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Solar Energy | Photovoltaic

Friday, December 10, 2010 | | 6 comments

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Solar energy is the most promising clean energy source for new generations. Sun has been shinning for about 5 billion years (without malfunctions) and it will continue shinning for another 4 to 5 billion years.

Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar radiation, along with secondary solar powered resources such as wind and wave power, hydroelectricity and biomass, account for most of the available renewable energy on earth. Only a minuscule fraction of the available solar energy is used.

Solar power is the conversion of sunlight into electricity, either directly using photovoltaics (PV), or indirectly using concentrated solar power (CSP). CSP systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. PV converts light into electric current using the photoelectric effect.

Solar Cells


Solar cell used in Skylab
Solar Cell used in Skylab
Though it sounds unusual at first, it is a fact that solar cells represent a very important part of information technology revolution in the last few decades. Without solar cells there wouldn't be so many communication satellites, which found the information technology revolution. The consequences are fast development in the field of information technologies and computer sciences, Internet etc. Data and voice transfer possibilities, which are offered to you by modern communications techniques, were made possible by a small piece of silicon in the form of a solar cell.

First solar cells

A physical phenomenon allowing light-electricity conversion - photovoltaic effect, was discovered in 1839 by the French physicist Alexandre Edmond Becquerel. Experimenting with metal electrodes and electrolyte he discovered that conductance rises with illumination.

Willoughby Smith discovered photovoltaic effect in selenium in 1873. In 1876, with his student R. E. Day, William G. Adams discovered that illuminating a junction between selenium and platinum also has a photovoltaic effect. These two discoveries were a foundation for the first selenium solar cell construction, which was built in 1877. Charles Fritts first described them in detail in 1883.

In 1918, a Polish scientist Jan Czochralski discovered a method for monocrystalline silicon production, which enabled monocrystalline solar cells production. The first silicon monocrystalline solar cell was constructed in 1941.

The photovoltaic effect in other materials was observed in 1932 in cadmium-selenide (CdS). Nowadays, CdS belongs among important materials for solar cells production.

In 1963, Sharp Corporation developed the first usable photovoltaic module from silicon solar cells. The biggest photovoltaic system at the time, the 242 W module field was set up in Japan. A year later, in 1964, Americans applied a 470 W photovoltaic field in the Nimbus space project.

Photovoltaics (PV)

The term photovoltaics derives from the Greek word "phos" meaning light and the word "volt" (named by Alessandro Volta). Photovoltaics is a science, which examines light-electricity conversion, respectively, photon energy-electric current conversion. In other words it stands for light-current conversion. Both direct and diffuse solar radiation take part of the process. The light to current conversion takes place within solar cells, which can be amorphous, poly- crystalline or single-crystal, according to their structure.
  • Single-crystal cells are made in long cylinders and sliced into round or hexagonal wafers. While this process is energy-intensive and wasteful of materials, it produces the highest-efficiency solar cells—as high as 25 percent in some laboratory tests. Because these high-efficiency solar cells are more expensive, they are sometimes used in combination with concentrators such as mirrors or lenses. Concentrating systems can boost efficiency to almost 30 percent. Single-crystal accounts for 29 percent of the global market for PV.
  • Polycrystalline cells are made of molten silicon cast into ingots or drawn into sheets, then sliced into squares. While production costs are lower, the efficiency of the solar cells is lower too—around 15 percent. Because the solar cells are square, they can be packed more closely together. Polycrystalline solar cells make up 62 percent of the global PV market.
  • Amorphous silicon (a-Si) is a radically different approach. Silicon is essentially sprayed onto a glass or metal surface in thin films, making the whole module in one step. This approach is by far the least expensive, but it results in very low efficiencies—only about five percent.

Photovoltaic Cells

The basic PV or solar cell typically produces only a small amount of power. To produce more power, solar cells (about 40) can be interconnected to form panels or modules. PV modules range in output from 10 to 300 watts. If more power is needed, several modules can be installed on a building or at ground-level in a rack to form a PV array. About 10–20 PV arrays can provide enough power for a household.

Application Example


Most common application of solar cells applies to pocket calculators power supply, parking meters power supply and similar appliances. Solar-module consists of many solar cells, which are electrically connected and placed between glass and tedlar plate, and framed by an (usualy) aluminium frame. A number of solar-modules and other components (batteries, charge regulators, and inverters) can form large photovoltaic systems.

The first PV system applications developed were applied as an energy source for satellites and later for orbital stations in space. Nowadays, photovoltaic systems are applied as an energy source in many cases. Most commonly applied photovoltaic systems can be found in remote and rural areas where no public grid is available. However, quite often grid-connected systems are constructed in urban areas.

