Topic 8: Energy, power and climate change

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Topic 8 Energy, power and climate change

• 8.4 Non-fossil Fuel Production

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Non-Fossil fuel production

• Nuclear Power
• Solar Power
• Hydroelectric Power
• Wind Power
• Wave Power

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Chain reactions
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• Chain reactions can only take place if more
  neutrons are released than were used during the
  nuclear reaction.
• Isotopes that produce an excess of neutrons in
  their fission support a chain reaction.
• This type of isotope is said to be fissionable,
• Only two main fissionable isotopes are used
  during nuclear reactions uranium-235 and
  plutonium-239.
• The minimum amount of fissionable material needed
  to ensure that a chain reaction occurs is called
  the critical mass.

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Controlled fission

• To maintain a sustained controlled nuclear
  reaction, there must be at least one neutron from
  each fission being absorbed by another
  fissionable nucleus.
• The reaction can be controlled by using control
  rods of material which absorbs neutrons.
• Control rods are commonly made of a strongly
  neutron-absorbent material such as boron or
  cadmium.

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Uncontrolled fission

• A fission reaction whereby the reaction is
  allowed to proceed without any moderation or
  control rods is called an uncontrolled fission
  reaction .
• If there are too many neutrons, the chain
  reaction would proceed at tremendous pace and
  result in an explosion.
• An example would be in an atomic bomb where the
  reactions are uncontrolled.
• In a nuclear reactor, if the fission process is
  not well controlled, the large amounts of energy
  would cause the fuel to melt and set fire to the
  reactor in what is called a meltdown.

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Fuel enrichment

• Uranium found in nature consists largely of two
  isotopes, U-235 and U-238. The production of
  energy in nuclear reactors is from the 'fission'
  or splitting of the U-235 atoms, a process which
  releases energy in the form of heat. U-235 is the
  main fissile isotope of uranium.
• Natural uranium contains 0.7 of the U-235
  isotope. The remaining 99.3 is mostly the U-238
  isotope which does not contribute directly to the
  fission process (though it does so indirectly by
  the formation of fissile isotopes of plutonium).

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• Some reactors, for example the Canadian-designed
  Candu and the British Magnox reactors, use
  natural uranium as their fuel.
• Most present day reactors (Light Water Reactors
  or LWRs) use enriched uranium where the
  proportion of the U-235 isotope has been
  increased from 0.7 to about 3 or up to 5.
• For comparison, uranium used for nuclear weapons
  would have to be enriched in plants specially
  designed to produce at least 90 U-235.

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Energy transformations in a nuclear power station
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Sankey diagrams for energy efficiency in a
nuclear power plant
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The nuclear fuel cycle
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Main stages in the nuclear fuel cycle

• Uranium recovery to extract (or mine) uranium
  ore, and concentrate (or mill) the ore
  to produce "yellowcake"
• Conversion of yellowcake into uranium
  hexafluoride (UF6)
• Enrichment to increase the concentration of
  uranium-235 (U235) in UF6
• Fuel fabrication to convert enriched UF6 into
  fuel (pellets) for nuclear reactors
• Use of the fuel in reactors (nuclear power,
  research, or naval propulsion)
• Interim storage of spent nuclear fuel
• Reprocessing of high-level waste (currently not
  done in the U.S.) 1
• Final disposition (disposal) of high-level waste

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Role of control rods

• The control rods, an important part of the
  reactor, regulate or control the speed of the
  nuclear chain reaction, by sliding up and down
  between the fuel rods or fuel assemblies in the
  reactor core. 

• The control rods contain material such as cadmium
  and boron.  Because of their atomic structure
  cadmium and boron absorb neutrons, but do not
  fission or split.

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• The temperature in the reactor core is carefully
  monitored and controlled. 
• When the core temperature goes down, the control
  rods are slowly lifted out of the core, and fewer
  neutrons are absorbed. 
• Therefore, more neutrons are available to cause
  fission.  This releases more energy and heat. 
• When the temperature in the core rises, the rods
  are slowly lowered and the energy output
  decreases because fewer neutrons are available
  for the chain reaction -- the control rods absorb
  neutrons that could otherwise hit uranium atoms
  and cause them to split. 
• To maintain a controlled nuclear chain reaction,
  the control rods are manipulated in such a way
  that each fission will result in just one
  neutron, since the other neutrons are absorbed by
  the control rods.

