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.
Firstly, it is time-consuming to inspect a large power plant area with a handheld tool. Some areas might also be missed during inspections. Handheld thermal imaging tools are better suited for inspections of smaller areas.
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 at a solar power plant in Southern Ontario.
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.
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.
The figure above 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.
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 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.
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.