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

1. Heat source

2. Steam generation

3. Turbines

4. Generator

5. Transmission lines

Hyperlink

Electrical energy is produced by coils rotating in a magnetic field.

World use of energy sources91% Non-renewable

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 fuelEnergy 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 productionIndustrialization 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

Each fission reaction releases neutrons that are used in further reactions when slowed down: low-energyneutrons (≈ 1 eV) cause further fission leading to a chain reaction.

Fast neutrons

Critical mass: minimum mass required for a sustained chain reaction

Controlled nuclear fission (power production)

Uncontrolled nuclear fission(nuclear weapons).

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.

graphite moderator

boron control rod

heat exchangerfuel element channel

steel

concrete

hot gas

reactor core

cold gas

charge face

The moderator slows the neutrons down to enable them to allow further fission

The control rods absorb neutrons to control the power level

The heat exchanger isolates the water from the coolant and lets the hot gas boil the water.

EK Fission fragments -> heat (gas, then water) -> EK(turbines)

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

1. photovoltaic cell (radiant to electrical)

There are 2 types of solar power

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 storageElectrical + PE to KE to

electrical energy.•It's a way of storing energy for when you need it in a hurry.

Wind powerThe wind blows the propeller around, which turns a generator to produce electricity

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.

Energy = ½ mv2

Mass per sec = ρx volume =

ρx Area x speed = ρπr2v

Energy/s = ½ ρπr2v x v2 = ½ ρπr2v3

The wind does not stop after passing through the turbine, therefore not all the energy can be harnessed (max = 59%)

Wave power (OWC)HyperlinkEnergy can be extracted from waves in a

number of waves including an Oscillating Water ColumnAdvantages   •The energy is free - no fuel needed, no waste produced. •Not expensive to operate and maintain. •Can produce a great deal of energy.

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 LMass = 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

a

λ

L

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

albedoSurface Typical

Albedo

Fresh asphalt 0.04

Conifer forest(Summer)

0.08,0.09 to 0.15

Worn asphalt 0.12

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

The albedo also varies with factors like

• season

• latitude

• 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

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.

The resonant or natural frequency of greenhouse gases is in the IR region

Black-body radiation isthe 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

tempsameatbodyblackfromPower

objectbyradiatedpower

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 modelThe 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 ofglobal warming.

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.

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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)

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.

One possible effect of the enhanced greenhouse effect is a rise in mean sea-level

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.