Boiling Water Reactor (BWR) - Advantages and Disadvantages

A boiling water reactor (BWR) is a type of light-water nuclear reactor developed by the General Electric Company in the mid 1950s.


1.Reactor pressure vessel 2.Fuel rods 3. Control rod 4.Circulating pump 5.Control rod drive 6.Fresh steam 7. Feedwater 8.High pressure turbine 9.Low pressure turbine 10.Generator 11.Exciter 12.Condenser 13.Cooling water 14.Preheater 15.Feedwater pump 16. Cooling water pump 17.Concrete shield

The above diagram shows BWR and its main parts.The BWR is characterized by two-phase fluid flow (water and steam) in the upper part of the reactor core. Light water (i.e., common distilled water) is the working fluid used to conduct heat away from the nuclear fuel. The water around the fuel elements also "thermalizes" neutrons, i.e., reduces their kinetic energy, which is necessary to improve the probability of fission of fissile fuel. Fissile fuel material, such as the U-235 and Pu-239 isotopes, have large capture cross sections for thermal neutrons.

In a boling water reactor, light water (H2O) plays the role of moderator and coolant, as well. In this case the steam is generted in the reactor it self.As you can see in the diagrm feed water enters the reactor pressure vessel at the bottom and takes up the heat generated due to fission of fuel (fuel rods) and gets converted in to steam.

Part of the water boils away in the reactor pressure vessel, thus a mixture of water and steam leaves the reactor core. The so generated steam directly goes to the turbine, therefore steam and moisture must be separated (water drops in steam can damage the turbine blades). Steam leaving the turbine is condensed in the condenser and then fed back to the reactor after preheating. Water that has not evaporated in the reactor vessel accumulates at the bottom of the vessel and mixes with the pumped back feedwater.
Since boiling in the reactor is allowed, the pressure is lower than that of the PWRs: it is about 60 to 70 bars. The fuel is usually uranium dioxide. Enrichment of the fresh fuel is normally somewhat lower than that in a PWR. The advantage of this type is that - since this type has the simplest construction - the building costs are comparatively low. 22.5% of the total power of presently operating nuclear power plants is given by BWRs.

Feedwater Inside of a BWR reactor pressure vessel (RPV), feedwater enters through nozzles high on the vessel, well above the top of the nuclear fuel assemblies (these nuclear fuel assemblies constitute the "core") but below the water level. The feedwater is pumped into the RPV from the condensers located underneath the low pressure turbines and after going through feedwater heaters that raise its temperature using extraction steam from various turbine stages.
The feedwater enters into the downcomer region and combines with water exiting the water separators. The feedwater subcools the saturated water from the steam separators. This water now flows down the downcomer region, which is separated from the core by a tall shroud. The water then goes through either jet pumps or internal recirculation pumps that provide additional pumping power (hydraulic head). The water now makes a 180 degree turn and moves up through the lower core plate into the nuclear core where the fuel elements heat the water. When the flow moves out of the core through the upper core plate, about 12 to 15% of the flow by volume is saturated steam.
The heating from the core creates a thermal head that assists the recirculation pumps in recirculating the water inside of the RPV. A BWR can be designed with no recirculation pumps and rely entirely on the thermal head to recirculate the water inside of the RPV. The forced recirculation head from the recirculation pumps is very useful in controlling power, however. The thermal power level is easily varied by simply increasing or decreasing the speed of the recirculation pumps.
The two phase fluid (water and steam) above the core enters the riser area, which is the upper region contained inside of the shroud. The height of this region may be increased to increase the thermal natural recirculation pumping head. At the top of the riser area is the water separator. By swirling the two phase flow in cyclone separators, the steam is separated and rises upwards towards the steam dryer while the water remains behind and flows horizontally out into the downcomer region. In the downcomer region, it combines with the feedwater flow and the cycle repeats.
The saturated steam that rises above the separator is dried by a chevron dryer structure. The steam then exists the RPV through four main steam lines and goes to the turbine.
Control systems
Reactor power is controlled via two methods: by inserting or withdrawing control rods and by changing the water flow through the reactor core.
Positioning (withdrawing or inserting) control rods is the normal method for controlling power when starting up a BWR. As control rods are withdrawn, neutron absorption decreases in the control material and increases in the fuel, so reactor power increases. As control rods are inserted, neutron absorption increases in the control material and decreases in the fuel, so reactor power decreases. Some early BWRs and the proposed ESBWR designs use only natural ciculation with control rod positioning to control power from zero to 100% because they do not have reactor recirculation systems.
Changing (increasing or decreasing) the flow of water through the core is the normal and convenient method for controlling power. When operating on the so-called "100% rod line," power may be varied from approximately 70% to 100% of rated power by changing the reactor recirculation system flow by varying the speed of the recirculation pumps. As flow of water through the core is increased, steam bubbles ("voids") are more quickly removed from the core, the amount of liquid water in the core increases, neutron moderation increases, more neutrons are slowed down to be absorbed by the fuel, and reactor power increases. As flow of water through the core is decreased, steam voids remain longer in the core, the amount of liquid water in the core decreases, neutron moderation decreases, fewer neutrons are slowed down to be absorbed by the fuel, and reactor power decreases.
Steam Turbines
Steam produced in the reactor core passes through steam separators and dryer plates above the core and then directly to the turbine, which is part of the reactor circuit. Because the water around the core of a reactor is always contaminated with traces of radionuclides, the turbine must be shielded during normal operation, and radiological protection must be provided during maintenance. The increased cost related to operation and maintenance of a BWR tends to balance the savings due to the simpler design and greater thermal efficiency of a BWR when compared with a PWR. Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7 second half life), so the turbine hall can be entered soon after the reactor is shut down.
Safety Like the pressurized water reactor, the BWR reactor core continues to produce heat from radioactive decay after the fission reactions have stopped, making nuclear meltdown possible in the event that all safety systems have failed and the core does not receive coolant. Also like the pressurized water reactor, a boiling-water reactor has a negative void coefficient, that is, the thermal output decreases as the proportion of steam to liquid water increases inside the reactor. However, unlike a pressurized water reactor which contains no steam in the reactor core, a sudden increase in BWR steam pressure (caused, for example, by a blockage of steam flow from the reactor) will result in a sudden decrease in the proportion of steam to liquid water inside the reactor. The increased ratio of water to steam will lead to increased neutron moderation, which in turn will cause an increase in the power output of the reactor. Because of this effect in BWRs, operating components and safety systems are designed to ensure that no credible, postulated failure can cause a pressure and power increase that exceeds the safety systems' capability to quickly shutdown the reactor before damage to the fuel or to components containing the reactor coolant can occur.
In the event of an emergency that disables all of the safety systems, each reactor is surrounded by a containment building designed to seal off the reactor from the environment.
Comparison with other reactors Light water is ordinary water. In comparison, some other water-cooled reactor types use heavy water. In heavy water, the deuterium isotope of hydrogen replaces the common hydrogen atoms in the water molecules (D2O instead of H2O, molecular weight 20 instead of 18).
The Pressurized Water Reactor (PWR) was the first type of light-water reactor developed because of its application to submarine propulsion. The civilian motivation for the BWR is reducing costs for commercial applications through design simplification and lower pressure components. In naval reactors, BWR designs are used when natural circulation is specified for its quietness. The description of BWRs below describes civilian reactor plants in which the same water used for reactor cooling is also used in the Rankine cycle turbine generators. A Naval BWR is designed like a PWR that has both primary and secondary loops.
In contrast to the pressurized water reactors that utilize a primary and secondary loop, in civilian BWRs the steam going to the turbine that powers the electrical generator is produced in the reactor core rather than in steam generators or heat exchangers. There is just a single circuit in a civilian BWR in which the water is at lower pressure (about 75 times atmospheric pressure) compared to a PWR so that it boils in the core at about 285°C. The reactor is designed to operate with steam comprising 12–15% of the volume of the two-phase coolant flow (the "void fraction") in the top part of the core, resulting in less moderation, lower neutron efficiency and lower power density than in the bottom part of the core. In comparison, there is no significant boiling allowed in a PWR because of the high pressure maintained in its primary loop (about 158 times atmospheric pressure).

