Measuring dust and soiling ratio is a new field of science. There are many opinions about it, but there is no large history of data and experience in measurements. Although the spot measurement of DustIQ is very precise, it is wise to do interpretation and use common sense when extrapolating the measurement value to the whole PV plant, or a part of it, for decision making. The DustIQ measurement value is true, but the question is whether this value is representative for the whole PV park.
How many measurement points
For giving a good representation of soiling across the PV plant, you should install multiple DustIQ’s. In fact, the IEC61724-1:2017 Standard for photovoltaic system performance monitoring prescribes that you should have as many soiling sensors as irradiance sensors. This makes intuitive sense; it follows the same logic as irradiance that is not constant across a solar plant, and you must have sufficient measurement locations to make a valid interpretation.
For the number of DustIQ’s, you can thus follow your policy for the number of pyranometers. Of course, every additional DustIQ adds up to a better overall measurement. For 1 MW plants, a single DustIQ placed in the middle could do, whereas for larger sites (say, 25 MW) one could imagine that one in the middle, and one near each corner would be a logical choice to understand the soiling pattern across the plant.
But, soiling is different from irradiation
Soiling is a different phenomenon to irradiation, and also very site-specific. Where you normally expect the same average global horizontal irradiation value for all pyranometers at a solar plant, this is not necessarily true for DustIQ’s; if the wind always blows from the East, one could expect more soiling (and thus a different cleaning interval policy) at the Eastern side of that plant. When wind direction changes over seasons, this pattern could shift during the year.
Also, it is known that the very ends of an array, and the bottom row of an array (closest to the ground), typically suffer more from soiling than the rest of the PV modules. Therefore, it is better not place DustIQ’s here, unless your purpose is to learn about and study soiling behaviour. Place them where the soiling is representative of the majority of the panels in the area to be monitored.
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