Energy, power and climate change

Energy, power and climate change

8.1 Energy degradation and power generation

1. Hot gas will cause the piston to move

2.But one stroke of the piston does not provide

much energy

3.The process needs to be cyclical

Cyclical processes

The continuous production of energy can be obtained from a cyclical process

Not all of the heat can be converted to

work

Some is transferred to the surroundings

Efficiency of heat engines

No heat engine can transfer all of it’s energy to work.

Some is always lost as heat to the surroundings.

Equation is not on the syllabus

Sankey diagrams

You must be able to construct and analyse Sankey diagrams to show where energy is degraded.

100% 25%

energy efficiency of a filament lamp

Production of electrical power

1. Heat source

2. Steam generation

3. Turbines

4. Generator

5. Transmission lines

The Generator

Hyperlink

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

8.2 World energy sources

Which energy resources produce CO2?

Which are renewable?

Which resources come from the sun?

What are the advantages and disadvantages of the types of energy sources? (Location, cost, pollution, energy density, continuity, availability….)

Define the energy density of a fuel

Energy density is measured in J kg–1.

World use of energy sources91% Non-renewable

Only approximate values are needed

Energy density of fuels

• Energy in GJ/tonne

• Uranium metal (U) 560,000

• Crude Oil 44.9

• Black Coal 29.0

• Wood 16.2

• Gas 54

Considerations of energy density

• Transport costs

• Storage

• Applications e.g. Nuclear submarines

CO2 emissions

• Emission indices (Kg CO2/GJ)

• LPG 60Natural Gas 58 Crude Oil 76Coal (electricity) 290

8.3 Fossil fuel power productionOutline the historical and geographical reasons for thewidespread use of fossil fuels

Students should appreciate that industrializationled to a higher rate of energy usage, leading toindustry being developed near to large deposits of fossil fuels.

Amount of fuel for power production

Discuss the energy density of fossil

fuels with respect to the demands of

power stations.

Students should be able to estimate the rate of fuel consumption by power stations.

Rate of coal use in a power station

• 1000 MW output of electricity

• Coal power stations are 40% efficient

• Coal has 29MJ/Kg

• Calculate the rate of use of coal

• (Approx 300 tonnes/hr)

Discuss the relative advantages and disadvantages associated with the transportation and storage of fossilfuels.

State the overall efficiency of powerstations fuelled by different fossil

fuels.

• Coal 35 – 42%

• Natural Gas 45 – 52%

• Oil 38 – 45%

Describe the environmental problemsassociated with the recovery of fossilfuels and their use in power stations.

8.4 Non-fossil fuel power production

Describe how neutrons producedin a fission reaction may be used toinitiate further fission reactions (chainreaction).Students should know that only low-energyneutrons (≈ 1 eV) favour nuclear fission.

They shouldalso know about critical mass.

Chain reactions

Each fission reaction releases neutrons that are used in further reactions.

Fast neutronsNeed to be

slowed down

Critical mass?

Distinguish between controllednuclear fission (power production)and uncontrolled nuclear fission(nuclear weapons).

Students should be aware of the moral and ethicalissues associated with nuclear weapons.

Describe what is meant by fuelenrichment.

Natural U-235 occurs as 0.7% abundance. (3300C)

Enriched fuel contains 2.3% U-235, therefore increases the temperature (6000C)of the core of the reactor, therefore increases the efficiency and power output/Kg

Describe the main energytransformations that take place in a

nuclear power station.

EK of fission

fragments

Nuclear power station

Discuss the role of the moderator and the control rods in the production of controlled fission in a thermal fissionreactor.

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 fissions

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.

What are the energy transformations?

Energy transformations in a reactor

• Fission fragments have EK

• This heats the fuel rod• The coolant (gas) takes the heat from the

rod• The hot gas goes to the heat exchanger• The hot gas turns the water to steam• The steam drives the turbines• The turbines drive the generator

Production of plutonium• Fissionable plutonium-239 can be

produced from non-fissionable uranium-238 by the reaction illustrated.

The bombardment of uranium-238 with neutrons triggers two successive beta decays with the production of plutonium.

Fast breeder reactors• The U-238 is

converted to Pu-239• The Pu-239 is

fissionable by fast neutrons

• Therefore, the reactor can breed its’ own fuel

• Doesn’t need a moderator (saves space)

• Very high operating temperature, cooled by liquid sodium

Risks of nuclear power• 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, as 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 (this takes a lot of energy).• This cannot come into contact with anything• Can be contained by a magnetic field.

Solar power

1. photovoltaic cell

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 e.g. a yacht.

2. Solar water heating The Sun is used to heat water in glass panels on the roof

This means you don't need to use so much gas or electricity to heat your water at home.

Solar PV cells

• Advantages • Solar energy is renewable and the Sun’s heat and light are free • Solar energy can be used to generate electricity in remote places

where other electricity supplies are hard to come by • It does not produce any carbon dioxide, which contributes to the

greenhouse effect • Energy is usually generated at or near to the location it will be used.

This keeps transmission and distribution costs to an absolute minimum

• Disadvantages• PV cells do not work so well when it is cloudy and do not work at

night • They only work in a very sunny country! Solar power works better in

hot places, so its use is therefore limited

Solar constant • The solar constant is the amount of

incoming solar electromagnetic radiation per unit area.

• It is measured by satellite to be roughly 1.4 kWm-².

• This value must be reduced if …..• You are not at the Equator• It is not mid summer• PV cells are about 10% efficient.

