The US solar industry has exploded in recent years. Ever since the Solar Investment Tax Credit (SITC) was enacted in 2006, we’ve seen rapid growth of solar power across residential, commercial, and utility sectors.
The increasing amount of photovoltaic (PV) systems being built also creates a need for more solar maintenance. Which is a key component to overall asset health by minimizing defects and optimizing the performance throughout its lifetime.
So in this article, we’ll go over why solar farms need frequent inspections and the 5 most common inspection methods for PV systems.
Let’s get started!
Table of Contents
Why Do Solar Power Plants Need Frequent Inspections?
Solar farms need regular inspection for several reasons. For one, most PV systems are contractually obligated to undergo an inspection once or twice per year.
On top of that, PV sites require commissioning inspections right after installation is complete. This is to verify that the PV system is operational, safe, and built according to Original Equipment Manufacturer (OEM) standards.
The same goes for asset transfers. When a PV system changes ownership, a solar inspection helps uncover any issues so that the buyer knows the true value of what they’re purchasing.
In addition, most solar equipment has a warranty period. A successful solar inspection can help you locate warrantable issues and give you documentable proof to make your warranty claim.
So, what exact issues are these frequent site inspections looking to uncover? Here are some of the most common problems that a solar inspection can identify:
- Cell anomalies
- Soiling
- Cracks
- Inverter anomalies
- Module delamination
- String outages
- Cable damage
- Shading
- Combiner anomalies
- Module failures
- Vegetation
- Diode anomalies
- Broken ballasts
- Reverse polarity
- Missing modules
- Junction box anomalies
- Internal short circuits
- Circuit anomalies
- Underperforming strings
- Tracker faults
- Broken racks
- Degradation
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Identifying these issues helps eliminate safety threats. For example, detecting a hot spot on a solar module could prevent potential fires and outages. It also helps you fix the issue before it evolves into a more costly repair.
By performing regular solar inspections, you can perform preventative (rather than reactive) maintenance. Plus, regular inspections help ensure that the solar site runs efficiently so you can optimize power output and maximize revenue.
And when you keep a detailed inspection record, you always have a baseline to go by from one inspection to the next.
Inspection Methods for Commercial and Utility-Scale PV Systems
Now that you know the important role of frequent PV inspections, let’s go over the 5 most common inspection methods. We’ll describe each one and tell you its unique advantages and disadvantages:
1. Drone Thermography
Drone thermography is the most recent innovation for inspecting ground mounted, commercial rooftop, and carport solar systems. It uses drones equipped with specialized sensors to detect the infrared (IR) band of the electromagnetic spectrum. This allows the drone to “see” differences in temperature on the surfaces of solar modules.
Equipping unmanned aircraft with thermal cameras like this allows you to capture detailed thermographic data across hundreds of acres in a single day. And when you add a high-quality visual RGB camera to the drone, you can superimpose a visual image over the thermal data to create an ever fuller picture.
This gives you a powerful tool for collecting qualitative and quantitative inspection data. The visual camera allows you to detect anomalies like cracks and dirt, while the thermal sensor can capture invisible heat differences and measure exact temperatures. Overlaying both types of data gives you incredible insight into the condition and status of every module throughout the system.
To perform drone thermography, you need to be a trained drone pilot and have the right equipment. Properly flying the inspection also requires the correct weather conditions. Specifically when the irradiance of the solar panels is at a good level (irradiance is the density of solar radiation on a surface). Measure the irradiance with an irradiance meter before flight. Ideally, it should be 600 W/m2. Any other level may result in lower-quality data.
That said, drone thermography isn’t the end-all, be-all solution for all photovoltaic inspections. It has some serious drawbacks—including the need for high technical training, expensive equipment, and more.
- Pros: inspects large areas quickly, captures high-quality thermal and visual data, reduce safety risks, easily scalable, no plant downtime required
- Cons: limited by weather conditions, requires technical training, significant equipment investment, flying may be restricted at sites near airports, no ability to inspect the underside of panels
“Drone technology maximizes productivity for solar professionals by conducting inspections 10x faster than manually possible. With PV systems encompassing hundreds of thousands of panels, performing aerial thermal and high-resolution inspections has become the most efficient way to detect anomalies.”
