Fossil Fuels, Hydroelectric & Geothermal Energy – Comprehensive Study Notes

Fossil Fuels: Overview

  • Objectives of the lesson
    • Describe how fossil fuels are formed
    • State the importance of energy, its various sources and uses
  • Definition & General Traits
    • World’s primary source of energy
    • Formed from prehistoric plants & animals over millions of years\text{millions of years} ⇒ non-renewable
    • Contain a high percentage of carbon
    • Everyday refined products include kerosene & propane

Major Types of Fossil Fuels

  • Coal – solid fuel derived from ancient terrestrial plants
  • Petroleum (Crude Oil) – liquid fuel sourced from marine organisms
  • Natural Gas – gaseous fuel created from decayed organic matter (often forms with oil)

Coal

  • Physical description
    • Black/brownish-black sedimentary rock
    • Occurs in rock strata called coal beds/seams (swamp deposits)
  • Chemical make-up (dominantly carbon) with appreciable
    • Hydrogen, Sulfur, Oxygen, Nitrogen

Formation Process (Coalification)

  1. Swamp with dense vegetation develops
  2. Flooding due to tectonic uplift/sea-level rise buries the swamp
  3. Death & burial of plant material
  4. Sedimentation – mud/sand stack over organic debris
  5. Slowed decomposition
    • Growing overburden weight
    • Material sealed from air (anaerobic)
    • Basin subsides ⇒ temperature rises
  6. Transformation sequence (diagenesis ➔ metamorphism)
    Peat → Lignite (brown coal) → Sub-bituminous → Bituminous → Anthracite\text{Peat}\,\rightarrow\,\text{Lignite (brown coal)}\,\rightarrow\,\text{Sub-bituminous}\,\rightarrow\,\text{Bituminous}\,\rightarrow\,\text{Anthracite}
    • Each step = higher temperature & pressure
  • Geological context
    • Originates in swampy forests
    • Major episodes occurred >>250 Ma, pre-dinosaur eras

Petroleum & Natural Gas

  • Definition: Naturally occurring liquid mixture of hydrocarbons in Earth’s crust (crude oil) + associated natural gas
  • Biological source: Plankton, algae & other marine organisms buried under sediment

Depositional Requirements

  1. Death of marine plants & animals
  2. Burial in O2_2-poor (anoxic) settings
  3. Avoid decomposition (not scavenged / not oxidised)
  4. Continuous accumulation of organic-rich sediments
  5. Unit subsides & deepens over geologic time
  6. At depths ≈ 2000 m2000\,\text{m} / temperatures ≈ 100∘C100^\circ\text{C} ⇒ kerogen starts expelling hydrocarbons
  7. Oil window: 2000–3800 m2000–3800\,\text{m} – kerogen ➔ crude oil
  8. Gas window: 3800–5000 m3800–5000\,\text{m} – liquids crack, methane (lightest HC) dominates
  9. 8000–10 000 m8000–10{\,}000\,\text{m} – thermal destruction of hydrocarbons
  • Time scale
    • Burial rate ≈ 50 m50\,\text{m} sediment / 1 Ma1\,\text{Ma}
    • ~60 Ma60\,\text{Ma} typically needed to form large oil pools
  • Organic source rule
    • Animal-rich debris → more oil
    • Plant-rich debris → more gas

Hydrocarbon Traps

Structural Traps
  • Caused by tectonic deformation
  • Anticlines or domes bend/fold reservoir & seal rock
Stratigraphic Traps
  • No tectonic folding
  • Hydrocarbons sealed by cap rock (e.g.\n salt dome)
Preservation & Leakage
  • Shallow traps (<1000 m1000\,\text{m}) exposed to meteoric water ⇒ oxygen & bacteria degrade HCs into H<em>2<em>2O, CO</em>2</em>2, tar
  • Deep traps (>1000 m1000\,\text{m}, >50^\circ\text{C}) still vulnerable to tectonic fracturing & leakage

Fossil-Fuel Power Plant Flow (Thermal)

