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.

                    Diesel power station

In diesel power stations diesel is used as the fuel and electricity generated by rotating prime mover by diesel engines. Fuel price is high, so these plants are used to supply low power demands and used as stand by power stations.

Arrangement of diesel power station

Fuel supply system

This system consist of following components. Fuel tank to store the fuel, then some amount of fuel store in the day tank for daily of shortage uses. Filters are used to remove the impurities and fuel transfer pumps are used to pump the fuel.   

Air intake system

Get fresh air from the atmosphere and send them to the diesel engine. It is used filters to remove particles from the air.

Exhaust system

Remove exhaust gas to the atmosphere. It is used silencer to reduce the noise level.

Cooling system

This system is consist of water pumps and cooling towers. Pumps circulate water through cylinders and head jackets then take excess heat from the engine and releases them in the cooling towers. Why cooling is needed? Because all the heat energy created within the engines are not completely converted into the work. Some part remain, if that remain part is not remove it may cause to make damages to the equipment’s.

Lubrication system

This system consist of lubrication oil tank, pumps, filters and also oil coolers to keep oil temperature in low value.

Engine starting system

Basically it is used compressed air for starting. In this case an air compressor is used to make compressed air. That compressed air is fed to some cylinders. Then they act as reciprocating air motors. Fuel is injected to the other cylinders. So after starting engine makes its own power.  

              Steam (thermal) power station

Stored energy of coal is converted to the heat energy by combustion. That heat energy is converted into the electricity.

Arrangement of steam power station

 

Coal and ash handling plant

Coal handling plant

In here coal is crushed into small particles to increase their surface area. Increasing of surface area help to burn them easily and completely. Then crushed coal is send to the boiler by conveyor belts.

Ash handling plant

After burning the coal in the boiler the ash is send to the ash handling plant and then to the ash storage.

Steam generating plant

Boiler

Combust the coal and by using that heat it makes high pressure and high temperature steam. The flue gases from the boiler is sent through super heater, economizer, air preheater and finally release to atmosphere through chimney.

There are two types of boilers

Water tube boiler- water flow through tubes inside the boiler and hot gases flow over that tubes.

Fire tube boiler- hot air flows through tubes inside the boiler which filled with water.

Super heater

Boiler send wet steam to the super heater. In here that steam is further heated (above the boiling temperature) with the help of temperature in flue gases. Therefore the thermal efficiency can be increased. Another advantage is damages to the turbine blades can be prevented. Because if there are water drops in the steam it make damages to the turbine blades. But output steam from super heater is completely dry. There are two types of super heaters radiant super heater and convection super heater.

Then the super-heated steam is fed into the turbine via main valve.

Economizer

This make water heated before supply to the boiler by using heat in the flue gases.

Air preheater

This fed air from the atmosphere and heated them before supply to the boiler by using flue gases.  

Steam turbine

Through main valve superheated dry steam is fed to the steam turbine. While passing through the turbine heat energy is converted to the mechanical energy. Then via an alternator that mechanical energy is converted to the electrical energy.

Cooling system

Steam after rotating the turbine is sent to the condenser then in cooling towers water is cooled. After cooling that water is sent through economizer (to heat to some temperature value) and then send to the boiler.     

                      Gas power station

Working fluid is air. Air is compressed and then heated by burning fuels. That compressed, heated air is fed to the gas turbine. In here gas is expanded and able to do mechanical work (rotating the turbine). In this type of power plants turbine shaft and air compressor shaft is mounted on a same shaft so that amount of generated power can be used to drive the air compressor. So it is clear that it need starting method. Electric motor known as cranking motor is used to start the compressor initially after starting a part of mechanical energy produced in the turbine is used to drive the compressor. 

Arrangement of gas power station

Compressor

This is a rotary type compressor and used to make compressed air. Initially, at the starting, this is driven by electric motor. But after that operate by using a part of energy that produce in the turbine. Atmospheric air is fed into the compressor via filters to remove dust and other particles.

Regenerator

Compressed air is flows through tubes inside this regenerator. Exhaust gas from the gas turbine flows through this regenerator so that part of their heat is absorbed by the compressed air.

Combustion chamber

Compressed air from the regenerator send to the combustion chamber for heating. Oil is injected with high pressure (atomizing air) so that they could properly mixed with air and easily and completely burned. This atomizing air get by an early stage of the compressor. Then ignite the air by high voltage spark plugs. Inside the combustion chambers there are flame detectors to detect whether oil mixed gas is fully burned or not. Resultant air is at about 3000⁰F but they are cooled to about 1300⁰F to 1500⁰ and then sent to the gas turbine.

Gas turbine

High pressure and high temperature gas passes through this turbine while expanding. So air able to some mechanical work (rotating the turbine). The exhaust gas should be in a proper temperature about 900⁰F to avoid making water droplets.  

                           Combined cycle power station

In combined cycle power stations there are two types of turbines gas turbine and a steam turbine. Gas turbine has three stages compressor, combustion chamber and turbine. Then flue gas from the gas turbine is sent to the steam power station. In some combined cycle power station flue gases flows through a diversion damper. Therefore that plants can be operated as both combined cycle and open cycle. (Ex: Kalanithissa CCP in Sri Lanka).

The flue gases from the gas turbine is sent to the heat recovery steam generator (HRSG). It recover heat from the flue gases and produce high pressure and low pressure steam.  

Heat recovery steam generator (HRSG)

Upper part of HRSG is LP part and the bottom part is HP part. Water from treatment plant is preheated and send to the feed water tank. Before store in feed water tank dearator is used to remove oxygen from the water. From this tank preheated LP preheater and HP economizer. Let’s take the LP part. After preheating that water is sent to the LP drum where saturated water and steam is in equilibrium. Saturated water from LP drum is taken out and continuously heated by LP evaporator. Saturated steam in the drum is taken out to drive the LP steam turbine.

Similar procedure for HP also. HP steam is fed to the HP turbine and then they are sent to the LP steam turbine.