Soiling from dust, pollen, sand, exhaust and other particulates reduces the amount of light PV panels can take in, leading to power output losses over time. As more solar PV plants optimize their operations, they are using dedicated soiling measurement sensors and stations to monitor soiling losses and inform their mitigation (cleaning) strategy.
We asked Damon Nitzel of OTT HydroMet to share his expertise about how soiling monitoring and mitigation can help boost ROI. OTT HydroMet is the parent company of Kipp & Zonen, the maker of the DustIQ Soiling Monitoring System.
How is PV module soiling affecting the solar energy industry?
Site underperformance has been the talk of industry leaders over the past few years. In their 2020 Solar Generation Index, kWh Analytics put a number to it, with performance coming in at 6.3% below expectations. Several individual contributors have identified module soiling as one of the major causes of this 6.3% loss rate.
The good news is that soiling is partially recoverable. That's caused the industry to start paying more attention to the specifics of module soiling, from predicting impact on future sites to mitigating impact on operating sites.
What are some examples of soiling variation from site to site?
Soiling can vary by region, locality and even within the same site. Local agriculture or flora can cause harvest dust, pollen and sap deposits. Panels near roadways or industrial areas may be soiled by salt spray, exhaust, emissions and grit. Panels that lie under a migration route may be dirtied with bird excrement. Panels near water or in a humid area may experience algae, moss and fungi.
Even if a site isn't located in a desert or near a road or farm, atmospheric dust can still be an issue. For example, dust from the Sahara frequently travels over 5,000 miles via trade winds to Florida, and recently even to Texas!
What technology is available to monitor soiling?
It's important to understand that soiling stations and sensors don't measure the dust and particulates on the panels. Rather, they provide a representative surface and output a value that's meant to define a region of soiling. Soiling data are used as a reference point to understand what percentage of power loss is due to soiling.
There are two main types of soil monitoring technology: traditional two panel systems, and newer optical sensor solutions.
Two Panel Clean-Dirty Systems
Two panel systems monitor soiling using a clean and a dirty panel. Comparing the power output of the two panels—the ratio of dirty to clean—determines the amount of transmission lost from soiling.
This is a straightforward method, but the industry has highlighted a couple of concerns: boots-on-the-ground upkeep and data inconsistencies. The clean panels must be kept clean, either manually or using an automated system. An automated system prevents daily labor, but it has water reservoirs that must be filled and moving parts that break down and require maintenance. Time between cleanings and automation downtime events create unknown time gaps where the clean panel is not clean.
Optical Sensors
To reduce boots-on-the-ground maintenance for water and moving parts, newer optical sensor solutions have come on the market. Unlike the two panel systems, they don't require a clean panel to use as a reference for measurements. Instead, they use an internal photodiode or camera to measure either an internal signal reflected from soiling or sunlight transmission loss through a glass surface.
The DustIQ Soiling Monitoring System, for example, emits a blue light and measures the amount of that light that is reflected from soiling on the glass surface. The large glass surface allows soiling to naturally deposit on the DustIQ like it deposits on the panel array it is mounted in. The large surface also makes room for two separate optical measurements to detect intra-panel variability and a built-in solar panel to adjust the quantitative output of the optical sensors to local soiling conditions. Because the measurements are all internal, the surface should only be cleaned when the panels are cleaned, eliminating the clean/dirty maintenance concerns.
Learn more about soiling monitoring systems: Basics & Benefits of Soiling Stations
What standards are available to give guidance on collecting module soiling data?
The main standard is IEC 61724-1 and there's a recently released 2021 revision. It includes requirements for using two panel clean-dirty systems and provides an option to use optical sensor technology instead.
How many soiling stations do I need for my solar PV site?
The IEC specification includes a table with a required minimum number of sensors per megawatt. It also specifies a minimum of two for all sites, creating measurement redundancy.
While the table is a good guideline, intersite variability is the biggest factor when trying to determine how many sensors to use and where to place them. Conditions—and therefore soiling rates—can vary widely across a sprawling solar PV site. You need sensors placed to collect representative samples of areas that soil differently. You also need to factor in how large a zone can be effectively cleaned at once.
The best approach is to start with the IEC recommendation as a minimum and go from there. In a fairly uniform environment, such a remote desert, the IEC minimums may be sufficient. However, if you have more complex terrain or variable conditions, you may need additional sensors as you evaluate the site.
How often should soiling sensor data be collected?
The new IEC 61724-1:2021 standard recommends averaging the instantaneously calculated soiling ratios over a daily period. The previous 2017 version suggested collecting 1-minute measurements for raw values, but in practice, an accurate daily value is typically what you need to determine an actionable soiling rate.
You can get that quality value either by averaging over a daily period or by using an optical sensor that can provide it in a single measurement. Whichever method you use, you need to be careful about data loss. If you're only collecting one measurement per day, it's easy to accidentally miss a day.
When is the right time to clean the solar PV panels?
