Low Emissions, Quick Energy Payback for Thin-Film PV

Manufacturers of new thin-film cadmium telluride modules have reinvented the solar photovoltaic (PV) field in recent years. But doubts have lingered about the overall life-cycle benefits of PV systems in general because of the intensive energy use of the manufacturing process as well as the heavy metals required for the panels (see EBN Vol. 10, No. 3). However, a new study shows all current PV technologies offer at least an 89% reduction of air emissions compared with conventional electricity, while also offering an energy-payback time of less than three years. Of the four main types of PV technology, thin-film modules offer the lowest life-cycle greenhouse gas emissions and heavy metal emissions.

The study, led by Vasilis Fthenakis of Brookhaven National Laboratory and Columbia University and released in January 2008 by Environmental Science & Technology, compiled life-cycle assessment data from numerous sources, including data from companies making single-crystal, multicrystal, and ribbon-silicon solar cells as well as data from one Arizona company, First Solar, which makes thin-film cadmium-telluride, or CdTe, PV systems. The analysis accounted not only for energy used in making the PV modules using average proportions of electricity from the grid (including some renewable energy), but also for support frames and other materials. It assumed that systems would be used in ground-mounted conditions under southern European light with 30-year lifetimes.

According to the study, emissions from any of the four PV technologies examined are “insignificant in comparison to the emissions that they replace when introduced in average European and U.S. grids.” The best-performing system was the CdTe thin-film technology, which, the study showed, leads to at least 89% and up to 98% reductions in greenhouse-gas emissions as well as other pollutant and heavy-metal emissions compared with grid-source electricity, which is sourced mostly from fossil fuels. When comparing the different types of PV technology against each other, CdTe had the lowest emissions of all heavy metals, in some cases showing more than a 50% reduction compared with other PV systems. Monocrystalline silicon consistently had the highest relative emissions for most pollutants.

The study does not update previous research on energy-payback time, or EPBT, the time it takes for a PV system to generate the amount of electricity used in its production. However, the most recent study, from 2006 and also led by Fthenakis, found that at 1.1 years CdTe systems have the lowest EPBT among PV systems, which can have EPBTs as high as 2.7 years for monocrystalline silicon PV.

The relatively high retail price per watt of photovoltaic-module generating capacity—currently $4.82 per watt, according to Solarbuzz, an industry consultant—has limited the growth in of photovoltaics (see EBN Vol. 17, No. 3). But along with the low environmental impact of CdTe systems comes a relatively low production cost, and a number of companies, including PrimeStar Solar, based in Golden, Colorado, are racing to bring new manufacturing capacity online. Fred Seymour, Ph.D., the director of PV technology for PrimeStar, told EBN that one of his competitors reports manufacturing costs of $1.12 per watt. “There’s ample room to go below that,” Seymour said. “We’re in the steep part of the learning curve with cadmium telluride.”

– Tristan Roberts

For more information:

Environmental Science & Technology
“Emissions from Photovoltaic Life Cycles”
http://pubs.acs.org/journals/esthag/index.html

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IMAGE CREDITS:
1. Chart: Environmental Science & Technology

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