  1. Combustion chamber – coal/oil/gas burnt → heat
  2. Steam boiler – water → high-pressure steam
  3. Steam valve – directs flow
  4. Steam turbine – steam → mechanical energy (rotation)
  5. Synchronous generator – rotation → AC electricity
  6. AC power output – delivered to grid
  7. Steam condenser – cools exhaust steam
  8. Pump – recycles water ⇒ closed cycle repeats

Hydroelectric Energy

  • Converts gravitational potential of water into electricity
  • Head (height difference) ∝ potential energy ⇒ higher head → more power

Key Components & Roles

  • Dam: creates reservoir, controls flow
  • Sluice gates: release water
  • Penstocks: large pipes; water accelerates under gravity (potential → kinetic)
  • Turbine: fast water spins blades (kinetic → mechanical)
  • Generator: rotor (electromagnet) inside stator coils (mechanical → electrical via induction)
  • Step-up transformer: boosts voltage for long-distance lines
  • Transmission lines: carry high-V electricity to substations
  • Step-down transformer: lowers V to safe 220 V220\,\text{V} / 110 V110\,\text{V} for users

Energy Transformation Chain

Gravitational PE→Kinetic→Mechanical→Electrical\text{Gravitational PE} \rightarrow \text{Kinetic} \rightarrow \text{Mechanical} \rightarrow \text{Electrical}

Advantages

  • Renewable (hydrologic cycle)
  • Clean – negligible GHG emissions
  • Reliable – 24/724/7 generation (independent of sun/wind)
  • Low operating cost once built
  • Energy storage possible (pumped-storage = “giant battery”)

Disadvantages

  • Environmental impact – flooding, river ecosystem disruption
  • Community displacement due to large reservoirs
  • High capital cost for construction
  • Drought-sensitivity – low inflow ⇒ reduced output
  • Location-specific – needs suitable topography & water supply

Additional Uses

  • Electricity supply
  • Water management – irrigation, flood control, storage
  • Transportation – navigation locks/channels around dams

Geothermal Energy

  • Harnesses heat from Earth’s interior
  • Found near volcanic / tectonically active regions & hot springs
  • Renewable & used for heating + power generation

Geological Heat Sources

  • Radiogenic decay of isotopes
  • Residual heat from Earth’s formation (≈4.5 Ga4.5\,\text{Ga})

System Components

  • Geothermal reservoir – hot water/steam trapped below surface
  • Production well – drills to reservoir; pressure drives steam upward
  • Turbine – converts thermal → mechanical
  • Generator – mechanical → electrical (EM induction)
  • Rotor vs Stator mnemonic
    • Rotor – ROTates (inside)
    • Stator – STays still (outer)
  • Step-up transformer – raises voltage → minimise line losses
Why High Voltage Reduces Losses

  • Ploss=I2RP_{loss}=I^2 R; using high V ⇒ low I

  • Lower current ⇒ less resistive heating ⇒ efficient long-distance delivery

  • Condenser – cools spent steam

  • Injection well – reinjects cooled water back underground (sustainability)

  • Step-down transformer – drops voltage to user level

Geothermal Power-Plant Types

  1. Dry Steam
    • Steam ≥230∘C\ge 230^\circ\text{C} directly drives turbine (no flashing/boiling step)
  2. Flash Steam
    • Water >180^\circ\text{C} pumped up; pressure drop ⇒ instant flash to steam (analogy: soda fizz)
  3. Binary Cycle • Geothermal fluid <170^\circ\text{C} heats secondary working fluid (e.g. isobutane, pentane) with lower boiling point • Closed-loop:
    1. Geo-water heats secondary
    2. Secondary boils → turbine
    3. Geo-water cooled & reinjected
    4. Secondary condenses → reused

Challenges / Disadvantages

  • Potential induced seismicity (earthquakes)
  • Possible release of CO<em>2<em>2 & H</em>2</em>2S

Direct-Use & Other Applications

  • District heating, greenhouses, spas
  • Geothermal heat pumps for heating/cooling
  • Electricity via the above plant types