17 April, 2007

Hydro Electric Plants - Classification, Advantages and Disadvantages

The classification of hydro electric plants based upon :
(a) Quantity of water available (b) Available head (c) Nature of load

The classification acording to Quantity of water available is
(i) Run-off river plants with out pondage : These plants does not store water; the plant uses water as it comes.The plant can use water as and when available.Since these plants depend for their generting capacity primarly on the rate of flow of water, during rainy season high flow rate may mean some quantity of water to go as waste while during low run-off periods, due to low flow rates,the generating capacity will be low.
(ii) Run-off river plants with pondage : In these plants pondage permits storage of water during off peak periods and use of this water during peak periods.Depending on the size of pondage provided it may be possible to cope with hour to hour fluctuations.This type of plant can be used on parts of the load curve as required,and is more useful than a plant with out storage or pondage.
When providing pondage tail race conditions should be such that floods do not raise tail-race water level,thus reducing the head on the plant and impairing its effectiveness.This type of plant is comparitively more reliable and its generating capacity is less dependent on avilable rate of flow of water.
(iii) Reservoir Plants :A reservoir plant is that which has a reservoir of such size as to permit carrying over storage from wet season to the next dry season.Water is stored behind the dam and is available to the plant with control as required.Such a plant has better capacity and can be used efficiently through out the year.Its firm capacity can be increased and can be used either as a base load plant or as a peak load plant as required.It can also be used on any portion of the load curve as required.Majority of the hydroelectric plants are of this type.

The classification according to availability of water head is
(i) Low-Head (less than 30 meters) Hydro electric plants :"Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river". Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. Hydro-electric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydro-electric technology is basically the same regardless of generating capacity.

(ii) Medum-head(30 meters - 300 meters) hydro electric plants :These plants consist of a large dam in a mountainous area which creates a huge reservoir. The Grand Coulee Dam on the Columbia River in Washington (108 meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on the Colorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000 MW) are good examples. These dams are true engineering marvels. In fact, the American Society of Civil Engineers as designated Hoover Dam as one of the seven civil engineering wonders of the modern world, but the massive lakes created by these dams are a graphic example of our ability to manipulate the environment - for better or worse. Dams are also used for flood control, irrigation, recreation, and often are the main source of potable water for many communities. Hydroelectric development is also possible in areas such as Niagra Falls where natural elevation changes can be used.

(iii) High-head hydro electric plants :"High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydro-electric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.
The classification according to nature of load is
(i) Base load plants :A base load power plant is one that provides a steady flow of power regardless of total power demand by the grid. These plants run at all times through the year except in the case of repairs or scheduled maintenance.

Power plants are designated base load based on their low cost generation, efficiency and safety at set outputs. Baseload power plants do not change production to match power consumption demands since it is always cheaper to run them rather than running high cost combined cycle plants or combustion turbines. Typically these plants are large enough to provide a majority of the power used by a grid, making them slow to fire up and cool down. Thus, they are more effective when used continuously to cover the power baseload required by the grid.

Each base load power plant on a grid is allotted a specific amount of the baseload power demand to handle. The base load power is determined by the load duration curve of the system. For a typical power system, rule of thumb states that the base load power is usually 35-40% of the maximum load during the year.Load factor of such plants is high.
Fluctuations, peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants.

(ii) Peak load plants :Power plants for electricity generation which, due to their operational and economic properties, are used to cover the peak load. Gas turbines and storage and pumped storage power plants are used as peak load power plants.The efficiency of such plants is around 60 -70%.

Advantages of hydroelectric plants

  • operation , running and maintenance costs are low.
  • Once the dam is built, the energy is virtually free.
  • No fuel is burnt and the plant is quite neat & clean.
  • No waste or pollution produced.
  • generating plants have a long lifetime.
  • Much more reliable than wind, solar or wave power.
  • Water can be stored above the dam ready to cope with peaks in demand.
  • unscheduled breakdowns are relatively infrequent and short in duration since the equipment is relatively simple.
  • hydroelectric turbine-generators can be started and put "on-line" very rapidly.
  • Electricity can be generated constantly
Disadvantages of hydroelectric plants
  • very land-use oriented and may flood large regions.
  • The dams are very expensive to build.However, many dams are also used for flood control or irrigation, so building costs can be shared.
  • Capital cost of generators, civil engineering works and cost of transmission lines is very high.
  • Water quality and quantity downstream can be affected, which can have an impact on plant life.
  • Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
  • fish migration is restricted.
  • fish health affected by water temperature change and insertion of excess nitrogen into water at spillways
  • available water and its temperature may be affected
  • reservoirs alter silt-flow patterns

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