By Shailang Marbaniang
Power plants are plants that generate electricity .The classification of these plants is based on the sources from which the energy is derived to produce electric power. Power plants are classified as under:-
a) Thermal – which use heat energy derived from combustion of solid or liquid fuels or nuclear fission to run steam turbines or natural gas or light fuel to run gas turbines.
b) Hydro – which use the potential energy of water to run water turbines.
c) Renewable – which use solar, wind, biogas, biomass, tidal energy, etc. as sources of power. This category also includes small hydropower below 25 MW capacity.
Again, based on the type of demand they supply, power plants may be divided into two types – base load stations and peaking stations. Base load stations are power stations that meet the base load demand or the demand that is more or less constant throughout the day while peaking stations are power stations that meet the peak demand or the demand that suddenly rises and falls, such as when people start to turn on the lights when it gets dark and switch them off when retiring to bed.
Power demand in an area fluctuates over the day – it is generally more in the morning and evening (peak hours) and less in the daytime and even lesser in the late night hours (off –peak hours).
Thermal Power Stations: Base load demand is generally met by thermal power stations. A thermal power station takes a long time to start as fuel has to be burnt and steam has to be generated for running the steam turbines, except for gas based stations where a mixture of fuel and air is burnt to produce pressurised air for running the gas turbines. Therefore thermal power stations, except gas based power plants, are not suitable for meeting peak demand. However, once started, the plant can run continuously at constant load for long periods of time. These plants are, therefore, suitable for meeting base load demand.
Hydropower Stations: Hydropower plants are classified into three categories –
(a) Purely Run-of–river (Purely ROR)
(b) ROR with pondage
(c) Storage.
Purely ROR plants cannot store any water since they do not have any pondage and utilise only the available flow in the river to generate power. These plants run almost continuously throughout the day and are suitable for meeting the base load demand. During lean period when the availability of water is very low, the output from these plants comes down drastically. An example of this type of plant is the Sonapani Mini hydel Project (1.5 MW capacity).
ROR plants with pondage have the capacity to store water for a short period of time, say, during the daytime when water is stored to meet the evening peak demand (6.00 PM to 10.00PM) or the morning peak demand (7.00AM to 10.00AM).The water level in the reservoir of this type of plant has large fluctuations everyday because of the small storage capacity of the reservoir. During the monsoons, these plants can also be used to meet the base load demand since they can be run for most part of the day as there is plenty of water flowing in the river. However, during the lean or dry season, these plants can be used only to meet the peak demand because of scarcity of water in the river. Examples of these plants are the Myntdu-Leshka (3x42MW) and the Ranganadi (3x135MW) Hydroelectric Projects in Meghalaya and Arunachal Pradesh respectively.
A storage type power plant has a large reservoir which can store water during the monsoon season for use during the dry or lean season.This is possible because of the favourable topography of the site which offers ample space for creating a large reservoir behind the dam . Few projects have such an advantage. Like an ROR plant with pondage, this type of plant can also be used to meet the base load demand during the monsoons when there is plenty of water flowing in the river. During the lean season, the water stored during the rainy season is utilised to meet the peak demand till the reservoir is almost depleted before it gets filled up again with the coming of the monsoon rains. Examples of these plants are the Umiam Stage- 1 (4x9MW) H.E Project in Meghalaya and the Kupli (4 X 50 MW) H.E Project in Assam-Meghalaya border.
The Plant Load Factor (PLF) or the Capacity Utilisation Factor of hydropower plants is very low compared to thermal power plants, since they are meant primarily to run for a few hours only for meeting peak requirements. While for thermal power plants the PLF may be 80% and above, this factor generally ranges anywhere from 25% to 60% for hydro power plants depending on the requirement of the system . For the projects located in the Umiam-Umtru basin, the PLF is around 30% while for Myntdu-Leshka it is much higher at about 45%.
Capital Investment : Hydropower plants require substantial investments during the construction stage since various components such as Dam, Tunnel, Penstocks (large pipes), Power House, etc. have to be constructed, unlike a thermal power plant where only the Power Station and auxiliary works need to be constructed. However, hydropower plants have low running costs because they use water, which is nature’s free gift, to run the turbines for generation of electricity unlike thermal power plants which use fossil fuel to generate energy. The tariff or the cost of energy per unit from hydropower stations drops drastically in about 12 years after interest and loan repayment, which constitute the major portion of the project cost, is over. On the otherhand, the tariff from a thermal power plant increases with the increase in fuel costs from time to time.
Environmental Issues : When it comes to environmental issues, hydro energy is regarded as clean and eco-friendly. On the other hand, thermal power is not regarded as eco-friendly since pollutants are discharged into the atmosphere during the burning of fossil fuels. However, with the energy crisis the world is now facing, we must strike a balance between good and bad and strive to find out environment-friendly solutions to this crisis so that the human race can move forward without jeopardising its very own existence.
