Notesale: Turn your study into money

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8
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e
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energy stored in fuels such as crude oil, coal, natural gas, wind energy, solar
Secondary energy is produced when primary energy is processed or exploited
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Specific energy, Es, is the amount of energy that can be extracted from a unit mass of fuel; it is measured in J kg-1
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Energy can be classified into two types, non-renewable and renewable
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This includes fossil fuels and nuclear fuels
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This includes solar energy and tidal energy
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A Sankey diagram shows how many energy units are lost, and how many are produced
as electricity
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Therefore, the efficiency for the plant in the above diagram is 35%
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Origin of fossil fuels:
The sun makes biomass through photosynthesis
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The biomass is buried under great pres sure and heat, becoming coal,
oal and natural gas over eons
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Topic 8 Page 4

Nuclear power
A nuclear reactor is a machine in which nuclear fission reactions take place, producing energy
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The reaction is self-sustaining, therefore it is a chain reaction
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For the chain reaction to get going, a critical mass of uranium-235 must be present (about 15kg for uranium-235)
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Neutrons slow down through collisions with the atoms of the moderator, the material surrounding the fuel rods (the tubes containing uranium)
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A heat exchanger is therefore needed to extract the heat from the moderator
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The water is turned into steam at high temperature and pressure, which produces electricity by turning the turbines of the generator
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If the rate of reactions needs to be increased, the control rods
are removed
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A blackened surface below the glass collects sunlight,
and water circulating in pipes underneath is heated
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Photovoltaic cells convert sunlight directly into direct current (dc) at an efficiency approaching 30%
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Topic 8 Page 6

Hydroelectric Power
Hydroelectric power is the power derived from moving water masses
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Consider a mass
of water that falls down a vertical height
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In a pumped storage system, the water that flows to lower heights is pumped back to its original height by using the generators of the plant as motors to
pump the water
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So:

The quantity

is known as the volume flow rate and so

(Not in data booklet!)
Where:
•
is the power available for generating electricity (in Watts)
• is the density of water (in kg m-3)
•
is the volume flow rate (in volume per second)
•
is the acceleration due to gravity
• is the average height of the dam (in metres)

Topic 8 Page 7

Wind power
Let us consider the mass of air that can pass through a tube of cross-sectional area with velocity in time
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Then the mass enclosed in
a tube of length
is
This is the mass that will exit the right end of the tube within a time interval equal to
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In practice, frictional and other losses (mainly turbulence) result in a smaller power increase
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Topic 8 Page 8

Characteristics of energy types
Advantages

Disadvantages

Fossil fuels

Relatively cheap
High power output (high energy density)
Variety of engines and devices use them directly and easily
Extensive distribution network is in place

Will run out
Pollutes the environment
Contributes to the greenhouse effect by releasing greenhouse gases into the
atmosphere

Nuclear power

High power output
Large reserves of nuclear fuels
Nuclear power stations do not produce greenhouse gases

Radioactive waste products difficult to dispose of
Major public health hazard should 'something go wrong'
Problems associated with uranium mining
Possibility of producing materials for nuclear weapons

Solar power

'Free'
Inexhaustible
Clean

Works during the day only
Affected by cloudy weather
Low power output
Requires large areas
Initial costs high

Hydroelectric

'Free'
Inexhaustible
Clean

Very dependent on location
Requires drastic changes to the environment
Initial costs high

Topic 8 Page 9

8
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Conduction is when energy is transferred from the hot side to the cold side of the solid
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The moving electrons collide with neighbouring molecules, transferring energy to them and increasing their average kinetic energy
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As water gets hotter it expands, becomes less dense and so rises to the top
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The process is similar for air, where it heats and expands, transferring warm air to the rest of the room
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Both conduction and convection require a material medium
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Radiation is important for Earth's climate
and its energy balance
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The Stefan-Boltzmann law works for both emission and absorption problems
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e
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When is 1 the body is a black body
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This body is a perfect
radiator as well as a perfect absorber
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Black bodies radiate electromagnetic radiation and is distributed over an infinite range of wavelengths
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Solids can be heated to incandescence (glowing) and different temperatures will have different visible radiation
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- The higher the temperature the smaller the wavelength of the maximum intensity
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The sun emits a total power of about
The solar constant S is

The solar constant is the intensity of the Sun's radiation at the position of Earth's orbit
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Albedo
The albedo, , of a body is the ratio of the power of radiation scattered from the body to the total power incident on the body:

High albedo indicates that the body reflects most of the radiation incident on it, while low albedo means that the body refle cts very little of the light incident on it
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The radiation that reaches the Earth has to go through the area of a disc of radius
The power through this disc is therefore:

(radius of the Earth)
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The Earth's surface receives radiation during the day when it faces t he Sun
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It is affected by factors such as the terrain, clouds, season, etc
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Energy Balance
Energy input to Earth must equal the energy output by the Earth
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The gases primarily responsible for this absorption (the greenhouse gases) are water
vapour, carbon dioxide, methane and nitrous oxide
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The resonance
allows the gas to heat up
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The enhanced greenhouse effect refers to the additional warming due to
increased quantities of the greenhouse gases in the atmosphere
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Greenhouse gases in the atmosphere can be natural as well as man-made (anthropogenic)
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Topic 8 Page 16

Mechanism of photon absorption
When solar radiation strikes the earth, that energy is carried in the form of photons
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If a photon is at a precise energy for an electron to jump to a
different energy level, then it will be absorbed
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The absorption and release of photons by the gases is called scattering
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They are ab le to be absorbed as internal energy
and kinetic energy

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