2.      STATE OF THE ART TECHNOLOGY

Hydro schemes use the kinetic energy of moving water to produce electricity. This water is put through a turbine, which in turn drives a generator that creates electricity. Some power plants are located on rivers, streams, and canals, but for a reliable water supply, dams are needed. Dams store water for later release for such purposes as irrigation, domestic and industrial use, and power generation. The reservoir acts much like a battery, storing water to be released as needed to generate power.

The amount of electricity which can be generated at a hydro-electric plant is dependent upon two factors. These factors are:

-        the vertical distance through which the water falls, called the "head", and

-        the flow rate, measured as the water volume per unit time.

The electricity produced is proportional to the product of the head and the rate of flow. Based on this, hydro-electric power plants can generally be divided into two categories:

-        "High head" power plants and

-        "Low 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.

"Low head" hydroelectric 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. A large volume of water must pass through the turbines of a low head hydro plant in order to produce a useful amount of power.

There are three main types of hydroelectric schemes, namely:

-        Run of the River

-        Diversion

-        Pumped Storage

Run of the River

In the Run of the River type of scheme, the turbine and generator are located either in the dam or found along side it. The dam uses the flow of the river to create the hydrostatic head. Run of the river generating stations cannot store water, thus their electric outputs vary with seasonal flows of water in a river.

Diversion scheme

In a diversion scheme the supply of water is taken from a dammed river or lake to a remote powerhouse containing the turbine and generator. A canal or low-pressure tunnel transports the water to this end point and then back to the river to continue its course.

Pumped Storage

Pumped storage is a scheme that incorporates two reservoirs. At times of low demand, generally when electricity is cheap like at night, electricity is bought to pump water from the lower to the upper basin. This extra water can then be released to create power at a time when demand is high and prices are high. This enables the scheme to perform with greater efficiency when matching supply and demand.

Although there are different forms of hydroelectric schemes, they are all based on the following components:

-        a dam

-        penstocks

-        forebay

-        turbine(s)

-        generator(s).

During operation the dam raises the level of the water to create a hydrostatic head, which means the difference between the level of the water in the dam and the level where it flows out from the scheme. This acts to trap the water, which can be released when necessary, enabling the water trapped to become stored energy.

The penstocks are pressurised pipes that take the water to the turbine. The forebay controls the rate of the flow, ensuring that adequate levels are attained in accordance with the turbine controls. This pressurised water then drives the turbine that in turn drives a generator and creates electricity.

There are two basic types of turbines: impulse and reaction. The turbine selected depends largely on the site conditions. A reaction turbine is a horizontal or vertical wheel that operates with the wheel completely submerged, a feature which reduces turbulence. In theory, the reaction turbine works like a rotating lawn sprinkler where water at a central point is under pressure and escapes from the ends of the blades, causing rotation. Reaction turbines are the type most widely used.

An impulse turbine is a horizontal or vertical wheel that uses the kinetic energy of water striking its buckets or blades to cause rotation. The wheel is covered by a housing and the buckets or blades are shaped so they turn the flow of water about 170 degrees inside the housing. After turning the blades or buckets, the water falls to the bottom of the wheel housing and flows out.