Determining how many years a solar panel will last can take, well, years. Intended to function for 25 years or more, solar panels must be made to withstand the elements. In some cases, panels continue working well after their planned operational lifetime. Nowhere is this more evident than on the sunny hillside in Golden, Colorado, where the National Renewable Energy Laboratory (NREL) operates the Outdoor Test Facility (OTF).
The OTF continues to run tests on photovoltaic (PV) panels made by companies still very much interested in the outcome. The OTF is equipped to perform accelerated testing on PV panels, which discovers problems in their design or in the materials used, said Dirk Jordan, a physicist who is a senior engineer at NREL. However, it is difficult to make long-term performance and degradation predictions for different environments with accelerated testing only. Multi-year outdoor testing is used to complement and validate accelerated testing. That is critical feedback to give to the manufacturer. Jordan studies the long-term performance of PV systems and analyzes the physics and chemistry of changes. For further information see the IDTechEx report on Energy Harvesting Microwatt to Gigawatt: Opportunities 2020-2040.
The gallery includes a cadmium telluride (CdTe) panel, a competitive solar technology developed at NREL with First Solar Inc, a publicly held PV manufacturer based in Arizona. In the early 1990s, the U.S. Department of Energy, which oversees NREL, funded initial research into CdTe as a solar material and tasked scientists at the laboratory with the project. Among the companies participating in researching the use of CdTe for solar panels was Solar Cells Inc, which later became First Solar.
The use of CdTe allows First Solar to manufacture PV panels more quickly, at a lower cost, and with a lower carbon footprint than silicon. CdTe solar panels are the second most common solar technology in the world after silicon, and they are widely used for utility-scale power generation.
First Solar’s technology reached a milestone at NREL this year after 25 years of continuously monitored performance testing; becoming the longest-running research project at the OTF.
“This NREL-developed technology resulted in a domestic manufacturing success story,” said Stephen Gorin, director of strategic partnerships for Materials, Chemical, and Computational Science at NREL. “Silicon manufacturing has all moved out of the United States, while First Solar has emerged as company with several billion dollars in revenue a year.”
Ben Kroposki, an electrical engineer who has worked at NREL for 27 years, installed the initial PV modules for First Solar at the OTF. “We always intended to keep the modules/arrays installed at the OTF as long as the technologies were relevant to get very long-term reliability data,” he said. Kroposki, now director of NREL’s Power Systems Engineering Center, conducted some of the initial analysis of how the CdTe solar panels performed.
A key metric of the performance of a solar cell is how long it will last. Solar cells lose efficiency over time. Having a measurement of that change enables manufacturers to offer a warranty on their panels. The initial studies on First Solar’s panels revealed that after five years, the company’s performance loss amounted to a relative 0.6% a year and their stability compared favorably to silicon modules.
Now, a quarter-century after the installation, the long-term study revealed a degradation rate of 0.5% a year, meaning the energy conversion efficiency of the panels was around 88% of the performance when it was first installed.
The initial absolute efficiency of the PV array installed in 1995 was low by today’s standards. But since then, Arizona-headquartered First Solar has steadily set and broken world records for efficiency. First Solar set the standing record for a CdTe solar cell, at 22.1%, in 2016. First Solar and NREL have continued to collaborate on developing CdTe solar cells, most recently publishing a paper last year that made a fundamental improvement by removing copper from the cells and unlocking further gains.
First Solar, which now operates the Western Hemisphere’s largest solar manufacturing footprint in Ohio as well as factories in Malaysia and Vietnam, is not selling the model of PV panels in operation at the OTF, so why does the ongoing research matter? Nick Strevel, the company’s vice president of product management, said the information gained has enabled First Solar to have greater confidence in its product. The encapsulation layer, for instance, which shields the semiconductor, is a 25-year-old technology proven to stop known thin-film defects such as corrosion.
“Our product encapsulation technology and materials today are far superior,” Strevel said. “This helps us understand a legacy performance baseline and provides further confidence in the superior long-term durability and long-term degradation performance of today’s product.”
Lou Trippel, director of product management for First Solar, said the lessons the company has learned from the experience at NREL “provided incremental confidence to help support a recently announced extension of our module power output warranty” from 25 years to 30 years.