Advantages

• The reactor vessel and associated components operate at a substantially lower pressure (about 75 times atmospheric pressure) compared to a PWR (about 158 times atmospheric pressure).
• Pressure vessel is subject to significantly less irradiation compared to a PWR, and so does not become as brittle with age.
• Operates at a lower nuclear fuel temperature.
• Fewer components due to no steam generators and no pressurizer vessel. (Older BWRs have external recirculation loops, but even this piping is eliminated in modern BWRs, such as the ABWR.)
• Lower risk (probability) of a rupture causing loss of coolant compared to a PWR, and lower risk of a severe accident should such a rupture occur. This is due to fewer pipes, fewer large diameter pipes, fewer welds and no steam generator tubes.
• Measuring the water level in the pressure vessel is the same for both normal and emergency operations, which results in easy and intuitive assessment of emergency conditions.
• Can operate at lower core power density levels using natural circulation without forced flow.
• A BWR may be designed to operate using only natural circulation so that recirculation pumps are eliminated entirely. (The new ESBWR design uses natural circulation.)

Disadvantages

• Complex operational calculations for managing the utilization of the nuclear fuel in the fuel elements during power production due to "two phase fluid flow" (water and steam) in the upper part of the core (less of a factor with modern computers). More incore nuclear instrumentation is required.
• Much larger pressure vessel than for a PWR of similar power, with correspondingly higher cost. (However, the overall cost is reduced because a modern BWR has no main steam generators and associated piping.)
• Contamination of the turbine by fission products.
• Shielding and access control around the steam turbine are required during normal operations due to the radiation levels arising from the steam entering directly from the reactor core. Additional precautions are required during turbine maintenance activities compared to a PWR.
• Control rods are inserted from below for current BWR designs. There are two available hydraulic power sources that can drive the control rods into the core for a BWR under emergency conditions. There is a dedicated high pressure hydraulic accumulator and also the pressure inside of the reactor pressure vessel available to each control rod. Either the dedicated accumulator (one per rod) or reactor pressure is capable of fully inserting each rod. Most other reactor types use top entry control rods that are held up in the withdrawn position by electromagnets, causing them to fall into the reactor by gravity if power is lost.

For flash animation of BWR operation Click here

Classification of Nuclear Reactors

Nuclear Reactors, specifically fission reacors, are classified by several methods, a brief outline of these classification schemes is given below.
Classification by use
Research reactors : Typically reactors used for research and training, materials testing, or the production of radioisotopes for medicine and industry. These are much smaller than power reactors or those propelling ships, and many are on university campuses. There are about 280 such reactors operating, in 56 countries. Some operate with high-enriched uranium fuel, and international efforts are underway to substitute low-enriched fuel.
Production reactors
Power reactors
Propulsion reactors

Classification by moderator material
Graphite moderated reactors
water moderated reactors

• Light water moderated reactors (LWRs)
• Heavy Water moderated reactors

Classification by coolant
Gas cooled reactor
Liquid metal cooled reactor
Water cooled reactor

• Pressure water reactor
• Boiling water reactor

Classification by type of nuclear reactionFast Reactors
Thermal reactors
Classification by role in the fuel cycleBreeder reactors
burner reactors
Classification by Generation
Generation II reactor
Generation III reactor
Generation IV reactor
Classification by phase of fuelSolid fueled
Fluid fueled
Gas Fueled

The Nuclear Fuel Cycle

• The nuclear fuel cycle is the series of industrial processes which involve the production of electricity from uranium in nuclear power reactors.
• Uranium is a relatively common element that is found throughout the world. It is mined in a number of countries and must be processed before it can be used as fuel for a nuclear reactor.
• Electricity is created by using the heat generated in a nuclear reactor to produce steam and drive a turbine connected to a generator.
• Fuel removed from a reactor, after it has reached the end of its useful life, can be reprocessed to produce new fuel.

The various activities associated with the production of electricity from nuclear reactions are referred to collectively as the nuclear fuel cycle. The nuclear fuel cycle starts with the mining of uranium and ends with the disposal of nuclear waste. With the reprocessing of used fuel as an option for nuclear energy, the stages form a true cycle.
Uranium
Uranium is a slightly radioactive metal that occurs throughout the earth's crust. It is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in many rivers and in sea water. It is, for example, found in concentrations of about four parts per million (ppm) in granite, which makes up 60% of the earth's crust. In fertilisers, uranium concentration can be as high as 400 ppm (0.04%), and some coal deposits contain uranium at concentrations greater than 100 ppm (0.01%). Most of the radioactivity associated with uranium in nature is in fact due to other minerals derived from it by radioactive decay processes, and which are left behind in mining and milling.
There are a number of areas around the world where the concentration of uranium in the ground is sufficiently high that extraction of it for use as nuclear fuel is economically feasible. Such concentrations are called ore.The below figure represents various stages in Nuclear Fuel cycle