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Role of moderator

• In addition to the need to capture neturons, the
  neutrons often have too much kinetic energy.
• These fast neutrons are slowed through the use of
  a moderator such as heavy water and ordinary
  water.
• Some reactors use graphite as a moderator, but
  this design has several problems.
• Once the fast neutrons have been slowed, they are
  more likely to produce further nuclear fissions
  or be absorbed by the control rod.
• Java applet nuclear reaction
• http//library.thinkquest.org/17940/texts/java/Rea
  ction.html

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A nuclear reactor

• Inside the "core" where the nuclear reactions
  take place are the fuel rods and assemblies, the
  control rods, the moderator, and the coolant.
• Outside the core are the turbines, the heat
  exchanger, and part of the cooling system.

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• The job of the coolant is to absorb the heat from
  the reaction.
• The most common coolant used in nuclear power
  plants today is water.
• In actuality, in many reactor designs the coolant
  and the moderator are one and the same.
• The coolant water is heated by the nuclear
  reactions going on inside the core.
• However, this heated water does not boil because
  it is kept at an extremely intense pressure, thus
  raising its boiling point above the normal 100
  Celsius.

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Heat exchanger

• A heat exchanger is a device built for efficient
  heat transfer from one medium to another
• The heated water rises up and passes through
  another part of the reactor, the heat exchanger.
• The moderator/coolant water is radioactive, so it
  can not leave the inner reactor containment.
• Its heat must be transferred to non-radioactive
  water, which can then be sent out of the reactor
  shielding.

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• This is done through the heat exchanger, which
  works by moving the radioactive water through a
  series of pipes that are wrapped around other
  pipes.
• The metallic pipes conduct the heat from the
  moderator to the normal water.
• Then, the normal water (now in steam form and
  intensely hot) moves to the turbine, where
  electricity is produced.

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• We are not able to convert all the internal
  energy of the system into useful work but we can
  extract some useful work through heat engines.
• The temperature of the reactor is typically
  limited to 570K. Higher temperature tend to
  damage the fuel rods.
• Typically the temperature of the water returning
  to the heat exchanger is 310K
• The efficiency of the nuclear plant is about 46
• With further energy used to drive pumps and
  pollution control devices, the efficiency is
  usually reduced to 34

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Plutonium-239

• U-238 is not fissile but it is useful because it
  can be used to produced Pu-239, a fissionable
  isotope.
• First, U-238 becomes U-239 by neutron capture
• Then U-239 goes through beta decay to become
  Neptunium

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• Then Neptunium beta decays into Plutonium
• And Pu-239 is fissionable and large amounts of
  energy is released

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Plutonium-239 as a nuclear fuel

• U-238 is 140 times more abundant than U-235.
• The neutrons given off in a U-235 reaction can be
  used to breed more fuel if the non-fissionable
  U-238 is placed in a blanket around the
  control rods containing U-235.
• On average, 2.4 neutrons are produced in a U-235
  reaction with 1 neutron required for the next
  fission and 1.4 left for neutron capture by
  U-238.

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• Suppose there were 100 fissions of U-235 and 240
  neutrons are produced.
• 100 neutrons will be needed to start the next
  fission of U-235 and 140 neutrons will be
  available for neutron capture.
• Suppose that some neutrons are lost and there are
  110 neutrons available for capture by
  non-fissionable U-238.
• This means that there will be 110 fissions of
  Pu-239.
• Therefore 100 U-235 will produce 110 fissions of
  Pu-239, which is a 10 increase in fuel.

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Safety and risks of nuclear power

• Problems associated with mining of Uranium
• Problems with disposal
• Risk of thermal meltdown
• Risk of nuclear programs as means of nuclear
  weapon production

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• Biggest risk for mining of uranium is the
  exposure of miners to radon-222 gas and other
  highly radioactive products, as well as water
  containing radioactive and toxic materials
• In 1950s, a significant number of american miners
  developed small cell lung cancer due to radon
  which was the cancer causing agent.

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• The are concerns over the disposal of waste
• - Low-level (radioactive cooling water, lab
  equipment and protective clothing)
• - Intermediate level (coolant)
• - High level (fuel rods)
• The products of fission called ash include
  isotopes of strongtium, caesium and krypton which
  are highly radioactive with half lives of 30
  years or less.

