The Fate of My Grandfather: The Same Story, from the Meter to the Soiling Sensor

You could point at the panel and say that it is either dusty or clean; however, you cannot tell how much energy you are losing or when it makes sense to clean visually. This is exactly what the soiling sensor is doing for you. It is comparing your clean reference cell to the soiled cell in the field and giving you your loss in terms of percentages. Our short video explains the “visual measurement” habit using an analogy.

What is the video about?

The video features two builders constructing pyramids in the desert. One refuses to use a measuring tape, saying, “I measure by hand, I estimate by eye. Real masters don’t need such tools.” The other warns him: these pyramids won’t be finished by hand and eye estimation; sooner or later you’ll use a measuring tape. In fact, in the future, no master will work without one.

The analogy is simple but apt: today, assessing dust “by eye” in solar power plants is like trying to measure a pyramid with a span. It might feel like it’s working, but it will slowly cost you money.

“I can see with my eyes, why do I need a sensor?” Why is this misleading?

The technician’s first words in the video were: “Why do we need a soiling sensor? I can see it with my own eyes.” This is exactly the objection we hear most often in the field. Here’s the problem:

• The eye can say “yes/no,” but not “how much.” A fine, homogeneous layer of dust, almost invisible to the naked eye can reduce production in a measurable way.
• Observations vary from person to person. Two technicians will evaluate the same panel differently. You cannot obtain a repeatable number.
• Cleaning is an economic decision. Early cleaning means costly water, labour, and equipment. Late cleaning means wasted energy. You can only strike this balance with a measurable level of contamination.
• Contracts and reporting require numbers. PR accounts, warranty and O&M contracts accept measurement data, not the statement “looked clean.”

How much loss does contamination cause in the panel?

Contamination is considered the second most important factor affecting PV efficiency after irradiance. International studies show that contamination takes away approximately 3-7% of annual PV production globally, resulting in billions of euros in lost revenue for the industry each year (IEA-PVPS; Ilse et al., Joule 2019).

More importantly, contamination is site and season specific. Losses may remain at a few points in rainy regions, while they can be much higher in dusty/desert climates. So, the assumption that “it was this much at the neighbouring power plant” is not useful. You need to measure it at your own site.

The main message here is, because the extent of the damage varies from field to field, you can only establish an accurate cleanup schedule with data measured on-site.

How does a soiling sensor work?

SEVEN Soiling Sensors are based on the industry-established comparative reference cell method:

1. Two PV reference cells are used.
2. One is kept clean (reference), the other is left to naturally get dirty under the same conditions as the panels.
3. The system calculates the soiling ratio by comparing the output of two cells.
4. This value is transmitted to the monitoring system/SCADA via Modbus RTU. You can track the loss in real time and plan the cleanup at the right time.