Solar energy is gaining momentum in the U.S., employing 231,474 workers in 2020 and showing increases in labor productivity ranging from 2% – 32% across market segments (National Solar Jobs Census 2020). This increase in productivity has to do with the increased demand for solar and renewable energy across the U.S. New technologies, such as drones, are emerging to help solar workers keep up.
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“Thanks to strong federal policies like the solar Investment Tax Credit, rapidly declining costs, and increasing demand across the private and public sector for clean electricity, there are now more than 100 gigawatts (GW) of solar capacity installed nationwide, enough to power 18.6 million homes.”
- Solar Energy Industries Association
Drones have brought about faster and more efficient methods of building, operating, and maintaining solar farms. Part of maintaining solar assets is routinely inspecting them for faulty panels within the PV (photovoltaic) system. The industry standard for inspecting these PV systems is a time-consuming, manual process known as I-V curve tracing. However, the days of labor-intensive, prolonged, and expensive PV inspections are falling behind us thanks to drone thermography.
Inspecting PV Systems for Anomalies: Aerial Thermography vs. I-V Curve Tracing
To fully understand how drone thermography outperforms I-V curve tracing, we should first visit a quick review of PV systems. PV systems are built in a hierarchy, starting at the smallest unit with a solar cell. These are arranged together in rows to form a solar panel. Panels are placed together to form a string, and multiple strings placed together are called solar arrays. A PV system will have multiple arrays as well as other components that keep the system running. These components and arrays must be inspected on a regular basis to ensure they are functioning properly.
Source: Wikimedia Commons
What is Drone Thermography?
For PV inspections, drones use a thermal camera to measure temperature and detect anomalies on the solar field. A thermal camera can detect the infrared band of the electromagnetic spectrum, revealing incongruent heat patterns across a solar field that signal possible irregularities in power production.
Drone thermography can assess a PV system down to the cell level with geo-referenced data that allows the operations team to pinpoint exactly where found issues are located. This differs from I-V curve tracing, which is conducted at the string level, requiring follow-up testing to identify the particular panel triggering the issue.
What is I-V Curve Tracing?
I-V curve tracing, also known as manual electric testing, is the current standard for inspecting PV systems. An I-V curve measures the relationship between the current (I) and voltage (V) to determine how much electrical power the PV system is producing. It is used to detect issues including series losses, shunt losses, mismatch losses, reduced current losses, and reduced voltage losses.
Source: Lukas Koester via Wikimedia Commons
To produce an I-V curve, a trained technician uses a handheld testing device to test individual panels. Each string of panels must be unplugged from the array and plugged into the testing device. The resulting I-V curve is graphed and assessed by the technician usually that same day onsite.
An irregularity in the curve alerts the technician to an issue within the string and depending on the issue, additional testing may be required to pinpoint the faulty panel and its precise location within the string. This can be extremely tedious and time-consuming work on large solar fields.
Drone Thermography vs. I-V Curve Tracing
Is drone thermography truly a better method for inspecting PV systems than I-V curve tracing? To find out, we’ve compared the following capabilities of drone thermography and I-V curve tracing:
- Coverage – the level of detail with which PV modules, combiners, and strings are assessed
- Collection Time – the amount of time needed to inspect PV systems per MW (megawatt)
- Power Production – how the inspection impacts the PV site’s ability to produce power
- Safety Restrictions – actions needed during the inspection to ensure worker safety
- Required Expertise – training, licensing, or other expertise needed by the inspector
- Weather Impacts – effect of weather on data collection and accuracy
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
Drone Thermography: Capabilities for PV System Inspections
First, let’s assess the capabilities of drone thermography for PV system inspections, including coverage, collection time, power production, safety restrictions, required expertise, and weather impacts.
Coverage
Through the aerial collection of thermal data, drones can inspect PV systems at the cellular level. Aerial thermography can detect anomalies within individual cells of each module, providing the most detailed analysis of the entire PV system.
Collection Time
Drones can complete thermal inspections at a pace of 10 minutes per MW. The data collection time may increase for sites larger than 5 MW to account for the time needed to recharge the drone batteries (which drain in about 30 minutes on common commercial drones); however, drones still provide a faster method than I-V curve tracing inspections conducted on-foot.
Power Production
It is mandatory for the site to stay powered online during the drone inspection. Keeping the PV systems online enables the drone to detect differences in temperature with the bonus of continuing to produce power and avoiding costly downtime. A drone can inspect the entire site while it is still powered, detecting more anomalies than only sampling a site—an industry practice where only 10-25% of the site is inspected to cut down on the time and cost of manual IV curve inspections.
Safety Restrictions
The drone pilot is not required to interact directly with the electrical system and can perform the inspection from a safe location. This reduces the risk of electrocution and minimizes other safety concerns such as heat exhaustion faced by technicians who inspect PV systems manually with handheld tools.
