7. Energy, Power and Climate Change

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7. Energy, Power and Climate Change

• Chapter 7.1 Energy degradation and power
  generation

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Degradation of energy

• Energy flows from hot bodies to cold bodies.
• The difference in temperature between two bodies
  can make a heat engine work, allowing useful
  mechanical work to be extracted in the process.

Some of the thermal energy transferred from the
hot to the cold body can be transformed into
mechanical work.

• Energy flow between two bodies is represented by
  a Sankey Diagram.

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Sankey Diagram

• The width of each arrow in the diagram is
  proportional to the energy carried by that arrow.
• Knowing the useful mechanical work done and the
  energy input we can calculate the machines
  efficiency

hot reservoir
800J
200J
600J
cold reservoir
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Degradation of energy

• As energy flows from the hot to the cold body,
  they will eventually reach the same temperature
  and the opportunity to do work will be lost.
• Heat engines are machines that use the heat
  transfer to do useful mechanical work.
• Any practical heat engine works in a cycle as the
  process must be repeated.

thermal energy absorbed
gas expands doing mechanical work
gas returned to its initial state, so that the
cycle can be repeated some thermal energy is
released from the engine
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Degradation of energy

• The problem with machines that operate in a cycle
  is that not all of the thermal energy transferred
  can be transformed into mechanical work.
• Some energy goes to the cold reservoir.
• Unless there is a colder reservoir, that energy
  cannot be used.
• This energy had become degraded.

Energy, while always being conserved, becomes
less useful, i.e., it cannot be used to perform
mechanical work this is called energy
degradation
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Electricity production

• Electricity is produced using electric generators
  by rotating a coil in a magnetic field so that
  magnetic field lines are cut by the moving coil.
• According to Faradays law an emf (voltage) will
  be created in the coil which can then be
  delivered to consumers.
• So, generators convert mechanical energy into
  electrical energy.

fossil fuels nuclear power reactors wind
energy hydroelectric turbines wave power
kinetic energy of rotation
electrical energy
generator
solar energy (photovoltaic cells)
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Energy sources

• Non-renewable sources are finite sources, which
  are being depleted, and will run out. They
  include fossil fuels (oil, natural gas and coal),
  and nuclear fuels (uranium). The energy stored in
  these sources is, in general, a form of potential
  energy, which can be released by human action.
• Renewable sources include solar energy, other
  forms indirectly dependent on solar energy (wind
  and wave energy) and tidal energy

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Energy sources

• Today, the main energy sources are those that
  rely on fossil fuels and emit large amounts of
  carbon dioxide. The world average energy
  production is give in the table below.

Fuel of total energy produced CO2 emission (g MJ-1)
Oil 40 70
Natural gas 23 50
Coal 23 90
Nuclear 7 -
Hydroelectric 7 -
Others lt1 -
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Energy density

• Energy density is the energy that can be obtained
  from a unit mass of the fuel. It is measured in J
  kg-1.
• If the energy is obtained by burning fuels, the
  energy density is simple the heat of combustion.

Substance Heat of combustion
Coal 30 MJ kg-1
Wood 16 MJ kg-1
Diesel oil 45 MJ kg-1
Gasoline 47 MJ kg-1
Kerosene 46 MJ kg-1
Natural gas 39 MJ m-3
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Energy density

• In a nuclear fission reaction, mass is converted
  directly into energy through Einsteins formula
  Emc2.
• For instance, 1kg of pure uranium-235 releases
  about 7x104 GJ. Natural uranium produces about
  490 GJ/kg and enriched uranium about 2100 GJ/kg.
• In a hydroelectric power station, considering
  that the water falls from a height of 100m the
  kinetic energy gained by 1kg of the water is 103
  J.
• This implies that the energy density of water
  used as fuel is much less than the energy
  density of fossil fuels.

