# 7. Energy, Power and Climate Change - PowerPoint PPT Presentation

PPT – 7. Energy, Power and Climate Change PowerPoint presentation | free to download - id: 7b4fcf-ZWI2Y

The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
Title:

## 7. Energy, Power and Climate Change

Description:

### 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

Number of Views:53
Avg rating:3.0/5.0
Slides: 64
Provided by: MikeSp150
Category:
Tags:
Transcript and Presenter's Notes

Title: 7. Energy, Power and Climate Change

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

2
• 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
• 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
• 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.

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

27
Nuclear Energy
• High power output
• Large reserves of nuclear fuels
• Nuclear power stations do not produce greenhouse
gases
• 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

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
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
• Free
• Inexhaustible
• Clean
• 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
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
• Free
• Inexhaustible
• Clean
• 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
• Free
• Inexhaustible
• Clean
• Ideal for remote island locations
• 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
• Free
• Inexhaustible
• Clean
• Reasonable energy density