Uranium MiningBoth excavation and in situ techniques are used to recover uranium ore. Excavation may be underground and open pit mining.
In general, open pit mining is used where deposits are close to the surface and underground mining is used for deep deposits, typically greater than 120 m deep. Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. As a result, the quantity of material that must be removed in order to access the ore may be large. Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine.
An increasing proportion of the world's uranium now comes from in situ leaching (ISL), where oxygenated groundwater is circulated through a very porous orebody to dissolve the uranium and bring it to the surface. ISL may be with slightly acid or with alkaline solutions to keep the uranium in solution. The uranium is then recovered from the solution as in a conventional mill.
The decision as to which mining method to use for a particular deposit is governed by the nature of the orebody, safety and economic considerations.
In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect against airborne radiation exposure.
Uranium MillingMilling, which is generally carried out close to a uranium mine, extracts the uranium from the ore. Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate which is shipped from the mill. It is sometimes referred to as 'yellowcake' and generally contains more than 80% uranium. The original ore may contains as little as 0.1% uranium.
In a mill, uranium is extracted from the crushed and ground-up ore by leaching, in which either a strong acid or a strong alkaline solution is used to dissolve the uranium. The uranium is then removed from this solution and precipitated. After drying and usually heating it is packed in 200-litre drums as a concentrate.
The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in mined out pit). Tailings contain long-lived radioactive materials in low concentrations and toxic materials such as heavy metals; however, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived. These materials need to be isolated from the environment.
Conversion
The product of a uranium mill is not directly usable as a fuel for a nuclear reactor. Additional processing, generally referred to as enrichment, is required for most kinds of reactors. This process requires uranium to be in gaseous form and the way this is achieved is to convert it to uranium hexafluoride, which is a gas at relatively low temperatures.
At a conversion facility, uranium is first refined to uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium. Most is then converted into uranium hexafluoride, ready for the enrichment plant. It is shipped in strong metal containers. The main hazard of this stage of the fuel cycle is the use of hydrogen fluoride.
Enrichment
Natural uranium consists, primarily, of a mixture of two isotopes (atomic forms) of uranium. Only 0.7% of natural uranium is "fissile", or capable of undergoing fission, the process by which energy is produced in a nuclear reactor. The fissile isotope of uranium is uranium 235 (U-235). The remainder is uranium 238 (U-238).
In the most common types of nuclear reactors, a higher than natural concentration of U-235 is required. The enrichment process produces this higher concentration, typically between 3.5% and 5% U-235, by removing over 85% of the U-238. This is done by separating gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium. The other stream is progressively depleted in U-235 and is called 'tails'.
There are two enrichment processes in large scale commercial use, each of which uses uranium hexafluoride as feed: gaseous diffusion and gas centrifuge. They both use the physical properties of molecules, specifically the 1% mass difference, to separate the isotopes. The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide.
Fuel fabricationReactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide which is sintered (baked) at a high temperature (over 1400°C). The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of fuel bundles.
In a fuel fabrication plant great care is taken with the size and shape of processing vessels to avoid criticality (a limited chain reaction releasing radiation). With low-enriched fuel criticality is most unlikely, but in plants handling special fuels for research reactors this is a vital consideration.
Power generation
Inside a nuclear reactor the nuclei of U-235 atoms split (fission) and, in the process, release energy. This energy is used to heat water and turn it into steam. The steam is used to drive a turbine connected to a generator which produces electricity. Some of the U-238 in the fuel is turned into plutonium in the reactor core. The main plutonium isotope is also fissile and it yields about one third of the energy in a typical nuclear reactor. The fissioning of uranium is used as a source of heat in a nuclear power station in the same way that the burning of coal, gas or oil is used as a source of heat in a fossil fuel power plant.
As with as a coal-fired power station about two thirds of the heat is dumped, either to a large volume of water (from the sea or large river, heating it a few degrees) or to a relatively smaller volume of water in cooling towers, using evaporative cooling (latent heat of vapourisation).
Used fuel
With time, the concentration of fission fragments and heavy elements formed in the same way as plutonium in a fuel bundle will increase to the point where it is no longer practical to continue to use the fuel. So after 12-24 months the 'spent fuel' is removed from the reactor. The amount of energy that is produced from a fuel bundle varies with the type of reactor and the policy of the reactor operator.
Typically, some 36 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic metres of gas.
Used fuel storage
When removed from a reactor, a fuel bundle will be emitting both radiation, principally from the fission fragments, and heat. Used fuel is unloaded into a storage pond immediately adjacent to the reactor to allow the radiation levels to decrease. In the ponds the water shields the radiation and absorbs the heat. Used fuel is held in such pools for several months to several years.
Depending on policies in particular countries, some used fuel may be transferred to central storage facilities. Ultimately, used fuel must either be reprocessed or prepared for permanent disposal.
Reprocessing
Used fuel is about 95% U-238 but it also contains about 1% U-235 that has not fissioned, about 1% plutonium and 3% fission products, which are highly radioactive, with other transuranic elements formed in the reactor. In a reprocessing facility the used fuel is separated into its three components: uranium, plutonium and waste, containing fission products. Reprocessing enables recycling of the uranium and plutonium into fresh fuel, and produces a significantly reduced amount of waste (compared with treating all used fuel as waste).
Uranium and Plutonium Recycling
The uranium from reprocessing, which typically contains a slightly higher concentration of U-235 than occurs in nature, can be reused as fuel after conversion and enrichment, if necessary. The plutonium can be directly made into mixed oxide (MOX) fuel, in which uranium and plutonium oxides are combined.
In reactors that use MOX fuel, plutonium substitutes for the U-235 in normal uranium oxide fuel.
Used fuel disposal
At the present time, there are no disposal facilities (as opposed to storage facilities) in operation in which used fuel, not destined for reprocessing, and the waste from reprocessing can be placed. Although technical issues related to disposal have been addressed, there is currently no pressing technical need to establish such facilities, as the total volume of such wastes is relatively small. Further, the longer it is stored the easier it is to handle, due to the progressive diminution of radioactivity. There is also a reluctance to dispose of used fuel because it represents a significant energy resource which could be reprocessed at a later date to allow recycling of the uranium and plutonium. (There is a proposal to use it in Candu reactors directly as fuel.)
A number of countries are carrying out studies to determine the optimum approach to the disposal of spent fuel and wastes from reprocessing. The general consensus favours its placement into deep geological repositories, initially recoverable.
Wastes
Wastes from the nuclear fuel cycle are categorised as high-, medium- or low-level wastes by the amount of radiation that they emit. These wastes come from a number of sources and include:

• low-level waste produced at all stages of the fuel cycle;
• intermediate-level waste produced during reactor operation and by reprocessing;
• high-level waste, which is waste containing fission products from reprocessing, and in many countries, the used fuel itself.

The enrichment process leads to the production of much 'depleted' uranium, in which the concentration of U-235 is significantly less than the 0.7% found in nature. Small quantities of this material, which is primarily U-238, are used in applications where high density material is required, including radiation shielding and some is used in the production of MOX fuel. While U-238 is not fissile it is a low specific activity radioactive material and some precautions must, therefore, be taken in its storage or disposal.

Nuclear Energy,Nuclear Fuels

Nuclei are made up of protons and neutron, but the mass of a nucleus is always less than the sum of the individual masses of the protons and neutrons which constitute it. The difference is a measure of the nuclear binding energy which holds the nucleus together.

Nuclear energy is energy released from the atomic nucleus. Atoms are tiny particles that make up every object in the universe. There is enormous energy in the bonds that hold atoms together.This binding energy can be calculated from the Einstein relationship: mass-energy equivalence formula E = mc², in which E = energy, m = mass, and c = the speed of light in a vacuum (a physical constant).The alpha particle gives binding energy of 28.3 MeV
Nuclear energy is released by several processes:

• Radioactive decay, where a radioactive nucleus decays spontaneously into a lighter nucleus by emitting a particle;
• Endothermic nuclear reactions where two nuclei merge to produce two different nuclei. The following two processes are particular examples:
• Fusion, two atomic nuclei fuse together to form a heavier nucleus;
• Fission, the breaking of a heavy nucleus into two nearly equal parts.

Nuclear Fuels
Nuclear fuel is any material that can be consumed to derive nuclear energy, by analogy to chemical fuel that is burned to derive energy. By far the most common type of nuclear fuel is heavy fissile elements that can be made to undergo nuclear fission chain reactions in a nuclear fission reactor; nuclear fuel can refer to the material or to physical objects (for example fuel bundles composed of fuel rods) composed of the fuel material, perhaps mixed with structural, neutron moderating, or neutron reflecting materials.
Not all nuclear fuels are used in fission chain reactions. For example, 238Pu and some other elements are used to produce small amounts of nuclear power by radioactive decay in radiothermal generators, and other atomic batteries. Light isotopes such as 3H (tritium) are used as fuel for nuclear fusion. If one looks at binding energy of specific isotopes, there can be an energy gain from fusing most elements with a lower atomic number than iron, and fissioning isotopes with a higher atomic number than iron.
The most common fissile nuclear fuels are natural urnium,enriched uranium,plutonium and 233U.Natural uranium is the parent material.The materials 235U,233U and 239Pu are called fissionable materials.The only fissionable nuclear fuel occuring in nature is uraium of which 99.3% is 238U and 0.7% is 235U and 234U is only a trace.Out of these isotopes only 235U will fission in a chain reaction.The other two fissionable materials can be produced artificially from 238U and 232Th which occur in nature are called fertile materials.Out of the three fissionable materials 235U has some advantages over the other two due to its higher fission percentage.Fissionable materials 239Pu and 233U are formed in the nuclear reactors during fission process from 238U and 232Th respectively due to absorption of neutrons with out fission.Getting 239Pu process is called conversion and getting 233U is called breeding.