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Fossil fuels

• Fossil fuels have been created over millions of
  years.
• They are produced by the decomposition of buried
  animals and plant matter under the combined
  action of the high pressure of the material on
  top and bacteria.
• Thermal energy produced when burning these fuels
  is used to power steam engines.
• Although these engines are generally efficient
  (30-40) they are also responsible for
  atmospheric pollution and contribute greenhouse
  gases to the atmosphere.

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Fossil fuel mining

• Coal is obtained by mining. This process releases
  a large number of toxic substances and the coal
  itself is high in sulphur content and traces of
  heavy metals.
• Rain can wash away these substances and cause
  environmental problems if this acidic water
  enters underground water reserves.
• Drilling for oil has also adverse environmental
  effects, with many accidents leading to leakage
  of oil both at sea and on land.

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Fossil fuel mining

• Advantages
• Relatively cheap (while they last)
• High power output (high energy density)
• Variety of engines and devices use them directly
  and easily
• Extensive distribution network is in place
• Disadvantages
• Will run out
• Pollute the environment
• Contribute to greenhouse effect by releasing
  greenhouse gases into atmosphere
• High cost of distribution due to high mass and
  volume of materials and high cost of storing
  (needs extensive storage facilities)
• Pose serious environmental problems due to
  leakages at various points along the production

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Nuclear power

• Nuclear fission is the process in which a heavy
  nucleus splits into lighter nuclei.
• When uranium-235 absorbs a neutron, it turns into
  uranium-236 which will decay into krypton and
  barium and will release another 3 neutrons
• In a nuclear fission reaction, mass is converted
  directly into energy through Einsteins formula
  Emc2.

Energy
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Nuclear reactors

• A nuclear reactor is a machine in which nuclear
  reactions take place, producing energy.
• The fuel of a nuclear reactor is typically
  uranium-235. The isotope of uranium that is most
  abundant is uranium-238. Natural uranium contains
  only about 0.7 of uranium-235.
• The uranium fuel in a reactor is made to contain
  about 3 of uranium-235 enriched uranium.
• When uranium-235 captures a neutron, two process
  can occur

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Nuclear reactors

• The are examples of induced fission.
• The fission does not proceed by itself neutrons
  must initiate it.
• The neutrons produced can used to collide with
  other uranium-235 nuclei in the reactor,
  producing more fission, more energy and more
  neutrons.
• The reaction is thus self-sustaining and called a
  chain reaction.
• For the chain reaction to get going a certain
  minimum mass of uranium-235 must be present,
  otherwise neutrons would escape without causing
  further reactions. This minimum mass is called
  critical mass.

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Nuclear reactors

• Uranium-235 will only capture neutrons if they
  are not too fast. The neutrons produced in the
  chain reaction are too fast to be captured and
  have to be slowed down (they have to go from 1MeV
  of kinetic energy to less than 1 eV).
• The slowing down of neutrons is achieved through
  collisions of the neutrons with atoms of the
  moderator, a material surrounding the fuel rods
  (tubes containing U-235). The moderator can be
  graphite or water, for example.
• The rate of reaction is determined by the number
  of neutrons available to be captured by U-235.
• To few neutrons would result in the reaction
  stopping
• Too many neutrons would lead to an
  uncontrollably large release of energy.

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Nuclear reactors

• Thus, the control rods (the material that can
  absorb excess neutrons whenever necessary) are
  introduced in the moderator.
• The control rods can be removed when not needed
  and reinserted when necessary again.
• The control rods ensure that the energy from
  nuclear reactions is released in a slow and
  controlled way as opposed to the uncontrollable
  release of energy that would take place in a
  nuclear weapon.

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Nuclear reactors
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Nuclear reactors

• In a Pressurized Water Reactor (PWR) water is
  kept under pressure to keep it from boiling, even
  at 300 C.
• The pressurized water is pumped through a closed
  system of pipes called the primary circuit.
• Heat from the primary circuit warms up water in
  the secondary circuit.
• The water in the secondary circuit comes to a
  boil and its steam turns the turbine.
• The water in the primary circuit returns to the
  reactor core after giving up some of its heat.

