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Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. Hydropower or water power is power derived from the energy of falling water, which may be harnessed for useful purposes. Since ancient times, hydropower has been used for irrigation and the operation of various mechanical devices, such as watermills, sawmills, textile mills, dock cranes, domestic lifts and paint making. Hydropower is the only renewable energy technology that is presently commercially viable on a large scale. It has four major advantages, namely it is renewable, it produces negligible amounts of greenhouse gases, it is the least costly way of storing large amounts of electricity, and it can easily adjust the amount of electricity produced to the amount demanded by consumers. Hydropower accounts for about 17% of global generation capacity and about 20% of the energy produced each year . Malaysia uses hydropower for 11% of its electricity .
Hydropower energy is widely used throughout the world, varying in size from small (mini-hydro) to mammoth schemes serving several countries or regions. Industrialized countries have utilized their hydro potential to a considerable extent, although in the developing world, particularly in the equatorial and tropical belts, the use of hydropower is an option with considerable potential for satisfying energy needs.
2.0 STATUS OF HYDROPOWER POTENTIAL
Malaysia receives an annual average rainfall of more than 2500 mm, mainly due to the Southwest and Northeast monsoons. The country is therefore rich in water resources when compared to the other regions of the world. The average annual water resources on a total land mass of 330,000km2 amount to 990 billion m³. Out of which, 360 billion m³, or 36% returns to the atmosphere as evapotranspiration, 566 billion m³, or 57% appear as surface runoff and the remaining 64 billion m³, or seven percent go to the recharge of groundwater. Of the total 566 billion m³ of surface runoff, 147 billion m³ are found in Peninsular Malaysia, 113 billion m³ in Sabah and 306 billion m³ in Sarawak.
Malaysia has a total land mass of 332,000 km² and its mean elevation is about 300m. The average rainfall is slightly more than 2,600mm per year. The total gross hydro potential is 414,000 GWh/year, of which about 85,000 GWh/year is available in Peninsular Malaysia. Hence, whilst Peninsular Malaysia has 39% of the land area, its share of hydropower resources is only slightly more than 20% . Geographically, the peninsula is relatively narrow, and its main range of low mountains, the Titiwangsa Range, runs along the interior to form the main watershed. Thus, the river basins formed are moderately small. The largest river basin is Sg. Pahang with a drainage area of 28,500 km². In addition, topographic features and rainfall are comparatively less favorable than Sabah and Sarawak. These are the main factors which contributes towards the limited hydropower resources in the peninsula. Of the 85,000 GWh/year gross potential, the utilized resources amount to 4,900 GWh/year, six percent whilst another 5,000 GWh/year, six percent has been identified.
The Sg. Perak river basin is the most developed in terms of hydropower development utilization (2,500 GWh/year), and it is reaching the limit of hydropower potential development. For Peninsular Malaysia, it has been estimated that the economic limit of hydropower utilizations is unlikely to exceed 10,000 GWh/year. Estimated that the economic limit of hydropower utilizations is unlikely to exceed 10,000 GWh/year.
2.1 Malaysia: Installed hydropower station
2.1.1 Peninsular Malaysia
Tenaga Nasional Berhad operates three hydroelectric schemes in the peninsular with an installed generating capacity of 1,911 (MW). 
Sungai Perak hydroelectric scheme, with 649 MW installed capacity:
â€¢ Bersia Dam 72 MW
â€¢ Chenderoh Power Station 40.5 MW
â€¢ Kenering Power Station 120 MW
â€¢ Sungai Piah Upper Dam 14.6 MW
â€¢ Sungai Piah Lower Dam 54 MW
â€¢ Temenggor Power Station 348 MW
â€¢ Terengganu hydroelectric scheme, with 400 MW installed capacity:
â€¢ Sultan Mahmud Power Station 4 x 100 MW = 400 MW
Terengganu hydroelectric scheme, with 400 MW installed capacity:
â€¢ Sultan Mahmud Power Station 400 MW
Cameron Highlands hydroelectric scheme, with 262 MW installed capacity:
â€¢ Sultan Yusof Jor Power Station 100 MW
â€¢ Sultan idris Woh Power Station 150 MW
â€¢ Odak Power Station 4.2 MW
â€¢ Habu Power Station 5.5 MW
â€¢ Kampong Raja Power Station 0.8 MW
â€¢ Kampong Terla Power Station 0.5 MW
â€¢ Robinson Falls Power Station 0.9 MW
Independent hydroelectric schemes
â€¢ Sg Kenerong Small Hydro Power Station in Kelantan at Sungai Kenerong, 20 MW
2.1.2 Sabah and Sarawak
â€¢ Bakun Dam 2400 MW (Under construction)
â€¢ Batang Ai Dam at Lubok Antu, Sarawak 25 MW
â€¢ Murum Dam in Sarawak 944 MW (proposed)
â€¢ Tenom Pangi Dam at Tenom, Sabah 66 MW
By using the hydropower as the alternative ways to replace the fossil fuel power station in order or produce electricity all around Malaysia, automatically we can conserved a lot of fossil fuel such as fuels, coals and gas that have been used for a long time for power generation for our country. Since these fossil fuels in being harvested every day in our country, and our fossil fuel resources is keep depleting day bay day. For coal, our country is buying the coal from Indonesia, and nowadays the price of coal is keep increasing it can contribute to the increasing price of the electricity to the consumers. Furthermore, hydroelectricity provides the clean electricity, so we can enjoy clean electricity without polluting our environment.
