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7. Energy, Power and Climate Change

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7. Energy, Power and Climate Change Chapter 7.1 Energy degradation and power generation * Active solar devices In other schemes, the pipes can be exposed directly ... – PowerPoint PPT presentation

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Title: 7. Energy, Power and Climate Change


1
7. Energy, Power and Climate Change
  • Chapter 7.1 Energy degradation and power
    generation

2
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.

3
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
4
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
5
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
6
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)
7
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

8
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 -
9
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
10
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.

11
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.

12
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.

13
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

14
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
15
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

16
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.

17
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.

18
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.

19
Nuclear reactors
20
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.

21
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/
22
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.

23
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.

24
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.

25
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.

26
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.

27
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

28
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).

29
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.

30
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.

31
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.

32
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.

33
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.

34
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.

35
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.

36
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.

37
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.

38
Photovoltaic cells
39
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.

40
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

41
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.

42
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.

43
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.

44
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.

45
The solar constant
46
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.

47
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

48
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.

50
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

51
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.

52
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.

53
Hydroelectric power
  • Advantages
  • Free
  • Inexhaustible
  • Clean
  • Disadvantages
  • Very dependent on location
  • Requires drastic changes to environment
  • Initial costs high

54
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.

55
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.

56
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

57
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

58
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.

59
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

60
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
61
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.

62
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.

63
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.
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