Theory 058/Engineering & Materials /Photovoltaics
Panel Decay Follows Humidity
Degradation is projected from temperature and irradiance. Water may be the carrier.
Photovoltaic module degradation is set by humidity-driven ion migration rather than by temperature alone, so installations in humid temperate climates can degrade faster than hot dry ones despite lower irradiance.
01The evidence
Potential-induced degradation depends on voltage, humidity and encapsulant, and field degradation rates vary more than temperature models predict.
02The tension
Lifetime models use temperature and irradiance as the stressors, with humidity as a correction. The correction is small relative to the observed variation.
03The mechanism
Ions migrate through the encapsulant under the electric field between the cell and the frame. The mobility of that migration depends on absorbed water, which sets the ionic conductivity of the polymer.
That makes humidity the controlling stressor, and predicts that degradation should correlate with moisture dose rather than with temperature.
04What this is not
Those models rank stressors by heat. This ranks them by moisture, which explains degradation in cool humid sites.
05Predictions
- P1Degradation should correlate with moisture dose.
- P2Humid temperate sites should out-degrade hot dry ones.
- P3Encapsulant water uptake should predict rate.
06Refutation
Degradation tracking temperature at controlled humidity would restore the thermal model.
07What it changes
It would change site selection and module qualification.