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Nuclear reactors

• Gas Cooled Reactors (GCR) use carbon dioxide as
  the coolant to carry the heat to the turbine, and
  graphite as the moderator.
• Like heavy water, a graphite moderator allows
  natural uranium (GCR) or slightly enriched
  uranium (AGR) to be used as fuel.

http//www.cameco.com/uranium_101/uranium_science/
nuclear_reactors/
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Nuclear reactors

• The energy released in the reaction is the form
  of kinetic energy of the produced neutrons (and
  gamma ray photons).
• This kinetic energy is converted into thermal
  energy (in the moderator) as the neutrons are
  slowed down by collisions with the moderator
  atoms.
• A coolant (e.g. water or liquid sodium) passing
  through the moderator can extract this energy,
  and use it in a heat exchanger to turn water into
  steam at high temperature and pressure.
• The steam can be used to turn the turbines of a
  power station, finally producing electricity.

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Plutonium production

• The fast neutrons produced in a fission reaction
  may be used to bombard U-238 and produce
  plutonium-239.
• This isotope of plutonium does not occur
  naturally.
• The reactions are

• The importance of these reactions is that
  non-fissionable material (U-238) is being
  converted to fissionable material (Pu-239)
  than can be used as the nuclear fuel in other
  reactors.

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Problems with nuclear reactors

• Both fuel and products of the reactions are
  highly radioactive, with long half-lives.
  Disposal of nuclear waste is a serious
  disadvantage of the fission process in commercial
  energy production.
• This material is currently buried deep
  underground in containers that are supposed to
  avoid leakage to the outside.
• Another problem is the possibility of accidents
  due to uncontrolled heating of the moderator.
• Such heating would increase T and hence the
  pressure in the cooling pipes, resulting in a
  explosion.
• This would lead to the leakage of radioactive
  material into the environment or, even worse,
  could lead to the meltdown of the entire core.

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Problems with nuclear reactors

• The positive aspect is that nuclear power does
  not produce larges amounts of greenhouse gases.
• The nuclei produced in a fission reaction are
  typically unstable and usually decay by beta
  decay.
• This decay produces an additional amount of
  energy.
• Even if the nuclear reactor shuts down,
  production of thermal energy continues because of
  the beta decay of the product nuclei.
• The energy produced in this way is enough to melt
  the entire core of the reactor if the cooling
  system breaks down.
• Another worry is that the fissionable material
  produced can be recovered and be used in a
  nuclear weapons programme.

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Uranium mining

• Like all types of mining uranium mining is
  dangerous.
• Uranium produces radon gas, a known strong
  carcinogen as it is an alpha emitter.
• Inhalation of this gas as well as of radioactive
  dust particles is a major hazard in the uranium
  mining industry.
• Mine shafts require good ventilation and must be
  closed to avoid direct contact with the
  atmosphere.
• The disposal of waste material from the mining
  processes is also a problem since the material is
  radioactive.

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Nuclear Energy

• Advantages
• High power output
• Large reserves of nuclear fuels
• Nuclear power stations do not produce greenhouse
  gases
• Disadvantages
• Radioactive waste products difficult to dispose
  of
• Major public health hazard should something go
  wrong
• Problems associated with uranium mining
• Possibility of producing materials for nuclear
  weapons

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Nuclear fusion

• A typical energy-producing nuclear fusion
  reaction is

• Deuterium (D) can be extracted from water using
  electrolysis and tritium (T) can be produced by
  bombarding lithium with neutrons.
• The problem with fusion is that, since D and T
  are both positively charged, the reacting nuclei
  repel.
• To get them close enough to each other for the
  reaction to take place, high temperatures must be
  reached around 108 K.
• At this temperature, hydrogen atoms are ionized
  and so we have a plasma (mixture of positive
  nuclei and electrons).

