IV Measurement uncertainties and the biggest contributor
During my time at TUV Rheinland, my first ask was to improve measurement accuracy and in particular the repeatability over long period of time of a year or more. This would lead me to learn something often ignored, although being by far, the biggest uncertainty contributor.
Uncertainties are estimated by taking into account each particular elements of a system and how each of those elements can be affected and what would be their contribution to the overall system.
Uncertainty contribution presentation by Dr. Christos Monokroussos
This process can get fairly complex as you can always dig deeper, but in general the main contributors in a laboratorial setup are the absolute irradiation measurement, spacial non-uniformity, spectral mismatch, thermal stability and by far the biggest: Human error.
Human error is the biggest risk to the measurement, is difficult to predict, difficult to track back. Think of an operator measuring repeatedly tens of panels a day, all having different sizes, properties. A misaligned reference cell, a misplaced temperature probe, a wrong software parameter and off you go of a few % without noticing it.
There are several ways to mitigate this kind of issue, the two most important ones are automation and traceability.
Automation applies to the hardware setup as well as the software. The setup can be made to mitigate potential errors, as for an example, mounting the reference device on a sliding rail that would prevent any miss-alignment while accommodating different module thickness. The software can be automated as to limit the amount of required input from the engineer performing the test as well as locking critical parameters that could offset the measurement. This goes atop of good training of the test engineer as well as regular tests and review. The automation doesn’t stop to the test itself but goes up to the report, as a whole measurement, all this hardly estimated uncertainties can be thrown off by just a simple typo, by a tired engineer retyping tests results on a Friday at 6 pm. 268.8W or 266.8W?
Traceability is the cornerstone for long term reliability. Each measurement, its results and the parameters used has to be recorded and more importantly, easily accessible. This can in fact be quite a challenge as it requires custom software to be made and not all labs can afford these resources. Continuous analysis of the measured samples helps to detect and understand long term drifts and fix them instead of going unnoticed. It is also part of measurement quality control. The daily and weekly measurement of a panel, called daily and weekly check, allows to ensure the system doesn’t deviate and if it does, to investigate. Simple things, as a sun stray light hitting the lab through a shade that were moved, a cable getting loose, a fly on an optic, can set off your whole setup.
My first task at TUV Rheinland Shanghai back in 2010 was to write these test processes, train the engineers and build the software required for automation and traceability. A lot were learnt and we were able to dramatically improve the quality of our measurement with these tools, as well as to detect and understand what was going on when something goes off. Now working at Avalon, given the previous experience, the understanding is clear as to what tools to provide our customer to ensure their own automation, traceability and measurement quality and the lesson learned are what we implement in the Nexun Solar Simulators.