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Principle of Power Generation & Power
Stations: Basic Structure and Functionality
Primary Sources of Electrical Energy
We can produce electrical energy by converting different energies available in nature. So we should look into various natural sources of energy which we use to produce electricity. Some common sources of energy are
- The Sun
- The Wind
- The Water Head
- The Fuel
- The Nuclear Energy Out of five above listed sources of energy we do not use the first two in very large scale. This is because there are some limitations. But in present days water head, fuel and nuclear energy are three most primarily used natural energy sources for producing electricity. We call these three sources collectively as conventional sources of energy. Energy of Sun The Sun is the basic source of energy. The sun is the source of both heat and light. We can use both heat and light for producing electricity. Energy of Sun Heat We focus sun rays at a small area with the help of concave mirror. The heat of the concentrated sun rays to heat up the water in boiler. The steam produced in boiler rotates a turbine. The turbine rotates an alternator to generate electricity. Although production cost of electricity is quite low here because no fuel like coal or diesel is required. However, production of electricity is not very popular. Because the area required for establishing this power plant is quite large even for smaller electrical energy generation. This is also because of unavailability of sun light in night and in cloudy weather. Also sun light varies time to time during a day. Over all the technique of producing electricity is not at all economical. Energy of Sun Light We can use sun light directly to produce electricity. This is done by photovoltaic cell. Here, sunlight directly strikes on the surface of voltaic cell. The photovoltaic cells are basically semiconductor p n junction cells. The potential difference or voltage appears across the junction of cell due to the incident sunlight. This potential difference or voltage creates electricity in the circuit connected to the solar panel system. Solar panels for producing electricity are becoming popular now days because of limitations of other conventional resources.
Energy of Wind We can use wind power to generate electricity. Where sufficient wind is available for long periods of time, we can construct efficient wind power mill to produce electricity. Here the wind mill rotates an electrical generator. As the speed of wind is not fixed we should not use the electrical energy produced by wind mill directly to the load. Instead we charge a battery connected to the system. We feed the output of the battery to load through an inverter. The main advantage of the system is that it has very low running cost, because of zero fuel cost and negligible maintenance cost. The main disadvantages of the system are variable output, unreliable because of variable wind pressure throughout a day as well as throughout a year and production rate of electricity is also quite low compared to conventional sources of energy. Energy of Water Head When we obstruct the natural flow of water from upstream to downstream in a river by constructing a dam across the river, a head is created in this water. When we allow to flow this stored water in a controlled way through the dam the potential energy stored in this high headed water gets released in the form of kinetic energy. This kinetic energy rotates a water turbine. An alternator coupled with the shaft of the turbine generates electrical energy. The power plants which use water head to produce electricity are referred as hydroelectric power plant. Water head is the most acceptable source of electricity because it is clean, it does not cause pollution in atmosphere, it is simple in construction, it is robust and demands very less maintenance. In addition to these reasons, the dam helps irrigation in the localities and controls flood. But the construction of dam needs huge monetary investment and complex engineering. Another drawback of the system is that we can not construct a hydroelectric plant at load centre, we only can construct it in downstream river which may be far away from the load centre. Energy of Fuel Till date the fuels are the main source of electricity. We can use three types of fuel for the purpose. Solid fuel such as coal, liquid fuel such as diesel and gaseous fuel such as natural gas. Whatever may be the form of fuel that is either solid, or liquid or gaseous the basic principles is same in this system. Here heat generated due to combustion of fuel in the furnace creates steam by boiling water in a vessel called boiler. This steam is then allowed to expand through nozzles in a turbine. This creates kinetic energy on the turbine blades which turns the turbine shaft. The alternator coupled with the shaft of the turbine generates electrical energy. We refer this system of producing electricity as thermal power generating plant. Although till date fuel is the main source of electricity generation but it has a limitation of availability in nature and it is true that the availability is diminishing day by day. Energy of Nuclear Nuclear fission releases a huge quantity of energy. This energy is used to produce steam which rotates a turbine coupled with an alternator. The alternator produces electrical power. In nuclear reaction the requirement of radio active material quite small for producing a large quantity of
- Fuel : The energy that finds its way into your TV,computer, or toaster starts off as fuel loaded into a power plant. Some power plants run on coal, while others use oil, natural gas, or methane gas from decomposing rubbish.
- Furnace : The fuel is burned in a giant furnace to release heat energy.
- Boiler : In the boiler, heat from the furnace flows around pipes full of cold water. The heat boils the water and turns it into steam.
