Energy production 2
Nuclear, hydro and renewables
Mikael Höök
2011-10-11
Nuclear energy
Nuclear binding energy can be released as
heat by two ways
• Splitting heavy nuclei into lighter (fission)
(basis for current nuclear power plants)
• Fusing light nuclei into heavier (fusion)
(not used commercially yet)
Nuclear binding energy
Less stable nuclei can be fused into more stable
ones, while heavy nuclear can be split into more
stable ones until one reaches Fe-56
Basic nuclear theory
• Energy is extracted from the nucleus by
splitting it into smaller fragments, thus
releasing binding energy as heat
• Neutrons are used to split the nuclei,
since they are neutral and can pass
through the electric potential barrier
Nuclear fuel chain
Nuclear fuel chain
1.
Mining
2.
Conversion
3.
Enrichment
4.
Fuel manufacture
5.
Use
Followed by
Reprocessing
or
Final disposal
Nuclear fuel cycle
Nuclear fuels
• Uranium and plutonium are used in
nuclear reactors
• Uranium is the only nuclear fuel that
exists naturally on earth
• 99.3% is U-238 and 0.7 % is U-235
• It is mined, processed and enriched
before it can be used in reactors
(See essay for mining and occurrence)
Chain reactions
For U-235 about 2-3 new neutrons are created for
each fission, making possible to sustain the reaction
Enrichment & neutron spectra
• Only U-235 can be split by thermal (slow)
neutrons
• With losses and absorption in material the
natural content of U-235 is generally to low
sustain chain reactions.
• Enrichment is needed! The concentration
of U-235 must be raised to about 3% for
most reactors
• Some reactors use 20% or more
Enrichment
Enrichment can
be done in many
ways.
•Electromagnetic
•Centrifuges
•Membranes
•Laser
The large Tricastin enrichment plant
in France (beyond cooling towers)
The four nuclear reactors in the
foreground provide over 3000 MWe
power for it
Enrichment
• Complicated process, as all isotopes
share the same chemical properties
• The only difference is mass
• Utilization of this mass difference can be
used for separating the different isotopes
• Example: Lighter nuclei moves somewhat
faster through membranes than heavier
Electromagnetic enrichment
• Simple, but very
energy intensive
• The ions will be
bent by the
magnetic field, and
the radius is
dependent on the
mass
Gaseous diffusion
• The process separates the lighter U235 isotopes from the
heavier U238. The gas is forced through a series of porous
membranes with microscopic openings. Because the U235
is lighter, it moves through the barriers more easily
• As the gas moves, the two isotopes are separated,
increasing the U235 concentration and decreasing the
concentration of U238
Gas centrifuges
• Strong centrifugal field of a rotating cylinder
sends the heavier isotope in UF6 to the outside
of the cylinder, where it can be drawn off, while
the U235 diffuses to the center of the cylinder
• Each centrifuge only
causes a small
enrichment, so they must
be used in cascades with
many thousand
centrifuges to yield
significant enrichment
Laser enrichment
• Ionization energy is mass dependent
• Finely tuned laser light will only be
absorbed by U-235, causing ionization that
allows separation from non-ionized U-238
Fuel manufacture
Westinghouse fuel factory i Västerås, Sweden
Nuclear fuel
Enriched uranium is sent to special factories that
mold it into fuel pellets
A fuel pellet is a few centimeter long cylind with
about 1 cm diameter
One pellet gives as much energy as 800 litres of
diesel fuel and a typical reactor contains about
15 milloner fuel pellets
Fuel rods
Fuel pellets are
stacked in fuel
tubes or fuel rods
The fuel rods are
later combined into
fuel elements
Fuel elements
Fuel elements contain
a number of fuel
rods
These are separated
with suitable
distances using
spreaders
Fuel elements are the
most important part
of a reactor
Reactor core
A bunch of fuel
elements are
stacked beside
each other in a
reactor tank
This forms the
reactor core
where heat is
extracted from
the nuclear fuel
Reactor types
Depending on the shape and layout of
the core, different types of reactors can
be made
Most reactors use water as the coolant in
the core, but some designs can use
other cooling medias
Nuclear reactors
The heat from the chain reactions cause water to boil
and steam turbines and generators can be driven to
produce electricity.
Nuclear power = fancy water boiling
Different reactor types
Most common reactor types in use
PWR = Pressurized Water Reactor
BWR = Boiling Water Reactor
(PWR and BWRs make up ~90% of world reactor fleet)
HWR = Heavy Water Reactor (can use natural
uranium as fuel, no need for enrichment)
•CANDU-reactors
Pressuried Water Reactor
High pressure
prevents boiling
in the reactor
Heat exchange
occurred in
secondary loop
that generates
steam
The stram is used
to drive turbines
and generators
Boiling Water Reactor
The nuclear heat boils
water into steam inside
the reactor
The steam is diverted to a
turbine and gives
electricity
The steam is finally
condensed to water and
returned to the reactor
for a new cycle
Advanced reactor types
Gas-cooled reactors
Fast reactors (can use U-238 as a fuel)
Molten metal-cooled reactors
Breed reactors (creates more fuel than
they use)
Most of them are complex and haven't
been used on large commercial scale.
