Energy and Environment / Sustainability: Efficiency, Emissions and Life Cycle
This domain links energy conversion to environmental load: efficiency, emission factors, greenhouse gas accounting, renewable resource potential and sustainability frameworks. Quantitative items usually involve carbon intensity or power calculations.
Core rules and concepts
- Conversion and capacity: efficiency = useful output/energy input; capacity factor = actual generation/(rated capacity x time); primary versus final energy accounting.
- Emissions: carbon intensity in kg CO2/kWh; combustion stoichiometry (44/12 kg CO2 per kg carbon); GWP values convert non-CO2 gases to CO2 equivalent (CH4 about 28, N2O about 265 under AR5).
- Renewables: wind power P = 0.5 rho A v^3 with the Betz limit of 16/27 (about 0.593); solar PV efficiency and capacity factor; biogas at roughly 55-65% CH4; small hydro.
- Sustainability tools: life cycle assessment (cradle to grave), carbon footprint, circular economy, extended producer responsibility, polluter-pays principle, SDG 7 and SDG 13.
Worked example
A 100 MW coal plant with 35% efficiency burns coal of 25 MJ/kg lower heating value and 70% carbon content. Fuel power = 100/0.35 = 285.7 MW; coal rate = 285.7 x 10^6/25 x 10^6 = 11.43 kg/s; carbon = 8.0 kg/s; CO2 = 8.0 x 44/12 = 29.3 kg/s. Per hour at 100 MWh, that is 29.3 x 3600/100,000 = 1.06 kg CO2/kWh. The reasoning step is separating fuel energy from electrical output using efficiency.
Traps that cost marks
Confusing capacity factor with efficiency; mixing GWP values from different IPCC assessment reports; using higher heating value where lower heating value is intended; and treating CO2 equivalent as identical to CO2 mass.