What is Solar Energy? Solar Photovoltaic Cells | Solar Thermal Power Plant

Solar energy is the abundant (virtually unlimited) source of renewable energy. If harnessed properly it could meet the present global energy demand.

There is a gradual depletion of fossil fuels and sharply increasing demand of energy. So the solar energy could well become an alternative source of energy to meet the future demand as earth receives 1.8 x 10¹¹ MW of solar power while the present global energy demand is about 1.6 x 10⁷ MW. 

The geographical location of India is ideal for solar energy. The country receives about 300 clear days in a year, which could produce enough power for more than total energy consumption of India.

Solar energy can be utilised in two ways, namely, solar thermal and solar photovoltaic. Solar thermal energy is used for cooking, heating, drying, timber seasoning, distillation, cooling, refrigeration, cold storage, and also for electricity generation. Some of the solar thermal devices are solar cooker, solar collectors, solar hot water systems, solar pond, solar hot air system, solar driers, solar still, solar air-conditioners, etc.

Solar energy is used for generating electricity by photovoltaic systems. Solar photovoltaic cells convert sunlight directly into electricity. This electricity can either be used directly or can be stored in batteries. This stored energy in batteries can be used during night hours when there is no sunlight.

Solar photovoltaic (SPV) systems can be used for a number of applications such as, rural electrification, water pumping, desalination, solar lighting, etc.

SOLAR ENERGY SUPPLY

Solar energy comes from the sun. The distance of sun from earth is 1.5 x 10⁸ km. So, how energy from sun comes to earth? Sun is a huge sphere of gaseous matter of diameter 1.39 x 10⁹ m and has temperature varying from 8 x 10⁶ to 40 × 10⁶ degree Kelvin. All substances above absolute temperature emit energy in the form of electromagnetic waves. Due to very high temperature, sun emits energy in the form of electromagnetic wave called solar radiation. These waves are transmitted to earth in the form of photons at a speed of light, i.e., 3 x 10⁸ m/s Photons are electromagnetic waves having packet of energy. Total energy received on earth has three spectral regions according to wavelength (λ).

1. Ultraviolet    6.4%       (λ < 0.38 μm)

2. Visible          48%    (0.38 μm < λ < 0.78 μm)

3. Infrared      45.6%         (λ > 0.78 μm)

Energy received from sun before entering earth's atmosphere is constant. This is called extraterrestrial radiation. It is expressed as solar constant and has value of 1367 w/m².

Solar radiation then passes through earth's atmosphere where it is scattered and some parts are absorbed. Ultraviolet rays of short wavelength are absorbed by ozone layer. Infra-red rays of long wave length are absorbed by CO₂ and water vapour. Scattering is due to air molecules, dust particles and water droplets. Thus solar radiation decreases while passing through earths atmosphere. It is called attenuation of radiation. 

The solar radiation in earth's atmosphere has following radiation: 

1. Beam radiation (I_b): It is the radiation received on earth's surface without change in direction. It is also called direct radiation.

2. Diffuse radiation (I_d): Scattered radiation received is called diffuse radiation. 

3. Albedo radiation (I_a): Radiation reflected by the ground or other objects on the ground.

4. Global radiation (I_g): I_g = I_b + I_d + I_a 

Solar radiation passes through different distance through atmosphere depending on position of sun and corresponds to different solar radiation intensity as listed below:

Symbol	Position of Sun	Solar Radiation Received W/m²
AM0	Extraterrestrial Radiation	1367 W/m²
AM1	Sun is overhead (also called zenith)	1105 W/m²
AM1.5	Sun is at 48° from zenith	1000 W/m²
AM2	Sun is at 60° from zenith	894 W/m²

About 30% of total solar radiation energy is reflected back to space by reflection from cloud by scattering and by reflection from earths surface. This is called albedo.

The amount of solar radiation available at a given location and time depends on latitude, longitude of location, time of day, month. Solar radiation increases from sunrise to noon and then decreases from noon to sunset.

Alternatively, the energy received by the land in one hour

= 1105 x 10⁴ watt-hour

= 11050 kWh

SOLAR PHOTO-VOLTAIC CELLS

Construction of Solar Cell

The structure of a typical solar photo-voltaic cell is shown in the Fig. 1.

Fig. 1. Solar Photo-Voltaic Cells

The main components of a solar photo-voltaic cell are:

(i) p-type material

(ii) n-type material

(iii) p-n junction

(iv) front metallic grid 

(v) back metal contact

(vi) anti-reflective coating

The p-type material is made of silicon and has a thickness of about 100 to 300 microns depending on technology. A thin layer of n-type silicon is formed at the top of the surface of p-type material by diffusing with an impurity like phosphorous which forms a p-n junction.

The n-type layer has a thickness of about 0.2 micron. The top surface of n-type material has a metallic grid structure to collect the current generated by falling photons and forms the negative terminal of the solar cell. It is in the form of thin metallic grid to allow the maximum photons to strike. The back metal contact is continuous layer which forms the positive terminal of the cell. When sunlight falls on the front surface, a major portion is reflected back. To prevent this an antireflective coating is provided on the front surface of the cell so that most of the light is absorbed by the cell. The blue colour of the cell is due to the anti reflective coating.

