Power Generation: Types, Renewable Sources, and Power Stations, Essays (university) of Energy Efficiency

Download Power Generation: Types, Renewable Sources, and Power Stations and more Essays (university) Energy Efficiency in PDF only on Docsity! 1 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 1. The Sun 2. The Wind 3. The Water Head 4. The Fuel 5. 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. 2 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 5 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. 6 Fig. 2: Pumped storage water plant What is renewable energy? Broadly speaking, the world's energy resources (all the energy we have available to use) fall into two types called fossil fuels and renewable energy:  Fossil fuels are things like oil, gas, coal, and peat, formed over hundreds of millions of years when plants and sea creatures rot away, fossilize, and get buried under the ground, then squeezed and cooked by Earth's inner pressure and heat. Fossil fuels supply about 80–90 percent of the world's energy.  Renewable energy means energy made from the wind, ocean waves, solar power, biomass (plants grown especially for energy), and so on. It's called renewable because, in theory, it will never run out. Renewable sources currently supply about 10–20 percent of the world's energy. 7 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. 10 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 1. Cost of the land: The cost of the land which is selected for the installation should be minimum or economical. 2. 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. 3. 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. 4. Availability of fuel: The availability of required fuel (coal) should be there because without fuel the plant will not work. 5. 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. 6. Scope for the future demand: The size of the land should be such that it is capable for the handling of future power demand. 7. 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 8. 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. 11 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 1. Coal Storage: It is the place where coal is stored which can be utilised when required. 2. 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. 3. 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. 4. 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. 5. 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. 6. Turbine: It is the mechanical device which converts the kinetic energy of the steam to the mechanical energy. 7. Generator: It is coupled with the turbine rotor and converts the mechanical energy of the turbine to the electrical energy. 8. Ash Storage: It is used to store the ash after burning of the coal. 9. Dust Collector: It collects the dust particle from the burnt gases before it is released to the chimney. 12 10. Condenser: It condensate the steam that leaves out turbine. It converts the low pressure steam to water. It is attached to the cooling tower. 11. Cooling Tower: It is a tower which contains cold water. Cold water is circulates to the condenser for the cooling of the residual steam from the turbine. 12. Chimney: It is used to release the hot burnt gases or smoke from the furnace to the environment at appropriate height. The height of the tower is very high such that it can easily throw the smoke and exhaust gases at the appropriate height. And it cannot affect the population living near the steam power plant. 13. Feed Water pump: It is used to transport the feed water to the boiler. Working steps of a steam power plant For better understanding, the step by step working of steam power plant is as follows  The pulverised coal is fed into the boiler where the pulverised coal is burnt into the furnace  Due to heat from the furnace, the water present in the boiler drum changes to the high pressure steam.  From the boiler this high pressure steam is passed to the super heater where it is again heated up to its dryness.  After that this super heated steam strikes the turbine blades with a high speed and the turbine blades starts rotating to at high speed. Here the stored potential energy of the steam is gets converted into mechanical energy.  A generator is coupled with the turbine rotor. As the turbine rotates, the generator also rotate with same speed and mechanical energy of the turbine gets converted into electrical energy.  Steam after hitting the turbines blades lost its most of the energy and leaves the turbine with low pressure steam.  This low pressure steam enters into the condenser. Cold water circulates in the condenser from the cooling tower. Here the low pressure wet steam is converted into water.  After that condensed water with the feed water passed to the economiser where it gets heated up by the economiser. And finally the feed water enters into the boiler by a feed water pump to repeat the cycle.  The burnt flue gases from the furnace passes through the super heater, economiser and air pre- heater. This heat of the flue gases is used to heat the steam in the super heater to its dryness, to heat feed water in the economiser before entering into the boiler and to heat air form the atmosphere in the air pre-heater before it enters into the furnace.  The ash from the furnace is transported to ash handling plant and finally to the ash storage. Efficiency of steam power plant  The power plant that operates on coal constitutes almost 41% of the world’s electricity generation.  It is the modified Rankine thermodynamic cycle on which the coal fired power plant operates.  The overall efficiency of the coal power plant ranges from 32% to 42%. And this is calculated by the superheat pressures, and super heat and reheat temperatures of steam.  Most of the large power plants that operates at steam pressures of 170 bar,570 ℃ superheat and 570 ℃ reheat steam temperatures can achieve the efficiency ranges from 35% to 38%. 15 Generator: A generator is mounted in the power house and it is mechanically coupled to the turbine shaft. When the turbine blades are rotated, it drives the generator and electricity is generated which is then stepped up with the help of a transformer for the transmission purpose. Surge Tank: Surge tanks are usually provided in high or medium head power plants when considerably long penstock is required. A surge tank is a small reservoir or tank which is open at the top. It is fitted between the reservoir and the power house. The water level in the surge tank rises or falls to reduce the pressure swings in the penstock. When there is sudden reduction in load on the turbine, the governor closes the gates of the turbine to reduce the water flow. This causes pressure to increase abnormally in the penstock. This is prevented by using a surge tank, in which the water level rises to reduce the pressure. On the other hand, the surge tank provides excess water needed when the gates are suddenly opened to meet the increased load demand. Fig. 5: illustration of a surge tank Types of Hydro-Power Plants Conventional Plants: Conventional plants use potential energy from dammed water. The energy extracted depends on the volume and head of the water. The difference between height of water level in the reservoir and the water outflow level is called as water head. Pumped Storage Plant: In pumped storage plant, a second reservoir is constructed near the water outflow from the turbine. When the demand of electricity is low, the water from lower reservoir is pumped into the upper (main) reservoir. This is to ensure sufficient amount of water available in the main reservoir to fulfil the peak loads. 16 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 (U235) or Thorium (Th232) 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. 17 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 1. 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