Global warming and climate change have become a worldwide issue, and these are the most debated topics among scientists and environmentalists around the world. Role of thermal power stations in global warming and climate change is well known.
The recently developed alternative sources of energy are thought to nullify the effects of thermal power in some way. The role of thermal power and renewable sources of power in climate change is described further.
Variation in the earth’s global climate or in regional climates over time is generally termed as climate change.
Climate change may be caused by the earth’s internal processes or external forces such as change in the intensity of sunlight or human activities.
In the present context, the term climate change refers to change in the modern climate only, including the rise in average surface temperature, commonly known as global warming.
Effects of climate change are already felt. Glaciers are recognised as the most sensitive indicators of climate change. They advance during cooling and retreat during warming. From the last century, glaciers and ice fields have been melting all over the world. Glaciers in the Himalayas have been retreating at a rate of 25 metres per year.
Melting ice has resulted in rising sea levels. It is feared that at the current rate of melting of ice, ocean levels may rise by 23 inches by 2100.
World primary energy demand increases with increase in population and economic development. Within the last 25 years, the total energy consumption in the world has almost doubled. Electricity is the most conventional form of energy in today’s world. It is mainly produced in power plants using conventional sources namely hydro energy, nuclear energy, and coal or other fossil fuels. However, in most of the countries, majority of plants use coal as primary energy. This is because installing a thermal power plant is in many ways more convenient than other power plants. Its gestation period is 3 to 4 years whereas in case of hydro power or nuclear power it may be 8 to 10 years or even more. So, to meet the immediate energy demand, thermal power is the best option. It can be located in any place unlike hydro power stations, which are site specific. Further, it is free from vagaries of weather and does not depend on rainfall unlike hydro power stations. Another factor that attracts thermal power is that coal is abundantly available in many countries. Also, the thermal power technology, over the years, becomes mature, reliable and easily available. Due to these reasons, thermal power shares more than 68% of total power produced in the world today. In our country share of thermal power is 69.5% (total thermal power installed capacity is 18,9497.78 MW out of total installed capacity of 272,687.17 MW from all sources) as on April, 2015.
Thermal power & GHG production
In thermal power plant, the heat energy from coal is used to produce steam that rotates a turbine. The turbine, in turn, rotates a generator, which produces electricity. Thus, the chemical energy stored in coal is converted to electricity. The coal or other fossil fuels are carbon rich energy sources. Coal, when burned in the boiler of the power plant produces carbon dioxide, a green house gas.
Coal–fired power stations are the least carbon efficient power stations in terms of the level of carbon dioxide produced per unit of electricity generated, and gas is the best. On an average 2.095 pounds of carbon dioxide per unit of electricity generated is produced in coal–fired power plants. It is 1.969 pounds per unit of electricity in case of oil-fired and 1.321 pounds per unit of electricity in case of gas-fired power plant. With coal-fired plants generating the majority of electricity in the world, they produce the greatest share of carbon dioxide emissions from electricity generation, approximately 80% of the total. It has been calculated that thermal power plants are responsible for about 41% of U.S. man-made carbon dioxide emissions.
The emission of carbon dioxide also depends on the efficiency of the power plant. The average efficiency of thermal power plant lies between 32 to 35%. The more efficient the plant, the less amount of carbon dioxide it emits. Substantial improvements in generation efficiency can be achieved in the future through the replacement of traditional power plants with more efficient technologies, such as supercritical boilers, combined–cycle units and combined heat and power systems.
Thermal power & climate change
It is now clear that the earth is becoming warmer day by day due to green house effect. Among the green house gases (GHGs), carbon dioxide is the main culprit. At the beginning of industrial revolution, the amount of carbon dioxide in the atmosphere was 280 ppm (parts per million). After industrialisation more amount of carbon dioxide was emitted to the atmosphere by burning of coal and other fossil fuels. In March, 2015 the level has crossed 400 ppm. If we continue to use the fossil fuels at the current level, the amount of carbon dioxide in the atmosphere is projected to reach 560 ppm by the end of 21st century.
Coal is used as fuel in many industries including thermal power stations. But the emission of carbon dioxide from thermal power stations is more than other industries. This is the only reason why the environmentalists now oppose thermal power, and engineers are thinking on alternative sources of energy.
Renewable sources of energy
In the background of increasing energy demand but scarce availability and environmental threats, the search for alternative sources of energy started towards the last part of the last century. The alternate sources which have already assumed a significant importance are solar, hydro, wind and biomass.
These energy sources are renewable in nature and environmentally benign. The magnitudes of all these sources are extremely large. The very idea of accessing these energy sources gives us a kind of confidence as far as energy security and sustainability is concerned, along with assisting in mitigating the climate change by way of reduction of carbon dioxide from power plants.
The potential for renewable sources to provide clean and inexhaustible energy that is accessible to all is now universally accepted. Intergovernmental Panel on Climate Change (IPCC) in its Fourth Assessment Report on ‘Mitigation of Climate Change’ has observed that technologies are available for mitigating the climate change, however these require appropriate policy and financial support. Renewable energy technologies have been identified as one of the key mitigation technologies for energy supply, transport, buildings, agriculture and waste management.
Renewable energy accounts for more than 10% of domestic energy production in the USA. According to the report the ‘Energy Revolution: A Sustainable World Energy Outlook’ renewable energy sources will account for 67% of the electricity produced in the developing Asia by 2050.
Sun is the principal source of almost all kinds of energy, both conventional and non–conventional. Although solar radiation is being utilised from time immemorial for drying, heating etc, direct production of electrical energy from it is a recent one. The solar energy that we receive on the earth everyday can produce 2500 times more power that we currently consume. But we should have the proper means and technology to harness the energy economically. Electricity is being generated from solar radiation either by photovoltaic cells or solar thermal power.
