Topic 8: Energy, power
and climate change
8.4 Non-fossil Fuel Production
Non-Fossil fuel production
•Nuclear Power
•Solar Power
•Hydroelectric Power
•Wind Power
•Wave Power
Chain reactions
• 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.
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 neutronabsorbent material such as boron or cadmium.
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.
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 U235 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).
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 U235 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.
Energy transformations in a nuclear
power station
Sankey diagrams for energy
efficiency in a nuclear power plant
The nuclear fuel cycle
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
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.
• 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.
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/Reaction.ht
ml
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.
• 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.
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.
• 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.
• 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%
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
Then Neptunium beta decays into Plutonium
And Pu-239 is fissionable and large amounts of energy is
released
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.
• Suppose there were 100 fissions of U-235 and 240
neutrons are produced.
• 100 neutrons will be needed to start the next fission of U235 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.
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
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.
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.
The biggest concern is Pu-239 which has a halflife 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.
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.
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.
Nuclear power using nuclear
fusion?
The basic problems in attaining useful nuclear
fusion conditions are
(1)to heat the gas to these very high temperatures
and
(2)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.
(3)the capture of this energy and its conversion to
electricity.
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
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.
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.
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.
If fusion energy does become practical, it offers
the following advantages:
(1)a limitless source of fuel, deuterium from the
ocean;
(2)no possibility of a reactor accident, as the
amount of fuel in the system is very small; and
(3)waste products much less radioactive and
simpler to handle than those from fission
systems.
Photovoltaic cells
• Photovoltaic devices make use of the
photoelectric effect.
• Solar photovoltaic modules use solar cells to
convert light from the sun into electricity.
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
What are the factors that would
affect the amount of solar
radiation that a place gets?
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
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
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.
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.
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
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
3 main schemes
• Water storage in lakes
• Tidal water storage
• Pump storage
Water storage in lakes
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.
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 earth’s
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.
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
Pumped storage
Generating Mode
Pumping Mode
Used in off-peak electricity demand period
Water is pumped from low reservoir to high
reservoir
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.
Installed wind power
capacity Ranking
1) Germany
2) US
3) Spain
4) India
5) China
6) Denmark
Check out:
http://www.world-wind-energy.info/
Basic features
1) Foundation
2) Tower
3) Nacelle
4) Rotor blades
5) Hub
6) Transformer (not part of
wind turbine)
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.
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.
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.
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
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
Formula
P = 0.5ρAv³
Where
P: is power in watts (W)
ρ: 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).
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.
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.
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 ρ 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).
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.
Simple animation of OWC:
http://www.daedalus.gr/DAEI/PRODUCTS/RET/General/O
WC/OWCsimulation2.htm
Offshore OWC
Onshore OWC
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.
http://www.darvill.clara.net/altenerg/wave.htm
http://www.alternative-energynews.info/technology/hydro/wave-power/
Energy
Potential energy of the wave over one period
Ep = 0.25 wρgA²λ
Kinetic energy of the wave over one period
Ek = 0.25 ρwgA²λ
Total energy over one period
ET = 0.5 wρgA²λ
Power
Power generated (work/time)
P = 0.5 wρgA²λ/T
Power per wavelength = 0.5 wρgA²f
Power per meter = 0.5 wρgA²v
where v is the speed of the wave
The density of seawater at the surface of the ocean
varies from 1020 to 1029kgm-3.
ρ= Water density
W = wave width, assumed to be the width of the chamber
A = wave amplitude
T = wave period
Λ= wavelength