PHOTON’s YIELD MEASUREMENT
A module’s nominal power is interesting, as is its efficiency – and, depending on the customer’s expertise, so are a few other pieces of technical module data. But the single most important factor for PV system operators is yield: how many kilowatt-hours per kW of installed power flow from the PV system to the inverter? This is exactly the question PHOTON Lab intends to answer with its module field tests.
Since 2005, a total of over 170 different module types has been installed on a piece of property – free of shadowing – were monitored constantly using an elaborate measurement system.
Two to three units of each module type are represented in the test to prevent potential faulty products or modules with below average results from distorting the results for the entire series. The modules are installed in Germany, facing south at a 28° angle and are mounted about 2.5 m above the ground, which means they have complete rear ventilation.
PHOTON Lab has developed its own electronics to perform fully automated measurements at each module’s output. This eliminates the possibility of errors due to false inverter adjustment or small cable cross-sections. The test set-up’s measurement tolerance is currently +/- 1.85 percent.
Every second, each module is measured to capture an IV curve with a nominal 14 bit resolution composed of 2,000 measurement points and the maximum power point (MPP). This measurement process takes about 10 milliseconds, which means almost 99 percent of the test module’s yield can be fed into the grid via a DC-DC converter, a DC bus and an inverter. This is important as it allows the test system to operate under real-world conditions and prevents modules from overheating due to permanent open-circuit operation.
In addition to data from the solar modules, the test field employs several highly accurate pyranometers to measure solar irradiation horizontally and at the module level every second, as well as other climate data such as ambient temperature, wind speed, precipitation and barometric pressure. Module and weather data is stored in synchronized databases to ensure precise correlation.
PHOTON test site Aachen, Germany
Real power is the decisive factor
For technical reasons, the solar modules in a certain series do not all have identical powers. That’s why nominal power is always listed with a certain tolerance range, which manufacturers determine using very different methodologies. For instance, a few manufacturers list a module’s nominal power at 100 W when the actual power of the module in question actually achieves this value. Other manufacturers, by contrast, list a 100 W nominal power for a series with a true power of between 95 and 105 W. Moreover, there are some manufacturers that list module power at 100 W when their products achieve 100 W at maximum but likely display lower nominal powers.
The measured yields of the individual modules installed on the PHOTON test site are standardized according to the maximum power under standard test conditions (STC), which is determined by PHOTON Lab prior to the installation using a Pasan Sun Simulator IIIb.