- Turbine : The steam flows at high-pressure around a wheel that's a bit like a windmill made of tightly packed metal blades. The blades start turning as the steam flows past. Known as a steam turbine, this device is designed to convert the steam's energy into kinetic energy (the energy of something moving). For the turbine to work efficiently, heat must enter it at a really high temperature and pressure and leave at as low a temperature and pressure as possible.
- Cooling tower : The giant, jug-shaped cooling towers you see at old power plants make the turbine more efficient. Boiling hot water from the steam turbine is cooled in a heat exchanger called a condenser. Then it's sprayed into the giant cooling towers and pumped back for reuse. Most of the water condenses on the walls of the towers and drips back down again. Only a small amount of the water used escapes as steam from the towers themselves, but huge amounts of heat and energy are lost.
- Generator : The turbine is linked by an axle to a generator, so the generator spins around with the turbine blades. As it spins, the generator uses the kinetic energy from the turbine to make electricity.
- Electricity cables : The electricity travels out of the generator to a transformer nearby.
- Step-up transformer : Electricity loses some of its energy as it travels down wire cables, but high-voltage electricity loses less energy than low-voltage electricity. So the electricity generated in the plant is stepped-up (boosted) to a very high voltage as it leaves the power plant.
- Pylons : Hugh metal towers carry electricity at extremely high voltages, along overhead cables, to wherever it is needed.
- Step-down transformer : Once the electricity reaches its destination, another transformer converts the electricity back to a lower voltage safe for homes to use.
- Homes : Electricity flows into homes through underground cables.
- Appliances : Electricity flows all round your home to outlets on the wall. When you plug in a television or other appliance, it could be making a very indirect connection to a piece of coal hundreds of miles away!
Types of power plants
Steam turbine Most traditional power plants make energy by burning fuel to release heat. For that reason, they're called thermal (heat-based) power plants. Coal and oil plants work much as I've shown in the artwork above, burning fuel with oxygen to release heat energy, which boils water and drives a steam turbine. This basic design is sometimes called a simple cycle. Gas turbine Natural gas plants work in a slightly different way that's quite similar to how a jet engine works. Instead of making steam, they burn a steady stream of gas and use that to drive a slightly different design of turbine (called a gas turbine ) instead.
Nuclear Nuclear power plants work in a similar way to simple cycle coal or oil plants but, instead of burning fuel, they smash atoms apart to release heat energy. This is used to boil water, generate steam, and power a steam turbine and generator in the usual way. Hydro While all these types of power plants are essentially thermal (generating and releasing heat to drive a steam or gas turbine), two other very common types don't use any heat whatsoever. Hydroelectric and pumped storage plants are designed to funnel vast amounts of water past enormous water turbines (think of them as very efficient water wheels), which drive generators directly. In a hydroelectric plant , a river is made to back up behind a huge concrete dam. The water can escape through a relatively small opening in the dam called a penstock and, as it does so, it makes one or more turbines spin around. For as long as the river flows, the turbines spin and the dam generates hydroelectric power. Although they produce no pollution or emissions, hydroelectric stations are very damaging in other ways: they degrade rivers by blocking their flow and they flood huge areas, forcing many people from their homes (the Three Gorges Dam in China displaced an estimated 1.2 million people). Pumped storage A pumped hydroelectric storage plant is a variation on a traditional hydropower plant that operates with two reservoirs: a lower and an upper one. Such a plant utilizes gravity to "store" electricity in the form of potential energy. In generating mode, water flows in traditional fashion from the upper reservoir to the lower, driving turbines and generating electricity. When there is a surplus of electricity in the grid, for example, when demand is low or wind/solar are producing more than needed, electricity is drawn from the grid and used to pump water from the lower reservoir back up to the upper reservoir. (See Figure 2 ) Thus, the system acts like a giant battery to store electricity until needed. A pumped hydroelectric storage plant typically uses reversible pump/turbines that can either generate electricity or pump water. And, although such equipment can be very efficient, the plant is still a net user of electricity. I.e., it takes more electricity to pump water from the lower to upper reservoir than is generated by the same amount of water flowing from the upper to lower reservoir. Some estimates put the energy loss at 15-30%; however, this is good for a storage system and comparable to battery storage. And, with the ability to store power for use when needed and take advantage of electricity price differentials between peak and off-peak hourse, these plants can be very cost effective. Essentially, electricity is generated when demand is high for peak price, and electricity is stored when demand is low for reduced price. Managed properly, such price differentials will more than offset losses in efficiency. Figure 2 illustrates electrical generation and demand by such a plant. Pumped storage plants are classified as either "closed loop" or "open loop". A closed loop system is one in which both reservoirs are independent of any free flowing water source. Open loop systems have one or both reservoirs associated with a free flowing water source. Although virtually all pumped storage plants in the US are of the open loop variety, the closed loop systems are currently preferred for their reduced environmental impact.
