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Inspection Parameters & Best Practices

As the use of drones for solar inspections continues to expand, there is a significant need for standard inspection parameters and best practices to ensure a successful inspection. Each inspection can vary and require special considerations, but standardizing variables such as flight altitude, frequency of inspections, maintenance, and parameters for appropriate weather conditions can better ensure a safe and effective inspection.

Correct Flight Altitude

One of the most important things to consider when performing a drone solar inspection is flying at the proper altitude. The drone must be high enough to get a clear view of the solar panels, but not so high that it is unable to see any potential problems. There is no "set it and forget it" altitude that is correct for every inspection. The flight altitude will vary depending on the type of inspection being performed, the drone equipment being used, and any airspace restrictions in the nearby area.

To determine a flight altitude, you need to ask two main questions. What is the purpose or main objective of this inspection, and what level of detail needs to be achieved in the results? The purpose of the inspection could be any one of several reasons, or a combination of several, such as warranty claims, evaluating for asset sale, maintenance, site construction, site commissioning, or post disaster evaluation.

The level of detail for the inspection is determined by calculating the ground sampling distance (GSD), the number of pixels that exist within a given distance measured from the ground. GSD calculations are made using the camera sensor height, flight height, lens focal length and the number of pixels you need to capture. International Electrotechnical Commission (IEC) TS 62446-3:2017 standards note a minimum of 5 pixels per cell are required for thermal inspections. As an example, if a GSD of 3 cm is calculated, to meet the IEC standard, the drone must be flown no higher than 72 feet above ground level (AGL).10 The smaller the calculated GSD the more detail captured, which means higher overall data quality. The larger the calculated GSD the less detail captured in your imagery

PROACTIVE MAINTENANCE

Proactive maintenance is the process of addressing normal routine issues before major failures occur. Often this means having a preventative maintenance plan in place that an O&M team can actively update. How often you schedule periodic inspections and maintenance depends greatly on several factors including weather exposure, equipment manufacturer, or contract specifications. Issues such as dust collection, vegetation growth, and component service life can all impact how often an asset manager will need to schedule inspections.11 For drone-based thermography, the most common inspection schedules are yearly, semi-annually, or quarterly. Additional attention should be given to older PV systems, which will require more frequent proactive maintenance inspections

Frequency of Inspections

Solar panels should be inspected regularly to ensure they are functioning properly. While they tend to require less maintenance than other renewable energy sources, they still require periodic maintenance inspections. The frequency of inspections will depend on the type of solar equipment, the environment in which it is located, and the type of maintenance requested, proactive or reactive.

A solar power meter records the irradiance levels of solar panels

REACTIVE MAINTENANCE

Even though planned maintenance is preferred, there will always be a need for reactive maintenance as well. These inspections will usually occur outside any planned schedule, often need to be executed very quickly, and require a very granular level of detail. For example, reactive maintenance would need to be performed after natural disaster events such as hailstorms, tornados, floods, earthquakes, hurricanes, lightning, and any abnormal output levels caused by defects. Excessive heat also impacts solar panels as well and could cause maintenance issues. Ambient temperatures above 77°F (or 25°C) can cause panels to lose 1% efficiency per degree, but during prolonged excessive heat events can possibly damage internal components.

APPROPRIATE WEATHER CONDITIONS

Another important consideration when conducting a solar inspection is the weather conditions. Inspections should only be flown in good weather conditions, such as sunny and clear skies to achieve optimal radiation levels across the solar panels.

A specialized tool, called a solar irradiance meter, is commonly used to confirm adequate irradiance levels are available to capture the highest quality thermal data. If the weather is not ideal, it could degrade the quality of the thermal data collected, making any anomalies much harder to detect.

The location of the flight is also very important when preparing for the proper weather conditions. Flying at high latitudes in winter months may produce low-irradiance conditions where the sun may never reach high enough levels.13 Low wind speeds are also important to keep the mission on course, preserve battery life, and to capture sharp images.

Sufficient weather conditions mean better data captured, higher quality post-processing, and more detailed data analysis to show defects and anomalies present.

Ideal conditions should fall within these parameters

Wind: Wind speed less than 15MPH (6.7 m/s).
Clouds: Clear skies, with a maximum cloud cover allowance of 2/8 oktas.
Humidity: Less than 60% humidity.
Solar Irradiance: Solar irradiance must be greater or equal to 600 Watts per square meter (600 W/m2).
Precipitation: No rain, dew, frost, or snow on panels
Time of Day: Peak sunlight hours, inspections should take place when the sun is at its brightest. Flights should not happen within 2 hours of sunrise or sunset.

An anemometer detects wind direction & speed

Identifiable Anomalies

When inspecting solar panels, it is important to identify all possible anomalies. Anomalies can be anything that is out of the ordinary, such as a crack in a panel or an offl ine string. Once the data is analyzed, all anomalies should be documented and reported to the appropriate stakeholders. This will help to ensure that the problem is fi xed in a timely manner. Anomalies can be categorized into 4 general types of issues:

MODULE LEVEL - external or internal attributes discovered during aerial drone inspections