Diesel Power Plant, Overview:
The diesel power plant is also known as standby power plant because the power generated by the diesel power plant is less as compare to the thermal and hydro power plant. This power plant can generate power between 5 to 50 MW. This power plant is usually used in emergency cases. When it comes to the generation of electricity; it is essential to rotate the rotor of alternator by prime mover. This prime mover can be driven by using different type of fuels. Diesel engine is one of the most popular as prime mover for the generation of electricity. So when prime mover of the alternator is diesel engine then the power station is known as diesel power station.
The various subsystem of diesel power plant is:
- Diesel engine
- Starting system
- Fuel system
- Air intake system
- Lubrication system
- Exhaust system
- Cooling system
Now we will discuss each part of the diesel power plant in detail:
The working principle of diesel engines was established in 1893 by inventor Rudlof Diesel.
The diesel engine provides maximum effect on minimum oil consumption. The diesel engine is the main component of the power plant. As we know that the engine is a device that will give us the mechanical energy by converting chemical energy (diesel). Diesel will be as input to the engine, the mechanical energy generated by the engine will be given to generator which will convert the mechanical energy into electrical energy. The diesel engines used for the diesel power plant may be four stroke or two stroke engines.
Two stroke diesel engine:
The construction of the two stroke diesel engine is similar to 2 stroke petrol engine except the fuel pump and fuel injector is there instead of carburettor and spark plug in petrol engine. The working of the diesel engine is similar to the two stroke petrol engine except that only air is supplied into the crank case in the diesel engine and diesel fuel is injected at the end of compression of air. It means that in petrol engine we supply the mixture of air and petrol but in case of diesel engine we only supply the air in to the crank. A two stroke engine is rugged and compact in construction simple in mechanical design, cheaper in cost, smaller flywheel required and develops more power for same speed and piston displacement.
Four stroke engine:
The four stroke engine has four cylinders mounted in a straight line. The camshafts operate the valves, each cylinder has four valves of which two are inlet valves and two are outlet valves. The fuel injection system is responsible for supplying the cylinders with fuel. The pistons are force down during the combustion stroke. The force exerted on the piston is transmitted through the connecting rod to the gear box in an inline four engines there is always one piston doing one of the four strokes that is the reason why four cylinder engines usually run more smoothly and their one and two cylinder counter parts. The four stroke diesel engine as the same stroke Otto cycle engine:
- Induction stroke
- Compression stroke
- Power stroke
- Axial stroke
Unlike the typical Otto cycle engine a diesel engine take only in air through the intake valves during the first stroke, during the second stroke the intake valves are closed and the air is compressed as the air is highly compressed the temperature of the air rises and almost reaches 1300° Fahrenheit. In the third stroke diesel fuel engine is injected directly into the cylinder. The fuel was instantly ignites because of the high temperature of the air the explosion pushes the piston down which transfer power to the crank shaft. The fourth stroke is to process where the spam fuel air mixture axes through the open access valves and the stroke cycle is repeated again and again. A big advantage of these valves is that they typically deliver 25 to 30 % better fuel economy than similarly performing gasoline engines.
However four stroke engines has a specific fuel consumption and more effective lubrication, more flexibility, less noisy exhaust, simple and better cooling, better scavenging and higher efficiency than a two stroke engine. We used four stroke engine in diesel power plant. Generally two stroke engine are favoured only for diesel power plant due to high power output, uniform turning moments, compactness and less capital cost. Now a day’s high diesel oil price and hence high operating cost of two strokes is favoured to use four stroke diesel engines for diesel electric power plant.
The diesel engine is available in size from 75 KW to 40 MW. The size and number of engine depend upon the plant capacity, purpose for which plant is being setup and the load characteristics.
Starting system provide initial rotation to the engine shaft to start the engine. Smaller units can be safely being started manually by hand cranking system. However larger units use compressed air for starting purpose. The starting system work until the engine is start when the engine starts working the starting system is closed. Battery driven motors can also be used for starting of the diesel engine. The engine can be stopped by stopping the fuel supply to the injection pump or stopping the action of the injection pump.
The diesel from the diesel tank is first supplied to the strainer from where it is transfer to the transfer pump which will transfer this diesel to the daily consumption tank. If the daily consumption is overflow then the diesel is again supply to the main diesel tank.
Fuel system consists of storage tank which are the main tank to store the fuel. The filter will remove the impurities from the fuel. During the transfer oil passes through the stiller of the filter. Another pipe called oil flow pipe is available to connect the day tank to the main tank to return the oil in the event of over flowing.
Fuel injection pump:
The fuel pump will suck the fuel from the tank. Fuel injection pump inject the oil from the day tank in to the engine cylinder at a high pressure of about 100 bar.
