Site Selection and Efficiency for Steam Power Stations

Choice of Site for Steam Power Stations:

In order to achieve overall economy, the following points should be considered while selecting a site for a steam power station :

(i) Supply of fuel. The steam power station should be located near the coal mines so that transportation cost of fuel is minimum.  However, if such a plant is to be installed at a place where coal is not available, then care should be taken that adequate facilities exist for the transportation of coal.

(ii) Availability of water. As huge amount of water is required for the condenser, therefore, such a plant should be located at the bank of a river or near a canal to ensure the continuous supply of water.

(iii) Transportation facilities. A modern steam power station often requires the transportation of material and machinery.Therefore, adequate transportation facilities must exist i.e.,the plant should be well connected to other parts of the country by rail, road. etc.

(iv) Cost and type of land. The steam power station should be located at a place where land is cheap and further extension, if necessary, is possible.  Moreover, the bearing capacity of the ground should be adequate so that heavy equipment could be installed.

(v) Nearness to load centres. In order to reduce the transmission cost, the plant should be located near the centre of the load. This is particularly important if  d.c. supply system is adopted. However, if a.c. supply system is adopted, this factor becomes relatively less important.  It is because a.c. power can be transmitted at high voltages with consequent reduced transmission cost.Therefore, it is possible to install the plant away from the load centres, provided other conditions are favourable.

(vi)Distance from populated area. As huge amount of coal is burnt in a steam power station, therefore, smoke and fumes pollute the surrounding area. This necessitates that the plant should be located at a considerable distance from the populated areas.

It is clear that all the above factors cannot be favorable at one place.However, keeping in view the fact that now-a-days the supply system is a.c. and more importance is being given to generation than transmission, a site away from the towns may be selected. In particular, a site by river side where sufficient water is available, no pollution of atmosphere occurs and fuel can be transported economically, may perhaps be an ideal choice.

Efficiency of Steam Power Station:

The overall efficiency of a steam power station is quite low (about 29%) due mainly to two reasons.Firstly, a huge amount of heat is lost in the condenser and secondly heat losses occur at various stages of the plant.The heat lost in the condenser cannot be avoided.  It is because heat energy cannot be converted into mechanical energy without temperature difference.  The greater the temperature dif-ference, the greater is the heat energy converted*  into mechanical energy.  This necessitates to keep the steam in the condenser at the lowest temperature.  But we know that greater the temperature difference, greater is the amount of heat lost.  This explains for the low efficiency of such plants.

(i) Thermal efficiency. The ratio of heat equivalent of mechanical energy transmitted to the turbine shaft to the heat of combustion of coal is known as  thermal efficiency  of steam power station.

Thermal efficiency, η(thermal)=Heat equivalent of mech. energy transmitted to turbine shaft/Heat of coal combustion

The thermal efficiency of a modern steam power station is about 30%.  It means that if 100 calories of heat is supplied by coal combustion, then mechanical energy equivalent of 30 calories will be available at the turbine shaft and rest is lost.  It may be important to note that more than 50% of total heat of combustion is lost in the condenser.  The other heat losses occur in flue gases, radiation, ash etc.

(ii) Overall efficiency. The ratio of heat equivalent of electrical output to the heat of combustion of coal is known as overall efficiency of steam power station i.e

    Overall  efficiency, η(overall)=Heat equivalent of electrical ouput/Heat of combustion of coal

The overall efficiency of a steam power station is about 29%.  It may be seen that overall efficiency is less than the thermal efficiency.  This is expected since some losses (about 1%) occur in the alternator.  The following relation exists among the various efficiencies.

                        Overall  efficiency = Thermal  efficiency ×  Electrical  efficiency