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

In this section we describe how useful energy can be obtained or electrical power generated from a variety of natural and human‑made resources. Each subsection focuses on a particular source and outlines the main conversion processes, key components and typical equations used to quantify the energy transfer.

(a) Chemical Energy Stored in Fossil Fuels

Fossil fuels (coal, oil, natural gas) contain high‑energy carbon‑hydrogen bonds. When oxidised they release chemical energy that is converted to heat, then to mechanical and electrical energy.

• Combustion reaction: \$CxHy + O2 \rightarrow CO2 + H_2O + \text{heat}\$
• Heat transfer: \$Q = mc\Delta T\$ (mass of fuel, specific heat, temperature rise)
• Steam generation: \$P_{\text{steam}} = \rho g h Q\$ (pressure from water column)
• Mechanical work: \$W = P_{\text{steam}} \times V\$ (volume of steam)
• Electrical generation: \$P{\text{el}} = \eta{\text{turbine}}\eta{\text{generator}}P{\text{steam}}\$

Typical plant layout: fuel → boiler → steam turbine → generator → transformer → grid.

(b) Chemical Energy Stored in Biofuels

Biofuels such as ethanol, biodiesel or biogas are produced from biological material. Their combustion follows similar chemistry to fossil fuels but with lower carbon emissions.

• Biomass conversion: \$CH2O + O2 \rightarrow CO2 + H2O + \text{heat}\$
• Energy density: \overline{10}–20 MJ kg⁻¹ (ethanol) vs. 30–40 MJ kg⁻¹ (petrol)
• Process: fuel → combustion chamber → boiler → turbine → generator

(c) Water Energy (Hydro, Waves, Tides)

Water can store potential and kinetic energy that is harnessed by turbines.

Hydroelectric Dams

• Potential energy: \$E_p = mgh\$ (mass of water, gravity, head height)
• Water flows through turbine → mechanical rotation → generator → electricity.

Wave Energy

• Waves carry kinetic energy: \$E_k = \frac{1}{8}\rho g H^2L\$ (wave height \$H\$, wavelength \$L\$)
• Devices (point absorbers, oscillating water columns) convert motion to mechanical work.

Tidal Power

• Energy from tidal range: \$E = \frac{1}{2}\rho g H^2A\$ (area \$A\$, tidal range \$H\$)
• Barrages or tidal stream turbines convert flow to electricity.

(d) Geothermal Resources

Heat from the Earth’s interior is extracted via steam or hot water.

• Geothermal steam: \$P_{\text{steam}} = \rho g h Q\$ (similar to hydro but with geothermal head)
• Binary cycle: heat transferred to secondary fluid with lower boiling point → turbine → generator.
• Typical plant: well → heat exchanger → turbine → generator.

(e) Nuclear Fuel

Controlled fission of heavy nuclei releases large amounts of energy.

• Fission reaction: \$^{235}\text{U} + n \rightarrow \text{fission fragments} + 3n + Q\$ (Q ≈ 200 MeV)
• Heat generation: \$Q_{\text{heat}} = \text{power density} \times \text{fuel volume}\$
• Heat → steam → turbine → generator.
• Key components: reactor core, moderator, control rods, coolant, steam turbine, generator.

(f) Solar Cells (Photovoltaic)

Solar cells convert incident photons directly into electric current.

• Photovoltaic effect: \$E_{\text{photon}} = h\nu\$ (photon energy)
• When \$E{\text{photon}} > Eg\$ (band gap), an electron is excited, creating an electron–hole pair.
• Built‑in electric field drives electrons to the external circuit → current \$I = q n v_d\$.
• Power output: \$P{\text{PV}} = V{\text{oc}} I_{\text{sc}} FF\$ (open‑circuit voltage, short‑circuit current, fill factor).

(g) Solar Thermal (Infrared & Other EM Waves)

Solar radiation heats a fluid, which is used to generate steam.

• Absorbed power: \$P_{\text{abs}} = \alpha I A\$ (absorptivity \$\alpha\$, irradiance \$I\$, area \$A\$)
• Heat transfer to water: \$Q = mc\Delta T\$
• Water → boiler → steam turbine → generator.
• Typical components: solar collector, boiler, turbine, generator.

Summary Table of Energy Conversion Processes