Solar radiation is directly converted into electricity by solar photovoltaic cells. The cell consists of two or more appropriately sandwiched thin layers of semiconducting material, usually silicon. When the solar cells are exposed to solar radiation, the incoming photons of radiation separate positive and negative charge carriers of the semiconducting material. This generates voltage and hence electricity. The higher the intensity of light, the greater is the flow of electricity. The electric output from a single cell is small. So a number of cells are connected in series or parallel to get the desired quantity. The module containing the cells is called a solar panel.
Solar thermal power station is like a conventional thermal power station having steam boiler, turbine and generator. In the conventional thermal power station, water is heated by coal, gas or petroleum oil to produce steam, which rotates the turbine and generator to produce electricity. But in the case of solar thermal power station, water is heated by heat derived from solar radiation. Sun rays are concentrated at solar receiver made of calcium carbide to have greater effectiveness.
To achieve this, sunrays are reflected from large mirrors, called heliostats, positioned at different positions at different angles so that the reflected rays concentrate at a point on the solar receiver. The surface of the solar receiver reaches to temperature as high as 10000C. In the receiver, a Heat Transfer Fluid (HTF) is heated. The HTF can be used directly in a small turbine to produce power or indirectly, to produce power when the heat is fed to a heat exchanger. The heat exchanger can transfer the heat in the HTF to high-pressure steam, which is fed to a steam turbine. Thus, although its principle is that of a thermal power station, here, no fuel is burnt, and hence there is no emission of carbon dioxide.
Energy obtained from a moving mass of air is known as wind energy. Wind has considerable potential as a global clean energy source, as it is abundant, and also non–polluting. Wind energy has been one of the primary energy sources used for milling grain, pumping water and so on. From the early wind mills used in India, China and Persia over 2000 years ago to the present use of wind for energy generation, wind has always played an important role in people’s lives.
Wind energy has attracted many investors throughout the world. Construction of the plant is very simple. A turbine with some blades coupled with a generator is installed atop a tower. When the turbine is rotated by the wind, electricity is generated. One essential feature for this plant is that there must be sufficient minimum wind speed available in most part of the year. Global installed capacity of wind energy was 369,600 MW by the end of 2014. Installed capacity of wind power is largest in China (114,604 MW) followed by the USA (65,879 MW), Germany (39,165 MW), Spain (22,987 MW), India (22,465 MW) and the UK (12,440 MW).
Hydropower is currently the most common form of renewable energy and plays an important part in global power generation. Its technology is well proven and reliable. It has many advantages over thermal power. It does not aid in global warming. Worldwide hydropower produced 3,288 TWh, just over 16% of global electricity production in 2008, and the overall technical potential for hydropower is estimated to be more than 16, 400 TWh/yr.
Electricity is now generated from biological sources like agricultural waste, plantations, municipality waste and bagasse etc. Biomass includes straw, stalks, stems, fines and agro-industrial processing residues such as shells, husks, de-oiled cakes, and also forestry residues. The conversion technologies used are combustion/incineration, gasification, pyrolysis etc., using gas or steam turbine, either in power alone or in co-generation mode. Co-generation is the multiple and sequential use of a fuel for production of steam and power in a process industry such as sugar mills, paper mills, rice mills etc., where biomass resources are either generated or consumed in their main processing/production process. Emission of carbon dioxide is minimal from this type of plants.
Development of RE in India
In India, the importance of the role of Renewable Energy (RE) to a sustainable energy base was recognised as early as in the 1970s. There has been a visible impact of renewable energy in the Indian energy scenario during the last few years. Renewable energy has been witnessing over 20% growth in the last five years. From the total renewable power installed capacity of 14,400 MW at the beginning of 2009, it has reached a capacity of 35,776.96 MW at the end of April 2015. This is apart from large hydro, which has an installed capacity of 41,632.43 MW as on April 2015. The potential and installed capacity of different sources of renewable energy in India is given in Table-1. Also, a total capacity of 1,174.5 MW of power plant in different renewable energy sources have been installed, which are not connected to the grid (off-grid power).
The growth of renewable energy in India is illustrated in Fig. – 1.
Apart from contributing about 12.96% in the national electricity installed capacity, renewable energy based decentralised and distributed applications have benefited millions of people in Indian villages by meeting their cooking, lighting and other energy needs in an environment friendly manner.
Renewable energy has been appropriately given the central place in India’s National Action Plan on Climate Change being finalised by the PM’s Council on Climate Change. India is perceived as an excellent country for developing Clean Development Mechanism (CDM) projects. As such, India has emerged as one of the most favoured destinations for CDM projects globally, with renewable energy projects having the major share.
India has achieved significantly in solar, wind, small hydro and bio energy. Wind energy continues to dominate India’s renewable energy industry. India occupies the fifth position in the world in wind energy with installed capacity of 23,444 MW. India is also doing experimental studies for other renewable sources like tidal and geothermal.
Fig. 1: Growth of renewable energy in India…
There is no doubt that increased concentration of carbon dioxide in the atmosphere leads to global warming and climate change. World wide concern over this has fixed targets to reduce green house gas emissions by between 25 to 40% by 2020. This will be difficult to achieve, if coal-fired plants remain in service, unless carbon capture and storage of emissions from coal fired power stations become viable. Some technology is available to limit carbon dioxide emissions, but it is extremely expensive. The extra cost means it is not economically feasible. In this situation, development of renewable power to mitigate climate change is absolutely necessary. India has taken a voluntary commitment of reducing emission intensity of its GDP by 20 to 25% from 2005 levels by 2020. The increased share of renewable energy in the coming years will contribute towards achieving this goal.
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