Test & measurement
Equally, if the current starts at a lower value, the modules may be degraded, the module is operating outside the set parameters, there are faulty connections or there is a manufacturing flaw. With the I-V curve available on-site, with real-time data generated by the measuring equipment, the operator can correct for that and re-do the measurement. The I-V reading is also used to see the performance of solar modules over time (with degradation indicated if the ‘knee’ of the I-V curve speeds up). This is particularly important in a large utility-scale solar energy plant where performance can suffer through degradation. Such I-V tests need to be carried out regularly – say once or twice a year – so that the energy plant owners can compare against the initial I-V curve to predict the performance of the PV modules over time. Every energy plant owner needs to know how much longer they can go on with their existing modules and when they should make plans to replace them.
THERMAL IMAGING
Another test that can be carried out is infra-red inspection using thermal imaging. This is another category 2 measurement where an inspector will check the heat distribution over a PV module. Because each module is a set of multiple different cells where the photovoltaic effect takes place, if one or more of the cells begins to malfunction this can compromise the efficiency of the whole module. Rather than generating energy, the cells will be dissipating energy generated by all the cells within a module,
Instrumentation Monthly October 2023
preventing energy from flowing out of the module to the rest of the system.
In practice, energy dissipation causes a cell to become hotter – ie the temperature distribution around the surface changes – and thermal imaging allows inspectors to identify where individual cells are heating up due to faulty operation. Anomalies within the I-V curve – such as steps, drops or irregular curves – are the first indicators that such a problem may be taking place.
In residential or commercial PV solar park installations, using a hand-held thermal imager makes it easy to identify such issues. However, in a large-scale array installation it is possible to use a fixed-mounted thermal imager (such as Fluke’s RSE 600 infra-red camera) attached to a drone to detect hotspots.
When modules are connected in a solar park in a string, tests are not carried out on individual modules but on the whole string. If the performance of one module is faulty then this will affect the operation of a stack of modules which can all be checked against one I-V curve. If a problem is revealed, then the operator will need to check each individual module to pinpoint the error. Ideally, thermal imaging should be carried out from the rear of PV modules (to avoid the effects of sunlight) but this is not always possible.
ENVIRONMENTAL INFLUENCES Finally, an absolutely essential point for an inspector to consider is what the environmental conditions are when measurements are taken. Put simply, output of a PV module will be higher when the sun is shining brightly and lower when it is not, so when an I-V curve is produced the irradiance will directly affect any readings and calculations. This is why measurement test conditions must always be standardised to 1,000W of solar energy per square metre of irradiance (1,000 W/m2
) and a module
temperature of 25°C, the so called Standard Test Conditions (STC)All specifications on data sheets relate to this standard test condition so that, in order to compare measurement results to the data sheets, the operator must always correct the results with the irradiance of 1,000W per square metre. Modern measurement devices are capable of making these adjustments automatically.
Fluke
www.fluke.com
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