- Shane Carey, Raptor Maps
2. I-V Curve Tracing
I-V curve tracing (aka electric testing) is the traditional method for inspecting PV systems. It measures the relationship between a module’s current (I) and voltage (V) on a line graph. Any irregularities in the I-V curve are traced back to issues in the module’s output—like shunt, series, or mismatch losses.
Trained technicians use specialized handheld I-V curve tracers to scan solar strings one by one. They measure what’s called the “fill factor” of a solar string’s I-V curve and then use a pass-fail threshold to determine its overall health. Usually, that threshold is 70%, so if a string scores below 70%, it’s marked for further investigation. Alternatively, you can set an even stricter performance factor threshold of 90% or 95% after normalizing for standard test conditions (STC).
Video Credit: Penn State Solar Center
Inspecting PV systems this way takes about 3.5 to 5 hours per MW when the module power and string length is at 1,000 volts. If the voltage is higher, it takes even longer. Because only one string can be checked at a time, crews typically only end up inspecting a small portion of a large PV site. Usually, it’s a random 5% to 15% sample size that can be used to make a base calculation on how many issues could be present throughout the entire site.
Then there are also safety risks involved. Since I-V curve tracing requires direct contact with electrical components, technicians must be licensed electrical workers and wear appropriate personal protective equipment (PPE) to keep themselves safe from hazards like electrocution.
Another common concern for technicians is heat exhaustion. Wearing heavy PPE during an inspection can cause you to overheat—especially in hot weather. As a result, additional personnel are typically involved to assess and monitor safety risks throughout the inspection, which only increases labor costs.
Source: Lukas Koester via Wikimedia Commons
- Pros: the ability to measure under all load conditions, electronically saved measurement data, achieves a detailed performance baseline
- Cons: slow inspection workflow, puts workers at risk of electrocution, a small range of identifiable anomalies, requires highly trained technicians
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.
3. Manned Aircraft
Manned aircraft like planes and helicopters can also be used to inspect solar farms. Similar to drones, manned aircraft can be equipped with mid-wave infrared (MWIR) thermal cameras to collect thermal data on solar arrays.
Typically, the aircraft takes off from a nearby airfield and then flies a designated flight pattern over the PV site(s). The aircraft carries out multiple scans to accumulate a database of thermal images that are then later reviewed and analyzed for hot spots, string outages, and more.
Manned aircraft inspections are ideal for large-scale PV systems or several sites that are in close proximity because it helps reduce the high cost of a single manned flight.
They are also best for fixed and single-axis ground-mounted systems, as well as rooftops. This is because you need the solar arrays to be at a consistent angle when inspecting from that high off the ground.
Manned aircraft inspections are also good for sites where visual line of sight (VLOS) can’t be maintained by a drone pilot. VLOS is a standard flying rule enforced by the FAA to keep drones from flying too far away from their operators.
But keep in mind, manned aircraft can’t fly under 500 feet unless the pilot has an acrobatics license. And their deployment time can often be weeks or months out.
- Pros: ideal for multiple PV sites at a time, quickest inspection method
- Cons: expensive, long lead time required, limited in the number of identifiable anomalies, not possible to inspect dual-axis trackers
4. Handheld Thermal Cameras
Another way to inspect PV systems is with handheld thermal cameras. These are similar to the IR sensors on drones, except that they are meant for close-distance measurements. They use an uncooled microbolometer detector sensitive to the 8–14 μm electromagnetic waveband to detect temperature anomalies.
To get the correct measurements with a handheld thermal camera, you need to have a solar irradiance of at least 500 W/m2 but preferably over 700 W/m2. You also need to angle the thermal camera at 5–60° to the module’s surface to avoid catching the reflection of you or the camera in the thermal image.