Photovoltaic systems are an excellent solution to electricity production regardless of your whereabouts - even at high latitudes of Himalayas or in Antarctica photovoltaic systems have been build. According to loads connected to the system and to the basic design principles, the following photovoltaic systems are used - direct coupled photovoltaic systems (systems without batteries - water pumping systems for example), standalone photovoltaic systems, grid-connected photovoltaic systems, hybrid systems (e.g. PV - wind or PV - diesel systems), concentrator photovoltaic systems. The applications below depict use of photovoltaic systems as an energy source in many interesting ways. 

Photovoltaic Power Plants
Photovoltaic power plants - Solar modules are nowadays parts of large standalone or grid-connected systems. Large photovoltaic power plants (MW range) have beeing constructed in Germany, Spain, USA, Italy, Netherlands etc. Worldwide more than 250 large PV power plants with peak power 1 MWp or more (each plant) are connected to the public grid(s).

Building Integrated Photovoltaics

Building integrated photovoltaics - Acronym of BIPV (Building Integrated Photovoltaics) refers to photovoltaic systems integrated with an object's building phase. It means that they are built/constructed along with an object. They are also planned together with the object. Yet, they could be built later on. Due to specific task cooperation of many different experts, such as architects, civil engineers and PV system designers, is necessary.

Photovoltaic Noise Barrier
Noise barriers - An efficient way of noise prevention by application of photovoltaic modules was first demonstrated in Switzerland in 1989. Later, the solution was applied also in some other European countries. Noise prevention is also a research subject of several projects conducted in European Union. Different photovoltaic noise barriers can be built considering motorway features, barrier construction, the height of the barrier, and other factor influence (environment etc.).



Photovoltaics: Design and Installation Manual
Photovoltaics: Design and Installation Manual

Practical Handbook of Photovoltaics: Fundamentals and Applications
Practical Handbook of Photovoltaics

Coal-Fired Steam Power Plant

Thursday, November 18, 2010 | | 3 comments

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The modern technology used for burning coal to generate electricity has evolved over a period of more than a century and until awareness grew of the environmental damage coal burning could produce, the coal-fired power station developed in a single direction. The basic principle underlying this type of power station is to burn coal in air and capture the heat released to raise steam for driving a steam turbine. The rotation of this steam turbine, in turn, drives a generator; the net result is electricity.

The traditional coal-fired power plant comprises two basic components. The first component is a furnace boiler designed to burn the coal and capture the heat energy released using a system of circulating water and steam. The second part of the system is a steam turbine generator which converts the heat energy captured by the steam into electrical energy. In other words, chemical energy held within the coal is first translated into heat energy and then into mechanical energy, and finally into electrical energy. Modern plants also include additional units to remove dust and acid emissions from the flue gases before they are released into the atmosphere.




 

Boiler

A power plant boiler is a device for converting the chemical energy in coal into heat energy and then transferring that heat energy to a fluid, steam. The efficiency of a coal-fired power plant increases as the pressure and temperature of the steam increases. This has led to a demand for higher temperatures and pressures as technology has developed and this has required, in turn, the development of materials with higher performance under increasingly stressful conditions. The most advanced boilers develop steam with a pressure of around 250bar and a temperature of 600°C.

Early boilers were made from iron, but as the demands on the system increased, special steels were used that could resist the conditions encountered in the power plant. These now dominate in modern boilers. Even so, oxygen dissolved in the water circulating within the boiler pipes can cause serious corrosion in steel at the elevated temperatures and pressures to which it is exposed, so the boiler water must be deoxygenated.

The first part of the boiler is a furnace in which combustion takes place. In the most common type of boiler, pulverised coal is injected with a stream of air into the furnace in a continuous process through a device known as a burner. The coal burns, producing primarily carbon dioxide while incombustible mineral material (ash) falls to the bottom of the furnace where it can be removed (some is also carried away by the hot combustion gases).

The heat generated during combustion (the temperature at the heart of the furnace may be as high as 1500°C) is partly radiant and partly convective, the latter carried off by the hot combustion gases. The radiant heat is collected at the walls of the furnace where water is circulated in pipes. Covective heat in the combustion gases is captured in bundles of tubes containing either water or steam which are placed in the path of the flue gas as it exits the furnace.

In a conventional boiler there is a drum positioned appropriately within the steam–water system containing both water and steam so that steam can develop as the temperature of the fluid rises. The most advanced designs, however, operate at such high temperatures and pressures that they do not pass through a stage in which water and steam co-exist. In these boilers the water turns directly to steam within the water tubes. This type of boiler exploits what is called a supercritical cycle, called so because the thermodynamic fluid (the water) enters what is known as the supercritical phase without passing through a condition in which both water and steam co-exist.

The boiler watertubes in the exhaust gas path are frequently divided into a number of different sections. (These sections have names, such as economiser, reheater or superheater.) The water or steam passes through them is a specific order determined by the design of the steam cycle. Traditional pulverised-coal boilers have been built with outputs of up to 1000MW.