Working Principle

A solar cell is a p-n junction diode under light illumination over the surface of the cell. When solar radiation is absorbed by p-n junction, electron-hole pairs are formed. The generation of electron-hole pairs will depend on solar radiation. The electrons from p-side will move to n-side and holes from n-side will move to p-side, so there is net positive charge on p-side and net negative charge on n-side.

This build-up of positive and negative charges produces a potential difference across the p-n junction due to light falling on it. This potential difference is called photo-voltage. The generation of photo-voltage due to light illumination is called photo-voltaic effect. Thus, a solar cell works on the principle of photo-voltaic effect. For a silicon p-n junction voltage generated is 0.5 to 0.6 volt.

I-V Characteristic

When there is no light falling on the solar cell there is no photo-voltage. When light falls on the solar cell photo-voltage is generated and current flows if a load is connected across the terminals of a solar cell. The I-V characteristic of a solar photo-voltaic cell is shown in the Fig. 2.

Fig. 2. I-V Characteristics

The four main parameters of the I-V characteristic of a solar cell are short-circuit current I_sc, open circuit voltage V_oc, maximum current I_m and maximum voltage V_m. V_m and I_m correspond to the maximum power point (MPP) P_m as shown in the Fig. 2.

I_sc is the current flowing through the solar cell when its terminals are short-circuited. V_oc is the potential difference across the open-circuited terminals of the solar cell. Ideal power (P_o) is the product of V_oc and I_sc.

P_o = V_oc × I_sc

Maximum power (P_m) that can be extracted by a solar cell is the product of V_m and I_m.

P_m = V_m × I_m

Fill Factor (FF) : It is defined as the ratio of maximum power P_m to ideal power P_o.

FF = P_m/P_o = V_m.I_m/V_oc.I_sc

Efficiency of a Solar Cell

The efficiency (η) of a solar cell is the ratio of output power (P_m) to input solar radiation power (P_rad).

η = P_m/P_rad

Standard solar radiation power (P_rad) is 1000 W/m² at a temperature of 25°C. Power ratings of the solar cells are based on this standard solar radiation. However, a solar cell will give less power because radiation falling on it is less than this standard.

Also the operating temperatures of a solar cell affects the performance of it. Solar cell produces less power as temperature increases beyond 25°C. A solar pv panel rated at 100 watts at standard temperature of 25°C will be an 83 watt panel at 43°C.

Comparison Between Solar Cell and Battery

Solar Cell	Battery
1. It converts solar radiation energy into electrical energy.	1. It converts chemical energy into electrical energy.
2. Cost of solar PV panel is higher than that of a battery.	2. Cost is lower.
3. Less maintenance required.	3. Regular maintenance required.
4. The voltage output is in the range of 0.5 to 0.6 volt	4. The voltage output is 1.2 V for alkaline batteries and 2.1 V for lead acid battery.
5. It is a semiconductor device having p and n-type materials forming a p-n junction.	5. It has two electrodes in a solution called electrolyte.
6. Size of solar cell I compact.	6. Size of battery is bulky.

Merits of Solar Photo-voltaic Cells

1. The energy of the solar radiation is directly converted into electrical energy.

2. It is a solid state semiconductor device and has no moving so no noise pollution.

3. It is easy to operate.

4. The maintenance cost is low.

5. It has long life span about 25 years.

Demerits of Solar Photo-voltaic Cells

1. Efficiency of a solar cell is very low and is about 15%.

2. Output from a solar photo-voltaic cell is d.c. so additional electronic device like inverter is required to convert it into a.c. 

3. The capital cost of a solar photo-voltaic system is high.

4. The disposal of solar photo-voltaic cell poses problem due to the presence of arsenic and cadmium.

5. Solar photo-voltaic cells cannot produce electricity when there is no sunlight as during night hours and cloudy days.

SOLAR THERMAL POWER PLANT

A solar thermal power plant utilises heat energy of solar radiation to generate electrical energy. The schematic diagram of a solar thermal power plant using a flat-plate solar collector is shown in the Fig. 3.

Fig. 3. Solar Thermal Power Plant

The main parts of the solar thermal power plant are:

• Flat plate solar collector
• Hot water tank
• Heat exchanger
• Turbine
• Generator
• Condenser
• Circulating pumps
• Cooling tower

When solar radiation falls on an array of solar collectors, it will heat the water passing through the water channel inside it. Cold water is fed to the collector from the water tank and hot water from the outlet of the collector is supplied to the tank. Thus the water is circulated through hot water tank and collector with the help of circulating pump. This increases the temperature of water in the hot water tank upto 90°C. The tank is air insulated to prevent heat loss.

