Point 7 of 7 — Renewable Energy Sources, Green Hydrogen and Climate AdaptationQuestion bank: 0 questions (0 with pictures)
07Worked examples
A wind turbine has blades 40 m long and the wind speed is 10 m/s; ρ = 1.225 kg/m³. Estimate the power in the wind. — A = π(40)² = 5,027 m²; P = ½(1.225)(5027)(10³) ≈30.8 MW, and the Betz limit gives at most 59% = 18 MW.
How much electricity does a 500 MW electrolyser consume in a day, and how much hydrogen does it make at 50 kWh/kg? — 500 × 24 =12,000 MWh; 12,000,000/50 =240 t of hydrogen.
A household solar water heater of 2 m² with 50% efficiency in 800 W/m² insolation heats 150 L. Find the temperature rise in 5 h. — Energy = 2 × 800 × 0.5 × 5 × 3600 = 14.4 MJ; ΔT = 14.4×10⁶/(150 × 4184) ≈23 °C.
Compare the CO₂ of 1 kg of green hydrogen with 1 kg of grey hydrogen. — Green: about0.5 kg CO₂ (only manufacturing and electricity losses) against 9–10 kgfor grey, a reduction of roughly 95%.
Why is hydrogen liquid at −253 °C and why is that a problem? — Its boiling point is 20 K because H₂ is a light non-polar molecule with only weak dispersion forces; cryogenic losses make transport expensive, which is why ammonia is preferred for shipping.
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