Nuclear Fission

Nuclear fission—also known as atomic fission—is a process in nuclear physics and nuclear chemistry in which the nucleus of an atom splits into two or more smaller nuclei as fission products, and usually some by-product particles, Hence, fission is a form of elemental transmutation. The by-products include free neutrons, photons usually in the form gamma rays, and other nuclear fragments such as beta particles and alpha particles. Fission of heavy elements is an exothermic reaction and can release substantial amounts of useful energy both as gamma rays and as kinetic energy of the fragments (heating the bulk material where fission takes place).

Nuclear fission produces energy for nuclear power and to drive explosion of nuclear weapons. Fission is useful as a power source because some materials, called nuclear fuels, generate neutrons as part of the fission process and undergo triggered fission when impacted by a free neutron. Nuclear fuels can be part of a self-sustaining chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon.

The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very tempting source of energy; however, the byproducts of nuclear fission are highly radioactive and remain so for millennia, giving rise to a nuclear waste problem.

Splitting the Uranium Atom:
Uranium is the principle element used in nuclear reactors and in certain types of atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a 235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and this causes the atom to fission. The fissioning of 236U can produce over twenty different products. However, the products' masses always add up to 236. The following two equations are examples of the different products that can be produced when 235U fissions:
235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY
235U + 1 neutron 2 neutrons + 92Sr + 140Xe + ENERGY

Let's discuss those reactions. In each of the above reactions, 1 neutron splits the atom. When the atom is split, 1 additional neutron is released. This is how a chain reaction works. If more 235U is present, those 2 neutrons can cause 2 more atoms to split. Each of those atoms releases 1 more neutron bringing the total neutrons to 4. Those 4 neutrons can strike 4 more 235U atoms, releasing even more neutrons. The chain reaction will continue until all the 235U fuel is spent. This is roughly what happens in an atomic bomb. It is called a runaway nuclear reaction.

Where Does the Energy Come From?
In the section above we described what happens when an 235U atom fissions. We gave the following equation as an example:
235U + 1 neutron 2 neutrons + 92Kr + 142Ba + ENERGY

You might have been wondering, "Where does the energy come from?". The mass seems to be the same on both sides of the reaction:
235 + 1 = 2 + 92 + 142 = 236
Thus, it seems that no mass is converted into energy. However, this is not entirely correct. The mass of an atom is more than the sum of the individual masses of its protons and neutrons, which is what those numbers represent. Extra mass is a result of the binding energy that holds the protons and neutrons of the nucleus together. Thus, when the uranium atom is split, some of the energy that held it together is released as radiation in the form of heat. Because energy and mass are one and the same, the energy released is also mass released. Therefore, the total mass does decrease a tiny bit during the reaction.
Fission in Nuclear Reactors
To make large-scale use of the energy released in fission, one fission event must trigger another, so that the process spreads thoughout the nuclear fuel as in a set of dominos. The fact that more neutrons are produced in fission than are consumed raises the possibility of a chain reaction. Such a reaction can be either rapid (as in an atomic bomb) or controlled (as in a reactor).
In a nuclear reactor, control rods made of cadmium or graphite or some other neutron-absorbing material are used to regulate the number of neutrons. The more exposed control rods, the less neutrons and vice versa. This also controls the multiplication factor k which is the ratio of the number of neutrons present at the beginning of a particular generation to the number present at the beginning of the next generation. For k=1, the operation of the reactor is said to be exactly critical, which is what we wish it to be for steady-power operation. Reactors are designed so that they are inherently supercritical (k>1); the multiplication factor is then adjusted to the critical operation by inserting the control rods.
An unavoidable feature of reactor operation is the accumulation of radioactive wastes, including both fission products and heavy "transuranic" nuclides such as plutonium and americium.
For flash animation on fission use these links Link1 Link2
For more details on this topic click these links Link1 Link2

Nuclear Power

Nuclear power is the controlled use of nuclear reactions to release energy for work including propulsion, heat, and the generation of electricity. Use of nuclear power to do significant useful work is currently limited to nuclear fission and radioactive decay. Nuclear energy is produced when a fissile material, such as uranium-235 (235U), is concentrated such that nuclear fission takes place in a controlled chain reaction and creates heat — which is used to boil water, produce steam, and drive a steam turbine. The turbine can be used for mechanical work and also to generate electricity. Nuclear power provides 7% of the world's energy and 15.7% of the world's electricity and is used to power most military submarines and aircraft carriers.

The United States produces the most nuclear energy, with nuclear power providing 20% of the electricity it consumes, while France produces the highest percentage of its electrical energy from nuclear reactors—80% as of 2006. In the European Union as a whole, nuclear energy provides 30% of the electricity.Nuclear energy policy differs between countries, and some countries such as Austria, Australia and Ireland have no nuclear power stations.

Concerns about nuclear power
The use of nuclear power is controversial because of the problem of storing radioactive waste for indefinite periods, the potential for possibly severe radioactive contamination by accident or sabotage, and the possibility that its use in some countries could lead to the proliferation of nuclear weapons. Proponents believe that these risks are small and can be further reduced by the technology in the new reactors. They further claim that the safety record is already good when compared to other fossil-fuel plants, that it releases much less radioactive waste than coal power, and that nuclear power is a sustainable energy source. Critics, including most major environmental groups, claim nuclear power is an uneconomic and potentially dangerous energy source with a limited fuel supply, especially compared to renewable energy, and dispute whether the costs and risks can be reduced through new technology.

There is concern in some countries over North Korea and Iran operating research reactors and fuel enrichment plants, since those countries refuse adequate IAEA oversight and are believed to be trying to develop nuclear weapons. North Korea admits that it is developing nuclear weapons, while the Iranian government vehemently denies the claims against Iran.

Several concerns about nuclear power have been expressed, and these include:

• Concerns about nuclear reactor accidents, such as the Chernobyl disaster
• Vulnerability of plants to attack or sabotage
• Use of nuclear waste as a weapon
• Health effects of nuclear power plants
• Nuclear proliferation

Nuclear Power Plant,Types, Advantages and Disadvantages

Nuclear Power Plant

Nuclear power is generated using Uranium, which is a metal mined in various parts of the world.

The structure of a nuclear power plant in many aspects resembles to that of a conventional thermal power station, since in both cases the heat produced in the boiler (or reactor) is transported by some coolant and used to generate steam. The steam then goes to the blades of a turbine and by rotating it, the connected generator will produce electric energy. The steam goes to the condenser, where it condenses, i.e. becomes liquid again. The cooled down water afterwards gets back to the boiler or reactor, or in the case of PWRs to the steam generator.