The hot water from the tank passes through a heat exchanger with the help of a hot water circulating pump and gives its heat energy to the working fluid used in the turbine. The working fluid is methyl chloride (CH₂Cl) or butane (C₂H₁₀) which has a boiling temperature below 90°C. When the fluid gets heated in the heat exchanger the vapour is formed which flows to a turbine working on Rankine cycle. The vapour expands over the turbine which utilises heat energy of the vapour to produce mechanical energy. An ac generator is coupled with the turbine. The generator converts the mechanical energy of the turbine into electrical energy. The vapour is condensed at the outlet of the turbine with the help of condenser. This increases overall efficiency of the turbine. Finally the organic fluid is pumped back from the condenser to the heat exchanger and the is repeated. The overall efficiency of this plant is about 2%.

Such type of a power plant has electrical power upto 150 kW capacity. However, electrical power of 10 MW and higher can be obtained by adopting concentrated type of collector where temperature of 400°C is obtained.

SOLAR ENERGY UTILISATION TECHNIQUES

Solar energy can be utilised for different useful purposes by adopting different techniques. The main techniques involved are of two types, namely, solar thermal conversion system and solar photo-voltaic system.

In solar thermal conversion system, energy of solar radiation is converted into heat energy which can be utilised for different purposes. In photo-voltaic system the energy of solar radiation is converted directly, into electrical energy which can be used for power generation, lighting, heating, charging batteries, etc.

Solar Thermal Utilisation Techniques 

Applications of solar energy based on solar thermal conversion system are:

• Solar pond
• Solar water heating
• Space cooling
• Refrigeration
• Solar dryer
• Solar distillation
• Solar cooker
• Solar thermal power plant

(1) Solar Pond

It is an artificially constructed pond, which is designed for collecting and storing solar heat. In the lower regions of the solar pond significant temperature increase is achieved. 

So the solar energy is stored in the thermal form but in the low grade (60 to 100°C). This low temperature energy can directly be used for space heating or industrial process heat. For other applications Rankine cycle engines may be used.

The schematic diagram of a non-convective solar pond is shown in the Fig. 4. In this type of solar pond the bottom of the pond is blackened to serve as an absorber. The bottom layer of salty water above the blackened surface may attain the temperature as high as 70 to 85°C. while the top layer of fresh water is at the normal temperature (about 27°C). The non-convective layer in between the two layers acts as a thermal insulator.

Fig. 4. Non Convective Solar Pond

The salty water of the bottom layer heats up. This heated water does not rise and stays trapped below. This is due to the density of salty water at the bottom layer is higher than that of water above.

This hot water from the bottom is taken out from the pond and used to evaporate an organic working fluid in a heat exchanger and then supplied back to the pond. The organic working fluid produces mechanical power in a rankine cycle which then drives the turbine. Coupled to this turbine an ac generator produces electricity. Some solar pond sites are mentioned below:

(i) A solar pond of an area of 1200 m² is located in Bhavnagar. Gujrat which was built in 1973.

(ii) Worlds second largest solar pond with dimensions of 100 m (length) x 60 m (width) x 3.5 m (depth) is located in Bhuj, Gujarat. It provides 90,000 litres of hot water at temperatures about 80°C daily.

(2) Solar Water Heating

Solar water heating system is used for heating of water with the help of energy of solar radiation. The detailed description is already given in the chapter. Over 3000 solar water heating systems of different capacities in the range of 100 litres per day to 10,000 litres per day are in operation in different parts of the country. These systems have following benefits:

• Solar energy is locally available and is free of cost.
• Solar energy is pollution free.
• Solar water heating systems do not require extra space as they can be mounted on roof tops, windows, etc. 
• Their installation, operation and maintenance is easy.
• Solar water heating systems of capacity upto about 150 litres per day (lpd) do not require circulating pump.

(3) Solar Dryers

Solar dryer is a solar thermal device which uses solar energy for drying of foodgrains and other agricultural products. Natural convection solar dryer consists of an enclosure with a transparent cover on the top and the sides. The internal surfaces of the enclosure are blackened. The product to be dried is placed inside the enclosure. The evaporation of moisture from the product takes place due to direct absorption of solar radiation by the product and heating of internal surfaces by the radiation incident on them.

Solar dryers are hygienic, free from dirt, dust, insects and also unaffected by adverse weather conditions. Also they do not need any fuel or electricity for drying. Because of these advantages solar dryers are replacing the traditional (open to sun drying) method in some parts of the country.

(4) Solar Distillation

Production of potable water from the saline water by distillation is already practised. There are various commercial desalination plants which use either fossil fuels or electricity as the energy source. As better option we have to use distillation system using solar energy. So, solar water still, a device to produce potable water by utilising solar heat energy can be used.

Distillation of saline water is done by exposing thin layers of the saline water to solar radiation, and condensing the water vapour deposited on a transparent cover and then collected in receiving troughs.