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Nuclear fusion

• The hot plasma must be confined in such way so
  that it doesnt come into contact with anything
  else as this would cause
• a reduction in temperature
• contamination of the plasma with other
  materials.
• These two effects would cause the fusion reaction
  to stop.
• The plasma is therefore confined magnetically in
  a tokamak machine (toroidal magnetic chamber).
• The magnetic field prevents the plasma from
  touching the container walls.

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Nuclear fusion

• Energy must be supplied to the fusion process to
  reach the high temperatures required.
• It has not yet been possible to produce more
  energy out of fusion that has first been put in,
  for sustained periods of time.
• For this reason, fusion as a source of
  commercially produced energy is not yet feasible.
• There are also technical problems with using the
  energy produced in fusion to produce
  electricity..
• Compared to nuclear fission, nuclear fusion has
  the advantage of plentiful fuels, substantial
  amount of energy produced and much fewer problems
  with radioactive waste.

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Solar power

• The Sun produces energy at a rate of about
  3.9x1026 W.
• This means that, on average, the Earth receives
  about 1400W per square metre of the surface of
  the outer atmosphere.

• Some of this radiation is reflected back into
  space, some is trapped by the atmospheres gases
  and about 1kW m-2 is received on the surface of
  the Earth.
• This amount assumes direct sunlight on a clear
  day and thus is the maximum that can be received
  at any one time.
• Averaged over a 24-hour time period, the
  intensity of sunlight is about 340 W m-2.
• This high-quality, free and inexhaustible energy
  can be put to various uses.

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Active solar devices

• The sunlight is used directly to heat water or
  air for heating in a house, for example.
• The surface is usually flat and covered by glass
  for protection the glass should be coated to
  reduce reflection.

• A blackened surface below the glass collects
  sunlight, and water circulating in pipes
  underneath gets heated.

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Active solar devices

• This hot water can then be used for household
  purposes, such as in bathrooms (the heated water
  is kept in well-insulated containers).
• Another possibility is to make the hot water,
  with the help of a pump, circulate through a
  house, providing a heating effect.

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Active solar devices

• In other schemes, the pipes can be exposed
  directly to sunlight, in which case they are
  blackened to increase absorption.
• The surface underneath the pipes is reflecting so
  that more radiation enters the pipes.
• Such a collector works not only with direct
  sunlight but also with diffuse light like in
  cloudy days.

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Active solar devices

• These simple collectors are cheap and are usually
  put on the roof of a house. Their disadvantage is
  that they tend to be bulky and cover too much
  space.
• More sophisticated collectors include a
  concentrator system in which the incoming solar
  radiation is focused, for example by a parabolic
  mirror, before it falls on the collecting
  surface.
• Such systems can heat water to much higher
  temperatures (500ºC to 2000ºC) than a simple flat
  collector.

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Active solar devices

• These high temperatures can be used to turn water
  into steam, which can drive a turbine, producing
  electricity.
• Obviously, back-up systems must be available in
  case of cloudy days.

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Photovoltaic cells

• The photovoltaic cell was developed in 1954 at
  Bell Laboratories for the use in the space
  programme to power satellites and probes sent to
  outer space.
• A photovoltaic cell coverts sunlight into DC
  current at an efficiency of about 30
• Although it was initially very expensive
  technology, currently the energy cost using
  photovoltaic cells is slightly higher than that
  produced by diesel-powered generators.

• The principle inherent to the working of a
  photovoltaic cell lies on the physics of
  semiconductors and must not be mistaken with the
  photoelectric effect.

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Photovoltaic cells
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Photovoltaic cells

• The price drop of this technology makes it more
  likely to become more dominant in electricity
  production around the world.
• Already, in places far from major power grids,
  its use is more economical than grid expansion.
• It can be used to power small remote villages,
  pump water in agriculture, power warning lights,
  etc.
• Their environmental ill effects are practically
  zero, with the exception on chemical pollution at
  the place of their manufacture.

