Soiling loss
Reduction in PV or CSP output caused by dust, pollen, bird droppings, snow or other deposits blocking light from reaching cells or mirrors.
Soiling is one of the most site-dependent losses in solar plants. Particles on the glass absorb and scatter light; uneven deposits, such as dirt accumulating at the lower frame edge, can also cause mismatch and activate bypass diodes.
Typical annual soiling losses:
- Temperate climates with regular rain: 1–3%.
- Agricultural areas with dust and pollen, or near roads and industry: 2–6%.
- Deserts in the Middle East, North Africa, India and the Atacama: daily soiling rates of 0.1–1% per day, giving 10–30% annual loss if modules are never cleaned.
IEC 61724-1 specifies monitoring with paired clean and soiled reference cells or modules. Low tilt angles shed dirt poorly; above about 15–20° rain washing is far more effective. Light rain can make losses worse by cementing dust, and dew-driven cementation is a known problem in coastal deserts.
Mitigation options include manual washing, truck-mounted brushes, waterless cleaning robots (now common in Gulf utility plants), anti-soiling glass coatings and tracker stow angles that promote shedding. The optimal cleaning interval balances cleaning cost against lost revenue and typically ranges from weekly in deserts to never in rainy climates. Soiling is a large component of the performance ratio gap between climates and is even more critical for CSP mirrors.
Related terms
All terms →- Performance ratio
Ratio of actual AC energy output to the theoretical output implied by plane-of-array irradiation and nameplate capacity; a location-independent quality metric for PV plants.
- Bypass diode
A diode wired in parallel with a group of cells in a PV module that lets string current flow around shaded or faulty cells, preventing hot spots.
- Concentrated solar power (CSP)
Power plants that use mirrors to concentrate direct sunlight onto a receiver, heating a fluid that drives a steam turbine, usually with built-in thermal storage.
- Specific yield
Annual energy produced per unit of installed PV capacity, expressed in kWh/kWp per year; combines solar resource and system performance.
- Degradation rate
The gradual annual loss of a PV module power output over its life, typically around 0.5% per year for crystalline silicon.