Some of the solar distillation plants are listed below:

• Salt works, Bhavnagar is situated in Gujrat having capacity of 1000 m³ per day is mainly used for drinking purpose for workers.
• Bitra Island, Lakshadweep has a solar distillation plant having capacity of 2000 m³ per day in an evaporating area of 750 m².
• Awania village has a solar distillation plant near Bhavnagar, Gujarat having capacity of 5000 m³ per day. It supplies drinking water to the village where saline water has a TDS (total dissolved salt) of 4500 Parts Per Million (PPM) and fluoride 10 PPM.
• A solar distillation plant at Bhaleri in Churu district of Rajasthan has capacity of 8000 m³ per day. Saline water has TDS of 3800 PPM, nitrates 340 PPM, fluoride 5 PPM.

(5) Solar Thermal Power Plant

Solar thermal power plant utilizes heat energy of solar radiation to generate electrical energy. The detailed description of this plant is already discussed in Article 2.13.

Solar PV Utilisation Techniques

Application of solar energy based on solar photo-voltaic system are:

• Solar PV water pumping system.
• Solar lantern.
• Solar street light.
• Solar home lighting system.
• Solar PV power plant.
• Solar PV power in satellite.

MERITS AND DEMERITS OF SOLAR ENERGY

Merits

1. Solar energy is a renewable energy source. The solar energy systems use radiation from the sun which is inexhaustible.
2. Solar energy does not use any fuel, so there is no pollution of air and water. It is clean source of energy.
3. The areas where there is no supply of electricity particularly the inaccessible areas, remote areas without grid connectivity can utilise solar energy for useful purposes.
4. Source of solar energy is vast in nature. The solar power received by earth is about 1.8 × 10¹¹ MW.

Demerits

1. Solar power depends on sunshine. It is not available during night hours and in cloudy days.
2. Intensity of solar radiation varies during the daytime. Maximum solar radiation is available only for 5 to 6 hours in a day.
3. The disposal of solar photo-voltaic cells causes problem due to the presence of arsenic and cadmium.
4. Solar energy is distributed and not concentrated, therefore, it is dilute form of energy source.
5. Solar reflectors are hazardous to eyesight.

Read more ...

Non Conventional Energy Sources

Renewable energy sources are called non-conventional energy sources. A renewable energy system converts energy found in sunlight, wind, falling water, sea waves, geothermal heat, biomass into a suitable form of energy. The renewable source can never be exhausted. Various forms of these energies are:

• Solar Energy
• Wind Energy
• Hydro Energy
• Bio Energy
• Geothermal Energy
• Ocean Energy

Solar Energy

The sun is the most abundant and unlimited source of energy. Energy from the sun is known as solar energy. This energy has very high potential. As a result, solar energy is one of the most important non-conventional sources of energy in India. Solar energy is useful in a number of ways. The solar cookers are used for cooking of food with practically no fuel cost. Solar concentrators can be used to produce steam for driving electric generators. Solar cell converts solar radiation into electrical energy by photoelectric effect. A large number of cells are connected in series/parallel to form a photovoltaic panel that are used to generate electrical energy on large scale.

Read More - Solar Energy

Wind Energy

Movement of atmospheric air is called wind. It has kinetic energy. About 1 to 3% of the solar energy falling on earth gets converted into wind energy. Wind energy is a popular form of non-conventional energy source. It is used for drawing water, which is an essential requirement in watering agricultural lands in the rural areas. Even, it can be utilized for electricity generation. In India, some states like Tamil Nadu, Gujarat, Odisha, and Maharashtra are regarded as superior areas with respect to this type of energy. Places that have regular and rapid wind flows are suitable for this kind of power generation.

Read More - Wind Energy and Wind Turbine

Small Hydro

Water is about thousand times heavier than air. Even a slow flowing water stream produces large amount of energy. Energy captured from falling water is called hydro energy. Small hydro power is considered as renewable or non conventional energy sources. Small quantity of water with high head as in case of hilly region and small quantity of water with low head as in case of canal can be tapped. The force of flowing and falling water is used to drive water turbine to generate energy.

Small hydro schemes are classified as micro hydro upto 100 kw plant capacity, mini hydro from 101 kW to 1000 kW and small hydro upto 25,000 kW plant capacity. The advantage of this scheme is that it can be harnessed almost everywhere in India from any nearby water stream or canal without causing submergence, deforestation or resettlement problem. The main advantage of these plants is high efficiency and long life. It is also cheapest, and cleanest source of energy. Civil works and installation of equipments need simple process and it employs local people and use locally available materials. Also gestation period is short.

India has a large potential of small hydro power of about 15,000 MW. The 130 KW small hydropower plant in Darjeeling set up in 1897, was the first in India. There are more than 4,000 potential sites of small hydro power in India and its development will reduce load on conventional sources of energy. So far, about 500 small hydro projects with a total capacity of 4,055 MW have been installed till March, 2015.

Bio-energy

1. Biomass: Biomass is an important source of energy which represents approximately 40% of the overall volume of fuel used in the country. It is broadly utilized in domestic circles for preparing food and warming. In rural areas, farming wastes, timber, wood, raw charcoal can be used as biomass. Many initiatives have been taken in India to make the right use of biomass in systematic way. The two principal segments of the biomass program are generation and usage of biomass.