Steam turbine

The steam turbine is the primary mechanical device in most conventional coal-fired power stations. Its job is to convert the heat energy contained in the steam exiting the boiler into mechanical energy, rotary motion. The steam turbine first appeared in power applications at the end of the nineteenth century. Before that steam power was derived from steam-driven piston engines.

The steam turbine is something of a cross between a hydropower turbine and a windmill. It, like them, is designed to extract energy from a moving fluid. The fluid is water, the same as the hydro turbine. In the case of a hydro turbine the water remains in the liquid phase and neither its volume nor its temperature changes during energy extraction. In the case of the steam turbine, energy extraction is from a gas, steam, rather than a liquid and involves both the pressure and the temperature of the fluid falling. This has a profound effect on the turbine design.

Both hydro and steam turbines exist in two broad types: there are impulse turbines which extract the energy from a fast-moving jet of fluid and reaction turbines which are designed to exploit the pressure of a fluid rather than its motion. A hydro turbine will be of one design or the other. In a steam turbine the two principles may be mixed in a single machine and they may even be mixed in a single turbine blade.


It is impossible to extract all the energy from steam using a turbine with a single set of turbine blades. Instead, a steam turbine utilises a series of sets of blades, called stages. Each stage is followed by a set of stationary blades (usually called nozzles) which control the steam flow to the next stage.

A single steam turbine stage consists of a set of narrow blades projecting from a central hub. (In concept, it is something like a steam windmill.) Ten or more sets of blades can be mounted on a single steam turbine shaft. This combination of shaft and blades is called a rotor. The turbine stages are separated by carefully designed stationary blades, or nozzles, which control the flow of steam from one set of rotating blades to the next. The precise shape of the blades in each set determines whether that set is impulse or reaction, or a cross between the two. The hub, blades and nozzles are enclosed in a close-fitting case to maintain the steam pressure.



In a steam turbine impulse stage, energy is extracted at constant pressure while the velocity of the steam falls as it flows across the blades. The steam is then expanded through a stationary control stage to increase its velocity again before energy is extracted from another set of impulse blades. In a steam turbine reaction stage, by contrast, both pressure and velocity of the steam fall as energy is extracted by the rotating blades.

Steam exiting the power plant boiler is at a high temperature and a high pressure. Both temperature and pressure fall as the steam passes through the turbine. The greater the temperature drop and the greater the pressure drop, the more energy can be captured from the steam. Consequently the most efficient power plants condense the steam back to water at the end of the turbine.

Even with a modern design it is impossible to capture all the energy from the steam efficiently with a single multiple-stage turbine. Coal-fired power plants use several. These are usually divided into high-, medium- and low-pressure turbines. The blades in these turbines get larger (longer) as the pressure drops; in fact, the low-pressure turbine may comprise several turbines operating in parallel to gain the most energy without making a single turbine impossibly large. All the turbines may be mounted on a single shaft, but it is common for the low-pressure turbines to be on a separate shaft rotating at a lower speed to reduce the forces exerted at the blade tips. Multiple turbines of this type can have aggregate outputs of over 1000MW.

As with boilers, the demands of modern power plant design have led to the development and introduction of high-performance materials that can cope with the extreme conditions encountered within a steam turbine. The high-pressure turbine blades have to be able to withstand extremes of both temperature and pressure and have to be able to resist the abrasive force of steam. At the low-pressure end of the turbine train the large size of the turbines means that the blade tip speeds are enormous, again requiring specially designed materials to withstand the centrifugal forces exerted on them.
A refinement which improves the overall efficiency is to return the steam to the boiler after it has passed through the high-pressure turbine, reheating it before delivering it to the medium-pressure turbine. Most modern steam turbine plants use this single reheat design (multiple reheat is also possible).
The theoretical maximum efficiency of a coal-fired power station is determined by the temperature difference between the steam entering the high-pressure turbine and the steam exiting the low-pressure turbine. The greater this temperature difference, the more energy can be extracted. With the most advanced technology, utilising the best boiler materials to achieve the highest-steam temperatures and pressures, a maximum efficiency of around 43–45% can be achieved. New supercritical designs may eventually push this as high as 55%. In the near future, however, the best that is likely to be achieved is something between 47% and 49%.

Generators

The turbine shaft, or shafts if there is more than one, are coupled to a generator which converts the rotary mechanical motion into the electrical energy that the plant is designed to provide. Generators, like steam turbines, first appeared during the nineteenth century. All utilise a coil of a conducting material, usually copper moving in a magnetic field to generate electricity.

The generators used in most power stations, including coal-fired power stations, are designed to deliver an alternating current (AC) to a power grid. An AC current is preferred because it allows the voltage to be raised or lowered easily using a transformer. For transmission of power over long distances it is preferable to use a very high voltage and a low current. The voltage is then reduced with a transformer before delivery to the consumer.

