqiao Dam failure in Southern China resulted in the deaths of 171,000 people and left millions homeless. Also, the creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963.The November 2009 Brazil and Paraguay blackout dam failure ,14 GW of output power was lost.
- Population relocation - Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost. Such problems have arisen at the Three Gorges Dam project in China, the Clyde Dam in New Zealand and the Ilisu Dam in Southeastern Turkey.
- Low Shortage - Since they are fed by rivers and streams, a dip in the level of water in these rivers results in reduced outputs of the power stations. In many countries which rely heavily on Hydroelectric power i.e. Kenya, suffer a lot due to insufficient rainfall during drought periods.
Largest Hydrolelectric Power Plants.The Three Gorges Dam complex on the Yangtze River in Hubei, China, has the world's largest generating capacity and generates the most electricity in the world. It includes 2 generating stations. They are the Three Gorges Dam (22,500 MW when completed) and Gezhouba Dam (3,115 MW). As of 2009, the total generating capacity of this complex is 21,515 MW. The whole project is planned to be completed in 2011, when the total generating capacity will be 25,615 MW. In 2008, this complex generated 97.9 TWh of electricity.
"The Itaipu power plant on the Paraguay River on the Brazil-Paraguay border currently produces second most hydroelectricity in the world. With 20 generator units and 14,000 MW of installed capacity, in 2008 the Itaipu power plant reached a new historic record for electricity production by generating 94.68 terawatt-hours (340,800 TJ).
Facts & Figures - Top 6 Power ProducersThree Gorges Dam
Date of completion: 2008/2011
Total Output (GW): 18,300 (October 2008) 22,500 (when complete)
Area Flooded: 632km2
Date of completion: 1984/1991/2003
Total Output: 14,000MW
Area Flooded: 1,350km2
Guri (Simón Bolívar)
Date of completion: 1986
Total Output: 10,200MW
Area Flooded: 4,250km2
Date of completion: 1984
Total Output: 8,370MW
Area Flooded: 3,014km2
Grand Coulee Country: United States
Date of completion: 1942/1980
Total Output: 6,809MW
Area Flooded: (No Data)
Sayano Shushenskaya (17 august 2009 - Stopped)
Date of completion: 1985/1989
Total Output: 6,400MW
Area Flooded: 621km2