2. Biogas: With the usage of agricultural and animal excretions over and above human body wastes, a number of 'Gobar Gas' plants have been built in many rural areas to make them self reliant in their energy requirements. The energy generated in this way is utilized for food preparation, lighting roads and houses, and satisfying the water supply requirements of the rural community. The plants have been established both at private and group levels.

The composition of biogas varies depending upon the origin of the anaerobic digestion process.

3. Biofuel: A new form of man-made fuel, which could be either bio-ethanol or bio-diesel. Both can be made by man using plant sugars or starch or animal fats/oil. While bio-ethanol and bio diesel can be used in its purest form as a fuel for vehicles, it is usually used as a gasoline/diesel additive.

Geothermal Energy

The core of the earth is very hot and it is possible to make use of this geothermal energy (in Greek it means heat from the earth). These are areas where there are volcanoes, hot springs, and geysers, and methane under the water in the oceans and seas. In some countries, such as in the USA water is pumped from underground hot water deposits and used to heat people's houses. Electric power generation from geothermal energy can be achieved through transfer of heat energy to a working fluid which operates the power cycle. Electric power can also be generated when hot geothermal water or steam is used to operate the turbines directly.

In India, North-western Himalayas and the Western Coast are considered geothermal areas. The Geological Survey of India has already identified more than 350 hot spring sites, which can be explored as areas to tap geothermal energy. The Puga valley in the Ladakh region has the most promising geothermal field. An experimental 1 kW generator is already in operation in this area. It is being used mainly for poultry farming, mushroom cultivation, and pashmina-wool processing, all of which need higher temperatures.

Ocean Energy 

Energy from ocean and seas are obtained mainly in three ways ocean thermal, tidal and wave energy. 

1. Ocean thermal energy: A large amount of solar energy is stored in the oceans and seas which absorb heat energy of solar radiation.

The surface water being directly exposed to solar radiation is heated more than that of deep ocean water. There a temperature difference of about 25°C exists between the surface of the ocean and the depths of about 1000 m to operate a heat engine, which produces electric power. The process of harnessing this type of energy is called OTEC (ocean thermal energy conversion). The surface water which is at a higher temperature could be used to heat some low temperature boiling fluid like ammonia, the vapours of which would run a heat engine.

Read More - Ocean Thermal Energy Conversion

2. Tidal energy: Tides are a result of the gravity of the sun, the moon and the rotation of the earth, working together. These tides can be used to produce electrical energy which is known as tidal energy. Tidal energy is a limitless and renewable source of energy. For producing electrical power from tidal energy, dams are built so that inward and outward water flow are used to power turbines to generate electricity. A major tidal power project costing 5000 crores is proposed to be set up in the Hansthal Creek in the Gulf of Kutch in Gujarat.

Read More - Tidal Energy and its Phenomenon

3. Wave Energy: Waves are a renewable source of energy that doesn't cause pollution. Energy from waves alone could supply the world's electricity needs. The total power of waves on the world's coastlines is estimated at 2 to 3 million megawatts (terra watts).

Ocean waves are caused due to wind as it blows across the open expanse of water, the gravitational pull from the sun and the moon, and changes in atmospheric pressure, etc. Waves created by the wind are the most common waves relevant for most wave energy technology. Wave energy conversion takes advantage of the ocean waves caused primarily by the interaction of winds with the ocean surface. Wave energy is not regular oscillating low-frequency energy source. They are a powerful source of energy, but are difficult to harness and convert into electricity in large quantities. This energy needs to be converted to a 50 Hertz frequency before it can be connected to the electric grid.

At present, there are only a handful of wave generator plants in operation around the world.

Read More - What is Wave Energy? Wave Energy Generation and Wave Energy Conversion Devices

Read more ...

Conventional Energy Sources

CONVENTIONAL SOURCES OF ENERGY

The main conventional sources of energy are:

(a) Fossil fuel

(b) Large hydro

(c) Nuclear fuel

Fossil Fuel

Petroleum, coal and natural gas are called fossil fuels because these are formed by decomposition of plants and animals buried under the earth for a very long time. These fuels cannot be formed in a short span of time and are called non-renewable sources of energy. Petroleum is refined in huge refineries to produce various fuels like petrol and diesel.

1. Petroleum

It is a dark coloured, viscous and foul smelling crude oil. This oil is lifeline of global economy. Thirteen countries of OPEC (Organisation of Petroleum Exporting Countries) have 67% of world oil reserves. USA is the world's largest consumer of oil using 30% of global total. Maximum use of oil is in transportation (63%), followed by industries (24%), residential and commercial building (8%) and electric utilities (8%). At the present rate of consumption, the world's crude oil reserves are estimated to be depleted in 40 years. Undiscovered oil deposits could meet world demand for another 30-40 years.

