Energy Production
This section covers Energy Production. The exam tests energy sources, power and efficiency, specific energy, Sankey diagrams, and the environmental and practical features of fossil, nuclear and renewable generation.
Core rules
- Energy is conserved but can be transferred to less useful stores. Power is energy transferred per unit time, P = E/t.
- Efficiency = useful output energy / total input energy × 100%, or useful output power / total input power × 100%.
- Specific energy is energy released per unit mass, often in MJ kg⁻¹. Fuels with high specific energy can supply more energy for a given mass.
- Sankey diagrams show input energy as a wide arrow; branches represent useful output and waste, with widths proportional to energy.
- Renewable sources include solar, wind, hydroelectric, wave and geothermal. Fossil fuels and nuclear fuel are non-renewable. Nuclear fission releases energy from mass defect.
Worked example
A power station receives 500 MW of energy from fuel per second and delivers 175 MW of electrical power. Its efficiency is 175/500 × 100% = 35%. The waste power is 500 − 175 = 325 MW. In one hour, useful electrical energy is E = Pt = 175 MW × 3600 s = 6.3 × 10¹¹ J, or 175 MWh in practical units.
Traps
Do not confuse energy with power. Efficiency cannot exceed 100%; if a calculation does, the input and output have been mixed up. A renewable source is not automatically low-impact or reliable. Capacity factor differs from maximum power rating. For fission, mass defect converts to energy; mass is not simply destroyed. Always identify whether the question asks for annual energy, instantaneous power or efficiency.