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• The biggest concern is Pu-239 which has a
  half-life of approx 24,000 years.
• It is also used in nuclear warheads
• Presently the disposal methods include deep
  storage underground.
• If these methods fail, there would be
  catastrophic consequences
• Radioactive waste would find its way into the
  food chain and underground water would become
  contaminated.

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• Provided that reactors are built to standard and
  maintained properly, no obvious pollutants escape
  into the atmosphere that would contribute to the
  greenhouse effect.
• However, even with expensive cooling towers and
  cooling ponds, thermal pollution from the heat
  produced by the exchanger process could
  contribute to global warming.
• The disadvantage of possible nuclear power plant
  containment failure is always present.
• Nuclear terrorism is a threat.

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Nuclear power using nuclear fusion

• The most probable way is to fuse deuterium and
  tritium.
• Deuterium atoms can be extracted from seawater
  and tritium can be bred from lithium.

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Nuclear power using nuclear fusion?

• The basic problems in attaining useful nuclear
  fusion conditions are
• to heat the gas to these very high temperatures
  and
• to confine a sufficient quantity of the reacting
  nuclei for a long enough time to permit the
  release of more energy than is needed to heat and
  confine the gas.
• the capture of this energy and its conversion to
  electricity.

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Nuclear power using nuclear fusion?

• Nuclear fusion was first achieved on earth in the
  early 1930s by bombarding a target containing
  deuterium, the mass-2 isotope of hydrogen, with
  high-energy deuterons in a cyclotron (Particle
  accelerator).
• To accelerate the deuteron beam a great deal of
  energy is required, most of which appeared as
  heat in the target.
• As a result, no net useful energy was produced.
• In the 1950s the first large-scale but
  uncontrolled release of fusion energy was
  demonstrated in the tests of thermonuclear
  weapons by the United States, the USSR, the
  United Kingdom, and France.
• This was such a brief and uncontrolled release
  that it could not be used for the production of
  electric power

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The problem with fusion is the sheer difficulty
of achieving the act.

• Why the very high temperatures?
• Atoms have a very strong repulsive force and it
  takes high temperatures and enormous amounts of
  energy to bring them close enough together to
  fuse.
• And this must be maintained for long periods to
  produce electricity.
• We have been researching fusion for over four
  decades and spent many millions of dollars,
  pounds and euros.
• It is possible that more money and time could
  produce successful fusion in another decade or
  so, but it may never be achievable.
• It might be wiser to spend that time and money on
  something which we know will succeed such as
  renewables.

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Why containment?

• At temperatures of 100,000 C, all the hydrogen
  atoms are fully ionized.
• The gas consists of an electrically neutral
  assemblage of positively charged nuclei and
  negatively charged free electrons.
• This state of matter is called a plasma.
• A plasma hot enough for fusion cannot be
  contained by ordinary materials.
• The plasma would cool very rapidly, and the
  vessel walls would be destroyed by the extreme
  heat.
• However, since the plasma consists of charged
  nuclei and electrons, which move in tight spirals
  around the lines of force of strong magnetic
  fields,
• the plasma can be contained in a properly shaped
  magnetic field region without reacting with
  material walls.

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Why is high temp maintained?

• Because fusion is not a chain reaction, these
  temperature and density conditions have to be
  maintained for future fusion to occur.

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• If fusion energy does become practical, it offers
  the following advantages
• a limitless source of fuel, deuterium from the
  ocean
• no possibility of a reactor accident, as the
  amount of fuel in the system is very small and
• waste products much less radioactive and simpler
  to handle than those from fission systems.

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Photovoltaic cells

• Photovoltaic devices make use of the
  photoelectric effect.
• Solar photovoltaic modules use solar cells to
  convert light from the sun into electricity.

