pv magazine re-examined the monitoring and mitigation of solar system pollution to answer the audience’s questions.

In June, Photovoltaic Magazine hosted a webinar that discussed in depth environmental monitoring, SCADA integration and solar panel cleaning, using robotics and automation to raise awareness of pollution losses, and emphasized the importance of long-term consideration in the development process sex. Special speakers include Damon Nitzel of OTT HydroMet, Troy Morlan of Nor-Cal Controls and Carla Dawson of Relysm.

The goal of the Photovoltaic Magazine webinar is to stimulate the audience's interest and dialogue. Sometimes, it's just not possible to include the discussion in a one-hour webinar time frame.

Recognizing this, we collected some questions that were not answered in the initial webinar and conducted interviews in collaboration with Nitzel, Morlan, and Dawson. The questions and content were provided entirely by you and your other attendees.

If you didn’t attend the webinar during the live broadcast, don’t worry. A complete replay and demonstration is available through this link. Any entries in italics below indicate issues related to slides or graphics in the presentation.

Q: What irradiance values ​​are considered when calculating the pollution ratio? More than 50 or 100 watts per square meter?

Answer: DustIQ uses internal LEDs to generate measurement signals and collect pollution data, which is not affected by irradiance, so irradiance values ​​are not considered in this presentation. This also means that DustIQ will provide data day and night as long as the sensor surface is dry. The pollution data provided are daily measurements at 1400 hours, so it is assumed that the irradiance at all measurement times is greater than 100 W m-2, and with a few exceptions, the sensor is dry at this time of the day.

Q: Can DustIQ measure uneven pollution in utility-scale sites?

Answer: DustIQ assumes uniform pollution. Installing multiple DustIQs provides a plant profile that can solve uneven pollution.

Q: To ensure accuracy, what is the optimal number of polluting stations in each area?

Answer: IEC 61724-1:2021 Class A places require a minimum number of pollution sensors per MW (see the table below). We recommend using this as a starting guide to determine the number of sensors needed so that the stain measurement can represent the stain of the entire array. Variability between and within the site (farm activities, maintenance of roads, atmospheric dust, car exhaust, etc.) and clean areas are all factors that need to be considered. Among them, additional pollution measurements may be required to represent the unique pollution areas inside a large website or A group of smaller websites.

Q: Questions about numbers. I think I'm confused because the cleaning cost for 10 years seems to be about $1.2 million. The 30-year savings are approximately US$3.6 million. This seems to be a net wash of $0. Is this accurate?

Answer: The cleaning cost has been included in the total savings calculation. Loss of income is expected income minus actual income. The actual revenue is the energy sold minus the cost of cleaning. When no action is taken, there is no cleaning cost, but the output is also reduced. When the cleaning cost is higher than the recycled production cost, this results in a net reduction (manual cleaning example). When cleaning costs are optimized, cleaning costs are lower than resumed production, which results in "savings" relative to no action (semi-automated example).

Q: How does manual cleaning result in a higher percentage of lost revenue compared to uncleaning?

A: In this case study, manual cleaning resulted in higher revenue loss because the cost of manual cleaning was higher than the cost of waiting for rain to clean. This does assume that the heavy rain event (the second event after manual cleaning, seen in the blue line in the demo) does completely clean the panel from a more polluted state than the actual data, so there may be a small ( 1-2%) error.

Areas with lower labor costs may not have the same situation, and manual cleaning will save income. In some cases, manual cleaning fees are more effective than semi-automatic tractor rates. For example, in Pakistan, manual cleaning is reported as US$200/MW.

Q: How often should we collect pollution data? Is it enough every day, or that high per minute?

A: It is measured every minute, the purpose is to obtain a correct daily value. In some cases, such as sand and dust storms, real-time data may warn of maintenance needs in advance, but most sites rarely use intraday data every day. One exception is the location of higher contamination rates where fully automated solutions are used.

Q: How long does it take to clean? Are you considering cleaning in one day?

