unit 8: energy production

non-renewable energy sources

• ones which cannot be produced again

• can be used up and become depleted.

primary energy

energy found in nature such as raw fuels and other forms of energy received as input to a system

secondary energy

energy transformed from primary energy sources through energy conservation processes

degraded energy

thermal energy that is released into the surrounding environment after any process

specific energy

• the amount of energy which can be extracted per unit mass of fuel

• measured in J Kg^-1

• equal to energy/mass

energy density

• the amount of energy which can be extracted per unit volume of a fuel

• measured in J m^-3

• equal to energy/volume

relationship between energy density and specific energy

Es/ED = 1/density of the fuel

useful energy

• primary and secondary energy

• not degraded energy

do gases have higher specific energy or energy density?

gases tend to have higher specific energy than energy density → makes gaseous fuels harder to transport as a greater volume is needed

examples of primary fuels

• fossil fuels

• sunlight

• flowing water

examples of secondary fuels

• hydrogen (as it has to be extracted)

• electricity

• biofuel (has been processed)

Sankey diagrams

• definition

• direction

• meaning of width

• meaning of length

a type of energy diagram where the width of each arrow is proportional to the amount of energy in that section.

what are primary sources of energy used for?

to generate electricity, a secondary form of energy

efficiency

• useful energy/power / input power/energy

• 0<e<1 or 0%<e<100%

renewable energy source examples

• Solar power

• Wind power

• Hydroelectric power

• Tidal power

• Geothermal

• Biomass

non-renewable energy source examples

• Coal

• Oil

• Natural gas

• Nuclear power

renewable energy sources

ones which cannot be depleted

purpose of a moderator in nuclear power plants

• to slow down neutrons so that fission reactions can occur

• increases reactivity

purpose of control rods in nuclear power plants

• absorb neutrons which stops fission

• fission requires a nucleus to absorb a neutron

• decreases reactivity so increases safety

purpose of fuel rods in nuclear power plants

to hold the fuel

what is a nuclear reactor filled with?

water to serve as a moderator, slowing down neutrons enabling fission

advantages of wind power

• renewable

• no harmful waste product

• cost effective in the long run

disadvantages of wind power

• expensive to build

• requires a large, windy area

• low power output

conduction

the transfer of heat through matter by transferring kinetic energy from particle to particle with no net movement of the particles

convection

• the transfer of thermal energy by the movement of a fluid

• hot fluids rise because it is less dense, the fluid then cools becomes more dense and sinks

• this creates a convection current

convection current in the atmosphere

• heat driven cycles

• warm air rises and cold air sinks

• due to points having different proximities to heat sources (the sun)

radiation

• the transfer of energy by electromagnetic waves / photons

• how the sun provides heat energy to Earth

• can happen in a vacuum

why can thermal energy sometimes not be seen?

• photons are emitted from everything that has a temperature

• this is only visible in infrared light

• something that appears less hot can have heat energy inside, kept there by an insulator

when can thermal energy be seen?

• if heated to a high enough temperate objects will give of visible light

• knowing that T∝λ^-1 and T∝f so λ∝f^-1

what is a black body radiation? what are thermal photons?

• the “glow” coming from an object due to its temperature

what are thermal photons?

• photons coming out of a black body

• all due to the temperature of the black body

what is a black body?

• A black body which absorbs all incident electromagnetic radiation is both the perfect absorber and the perfect emitter of radiation.

• The radiation emitted by such a body at constant temperature is called black-body radiation.

what is emissivity (e)?

• power per unit area radiated by the object / power per unit area radiated by a black body at the same temperature

• when e=1 it is perfect emissivity

• for black bodies e is closer to 1

• varies with surface material

which law shows the power emitted from a black body?

• Stephan-Boltzmann Law

• P=eσAT^4

• σ = Stephan-Boltzmann constant

• T must be in Kelvins

Wien’s displacement law

• the energy radiated by a body is electromagnetic radiation and distributed over an infinite range of wavelengths

• most energy is radiated at a specific wavelength λmax

• λmax = 2.9×10^-3/T (Kelvin)

earth’s energy balance

Earth’s energy balance describes how the incoming energy from the sun is used and returned to space. If incoming and outgoing energy are in balance, the Earth’s temperature remains constant.

albedo

• The Albedo (α) of a planet is defined as the ratio between the total scattered (reflected) radiation and the total incident radiation of that planet.

solar constant

• The solar constant is the amount of energy that normally falls on a unit area (1m^2) of the Earth’s atmosphere per second when the Earth is at its mean distance from the sun.

• The solar constant is approximately 1366 W/m^2.

greenhouse gases

Greenhouse gases absorb infrared radiation because its molecules have natural frequencies in the infrared region and readily absorb infrared radiation due to resonance.

greenhouse gas effect

• The greenhouse gases present in the atmosphere absorb infrared radiation and reflect it back towards the Earth’s surface.

• Thus, heat energy becomes trapped inside Earth’s atmosphere and accumulates, leading to the greenhouse effect and an increase in average mean temperatures on Earth.