3.0 HOW IT WORKS?
Hydropower plants harness water’s energy and use simple mechanics to convert that energy into electricity. Hydropower plants are actually based on a rather simple concept of water flowing through a dam turns a turbine, which turns a generator. 
Figure1. Illustration on how hydropower station works.
The basic components of a conventional hydropower plant:
â€¢Dam – Most hydropower plants rely on a dam that holds back water, creating a large reservoir.
â€¢Intake – Gates on the dam open and gravity pulls the water through the penstock, a pipeline that leads to the turbine. Water builds up pressure as it flows through this pipe.
â€¢Turbine – The water strikes and turns the large blades of a turbine, which is attached to a generator above it by way of a shaft. The most common type of turbine for hydropower plants is the Francis Turbine, which looks like a big disc with curved blades. A turbine can weigh as much as 172 tons and turn at a rate of 90 revolutions per minute (rpm), according to the Foundation for Water & Energy Education (FWEE).
â€¢Generators – As the turbine blades turn, so do a series of magnets inside the generator. Giant magnets rotate past copper coils, producing alternating current (AC) by moving electrons.
â€¢Transformer – The transformer inside the powerhouse takes the AC and converts it to higher-voltage current.
â€¢Power lines – Out of every power plant come four wires: the three phases of power being produced simultaneously plus a neutral or ground common to all three.
â€¢Outflow – Used water is carried through pipelines, called tailraces, and re-enters the river downstream.
The water in the reservoir is considered stored energy. When the gates open, the water flowing through the penstock becomes kinetic energy because it’s in motion. The amount of electricity that is generated is determined by several factors. Two of those factors are the volume of water flow and the amount of hydraulic head. The head refers to the distance between the water surface and the turbines. As the head and flow increase, so does the electricity generated. The head is usually dependent upon the amount of water in the reservoir. 
3.1 Improve Hydroelectricity System.
3.1.1 Pumped-Storage Hydroelectricity Plants
There’s another type of hydropower plant, called the pumped-storage plant. In a conventional hydropower plant, the water from the reservoir flows through the plant, exits and is carried downstream. A pumped-storage plant has two reservoirs:
Figure 2. Illustration show how Pumped-Storage Hydroelectricity Plants works.
â€¢ Upper reservoir – Like a conventional hydropower plant, a dam creates a reservoir. The water in this reservoir flows through the hydropower plant to create electricity.
â€¢ Lower reservoir – Water exiting the hydropower plant flows into a lower reservoir rather than re-entering the river and flowing downstream.
Using a reversible turbine, the plant can pump water back to the upper reservoir. This is done in off-peak hours. Essentially, the second reservoir refills the upper reservoir. By pumping water back to the upper reservoir, the plant has more water to generate electricity during periods of peak consumption. 
3.2 The Generator
The heart of the hydroelectric power plant is the generator. Most hydropower plants have several of these generators. The generator, as you might have guessed, generates the electricity. The basic process of generating electricity in this manner is to rotate a series of magnets inside coils of wire. This process moves electrons, which produces electrical current.
Figure 3. Illustration on Generator used in the hydro station facilities
As the turbine turns, the excitor sends an electrical current to the rotor. The rotor is a series of large electromagnets that spins inside a tightly-wound coil of copper wire, called the stator. The magnetic field between the coil and the magnets creates an electric current.
Hydropower is a fuelled by water, so it’s a clean fuel source. Hydropower doesn’t pollute the air like power plants that burn fossil fuels, such as coal or natural gas. Since fuels, gas and coals power plant is produce greenhouse gases and release it to the environment and cause air pollution and contributing on the greenhouse effect, furthermore the efficiency of the fossil fuels power plant is very low and it is about fewer than 20%, and lots of energy is wasted because the power plant cannot convert fossil fuel into electricity efficiently. The cost of fuels also effected the fossil fuels power plant since the fossil fuel used to fired up the power station is keep increasing, it also contributing to the increasing of the electricity tariff to the energy consumers around Malaysia, because the electricity tariff is depend on the fossil fuel price. So hydroelectricity is a alternative ways to replace the fossil fuel power plant, because the source to produce the electricity is free and also the output of the power can be controlled, so it is no wastage of energy and most important it is a clean energy.
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