Energy and power: energy sources, generation, conversion and environmental impact
What the exam tests
Energy sources and conversion chains, efficiency, power calculations, capacity factor, and environmental impact.
Sources and conversion
- Fossil: chemical → thermal → mechanical → electrical; emissions include CO2, SO2 and NOx.
- Nuclear fission: U-235 chain reaction; no CO2 at operation but radioactive waste.
- Hydro: potential → kinetic → mechanical; high efficiency, site-dependent.
- Wind: kinetic → mechanical; power P = ½ρAv³, so doubling rotor diameter quadruples power and doubling wind speed multiplies it by eight.
- Solar PV: photons → electron–hole pairs; typical efficiency 15–22%; output depends on irradiance and temperature.
- Solar thermal, biomass, geothermal and tidal are further options.
Efficiency and systems
η = useful output/input. Carnot limit η_C = 1 − T_c/T_h using absolute temperatures. Combined-cycle gas turbines reach about 60%. Electrical transmission losses depend on I²R; high voltage reduces current for the same power. Energy storage includes batteries, pumped hydro, flywheels and hydrogen. Capacity factor = actual annual output/(rated power × 8760 h). Intermittent sources need backup or storage.
Environmental impact
Life-cycle assessment covers manufacturing, operation and decommissioning. Compare gCO2/kWh, land use, water use and material criticality. Mitigation includes efficiency, recycling, carbon capture and demand management.
Worked example
A wind turbine has rotor diameter 80 m and rated power 2 MW. Area = π×40² = 5027 m². At ρ = 1.225 kg/m³ and v = 10 m/s, available power = ½×1.225×5027×1000 = 3.08 MW. With Cp = 0.45 and generator efficiency 0.95, electrical output is about 3.08×0.45×0.95 = 1.32 MW. At 12 m/s the available power becomes 5.32 MW, so the turbine reaches its 2 MW rating and pitch control limits output.
Traps
- Using Celsius in the Carnot formula.
- Mixing kWh (energy) and kW (power).
- Forgetting the v³ dependence of wind power.
- Quoting installed capacity as actual production.
- Ignoring life-cycle emissions.