The great difference between a conventional and a nuclear power plant is how heat is produced. In a fossile plant, oil, gas or coal is fired in the boiler, which means that the chemical energy of the fuel is converted into heat. In a nuclear power plant, however, energy that comes from fission reactions is utilized.
How it works

• Nuclear power stations work in pretty much the same way as fossil fuel-burning stations, except that a "chain reaction" inside a nuclear reactor makes the heat instead.
• The reactor uses Uranium rods as fuel, and the heat is generated by nuclear fission. Neutrons smash into the nucleus of the uranium atoms, which split roughly in half and release energy in the form of heat.
• Carbon dioxide gas is pumped through the reactor to take the heat away, and the hot gas then heats water to make steam.
• The steam drives turbines which drive generators. Modern nuclear power stations use the same type of turbines and generators as conventional power stations.

In Britain, nuclear power stations are built on the coast, and use sea water for cooling the steam ready to be pumped round again. This means that they don't have the huge "cooling towers" seen at other power stations.
The reactor is controlled with "control rods", made of boron, which absorb neutrons. When the rods are lowered into the reactor, they absorb more neutrons and the fission process slows down. To generate more power, the rods are raised and more neutrons can crash into uranium atoms.
Nuclear Power Plant TypesSeveral nuclear power plant (NPP) types are used for energy generation in the world. The different types are usually classified based on the main features of the reactor applied in them. The most widespread power plant reactor types are:

• Light water reactors: both the moderator and coolant are light water (H2O). To this category belong the pressurized water reactors (PWR) and boiling water reactors (BWR).


• Heavy water reactors (CANDU): both the coolant and moderator are heavy water (D2O).


• Graphite moderated reactors: in this category there are gas cooled reactors (GCR) and light water cooled reactors (RBMK).


• Exotic reactors (fast breeder reactors and other experimental installations).


• New generation reactors: reactors of the future.

Advantages

• Nuclear power costs about the same as coal, so it's not expensive to make.
• The amount of fuel required is quite small ,therfore there is no problem of transportation, storage etc.
• Does not produce smoke or carbon dioxide, so it does not contribute to the greenhouse effect.
• Produces huge amounts of energy from small amounts of fuel.
• Produces small amounts of waste.
• The output control is most flexible.
• Nuclear power is reliable.

Disadvantages

• The fuel used is expensive and is difficult to recover.
• The fission by-products are generally radio active and may cause a dangerous amount of radio active pollution.
• Although not much waste is produced, it is very, very dangerous. It must be sealed up and buried for many years to allow the radioactivity to die away.
• The initial capital cost is very high as compared to other power plants.
• Nuclear power is reliable, but a lot of money has to be spent on safety - if it does go wrong, a nuclear accident can be a major disaster. People are increasingly concerned about this - in the 1990's nuclear power was the fastest-growing source of power in much of the world. In 2005 it was the second slowest-growing.
• The cooling water requirements of a nuclear power plant are very heavy.

Pelton Wheel

Pelton wheel, also called a Pelton turbine, is one of the most efficient types of water turbines. It was invented by Lester Allan Pelton (1829-1908) in the 1870s, and is an impulse machine, meaning that it uses Newton's second law to extract energy from a jet of fluid.

The pelton wheel turbine is a tangential flow impulse turbine, water flows along the tangent to the path of the runner. Nozzles direct forceful streams of water against a series of spoon-shaped buckets mounted around the edge of a wheel. Each bucket reverses the flow of water, leaving it with diminished energy. The resulting impulse spins the turbine. The buckets are mounted in pairs, to keep the forces on the wheel balanced, as well as to ensure smooth, efficient momentum transfer of the fluid jet to the wheel. The Pelton wheel is most efficient in high head applications.

Since water is not a compressible fluid, almost all of the available energy is extracted in the first stage of the turbine. Therefore, Pelton wheels have only one wheel, unlike turbines that operate with compressible fluids.

Applications

Peltons are the turbine of choice for high head, low flow sites. However, Pelton wheels are made in all sizes. There are multi-ton Pelton wheels mounted on vertical oil pad bearings in the generator houses of hydroelectric plants. The largest units can be up to 200 megawatts. The smallest Pelton wheels, only a few inches across, are used with household plumbing fixtures to tap power from mountain streams with a few gallons per minute of flow, but these small units must have thirty meters or more of head. Depending on water flow and design, Pelton wheels can operate with heads as small as 15 meters and as high as 1,800 meters.

In general, as the height of fall increases, less volume of water can generate a bit more power. Energy is force times distance, in the instance of fluid flow power is expressed as P = Constant x Pressure x Volume/t. The power P grows linearly with flow rate and grows with f(Pressure^3/2.) Thus it is usually best to seek as much head or pressure as possible in hydro designs then go for flow rate.

Kaplan Turbine

The Kaplan turbine is a propeller-type water turbine that has adjustable blades. It was developed in 1913 by the Austrian professor Viktor Kaplan.

The Kaplan turbine was an evolution of the Francis turbine. Its invention allowed efficient power production in low head applications that was not possible with Francis turbines.
Kaplan turbines are now widely used throughout the world in high-flow, low-head power production.

The Kaplan turbine is an inward flow reaction turbine, which means that the working fluid changes pressure as it moves through the turbine and gives up its energy. The design combines radial and axial features.



The above figures shows flow in a Kaplan turbine. In the picture, pressure on runner blades and hub surface is shown using colormapping (red = high, blue = low).
The diameter of the runner of such machines is typically 5 to 8 meters.

The inlet is a scroll-shaped tube that wraps around the turbine's wicket gate. Water is directed tangentially, through the wicket gate, and spirals on to a propeller shaped runner, causing it to spin.

The outlet is a specially shaped draft tube that helps decelerate the water and recover kinetic energy.

The turbine does not need to be at the lowest point of water flow, as long as the draft tube remains full of water. A higher turbine location, however, increases the suction that is imparted on the turbine blades by the draft tube. The resulting pressure drop may lead to cavitation.

Variable geometry of the wicket gate and turbine blades allow efficient operation for a range of flow conditions. Kaplan turbine efficiencies are typically over 90%, but may be lower in very low head applications.

Applications
Kaplan turbines are widely used throughout the world for electrical power production. They cover the lowest head hydro sites and are especially suited for high flow conditions.

Inexpensive micro turbines are manufactured for individual power production with as little as two feet of head.

Large Kaplan turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%. They are very expensive to design, manufacture and install, but operate for decades.

Variations
The Kaplan turbine is the most widely used of the propeller-type turbines, but several other variations exist:

Propeller turbines have non-adjustable propeller vanes. They are used in low cost, small installations. Commercial products exist for producing several hundred
watts from only a few feet of head.
Bulb or Tubular turbines are designed into the water delivery tube. A large bulb is centered in the water pipe which holds the generator, wicket gate and runner. Tubular turbines are a fully axial design, whereas Kaplan turbines have a radial wicket gate. Pit turbines are bulb turbines with a gear box. This allows for a smaller generator and bulb.
Straflo turbines are axial turbines with the generator outside of the water channel, connected to the periphery of the runner.
S- turbines eliminate the need for a bulb housing by placing the generator outside of the water channel. This is accomplished with a jog in the water channel and a shaft connecting the runner and generator.Tyson turbines are a fixed propeller turbine designed to be immersed in a fast flowing river, either permanently anchored in the river bed, or attached to a boat or barge.

