Dry season is soiling season: A six-point checklist for Southern African solar plants

Six things: whether your soiling measurement works, how hard your cleaning water is at the nozzle, which cleaning interval your tariff justifies, whether a coating has been proven on your own strings, where your worst dust and droppings come from, and whether crews and equipment are ready.

Across Southern Africa’s summer-rainfall interior, the dry months are winter, and that is when soiling does its damage. The measured South African rates and rain findings are in soiling losses in South Africa.

In the Western Cape, which gets its rain in winter, the dry months run from about November to March, so work through the list in October. Elsewhere, work through it before May, and use it again in October to review the season just ending.

Why do the dry months set the soiling bill? Rain resets loose dust. How much it helps, including at Kalkbult near De Aar, is in soiling losses in South Africa. Without rain, the loss grows until someone cleans, and the region has long dry spells. South Africa averages about 490 mm of rain a year, about half the world average (WWF-SA), and parts of the Northern Cape get under 70 mm (SAEON). Dew makes it worse: it dissolves the soluble part of the dust, which recrystallises into bonds that dry cleaning can’t break (IEA PVPS 2022).

1. Is your soiling measurement telling the truth?

A soiling station compares a reference device cleaned on schedule with one left to soil. If the clean one misses its wash, the ratio drifts towards 1 and hides the loss, so check the cleaning log, and check any optical sensor against the modules around it.

How many sensors a site needs, and how long a dry spell you need to fit a soiling rate, are in soiling ratio, rate and loss explained. Check the irradiance sensors too: IEC 61724 recommends cleaning them weekly, and performance ratio only tracks module soiling well when the plane-of-array sensor itself is clean (IEA PVPS 2022). See soiling measurement and how to measure PV soiling.

2. How hard is the water at the nozzle?

Test the water in the bowser or tank the crew will use, as well as at the borehole. Read hardness with test strips in mg/L as CaCO₃; USGS classes anything above 180 mg/L as very hard. Compare it with your module maker’s limit on the same basis. Maker limits are in water quality for solar panel cleaning.

Bad water costs output; see the Malaysia case. Chemitek Water Softening Agent is dosed to the measured hardness, for example 1:500 at 250 ppm or 1:1,500 at 125 ppm, and acts in about 20 minutes. Once it is in the tank, check hardness with strips, because a TDS meter reads the agent itself.

3. What interval does your tariff justify?

The right interval depends on your tariff, soiling rate and what a clean costs, and it changes with the season. The calculator works it out for your plant. A calendar copied from another plant or a module manual ignores both. The cleaning frequency guide sets out how to use the result.

4. Will a coating help this dry season?

The dry months are where coatings earn their keep. See the Vietnam case. In Morocco’s peer-reviewed Green Energy Park trial, Solar Wash Protect also did best in the dry months; see the Morocco trial.

Coatings don’t suit every site. In a Northern Cape trial, a hydrophobic coating increased dust soiling, opening a performance-ratio gap of up to 5.5% (du Plessis et al. 2020), and long-term field results for coatings are typically 20–50% less soiling (Ilse et al. 2019). Prove one on a few strings first; on utility-scale and large C&I plants, a free pilot does exactly that.

Then restore before you coat: the first coat needs fully clean glass. Chemitek Antistatic Solar Armor 2.0 goes into the water of rotary brushes, robots or tractors at 1 kg per 1,000 L, with at least 12 months of protection (Chemitek). For manual crews, Chemitek recommends Solar Wash Protect, which protects for about six months.

5. Where do your droppings and cement dust come from?

Map the point sources, because they make your worst rows.

• Birds. Look for roosts and perches: cables, fences and lighting masts. NREL puts soiling near bird colonies at up to 0.5% a day, against a typical 0.05%. Spot-clean those zones between full washes. See bird droppings.
• Cement, limestone and quarry dust. Rows downwind of cement works, quarries and precast yards collect alkaline dust that cures into a film water can’t shift. Chemitek Inorganic Removal Agent goes onto dry glass, starting at 1 kg to 2 L of water, and is brushed in and rinsed off. In a Chemitek case study in Israel, coated modules still cleaned fully two weeks after cement removal, while cement had stuck to the uncoated ones (case study). See cement dust.
• Roads and farmland. In Chemitek’s Scatec case study in Honduras, the string nearest a road collected much more dust than its neighbours, and NREL puts heavy farmland dust at 0.36% a day.

6. Are crews and equipment ready?

• Brushes and pressure inside your module maker’s rules. See warranty-safe methods and equipment selection.
• Water supply. A wet wash uses about 1 L/m² in US literature and about 3 L/m² in a South African dissertation (NREL 2018; Naicker 2018): roughly 50–150 m³ for one full wash of a 10 MWp plant.
• Dosing pumps set to each product’s ratio and checked before the first wash.
• Products in date. Solar Wash Protect keeps three years closed and six months opened, and diluted solution must be used within 48 hours (Chemitek).
• Timing. Keep the water within the module maker’s temperature window, and clean early, late or under cloud so no product dries on hot glass. See warranty-safe methods.
• A method statement and trained crews. LONGi’s sample letter of conformity lists trained staff among its conditions. See warranty-safe methods.
• Run-off rules. Check your environmental authorisation against each product’s safety data sheet.

Where RenewChem fits. Our engineers work through this list with your O&M team: soiling sensors, water at the nozzle, an interval set from your tariff and soiling rate, and a coating piloted on your own strings before the dry months. See solar panel cleaning and cleaning strategy. If your crews only need the products, order them from us on their own from the product range.