LARGE SCALE HYDROELECTRIC SCHEMES

1.      SCIENTIFIC AND TECHNICAL INFORMATION

1.1        Basic Principles of Large Hydropower

Hydropower is one of the oldest and most widely used forms of renewable energy, exploiting the potential or kinetic energy of flowing water. Hydro comes from the Greek word for water. It refers to any power generated by the movement of water, therefore the term hydroelectric power is applied to electrical power generated from inland waters such as streams, rivers and lakes.

The basic principle of hydroelectric energy is based on the water (hydrologic) cycle. Water evaporates from the earth's surface when it is heated by sunlight, and eventually condenses in the atmosphere to form clouds. Finally it is brought back to the surface by rain. This unending natural process resulting from the action of the sun's radiant energy on the earth's atmosphere provides a continual renewable supply of water on the earth's surface. A portion of this water returns to the sea during the cycle in the form of surface water runoff. It relinquishes a definite potential energy as the runoff flows to lower altitudes, which can be harnessed and used as a viable energy source.

Hydropower systems use the energy in flowing water to produce electricity or mechanical energy. The water that flows via a channel or penstock to a waterwheel or turbine strikes the bucket of the wheel, causing the shaft of the waterwheel or turbine to rotate. When generating electricity, the rotating shaft, which is connected to an alternator or generator, converts the motion of the shaft into electrical energy. This electrical energy may be used directly, stored in batteries, or inverted to produce utility-quality electricity. The amount of power a specific hydropower site can produce depends on its head (vertical distance between the reservoir or river to the turbine), the volume of water available and the flow rate.

Though there is no international consensus on the classification of hydropower, hydroelectric schemes are generally divided into two broad categories based on their capacity: large scale - more than 10 MW and small scale - less than 10 MW.

Hydropower is currently the world's largest renewable source of electricity, accounting for 6% of the worldwide energy supply and about 20% of the world's electricity. In spite of their drawbacks hydroelectric schemes can provide cheap and clean electricity, deploying a proven technology that offers reliable and flexible operation. Though the potential for hydroelectric systems is geographically predetermined, as it depends on the availability of suitable water flow, the theoretical worldwide potential for hydropower is about four times bigger than that currently under exploitation. Most probably, the actual amount of electricity generated by hydropower will never reach this theoretical potential.