With the UK's commitment to reducing its greenhouse gas emissions by 80% by 2050, it stands to reason that renewable energy systems will increasingly feature in our built environment.
Solar power is one of several technologies that will help meet this goal and despite its chequered take-up in the UK, an upward trend has been established. Indeed in 2012, the Government estimated that 4 million homes will be solar powered by 2020, representing an installed solar power capacity of 22,000 MW.
Solar farms are also becoming relatively common in the landscape and, with the introduction of the Feed-in-Tariff making it financially attractive to invest in solar power, growth in the commercial and industrial markets have been stimulated too. So there is no doubt that the maintenance of these installations will soon become standard practice for future generations of building services and environmental engineers.
Fast and effective defect detection
For maximum power generation, system life and best return on investment, every solar cell on a solar panel must be working. To provide this assurance, those involved in maintaining these installations are increasingly employing thermal imaging as their preferred method for locating defects. Thermal imaging allows anomalies to be seen clearly and, unlike other methods, can be used to scan installed solar panels during normal operation. It is also a highly time efficient process as a large area can be scanned in minutes.
If a solar cell produces less electricity it usually generates more heat than other cells and that difference is easy to spot with thermal imaging. The cause of this inefficiency commonly lies in the uniformity, or lack of it, in the silicon wafer used. Often the source of the problem can be traced back to the production stage so thermal imaging is also a vital verification tool for post-installation inspection.
Cell defects can render the entire solar panel useless as a critical threshold needs to be reached by the combined performance of the cells to allow direct current from the sun to be converted to alternating current. And the loss of one complete panel in the installation for any length of time can be the difference between asset and liability.
There are many other possible causes for a solar panel's poor performance however; broken cells, broken glass, water leakage, damaged solder, defective sub-strings and bypass diodes, delamination of the semiconductor material and poor connections, to name but a few. By scanning the panel with a thermal imaging camera the effect of these problems can be clearly seen. Importantly it also shows the precise location of the fault. And by using a camera with a high resolution it is even possible to identify the cause.
Choosing the right camera
Handheld thermal imaging cameras typically have an uncooled detector that is sensitive in the 8 - 14μm waveband. However, glass is not transparent in this region. So when solar cells are inspected from the front, a thermal imaging camera sees the heat distribution on the glass surface but only indirectly, the performance of the underlying cells.
As a result, the temperature differences that can be measured and seen on the solar panel's glass surface are small. In order for these differences to be visible, the thermal imaging camera chosen needs a thermal sensitivity of <80mK. It should also allow manual adjustment of the level and span function to optimise visual contrast.
New technology from FLIR is also helpful in this regard. Called multispectral image enhancement (MSX), this feature produces a detailed and texture rich image. Unlike traditional thermal fusion that inserts a thermal image into a visible light picture, this patent pending technology embeds a digital camera image onto thermal video and stills.
The result is superb image quality that allows not only the location of the fault to be determined but also its nature.
On site considerations
The emissivity of any material is the relative ability of its surface to emit energy by radiation. It is therefore vital that this value is factored in to any thermal measurement and professional thermal imaging cameras will allow this to be pre-programmed.
As with all highly reflective material, the glass on a solar panel requires particular attention as any thermal image of its surface will also pick up the radiated temperature of surrounding objects as well as the sun itself. In the worst case, this results in false hotspots and measurement errors.
Choosing the best viewing is therefore essential. Pointing the thermal imaging camera at the solar panel perpendicularly provides the best result as the emissivity value is at its highest. However this angle runs the risk of radiated temperature from the camera and its operator skewing results. A view angle of 5? to 60? is therefore considered to be the best compromise.
An even better way is to avoid reflection altogether by inspecting the solar panel from the back. This isn't possible with solar panels fitted to a roof, of course, but increasingly installers are mounting panels on poles to take advantage of this more reliable inspection technique.
Roof panel inspection
To simplify the process of inspecting roof mounted solar panels, building service engineers with the Belgian company Visiotherm have developed an ingenious solution using a retractable pole system.
Whilst traditional access lifts and ramps are clearly suitable for the task they require a lot of ground space and, if they need to be hired, this significantly increases the cost of inspection. To overcome this problem Visiotherm developed an affordable, retractable pole which requires only 3 square feet of ground space to be set up, allowing much greater application flexibility.
On top of the pole is a remotely controlled pan and tilt mechanism which points the camera at the target. The camera itself, in this case a FLIR T640 with MSX technology, is then controlled through a wireless connection to a PC that runs the FLIR Tools application. In this way, the operator can change emissivity settings, adjust level and span, add temperature measurement points and perform other tasks from ground level.
Developing skills
Whilst thermal inspection in overcast conditions may appear a solution to reflection problems, it is also a mistake to undertake the task without sufficient sunlight; although this can be offset by choosing a camera with a high thermal sensitivity.
Experienced solar panel inspectors recommend that thermal imaging is best performed when the solar irradiance is 700W/m2 or higher. As a rule of thumb, this is the amount of light needed for the solar modules to operate in an optimum range for detecting defects.
Calm conditions are also desirable as airflow on the module's surface will cause convective cooling, reducing the thermal gradient. The cooler the air temperature, the higher the potential thermal contrast, so early morning inspection is certainly the best option.
Determining the optimum conditions for thermal imaging and capturing meaningful data is a skill so the importance and value of dedicated training should never be underestimated.