For more details on this topic Click here

Francis Turbine

The Francis turbine is a type of water turbine that was developed by James B. Francis. It is an inward flow reaction turbine that combines radial and axial flow concepts.
Francis turbines are the most common water turbine in use today. They operate in a head range of ten meters to several hundred meters and are primarily used for electrical power production.

The Francis turbine is a reaction turbine, which means that the working fluid changes pressure as it moves through the turbine, giving up its energy. A casement is needed to contain the water flow. The turbine is located between the high pressure water source and the low pressure water exit, usually at the base of a dam.

The inlet is spiral shaped. Guide vanes direct the water tangentially to the runner. This radial flow acts on the runner vanes, causing the runner to spin. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions.
As the water moves through the runner its spinning radius decreases, further acting on the runner. Imagine swinging a ball on a string around in a circle. If the string is pulled short, the ball spins faster. This property, in addition to the water's pressure, helps inward flow turbines harness water energy.At the exit, water acts on cup shaped runner features, leaving with no swirl and very little kinetic or potential energy. The turbine's exit tube is shaped to help decelerate the water flow and recover the pressure.
Application
Large Francis turbines are individually designed for each site to operate at the highest possible efficiency, typically over 90%.Francis type units cover a wide head range, from 20 meters to 700 meters and their output varies from a few kilowatt to 1000 megawatt. Their size varies from a few hundred millimeters to about 10 meters.
In addition to electrical production, they may also be used for pumped storage; where a reservoir is filled by the turbine (acting as a pump) during low power demand, and then reversed and used to generate power during peak demand.
Francis turbines may be designed for a wide range of heads and flows. This, along with their high efficiency, has made them the most widely used turbine in the world.

For more details on this topic Click here

Water Turbines and its Classification

Water turbine is a device that convert the energy in a stream of fluid into mechanical energy by passing the stream through a system of fixed and moving fan like blades and causing the latter to rotate. A turbine looks like a large wheel with many small radiating blades around its rim.

Classification of Water turbines
According to the type of flow of water : The water turbines used as prime movers in hydro electric power stations are of four types.They are

• axial flow : having flow along shaft axis
• inward radial flow : having flow along the radius
• tangential or peripheral : having flow along tangential direction
• mixed flow : having radial inlet axial outlet

If the runner blades of axial flow turbines are fixed,those are called propeller turbines.
According to the action of water on moving blades water turbines are of 2 types namely impulse ad reaction type turbines.
Impulse Turbines :These turbines change the direction of flow of a high velocity fluid jet. The resulting impulse spins the turbine and leaves the fluid flow with diminished kinetic energy. There is no pressure change of the fluid in the turbine rotor blades. Before reaching the turbine the fluid's Pressure head is changed to velocity head by accelerating the fluid with a nozzle. Pelton wheels and de Laval turbines use this process exclusively. Impulse turbines do not require a pressure casement around the runner since the fluid jet is prepared by a nozzle prior to reaching turbine. Newton's second law describes the transfer of energy for impulse turbines.
Reaction Turbines : These turbines develop torque by reacting to the fluid's pressure or weight. The pressure of the fluid changes as it passes through the turbine rotor blades. A pressure casement is needed to contain the working fluid as it acts on the turbine stage(s) or the turbine must be fully immersed in the fluid flow (wind turbines). The casing contains and directs the working fluid and, for water turbines, maintains the suction imparted by the draft tube. Francis turbines and most steam turbines use this concept. For compressible working fluids, multiple turbine stages may be used to harness the expanding gas efficiently. Newton's third law describes the transfer of energy for reaction turbines.
According to the Head and quantity of water available the water turbines are of 2 types.Those are high head - low flow and low to medium head and high to medium discharge turbines.
According to the name of the originator water turbines are of 3 types namely Pelton Wheel,Francis tubine and Kaplan turbine.

Hydro Power Plant Working

A hydroelectric power plant harnesses the energy found in moving or still water and converts it into electricity.
Moving water, such as a river or a waterfall, has mechanical energy. ‘Mechanical energy is the energy that is possessed by an object due to its motion or stored energy of position.’ This means that an object has mechanical energy if it’s in motion or has the potential to do work (the movement of matter from one location to another,) based on its position. The energy of motion is called kinetic energy and the stored energy of position is called potential energy. Water has both the ability and the potential to do work. Therefore, water contains mechanical energy (the ability to do work), kinetic energy (in moving water, the energy based on movement), and potential energy (the potential to do work.)
The potential and kinetic/mechanical energy in water is harnessed by creating a system to efficiently process the water and create electricity from it. A hydroelectric power plant has eleven main components. The first component is a dam.

The dam is usually built on a large river that has a drop in elevation, so as to use the forces of gravity to aid in the process of creating electricity. A dam is built to trap water, usually in a valley where there is an existing lake. An artificial storage reservoir is formed by constructing a dam across a river.Notice that the dam is much thicker at the bottom than at the top, because the pressure of the water increases with depth.

The area behind the dam where water is stored is called the reservoir. The water there is called gravitational potential energy. The water is in a stored position above the rest of the dam facility so as to allow gravity to carry the water down to the turbines. Because this higher altitude is different than where the water would naturally be, the water is considered to be at an altered equilibrium. This results in gravitational potential energy, or, “the stored energy of position possessed by an object.” The water has the potential to do work because of the position it is in (above the turbines, in this case.)

Gravity will force the water to fall to a lower position through the intake and the control gate. They are built on the inside of the dam. When the gate is opened, the water from the reservoir goes through the intake and becomes translational kinetic energy as it falls through the next main part of the system: the penstock. Translational kinetic energy is the energy due to motion from one location to another. The water is falling (moving) from the reservoir towards the turbines through the penstock.

The intake shown in figure includes the head works which are the structures at the intake of conduits,tunnels or flumes.These structures include blooms,screens or trash - racks, sluices to divert and prevent entry of debris and ice in to the turbines.Booms prevent the ice and floating logs from going in to the intake by diverting them to a bypass chute.Screens or trash-racks(shown in fig) are fitted irectly at the intake to prevent the debris from going in to the take.Debris cleaning devices should also be fitted on the trash-racks.Intake structures can be classified in to high pressure intakes used in case of large storage reservoirs and low pressure intakes used in case of small ponds.The use of providing these structures at the intake is,water only enters and flows through the penstock which strikes the turbine.
Control gates arrangement is provided with Spillways.Spillway is constructed to act as a safety valve.It dischargs the overflow water to the down stream side when the reservoir is full.These are generally constructed of concrete and provided with water discharge opening,shut off by metal control gates.By changing the degree to which the gates are opened,the discharge of the head water to the tail race can be regulated inorder to maintain water level in reservoir.
The penstock is a long shaft that carries the water towards the turbines where the kinetic energy becomes mechanical energy. The force of the water is used to turn the turbines that turn the generator shaft. The turning of this shaft is known as rotational kinetic energy because the energy of the moving water is used to rotate the generator shaft. The work that is done by the water to turn the turbines is mechanical energy. This energy powers the generators, which are very important parts of the hydroelectric power plant; they convert the energy of water into electricity. Most plants contain several generators to maximize electricity production.
The generators are comprised of four basic components: the shaft, the excitor, the rotor, and the stator. The turning of the turbines powers the excitor to send 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. “A voltage is induced in the moving conductors by an effect called electromagnetic induction.” The electromagnetic induction caused by the spinning electromagnets inside the wires causes electrons to move, creating electricity. The kinetic/mechanical energy in the spinning turbines turns into electrical energy as the generators function.
The transformer, another component, takes the alternating current and converts it into higher-voltage current. The electrical current generated in the generators is sent to a wire coil in the transformer. This is electrical energy. Another coil is located very close to first one and the fluctuating magnetic field in the first coil will cut through the air to the second coil without the current. The amount of turns in the second coil is proportional to the amount of voltage that is created. If there are twice as many turns on the second coil as there are on the first one, the voltage produced will be twice as much as that on the first coil. This transference of electrical current is electrical energy. It goes from the generators to one coil, and then is transferred through an electromagnetic field onto the second coil. That current is then sent by means of power lines to the public as electricity
Now, the water that turned the turbines flows through the pipelines (translational kinetic energy, because the energy in the water is being moved,) called tailraces and enters the river through the outflow. The water is back to being kinetic/mechanical/potential energy as it is in the river and has to potential to have the energy harnessed for use as it flows along (movement.)
To see the flash animation of hydro power plant working Click here
This Link describes eah part of hydro plant. To know more about hydro electric plant see these links Link1 Link2