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Solar heating panels

• Solar thermal panels contain liquid that
  circulates through special panels and is heated
  by sunlight, this then passes through a coil in
  the water tank which in turn heats the water
  stored in the tank

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What are the factors that would affect the amount
of solar radiation that a place gets?
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The main factors are

• Geographic location
• Time of day (altitude of the sun from the sky)
• Season
• Local landscape
• Local weather
• The distance of earth from the sun

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• Because the Earth is round, the sun strikes the
  surface at different angles ranging from 0º (just
  above the horizon) to 90º (directly overhead).
• When the sun's rays are vertical, the Earth's
  surface gets all the energy possible.
• The more slanted the sun's rays are, the longer
  they travel through the atmosphere, becoming more
  scattered and diffuse.
• Because the Earth is round, the frigid polar
  regions never get a high sun, and because of the
  tilted axis of rotation, these areas receive no
  sun at all during part of the year

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• The Earth revolves around the sun in an
  elliptical orbit and is closer to the sun during
  part of the year.
• When the sun is nearer the Earth, the Earth's
  surface receives a little more solar energy.
• The Earth is nearer the sun when it's summer in
  the southern hemisphere and winter in the
  northern hemisphere.
• However the presence of vast oceans moderates the
  hotter summers and colder winters one would
  expect to see in the southern hemisphere as a
  result of this difference.

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• The 23.5º tilt in the Earth's axis of rotation is
  a more significant factor in determining the
  amount of sunlight striking the Earth at a
  particular location.
• Tilting results in longer days in the northern
  hemisphere from the spring (vernal) equinox to
  the fall (autumnal) equinox and longer days in
  the southern hemisphere during the other six
  months.
• Days and nights are both exactly 12 hours long on
  the equinoxes, which occur each year on or around
  March 23 and September 22.

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• Countries like the United States, which lie in
  the middle latitudes, receive more solar energy
  in the summer not only because days are longer,
• but also because the sun is nearly overhead.
• The sun's rays are far more slanted during the
  shorter days of the winter months. Cities like
  Denver, Colorado, (near 40º latitude) receive
  nearly three times more solar energy in June than
  they do in December

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• The rotation of the Earth is responsible for
  hourly variations in sunlight.
• In the early morning and late afternoon, the sun
  is low in the sky. Its rays travel further
  through the atmosphere than at noon when the sun
  is at its highest point.
• On a clear day, the greatest amount of solar
  energy reaches a solar collector around solar
  noon

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3 main schemes

• Water storage in lakes
• Tidal water storage
• Pump storage

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Water storage in lakes
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Water storage in lakes

• The Three Gorges Dam on the Yangtze River will be
  the largest hydroelectric dam in the world when
  it is complete in 2009.
• It will generate 18200MW
• The dam is more than 2 km wide and has a height
  of 185m.
• Its reservoir will stretch over 600km upstream
  and force the displacement of more than
  1.3million people.

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Tidal water storage

• Have been built in Russia and France and in
  developmental stage in other countries
• Source of energy is the kinetic energy of the
  earths rotation.
• Coastal estuaries that have a large vertical
  range in tides are potential sites for tidal
  power stations
• The station in France has a tidal range of 8.4m
  and generates 10MW of electrical energy for each
  of the 24 turbines.

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Tidal water storage

• A dam is built to catch the high tide.
• A sluice gate is opened to let the high tide
  water in
• The water is released at low tide, and the
  gravitational potential energy is used to drive
  turbines which produce electrical energy

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Pumped storage
Generating Mode
Pumping Mode

• Used in off-peak electricity demand period
• Water is pumped from low reservoir to high
  reservoir

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Energy transformations

• Water trapped in reservoirs have gravitational
  potential energy
• Water falls through a series of pipes where its
  potential energy gets converted to rotational
  kinetic energy that drives a series of turbines
• The rotating turbines drive generators that
  convert the kinetic energy into electrical energy
  by electromagnetic induction.

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• Installed wind power capacity Ranking
• Germany
• US
• Spain
• India
• China
• Denmark

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• Check out
• http//www.world-wind-energy.info/

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Basic features

1. Foundation
2. Tower
3. Nacelle
4. Rotor blades
5. Hub
6. Transformer (not part of wind turbine)

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1) Foundation and 2) Tower

• Guarantee the stability of a wind turbine a pile
  or flat foundation is used, depending on the
  consistency of the underlying ground.
• The tower carry the weight of the nacelle and the
  rotor blades, AND must also absorb the huge
  static loads caused by the varying power of the
  wind.
• Generally, a tubular construction of concrete or
  steel is used. An alternative to this is the
  lattice tower form.

