Describe how useful energy may be obtained, or electrical power generated, from: (a) chemical energy stored in fossil fuels (b) chemical energy stored in biofuels (c) water, including the energy stored in waves, in tides and in water behind hydroelec

1.7.3 Energy Resources

Scope (Cambridge IGCSE 0625) – The syllabus requires that students understand how useful energy or electrical power is obtained from three main resources and how each fits into the generic conversion chain:

• chemical energy stored in fossil fuels (coal, oil, natural gas)
• chemical energy stored in bio‑fuels (ethanol, biodiesel, biogas, wood)
• energy stored in water – potential energy behind a dam, kinetic & potential energy of surface waves, and kinetic/potential energy of tidal motions

All three follow the same overall sequence:

Primary energy (chemical / potential / kinetic) → heat or pressure → mechanical rotation → electrical generator → grid.

Key equations (syllabus notation)

(a) Chemical energy stored in fossil fuels

Why fossil fuels release heat

• Fossil fuels consist mainly of long‑chain hydrocarbons (C–H bonds). Breaking these bonds and forming CO₂ and H₂O releases the stored chemical energy.
• Typical calorific values (MJ kg⁻¹): coal ≈ 30, gasoline ≈ 44, natural‑gas ≈ 55.

Typical combustion reaction (general hydrocarbon \(C{x}H{y}\))

\[

\mathrm{C{x}H{y}+ \Bigl(x+\frac{y}{4}\Bigr)O{2}\;\longrightarrow\;x\,CO{2}+ \frac{y}{2}\,H_{2}O+\text{heat}}

\]

Conversion pathway

Key performance numbers

• Overall thermal‑to‑electrical efficiency of modern coal/oil/gas plants: 30–35 %.
• Losses arise from:

  • Heat loss in the boiler (radiation, convection)
  • Steam‑turbine mechanical friction
  • Generator copper & iron losses

Example calculation

A 500 MW coal plant burns 100 kg s⁻¹ of coal (ΔH₍comb₎ = 32 MJ kg⁻¹).

\[

Q = 100 \times 32\times10^{6}=3.2\times10^{9}\ \text{J s}^{-1}=3.2\ \text{GW}

\]

\[

P_{\text{el}} = 0.35 \times 3.2\ \text{GW}\approx 1.12\ \text{GW}

\]

(The plant would be rated at the nearest standard size, e.g. 1 GW.)

Advantages & disadvantages

(b) Chemical energy stored in bio‑fuels

What are bio‑fuels?

• Derived from recent biological material (plants, animal waste).
• Typical syllabus fuels: ethanol, biodiesel, biogas (CH₄), and dry wood.

Energy densities (MJ kg⁻¹)

Bio‑fuels are generally lower in energy density than fossil fuels, and many contain moisture, which further reduces the usable heat.

Combustion chemistry (simplified)

\[

\mathrm{CH{2}O + O{2}\;\longrightarrow\;CO{2}+H{2}O+\text{heat}}

\]

Conversion pathway (identical to fossil‑fuel plants)

Performance

• Overall efficiency typically ≈ 30 % – slightly lower than fossil‑fuel plants because of lower calorific value and higher moisture.
• Advantages: renewable (if feedstock is sustainably managed), lower net CO₂ emissions.
• Disadvantages: larger fuel volume required, possible competition with food production, still emit CO₂ when burned.

Example – Ethanol power plant

Fuel flow = 150 kg s⁻¹, ΔH₍comb₎ = 27 MJ kg⁻¹.

\[

Q = 150 \times 27\times10^{6}=4.05\times10^{9}\ \text{J s}^{-1}=4.05\ \text{GW}

\]

\[

P_{\text{el}} = 0.30 \times 4.05\ \text{GW}\approx 1.22\ \text{GW}

\]

(c) Water‑based energy (hydroelectric, wave and tidal)

Why water is an excellent energy carrier

• High density (≈ 1000 kg m⁻³) → large mass for modest volumes.
• Both gravitational potential (height) and kinetic energy (velocity) can be harvested.

1. Hydroelectric dams (potential energy)

• Energy stored in a volume \(V\) of water at height \(h\):

  \[ E_{\text{p}} = \rho g V h \]

• Power from a steady flow \(\dot V\):

  \[ P = \rho g h \,\dot V \] (ideal, neglecting losses)

• Typical overall efficiency: 80–90 % – high because there is no combustion and mechanical losses are relatively small.

2. Wave energy (kinetic + potential of surface waves)

• Average power per metre of wave crest in deep water:

  \[ P{\text{wave}} = \frac{1}{8}\,\rho g H^{2}\,c{g} \]

  where \(H\) = wave height (peak‑to‑trough) and \(c_{g}\) = group velocity.

• Common conversion devices:

  • Point absorbers – vertical buoys driving linear generators.
  • Oscillating water columns – trapped air column drives a turbine.
  • Attenuators – flexible floating structures.

• Typical prototype efficiency: 20–40 % (still under development).

3. Tidal power

3.1 Tidal‑range (barrage) plants

• Energy released each half‑cycle:

  \[ E = \frac12\,\rho g\,A\,H^{2} \]

  (\(A\) = basin area, \(H\) = difference between high‑ and low‑tide levels).

• Two cycles per tidal day → total daily energy ≈ \(2E\).
• Overall efficiency: 30–40 %.

3.2 Tidal‑stream turbines

• Extract kinetic energy from fast‑moving tidal currents:

  \[ P = \frac12\,\rho A v^{3} \]

  (\(v\) = current speed, \(A\) = swept area of the rotor).

• Typical efficiency: 25–35 %.

Advantages & disadvantages of water‑based technologies

Summary of water‑based energy

Key points to remember for the IGCSE exam

• All resources follow the chain: primary energy → heat/pressure → mechanical rotation → electricity.
• Fossil‑fuel and bio‑fuel plants use the same thermal‑power cycle; the only differences are calorific value, moisture content and associated emissions.
• Water‑based systems do not involve combustion, which explains their higher efficiencies and lack of local emissions.
• Memorise the seven core equations listed in the “Key equations” box and the symbols used (e.g., \(h\) for head, \(\dot V\) for flow rate).
• Typical efficiency ranges:

  • Coal/oil/gas plant – 30–35 %
  • Bio‑fuel plant – ≈ 30 %
  • Hydroelectric – 80–90 %
  • Wave – 20–40 % (prototype)
  • Tidal‑range – 30–40 %
  • Tidal‑stream – 25–35 %

• Be able to discuss at least one advantage and one disadvantage for each resource type.

Suggested revision diagrams

• Flow‑chart of a thermal power plant (fuel → boiler → steam turbine → generator).
• Cross‑section of a hydroelectric dam showing head, penstock, turbine and generator.
• Schematic of a point‑absorber wave device.
• Diagram of a tidal‑range barrage with sluice‑gate turbines.
• Illustration of a tidal‑stream turbine in a fast‑moving current.