What's the difference between fossil fuels and renewable energy?
In theory, fossil fuels exist in limited quantities and renewable energy is limitless. That's not quite the whole story, however. The good news is that fossil fuels are constantly being formed. New oil is being made from old plants and dead creatures every single day. But the bad news is that we're using fossil fuels much faster than they're being created. It took something like 400 million years to form a planet's worth of fossil fuels. But humankind will use something like 80 percent of Earth's entire fossil fuel supplies in only the 60 years spanning from 1960 to 2020. When we say fossil fuels such as oil will "run out," what we actually mean is that demand will outstrip supply to the point where oil will become much more expensive to use than alternative, renewable fuel sources. Just as fossil fuel supplies aren't exactly finite, neither is renewable energy completely infinite. One way or another, virtually all forms of renewable energy ultimately come from the Sun and that massive energy source will, one day, burn itself out. Fortunately, that won't happen for a few billion years so it's reasonable enough to talk of renewable energy as being unlimited.
What are the different types of renewable energy?
Almost every source of energy that isn't a fossil fuel is a form of renewable energy. Here are the main types of renewable energy: Solar power For as long as the Sun blazes (roughly another 4–5 billion years), we'll be able to tap the light and heat it shines in our direction. We can use solar power in two very different ways: electric and thermal. Solar electric power (sometimes called active solar power) means taking sunlight and converting it to electricity in solar cells (which work electronically). This technology is sometimes also referred to as photovoltaic ( photo = light and voltaic = electric, so photovoltaic simply means making electricity from light) or PV. Solar thermal power (sometimes called passive-solar energy or passive-solar gain) means absorbing the Sun's heat into solar hot water systems or using it to heat buildings with large glass windows. Wind power Depending on where you live, you've probably seen wind turbines appearing in the landscape in recent years. There are loads of them in the United States and Europe, for example. A turbine is any machine that removes kinetic energy from a moving fluid (liquid or gas) and converts it into another form. Windmills, based on this idea, have been widely used for many hundreds of years. In a modern wind turbine, a huge rotating blade (similar to an airplane propeller) spins around in the wind and turns an electricity generator mounted in the nacelle (metal casing) behind. It takes roughly several thousand wind turbines to make as much power as one large fossil fuel power plant. Wind power is actually a kind of solar energy, because the winds that whistle round Earth are made when the Sun heats different parts of our planet by different amounts, causing huge air movements over its surface.
Hydroelectric power Hydro means water, so hydroelectricity means making electricity using water—not from the water itself, but from the kinetic energy in a moving river or stream. Rivers start their lives in high ground and gradually flow downhill to the sea. By damming them, we can make huge lakes that drain slowly past water turbines, generating energy as they go. Water wheels used in medieval times to power mills were an early example of hydro power. You could describe them as hydromechanical , since the water power the milling machines used was transmitted by an elaborate systems of wheels and gears. Like wind power, hydroelectric power is (indirectly) another kind of solar energy, because it's the Sun's energy that drives the water cycle , endlessly exchanging water between the oceans and rivers on Earth's surface and the atmosphere up above. Ocean power The oceans have vast, untapped potential that we can use in three main ways: wave power, tidal barrages, and thermal power. Wave power uses mechanical devices that rock back and forth or bob up and down to extract the kinetic energy from moving waves and turn it into electricity. Surfers have known all about wave power for many decades! Tidal barrages are small dams built across estuaries (the points on the coast where rivers flow into the sea and vice versa). As tides move back and forth, they push huge amounts of water in and out of estuaries at least twice a day. A barrage with turbines built into it can capture the energy of tidal water as it flows back and forth. The world's best-known tidal barrage is at La Rance in France; numerous plans to build a much bigger barrage across the Severn Estuary in England have been outlined, on and off, for almost a century. Thermal power involves harnessing the temperature difference between warm water at the surface of the oceans and cold water deeper down. In a type of thermal power called Ocean thermal energy conversion (OTEC), warmer surface water flows into the top of a giant column (perhaps 450m or 1500ft tall), mounted vertically some miles out to sea, while cooler water flows into the bottom. The hot water drives a turbine and makes electricity, before being cooled down and recycled. It's estimated that there is enough thermal energy in the oceans to supply humankind's entire needs, though little of it is recovered at the moment. Biomass Biomass is the name given to any crop grown for the purpose of making energy. Biofuels are one example. Other examples include burning animal waste in a furnace to generate electricity. Biofuels are controversial because they often take up land that could be used to grow food, but they are generally a cleaner and more efficient way of making power than using fossil fuels. Because plants absorb carbon dioxide while they're growing and give it out when they're burned, biomass can provide energy without adding to the problem of global warming. Geothermal energy Earth may feel like a pretty cold place at times but, inside, it's a bubbling soup of molten rock. Earth's lower mantle, for example, is at temperatures of around 4500°C (8000°F). It's relatively
Fig. 3 Site Selection of a steam turbine The site selection of steam power plant depends upon various factors. Let’s discuss about these factors one by one
- Cost of the land: The cost of the land which is selected for the installation should be minimum or economical.