The need to generate an AC voltage determines the speed at which the generator rotates. This must be an exact multiple of the grid frequency (normally grids operate at either 50 or 60Hz). For grids operating at 50Hz the traditional generator speed is 50 cycles per second, or 3000rpm. The equivalent 60Hz machine rotates at 3600rpm. This speed, in turn, determines the operating speed of the steam turbine. Large low-pressure steam turbines may operate at half these speeds.


Generators may be as large as 2000MW, and large generators are normally built to suit a particular project. Modern generators operate with an efficiency of greater than 95%. The remaining 5% of the mechanical input energy from the turbine is usually lost as heat within the generator windings and magnetic components. Even though the percentage is small, this still represents an enormous amount of energy; perhaps 50MW in a 1000-MW machine. Hence generators require very efficient cooling systems in order to prevent them overheating. Avariety of cooling mediums are used, including hydrogen which is extremely efficient because of its low density and high specific heat.

The broad outline of generator design has changed little over a century. However new materials have improved efficiencies. The latest developments involve the use of superconducting materials to reduce energy and increase efficiencies.

Coal Fired Steam Power Plant Video




Coal-Fired Steam Power Plant Reference Books:



Steam Plant Operation
Steam Plant Operation
Steam Turbines: Theory and Design
Steam Turbines: Theory and Design

Steam Plant Calculations Manual, 2nd Edition (Dekker Mechanical Engineering, No. 87)
Steam Plant Calculations Manual,...
 
Power Plant Engineering
Power Plant Engineering
     

HYDROGEN FUEL CELL

Friday, June 4, 2010 | | 5 comments

There are many renewable energy sources in the world such as hydrogen, solar, wind, biomass, hydropower, and geothermal energy. Many people think that hydrogen will be the most important fuel of the future because it meets so many requirements of a good energy system. Experts agree the ideal energy system should include the characteristics listed below:
  • should rely on domestic energy sources.
  • should be able to utilize a variety of energy sources.
  • should produce few harmful pollutants and greenhouse gas emissions.
  • should be energy efficient (high energy output from the energy input).
  • should be accessible (easy to find, produce or harness).
  • should result in stable energy prices.

Hydrogen is a versatile energy carrier that can be used to power nearly every end-use energy need. The fuel cell — an energy conversion device that can efficiently capture and use the power of hydrogen — is the key to making it happen.
  • Stationary fuel cells can be used for backup power, power for remote locations, distributed power generation, and cogeneration (in which excess heat released during electricity generation is used for other applications).
  • Fuel cells can power almost any portable application that typically uses batteries, from hand-held devices to portable generators.
  • Fuel cells can also power our transportation, including personal vehicles, trucks, buses, and marine vessels, as well as provide auxiliary power to traditional transportation technologies. Hydrogen can play a particularly important role in the future by replacing the imported petroleum we currently use in our cars and trucks.

What Is A Fuel Cell

A fuel cell is a device that produces a chemical reaction between substances, generating an electric current in the process. It is an electrochemical energy conversion device. Everyone uses another electrochemical energy conversion device, i.e. a battery. A battery contains substances that produce an electric current as they react. When all of the substances have reacted, the battery is dead; it must be replaced or recharged.

With a fuel cell, the substances (in this case, hydrogen and oxygen) are stored outside of the device. As long as there is a supply of hydrogen and oxygen, the fuel cell can continue to generate an electric current, which can be used to power motors, lights, and other electrical appliances.

Why Fuel Cells?

Fuel cells directly convert the chemical energy in hydrogen to electricity, with pure water and potentially useful heat as the only byproducts.
  • Hydrogen-powered fuel cells are not only pollution-free, but also can have two to three times the efficiency of traditional combustion technologies.
  • A conventional combustion-based power plant typically generates electricity at efficiencies of 33 to 35 percent, while fuel cell systems can generate electricity at efficiencies up to 60 percent (and even higher with cogeneration).
  • The gasoline engine in a conventional car is less than 20% efficient in converting the chemical energy in gasoline into power that moves the vehicle, under normal driving conditions. Hydrogen fuel cell vehicles,which use electric motors, are much more energy efficient and use 40-60 percent of the fuel’s energy — corresponding to more than a 50% reduction in fuel consumption, compared to a conventional vehicle with a gasoline internal combustion engine. 
In addition, fuel cells operate quietly, have fewer moving parts, and are well suited to a variety of applications.

How Do Fuel Cells Work?


A single fuel cell consists of an electrolyte sandwiched between two electrodes, an anode and a cathode. Bipolar plates on either side of the cell help distribute gases and serve as current collectors.

There are many types of fuel cells, but the most important technology for transportation applications is the polymer electrolyte (or proton exchange) membrane or PEM cell. A PEM fuel cell converts hydrogen and oxygen into water, producing an electric current during the process.

