Diesel power plants
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Diesel engine (Internal combustion engine) driven generators are commonly seen almost every where. They serve as emergency backup power for hospitals, offices and industries. In addition diesel power plants are used as back up power by most of the nuclear power plants due to their reliability and ability to maintain plant safety in emergencies. Steam power plants also rely on diesel plants fire emergency auxiliary systems. Apart from these, diesel power plants also exist in some countries.
A simple diesel power plant consists of a diesel engine coupled with an alternator. The diesel engine is used to run the generator, which converts the mechanical energy provided by the diesel engine into electrical energy.
Layout of Diesel Engine Power Plant
Components of a diesel power plant.
A diesel power plant needs the following equipments for their operation:
- Diesel engine
- Speed governing system
- Air intake system
- Exhaust system
- Starting system
- Cooling system
- Engine cooling system
- Fuel system
1. Diesel engine
The diesel engine used for a diesel electric plant may be two stroke or four stroke engine based on the requirement. A two stoke engine can produce more power for the same speed and piston displacement and costs less than a four stroke diesel engine. A four stroke engine has comparatively higher efficiency than a two stroke engine.
Advantages of four stroke engine
- More efficiency and less fuel consumption than a two stroke engine.
- More effective lubrication.
- Low heat stress and better cooling.
- Scavenging pumps not necessary.
Advantages of two stroke engine
- More power can be developed for a given size of engine
- Less cost, weight and size.
- More flexible and simple design.
The diesel engines are mostly compression ignition type. The overload capacity of the plant is very low and its operation at loads less than the full loads is uneconomical. Therefore it is important to determine the plant capacity before the installation of diesel engine of specific size.
Diesel power plant
2. Speed governing system
Diesel engines are provided with either non isochronous or isochronous governors. A non isochronous governors may be mechanical type in which fly weights are used or it may be a hydraulic or electric relay. Isochronous governors are relay type used for the diesel engines having parallel operation.
The speed regulation can be given by
S = (No – Nf) / Nf
No – No load speed
Nf – Full load speed
3. Air intake system
Air requirement of diesel plants are high. It is around 4 - 8 m3 per kWh. Small diesel engines can be place in a room. Large engines require more air and must be placed in open air or in an engine room with air drawn from outside. Intake air must be free from dust. So air filters are provided to clean the inlet air. In cold climates, intake air is preheated by the exhaust gases before injection.
Small portable Diesel Engine Power Plant
4. Exhaust system
The diesel engine must exhaust the air that has been taken in. The functions of the exhaust system is to discharge the engine exhaust well above the ground level, keep the noise level within the specified level and isolate the engine vibrations from the building. The exhaust system must be able to carry around 0.17 to 0.23 m3 of air per minute per house power developed.
5. Starting system
Small engines are provided with electric starter driven by batteries. Air motor is used for medium sized unit. In power plants compressed air is used to start the diesel engine. A 0.45 m3 tank with air at 20 atmospheric pressure can start a 1200 hp engine 5 to 10 times without being recharged.
6. Cooling system
The extra heat, not used for doing work should be removed from the engine. Otherwise, the lubricating oil will disintegrate the lubricating oil film on the cylinder walls and damage the cylinder liners, heads, walls, piston and rings. Small engines can be air cooled. In case of large engines, forced water cooling is mostly employed. The cooling water required for the cooling of engine is around 2 to 4 liters per bhp per minute.
7. Lubricating oil system
Lubricating oil is one of the most important components of the diesel engine. The life of the engine depends on the lubrication. Lubricating oil should be able to withstand the heat produced. Forced fed lubrication is mostly used. The lubricating oil should be free from impurities. Therefore strainer is used to remove larger impurities and filter is used to remove smaller impurities.
8. Fuel system
In small engines, the fuel is stored in small tanks mounted in the engine. In case of large power plants fuel is stored in large outdoor tanks placed away from the plant. Fuel is drawn from these tanks daily or at suitable intervals and stored in day tank. The engine fed from the day tank through strainers.
An alternator is used to convert the mechanical energy supplied by the diesel engine to electrical energy. Salient pole synchronous machines with 6 to 28 poles, 52 to 35000 kVA, 0.8 pf lagging are normally used. The alternator is coupled with the diesel engine. Alternator is provided with automatic voltage controllers to permit close voltage regulation and satisfactory parallel operation.
- Low capital cost per kW.
- Simple and cheap design.
- Less space requirement compared to thermal power plants.
- Less starting and stopping time
- Good efficiency (40 – 50 %).
- Small diesel plants are portable.
- These power plants are free from ash.
- Water requirement for cooling system is less.
- Operating cost is high due to the higher fuel cost.
- These power plants have limited capacity.
- High repair and maintenance costs.
- Life of the plant is less.
- Noise and air pollutions are more.
- Diesel plants are more efficient only when it is operated at full load.
- Diesel power plants are used for starting auxiliaries in thermal power plants.
- Diesel power plants serve as back up power for industries, hospitals, business etc.
- They are used as stand by power when power from grid is not available.
- Diesel power plants are used as back up power by most of the nuclear power plants due to their reliability and ability to maintain plant safety in emergencies.