India has total reserves of 1201 million metric tonnes of crude oil. India has a total installed capacity of 1200 MW of oil based power plant and price per kWh is about 8. So these power plants are mainly used during peak hours.

Some of the crude oil producing locations in India are:

1. Ankleshwar and Kalol in Gujarat

2. Rudrasagar and Lakwa in Assam

3. Bombay high (off-shore area) 

The oil wells of Bombay high are producing about 22 million tons of crude petroleum oil every year which is about 50% of country's total oil requirement. The crude petroleum is refined by the process of fractional distillation to obtain useful products as listed below:

(i) Petroleum gas (below 40°C) used as LPG (ii) Petrol (40"C to 170°C) for light vehicles
(iii) Kerosene (170°C to 250°C) for household and industrial use
(iv) Diesel oil (250°C to 350°C) for heavy vehicles.

(v) Residual oil:
lubrication oils • paraffin wax • asphalt 

(vi) Fuel oil (350°C to 400°C) for boilers and furnaces.

2. Coal

Coal is the most common and main source of energy. It is a complex mixture of carbon, hydrogen and oxygen. Small amount of nitrogen and sulphur compounds are also present in coal.

Coal is the most abundant fossil fuel of the world. About 50% of coal reserves are located in USA, Russia and China alone. At the present rate of consumption, world coal reserves will be exhausted in 154 years. India has about 5% of world's coal reserve and coal India is available mainly in Bihar, Jharkhand, West Bengal, Odisha, Madhya Pradesh and Chhatisgarh. The big coal mines India are at Jharia and Bokaro in Jharkhand and at Raniganj in West Bengal. Coal is main energy source of power production in India, generating approximately 57% of total electricity by coal based thermal power plants. These power plants are generally installed near coal mine areas to reduce transportation cost of coal. Total installed capacity of coal powered thermal power plants is 1,64,636 MW by year 2015.

Depending upon the carbon contents of coal, it is classified as follows:

S No.	Type of coal	Carbon content (%)
1	Peat	60
2	Lignite (soft coal)	70
3	Bituminus (household coal)	80
4	Anthracite	90

From the above table it is clear that peat is poor quality of coal and anthracite is best quality of coal.

Coal is used as fuel in thermal power plant. Coal occurs in coal mines and is brought to power plant by means of railways and road transportation where it stored in coal yard or coal storage. Then it is sent to coal handling plant for different processes. Coal is pulverized by crushers and magnetic material present are separated by big magnets before coal is being fed to boiler furnace by conveyor belt. Combustion of coal takes place at furnace to produce hot gases called flue gas. Air is fed to furnace bed by Forced Drought (FD) fan as sufficient air is required for proper burning of coal. Water from nearby pond or river is processed by water treatment plant and clean water is fed to boiler where it is heated by flue gas to produce steam. High pressure and temperature steam is fed to steam turbine through a set of stationary nozzles and steam is expanded in the turbine causing turbine to rotate. Thus heat energy of steam is converted into mechanical energy the turbine. Steam turbine is coupled with alternator which converts mechanical energy into electrical energy. Alternator is synchronized with grid and electrical energy can be transmitted to load centre by transmission lines. Generally thermal power plant is located near load centre to minimise transmission cost.

After doing useful work over turbine, steam is cooled into hot water at turbine outlet by condenser and this hot water is again fed back into boiler and the process is repeated. Sufficient water is required for steam production and for cooling, so this type of plant is installed near river or other water bodies.

Efficiency of thermal power plant is low and is of the order of 26-30% due to heat losses at various stages. Heat energy of flue gases is recovered at different points by super heater, economizer and air pre-heater to improve efficiency before the gas is allowed to atmosphere via chimney. This gas pollutes air as it contains suspended micro solid particles. Pollution is minimized by Electro Static Precipitator (ESP) where particles are charged and settled down. Left out of burned coal appears as hot ash which is cooled and treated in ash handling plant and is then disposed of.

Main drawback is maximum global warming caused by coal due to carbon dioxide (CO₂) gas emission to the atmosphere.

3. Natural Gas

Methane, ethane and propane are main constituents of natural gas. It is found deep under the earth crust above petroleum deposits. It is a product of petroleum mining and is used as industrial and domestic fuel.

Large Hydro

Potential energy of water is used to produce electricity. Water from river or lake is stored in a reservoir at a sufficient height or head to get a considerable amount of energy. Water head is created by constructing a dam across the river or lake. This type of power mainly depends on rain fall of catchment area. Also the dam is to be built in narrow canyon so that capital cost and gestation period are reduced and it is economical. In view of this, hydropower plants are generally constructed in hilly areas where there is high head and narrow canyon between two hills. Hydro potential of India is around 89,000 MW at 60% load factor which is equivalent to installed capacity of 1,48,700 MW. At present we are using about 25% of hydropower in our country.

The main parts of a hydroelectric power plant are a dam to store water in a reservoir, a penstock for delivering water, electric generators, a valve house which contains the main sluice valve, automatic isolating valves and control equipments A surge tank is located just before the valve house to protect the penstock from pressure surge, called water hammering, in case the turbine gates are suddenly closed.