In a Polymer Electrolyte Membrane (PEM) fuel cell, which is widely regarded as the most promising for light-duty transportation, hydrogen gas flows through channels to the anode, where a catalyst causes the hydrogen molecules to separate into protons and electrons.  The anode is the negative side of the fuel cell. The anode has channels to disperse the hydrogen gas over the surface of the catalyst. Hydrogen gas under pressure enters the fuel cell on the anode side and reacts with the catalyst.

The polymer electrolyte membrane (PEM) is a specially treated material that conducts positive ions (protons), but blocks electrons from flowing through the membrane.  While the protons are conducted through the membrane to the other side of the cell, the stream of negatively-charged electrons follows an external circuit to the cathode. This flow of electrons is electricity that can be used to do work, such as power a motor.

On the other side of the cell, oxygen gas, typically drawn from the outside air, flows through channels to the cathode.  The cathode is the positive side of the fuel cell. It has channels to distribute oxygen gas to the surface of the catalyst. The oxygen reacts with the catalyst and splits into two oxygen atoms. Each oxygen atom picks up two electrons from the external circuit to form an oxygen ion (that combines with two hydrogen ions (2H+) to form a water molecule (H2O). This union is an exothermic reaction, generating heat that can be used outside the fuel cell.

The power produced by a fuel cell depends on several factors, including the fuel cell type, size, temperature at which it operates, and pressure at which gases are supplied. A single fuel cell produces approximately 1 volt or less — barely enough electricity for even the smallest applications. To increase the amount of electricity generated, individual fuel cells are combined in series to form a stack. (The term “fuel cell” is often used to refer to the entire stack, as well as to the individual cell.) Depending on the application, a fuel cell stack may contain only a few or as many as hundreds of individual cells layered together. This “scalability” makes fuel cells ideal for a wide variety of applications, from laptop computers (50-100 Watts) to homes (1-5kW), vehicles (50-125 kW), and central power generation (1-200 MW or more).

What Is Hydrogen?


Hydrogen is the simplest element known to exist. An atom of hydrogen has oneproton and one electron. It is the lightest element and a gas at normaltemperature and pressure. Hydrogen is also the most abundant gas in the universe and the source of all the energy we receive from the sun. Hydrogen has the highest energy content of any common fuel by weight, but the lowest energy content by volume.

The sun is basically a giant ball of hydrogen and helium gases. In the sun.s core, the process of fusion is continually taking place. During fusion, the protons of four hydrogen atoms combine to form one helium atom with two protons and two neutrons, releasing energy as radiation.

This radiant energy is our most important energy source. It gives us light and heat and makes plants grow. It causes the wind to blow and the rain to fall. It is stored as chemical energy in fossil fuels. Most of the energy we use originally came from the sun.s radiant energy.

Hydrogen as a gas (H2), however, doesn’t exist naturally on earth. It is found only in compound form. Combined with oxygen, it is water (H2O). Combined with carbon, it forms organic compounds such as methane (CH4), coal, and petroleum. It is found in all growing things. biomass. Hydrogen is also an abundant element in the earth.s crust.


How Is Hydrogen Made?


Since hydrogen gas is not found naturally on earth, it must be manufactured. There are many ways to do this. The fact that hydrogen can be produced using so many different domestic resources is an important reason why it is a promising energy carrier. In a hydrogen economy, we will not need to rely on a single resource or technology to meet our energy needs.

Steam Reforming


Industry produces hydrogen by steam reforming, a process in which high-temperature steam separates hydrogen atoms from carbon atoms in methane (CH4), as shown below.

     CH + H2O (steam)  ---> 3H + CO
     CO + H2O (steam) ---> CO2 + H2



Today, most of the hydrogen produced by steam reforming isn.t used as fuel but in industrial processes. Steam reforming is the most cost-effective way to produce hydrogen today and accounts for about 95 percent of the hydrogen produced in the U.S. Because of its limited supply, however, we cannot rely on natural gas to provide hydrogen over the long term. Instead, we will need to produce hydrogen using other resources and technologies, such as those listed below.

Electrolysis


One way to make hydrogen is by electrolysis.splitting water into its basic elements hydrogen and oxygen. Electrolysis involves passing an electric current through water (H2O) to separate the water molecules into hydrogen (H2) and oxygen (O2) gases.

The electricity needed for electrolysis can come from a power plant, windmill, photovoltaic cell or any other electricity generator. If the electricity is produced by renewable energy or nuclear power, there is no net increase in greenhouse gases added to the atmosphere. Hydrogen produced by electrolysis is extremely pure, but it is very expensive because of equipment costs and other factors. On the other hand, water is renewable and abundant in many areas.

Technological advances to improve efficiency and reduce costs will make electrolysis a more economical way to produce hydrogen in the future.

Photoelectrochemical Production


Photoelectrolysis uses sunlight to split water into hydrogen and oxygen. A semiconductor absorbs energy from the sun and acts as an electrode to separate the water molecules.

Biomass Gasification


In biomass gasification, wood chips and agricultural wastes are super-heated until they turn into hydrogen and other gases. Biomass can also be used to provide the heat.

