Topic 8: Energy Production

8.1 Energy Production

Energy measurements

  • Specific energy (ES): "Amount of energy a fuel releases per unit mass", in J kg^-1.

  • Energy density (ED): "Amount of energy a fuel releases per unit volume", in J m^-3.

Both useful in the choice of a fuel, but other factors must be taken into account, such as transportation, availability, safeness etc.


Energy classifications

  • Primary energy: Found in nature, which has not yet been transformed or converted.

    • Examples: Fossil fuels (oil, coal and natural gas), Solar energy, Kinetic energy of air.​

  • Secondary energy: Resultant from the transformation of a primary source.

    • Examples: Electricity and Kinetic energy from wheel.​

  • Transformation process: From primary energy form into secondary energy form.​

  • Renewable sources: "Rate of use is less than the rate of production of the source".

    • Examples: ​Biomass, solar, hydroelectric, wind.

  • Non-renewable sources: "Rate of depletion is greater than the rate of production".

    • Examples: ​Fossil fuels and nuclear. 


Energy sources


Fossil fuels (oil, coal and natural gas): Burning fossil fuels in thermal power plants. 

  • Process: ​

  1. Initial chemical potential energy (decomposition of dead animals and plants)     

  2. Kinetic energy of moving steam          

  3. Kinetic energy of turbines        

  4. Electrical energy (generator).

Nuclear power  (uranium and plutonium): Induced fission in thermal fission reactors.

  • Fission reaction

  • Uranium enrichment: U-238 absorbs neutrons. Hence, uranium needs to be enriched, i.e. the percentage of U-235 needs to be increased to up to 3%, producing yellow cake, thus achieving critical mass and being able to have a chain reaction. 



  • Moderator (graphite or H20): reduce the speed of neutrons, because very fast moving neutrons do not induce fission.

  • Heat exchanger: allows coolant (H20 or C02) to transfer heat from reactor to water.

  • Control rods: absorb neutrons, so that the reaction does not go out of control.

  • Safety issues:​​

    • Radioactive waste: buried deep underground in containers, supposed to avoid leakage

    • Fukushima: reactor explosion.

    • Chernobyl: thermal meltdown of the reactor.

Solar power

  • Solar heating panel: sunlight directly used to heat up water.

  • Photovoltaic cells: sunlight directly converted into direct electrical current (dc), as the light incident on the panel releases electrons and establishes a pd across the cell.


Wind power

  • Formula: maximum theoretical value of the available power = ρAv^3.

    • Assumptions: all wind is stopped, no friction or turbulence.

  • Ideal places: off-shore and top of hills, due to higher wind speeds.


Hydroelectric power

  • Process: potential energy of a mass of water transformed into electricity.

  • Formula: P = mgh/∆t = ρ∆Vgh/∆t = ρQgh, where Q = volume flow rate and ρ the water density.

  • Reverse process (pumped storage system): storing energy in large scale in the case of necessity (emergency), what requires more energy than it will be gained.


Sankey diagram

  • Use: represents energy and power flow, from left to right.

  • Representation: each energy source and "loss" is represented by an arrow.

    • Arrow drawn to scale (both vertical and horizontal).

    • Energy flow always from left to right.

    • Degraded/"lost" energy to the bottom.


8.2 Thermal Energy Transfer

There are three types of thermal energy transfer: conduction, convection and radiation. While conduction and convection will only occur if there is a difference in temperature, radiation will always occur. Temperature will be constant if radiant heat from surroundings equals heat lost by radiation. 



  • Definition: moving electrons collide with neighboring molecules, transferring energy to them and so increasing their average kinetic energy, and, consequently, the temperature.

  • Occurrence: normally in solids, because fluids have weaker inter-atomic bonds and their atoms are further apart.

  • Materials: good thermal conductors are normally good electrical conductors (e.g. copper), since the mechanism is similar.



  • Definition: movement of groups of atoms or molecules within fluids that arise through variations in density, leading to thermal energy transfer.

  • Movement direction: Mass of fluid is bottom heated, what decreases density, causing mass to go up, while mass that goes down is heated, leading to the formation of a convection current.   

Thermal radiation

  • Definition: transfer of energy by means of electromagnetic waves, and so, it does not require a medium, like conduction and convection do.

  • Black body: at a certain temperature, black bodies absorb all incoming energy, while at other, it emits great amount of radiation.

  • Intensity (I) definition: "The power received per unit area from a radiating source".

    • I = P/A

  • Stefan-Boltzmann's law: P = eσAT^4, where σ = Stefan-Boltzmann's constant = 5.67 x 10^-8 W m^-2 K^-4 and e = emmissivity of a body.

  • Emmissivity:

    • For a black body: e = 1.

    • Grey bodies: all other bodies, 0 < e < 1 

  • Albedo (α): the ratio of the power of radiation scattered from the body to the total power incident on a body

    • Relationship between albedo and emissivity: α + e = 1.

    • Albedo of a planet depends on cloud cover, ice, water, color and nature of the soil and latitude.

  • Solar constant (S): the solar radiation per second per m^2 at the top of the Earth's atmosphere (= 1400 W m^-2).

  • Tip: Although the intensity of the Sun's radiation at the position of the Earth is approximately 1400 W m^-2, the average power received per unit are on the Earth is 350 W m^-2. This occurs, because the solar radiation is captured by a disc of area πR^2, where R is the radius of the Earth, but it is distributed (when averaged) over the entire Earth's surface, which has an area four times as large (4πR^2). The power received per unit area on the Earth may be further reduced if we take the effect of the albedo into account.

  • Graph: Radiation distributed over a range of wavelengths​.

    • Wien's displacement law: T x λmax = 2.90 x 10^-3 K m. ​

Greenhouse effect

  • Definition: the warming of the Earth caused by infrared radiation, emitted by the Earth's surface, which is absorbed by various gases in the Earth's atmosphere and is then partially re-radiated towards the surface.

  • Greenhouse gases: The gases primarily responsible for this absorption are water vapor (H2O), carbon dioxide (CO2), methane (CH4) and nitrous oxide (NO2).

  • Temperature balance: energy input to Earth must be equal to energy output by the Earth.

    • Natural balance on the Earth, due to naturally occurring greenhouse gases.​

  • Enhanced greenhouse effect: increased level of greenhouse gases, commonly attributed to human cause:

    • Water vapor (H2O): irrigation.

    • Carbon dioxide (CO2): burning fossil fuels in power plants and cars.

    • Methane (CH4): flooded rice fields, farm animals, processing of coal.

    • Nitrous oxide (NO2): burning fossil fuels, manufacture of cement, fertilizers.


The wavelength at which the curve peaks can be associated with temperature through Wien's law

Earth's Energy Balance

3.png is a student initiative to provide free material to help international students prepare for the IB exams.

For any feedback or comments, contact a fellow IB alumni:

Maria Eduarda Lopes |

Disclaimers: the IB organization does not endorse this website's content.

All images are under copyright that allows them to be shared.

(the source and copyright details may be downloaded by clicking on the images)