- Population density of the land: The distance of the steam power plant from the public area should be at appropriate distance. So that in case of any failure or hazard happen in the plant, the population of the area near to the power plant should not be affected.
- Availability of water sources: There should be a plenty of water sources in the selected area. Since the power plant requires a large amount of water for the generation of steam.
- Availability of fuel: The availability of required fuel (coal) should be there because without fuel the plant will not work.
- Type of land: The land which is selected for the power plant installation should be plain enough and it is suitable for the strong foundation for the various machinery of the plant.
- Scope for the future demand: The size of the land should be such that it is capable for the handling of future power demand.
- Availability of Ash handling facility: Proper ash handling facility should be available near the power plant to minimise the adverse effect of the ash produced in the steam power plant
- Availability of transportation facility: The transportation facility is must in the installation for the power plant, because any material cannot be transported to the power plant form its required location in lack of transport. There should be easy availability of proper transportation facility at the selected site.
Construction or Layout of Steam Power Plant
There are so many components present in the steam power plant which performs their specialized function for the efficient working. The various component of the steam power plant are: Fig 4: Component of a thermal power plant
- Coal Storage: It is the place where coal is stored which can be utilised when required.
- Coal Handling: Here the coal is converted into the pulverised form before feeding to the furnace. A proper system is designed to transport the pulverised coal to the boiler furnace.
- Boiler: It converts the water into high pressure steam. It contains the furnace inside or outside the boiler shell. The combustion of coal takes place in the furnace.
- Air-preheater: It is used to pre-heat the air before entering into the boiler furnace. The pre heating of air helps in the burning of fuel to a greater extent. It takes the heat from the burnt gases from the furnace to heat the air from the atmosphere.
- Economiser: As its name indicates it economises the working of the boiler. It heats the feed water to a specified temperature before it enters into the boiler drum. It takes the heat from the burnt gases from the furnace to do so.
- Turbine: It is the mechanical device which converts the kinetic energy of the steam to the mechanical energy.
- Generator: It is coupled with the turbine rotor and converts the mechanical energy of the turbine to the electrical energy.
- Ash Storage: It is used to store the ash after burning of the coal.
- Dust Collector: It collects the dust particle from the burnt gases before it is released to the chimney.
The efficiency of the super critical power plants operating at 220 bar steam pressure, 600/ ℃ superheat/reheat temperatures can achieve is 42%. The efficiency in the range of 45 to 48% can be achieved by the ultra-super critical power plants operating at 300 bar pressure, 600/600 ℃ superheat/reheat steam temperatures.
Advantages and Disadvantages
The advantages and disadvantages of steam power plant are as follows: Advantages As compared with power generating plant, it has low initial cost and hence economical. Less land area is required as compared with the hydro power plant. Coal is used as fuel and the cost of coal is cheaper than petrol and diesel fuel. So the power generation cost is economical. This power plant has easy maintenance cost. Steam power plant can be installed in any area where water sources and transportation facility are easily available. Disadvantages The running cost of steam power plant is comparatively high because of fuel, maintenance etc If we talk about the overall efficiency of steam power plant, than is about 35 % to 41% which is low. Due to the release of burnt gases of the coal or fuel, it contributes to the global warming to a larger extent. The heated water that is thrown in the rivers, ponds etc puts and adverse effect on the living organism of water and disturbs the ecology.
Case Study 2 : Hydro Power Plant: layout, Working, and Types.
Generation of electricity by hydropower (potential energy in stored water) is one of the cleanest methods of producing electric power. In 2012, hydroelectric power plants contributed about 16% of total electricity generation of the world. Hydroelectricity is the most widely used form of renewable energy. It is a flexible source of electricity and also the cost of electricity generation is relatively low. This section talks about the layout, basic components and working of a hydroelectric power station.
