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TOPIC 8
ENERGY PRODUCTION
ENERGY SOURCES
PRIMARY SOURCES: all types of unprocessed energy sources that can be found in
nature
SECONDARY SOURCES: processes / exploited to mechanical work / electrical forms
EG: electricity
RENEWABLE SOURCES: readily replenished - a fuel that is creased faster / equally fast
as it is consumed EG: solar energy / wind energy
NON-RENEWABLE SOURCES: a fuel that is consumed faster than it is created - will be
depleted EG: fossil fuels / nuclear fuel
RENEWABLE ENERGY SOURCES NON-RENEWABLE ENERGY SOURCES
● solar power ● wind power ● hydroelectric power ● tidal power ● geothermal ● biomass ● coal ● oil ● natural gas ● nuclear power
SPECIFIC ENERGY (Es): amount of energy that can be extracted from a unit mass of
fuel ⇒ energy / mass
ENERGY DENSITY (Ed): amount of energy that can be extracted from a unit volume of
fuel mass ⇒ energy / volume
FUEL SPECIFIC ENERGY (Es)
J kg¯¹
ENERGY DENSITY (Ed)
J m¯³ uranium - 235 7.0 x 10¹³ 1.3 x 10¹⁸ hydrogen 1.4 x 10⁸ 1.0 x 10⁷ natural gas 5.4 x 10⁷ 3.6 x 10⁷ gasoline 4.6 x 10⁷ 3.4 x 10¹⁰ kerosene 4.3 x 10⁷ 3.3 x 10¹⁰ diesel 4.6 x 10⁷ 3.7 x 10¹⁰ coal 3.2 x 10⁷ 7.2 x 10¹⁰
SANKEY DIAGRAMS: show the energy transfer & loss
⇒ width of each arrow is proportional to the amount of energy transfer
ENERGY DEGRADATION
● while energy may be completely converted into work in a single process, a CYCLICAL PROCESS is required to convert thermal energy to work continuously ● in accordance with the 2ND LAW OF THERMODYNAMICS , some energy will be lost to the surroundings & cannot be used to perform useful work
DEGRADED ENERGY: unavailable energy
FOSSIL FUELS: produced by the decomposition of buried animals & plant material due
to pressure & bacteria EG: oil / coal / natural gas
NUCLEAR REACTOR
NUCLEAR FUEL: the fuel is held in rods so that the neutrons released will fly out & cause
nuclear fission in other rods
GRAPHITE CORE: slows the neutrons down so that they are more likely to be absorbed
into a nearby fuel rod
CONTROL RODS: raised / lowered to stop neutrons from travelling between fuel rods &
therefore, change the speed of the chain reaction ⇒ absorb any excess neutrons
MODERATOR: slows down the fast moving neutrons so they can be absorbed by fuel rods
⇒ collide with H₂O molecules & transfer KE so slow down ordinary / heavy H₂O
COOLANT: heated by the energy released from the fission reactions & is used to boil H₂O
to drive turbines in the power station
CONCRETE SHIELD: the daughter products of the fission reaction are radioactive & can
be hazardous ⇒ many of the features of the reactor are designed to control the speed of the reaction & the T°C inside the shielding ⇒ an uncontrolled fission reaction can cause an explosion
RISKS WITH NUCLEAR POWER
● fission of U-238 ending up in Plutonium- ADVANTAGE: non-fissionable U-238 can be used to produce energy RISK: Pu-239 can be used as a NUCLEAR WEAPON ● NUCLEAR WASTE is ↑ radioactive with long half-life ○ currently, buried deep underground in containers avoiding leakage
ADVANTAGES DISADVANTAGES
- ↑ power output ⇒ ↑ energy density
- large reserves of nuclear fuels
- nuclear power stations do not produce ghgs
- radioactive waste products are 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
SOLAR PANELS: collect heat from the Sun which is used to heat the H₂O
in pipes underneath ● flat collecting surface ● coating: ↓ reflection ● black surface below collects sunlight which heats up the H₂O in the pipe underneath for use in the house ADVANTAGE: cheap DISADVANTAGE: bulky
PHOTOVOLTAIC CELLS: convert light directly into electricity
● sunlight incident on photovoltaic cell makes the e¯ excited ⇒ establish a potential difference
ADVANTAGES DISADVANTAGES
- ‘free’
- inexhaustible
- clean