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Photovoltaic cells

• Advantages
• Free
• Inexhaustible
• Clean
• Disadvantages
• Works during the day only
• Affected by cloudy weather
• Low power output
• Requires large areas
• Initial cost high

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The solar constant

• The suns total power output, also know as
  luminosity, is
• P 3.9 x 1026 W
• On Earth, we receive only a very small fraction
  of this total power output.
• The average distance between the sun and the
  earth is r1.5x1011m.
• The suns power is distributed uniformly over the
  surface of an imaginary sphere of radius
  r1.5x1011m.

• The power that is collected by area A is the
  fraction

• Note that 4?r2 is the surface area of the
  imaginary sphere.

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The solar constant

• The power per unit area received at a distance r
  from the sun is called the intensity, I, and so

• This amounts to about 1400 W/m2 and is known as
  the solar constant.

• Its the power received by one square metre
  placed normally to the path of the incoming rays
  a distance 1.5x1011 m from the sun.

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The solar constant

• This amount varies as the suns output is not
  constant (variation of 1.5).
• Also, Earth does not keep a constant distance
  from the sun as the orbit is elliptical
  (additional variation of 0.4).
• To find the radiation received on Earths
  surface, we must take into account the reflection
  of the radiation from the atmosphere and the
  earths surface itself, latitude, angle of
  incidence and average between day and night.
• It is useful to define the total amount of energy
  received by one square metre of the earths
  surface in the course of one day.
• This is called the daily insolation.

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The solar constant

• The reduction of the daily insolation in the
  winter for high latitudes can be explained by the
  shorter length of daylight and the oblique
  incidence of light.

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The solar constant
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Hydroelectric power

• Hydropower, the power derived from moving water
  masses, is one of the oldest and most established
  of all renewable energy sources
• Although dependent of sites, its capable of
  producing cheap electricity.
• Turbines driven by falling water have a long
  working life without major maintenance costs.
• It has high initial costs but its widely used
  all over the world.

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Hydroelectric power

• Hydroelectric power stations are, however,
  associated with massive changes in the ecology of
  the area surrounding the plants.
• To create a reservoir behind a newly constructed
  dam, a vast area of land must be flooded

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Hydroelectric power
49
Hydroelectric power

• The principle behind hydropower is very simple.
• Consider a mass m of water that falls down a
  vertical height h. The potential energy of the
  mass is mgh, and it gets converted into kinetic
  energy when the mass descends the vertical
  distance h.
• The mass is given by ???V, where ? is the waters
  density (1000kg/m3) and ??V is the volume it
  occupies.

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Hydroelectric power

• The rate of change of this potential energy, that
  is, the power P, is given by the change in
  potential energy divided by the time taken for
  that change, so

• The quantity Q ?V/?t is known as the volume flow
  (volume per second) and so

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Hydroelectric power

• Within a time equal to ?t, the mass of water that
  will flow through the tube is
• m??V ?Qgh

?
?V

• This is the power available for generating
  electricity and its clear that hydropower
  requires large volume flow rates, Q, and large
  heights, h.

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Hydroelectric power

• A different number of schemes are available for
  extracting the power of water.

1. Water can be stored in a lake, which should be at
   as high as elevation as possible to allow for
   energy release when the water is allowed to flow
   to lower heights.
2. In a pump storage system, the water the water
   that flows to lower heights is pumped back to its
   original height using the excess of electricity
   from somewhere else. This is the only way to
   store energy on a large scale for use when demand
   is high.
3. Finally, tidal storage systems take advantage of
   tides the flow of water during a tide turns
   turbines, producing electricity.

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Hydroelectric power

• Advantages
• Free
• Inexhaustible
• Clean
• Disadvantages
• Very dependent on location
• Requires drastic changes to environment
• Initial costs high

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Wind power

• Wind power devices have no adverse effects,
  although there is some evidence that
  low-frequency sound emitted during the operation
  of wind turbines affects peoples sleeping).
• However, a very large number of them is not an
  attractive sight to many people, and there is a
  noise problem.

