Soiling can significantly reduce PV plant performance, depending on location and time of year [1,2,3]. Cleaning PV plants is luckily possible, but when and where should the modules be cleaned?
To answer this question, Kipp & Zonen has developed the DustIQ, a novel soiling measurement system that can accurately measure soiling amounts, while avoiding daily cleaning and moving parts or liquids, unlike other existing commercial systems . It can also provide multiple sensors across a plant for a similar cost.
The DustIQ system consists of a glass plate – the same as those in PV modules – mounted next to or inbetween the modules to ensure that the instrument follows the same soiling and cleaning patterns as the modules. Below this glass plate, are two sensor heads, which use Optical Soiling Measurement (OSM) technology to measure the scattered light from soiling on top of the glass plate (figure 1).
With an on-board calculation, the transmission loss due to soiling is determined from the two OSM sensors. Transmission loss and Soiling Ratio (SR) are related to each other: Around solar noon the transmission loss = 100% – Soiling Ratio. This SR is defined as the ratio in short circuit current, Isc, or maximum power, Pmax, between a soiled and a cleaned PV panel in the IEC 61724-1 . For a completely clean panel, the SR is 100% and for a soiled panel it is closer to 0%. Even if the soiling is constant, the SR is not constant over the day but is largest around solar noon, and grows smaller closer to sunset and sunrise [3,6,7].
A unique feature is that the soiling measurement does not use the irradiance of the sun, therefore it is independent of the sun's position and other weather conditions. The system communicates using the Modbus RTU protocol, like all Kipp & Zonen Smart instruments with suitable inverters and data acquisition systems.
The color of soiling can vary per location and influences the amount of scattered light measured with OSM and the corresponding transmission loss. An example of different dust colours is shown in figure 3 (a). We have also measured these dust colours in the lab at Kipp & Zonen and found that for very different dust colours, such as white and black dust, there is a significant difference in response, as shown in figure 3 (b). Similarly, when the visible difference in colour is small, such as the beige Arizona Test Dust compared to light grey or other beiges, the difference in response will be fairly small.
We have expanded the DustIQ with an onboard silicon cell, as shown in figure 2, which provides the possibility of a local dust calibration. This onboard silicon cell has the same optical and spectral properties as a PV module and allows for on-site calibration by relating the signal loss to the DustIQ signal due to the total ensemble of soiling types.
Local dust calibration
The prerequisites for the calibration is that the DustIQ is uniformly soiled with a loss of at least 5-10 %, that there is bright sunlight/clear sky and that the time is within two hours of solar noon. The DustIQ should have a clear view of the sky (for instance: no trees blocking the sun).
A step by step guide for performing the local dust calibration is in the box and given below:
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- Step 1: At the back of the DustIQ hold the calibration button for (at least) five seconds and release to start the calibration (figure 4).
- Step 2: Wait for 50 seconds until the blue sensor LEDs blink steadily. This indicates the soiled signal has been measured. Take care not to block the sunlight falling on the DustIQ (both the OSM sensors and the silicon cells) during this phase. The uniformly soiled DustIQ during this procedure looks as shown in figure 5.
- Step 3: Clean sensor 1 (left in picture and close to the silicon cells) and the silicon cells (see figure 6). Leave sensor 2 soiled (right in the picture and close to the logo). Clean using soap and distilled or demineralized water. Clean thoroughly and dry thoroughly.
- Step 4: Press the buton again, and the clean sensor 1 signal will be measured. Take care not to block the sunlight falling on the DustIQ (both the OSM sensors and the silicon cells) during this phase. When the blue sensor LEDs blink steadily (approx. 50 seconds), the measurement has been performed.
- Step 5: Now clean sensor 2. A completely cleaned DustIQ is shown in figure 7.
- Step 6: Press the button again. Now both clean signals will be measured. Take care not toblock the sunlight falling on the DustIQ (both the OSM sensors and the silicon cells) during this phase. Wait for 50 seconds until the blue sensor LEDs blink steadily.
- Step 7: Within minutes of finishing the calibration procedure, press the button again to save the new calibration settings. The settings will be saved in the memory and be used directly. If the button is not pressed, the settings are not saved and the original settings will be used.
Kipp & Zonen acknowledges the contribution of Dr. A. Alami Merrouni from IRESEN Green Energy Park
 A. Sayyah, M. Horenstein, and M. Mazumder, “Yield loss of photovoltaic panels caused by depositions”, Solar Energy 107, 2014.
 M. Maghami et al., “Power loss due to soiling on solar panel”, Renewable and Sustainable Energy Reviews 59, 2016.
 J. Zorrilla-Casanova, M. Philiougine, J. Carretero, P. Bernaola, P. Carpena, L. Mora-Lopez, and M. Sidrach-de-Cardona, “Analysis of dust losses in photovoltaic modules”, World Renewable Energy Congress, 2011.
 M. Gostein, T. Duster, and C. Thuman, “Soiling Station Employing Module Power Measurements”, Proc. IEEE PVSC, 2015.
 International Standard IEC 61724-1.
 M. Gostein, J. Caron, and B. Littmann, “Measuring Soiling Losses at Utility-scale PV Power Plants”, IEEE Photovoltaic Specialists Conference, 2014.
 M. Korevaar, J. Mes, P. Nepal, G. Snijders, X. van Mechelen, “Novel Soiling Detection System for Solar Panels”, Proc. EU PVSEC, 2017.
 DustIQ local dust calibration procedure: http://www.kippzonen.com/Download/981/DustIQ-Calibration-Procedure-0386900-model
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