Thermal imaging is a vital tool for detecting defects and anomalies at solar power plants. However, capturing thermal images with handheld equipment is time-consuming and susceptible to human error. Unmanned aerial vehicles with dual-camera attachments flying on preprogrammed routes provide a precise and repeatable way to complete inspections efficiently.

As a cleaner form of energy, solar power generation relies on several major components including photovoltaic (PV) modules, high-voltage power equipment, inverters, power cables, and direct current-alternating current (DC-AC) string boxes. To ensure smooth power generation, the equipment requires proper maintenance so that everything works well. Equipment inspection is an important part of the process.

There are three types of common anomalies in solar power generation: PV module string anomalies; single PV module anomalies and diode circuit anomalies; and anomalies contributed by environmental factors (such as shadowing, biological contamination, etc.). Some anomalies, such as PV string anomalies and anomalies contributed by environmental factors, can either be detected by observing an increased or decreased amount of power generated, as shown on the monitoring system, or they can be detected visually. Some anomalies such as single PV module anomalies or diode circuit anomalies, on the other hand, cannot be detected through such means. In either case, thermal imaging is an ideal tool for inspection, as anomalies can be precisely located through this method and repaired.

Thermal Imaging

Thermal imaging works by turning the infrared radiation detected from an object into electronic signals, which are then processed into an image. It can help locate anomalies in a solar power plant because anomalies on a PV module show a difference in temperature.

Handheld thermal imaging tools are commonly used for solar power plant inspections (Figure 1). However, handheld equipment suffers from a couple of drawbacks.

1. Thermal image of PV modules as seen through a hand-held thermal imaging tool. Courtesy: ECOVE

Firstly, it is time-consuming to inspect a large power plant area with a handheld tool. Some areas might also be missed during inspections (Figure 2). Handheld thermal imaging tools are better suited for inspections of smaller areas.

2. It is not only time-consuming to inspect massive areas of solar power modules with handheld thermal imaging tools, but it is also restrictive in terms of the amount of area that can be inspected. Courtesy: ECOVE

Secondly, there are safety concerns. Many solar power plants are not installed on the ground but rather on rooftops or on water surfaces. To capture clear pictures when using thermal imaging tools, it is necessary to keep the camera a certain distance away from the equipment during the inspection and while taking pictures. Due to limited operative space on rooftops and on water surfaces, inspecting with handheld tools increases the safety risks of the inspector.

Given such drawbacks, combining unmanned aerial vehicles (UAVs) with thermal imaging provides a more efficient and safer alternative for inspections. Maintenance staff can inspect more PV modules within the same amount of time and precisely locate anomalies without increasing safety hazards. The following sections will briefly introduce the characteristics of thermal imaging UAVs adopted by ECOVE, a company experienced in solar power plant maintenance, as well as some real cases with images showing the anomalies found during inspections conducted across solar power plants in Taiwan.

Thermal Imaging UAVs

The multi-axis UAV discussed here is a customized dual-camera design that can take thermal imagery and visible light imagery (Figure 3). It can fly under 15 meter/second wind conditions, with an 8 kilometer maximum operation distance and 40-minute flight duration per inspection.

3. The unmanned aerial vehicle (UAV) shown here includes a dual camera that allows capturing both visible and thermal images. Courtesy: ECOVE

Based on preconfigured flight routes, the UAV can complete the inspection of 1 MW of solar modules in about one hour (actual time varies according to different setting parameters). Imagery can not only be viewed on the controller in real time through the UAV’s built-in Wi-Fi connection, but it can also be simultaneously stored in the camera memory, which allows further processing through professional software for report compilation purposes.

Anomaly Cases and Insights on Thermal UAVs

Below we will share a few cases showing three types of anomalies as seen from a thermal UAV, which include a PV module string anomaly, a single PV module/diode anomaly, and an anomaly due to environmental factors.

PV Module String Anomaly. A PV module string anomaly poses the greatest threat to power generation. Depending on the scale of the power plant, as many as thousands of modules can suffer from this type of anomaly, bringing considerable loss to the plant. Therefore, PV module string anomaly deserves particular attention.