Safety concerns for drone pilots are covered by the Federal Aviation Administration’s rules for commercial drone pilots, also known as the Part 107 rules. These rules standardize requirements for safe flight in different types of airspace and account for environmental factors that could impact flight such as weather and visibility. Drone pilots must hold an FAA Part 107 certificate to operate legally and safely.
Required Expertise
Most solar companies choose to outsource their drone solar inspections to qualified professionals. A drone pilot’s expertise lies in their knowledge of drone systems, how to operate the drone, and how to collect aerial data that yields precise results. A qualified drone pilot obtains this experience through the FAA Part 107 certification process along with field experience that produces logged flight hours.
A drone pilot will be able to arrive on site with their equipment and assess how to fly the site to collect the highest quality thermal data. Ideally, they will have an understanding of thermography, but the drone pilot does not need expertise in electrical systems since they will not come into direct contact with the solar arrays.
Once the drone data is collected, the images are processed and analyzed using advanced software solutions. Thermal imaging software makes it easy for O&M teams to identify anomalies, with reports that are easily understood by personnel with varying levels of expertise in solar energy.
Weather Impacts
The minimum level of irradiance recommended for drone thermography is 600 watts per square meter. Irradiation levels have to do with the amount of light striking a surface and may be too low in certain weather conditions, like an overcast day.
Drone pilots should also set limitations when flying in windy conditions, not flying when wind speeds exceed 15 MPH (6.7 M/S) is a best practice. Wind speeds over 15 MPH can potentially create a cooling effect over the solar modules, causing it to be more difficult to locate anomalies. Additionally, the panels must be dry with no moisture from recent rain or snow.
I-V Curve Tracing: Capabilities for PV System Inspections
Now we can assess the capabilities of I-V curve tracing for PV system inspections, including coverage, collection time, power production, safety restrictions, required expertise, and weather impacts.
Coverage
I-V curve tracing inspects PV systems at the combiner and string level. Individual module inspections are not typically completed unless an anomaly is detected at the combiner or string level first. The systems are initially inspected in large sections rather than module-by-module because the latter method would be extremely costly and time-consuming for a technician making manual assessments on foot.
Collection Time
A technician can collect the necessary data in 3-5 hours on average per MW. The data is analyzed onsite immediately following the collection. However, it may take days to collect all the data needed for a large site, before the issues can be detected. Additionally, data collection and interpretation during I-V tracing is more prone to human error since the data is highly sensitive and distorted by environmental factors (such as weather).
Power Production
The combiner or string must be isolated from the rest of the system and turned offline during the inspection. This temporarily prevents the system from generating new power, which results in lost revenue for the solar asset owner.
Safety Restrictions
Since the IV curve tracing technician has direct contact with electrical components, they must be a licensed electrical worker. They also need to dawn the appropriate personal protective equipment (PPE) to keep themselves safe from hazards such as electrocution.
A common concern for technicians is heat exhaustion brought on by conducting the inspection in hot weather while wearing heavy PPE. Additional personnel are typically involved to assess and monitor safety risks throughout the inspection, increasing labor costs.
Required Expertise
I-V tracing is highly skilled work requiring in-depth training. Collecting and interpreting I-V data requires a skilled technician, licensed for electrical work, with knowledge of environmental factors.
Weather Impacts
The minimum level of irradiance for I-V curve tracing is 400 watts per square meter. To reach a high enough irradiance, there must be enough sunlight reaching the surface of the solar panels. The time of day, air temperature, cloud cover, humidity, and rain can all impact the accuracy of an I-V curve test, and the test may need to be rescheduled if the weather is not suitable at the scheduled time.
Verdict: Drone Solar Inspections are the Most Efficient Method of Inspection
In summary, drone thermography offers a more scalable and efficient method of PV system inspection than I-V curve tracing. Drone thermography generates more precise data that makes it easier to identify and locate issues within a solar array. The data is collected faster in the air than on foot during a traditional I-V curve test and using drones keeps the workers safer since they do not have to come into direct contact with any electrical systems.
Drone operators are uniquely skilled workers who can assist the solar site’s O&M team with both data collection and thermal imagery interpretation. Solar companies that hire drone pilots are improving their maintenance workflows while saving time and money.
From data collection to report generation, the entire process of drone thermography appeals to solar companies because of the minimum safety risks, faster inspection times, and improved quality of data.
To determine whether or not drone thermography could save your solar company money, check out our Ultimate ROI Guide For Infrared Drone Solar Inspections next. Here at The Drone Life, we have licensed drone pilots who are experienced in the solar industry working in the northeast U.S. You can learn more about our drone services for solar farms here.
What's included in YOUR GUIDE...
Techniques for Exposing Warrantiable Items on Your PV Sites.
A Proven Roadmap for Successfully Implementing Drones in Solar.
Strategies for Identifying DC Losses and Avoiding Downtime.