Layout and Working of Hydro Power Plant
Fig. 5: Hydro power plant The above image shows the typical layout of a hydroelectric power plant and its basic components. Dam and Reservoir: The dam is constructed on a large river in hilly areas to ensure sufficient water storage at height. The dam forms a large reservoir behind it. The height of water level (called as water head) in the reservoir determines how much of potential energy is stored in it. Control Gate: Water from the reservoir is allowed to flow through the penstock to the turbine. The amount of water which is to be released in the penstock can be controlled by a control gate. When the control gate is fully opened, maximum amount of water is released through the penstock. Penstock: A penstock is a huge steel pipe which carries water from the reservoir to the turbine. Potential energy of the water is converted into kinetic energy as it flows down through the penstock due to gravity. Water Turbine: Water from the penstock is taken into the water turbine. The turbine is mechanically coupled to an electric generator. Kinetic energy of the water drives the turbine and consequently the generator gets driven. There are two main types of water turbine; (i) Impulse turbine and (ii) Reaction turbine. Impulse turbines are used for large heads and reaction turbines are used for low and medium heads.
Run-Of-River Plant: In this type of facility, no dam is constructed and, hence, reservoir is absent. A portion of river is diverted through a penstock or canal to the turbine. Thus, only the water flowing from the river is available for the generation. And due to absence of reservoir, any oversupply of water is passed unused.
Advantages and Disadvantage of a Hydroelectric Power Plant
Advantages No fuel is required as potential energy is stored water is used for electricity generation Neat and clean source of energy Very small running charges - as water is available free of cost Comparatively less maintenance is required and has longer life Serves other purposes too, such as irrigation Disadvantages Very high capital cost due to construction of dam High cost of transmission – as hydro plants are located in hilly areas which are quite away from the consumers
Case Study 3 : Nuclear Power Plant layout, Working and Types
In a nuclear power plant , heat energy is generated by a nuclear reaction called as nuclear fission. Nuclear fission of heavy elements such as Uranium or Thorium is carried out in a special apparatus called as a nuclear reactor. A large amount of heat energy is generated due to nuclear fission. Rest parts of a nuclear power plant are very similar to conventional thermal power plants. It is found that fission of only 1 Kg of Uranium produces as much heat energy as that can be produced by 4,500 tons of high grade coal. This considerably reduces the transportation cost of fuel, which is a major advantage of nuclear power plants. Also, there are large deposits of nuclear fuels available all over the world and, hence, nuclear power plants can ensure continued supply of electrical energy for thousands of years. About 10% of the total electricity of the world is generated in nuclear power plants. How Does A Nuclear Power Plant Work? Heavy elements such as Uranium (U^235 ) or Thorium (Th^232 ) are subjected to nuclear fission reaction in a nuclear reactor. Due to fission, a large amount of heat energy is produced which is transferred to the reactor coolant. The coolant may be water, gas or a liquid metal. The heated coolant is made to flow through a heat exchanger where water is converted into high- temperature steam. The generated steam is then allowed to drive a steam turbine. The steam, after doing its work, is converted back into the water and recycled to the heat exchanger. The steam turbine is coupled to an alternator which generates electricity. The generated electrical voltage is then stepped up using a transformer for the purpose of long distance transmission. The image below shows basic components and layout of a nuclear power station.
Fig 6: Component of a nuclear power plant
Basic Components of a Nuclear Power Plant
Nuclear Reactor A nuclear reactor is a special apparatus used to perform nuclear fission. Since the nuclear fission is radioactive, the reactor is covered by a protective shield. Splitting up of nuclei of heavy atoms is called as nuclear fission, during which huge amount of energy is released. Nuclear fission is done by bombarding slow moving neutrons on the nuclei of heavy element. As the nuclei break up, it releases energy as well as more neutrons which further cause fission of neighboring atoms. Hence, it is a chain reaction and it must be controlled, otherwise it may result in explosion. A nuclear reactor consists of fuel rods, control rods and moderator. A fuel rod contains small round fuel pallets (uranium pallets). Control rods are of cadmium which absorb neutrons. They are inserted into reactor and can be moved in or out to control the reaction. The moderator can be graphite rods or the coolant itself. Moderator slows down the neutrons before they bombard on the fuel rods. Two types of nuclear reactors that are widely used
- Pressurised Water Reactor (PWR) This type of reactor uses regular water as coolant. The coolant (water) is kept at very high pressure so that it does not boil. The heated water is transferred through heat exchanger where water from secondary coolant loop is converted into steam. Thus the secondary loop is completely free from radioactive stuff. In a PWR, the coolant water itself acts as