Hydroelectric power Hyperlink

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 storageHyperlink

•Tidal Power is renewable•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

•It's a way of storing energy for when you need it in a hurry.•The biggest one is at Dinorwig, in Wales•Expensive to build•Most power stations take a long time to turn up to full power. Pumped Storage reservoirs mean that we can quickly get more energy for half an hour or so, to keep us going until the other power stations catch up

Dinorwig has the fastest "response time" of any pumped storage plant in the world - it can provide 1320 Mega Watts in 12 seconds. That's a lot of cups of tea!

Buy when cheap

Sell when expensiveGPE

KE Electric

Question

• How much water must fall per second to produce 1,400 MW of electricity, if it falls through a height of 200m? Assume the turbine is 60% efficient.

Wind powerThe wind blows the propeller round, 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 = 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%)

Questions

• A wind generator is designed to work in winds of 10km/hr with a blade length of 3m. How much power can it produce?

• What would be the power output at 20km/hr?

• What would be the power output if the blade length were increased to 6m?

• ρair = 1.3Kgm-3

Wave power (OWC)Hyperlink

Advantages   •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. •Needs a suitable site, where waves are consistently strong. •Some designs are noisy. But then again, so are waves, so any noise is unlikely to be a problem.•Must be able to withstand very rough weather

Waves

a

λ

L

Volume of water in red area = a x λ/2 x L Mass = Volume x density(ρ)

Loss of GPE of the wave = mgh = (a x λ/2 x L x ρ) x g x a

Number of waves per sec = Frequency = v/λ

Power = loss of GPE per sec = a2 x λ/2 x L x ρ x g x v/λ

Power per unit length = ½ a2ρgv

OWC question

• Waves of amplitude 2m reach the beach every 15 seconds. The wavelength of the wave is 80m. Calculate

• The speed of the wave.

• The power per metre of the waves along the shore.

• The power available from a 3km beach.

8.5 Greenhouse effectHyperlink

Short λ

not absorbed

Long λ

absorbed

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.

When the energy reaches the Earth, what happens to it?

Albedo

the fraction of the incident sunlight that is reflected

Variations in albedoSample albedos

Surface TypicalAlbedo

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 and cloud cover

The average value on Earth is 0.3

Why does the reflected radiation not escape into space?

Greenhouse gases

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 major components of the atmosphere N2 and O2 are two-atom molecules too tightly bound together to

vibrate and thus they do not absorb heat and do not contribute to the greenhouse effect.

The resonant frequency of greenhouse gases is in

the IR region

Microwave simulation

Hyperlink

Start of IR region 0.7 nm

Sources of greenhouse gases

• Burning of fossil fuels and deforestation leading to higher carbon dioxide concentrations

• Livestock CO2 and CH4

• Fertilisers N2O• CFC’s in refrigeration and fire extinguishers• When these gases are ranked by their contribution to the

greenhouse effect, the most important are:• water vapour, which contributes 36–70% • carbon dioxide, which contributes 9–26% • methane, which contributes 4–9% • ozone, which contributes 3–7%

How much heat does the Earth radiate?

The nature of black-bodyradiation.

λmax x T = Wien’s constantMicrosoft Equation 3.0

Stefan–Boltzmann law

P = Power output

σ = Stefan–Boltzmann constant

A = Surface area of emitting body

T = Temperature of the emitter

Black body simulation

Emissivity

• The Earth is not a perfect Black Body radiator

• The emissivity is defined as

tempsameatbodyblackfromPower

objectbyradiatedpower

Therefore the Earth is not a perfect absorber or emitter of heat. Black objects have a high emissivity, white low.

Values of emissivityAluminium: anodised 0.77

Aluminium: polished 0.05

Asbestos: board 0.96

Asbestos: fabric 0.78

Asbestos: paper 0.93

Asbestos: slate 0.96

Brass: highly polished 0.03

Brass: oxidized 0.61

Brick: common .81-.86

Brick: common, red 0.93

Brick: facing, red 0.92

Brick: fireclay 0.75

Brick: masonry 0.94

Brick: red 0.90

Carbon: candle soot 0.95

Carbon: graphite, filed surface 0.98

What is the effect of the absorbed radiation on the temperature of the Earth?

Surface Heat capacity Cs

Surface heat capacity 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 modelStudents should appreciate that the change of aplanet’s temperature over a period of time is givenby:(incoming radiation intensity – outgoing radiation intensity) × time / surface heat capacity.

Download

Greenhouse simulation

Predictions

Met office prediction

Describe some possible models ofglobal warming.

Students must be aware that 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 and volcanic activity.

State what is meant by the enhancedgreenhouse effect.

It is sufficient for students to be aware that

enhancement of the greenhouse effect is caused by human activities.

Identify the increased combustion offossil fuels as the likely major cause of

the enhanced greenhouse effect

Students should be aware that, although debatable, 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.

Describe the evidence that linksglobal warming to increased levels of

greenhouse gases.

• For example, 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).

Evidence of Global warming

The concentration of carbon dioxide measured at Mauna Loa

Observatory in Hawaii

Cyclical change?

Outline some of the mechanismsthat may increase the rate of global

warming.Students should know that:

• global warming reduces ice/snow cover, which

in turn changes the albedo, to increase rate of

heat absorption

• temperature increase reduces the solubility

of CO2 in the sea and increases atmospheric

concentrations

• deforestation reduces carbon fixation.

Define coefficient of volume expansion

Students should know that the coefficient of

volume expansion is the fractional change in

volume per degree change in temperature.

State that one possible effect of theenhanced greenhouse effect is a rise

in mean sea-level.

Outline possible reasons for apredicted rise in mean sea-level.

Students should be aware that 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.

Identify climate change asan outcome of the enhancedgreenhouse effect.

i.e. man made