Charles Chamberlin
Twenty years ago Jim Zoellick measured the performance of every one of the 192 photovoltaic modules just before they were installed in the Schatz Solar Hydrogen Project array. Nine years ago Antonio Reis and Nate Coleman retested every module after it had been in service for eleven years. This year Mark Rocheleau, Marc Marshall, and Scott Rommel tested every module for the third time after twenty years of service.
These painstaking measurements provide a unique opportunity to track the degradation of the performance of individual modules over twenty years of service in the cool, coastal, marine environment 150 meters from the Pacific Ocean at HSU’s Telonicher Marine Laboratory in Trinidad, CA.
In 1990 when the modules were brand new, right out of the box, Jim determined the current versus voltage (I-V) curves, the maximum power output, and other performance parameters for each of these ARCO M75 modules. Even then the average peak power under full solar illumination at normal operating cell temperature was 39.88 W, 14.1% below the 46.4 W nameplate rating; the power ranged from 37.79 W to 42.61 W.
Eleven years later in 2001, Antonio and Nate found that the average peak power had fallen by 4.3% to 38.13 W and ranged from 31.4 W to 41.55 W. They attributed the drop in performance to the visually observable physical defects in the modules. The modules’ ethylene vinyl acetate encapsulant had delaminated, leading both to corrosion of the cells and to the occurrence of localized hot spots.
These degradation processes continued and appear to have accelerated over the intervening nine years. When Mark, Marc, and Scott retested every module again this year, the average peak power had declined from the 2001 levels by an additional 12.4% to 33.43 W and ranged from 9.64 W to 37.95 W. Not only is the average peak power steadily falling, but the variation among the modules is also dramatically increasing. The impact of increased variation is to accentuate the mismatch among the modules, which in turn reduces the peak power output of the array as a whole.
Though the mismatch among our modules diminishes the array power, it caught the attention of National Semiconductor. National has developed a product they call SolarMagic™ power optimizer, a technology specifically designed to reduce power loss in mismatched photovoltaic arrays. Chris Goeltner of National contacted SERC and said our array is just what National is looking for to test their product. After some discussion National decided to fund a project and next month, we will begin modifying the array in preparation for the installation of SolarMagic in half of the array. We will then compare the performance of the unmodified half to the half outfitted with SolarMagic. The testing will continue for about one year; we’ll let you know how things are going in future newsletters.