When gates of the dam are opened, water from reservoir flows down through. large pipes called penstock where potential energy of water is converted into kinetic energy. This water is led to the water turbine through penstock. Penstock is a big pipe made of concrete or steel and its other end opens at a spacing at the bottom called spiral casing where guide vanes are mounted to control flow of water from penstock to turbine. Flow of water can be regulated by changing the angle of opening of guide vanes. Water is impinged on runner blades of hydraulic turbine after guide vanes are opened at spiral casing. The turbine starts to rotate and converts kinetic energy of water into mechanical energy. The turbine is coupled with alternator which converts mechanical energy into electrical energy. After doing useful work, water at the runner outlet of turbine is collected in draft tube and discharged to tail race of river at the downstream side of the dam. Generation of electric power is controlled in control room. Control of turbine and its auxiliaries are facilitated at turbine floor.

Flowing water of river carries grass, debris etc., along with it. This will reduce the passage of water in penstock and decrease the pressure in it. Reduction of pressure in penstock effect generation drastically. Trash racks are placed in water inlet at forebay which filter out grass, debris, etc., present in water. Also trash racks are choked many times. In this case, diving teams are called upon to clean these racks. The most significant operating characteristics of hydropower plant are quick starting and loading, long life and low operating and maintenance cost. Hydraulic turbine operates at low pressure and low speed. Therefore generators are usually salient type rotor with large no of poles. To maintain the generator voltage at constant frequency, turbine must rotate the generator at constant speed given by the formula

n = 120 f/P

where f is frequency and P is number of poles of generator. A number of control schemes are used to regulate the flow of water in order to make the turbine speed constant. 

Hydropower

The potential energy (PE) of water in the reservoir is proportional to the mass (m) of water and difference in height between the water impoundment and water outflow. This height difference is called head (h).

mass of water m = volume × density

PE = mgh = volume × pgh 

Power is rate of energy, therefore available hydropower is given by

P= Energy/time = volume/t × pgh

   =Q ρgh, watt

where,

Q = rate of flow of water in m³/s 

h = head in m

ρ = density of water = 1000kg/m³ (approximately)

g= acceleration due to gravity = 9.81 m/s² 

P= 9.81 Qh x 1000 watt = 9.81 Qh, kW 

If n is overall efficiency of hydropower plant, the electrical power output is

P= 9.81 Qhη, kW

where, 

η = η_P . η_t . η_g

η_P = penstock efficiency

η_t = turbine efficiency 11 generator efficiency

η_g = generator efficiency

Design of penstock depends on head. From the expression of power of hydro-plant, it is evident that the length of penstock is large and its diameter is less if the head is high. The Pelton wheel turbine is an impulse hydraulic turbine and is normally used for high head hydro-plant. Penstock of low head plant has large diameter and less length as it requires more quantity of water to be discharged for same power. A Kaplan turbine is a propeller or axial type hydraulic turbine and is used for low head plant. The Francis turbine is a radial hydraulic turbine commonly used for medium head plant. In general, efficiency of hydraulic turbine lies in between 80 to 94 percent during normal operation and generator efficiency is from 95 to 99 percent.

Generation in hydro-plant depends on storage of water. During rainy season there is sufficient water in reservoir. Power is generated through out the day and plants are operated as base load plant. When there is less water in dam, particularly during low flow period, plants are operated in peak hours as peak load plants. Pumped storage plant also supply peak load. Large hydro plant is a multi-utility project. Besides producing power, it also helps in flood control, irrigation and supply of drinking water.

Nuclear Fuel

Heavier atoms like Uranium and Thorium are unstable and they produce huge amount of heat energy by nuclear fission process. The heat enrgy liberated by complete fission of one kg of Uranium is equivalent to heat energy produced by burning 4,500 tonnes of good quality coal. Uranium reserves in world is small at present.

Nuclear power is the fourth largest source of electricity in India after thermal, hydroelectric and renewable sources. By the year 2015, India has 21 nuclear reactors in operation in six nuclear power plants, generating 5,780 MW. India is involved in the development of nuclear fusion reactor through its participation in ITER project. India has small reserve of uranium. Available reserve supply fuel for 10000 MW. The cost of nuclear energy is about 4 per kWH as compared to solar energy at 20 per kWH. About 2.25% of energy produced in India is obtained from nuclear power plants.

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Energy Resources

Energy resources play very important role in our life. They are vital for all development activities (for example, cooking, heating, cooling, lighting, travelling, etc.). Also they are required in maintaining present living standards as well as developing further in all spheres of human life. Therefore, there is a great need to study the energy resources.

