Leveraging Drones for PV Plant Inspections: Page 3 of 3

Camera and Image Considerations

Thermal imaging drones previously required aftermarket integration, but turnkey systems with high-quality cameras are now commonplace.

Camera types. There are three main considerations when deciding which thermal camera package to purchase with your drone: resolution, radiometry and lens size.

Thermal cameras for drones are typically available in two resolutions: 640 by 512 pixels and 336 by 256 pixels. The higher resolution will cover a footprint four times larger in area than the lower resolution, resulting in a significant time savings when inspecting a PV system.

Many cameras are available in both nonradiometric and radiometric versions. Radiometric cameras are more accurate, typically within 2°F–4°F. Most important, radiometric cameras have infrared measurements in each pixel, required for automated analysis. The camera you select should output radiometric JPEG or thermal TIFF files.

Drone thermal cameras are available with a variety of lenses, from 7 mm to 25 mm and beyond. A 7 mm lens causes fisheye effect, while a 25 mm lens has an extremely narrow field of view. For PV system inspection, 13 mm lenses tend to be the most versatile.

Raptor Maps makes software to analyze drone data and report findings.  It applies machine learning and artificial intelligence to automate the processing of thermal infrared and color images. To date, its software has processed data from more than 5 million modules, in sites ranging from small rooftop systems in Europe and the Northeast US to utility-scale plants in the Southwest US. Based on the data from these millions of inspected modules, we recommend the following camera type for drone-based solar site inspections: A 640-pixel resolution, radiometric camera with a 13 mm lens consistently produces high-quality input data.

Color images. Color photos are an important complement to thermal imaging. Software can overlap RGB color drone images to create large high-resolution site maps. This is particularly useful for older sites that may not have accessible as-built drawings or for newer sites that may not have updated high-resolution aerial or satellite imagery in services such as Google Maps.

Flying a drone at 400 feet with an RGB camera to make a map can take as little as 10 minutes on a 20 MW site. Color images also help software identify the root cause of issues at the module level. For example, soiling and cracking issues may appear similar in a thermal image, but a color image enables you to easily distinguish them.

Plan Your Data Capture

Whether you are considering drone training or have been authorized to fly an sUAS, the following best practices will help you do so safely and capture high-quality thermal images and color photographs for solar installation inspections.

Check the airspace. Part 107 of the Federal Aviation Regulations authorizes sUAS pilots to fly up to 400 feet in uncontrolled airspace. For flights in controlled airspace, such as those in proximity to airports, the Federal Aviation Administration (FAA) has published online maps to aid in the waiver process. When applying for a waiver, allow yourself up to 90 days to complete the application, and remember that submitting an airspace authorization does not obligate you to fly. The FAA is also beta-testing the new industry-developed Low Altitude Authorization and Notification Capability (LAANC) application, which allows real-time processing of airspace notifications and automatic approval of requests. SunPower was the first solar company to utilize LAANC, in October 2017.

Plan a flight pattern. You can plan the entire flight with a few taps on your electronic device. Flight planning apps, also known as ground station apps, allow you to box the boundaries of the PV system you are going to survey and automatically create a lawnmower pattern for complete coverage. You can fly the drone manually if you are inspecting for a specific issue, such as a combiner box failure that is impacting multiple strings. However, capturing the complete site is typically recommended, because there are many issues you cannot simply detect on the tablet’s screen—they require analysis. Additionally, the tracking of features helps software-based analytics solutions measure motion and automatically localize defects. Generally, you need high overlap in the direction of flight and low overlap between passes. Choose a non-oblique direction, either parallel or perpendicular.

Plan altitude and heading. Drones give you the ability to tune the resolution to achieve your desired granularity. Checking for functional module strings during commissioning may require a high-altitude survey, while a warranty claim for a batch of modules with defective cells may require low-altitude flights that result in a higher resolution. As an example, the recommended camera setup flying at 130 feet above the array will spot a 6-inch defect in a given module, while the same setup flying at 30 feet will spot a 1.5-inch defect.

Follow safety procedures. Routine on-site practices already incorporate many of the safety considerations for drone flights. These include awareness of surrounding hazards, knowledge of who is on the job site and a strong safety culture. Recommended personal protective equipment for drone operators includes helmets, sunglasses and safety vests.

Fly in good conditions. Sunny days with calm winds are the best time to fly your drone. The National Renewable Energy Laboratory recommends an irradiance of 600 W/m2, so pay extra attention when flying at high latitudes in the winter during low-irradiance conditions. Also keep in mind that calmer wind conditions make it easier to keep your drone on course and preserve its battery life.

Avoid glare. Glare is the reflection of sunlight into the camera, which results in false readings. Software for automated defect identification and localization can tolerate a gimbal tilt by up to 20° off nadir to avoid glare. Set the drone to maintain its heading so that it does not turn around with every pass. This keeps the camera angle consistent relative to the tilt of the modules.

Avoid motion blur. While it may be tempting to set the drone to the maximum speed, this can result in blurry images. If you want to survey the PV system faster, increase the flight altitude instead.

Check data quality. Check your data before leaving the field to ensure that it is free from glare and motion blur, contains files in the correct format and covers your entire flight. Finally, always take the time to back up the memory card. This simple and often overlooked step can make the difference between completing an aerial solar asset survey successfully and having to spend time and money repeating it.

Drone Operator Trends

In 2016, the FAA updated Part 107 of the Federal Aviation Regulations, which covers the use of commercial drones, and removed major roadblocks to the commercial operation of drones. Operators are no longer required to hold a traditional pilot’s license. Instead, they must pass a written aeronautical knowledge test at an FAA-approved testing center, pass a TSA background check and be at least 16 years old to qualify for a remote pilot certificate.

In the 18 months since enactment of these new regulations, the FAA has authorized more than 100,000 remote pilots to fly an sUAS in the US. Several drone schools with weekend courses train new operators. This has made it easier for solar professionals to train their technicians to use drones and to find qualified regional drone service providers. The quick pace of advancement and the rapid maturation of the drone industry make leveraging its technology for PV system O&M activities increasingly applicable to both large and midsize solar EPC firms and integration companies.

Nikhil Vadhavkar / Raptor Maps / Boston, MA / raptormaps.com

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