New research helps identify optimal cleaning cycle to reduce soiling in MENA region

Researchers led by the German University of Technology in Oman have looked into the effect of dust accumulation on PV systems and claim to have identified an optimal cleaning cycle in economic terms.

The scientists have conducted the research on an experimental setup located in an area next to their campus. “The research might be valid to countries with dry weather, humidity during summer, and high temperature,” the research's corresponding author, Ali Al Humairi, told pv magazine. 

“Photovoltaic energy is considered the most viable renewable energy source in the Middle East and North Africa region due to the high solar irradiation level and the number of clear sky days during the year,” the group said. “However, environmental factors such as dust limit the optimum utilization of the source.”

The experimental setup included two identical strings of nine PV modules connected in series, with one string being dry-cleaned daily and the other not. The 5.85 kW ground-mounted system was south-oriented and had a tilt of 17 degrees. The modules were based on polycrystalline cells, and each had a peak power of 325 W. The system included an inverter with 98.5% efficiency.

The observation of electrical and weather parameters began in November 2020 and ended in April 2021. “The experiment was conducted in the winter and spring seasons, which generally have less soiling rate and air contamination,” the researchers explained.

Comparing the cleaned string to the non-cleaned string, the academics found that dust led to up to a 28% reduction in the PV current performance and up to a 24.2% reduction in the PV power. Overall, the average difference in the current performance was 14%, and in PV output it was calculated at 11%.

“The difference between the uncleaned and the cleaned modules’ output current has increased exponentially during this period,” they said regarding the current. “In November, the difference in current is about 2%, which increased with time; in December and January, it is about 5% and 10%, respectively. The momentum intensity slightly dropped in February and recorded a difference of 18%. This was followed by a less momentum increment in March and April, resulting in a difference of 22% and 28%, respectively.”

As for the PV power output, they found no substantial effect in the first three months, with the difference being 0.1% in November, 1.9% in December, and 7.7% in January. However, it was much more noticeable in the next three months – with a 14.7% difference in February, 19.3% in March, and 24.2% in April.

For its economic analysis, the team used a fixed rate tariff of $0.11 per kWh. The cleaning rate was set at $1.30 per hour per worker, and according to the paper, one person could clean the whole system in one hour. Using this data, they have found the recommended cleaning interval to be once every one or 1.5 months, resulting in 8 to 12 cleaning cycles per year.

The group presented its findings in the paper “Experimental Investigation Of The Soiling Effect On The PV Systems Performance And The Cleaning Intervals In Oman,” published in Solar Energy Advances. It also included scientists from the Sultan Qaboos University, Muscat University, and Germany’s Duisburg Essen University.

“The effect of the accumulated dust was evident in the third month of the experimental period, indicating the necessity of conducting a cleaning cycle for fewer than three months to avoid losses,” the researchers concluded. “However, the results could vary depending on the location, season, geographical and meteorological conditions.”