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

Understanding how useful energy can be obtained from various natural and man‑made sources is essential for the IGCSE Physics syllabus. The following notes describe the principal methods of generating electrical power or useful heat from each resource.

(a) Chemical energy stored in fossil fuels

Fossil fuels (coal, oil, natural gas) are hydrocarbons formed from ancient organic matter. Their chemical energy is released by combustion:

• Fuel + \$O_{2} \rightarrow\$ CO₂ + H₂O + heat
• The heat raises the temperature of water in a boiler, producing high‑pressure steam.
• Steam drives a turbine which is coupled to an electrical generator.

(b) Chemical energy stored in biofuels

Biofuels (e.g., ethanol, biodiesel) are derived from recent biological material. The conversion process is similar to fossil fuels:

1. Fermentation or trans‑esterification produces the fuel.
2. Combustion in a boiler releases heat: \$C{x}H{y} + O{2} \rightarrow CO{2} + H_{2}O + Q\$.
3. Steam drives a turbine → generator.

Because the carbon originated recently, the net increase in atmospheric CO₂ can be lower than for fossil fuels, depending on cultivation and processing methods.

(c) Water – waves, tides and hydroelectric dams

Water stores kinetic and potential energy that can be converted to electricity.

Wave energy

• Surface waves carry energy \$E = \frac{1}{2}\rho g A^{2} \lambda\$, where \$A\$ is amplitude and \$\lambda\$ wavelength.
• Oscillating water columns or floating buoys convert the up‑and‑down motion into rotary motion that drives a generator.

Tidal energy

• Gravitational interaction between Earth, Moon and Sun creates predictable high and low tides.
• Tidal barrages trap water at high tide; when released, the water flows through turbines.

Hydroelectric dams

• Water stored at height \$h\$ possesses gravitational potential energy \$E = mgh\$.
• When released, it flows through penstocks, turning turbines.
• Typical arrangement: reservoir → penstock → turbine → generator → transmission lines.

(d) Geothermal resources

Heat from the Earth’s interior can be accessed in two main ways:

1. Dry steam plants: Steam naturally rises from hot rocks, is piped directly to turbines.
2. Binary cycle plants: Hot water transfers heat to a secondary fluid with a lower boiling point; the secondary fluid vaporises and drives the turbine.

In both cases the turbine is coupled to a generator. The process is continuous and not dependent on weather.

(e) Nuclear fuel

Energy is released from the nucleus via fission of heavy atoms (e.g., \$^{235}\$U). The reaction:

\$^{235}\text{U} + n \rightarrow ^{236}\text{U}^* \rightarrow \text{fission fragments} + 2-3\,n + 200\ \text{MeV}\$

Key steps in a nuclear power plant:

• Fission heats water in a primary loop, creating high‑pressure steam.
• Steam drives a turbine → generator.
• Heat exchangers transfer waste heat to a secondary cooling system (often a cooling tower).

(f) Light from the Sun – solar cells (photovoltaic)

Solar cells convert photons into electrical energy via the photovoltaic effect.

• When a photon with energy \$E_{\text{ph}} \geq \phi\$ (work function) strikes a semiconductor, it excites an electron from the valence band to the conduction band, creating an electron–hole pair.
• Built‑in electric field at the p‑n junction separates the charges, producing a current \$I\$ when the circuit is closed.
• Power output \$P = VI\$, where \$V\$ is the cell voltage (typically 0.5–0.6 V per cell).

(g) Infrared and other electromagnetic waves – solar thermal panels

Solar thermal systems absorb infrared and visible radiation to heat a fluid, which then produces steam to drive a turbine.

1. Sunlight strikes a collector (often a dark‑coated metal tube with a glass cover).
2. Fluid (water or oil) circulates, absorbing heat: \$Q = mc\Delta T\$.
3. Heated fluid passes to a boiler where water is turned into high‑pressure steam.
4. Steam drives a turbine → generator.

Comparison of Energy Resources

Key Points to Remember

• All energy conversion processes obey the conservation of energy; efficiencies are limited by practical losses (heat, friction, resistance).
• Electrical power \$P\$ is related to energy \$W\$ and time \$t\$ by \$P = \dfrac{W}{t}\$.
• Renewable resources (water, geothermal, solar, biofuels) are preferred for long‑term sustainability, but each has site‑specific constraints.
• Understanding the role of the boiler, turbine and generator is central to most thermal power schemes, whether the heat originates from combustion, nuclear fission or solar radiation.