The fuel oil may be supplied to the power plant site by road, rail, tank etc. The function of the fuel system is to transfer the fuel from the storage tank to the consumption tank and to increase the pressure of the fuel. It also measure and control the fuel supply. To inject and automize the fuel injector is used which will spray the diesel in form of small particles the fuel in engine cylinder.
Air Intake System:
It consists of air supply pipe, air filters and super charge in case of super charge engine. The air intake system consists of the compressor which wills intake the pressurized air and supply to the engine for the combustion. The air filter removes the dust from the air. Air filters are of dry type made up of wool or cloth.
The fuel and air both go in to the engine. They will mix burn and generate heat and this heat energy is converted into mechanical energy.
The function of the air intake system is:
- To clean the air
- To supply air for super charging in case super charge engine
- To reduce the noise of air take system
It consists of lubricating oil tank, pumps, filters and lubricating oil cooler. In lubrication system we provide the require lubricants to the engine. The size of the diesel engine is very large. Due to which require separate lubricant system
Lubrication system consists of:
- Lubricant oil tank
- Oiler cooler
Lubricant oil tank:
The lubricant oil is used to store oil and this Lubricant oil is suck from the oil tank by the pump and is passed through the filter to remove impurities. This oil goes into the engine system. It will occupy various positions where we have relative motion like piston and cylinder so these are the parts where friction occurs. This friction will be reduces by the lubricant oil. Because of the heat generated by the air and fuel system this lubricant oil will be heated which may change its properties. When the properties changes the lubrication will not full fill its work. So for this purpose we will require the lubrication cooling system. The lubricant oil is taken out from the engine which will pass through the filter to remove the impurities and then pass through the oil cooler. The Oil cooler is also there to keep the temperature of the oil as low as possible. The cool oil will be again given to the oil tank.
The function of the lubricating system is to provide proper amount of the oil to minimize friction and wearing of the rubbing parts.
The exhaust system consists of piping from the engine to a point where exhaust gases may be discharged without danger or annoyance. The exhaust gasses will be generated by burning of the fuels. The exhaust system leads the engine exhaust gases the atmosphere. A silencer is usually incorporated in this system reduce the noise level.
The purpose of the cooling system is to provide amount of water circulation all around the engines to remove the part of heat from engine cylinder keeping the low temperature at predefined level. The cooling system consists of water jackets, water pump, surge tank, cooling tower, raw water pump and heat exchanger. Cooling system is instrumental for the liquid cooling process. In this system the cooling jackets are provided on the engine cylinder valvesand head through which the cool water is circulated. The heat is absorbed by the cool water.
The heat is transfer from the cylinder valves to the coolant by the process of convection and conduction. The hot water from the jackets is passed to the surge tank. This hot water from the surge tank is supplied through the pump to the heat exchanger which will convert this hot water in cool water and is again supply to the jackets.
The function of the generator or alternator is to convert the mechanical energy shaft power of the engine in to the electrical energy. The generator shaft is coupled with the engine shaft. It consists of automatic voltage regulator to allow close voltage regulation and satisfactory parallel operation. The generator used in the diesel power plants are of rotating fields, salient pole construction, speed ranging from 214 to 1000 rpm (poles 28 to 6) and capacities ranging from 25 to 50000 KV at 0.8 power factor lagging.
- The size of the plant is comparatively small for the same capacity of the thermal power plant
- Diesel engine can be start up and brought in to the service within minute. It does not require any sort of preparation or warming time. Also plant can be respond to varying loads without any difficulty.
- The cooling water required for same capacity is considerably less than thermal power plant.
- Diesel power plant can be located very near to the load centre without causing much nuisance to the surrounding. Hence diesel power plant are admirably suited to load centre location.
- Quick installation and commission as compared to steam and hydro power plant
- The thermal efficiency of the diesel power plant is more about 40% than steam power plant in the range of 150 MW capacities.
- Diesel power plant maintains high operating efficiency in the load range of 50 % to 100 % of full load.
- The overall capital cost including installation per unit of installed capacity is less than that of the steam power plant
- Quick starting and easy pick up of loads in a very short span of time
- The layout design and construction of foundation and building for diesel power plant is simple and cheap.
- Diesel power plant requires less space because of minimum auxiliaries and free from ash handling problem
- In compare with a steam power plant diesel power plant efficiency fall off very little with use
- Diesel power plant is readily available at standard set in the range of 500 KW to 40 MW. Also plant can be easily extended to the given power requirements
- Easy load operation
- Required minimum labours
- Simple fuel handling
- Efficient ash disposal
- High level of efficiency
- The power generation capacity of these power plants are limited to 50 MW
- It is expensive due to the diesel as compare to the coal and hydro power plant
- Maintenance cost of the diesel power plant is also very high
- Life of this power plant is very short