An added benefit about handheld thermal cameras is that they can be used to inspect areas that are hard to reach otherwise, like the underside of panels, cable connections, combiner boxes, and more.
But on the downside, handheld thermal camera inspections are a time-consuming process not suitable on a large scale because they require walking through rows of solar panels and scanning them one by one. Plus, the data can be hard to organize. So most crews only measure a sample size of the PV system and use the data to extrapolate how many defects are on the entire site.
- Pros: non-invasive inspection method, relatively inexpensive equipment, ability to locate anomalies down to cell level, easy maneuverability
- Cons: very time consuming, data hard to organize, not scalable
HAVE QUESTIONS ABOUT droneS? START HERE.
5. Electroluminescence Testing
Finally, you can also inspect PV systems through electroluminescence (EL) testing. This is a powerful way to uncover microcracks and other defects that are hard for I-V curve tracing and IR scanning (let alone the naked eye) to pick up.
Electroluminescence refers to the light emission that occurs when an electrical current passes through a PV cell. As the current goes through, a radiative recombination of carriers produces some light, a small amount of which you can capture with an external detector.
Taking an EL image with an external detector is like taking an X-ray photo. It allows you to look at a solar cell’s inner structure in a non-destructive way that’s otherwise impossible to do.
Video Credit: 2DegreesKelvin
EL cameras come in multiple resolutions just like regular digital cameras and can quickly detect microcracks (cracks in the silicon wafers), cell cracks, dead cells, diode failures, and more. But this inspection method still requires you to inspect one solar array at a time. So an EL inspection of an entire solar farm would take a lot of time.
- Pros: finds defects not apparent through I-V curve tracing and IR scanning, non-destructive, ability to test one panel relatively quickly, very accurate
- Cons: slow for testing an entire PV site, requires highly skilled technicians, defect origins are not identifiable,
Bonus PV Inspection Method: Visual
One additional inspection method that doesn’t require any specialized equipment is a visual inspection. This involves having workers walk through solar arrays looking for any anomalies with only their eyes. Some issues that can be found by sight include soiling, cracking, delamination, vegetation, and disconnected cables.
However many of the solar faults which have the most effect on system performance cannot be seen with the naked eye. And workers are severely limited in the amount of ground that can be covered, especially at sites with hazardous terrain. For large PV systems, visual inspections are unfortunately not the most practical method for achieving a highly detailed understanding of the site’s condition.
- Pros: no additional equipment is required, quick deployment, not subject to the majority of weather limitations
- Cons: slow and hazardous, limited in the number of identifiable anomalies, difficulty in keeping track of anomaly locations
Which PV Inspection Method is Best?
Well, there you have it! The 5 most common inspection methods for solar farms. If you’re still on the fence about which is best for your own operations and maintenance, here are some key questions to keep in mind:
- Which inspection method(s) do you currently use?
- How often are your sites inspected and to what extent?
- Are your existing O&M techniques a financial burden?
- How many sites do you manage and are they within close proximity to one another?
- Do you have internal inspection teams or do you subcontract to a third party?
- Do your current inspection practices involve unnecessary safety risks for workers?
In the end, each inspection method has its own use cases, unique advantages, and disadvantages. But in recent years, the solar industry as a whole has been switching to drone thermography as the gold standard for maintaining solar assets. This is because the benefits of using drones usually outweigh those of other inspection methods. In fact, a study in ScienceDirect concludes that drones are hands down the best tool for inspecting PV systems.
Getting Started with Drone Photovoltaic Inspections
If you’re interested in hiring a drone service provider to inspect your PV system, we’re here to help. At The Drone Life, we specialize in infrared solar inspections. Our drone experts can help develop an inspection workflow that includes data collection, reporting, and identifying all kinds of PV faults: cell anomalies, cracking, soiling, string outages, shading, reverse polarity, and much more!
Our detailed inspection reports label all issues affecting system performance which allows your field team to start making repairs immediately. This way, you can eliminate system inefficiencies and make sure your solar farm is operating at maximum capacity. Contact us today for a free consultation.
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.