Pumped Storage Plants

"Pumped Storage" is another form of hydro-electric power. Pumped storage facilities use excess electrical system capacity, generally available at night, to pump water from one reservoir to another reservoir at a higher elevation. During periods of peak electrical demand, water from the higher reservoir is released through turbines to the lower reservoir, and electricity is produced . Although pumped storage sites are not net producers of electricity - it actually takes more electricity to pump the water up than is recovered when it is released - they are a valuable addition to electricity supply systems. Their value is in their ability to store electricity for use at a later time when peak demands are occurring. Storage is even more valuable if intermittent sources of electricity such as solar or wind are hooked into a system.

Pumped storage plant is a unique design of peak load plant in which the plant pumps back all or portion of its water supply during lo load period.The usual construction is a lowand high elevation reservoirs connected through a penstock.The generating pumping plant is at the lower end.The plant utilises some of the surplus energy generated by the base load plant to pump water from low elevation to highelevation reservoir during off peak hours.During peak load period this water is used to generate power by allowing it to flow from high elevation reservoir through reversible hydraulic turbine of this plan to low elevation reservoir.Thus same water is used again and again and extra water is required only to take care of evaporation and seepage.

The main important point in this plant is reversible turbine/generator assemblies act as pump and turbine (usually a Francis turbine design).During low load periods it acts as pump and pumps water from low to high elevation reservoir.During peak load periods it acts as turbine when water flows from high to low elevation reservoir.
To see the flash animation of pumped storage plant working Click here
Advantages

• Without some means of storing energy for quick release, we'd be in trouble.
• Little effect on the landscape.
• No pollution or waste

Disadvantages

• Expensive to build.
• Once it's used, you can't use it again until you've pumped the water back up. Good planning can get around this problem.

For more details on this topic click here Link1 Link2

Hydro Electric Plants - Classification, Advantages and Disadvantages

Classification
The classification of hydro electric plants based upon :
(a) Quantity of water available (b) Available head (c) Nature of load

The classification acording to Quantity of water available is
(i) Run-off river plants with out pondage : These plants does not store water; the plant uses water as it comes.The plant can use water as and when available.Since these plants depend for their generting capacity primarly on the rate of flow of water, during rainy season high flow rate may mean some quantity of water to go as waste while during low run-off periods, due to low flow rates,the generating capacity will be low.
(ii) Run-off river plants with pondage : In these plants pondage permits storage of water during off peak periods and use of this water during peak periods.Depending on the size of pondage provided it may be possible to cope with hour to hour fluctuations.This type of plant can be used on parts of the load curve as required,and is more useful than a plant with out storage or pondage.
When providing pondage tail race conditions should be such that floods do not raise tail-race water level,thus reducing the head on the plant and impairing its effectiveness.This type of plant is comparitively more reliable and its generating capacity is less dependent on avilable rate of flow of water.
(iii) Reservoir Plants :A reservoir plant is that which has a reservoir of such size as to permit carrying over storage from wet season to the next dry season.Water is stored behind the dam and is available to the plant with control as required.Such a plant has better capacity and can be used efficiently through out the year.Its firm capacity can be increased and can be used either as a base load plant or as a peak load plant as required.It can also be used on any portion of the load curve as required.Majority of the hydroelectric plants are of this type.

The classification according to availability of water head is
(i) Low-Head (less than 30 meters) Hydro electric plants :"Low head" hydro-electric plants are power plants which generally utilize heads of only a few meters or less. Power plants of this type may utilize a low dam or weir to channel water, or no dam and simply use the "run of the river". Run of the river generating stations cannot store water, thus their electric output varies with seasonal flows of water in a river. A large volume of water must pass through a low head hydro plant's turbines in order to produce a useful amount of power. Hydro-electric facilities with a capacity of less than about 25 MW (1 MW = 1,000,000 Watts) are generally referred to as "small hydro", although hydro-electric technology is basically the same regardless of generating capacity.

(ii) Medum-head(30 meters - 300 meters) hydro electric plants :These plants consist of a large dam in a mountainous area which creates a huge reservoir. The Grand Coulee Dam on the Columbia River in Washington (108 meters high, 1270 meters wide, 9450 MW) and the Hoover Dam on the Colorado River in Arizona/Nevada (220 meters high, 380 meters wide, 2000 MW) are good examples. These dams are true engineering marvels. In fact, the American Society of Civil Engineers as designated Hoover Dam as one of the seven civil engineering wonders of the modern world, but the massive lakes created by these dams are a graphic example of our ability to manipulate the environment - for better or worse. Dams are also used for flood control, irrigation, recreation, and often are the main source of potable water for many communities. Hydroelectric development is also possible in areas such as Niagra Falls where natural elevation changes can be used.

(iii) High-head hydro electric plants :"High head" power plants are the most common and generally utilize a dam to store water at an increased elevation. The use of a dam to impound water also provides the capability of storing water during rainy periods and releasing it during dry periods. This results in the consistent and reliable production of electricity, able to meet demand. Heads for this type of power plant may be greater than 1000 m. Most large hydro-electric facilities are of the high head variety. High head plants with storage are very valuable to electric utilities because they can be quickly adjusted to meet the electrical demand on a distribution system.
The classification according to nature of load is
(i) Base load plants :A base load power plant is one that provides a steady flow of power regardless of total power demand by the grid. These plants run at all times through the year except in the case of repairs or scheduled maintenance.

Power plants are designated base load based on their low cost generation, efficiency and safety at set outputs. Baseload power plants do not change production to match power consumption demands since it is always cheaper to run them rather than running high cost combined cycle plants or combustion turbines. Typically these plants are large enough to provide a majority of the power used by a grid, making them slow to fire up and cool down. Thus, they are more effective when used continuously to cover the power baseload required by the grid.

Each base load power plant on a grid is allotted a specific amount of the baseload power demand to handle. The base load power is determined by the load duration curve of the system. For a typical power system, rule of thumb states that the base load power is usually 35-40% of the maximum load during the year.Load factor of such plants is high.
Fluctuations, peaks or spikes in customer power demand are handled by smaller and more responsive types of power plants.

