What NREL's soiling research says about lost solar output and how commercial owners can set a site-specific cleaning schedule.
Studies Cited By NREL Report About 0.05% Output Loss Per Day From Soiling, With Wide Variation By Site. This Paper Turns That Research Into A Practical Way To Set A Cleaning Schedule For Commercial Arrays.
Download PDFExecutive Summary
Dust, pollen, soot and bird droppings on a solar array cut its output. How much, and whether cleaning pays for itself, depends on the site. This paper summarizes the soiling research collected in NREL's Best Practices for Operation and Maintenance of Photovoltaic and Energy Storage Systems, 3rd Edition and turns it into a practical way for Bay Area commercial owners to set a cleaning schedule. Every figure below is quoted from that report, with the original study named.
Key Points
- Soiling Costs Energy: It "reduces the energy output of the PV array and can lead to localized hot-spot failures if the soiling is uneven." NREL/TP-7A40-73822 §5.7.1
- Typical Daily Loss: "Studies report about 0.05% reduction in output per day due to soiling." NREL/TP-7A40-73822 §5.7.1
- Site Matters: Reported rates range from 0.01%/day in a clean desert to 0.36%/day near heavy agriculture and up to 0.5%/day with large bird populations. NREL/TP-7A40-73822 §5.7.1
- No Universal Schedule: NREL recommends a local, site-specific cost-benefit analysis. NREL/TP-7A40-73822 §5.7.1
What Soiling Is
Soiling is any material that collects on module glass and blocks light. NREL notes that losses are about twice as high in the morning and evening as at midday, because the shadow cast by each dirt particle grows at high incident angles. Rain helps: heavy rains result in "a nearly complete cleaning effect," while light rains clean much less effectively "and can even increase soiling if dust then sticks to sparse water droplets." NREL/TP-7A40-73822 §5.7.1
Published Loss Ranges
| Finding | Value | Study As Cited By NREL |
|---|---|---|
| Annual Losses Reported | 4.3% to 7.5%; many around 6%/year | Literature summary |
| Typical Daily Loss | About 0.05%/day | Literature summary |
| Daily Rate, 186 Systems | 0.051%/day | Study recounted by Naeem (2014) |
| Naeem Experiments | 0.061%/day | Naeem (2014) |
| Heavy Agriculture Area | 0.36%/day | Literature summary |
| Clean Desert Area | 0.01%/day | Literature summary |
| Large Bird Populations | As high as 0.5%/day | Literature summary |
How Much Cleaning Helps
For a system in Mesa, Arizona, with 1.91% annual soiling loss, Naeem (2014) simulated one annual cleaning reducing loss to 1.52%/year (-20%), two to 1.32%/year (-31%) and three to 1.20%/year (-37%). Naeem also concluded routine cleaning was not cost-effective for residential, commercial or utility plants in that study, with cleaning costs of $0.25/m² to $1/m² and about 1 liter/m² of water. NREL/TP-7A40-73822 §5.7.1 NREL adds that "where special conditions ... occur, cleaning will be required on a schedule that depends on the source and nature of the soiling."
Bay Area Factors
NREL lists soiling sources that may call for prevention or cleaning. Several are common at Bay Area commercial sites: construction dust (clean after nearby construction ends), pollen (clean after pollen season), bird populations, diesel soot near bus depots and city traffic, and industrial sources such as kitchen exhaust, where "a filter added to a fryer can reduce oil in kitchen exhaust air." NREL/TP-7A40-73822 §5.7.1
Building A Cleaning Schedule
| Factor (NREL) | Effect On Interval |
|---|---|
| Cost Of Cleaning | Mobilization fee plus per-area cost; higher cost favors fewer cleanings |
| Soil Accumulation Rate | Faster soiling favors more frequent cleaning |
| Capacity Factor | Better solar resource, higher reward for cleaning |
| Value Of Power ($/kWh) | Higher value, higher reward |
| Module Efficiency | Lower efficiency means more area cleaned for the same benefit |
- Track Production Before And After Each Cleaning
- Inspect After Construction Nearby, Pollen Season And The Dry Season
- Treat Bird Droppings Promptly Because Uneven Soiling Risks Hot Spots
- Consider Condition-Based Cleaning Triggered By Monitoring
NREL notes cleaning "may be on a defined interval or 'condition based'," and that the frequency "may be seasonal, depending on local rainfall and dust characteristics." NREL/TP-7A40-73822 §5.7.1
Sources
- NREL et al., Best Practices For Operation And Maintenance Of Photovoltaic And Energy Storage Systems, 3rd Edition, NREL/TP-7A40-73822, December 2018, §5.7.1 (citing Naeem 2014). osti.gov/servlets/purl/1489002
This paper summarizes published research for general information and is not legal advice. Requirements may change and vary by city; consult the current text, your local agency and qualified professionals.