Photobiological Production

 Scientists have discovered that some algae and bacteria produce hydrogen under certain conditions, using sunlight as their energy source. Experiments are underway to find ways to induce these microbes to produce hydrogen efficiently.

Coal Gasification With Carbon Sequestration

 In this process, coal is gasified (turned into a gas) with oxygen under high pressure and temperature to produce hydrogen and carbon monoxide (CO). Steam (H2O) is added to the CO to produce hydrogen and carbon dioxide (H2 and CO2). The carbon dioxide is captured and sequestered (stored) to prevent its release into the atmosphere.

Nuclear Thermochemical


In this experimental process, the heat from a controlled nuclear reaction is used to decompose water into hydrogen and oxygen in a series of complex chemical reactions.

Watch Video





 

ReferenceBooks

Fuel Cell Fundamentals


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Next:  Fuel Cell Applications

NUCLEAR POWER PLANT

Tuesday, April 6, 2010 | | 0 comments

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Atomic Structure

Atoms are tiny particles that make up every object in the universe. An atom consists of an extremely small, positively charged nucleus surrounded by a cloud of negatively charged electrons. Although typically the nucleus is less than one ten-thousandth the size of the atom, the nucleus contains more than 99.9% of the mass of the atom!

Fig 1. Structure of Helium Atom

Nuclei consist of positively charged protons and electrically neutral neutrons held together by the so-called strong or nuclear force. This force is much stronger than the familiar electrostatic force that binds the electrons to the nucleus, but its range is limited to distances on the order of a few x10-15 meters.

Nuclear Structure





Fig 2. Nuclear Structure

The number of protons in the nucleus, Z, is called the atomic number. This determines what chemical element the atom is. The number of neutrons in the nucleus is denoted by N. Theatomic mass of the nucleus, A, is equal to Z + N. A given element can have many different isotopes, which differ from one another by the number of neutrons contained in the nuclei. In a neutral atom, the number of electrons orbiting the nucleus equals the number of protons in the nucleus. Since the electric charges of the proton and the electron are +1 and -1 respectively (in units of the proton charge), the net charge of the atom is zero. At present, there are 112 known elements which range from the lightest, hydrogen, to the recently discovered and yet to-be-named element 112. All of the elements heavier than uranium are man made. Among the elements are approximately 270 stable isotopes, and more than 2000 unstable isotopes.

Nuclear Power

Nuclear energy is energy in the nucleus (core) of an atom. There is enormous energy in the bonds that hold atoms together. Nuclear energy can be used to make electricity. But first the energy must be released.

The energy can be released from atoms in two ways:
  • nuclear fusion
  • nuclear fission.

Nuclear Fusion


Fig 3. Nucelar Fusion

In nuclear fusion, energy is released when atoms are combined or fused together to form a larger atom. This is how the sun produces energy. An example of a fusion reaction important in thermonuclear weapons and in future nuclear reactors is the reaction between two different hydrogen isotopes to form an isotope of helium.


Fig 4. Nucelar Fusion Example

This reaction liberates an amount of energy more than a million times greater than one gets from a typical chemical reaction. Such a large amount of energy is released in fusion reactions because when two light nuclei fuse, the sum of the masses of the product nuclei is less than the sum of the masses of the initial fusing nuclei. Once again, Einstein's equation, E=mc2, explains that the mass that is lost it converted into energy carried away by the fusion products.

Even though fusion n is an energetically favorable reaction for light nuclei, it does not occur under standard conditions here on Earth because of the large energy investment that is required. Because the reacting nuclei are both positively charged, there is a large electrostatic repulsion between them as they come together. Only when they are squeezed very close to one another do they feel the strong nuclear force, which can overcome the electrostatic repulsion and cause them to fuse.

Fusion reactions have been going on for billions of years in our universe. In fact, nuclear fusion reactions are responsible for the energy output of most stars, including our own Sun. Scientists on Earth have been able to produce fusion reactions for only about the last sixty years. At first, there were small scale studies in which only a few fusion reactions actually occurred. However, these first experiments later lead to the development of thermonuclear fusion weapons (hydrogen bombs).

Fusion is the process that takes place in stars like our Sun. Whenever we feel the warmth of the Sun and see by its light, we are observing the products of fusion. We know that all life on Earth exists because the light generated by the Sun produces food and warms our planet. Therefore, we can say that fusion is the basis for our life.

Nuclear Fission

 The word fission means to split apart. Fission is a nuclear process in which a heavy nucleus splits into two smaller nuclei. An example of a fission reaction that was used in the first atomic bomb and is still used in nuclear reactors is.