Answer: For this case study, just for the sake of simplicity, we do assume that the entire 100 MW site is cleaned up in one day. In fact, 1-2 weeks will be a more realistic cleaning time frame. However, given that there is a consistent pollution rate in this data, we believe that if we clean 1/12 times a day, the results will not be significantly different.

Q: Have you considered the potential nonlinear losses associated with uneven pollution (mismatches, hot spots, etc.) in your model?

Answer: For this case study, we assume that the pollution measurement value 100% represents the pollution on the panel. A single sensor on a 100 MW site is unlikely to meet this assumption, but using multiple sensors to increase the pollution data density will bring the pollution data closer to the 100% representation of the actual pollution on the panel.

We anticipate the variability within the site and recommend the installation of pollution monitoring equipment in areas that represent unique conditions. Compared to cleaning the entire site on average and treating it as a whole, local site cleaning of more heavily polluted areas is likely to be the most effective method, or to recover the most pollution losses.

Q: For automatic robot cleaning, how is the cleaning cycle considered? One cycle per day or less?

Answer: Generally, automated solutions are expected to be cleaned at least once a day. But why limit yourself to a set timetable? By cleaning only when it is needed to optimize production, whether it is multiple times a day or skipping days, pairing automation and pollution data will save power and lifespan of automation equipment and solar panel surfaces.

Q: In the case that robot non-water cleaning is the expected long-term solution, is there no negative impact on pollution?

Answer: It must be cleaned with water at a certain point in time. Dry cleaning is very suitable to solve the daily pollution problem, but in the end you have to clean it with water, because dry cleaning cannot avoid cementation. Even in the driest areas in the world, there is dew, which can cause cementation.

Q: Is the equipment used to clean double-sided panels the same as the equipment used to clean single-sided panels?

Answer: There are several ways to do this:

Question: Is the water used in the cleaning process "recycled" through the cleaning process, or does it flow to the ground?

Answer: In current cleaning methods, water will not be recycled. It was wasted and there is room for improvement.

Q: Does the robot and tractor cleaning research consider corrective and preventive maintenance estimates for these devices?

Answer: Yes. The value shown in the webinar takes into account the operating expenditure value of the solar panel cleaning robot, etc. The financial research takes into account the O&M of the machine and other factors.

Q: The cleaning brush may be very small to scrub, but it may be very abrasive. What is the rate of decrease in transmittance due to erosion caused by abrasive contamination (reflective scratches on the panel glass)?

A: This is a thing worth studying. Before buying any cleaning machinery, you should understand the degree of wear, or better yet, check whether the machinery has been certified by the photovoltaic panel manufacturer. For the system shown, we have no wear level because we have the certification of the photovoltaic panel manufacturer.

Q: Is it possible to use air for cleaning?

A: Air purification is being used, with partial effect. It is only suitable for very arid areas and uses some kind of brush or fabric to clean the photovoltaic panels together with the air. Clean air alone cannot accomplish this task.

Question: Is there any relationship between the contamination rate and module degradation during the entire life cycle of the site? When determining the module pollution loss during the life cycle of the site, has the weather forecast been taken into consideration?

A: As far as I know, there is no news about pollution-related degradation, but there may be, because polluted areas usually become hotter. It is meaningful to consider the weather forecast to determine the pollution loss during the entire life cycle. Rain water is considered a cleaning mechanism for stains.

Q: Have you published any data comparing DustIQ pollution measurement with two panel settings (clean module/dirty module) and field IV curve measurement?

A: As far as I know, Spain and Morocco have already done some work, and Saudi Arabia also has a method. Publications on Morocco are in preparation, but it has been going on for some time and progress is slow. Moroccan publications focus more on the angle of incidence effect rather than the true pollution value.

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More articles by Tim Sylvia

These are some good things from Tim, thank you for sharing the Q&A. With a simple case study alone, the difference in income is surprising. Take your article as an example. This is an interesting study..."In this case study, manual cleaning resulted in higher revenue loss because the cost of manual cleaning was higher than the cost of waiting for rain to clean. ."

It’s nice to see the data being used to judge whether it’s worth it. I like to get knowledge and data from these questions and answers, and always thank these amazing experts for their answers.

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