Humans first obtained energy from biomass (wood and cow dung), then coal and finally oil and natural gas. These fossil fuels (coal, oil and natural gas) were formed roughly 300 million years ago in the carboniferous period. Carboniferous period was part of Paleozoic Era i.e., a period even before dinosaurs lived on earth in Jurassic Period. At the time of carboniferous period the land was covered with huge trees, ferns and other large leafy plants. The trees and plants were buried in the oceans and pressed down by dirt and rocks over a long period. Gradually the dead trees and plants which were buried under the pressure, transformed into peat and then to coal. Likewise sea animals buried under enormous water pressure and remained there for millions of years. Then they were transformed into oil and natural gas (petroleum).

In addition to these conventional/fossil fuels (coal, oil and natural gas) nuclear and hydro-power are also used for generating electricity. Large Hydro Power Plants are being used for over a hundred years.

They use energy of moving water, which is a renewable source of energy as the water is not consumed up but replenished or filled up again and again. Though the large hydro power plants are renewable energy sources they come under the category of conventional energy sources. It is so because they are being used to produce electrical energy commercially for over a hundred years. It will be appreciated later that Small Hydro Power (SHP) Plants are classified as non-conventional energy source.

Thus, we can recognise the fact that thermal, large hydro and nuclear are the base of conventional energy. However, the main drawbacks of the fossil-fuel based thermal power plants are limited supply of fuels and environmental pollution. Global Warming is caused by greenhouse gas (GHG) emissions from fossil fuel based generating systems. Main drawbacks of large hydro power plants are long gestation periods, submergence of large area along with vegetation, shifting of people and other environmental effects. Main problem in nuclear power plants is danger of nuclear radiations causing health hazards.

You are aware that our standards of living are rising rapidly. As more and more people now have many electrical gadgets. Many people who were not having electricity connections in their homes are now connected with electric supply. Though some of them may not be having continuous regular supply of electricity for 24 hours a day (i.e., 24 × 7 regular supply).

It is hoped that the quality and quantity of electric supply will improve greatly. The demand for electricity will also increase as the number of electrical gadgets used by a common household is ever-increasing.

To overcome the problem of conventional energy sources and to meet huge demand of energy in future, most countries including India have focused to develop energy from renewable sources. Renewable energy sources are also called non-conventional sources of energy. These sources are continuously regenerated (renewed or replenished) by natural processes. Solar energy, wind energy, biomass energy, tidal energy, Ocean Thermal Energy Conversion (OTEC) are various forms of renewable energy resources. Various other sources of renewable energy are also available substantially. 

It may be stated that the future of solar, wind, biomass, small hydropower, tidal and other renewable energy sources is bright and these will play an important role in the world energy scenario.

BASICS OF ENERGY AND POWER

Energy is the ability to do work. In other words, each and every work needs a fixed amount of energy. The rate of energy is called power. With more power, the same work can be completed in a lesser time and with less power the same work will take more time to complete. Energy, power and time are related as given by following expression: 

Energy = Power × Time

Power is the rate of energy at which it is generated or consumed. So mathematically, it can be expressed as

p = e/t

where,

p = power, e = energy, and t = time

SI unit of energy, power and time are joule, watt and second respectively.

Main Energy Types

Law of conservation of energy states that energy can neither be created nor be destroyed. Only the one form of energy can be converted to other forms of energy. The main forms of energy are:

1. Mechanical energy

2. Heat energy

3. Electrical energy

These forms of energy can be easily converted from one form to another.

Units of Different forms of Energy and their Relation

1. Mechanical Energy : Energy due to motion (kinetic energy) or inherent position of a body (potential energy) is called mechanical energy. The unit of mechanical energy is joule. One joule is defined as amount of energy required if power of one watt is used for one second.

2. Heat Energy: Internal energy in substance due to vibration and movement of atoms and molecules within the substance is called heat or thermal energy. Heat is considered as low quality energy because it is dispersed. The unit of heat energy is calorie. Amount of heat energy required to raise the temperature of one gram of water by 1°C is one calorie. 

1 kilocalorie (kcal) = 1000cnlorie .

Nowadays joule is used as unit of heat energy and it is internationally accepted.

1cal = 4.187joule (joule 's constant) 1kilojoule = 1000 joules

3. Electrical Energy: Energy due to flow of electric current is called electrical energy. Electrical energy is a high quality form of energy because it can be transported by conductive wires, can be stored in batteries and is used to make other forms of energy.

Unit of Electrical Energy: You must be familiar with the unit of electrical energy as one kilowatt-hour (1 kWh), because you pay your electricity bill by the units (kilowatt-hours) you consume over a billing period of one month or two months.

1 kilowatt-hour (kWh) = 1000 watt-hours (Wh) 

1 watt-hour (Wh) = 1 watt × 3600 seconds = 3600 joules (J)

1 kilowatt-hour (kWh) = 3, 600, 000 joules (J) = 3600 kilojoules (kJ)

Relation

1 joule (J) = 1 watt-second 

or

1watt = 1 joule/second

1 kilo joule (kJ) = 1000 watt-seconds 

1kWh = 36, 00, 000 joules

            = 36,00,000/4.187 calorie

            = 860 kilocalories (kcal)

1 kWh = 3600 kJ = 860 kcal

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