Fig 5. Nuclear Fission

An atom's nucleus can be split apart. When this is done, a tremendous amount of energy is released. The energy is both heat and light energy. This energy, when let out slowly, can be harnessed to generate electricity. When it is let out all at once, it makes a tremendous explosion in an atomic bomb. Example of this reaction is as follow:


Fig 6. Nuclear Fission Example


The products shown in the above equation are only one set of many possible product nuclei. Fission reactions can produce any combination of lighter nuclei so long as the number of protons and neutrons in the products sum up to those in the initial fissioning nucleus. As with fusion, a great amount of energy can be released in fission because for heavy nuclei, the summed masses of the lighter product nuclei is less than the mass of the fissioning nucleus.

Fission occurs because of the electrostatic repulsion created by the large number of positively charged protons contained in a heavy nucleus. Two smaller nuclei have less internal electrostatic repulsion than one larger nucleus. So, once the larger nucleus can overcome the strong nuclear force which holds it together, it can fission. Fission can be seen as a "tug-of-war" between the strong attractive nuclear force and the repulsive electrostatic force. In fission reactions, electrostatic repulsion wins.

How nuclear Power Plant Work:


Nuclear power plants use nuclear fission to produce electricity. To turn nuclear fission into electrical energy, the first step for nuclear power plant operators is to be able to control the energy given off by the enriched uranium and allow it to heat water into steam.

Enriched uranium is typically formed into inch-long (2.5-cm-long) pellets, each with approximately the same diameter as a dime. Next the pellets are arranged into long rods, and the rods are collected together into bundles. The bundles are submerged in water inside a pressure vessel. The water acts as a coolant. For the reactor to work, the submerged bundles must be slightly supercritical. Left to its own devices, the uranium would eventually overheat and melt.

To prevent overheating, control rods made of a material that absorbs neutrons are inserted into the uranium bundle using a mechanism that can raise or lower the control rods. Raising and lowering the control rods allow operators to control the rate of the nuclear reaction. When an operator wants the uranium core to produce more heat, the control rods are raised out of the uranium bundle (thus absorbing fewer neutrons). To create less heat, they are lowered into the uranium bundle. The rods can also be lowered completely into the uranium bundle to shut the reactor down in the case of an accident or to change the fuel.

The uranium bundle acts as an extremely high-energy source of heat. It heats the water and turns it to steam. The steam drives a turbine, which spins a generator to produce power. Humans have been harnessing the expansion of water into steam for hundreds of years.

Nuclear Reactor


There are Two Types of Nuclear Reactors:
  • The Boiling Water Reactor (BWR)
  • The Pressurized Water Reactor (PWR)

The Boiling Water Reactor (BWR)





Fig 7. The Boiling Water Reactor (BWR)

The Boiling Water Reactor (BWR) actually boil the water. In both types, water is converted to steam, and then recycled back into water by a part called the condenser, to be used again in the heat process. Since radioactive materials can be dangerous, nuclear power plants have many safety systems to protect workers, the public, and the environment.

These safety systems include shutting the reactor down quickly and stopping the fission process, systems to cool the reactor down and carry heat away from it, and barriers to contain the radioactivity and prevent it from escaping into the environment. Radioactive materials, if not used properly, can damage human cells or even cause cancer over long periods of time.


The Pressurized Water Reactor (PWR)

Fig 7. The Pressurized Water Reactor (PWR)

The Pressurized Water Reactor (PWR) keep water under pressure so that it heats, but does not boil. Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.       

The steam from the reactor goes through a secondary, intermediate heat exchanger to convert another loop of water to steam, which drives the turbine. The advantage to this design is that the radioactive water/steam never contacts the turbine. Also, in some reactors, the coolant fluid in contact with the reactor core is gas (carbon dioxide) or liquid metal (sodium, potassium); these types of reactors allow the core to be operated at higher temperatures.

Watch Nuclear Power Plant Video



Nuclear References:

Fundamentals of Nuclear Science and Engineering Second Edition
Fundamentals of Nuclear Science and Enginee...
Introduction to Nuclear Engineering (3rd Edition)
Introduction to Nuclear Engineering (3rd Ed...
Nuclear Weapons: What You Need to Know
Nuclear Weapons: What You Need to Know
Nuclear Energy (Landolt-Börnstein: Numerical Data and Functional Relationships in Science and Technology - New Series / Advanced Materials and Technologies)
Nuclear Energy (Landolt-Börnstein: Num...

HYDRO POWER PLANT

Wednesday, March 31, 2010 | | 20 comments

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Use of hydropower peaked in the mid-20th century, but the idea of using water for power generation goes back thousands of years. A hydropower plant is basically an oversized water wheel. More than 2,000 years ago, the Greeks are said to have used a water wheel for grinding wheat into flour. These ancient water wheels are like the turbines of today, spinning as a stream of water hits the blades. The gears of the wheel ground the wheat into flour.

Hydropower plants harness water's energy and use simple mechanics to convert that energy into electricity. Hydropower plants are actually based on a rather simple concept -- water flowing through a dam turns a turbine, which turns a generator.