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3) Nacelle and 5) Hub

• The nacelle holds all the turbine machinery.
• Because it must be able to rotate to follow the
  wind direction, it is connected to the tower via
  bearings.
• The build-up of the nacelle shows how the
  manufacturer has decided to position the drive
  train components (rotor shaft with bearings,
  transmission, generator, coupling and brake)
  above this machine bearing.

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4) Rotor and rotor blades

• The rotor is the component which, with the help
  of the rotor blades, converts the energy in the
  wind into rotary mechanical movement.
• Currently, the three-blade, horizontal axis rotor
  dominates. The rotor blades are mainly made of
  glass-fibre or carbon-fibre reinforced plastics
  (GRP, CFRP).
• The blade profile is similar to that of an
  aeroplane wing. They use the same principle of
  lift on the lower side of the wing the passing
  air generates higher pressure, while the upper
  side generates a pull.
• These forces cause the rotor to move to rotate.

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FYI

• Significant areas of the world have mean annual
  windspeeds of above 4-5 m/s (metres per second)
  which makes small-scale wind powered electricity
  generation an attractive option.
• It is important to obtain accurate windspeed data
  for the site in mind before any decision can be
  made as to its suitability

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Power calculation

• The power in the wind is proportional to
• the area of windmill being swept by the wind
• the cube of the wind speed
• the air density - which varies with altitude

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Formula

• P 0.5?Av³
• Where
• P is power in watts (W)
• ? is the air density in kilograms per cubic
  metre (kg/m3), (about 1.225 kg/m3 at sea level,
  less higher up)
• A is the swept rotor area in square metres (m2)
• V is the windspeed in metres per second (m/s).

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• The actual power that we can extract from the
  wind is significantly less than what the previous
  formula suggests. The actual power will depend
  on several factors, such as
• the type of machine and rotor used,
• the sophistication of blade design,
• friction losses, and
• the losses in the pump or other equipment
  connected to the wind machine.

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• There are also physical limits to the amount of
  power that can be extracted realistically from
  the wind.
• It can been shown theoretically that any windmill
  can only possibly extract a maximum of 59.3 of
  the power from the wind (this is known as the
  Betz limit).
• In reality, this figure is usually around 45
  (maximum) for a large electricity producing
  turbine and around 30 to 40 for a windpump.

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• Modifying the formula for Power in the wind we
  can say that the power which is produced by the
  wind machine can be given by
• Pm 0.5 Cp ? AV³
• Where
• Pm is power (in watts) available from the
  machine
• Cp is the coefficient of performance of the wind
  machine (power efficiency)
• rho is the air density in kilograms per cubic
  metre (kg/m3), (about 1.225 kg/m3 at sea level,
  less higher up)
• A is the swept rotor area in square metres (m2)
• V is the windspeed in metres per second (m/s).

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Wave Power

• Describe the principle of operation of an
  oscillating water column (OWC) ocean-wave energy
  converter
• Determine the power per unit length of a
  wavefront, assuming a rectangular profile for the
  wave.
• Solve problems involving wave power.

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• Simple animation of OWC
• http//www.daedalus.gr/DAEI/PRODUCTS/RET/General/O
  WC/OWCsimulation2.htm
• Offshore OWC Onshore OWC

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• As the wave enters a capture chamber, the air
  inside the chamber is compressed
• and the high velocity air provides the kinetic
  energy needed to drive a turbine connected to a
  generator.
• As the captured water level drops, there is a
  rapid decompression of the air in the chamber
  which
• again turns the turbine that has been specially
  designed with a special valve system which turns
  in the same direction regardless of the direction
  of the air flowing across the turbine blades.

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• http//www.darvill.clara.net/altenerg/wave.htm
• http//www.alternative-energy-news.info/technology
  /hydro/wave-power/

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Energy

• Potential energy of the wave over one period
• Ep 0.25 w?gA²?
• Kinetic energy of the wave over one period
• Ek 0.25 ?wgA²?
• Total energy over one period
• ET 0.5 w?gA²?

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Power

• Power generated (work/time)
• P 0.5 w?gA²?/T
• Power per wavelength 0.5 w?gA²f
• Power per meter 0.5 w?gA²v
• where v is the speed of the wave
• The density of seawater at the surface of the
  ocean varies from 1020 to 1029kgm-3.

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• ? Water density
• W wave width, assumed to be the width of the
  chamber
• A wave amplitude
• T wave period
• ? wavelength