Normally, PV module strings are not maintained on a daily, weekly, or monthly basis, so it is difficult to detect anomalies right away. Even though anomalies can be judged by looking at the increased or reduced amount of power generation as shown on the monitoring system, this does not work when an anomaly occurs right at the beginning when the power plant is up and running, since there would be no differential figures for comparison. It is suggested, therefore, to use thermal imagery to locate anomalies whenever power generation abnormality is detected from the monitoring system, rather than wait for routine inspections to be completed. This can help prevent unnecessary power losses.

Figures 4 and 5 are thermal images showing two types of PV module string anomalies. They depict a single-string anomaly and multiple-string anomaly, respectively. Because circuit anomalies in single strings or small-scale strings do not significantly impact power generation, they usually go undetected without installation of a string monitoring system. Also, this type of anomaly is not only nearly impossible to be detected by the naked eye, but it is also difficult to be detected with handheld thermal imaging tools. However, with aerial thermal imaging by a UAV, anomalies can be quickly spotted.

4. This thermal image shows an anomaly from a single PV module string. Courtesy: ECOVE


5. This thermal image shows an anomaly from multiple PV module strings. Courtesy: ECOVE

Single PV Module Anomaly or Diode Circuit Anomaly. Figures 6 and 7 are thermal images showing a single PV module anomaly and a diode anomaly, respectively. These anomalies can neither be spotted under visible light nor be detected through a monitoring system. They can only be spotted with thermal imaging, either by handheld thermal imaging tools or thermal UAV. Again, thermal UAV is far more efficient in this respect.

6. This thermal image shows an anomaly of a single PV module. Courtesy: ECOVE


7. This thermal image shows an anomaly of a diode circuit. Courtesy: ECOVE

Anomalies Due to Environmental Factors. Some environmental factors contribute to the loss of power generation, including biological contamination and tree or building shadowing, as shown in Figures 8 and 9. Even though these types of anomalies can be detected visually during routine inspections, thermal imaging brings far more detection efficiency. Once the anomaly is spotted, operators can refer to the power generation figures as shown on the monitoring system to further determine the type of module cleaning plan or vegetation removal plan needed. Anomalies caused by environmental factors do not pose as immediate threat to equipment function; however, if left untreated or uncleaned, there may be partial hotspots on PV modules, which in the long run would affect power generation.

8. This thermal image shows an anomaly of PV modules due to biological contamination, affecting normal power generation. Courtesy: ECOVE


9. This thermal image shows an anomaly of PV modules due to tree shadowing, affecting normal power generation. Courtesy: ECOVE

UAVs Save Time and Optimize Performance

From our experience, thermal UAVs can help solar power plant maintenance operators save at least half the time required to perform manual inspections. The dual-camera modules on the UAV can not only take thermal images, but also capture visible light images. This can aid inspectors when analyzing the images, as defects or anomalies shown on both pictures can confirm problems, helping to identify causes and avoid misreadings.

Another feature of thermal UAV is that it can conduct full range inspections and area screening. The screened results can then be post-edited with professional software to produce complete images of the entire solar power plant (Figure 10) in gray scale and in thermal mode. Comparison between gray-scale and thermal images is recommended because sometimes an anomaly seen on a thermal image is not necessarily a real anomaly. Gray-scale images provide a way to double-check and avoid misreading the results.

10. Images taken by UAV can be pieced together during the post-editing process to produce thermal (right) and gray-scale (left) pictures showing the full-sized solar power plant. Anomalies can be detected by comparing the two pictures. Courtesy: ECOVE

Lastly, pre-configured flight routes allow thermal UAV to accurately inspect the same areas every time, reducing the chance of human error during inspection. Quality and standardized data is especially important if the operator wishes to develop a database for future machine learning programs.

Challenges remain for carrying out solar farm inspections with thermal UAVs, including ambient temperatures, sunlight angle, and UAV flight parameters, which could either reduce the validity of inspection results or impact results judgment. But overall, thermal imaging through UAVs is a great maintenance tool for solar farms in that it can locate anomalies with precision, save more than one half of the inspection time than when done by hand, ensure inspection staff security, and provide useful data for future smart applications.

Cheng Hong Tsai and Jie-Ming Yang are senior engineers with ECOVE, a CTCI company that provides environmental services, specializing in energy-from-waste, waste management, wastewater recycling, solar power, and PET (polyethylene terephthalate) recycling.