Worldwide, hydropower plants produce about 24 percent of the world's electricity and supply more than 1 billion people with power. The world's hydropower plants output a combined total of 675,000 megawatts, the energy equivalent of 3.6 billion barrels of oil, according to the National Renewable Energy Laboratory. There are more than 2,000 hydropower plants operating in the United States, making hydropower the country's largest renewable energy source.

Here are the basic components of a conventional hydropower plant:



Hydropower plant parts


Dam

- Most hydropower plants rely on a dam that holds back water, creating a large reservoir. Often, this reservoir is used as a recreational lake, such as Lake Roosevelt at the Grand Coulee Dam in Washington State.


Fig 1.  Dam

Intake

Gates on the dam open and gravity pulls the water through the penstock, a pipeline that leads to the turbine. Water builds up pressure as it flows through this pipe.

Turbine

The water strikes and turns the large blades of a turbine, which is attached to a generator above it by way of a shaft. The most common type of turbine for hydropower plants is the Francis Turbine, which looks like a big disc with curved blades. A turbine can weigh as much as 172 tons and turn at a rate of 90 revolutions per minute (rpm), according to the Foundation for Water & Energy Education (FWEE).

Power Developed by a Turbine


Pt = 9.81 x Q x H x ? (KW)
where:
Q = Discharge, m3/sec
H = Net Head, m
h = Efficiency of Turbine

Types of turbine:


Kaplan

Fig 2 Kaplan Turbine

* Power: 100 Kw to 7 Mw
* Head: from 1.80m to 25m
* Runner blades : 4 /5 /6
* Diameter: 700 to 4000mm
* Simple or double regulation
* Arrangement:
* Vertical
* S type
* Horizontal ( Pit)
* Inclined simple regulated
* Syphon intake

Francis


Fig 3. Francis Turbine

* Power: 100 Kw to 15 Mw
* Head: from 15m to 200m
* Diameter: 250 to 3500mm
* Arrangement:
* Vertical shaft
* Horizontal shaft
* Semi spiral casing or full spiral casing
* Double francis (2 runners )


Pelton


Fig 4. Pelton Turbine

* Power: 100 Kw to 10 Mw
* Head: from 100m to 1000m
* Diameter : up to 1800mm
* Arrangement:
* Vertical 3 jets /4 jets
* Horizontal 1 jet /2 jets
* Double (horizontal 4 jets)



Generators

The heart of the hydroelectric power plant is the generator. Most hydropower plants have several of these generators. As the turbine blades turn, so do a series of magnets inside the generator. Giant magnets rotate past copper coils, producing alternating current (AC) by moving electrons.

Fig 5. Generator

The generator, as you might have guessed, generates the electricity. The basic process of generating electricity in this manner is to rotate a series of magnets inside coils of wire. This process moves electrons, which produces electrical current.


Inside a hydropower plant generator

The Hoover Dam has a total of 17 generators, each of which can generate up to 133 megawatts. The total capacity of the Hoover Dam hydropower plant is 2,074 megawatts.


Each generator is made of certain basic parts:

* Shaft
* Excitor
* Rotor
* Stator

As the turbine turns, the excitor sends an electrical current to the rotor. The rotor is a series of large electromagnets that spins inside a tightly-wound coil of copper wire, called the stator. The magnetic field between the coil and the magnets creates an electric current.

Transformer

The transformer inside the powerhouse takes the AC and converts it to higher-voltage current.

Power lines

Out of every power plant come four wires: the three phases of power being produced simultaneously plus a neutral or ground common to all three. (Read How Power Distribution Grids Work to learn more about power line transmission.)

Outflow

Used water is carried through pipelines, called tailraces, and re-enters the river downstream. The water in the reservoir is considered stored energy. When the gates open, the water flowing through the penstock becomes kinetic energy because it's in motion. The amount of electricity that is generated is determined by several factors. Two of those factors are the volume of water flow and the amount of hydraulic head. The head refers to the distance between the water surface and the turbines. As the head and flow increase, so does the electricity generated. The head is usually dependent upon the amount of water in the reservoir.

The Largest Hydroelectric Power Plant:


  • The largest hydroelectric power plant in the world is the Itaipu power plant, jointly owned by Brazil and Paraguay. Itaipu can produce 12,600 megawatts.
  • The second largest hydroelectric power plant is the Guri power plant, located on Caroni River in Venezuela. It can produce 10,300 megawatts.
  • The largest U.S. hydroelectric power plant is the Grand Coulee power station on the Columbia River in Washington State. It can produce 7,600 megawatts and is currently being upgraded to produce 10,080 megawatts.

Watch hydro power plant video:







Relevance Books

 
  
Hydro-Electric Power...
Hydro-Electric Power...
Micro-Hydro Design Manual: A Guide to Small-Scale Water Power Schemes
Micro-Hydro Design Manual

Renewable Energy
Renewable Energy

Serious Microhydro: Water Power Solutions from the Experts
Serious Microhydro
 

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