Type of Turbines

Hydro power plants can be classified in various way,

1.According to Construction Features

Impoundment
An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.

Diversion
A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Pumped Storage
When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

2.According to Size of Hydropower Plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.

Large Hydropower
Although definitions vary, DOE defines large hydropower as facilities that have a capacity of more than 30 megawatts.

Small Hydropower
Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 0.1 to 30 megawatts.

Micro Hydropower
A micro hydropower plant has a capacity of up to 100 kilowatts (0.1 megawatts).

For some good pictures of various types of hydro power plants visit EERE : Department of Wind and Hydropower Technology Programme website

3.According to Hydro power Turbines

There are two main types of hydro turbines: impulse and reaction. The type of hydropower turbine selected for a project is based on the height of standing water—referred to as "head"—and the flow, or volume of water, at the site. Other deciding factors include how deep the turbine must be set, efficiency, and cost.

All types of turbines are classified into 2 large divisions.

A.Reaction B.Impulse

A.Reaction :

Reaction turbines are acted on by water, which changes pressure as it moves through the turbine and gives up its energy. They must be encased to contain the water pressure (or suction), or they must be fully submerged in the water flow.

Newton's Third Law i.e 'Every Action has Equal and Opposite Reaction' describes the transfer of energy for reaction turbines.Most water turbines in use are reaction turbines. They are used in low and medium head applications.

B.Impulse :

Impulse turbines change the velocity of a water jet. The jet impinges on the turbine's curved blades which reverse the flow. The resulting change in momentum impulse) causes a force on the turbine blades. Since the turbine is spinning, the force acts through a distance (work) and the diverted water flow is left with diminished energy.

Prior to hitting the turbine blades, the water's pressure (potential energy) is converted to kinetic energy by a nozzle and focused on the turbine. No pressure change occurs at the turbine blades, and the turbine doesn't require a housing for operation.

Newton's second law describes the transfer of energy for impulse turbines.

Impulse turbines are most often used in very high head applications.

Examples :

Reaction turbines:

• Francis
• Kaplan, Propeller, Bulb, Tube, Straflo
• Tyson
• Water wheel

Impulse turbines:

• Pelton
• Turgo
• Michell-Banki (also known as the Crossflow or Ossberger turbine)

4.According to Head or reservoir level

Ludins propose classification of hydro-plants based on head.

1.Low Head Plants : Less than 15 m head
2.Medium Head Plants : 15 m - 70 m
3.High Head Plants : 71 m - 250 m
4.Very High Head Plants : Above 250 m

Hydro power plants can also be classified based upon location,operation and turbine placement.

Why Hydropower ?

Advantage of Hydro Power or 'power from water' is

• The chief advantage of hydro systems is elimination of the cost of fuel. Hydroelectric plants are immune to price increases for fossil fuels such as oil, natural gas or coal, and do not require imported fuel.
  

• Hydroelectric plants tend to have longer lives than fuel-fired generation, with some plants now in service having been built 50 to 100 years ago.
  

• Labour cost also tends to be low since plants are generally heavily automated and have few personnel on site during normal operation.
  

• Hydroelectric plants generally have small to negligible emissions of carbon dioxide and methane due to reservoir emissions, and emit no sulphur dioxide, nitrogen oxides, dust, or other pollutants associated with combustion.
  

• Since the generating units can be started and stopped quickly, they can follow system loads efficiently, and may be able to reshape water flows to more closely match daily and seasonal system energy demands.
  

• Hydroelectric plants with reliable hydrological histories are dispatchable and can be considered firm capacity. Consequently, in normal water years hydroelectric plants designed for a firm load will have a useful amount of surplus energy that may be exportable if transmission is available.
  

• Pumped storage plants currently provide the most significant means of storage of energy on a scale useful for a utility, allowing low-value generation in off-peak times (which occurs because fossil-fuel plants cannot be entirely shut down on a daily basis) to be used to store water that can be released during high load daily peaks. Operation of pumped-storage plants improves the daily load factor of the generation system.
  

• Reservoirs created by hydroelectric schemes often provide excellent leisure facilities for water sports, and become tourist attractions in themselves.
  

• Multi-use dams installed for irrigation, flood control, or recreation, may have a hydroelectric plant added with relatively low construction cost, providing a useful revenue stream to offset the cost of dam operation.

• Hydro power plants convert about 90 percent of the energy in falling water into electrical energy. This is much more efficient than fossil-fueled power plants, which lose more than half of the energy content of their fuel as waste heat and gases.

• Hydro generates few GHG and no other air pollutants.

• Wildlife preserves can be created around reservoirs, which can provide stable habitats for endangered and threatened species(Eg. catch rates for game fish like walleye and small mouth bass are substantially higher on hydro power reservoirs than natural lakes.)

• Flood prevention

• Humidity and temperature increase, which is beneficial to crops(Eg.Three Gorge Dam in China)

As correctly said : - "Hydropower is a clean and renewable source of energy that can contribute to fighting climate change and air pollution in countries around the world, as well as providing electricity and water in areas where it is most needed," adds International Hydropower Association president Dogan Altinbilek....Hydropower: An Essential Part of the Solution to Climate Change,Montreal, Canada December 6, 2005(Source : http://www.ich.no)

Though,as usual,there is a anti-opinion about ,whether hydropower a less Green House Gas emitter than a Fossil Fueled power house.According to Dr.Fearnside,GHG emitted from hydro power terms are greater than fossil fuel plants.He explain this with the help of CO2 compressed in CocoCola .When you open a bottle of CocoCola,a lot of CO2 is emitted due to the sudden reduction of pressure.Similarly when water comes out of the spillways and turbine,pressure is reduced and hence a lot of methane is released . As 1 ton of CO2 is equivalent to 21 tons of CO2,hence the GHG emission from the hydro power plants are worth noticiable.You can read the full article about "Why Hydropower is Not Clean Energy" by Dr. Philip M. Fearnside

But though there is a debate about whether hydro power's ability to be a clean energy or not,its potential of becoming the most efficient alternative energy source is never doubted nor contested.

Thanks
Mrinmoy
Research Scholar
School of Water Resources Engg,Jadavpur University
http://www.baipatra.ws