Transcript Energy, power and climate change

Energy, power and climate change
Great website
8.1 Energy degradation and power
generation
1. Hot gas will cause the
piston to move. Thermal
energy may be completely
converted to work in a single
process
2.But one stroke of the piston does not
provide much energy. Continuous conversion
of this energy into work requires a cyclical
process and this involves the transfer of
some energy from the system. This is called
DEGRADED ENERGY and is not available to
do useful work
SANKEY Diagram – 25%
efficient – note the degraded
energy – brown arrows
Production of electrical power
Hyperlink
1. Heat source
2. Steam generation
3. Turbines
4. Generator
5. Transmission lines
Electrical energy is produced by coils rotating in a
magnetic field.
World use of energy sources
91% Nonrenewable
Oil, Coal, Gas,
- emit CO2
Nuclear.
The SUN is
the prime
energy
source for
world energy.
Only approximate values are needed
Energy density of fuels: The amount of energy
that can be extracted per Kg of Fuel –
influences choice of fuel
Energy in
MJ/Kg
• Nuclear/Uranium
90,000,000
• Crude Oil
42
• Coal
22-33
• Wood
17
• Gas
54
NOTE: The energy density values will be
provided in the questions on the paper.
8.3 Fossil fuel power production
Industrialization in the 19th century led to a higher rate of energy
usage, leading to industry being developed near to large deposits
of fossil fuels.
Efficiency
• Coal 35 – 42%
• Natural Gas 45 – 52%
• Oil 38 – 45%
Advantages – easy to transport, used in home for heating, cheap
in comparison with other sources.
Disadvantages – coal fired stations need large amounts of fuel,
pollution – acid rain, greenhouse gases, oil spills and fires, mining
dangers for health.
Nuclear power
Fast neutrons
Each fission reaction
releases neutrons that
are used in further
reactions when slowed
down: low-energy
neutrons (≈ 1 eV) cause
further fission leading to a
chain reaction.
Critical mass: minimum mass required for a sustained
chain reaction
Uncontrolled nuclear
fission
(nuclear weapons).
Controlled nuclear fission
(power production)
Natural U-235 (used for fission) occurs as 0.7% abundance (99.3% U-238)
Enriched fuel contains 2.3% U-235, used in power production and increases
the efficiency and power output/Kg of nuclear reactors.
EK Fission fragments -> heat (gas, then water) -> EK(turbines)
The control rods absorb
neutrons to control the
power level
charge face
boron control rod
hot gas
graphite moderator
reactor core
fuel element channel
heat exchanger
The moderator slows the
neutrons down to enable
them to allow further
fission
concrete
steel
cold gas
The heat exchanger isolates
the water from the coolant
and lets the hot gas boil the
water.
Fast breeder reactors using plutonium
• The U-238 can
capture neutrons
and is converted to
Pu-239
• The Pu-239 is
fissionable by fast
neutrons
• Therefore, the
reactor can breed
its’ own fuel
Risks and safety issues:
• Meltdown – This is when the power goes out of control and the reactor blows up. This
may happen if the coolant is “interrupted”, or the control rods are removed.
• The waste produced is radioactive, and is hazardous to living things. It is expensive to
store. The half life of some products is very long.
• Uranium mining - Because uranium ore emits radon gas, uranium mining can be more
dangerous than other underground mining.
• The plutonium produced can be used for weapons manufacture.
Nuclear fusion
• The plasma needs to be at a temperature of
about 108K and has a high density.
• It is difficult to confine and maintain the
plasma.
• Can be contained by a magnetic field.
Solar power
There are 2 types of solar power
1. photovoltaic cell (radiant to
electrical)
In a sunny climate, you can get enough power to run a
100W light bulb from just one square metre of solar
panel. Good for remote situations.
2. Solar water heating
The Sun is used to heat water
in glass panels on the roof
(radiant to thermal).
This means you don't need to use so much gas or electricity to heat your water at home.
Solar power received on earth is less if:
• You are not at the Equator
• It is not mid summer
Hydroelectric power RENEWABLE
(PE to KE to Electrical energy
water storage in lakes
Advantages
•Once the dam is built, the energy is virtually free.
•No waste or pollution produced.
•Much more reliable than wind, solar or wave power.
•Water can be stored above the dam ready to cope
with peaks in demand.
•Hydro-electric power stations can increase to full
power very quickly, unlike other power stations.
•Electricity can be generated constantly
•Disadvantages
•The dams are very expensive to build.
However, many dams are also used for flood control or irrigation, so building
costs can be shared.
•Building a large dam will flood a very large area upstream, causing problems
for animals that used to live there.
•Finding a suitable site can be difficult - the impact on residents and the
environment may be unacceptable.
•Water quality and quantity downstream can be affected, which can have an
impact on plant life.
Tidal water storage (PE to KE)
•Doesn't cause pollution, doesn't need fuel
•A tidal barrage is very expensive to build
•Only works when tide is going in or out
•A tidal barrage affects a large area
•There are very few places that you could
sensibly build a Tidal barrage
•Underwater turbines may be a better bet than a
barrage - they are cheaper and don't have the
huge environmental impact
Pump storage
Electrical + PE to KE to
electrical energy.
•It's a way of storing energy
for when you need it in a
hurry.