(ii) Peak load plants :Power plants for electricity generation which, due to their operational and economic properties, are used to cover the peak load. Gas turbines and storage and pumped storage power plants are used as peak load power plants.The efficiency of such plants is around 60 -70%.
Advantages of hydroelectric plants

• operation , running and maintenance costs are low.
• Once the dam is built, the energy is virtually free.
• No fuel is burnt and the plant is quite neat & clean.
• No waste or pollution produced.
• generating plants have a long lifetime.
• Much more reliable than wind, solar or wave power.
• Water can be stored above the dam ready to cope with peaks in demand.
• unscheduled breakdowns are relatively infrequent and short in duration since the equipment is relatively simple.
• hydroelectric turbine-generators can be started and put "on-line" very rapidly.
• Electricity can be generated constantly

Disadvantages of hydroelectric plants

• very land-use oriented and may flood large regions.
• The dams are very expensive to build.However, many dams are also used for flood control or irrigation, so building costs can be shared.
• Capital cost of generators, civil engineering works and cost of transmission lines is very high.
• Water quality and quantity downstream can be affected, which can have an impact on plant life.
• Finding a suitable site can be difficult - the impact on residents and the environment may be unacceptable.
• fish migration is restricted.
• fish health affected by water temperature change and insertion of excess nitrogen into water at spillways
• available water and its temperature may be affected
• reservoirs alter silt-flow patterns

Top 10 Rules for Saving Energy

1. DO shut off the lights when you’re done using them, and turn off the TV, computer, video games and other electrical stuff when you leave the room.






2. DO lower the thermostat during the winter. To keep warm without wasting energy, put on a sweatshirt or snuggle under a blanket.





3. DON'T leave the refrigerator door open. Every time you open the door, up to one-third of the cold air can escape.





4. DO replace a burnt-out light bulb with a new compact fluorescent bulb. Fluorescent bulbs use 75 percent less energy, and they last 10 times longer.




5. DO remind grown-ups to use cold water in the washing machine. Hot water won’t get the clothes any cleaner, and it wastes a lot of energy.





6. DO turn off dripping faucets. One drop per second can add up to 165 gallons of hot water a month - that's more than one person uses in two weeks!





7. DON’T take a long bath – take a short shower instead. It might take 25 gallons of hot water to fill the bathtub, compared to only seven gallons for a quick shower.




8. DO close the curtains during hot summer days to block the sun. During the winter, keep the curtains open.





9. Help a grown-up put plastic sheeting on windows. Blocking cold drafts is called “weatherizing” and it can save a lot of energy.





10. DO help your mom or dad plant a tree to help shade your house on hot summer days.

What Is Renewable Energy?

All the energy we use comes from the earth. The electricity we use every day doesn't come directly from the earth, but we make electricity using the earth's resources, like coal or natural gas.
Both coal and natural gas are called “fossil fuels” because they were formed deep under the earth during dinosaur times.
The problem is that fossil fuels can't be replaced - once we use them up, they're gone forever. Another problem is that fossil fuels can cause pollution.
Renewable energy is made from resources that Mother Nature will replace, like wind, water and sunshine.
Renewable energy is also called “clean energy” or “green power” because it doesn’t pollute the air or the water.

Why don’t we use renewable energy all the time?
Unlike natural gas and coal, we can’t store up wind and sunshine to use whenever we need to make more electricity. If the wind doesn’t blow or the sun hides behind clouds, there wouldn’t be enough power for everyone.
Another reason we use fossil fuels like coal and natural gas is because they’re cheaper. It costs more money to make electricity from wind, and most people aren’t willing to pay more on their monthly utility bills.

How can we use renewable energy?
You might be using renewable energy today without knowing it! Iowa is home to more than 600 wind turbines, creating enough electricity to power 140,000 homes. Wisconsin and Minnesota also have lots of wind farms – and the number is growing every day.

Hydro Power - Introduction and Types

Hydro power has played an important historical role in the industrialization of society from grinding flour to powering industry. Hydro energy originates from the sun, and hence, is renewable and its fuel is free.

“Hydro” means “water” in Latin – so “hydro power” is made from water.Hydropower is the capture of the energy of moving water for some useful purpose.The analysis of hydroelectric generation begins with the potential energy of the water. The gravitational potential energy (PE) is defined based on a material’s mass (m) and height (H) from a reference point.

PE = m.g.H

where g is gravitational constant. The power generation (P) depends upon the period (T) over which the water is discharged through that height, often times referred to as the head.

The water mass may be expressed in terms of its density (ρ) and volume (V), i.e., m=ρV.Often,the volume of water is measured in acre-feet which is the volume occupied by a foot of water covering an acre of area; one acre-foot is equivalent to 43,560 ft³. The standard density of water is 1,000 kg/m³ or 62.4 lbm/ft³. The power can then be represented in terms of the mass flow rate or volumetric flow rate
The electric power output is reduced by the hydraulic turbine-generator efficiency.
There are many forms of water power:

• Waterwheels , used for hundreds of years to power mills and machinery
• Hydroelectric energy, a term usually reserved for hydroelectric dams.
• Tidal power, which captures energy from the tides in horizontal direction
• Tidal stream power, which does the same vertically
• Wave power, which uses the energy in waves

Hydroelectric power
Hydroelectric power now supplies about 715,000 MW or 19% of world electricity (16% in 2003). Large dams are still being designed. Apart from a few countries with an abundance of it, hydro power is normally applied to peak load demand because it is readily stopped and started. Nevertheless, hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations are either already being exploited or are unavailable for other reasons, such as environmental considerations.
Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.
Hydroelectric power can be far less expensive than electricity generated from fossil fuel or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.
The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.

Tidal power
Harnessing the tides in a bay or estuary has been achieved in France (since 1966), Canada and Russia, and could be achieved in other areas with a large tidal range. The trapped water turns turbines as it is released through the tidal barrage in either direction. Another possible fault is that the system would generate electricity most efficiently in bursts every six hours (once every tide). This limits the applications of tidal energy.

Tidal stream powerA relatively new technology, tidal stream generators draw energy from currents in much the same way that wind generators do. The higher density of water means that a single generator can provide significant power. This technology is at the early stages of development and will require more research before it becomes a significant contributor.
Several prototypes have shown promise. In the UK in 2003, a 300 kW Periodflow marine current propeller type turbine was tested off the coast of Devon, and a 150 kW oscillating hydroplane device, the Stingray, was tested off the Scottish coast. Another British device, the Hydro Venturi, is to be tested in San Francisco Bay.
The Canadian company Blue Energy has plans for installing very large arrays tidal current devices mounted in what they call a 'tidal fence' in various locations around the world, based on a vertical axis turbine design.

Wave powerHarnessing power from ocean surface wave motion might yield much more energy than tides. The feasibility of this has been investigated, particularly in Scotland in the UK. Generators either coupled to floating devices or turned by air displaced by waves in a hollow concrete structure would produce electricity. Numerous technical problems have frustrated progress.
A prototype shore based wave power generator is being constructed at Port Kembla in Australia and is expected to generate up to 500 MWh annually. The Wave Energy Converter has been constructed (as of July 2005) and initial results have exceeded expectations of energy production during times of low wave energy. Wave energy is captured by an air driven generator and converted to electricity. For countries with large coastlines and rough sea conditions, the energy of waves offers the possibility of generating electricity in utility volumes. Excess power during rough seas could be used to produce hydrogen.

For more information on this topic click here Link1 Link2 Link3