Hydro power generation

Theory is electricity is generated by using the potential energy of the water. Fuel to the plant is water. So there is no any fuel cost. Environment friendly because there are no emissions during operation. Hydro power stations can start quickly and it take less time to synchronize to the grid. So these plants can be used for peak loads and for frequency control. Although the operation cost is less it require high initial cost due to civil engineering works. Usually hydroelectric power projects are constructed as multipurpose such as irrigation, flood control etc.

Different types of hydroelectric power plants

Run of river – use water when it comes

Pondage - medium head type

Reservoir – high head type

Pumped storage type- there are two reservoirs, upper reservoir and lower reservoir. During non-peak periods where the electricity demand is less the excess energy is used to pump water to the upper reservoir. During peak hours electricity is produced by using the water in upper reservoir. Its outer flow is to the lower reservoir.  

In addition some hydro power schemes are constructed as cascaded form. It means outer flow of one reservoir is the inflow of another reservoir.

Energy input to the power station is

P= ϸghw watts

Here

ϸ= density of water (m³)

g=acceleration of gravity (m/s²)

h=net head (m)

w=water flow (m³/s)

Arrangement of hydroelectric power plant

Dam

Blocks the water flow and create a water head. There are basically two types.

Concrete or masonry dam – for narrow canyon

Earth dam – for wide valley

Spillways

Build on the top of the dam and controlled by gates. Spillways are used to discharge the excess water from the reservoir to protect the dam (during high rainy seasons in catchment area).

Pressure channel

This is also known as the low pressure channel. Bring the water from reservoir to the surge tank.

Surge tank

This is a kind of well. It help to prevent damages to the high pressure channel (penstock) during immediate shut downs. Because at an immediate shout down water flow into the power house is suddenly stop so water pressure inside the penstock will increases suddenly. At those situations water level of the surge tank is increase and control the pressure inside the penstock. Otherwise penstock may be damaged due to the high pressure.

Penstock

This is the high pressure tunnel that carry water to power house.

Different types of turbines in hydro power plants

Pelton            - for high heads

Francis           –for low and medium heads

Kaplan           -for low head with large quantity of water

Water turbines can be classified into two main groups as impulse type and reaction type.

Impulse type water turbines

Example is Pelton wheel. The entire potential energy in the water is converted into the kinetic energy of the water. The output water jet from the nozzle drives the Pelton wheel. Pelton wheel has buckets mounted on it. When water hit on it wheel start rotating. 

Active power controlling of impulse type water turbines

Inside the nozzle there is a needle. It is used to control the water flow into the turbine. Position of the needle is controlled by a governor. So when the load increased the water flow into the turbine is increased and vice versa when load is decreased.

Reaction type water turbines

In this type pressure energy is not completely converted into the kinetic energy. Instead fraction of pressure energy is converted into the kinetic energy.

Ex: Francis and Kaplan wheels

In this Francis type turbine consist of outer ring and inner ring. Outer ring is consist of guide blades and inner ring is consist of rotating blades (runner). Position of guide blades can be controlled by governor for active power controlling. Because it controlled the water flow into the runner (on the other hand to the turbine). Water enters to the turbine radially and change its direction to the downward while passing through the runner. So pressure and velocity of the water reduces and it makes runner rotating due to the created reaction.