to NREL's South Table Mountain Campus in Golden, Colorado, as golden samples. There, the output performance of each is measured and the laboratory certifies that the module produces, for example, 302 watts, plus or minus 1.1%.
Back at the factory, the modules are used to check that output against the company's solar simulator, which is essentially a large table that illuminates the module with light approximating sunlight.
"One of the biggest uncertainties in measuring power output is how accurately can you measure the light output of your simulator," said Levi. "With this golden module, the manufacturer can set the light output of their simulator very accurately. Then they put that module in a safe place and they get out another module, just like the first one. They measure that module on the simulator and that's their ‘silver module.' That one they'll use every morning, or maybe several times a day, to set the intensity of their simulator."
"The simulator sits at the end of the production line," Levi continued." Every module that comes off the production line gets tested, and its power output gets assigned, based on that simulator—which is ultimately traceable back to our measurement of their module."
The main work by Carl Osterwald, an NREL electrical engineer, and Larry Ottoson, a technician at the laboratory's Outdoor Test Facility, reduced the margin of error by eliminating anything that was causing errors in the calibrations, such as eliminating light leaks. "Not any one of our systems by itself could get better than 2%," said Osterwald. "So, we had to use a combination method. It was a combination of a whole bunch of things. We completely changed the calibration procedure for PV modules."
One change considered the fact that light from a simulator isn't uniform, something that can dominate the uncertainty. In the new procedure, the current produced by a module is first calibrated outdoors, which provides a very uniform light source. Then, that carefully calibrated