- ↓ power output
- works during the day only
- affected by cloudy weather
- ↓ power output
- requires large area
- initial $$ ↑
WIND POWER: power derived from moving masses of air
⇒ typically ∼30% of the power carried in wind is converted to energy
BIGGEST LOSS OF EFFICIENCY: wind cannot be stopped completely by the turbine
⇒ not all KE is transferred
ADVANTAGES DISADVANTAGES
- ‘free’
- inexhaustible
- clean
- dependent on local wind conditions
- aesthetic problems - large infrastructures
- noise problems 1. mass of air = ƿ AvΔT 2. KE of air = ½mv² = ½ ƿ AvΔtv² = ½ ƿ AΔtv³ 3. power = E/Δt = ½ ƿ Av³
THERMAL ENERGY TRANSFER
CONDUCTION: transfer of energy due to ↑ energy e¯ colliding with neighbouring
molecules
CONVECTION
EG: air over a hot radiator in a room is heated, expands & rises, transferring warm air to the rest of the room - cold air takes its place through convection currents & the process repeats
RADIATION: transfer of energy through electromagnetic induction
BLACK-BODY RADIATION
● all bodies with a finite (Kelvin) T°C radiate energy in the form of EM radiation (due to oscillating electric charges)
STEFAN-BOLTZMAN LAW
EMISSIVITY: measures how effectively a black-body radiates ⇒ 0 ≤ 𝜺 ≤ 1
BLACK BODY: perfect absorber & radiator ⇒ none of the incident radiation is reflected
(not shiny) ⇒ 𝜺 = 1 ● anything which has a black & dull surface is close to a black body EG: coal at ↓ T°C: radiates a little bit & looks dull at ↑ T°C: radiates a lot & looks bright
ABSORPTION VS RADIATION
SURFACE T°C OF A BLACK BODY: Tb SURROUNDING T°C: Ts POWER ABSORBED: Pabs = 𝜺𝝈ATs⁴ POWER RADIATED: Prad = 𝜺𝝈ATb⁴ THERMAL EQUILIBRIUM: Tb = Ts / Pabs = Prad ● EM radiation with different 𝝺 is emitted - a continuous distribution of 𝝺 ● most energy is radiated at a specific 𝝺max determined by the **WIEN’S DISPLACEMENT LAW *** a body with ↑T°C emits more at short 𝝺 INTENSITY: power of radiation received per unit area
SOLAR CONSTANT (S): amount of solar radiation at the mean distance from the Sun to
the Earth ***** assuming the Sun is a black body - calculate the power radiated by using the Stefan-Boltzman law P = 3.9 x 10²⁶ W
ENERGY BALANCE
● at EQUILIBRIUM , the energy input must be equal to the energy output ● the effects of the Earth's atmosphere on its mean surface T°C ⇒ GREENHOUSE EFFECT
TEMPERATURE AT EARTH’S SURFACE
1) the Earth radiates power from the entire surface area of its spherical shape, so the power radiates is: 2) assume the Earth is a black body ⇒ 𝜺 = 1 3) equating the incident & outgoing intensities, we get: 4) solving the equation, we get: 5) T = -17°C ***** actual average T°C of the Earth surface = 288K / 15°C
THE GREENHOUSE EFFECT
GREENHOUSE GASES: CO₂ / CH₄ / H₂O / N₂O ⇒ found in the Earth’s atmosphere
⇒ both natural & man-made origins ● by using WIEN’S DISPLACEMENT LAW with T=288K, we find that the peak 𝝺 that is
radiated by the Earth is in the INFRARED REGION
● infrared radiation is STRONGLY ABSORBED by greenhouse gases ○ e¯ transition between the MOLECULAR ENERGY LEVELS ○ the energy difference corresponds to energy of photons with infrared 𝝺 ● the absorbed radiation is eventually RE-RADIATED IN ALL DIRECTION ⇒ some of the re-radiated energy is absorbed by the Earth again causing the surface to be warmer GREENHOUSE GAS NATURAL SOURCES ANTHROPOGENIC SOURCES H₂O evaporation of H₂O from oceans / rivers / lakes irrigation O₂ forest fires / volcanic eruptions / evaporation of H₂O from oceans burning fossil fuels in power plants & cars / burning forests CH₄ wetlands / ocean / lakes & rivers / termites flooded rice fields / farm animals / processing of coal / natural gas & oil / burning biomass N₂O forests / oceans / soil & grasslands burning fossil fuels / manufacture of cement / fertilisers / deforestation (↓ of nitrogen fication in plants)
