In this article we will discuss about:- 1. Introduction to Diesel Electric Power Plant 2. Selection of Site for Diesel Electric Power Plant 3. Maintaining Records (Log Sheets) 4. Layout 5. Performance and Thermal Efficiency 6. Supercharging and Its Advantages 7. Merits and Demerits 8. Applications.
Introduction to Diesel Electric Power Plant:
Diesel electric power plant is a power plant in which a diesel engine is used as the prime mover for the generation of electrical energy. The diesel engine obtains energy from a liquid fuel usually called the diesel oil and converts it into mechanical energy. An alternator or a dc generator mechanically coupled to the diesel engine converts the mechanical energy into electrical energy.
It is noteworthy that the main difference between a diesel engine and a steam turbine is that in the first case the chemical energy of combustion is released inside the cylinder whereas in the second case the energy developed during combustion of fuel is first transformed into steam which in turn develops mechanical power in the turbine.
As on July 31, 2010, and as per the Central Electricity Authority the total installed capacity of diesel based power plants in India is 1,199.75 MW. Normally the diesel based power plants are either operated from remote locations or operated to cater peak load demands. Here is some list of presently operating plants.
Selection of Site for Diesel Electric Power Plant:
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The following factors should be considered while selecting a site for diesel electric power plant:
I. Distance from the load centre – The site should be as near to the load centre as possible in order to avoid transmission costs and losses.
II. Availability of land – The land should be available at cheap rate to keep the capital cost of the plant to the reasonable one.
III. Availability of fuel – The fuel should be easily available and at reasonable rate.
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IV. Availability of transportation facilities – The transportation facilities should be available.
V. Availability of water – Water should be available in sufficient quantity for cooling purposes.
VI. Distance from populated area – The site should be away from thickly populated area because of noise and nuisance caused from exhaust.
VII. Type of land – The land should be of high bearing capacity to withstand the load of the plant and also vibrations transmitted to the foundations from compressors and diesel engines.
Maintaining Records of Diesel Electric Power Plant(Log Sheets):
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A correct record of instrument readings (ammeter in each phase, voltmeter, and wattmeter) and conditions of operation (oil pressure, water pressure, inlet and outlet temperatures of water and oil etc.) is maintained at regular intervals, say every half an hour. Such records form log sheets. A separate log sheet is used for each unit. The data so recorded enables the personnel to check satisfactorily working of the plant. Such records also assist in evaluation of performance of the plant.
Maintenance includes cleaning of fuel oil from dirt and impurities by means of filters. Occasionally all the fuel is drained and the fuel tanks are cleaned thoroughly. The temperature and flow of coolant, lubricating oil and exhaust gases are also checked at regular intervals and recorded.
Layout of Diesel Electric Plant:
For planning the layout of the diesel electric power plant the following points require consideration:
(i) Approximate dimensions of the equipment.
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(ii) Location of equipment and minimum clearances between its parts.
(iii) Provision of natural lighting and proper ventilation.
(iv) Possibility of future additions to the building with a minimum changes.
(v) The spacing required between two units, the distance between their centre lines, the distance between the centre line of the end unit and the wall, the distance between the head end of the engine and the wall, and the generator end and the wall.
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(vi) The location of switchboard, station auxiliary transformers, battery room, fuel-oil tank, compressed-air cylinder bottle for starting of engine, compressors, lubricating oil system and the cooling arrangements for cylinder jackets and suction and exhaust arrangements for the engine.
(vii) Provision for a small workshop, stores, office etc.
(viii) Provision for installation of at least one spare unit in future.
(ix) Provision for bringing the equipment into the building.
(x) Provision of space for erection work, and for handling the equipment for maintenance.
The general layout of a medium size diesel electric power plant is illustrated in Fig. 5.2. The units are usually placed parallel to each other in order to keep the length of electrical connections for generators to control board and air ducts and exhaust pipes minimum. The generating units (diesel engine- generator sets) are placed on large concrete slabs preferably reinforced. The foundation should be firm and subsoil solid. The air intake and filters and the exhaust mufflers should not be located in the engine room.
