A photovoltaic (PV) cell absorbs photons and knocks electrons loose across a semiconductor
junction, producing current directly. A solar-thermal collector instead absorbs sunlight as heat
and uses it to warm a working fluid (usually water or glycol) for heating or to drive a turbine.
Both lose output as the sun's rays hit the collector at a shallower angle.
Effective irradiance: G_eff = G · cos(θ_AOI)
θ_AOI = |sun elevation − panel tilt| (simplified planar case)
PV power: P = η_pv · A · G_eff (η_pv ≈ 20%)
Thermal power: P = η_th · A · G_eff (η_th ≈ 55%, lower-grade heat)
- Photovoltaic / Solar thermal toggle — switches the collector type and what the output represents (electricity vs. heat).
- Sun elevation angle — how high the sun sits in the sky; moves the sun and its light rays.
- Panel tilt — collector angle; matching it to the sun's elevation minimizes the angle of incidence and maximizes output.
- Irradiance — raw sunlight intensity available before the angle-of-incidence loss is applied.
Real-world relevance: rooftop solar installers tilt panels toward the average solar elevation
for their latitude, and solar water heaters use the same cosine-loss physics to size thermal
collectors for homes and swimming pools.