This is really the crux of what everyone discussing this topic is trying to figure out. There are dozens of variables that factor in, and therefore no one recommendation that works for every site.
When you monitor soiling, you can predict how dirty your panels are going to be in the future and what percentage of power loss you can expect from that soiling. But the answer isn't always to rush out and clean the panels when the power loss hits a specified percentage. You need to determine how much it will cost to clean the panels and if the sale of the recouped power will make it worth your while. At what point will selling the recouped power overcome the cost of cleaning?
You can also use the soiling data to forecast trends over time, in order to create efficient cleaning schedules that make sense for your plant. You can forecast and factor in weather patterns—if there's a rain event in the forecast, for example, you're better off waiting for that natural cleaning. That's significant money saved!
Do bifacial panels soil on the rear side?
They do, but it's still a newer section of the industry and there aren't hard numbers yet. Anecdotally, some customers have shared that the rear side soils at roughly one-fifth the rate of the top side for single axis tracker sites.
You can evaluate this at your own site by using the DustIQ Optical Soiling Sensor upside down. Rear side soiling is definitely a concern for the industry and there's more research to do.
My site only soils 1-2% per year. Should I monitor soiling?
That's a matter of user preference, but monitoring soiling can help with diagnosing power loss issues at your plant. If you're not monitoring soiling but your site is experiencing the industry average 6.3% power loss, how do you know what else to look for?
Soiling can also build up over time, especially certain types of soil like pollen and algae. In one year you may only have a 1% loss, but in year four you may be at a 4 or 5% loss. If you're actively monitoring soiling, you'll know about that build-up and can provide that data to the site operator. They can then understand the effects of the soiling and how fast they can expect it to happen again once they clean. Even though the soiling timescale is longer, there's still value in having a better understanding of what's going on at your site.
Can soiling data be used for more than calculating cleaning ROI?
Absolutely. Monitoring soiling also makes it more efficient to troubleshoot performance losses. If you know the percentage of power loss that can be attributed to soiling, it's easier to narrow down the other causes.
For example, if you track soiling trends and there's a sudden, significant power loss deviation, that's an indication that something at the site is wrong. This might be a weather event, damaged wires, an inverter failure or a blown fuse. Or perhaps the power loss decline is more subtle. If you know the percentage that can be attributed to built-up soiling, you know to look at other possible causes, like module and equipment degradation.
This not only saves operators time in diagnosing issues, but it helps prevent unnecessary truck rolls to the site when trying to troubleshoot power losses. Every time you save a truck roll, you're saving money.
You can also learn from soiling data collected over the years when building future sites. You'll have a more accurate picture of actual versus predicted performance, and can reduce risks that can result in better financing terms. This is a less direct benefit of soiling stations, but still a potential way to increase ROI.
What do I need to know to determine the optimal cleaning method for my solar PV site?
Water availability and the cost of labor are the two main factors when determining your cleaning method. Can you clean manually, using labor with dry or wet brushes, or should you move away from that and include automation? Semi-automated and fully automated cleaning systems are typically water-based, but can sometimes reduce the amount of water needed because it’s more targeted on the panel.
The best course of action is to consider any limitations you have locally. Get in touch with multiple vendors and talk with them about their methods. Make sure they're taking into account all the variables that affect cleaning costs.
How long does it take to clean a solar PV site?
This depends on the cleaning method. If you have two people manually cleaning a 100 megawatt site, it's going to take a long time, but if you have a thousand people cleaning it, not so much. There are fully automated solutions that can clean an entire site the same day because every row has its own cleaning robot, but the upfront cost to purchase the robots is high.
The middle ground is semi-automated cleaning solutions. For a 100 megawatt site, it may take between 10 to 15 days to clean using two tractors with big rotational brushes and two to four employees running the cleaning solution.
The best solution again is to talk with your local cleaning companies and discuss the specifics for each site. This will give you the most accurate cost estimates.
How important is it to consider soiling for new projects?
Don't wait to get through the development process before you start considering how you're going to mitigate soiling. By talking and thinking about soiling during the project development phase, you can design your site in a way that allows you to use a lower cost and/or lower water cleaning solution.
While soiling has historically been more the province of O&M, there is a trend towards making sure that soiling is a bigger part of the development process. If you're incurring capital expenses for cleaning, such as purchasing semi-automated equipment or fully automated robots, you can potentially take advantage of local subsidies to make these solutions more economical. In the United States, you may be able to take advantage of current investment tax credits (ITC) to offset some of the capital cost by installing cleaning systems during development rather than after commissioning during operations.
Learn more about how soiling monitoring and mitigation can increase ROI.
OTT HydroMet has an excellent whitepaper that goes into depth about solar module soiling and its effects. You can download it here.
Nor-Cal's Solar PV Operations training also covers soiling stations, sensors, MET stations, and how the data can be utilized to measure and improve solar power plant performance. You can see upcoming training dates here.