Performance and Thermal Efficiency of Diesel Electric Power Plant:
Only 34 per cent of the energy supplied to the diesel electric power plants in the form of fuel is converted into useful work, remaining is lost in cooling water (about 30%), through exhaust gases (about 25%) and in radiation, friction etc. (11%). Thus the full-load thermal efficiency of a diesel power plant comes out to be about 34%. At part load, the thermal efficiency decreases appreciably and specific fuel consumption increases, as shown in Fig. 5.3. So part-load operation of diesel power plants is not economical. Specific fuel consumption at full load is roughly 0.23 kg per kWh.
Supercharging of Diesel Engine and Its Advantages:
Supercharging, also known as boosting, can be defined as the admittance of an air charge into the cylinder larger than that obtainable as a result of the regular suction stroke. It is accomplished by external means as compared to regular suction, called the natural aspiration. The main object of supercharging is to increase the power output of the diesel engine without increasing its rotational speed or the cylinder dimensions.
This is achieved by increasing the charge of air causing burning of more fuel and resulting in increase in mean effective pressure. The charge or weight of air for a given volume of air depends solely on pressure (varies directly as pressure) and temperature (varies inversely as temperature).
Thus an air charge can be increased either by increasing pressure P or reducing the temperature T for a given volume. It can be shown that a large temperature drop is required to have a reasonable increase in weight—a drop of at least 38° would be required to give a 20% weight increase.
Lowering of compression temperature will also make the starting very difficult. On the other hand, quite moderate pressure increase will give the desired result. An increase of about 0.003 MPa gauge result in an increase of weight by 27%. It is this method which is utilized in supercharging for increasing the weight of the air.
Supercharging offers advantages of an increased power output (30-50%) at the given speed, fuel economy due to better combustion, higher mechanical efficiency, better scavenging action (in the case of two-stroke engines), increased volumetric efficiency and reduced possibility of knocking in diesel engines.
Supercharging is done by means of blowers either of positive displacement type (such as piston cylinder type, root blowers or vane blowers), or of the centrifugal type or of the turbo-type. The blowers are usually driven from the engine crank shaft by means of spur, helical or her- ring-bone gears, silent chains or V-belts at a speed of 2- 3 times the engine speed. In the case of a turbo-charger, the super-charger is coupled to a gas turbine in which the exhaust of the engine is expanded.
The positive displacement blowers are employed for low and medium speed engines with speeds not exceeding 4,000 rpm. Centrifugal blowers are employed both on low and high speed engines. Centrifugal blowers driven by exhaust gas turbines are small and light and are employed for stationary, locomotive, marine and aircraft engines.
There is an economical limit to the supercharging, which is about 40% at present. With the exceeding limit of supercharging, the power absorbed by the engine driven blowers, increases rapidly without providing appreciable gain. The other limiting factors are ability of engine to withstand the increased pressure and temperature stresses. However, these difficulties can be overcome by providing better cooling system, improving methods of lubrication and making metallurgical improvements.
The use of supercharging on the diesel engines has progressed rapidly as increase in compression pressure does not cause diesel knock in the engine. Actually it results in smoother running of the engine.
Merits and Demerits of Diesel Electric Power Plant:
Diesel electric power plants have many advantages over other types of power plants, as given below:
(i) The design and installation of such power plants is very simple.
(ii) Such plants can be located at any place.
(iii) Such plants can be quickly procured, installed and commissioned.
(iv) The layout, design and construction of foundations and buildings for such power plants are simple and cheap.
(v) Such plants require less space for fuel storage and are free from ash handling problems.
(vi) Such plants can respond to varying loads without any difficulty.
(vii) Such plants occupy less space because of minimum auxiliaries.
(viii) No standby losses.
(ix) The quantity of water required for cooling is limited.
(x) Such plants can be started and put on load quickly.
(xi) Such plants maintain high operation efficiency irrespective of load.
(xii) The overall capital cost including installation per unit of installed capacity is lesser than that for steam power plants.
(xiii) The efficiency of such power plants falls off very little with the usage in comparison with steam power plants.
(xiv) Thermal efficiency of such power plants is higher (about 40%) than that of a steam power plant.