Wind power
The wind blows the
propeller around,
which turns a
generator to produce
electricity
Energy = ½ mv2
Mass per sec = ρx volume =
ρx Area x speed = ρπr2v
The wind does not stop
after passing through the
turbine, therefore not all
the energy can be
harnessed (max = 59%)
Energy/s = ½ ρπr2v x v2 = ½ ρπr2v3
Wind Power is renewable
•Doesn't cause pollution, doesn't need fuel
•Need a lot of generators to get a sensible amount of power
•Need to put them where winds are reliable.
Wave power (OWC)
Energy can be extracted from waves in a
number of waves including an
Oscillating Water Column
Advantages
•The energy is free - no fuel needed, no
waste produced.
•Not expensive to operate and maintain.
•Can produce a great deal of energy.
Hyperlink
Disadvantages
•Depends on the waves - sometimes you'll get loads of energy, sometimes
almost nothing. Must be able to withstand very rough weather
•Needs a suitable site, where waves are consistently strong.
Volume of water in red area = a x λ/2 x L
Mass = Volume x density(ρ)
Number of waves per sec = v/λ
Loss of GPE of the wave per sec = mgh =
(a x λ/2 x L x ρ) x g x a x v/λ
L
a
λ
Power per unit length = ½ a2ρgv
Solar constant
• The sun radiates 3.9x1026W
• The Earth is a distance of 1.5x1011m from
the sun
• Calculate the power per m2 reaching the
Earth.
I = 3.9x1026W
4π(1.5x1011)2
albedo
Surface
Typical
Albedo
The albedo also varies
with factors like
Fresh asphalt
0.04
Conifer forest
(Summer)
0.08,0.09 to 0.15
• season
Worn asphalt
0.12
• latitude
Deciduous
trees
0.15 to 0.18
Bare soil
0.17
Green grass
0.25
Desert sand
0.40
New concrete
0.55
Fresh snow
0.80–0.90
• cloud cover
The average value on
Earth is 0.3
Greenhouse effect and greenhouse gases
Short λ not
absorbed
Long λ
absorbed
Main greenhouse gases: water vapour, carbon dioxide, methane, N2O.
Each has a natural - livestock and plants, and man made origin Burning of fossil fuels, fertilisers and deforestation.
Molecular mechanisms: Absorption of IR radiation
The resonant or
natural frequency of
greenhouse gases is in
the IR region
Carbon dioxide, water vapour , methane , nitrous oxide , and a few
other gases are greenhouse gases. They all are molecules composed
of more than two component atoms, bound loosely enough together
to be able to vibrate with the absorption of heat.
Black-body radiation is
the radiation emitted by a perfect emitter.
λmax x T = Wien’s constant
Stefan–Boltzmann law
P = Power emitted from a surface.
σ = Stefan–Boltzmann constant
A = Surface area of emitting body
T = Temperature of the emitter
Emissivity
• The Earth is not a perfect Black Body radiator
(or absorber).
• The emissivity (e) is defined as
power  radiated  by  object
Power  from  black  body  at  same  temp
Surface heat capacity (Cs) is
the energy required to raise
the temperature of unit area
of a planet’s surface by one
degree, and is measured in
J m–2 K–1.
Climate change model
The change of a planet’s temperature over a period of time is given by:
(incoming radiation intensity – outgoing radiation
intensity) × time / surface heat capacity.
Predictions
Describe some possible models of
global warming. Hyperlink
A range of models has been suggested to explain global
warming, including:
• Changes in the composition of greenhouse gases in the
atmosphere.
• Increased solar flare activity.
• Cyclical changes in the Earth’s orbit.
• Volcanic activity.
Enhancement of the greenhouse effect is
caused by human activities.
The generally accepted view of most scientists is that
human activities, mainly related to burning of fossil fuels,
have released extra carbon dioxide into the atmosphere.
Evidence of Global warming
International ice core research
produces evidence of
atmospheric composition and
mean global temperatures over
thousands of years (ice cores
up to 420,000 years have been
drilled in the Russian Antarctic
base, Vostok).
Some of the mechanisms that may
increase the rate of global warming.
• Global warming reduces ice/snow cover, which in turn
reduces the albedo, which increases the rate of heat
absorption. It also increases evaporation.
• Temperature increase reduces the solubility
of CO2 in the sea and increases atmospheric
concentrations
• Deforestation reduces carbon fixation and uptake of CO2
The coefficient of volume expansion(γ) is the
fractional change in volume (V) per degree
change in temperature (T).
γ=1/Vo(ΔV/ΔT)
One possible effect of the enhanced
greenhouse effect is a rise in mean sea-level
Possible reasons for a predicted rise
in mean sea-level.
Precise predictions are difficult to make
due to factors such as:
• anomalous expansion of water
• different effects of ice melting on sea
water compared to ice melting on land.
Climate change is an outcome of the
enhanced greenhouse effect.
Possible solutions to reduce the
enhanced greenhouse effect
•
•
•
•
•
Greater efficiency of power production.
Replacing coal and oil with natural gas.
Carbon dioxide capture and storage.
Use of hybrid vehicles.
Increase use of renewable energy sources and
nuclear power.
• Use of combined heating and power systems.
International efforts:
• Intergovernmental panel on climate change.
• Kyoto protocol
• Asia-Pacific partnership on clean development and climate.