• The blades are susceptible to stresses in high
  winds, and damage due to metal fatigue frequently
  occurs.
• Generally, about 25 of the power of carried by
  the wind can be converted into electricity.

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Wind power

• Wind speed is the crucial factor for these
  systems, the power extracted being proportional
  to the cube of the wind speed.
• Wind speed of up to about 4m/s are not
  particularly useful for energy extraction.
• Serious power production from winds occurs at
  speeds from 6 to 14 m/s.
• The dependence of the power on the area of the
  blades and the cube of the wind speed can be
  easily understood.

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Wind power

• Consider the mass of air that can pass through a
  tube of cross-sectional area A with velocity v in
  time ?t. Let ? be the density of air.
• Then the mass enclosed in a tube of length v?t is
  ?Av?t.

• This is the mass that will exit the right end of
  the tube within a time interval equal to ?t.

• The kinetic energy of this mass of air is thus

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Wind power

• The kinetic energy per unit time is the power,
  and so dividing by ?t we find

• Which shows that the power carried by the wind is
  proportional to the cube of the wind speed and
  proportional to the area spanned by the blades.
• The power extracted by the turbine is
• Where Cp is know as the power coefficient. It is
  simply an efficiency factor that determines how
  much of the available wind power the wind turbine
  can extract. Theoretically, it varies between
  0.35 and 0.45

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Wind power

• In practice, frictional and other losses (mainly
  turbulence) result in a smaller power increase.
• The previous calculations also assume that all
  the wind is actually stopped by the wind turbine,
  extracting all of the winds kinetic energy,
  which in practice is not the case.

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Wind power

• Advantages
• Free
• Inexhaustible
• Clean
• Ideal for remote island locations
• Disadvantages
• Works only if there is wind not dependable
• Low power output
• Aesthetically unpleasant (and noisy)
• Best locations far from large cities
• Maintenance costs high

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Wave power

• It has been realised that deep-water, long
  wavelength sea waves carry a lot of energy.
• Water waves are very complex and belong to a
  class of waves called dispersive, i.e., the speed
  of the wave depends on the wavelength.

• A water wave of amplitude A carries an amount of
  power per unit length of its wavefront equal to

Where ? is the density of water and v stands for
the speed of energy transfer in the wave
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Wave power

• Many devices have been proposed to extract the
  power out of waves.
• The one discussed here is the oscillating water
  column (OMC).
• As a crest of the wave approaches the cavity in
  the device, the column of water in the cavity
  rises and pushes the air above it upwards. The
  air then turns a turbine.
• As a trough of the wave approaches the cavity,
  the water in the cavity falls and the air drawn
  turns the turbine again.

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Wave power

• Wave patterns are irregular in wave speed,
  amplitude and direction.
• This makes it difficult to achieve reasonable
  efficiency of wave devices over all the
  variables.
• For many wave devices, it is difficult to couple
  the low frequency of the waves (typically 0.1 Hz)
  to the much higher generator frequencies (50-60
  Hz) required for electricity production.
• The OWC solves this problem by changing the speed
  of the air by adjusting the diameter of the
  valves through which the air passes.
• In this way, very high speeds can be attained,
  thus coupling the low-frequency water waves with
  the high-frequency turbine motion.

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Wave power

• Advantages
• Free
• Inexhaustible
• Clean
• Reasonable energy density
• Disadvantages
• Works only in areas with large waves
• Irregular wave patterns make it difficult to
  achieve reasonable efficiency
• Difficult to couple low-frequency water waves
  with high-frequency turbine motion
• Maintenance and installation costs high
• Transporting the produced power to consumers
  involve high costs
• Devices must be able to withstand hurricane and
  gale-force storms.