(xv) Variety of fuels, such as residual fuel oil, low sulphur heavy stock etc. can be used.
(xvi) Less fire hazard.
(xvii) Such plants being simpler in operation require less operating and supervising staff as compared to that for steam power plants.
However, the diesel electric power plants have some drawbacks as given below:
(i) Operating cost, due to high cost of diesel oil as fuel, is very high.
(ii) Maintenance and lubrication cost is also high as compared in the case of other plants.
(iii) Diesel plants cannot supply overloads continuously whereas steam plants can work under 25% overload continuously.
(iv) Diesel unit’s capacity is limited. These cannot be constructed in large size.
(v) Noise from the exhaust is a serious problem.
(vi) Their useful life is very short (say, about 10 years).
Applications of Diesel Electric Power Plant:
The size limitations not-withstanding, the escalating oil costs have made the operation of diesel electric power plants very uneconomical. In view of this, such plants play a very minor role in electric power generation.
Their principal fields of use are given below:
1. Central Power Stations:
Diesel power plants are installed where supply of coal and water is not available in sufficient quantity or where power is to be generated in small quantity. Power stations of this type in common use are of capacities up to 10 MW. The limit will be decided by the economics of the cost of the plant and the local conditions such as fuel costs, availability of water supply etc.
2. Standby Power Stations:
Diesel power plants may be used as standby plants where continuity of power supply is essential such as in hospitals, telephone exchanges, radio stations, cinemas etc.
3. Peak Load Plants:
Diesel power plant can be employed to supply the peak load on the power system while the base load is supplied by a nuclear or hydro power plant. The base load factor will thus be improved and cost of electrical energy per unit will be reduced. Diesel power plant is particularly suitable for supplying a load of small load factor. Its characteristics of quick starting and absence of standby losses make such plants very suitable for supplying peak loads.
4. Emergency Plants:
A small diesel power plant may be installed in a large power station to supply essential auxiliaries in case of failure of main supply. Arrangements can be made to start the diesel plant automatically.
5. Private Power Plant For Small Industries:
Diesel electric power plants can be employed as private industrial plants for supply of electric power specially if the capacity requirements are within the limits, set by the size of the diesel units available. The diesel power plants have certain advantages over the steam power plants.
The diesel power plant needs much less water than a steam plant of equal capacity, its other advantages are:
i. A diesel power plant can be started and stopped quickly, as and when required.
ii. It does not require any warming period, and need to be kept running for a long time before picking up load. There are thus no standby losses, as in steam power plants.
iii. Diesel power plant does not require a large amount of cooling.
iv. The diesel engines have a lower installation cost per kW as compared with the same size of the steam unit.
v. The diesel power plant needs less operating and supervising staff than a steam power plant.
vi. There are fewer auxiliaries as compared to a steam power plant.
vii. Buildings and foundations are relatively simple.
viii. The diesel engines maintain their efficiency even at fractional loads whereas the efficiency of a steam power plant decreases rapidly below at 75% load.
6. Power Plants for Sparsely Populated Areas:
A diesel plant may be employed to supply a sparsely populated area where only one transmission line is economically justifiable, to maintain continuity of supply in case the line should fail. Continuity of supply can be maintained by starting the diesel plant as soon as the transmission line fails or the voltage drops below the permissible limit. In either event the diesel power plant can be made to start automatically.
7. Nursery Power Plants:
In India, a large number of power projects are being developed under various five year plans, and they are interconnected wherever possible to form a grid system. This assists to make the power available to large area and to a number of places. Unless there is an enough load, it is, however, not possible to extend the grid immediately to every village and corner of the country.
During the process of development, a small diesel unit may be temporarily installed to supply the load of a village or town including the farm load. When the load develops and the grid supply becomes available, the concerned town or village can be connected to the grid and the diesel engine set can be moved to another area which requires power on a small scale and where the grid supply is not available. The diesel power plants used in this way are known as ‘nursery power plants’.
8. Mobile Plants:
Mobile diesel electric power plants mounted on trailers are employed for use in emergency and for temporary purposes.