Renewable Energy Technologies
This domain tests conversion physics, resource assessment and performance rating for solar, wind, hydro, geothermal, biomass and heat pumps. Most questions ask you to take a rated value, apply the real-world derates and produce a defensible energy estimate.
Core rules and concepts
- PV rating conditions. Standard test conditions are 1,000 W/m² irradiance, 25 °C cell temperature and AM 1.5 spectrum. The power temperature coefficient for crystalline silicon is typically -0.25 to -0.45 %/°C, so hot modules lose output even in strong sun.
- PV system losses. The performance ratio, usually 0.75-0.85, bundles soiling, mismatch, wiring, inverter and availability losses. Bifacial modules add a 5-15% rear-side gain depending on albedo and mounting height; single-axis tracking adds a further 15-25% over fixed tilt in high-DNI sites.
- Wind physics. Power in the wind is 0.5·ρ·A·v³. The Betz limit is 16/27, about 59.3%; modern turbines reach Cp of 0.35-0.50. Typical cut-in is 3 m/s, rated power at 11-14 m/s, cut-out at 25 m/s. Because output scales with the cube of speed, a 10% wind resource error is a 33% energy error.
- Resource extrapolation. Hub-height wind is estimated from measured height using a shear exponent, commonly 0.10-0.25 offshore and land, and 0.14 for the one-seventh power law. Wake losses of 5-15% apply within arrays.
- Other technologies. Hydro output is proportional to head times flow. CSP with thermal storage is dispatchable and has a high capacity credit. Geothermal runs at 85-95% capacity factor. A heat pump with COP 3-4 delivers three to four units of heat per unit of electricity.
Worked example
A 450 W module with a -0.35 %/°C power coefficient operates at 55 °C cell temperature under 900 W/m² plane-of-array irradiance. Irradiance factor = 900/1,000 = 0.90. Temperature derate = (55 - 25) × -0.35% = -10.5%, so the factor is 0.895. Output = 450 × 0.90 × 0.895 ≈ 362 W. Adding a 97% inverter efficiency and 3% wiring loss gives roughly 341 W at the meter, about 76% of nameplate.
Traps that cost marks
- Using ambient air temperature instead of cell temperature; cells run 20-30 °C above ambient at full sun.
- Applying the temperature coefficient to irradiance changes as well as temperature.
- Quoting the Betz limit as an achieved efficiency.
- Extrapolating wind speed with the wrong shear exponent or from the wrong measurement height.
- Comparing capacity factors across technologies without noting dispatchability and firmness.