Thermal Imaging Cameras for Solar Panel Inspection

Fig. 1a: If a solar cell produces less electricity, it usually produces more heat than the other cells. That means that ineffective cells can be spotted very easily with a thermal camera

Solartechnik Stiens is a relatively new company. Founded in 2004, it has grown from the two employees it had on the day it was founded to its current staff of 55. Its main office building at Kaufungen, which was opened in 2008, has a 46-kWp solar system on the roof. The upper half of the building constantly follows the course of the sun. Every 10 minutes the upper story of the office building moves almost imperceptibly. It turns through 180° from sunrise to sunset, directing the solar panels toward the sun throughout the sunshine hours, thus making optimum use of the power of the sun.

Thermal imaging: usability is key

Fig. 1b: If a solar cell produces less electricity, it usually produces more heat than the other cells. That means that ineffective cells can be spotted very easily with a thermal camera

To prevent defective solar panels from being delivered to their clients, Solartechnik Stiens has acquired a Flir T335 thermal imaging camera, which they can use to check panels before delivery. The outstanding features of the T-Series portable thermal imaging camera are its ergonomics, weight, and ease-of-use. Usability is key: the engineers of the camera have translated user feedback concerning comfort and clarity into a series of comprehensive and innovative features.

The T335 has an uncooled microbolometer detector that produces crisp thermal images with a resolution of 320 × 240 pixels. It can accurately measure temperatures from –20 °C to 650 °C at a thermal accuracy of less than 50 mK. “The quality of the images is remarkable, but even more important for our application is the practical design”, explains Benjamin Kimpel, Service Technician at Solartechnik Stiens. “Especially the fact that it is lightweight and that it has a tiltable lens unit makes it the ideal tool for fieldwork, checking already installed solar panels for flaws.”

Fig. 2a: To prevent defective solar panels from being delivered to their clients, Solartechnik Stiens has acquired a Flir T335 thermal imaging camera, which they can use to check panels before delivery

Detection of ineffective solar cells

Thermal cameras are an ideal tool for checking solar panels for bad cells, according to Mr. Kimpel. “If a solar cell produces less electricity, it usually produces more heat than the other cells. That means that you can very easily spot such ineffective cells with a thermal camera.” The cause for solar cell inefficiency usually lies in the lack of uniformity of the semi-conductor material that is used; for most solar panels this semi-conductor is silicon. The multicrystalline silicon wafers used in most solar cells can be very prone to develop these non-uniformities. If a solar cell has a higher concentration of such non-uniformities it produces less electricity and more heat than others, dragging the performance of the entire panel down.

Fig. 2b: To prevent defective solar panels from being delivered to their clients, Solartechnik Stiens has acquired a Flir T335 thermal imaging camera, which they can use to check panels before delivery

Conversion of the DC output into AC requires a certain amount of electric current, so if inefficient cells drag the electricity production of the entire panel below this critical threshold the entire panel can be rendered completely useless. But there are many other possible causes for a solar panel’s performing poorly, such as broken cells, broken glass, water leakage, broken soldering points, defective sub strings, defective bypass diodes, delamination of the semi-conductor material, defective connectors, to name just a few. Whatever the cause may be, thermal imaging cameras can help the operator to find out where the cause lies and can often even indicate what the cause might be. In all of the possible scenarios a thermal camera can play an important part in finding out what is wrong.

Fig 3: The viewing angle should be chosen carefully to avoid misinterpretation of thermal images due to heat emitted by surrounding objects that are reflected in the glass

Identifying defects

“We want to prevent such defective solar panels from reaching our clients”, explains Mr. Kimpel. “And we also want to be able to monitor their performance after they are installed.” Identifying these defects requires efficient, cost effective test and measurement methods for characterizing a cell’s performance and its electronic structure. “We invited several different suppliers to demonstrate their thermal imaging cameras. We set up a test situation with an operating solar panel and we wanted to see which thermal imaging camera would spot the hot spot first. The Flir T335 achieved the best result in our comparative test by finding the hot spot long before the others did.”

Fig 4: The viewing angle should be chosen carefully to avoid misinterpretation of thermal images due to heat emitted by surrounding objects that are reflected in the glass

“But training is just as important”, explains Mr. Kimpel: “When you use a thermal imaging camera to look at solar panels from the front side then you have to be very careful not to jump to false conclusions due to reflection.” The viewing angle should be chosen carefully to avoid misinterpretation of thermal images due to heat emitted by surrounding objects that are reflected in the glass. Pointing the thermal imaging camera at the solar panel perpendicularly would provide the best result, because emissivity is at its highest when the angle is perpendicular, and decreases with an increasing angle, but in the case of a perpendicular angle the reflection of the camera and the operator might be seen in the thermal image. Therefore a viewing angle of 5 to 60° (where 0° is perpendicular) is seen as a good compromise. An even better way to solve this problem is to avoid reflection altogether. This is not always an option in solar panels that have been installed on a roof, but with the solar panels on the parking lot of Solartechnik Stiens, which are mounted on poles, it is possible to take a thermal imaging camera and point it at the back of the solar panel. The back of the panel does not reflect so much thermal radiation and thus permits more accurate measurement of the temperature of the solar panel.

MeterLink simplifies work

Along with the thermal imaging camera Solartechnik Stiens also bought an Extech clamp meter, because it can be connected to the thermal imaging camera through a MeterLink connection. This technology simplifies work in electrical or building inspections by making it possible to transfer the data measured by the clamp meter via Bluetooth to the camera. This saves time and eliminates the risk of erroneous records or notes. “Before we could connect the camera with the clamp meter via MeterLink we had to manually write down the measurement values of the clamp meter in our notebook. With MeterLink that information is automatically combined in one report.” Another useful feature of the T335 is the possibility of combining a visual image with a thermal image. The camera has two different methods: Thermal Fusion and Picture-in-Picture.

These two features of the camera make it easy to create compelling reports and communicate trouble spots to colleagues or clients. The Picture-in-Picture feature allows the user to overlay the thermal image directly over the corresponding visible image taken with the integrated 3.1 megapixel photo camera. This functionality combines the benefits of both the infrared image and visual picture at the push of a button. The user can simply move, resize, and reshape thermal images inside the visual image.

Contact www.flir.com

Pictures: Flir

More information about various thermal imaging cameras can be found in an expert article at www.chemietechnik.de