Claims for Solar Cell Efficiency Put to Test at NREL

The sheet of paper taped to the door of Keith Emery's office tells the story. On the paper is a simple fever chart showing the improvements made in increasing the efficiency of two dozen types of solar cells. Some of the lines marking record efficiencies date to the mid-1970s. Others start much more recently, with the advent of newer technologies.

More than anyone at the Energy Department's National Renewable Energy Laboratory (NREL), Emery has seen the lines on that chart move ever higher. Since joining what at the time was the Solar Energy Research Institute, Emery has made the process of recording record efficiencies of solar cells and modules more accurate.

"The community requires—demands—independent measurements because it's very easy to unintentionally inflate the measurements," said Emery, a principal engineer at the lab. "Most of the measurement errors a researcher would make would tend to make the efficiency higher, not lower. So it's our job to put everybody on the same playing field. That's what we've been working on as a group."

NASA originally was in charge of determining record efficiencies for solar cells. After taking over that task in 1980, Emery visited NASA's Jet Propulsion Laboratory in California where he was given a solar simulator, and the John H. Glenn Research Center in Ohio where he was given all of their primary terrestrial reference solar cells. Emery, who holds a bachelor's degree in physics and a master's in electrical engineering, was just getting started.

The original system for testing solar modules was set up atop a mobile home, and at that time the computers were bulky and unrepairable. One of the biggest challenges in the first 10 years of the program, Emery said, was the incompatible computer platforms. One computer used a cassette tape to back up its files. Another was booted up by entering binary codes with paddle switches and saved data to reel-to-reel tapes.

Since Emery's early days verifying how much electricity a solar cell generates, he's been joined in his work by scientists in Germany (at the Fraunhofer Institute for Solar Energy Systems, or ISE), in Japan (at the National Institute of Advanced Industrial Science and Technology, or AIST), and in Italy (at the European Solar Test Installation).

Together, Emery and four counterparts around the globe determine which companies or laboratories are included in "Solar cell efficiency tables," which is published twice a year in the journal Progress in Photovoltaics. To secure a spot in the efficiency tables, claims must be independently recognized by a recognized test center.

The scientists involved with measuring the efficiencies of solar cells have visited each other's labs to familiarize themselves with the available equipment and general procedures. An inquisitive group, the researchers toss out question after question. How did you determine what the edge of the cell was for area measurements?  What kind of aperture did you use? What stabilization criteria did you use?

Having a select group of laboratories handle the validation of efficiency records gives the results credibility.

"The measurement labs have exactly zero enforcement," Emery said. "It's peer pressure that says, I'm not believing your result until you get an independent measured result. It is absolutely peer pressure that is making this whole thing work. This field is incredibly skeptical."

The efficiency of a solar cell or module is determined by the percentage of sunlight that's converted to electricity. To test how effective a photovoltaic (PV) device is, NREL uses solar simulators that can produce from 1-sun—which equals the amount of sunlight hitting the earth on a clear day, or 1,000 watts per square meter—to about 2,000 suns.