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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.

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. 

 

Conduction

  • 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.

 

Convection

  • 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.

Topic 8: Energy Production

8.1 Energy Production

8.2 Thermal Energy Transfer