Social acceptance
Social acceptance is important for nuclear energy
Connection to nuclear weapons and the accidents at
Chernobyl and Fukushima casts dark shadows over
nuclear energy
Spend nuclear fuel
After a single pass in a reactor, only a
few percent of the energy content has
been extracted
About 97% of the recoverable energy
remains in the nuclear fuel
This can be used if the spent nuclear fuel
is reprocessed
The alternative is to send it for final
disposal
Fission fragments
Nuclear fission gives
many different fission
fragments
Fission fragments are
highly radioactive due
to large neutron surplus
but relatively shortlived
Must be handled carefully
and lacks use
Transuraniums
Some neutrons can be captured by U-238
and used to generate heavier than
uranium-compounds, called transuranium
elements
Mainly it is U-238 that is transformed into
Plutonium-239
Pu-239 is fissile and a attractive fuel for
reactors or nuclear weapons
Medium radioactivity, but very long lifetimes
Reprocessing
Spent nuclear fuel can be
reprocessed and
recycled
Fission fragments and
other non-usuable
elements are removed,
while reusable elements
are recycled
Pu-239 and U-235 is
singled out and recycled
Upparbetningsanläggning
ar
Sellafield, Storbritannien
La Hague, Frankrike
Final disposal
Spend nuclear fuel is
encapsuled to
prevent it from
being spread by
wind, water and
weather
Often buried at large
depths in
inaccessible final
repositories
Summary: nuclar
Working and commercially proven since
1960s
Social acceptance is the largest problem
The future is pretty unclear and very
dependent on the path of development
that is chosen by companies and
countries
Renewables
Comes in many forms
Biomass (combustion)
Geo/solar thermal power
Photovoltaic cells
Wind and hydropower
Tidal and wave power
Biomass
Similar to combustion of coal or any
other solid fuel
Generally lower energy content than
coals, but non-fossil in nature
Small technical differences in
combustion due to other fuel
properties (moisture content, etc.)
Geothermal power
By drilling deep
holes it is possible
to extract heat
from the Earths
core
High pressure
water or steam
can be tapped
and used for
heating or in
power plants
Several tests conducted, but seldom successful...
Some examples
Geothermal power plant at
Hellisheidi on Iceland
Geothermal powerplant
in Aberdeen, Scotland
Solar heating
Solar heating 2
Efficiency: 30-60%
Very simple and reliable technology
Can be combined with steam turbines to
produce electricity
Solar focusing power plant
Parabolic trough systems
Parabolic mirrors focus solar radiation to pipes
containing water or other liquids that can be stored
or used in steam cycles
Sun-tracking systems
Solar collectors
and photovoltaic
cells usually has
motors that turn
them towars the
sun at all times
Thus, the entire
surface can be
used as long as
the sun is
available
Photovoltaics
Solar energy can also be directly
transformed into electricity via
photovoltaic cells
Semiconductor material can transform
photons into electric current
Low efficiency. ~10%
Generates only direct current
Silicon solar cell
PV-cell market
About 90% av alla PV-cells are based on
thick silicon
The remaining 10% är thinfilm cells, but
even these are dominated by
amorphous silicon
Only a few percent is non-silicon
technologies such as CIGS, Grätzel
eller CdTe-cells
Solar electricity
Advantages
•Electricity directly!
•Small and flexible shapes
Drawbacks
•Energy costly production
•Intermittent
•Low efficiency
•Current cells often require rare metals
Wind power
Energy is extracted from
the wind by slowing it
down and transforming it
to mechanical motion via
lift/drag forces.
Similar to air plane wings. Blades can
be placed in both vertical or horizontal
assemblies.
Vertical wind turbines
•Independent of wind direction
•Low theoretical efficiency, but
not in practice
•Silent
•Easy to service
Drawbacks:
•Pulsating torque
•Difficult to mount in towers
•Generally quite small
Horizontal wind turbines
•Good stability
•Easy to mount in towers
•Easy to place
Drawbacks:
•Noisy (tips move at 6 times the wind speed)
•Expensive to install
•Bird killing & landscape issues
Wind power
• Intermittent source with highly varying
power production
• Varies cubically depending on the wind
speed
• Wind is present everywhere and the
basic principles of wind power are
simple and relatively easy to apply on
an industrial scale
Ocean/tidal currents
Similar to wind turbines,
only water instead of air
Drawbacks:
•Low rotational speed
•Special generators
Advantages:
•Much more predictable
than winds.
•Less visual impact
•No noise
Tidal barrages
Tidal power can also
be extracted by
capturing the water in
a dam and lead it out
through a turbine.
Only suitable in some
places, typically large
bays with narrow
entrance
La Rance 240 MW
Hydropower
Transforms moving water into
mechanical energy by turbines.
But other solutions also exist, such as
water wheels.
Very reliable, predictable and efficient
Turbine types
Two main types of turbines:
Impulse turbines:
Changes direction of flow and is moved by the
resulting impulse. F = m*a
Examples: Pelton, de Laval, Turgo
Reaction turbines:
Moves because of the weight or pressure of the fluid.
Works according to Newtons third law.
Examples: Francis, Tyson, Kaplan, Propellers
Turbine types 2
Turbines
Turgo turbine,
optimized for
medium heights
Kaplan turbine,
optimized for
variable flows
Summary
Many of the renewable energy sources
cannot deliver energy on demand =>
requires backup power
Other only work on very special sites
(geothermal, tidal or hydropower)
Several engineering issues remains to be
solved to give cheaper and more
realiable systems
Conclusions
• Turbines are essential for nearly all power
production
• Wind and water power are very similar, the
only difference is the fluid
• Photovoltaics can generate electricity
directly, without having to use generators
and turbines
• Only nuclear, geothermal and solar thermal
can generate heat