Solar energy payback will be one year within a decade, says NREL
GOLDEN, Colorado, US, May 11, 2005 (Refocus Weekly)
Solar PV systems can repay their energy investment in two years, according to the U.S. government.
The payback for multicrystalline PV modules is four years for systems which
use current technology, which will drop to two years for technology coming onto
the market, says a fact sheet released by the National Renewable Energy
Laboratory of the U.S. Department of Energy. For thin-film solar modules, the
payback is three years using current technology and one year for anticipated
thin-film technology.
The energy payback is the same for both rooftop and ground-mounted PV systems,
depending on the technology and type of framing used.
“Based on models and real data, the idea that PV cannot pay back its energy
investment is simply a myth,” it says. Researchers found that fabrication of
PV systems and fossil fuel production have similar energy payback periods when
the costs for mining, transportation, refining and construction are included.
It takes 120 kWh to manufacture every m2 of frameless amorphous-silicon PV
modules, according to one analysis, with another 120 kWh/m2 for a frame and
support structure for a rooftop-mounted grid-connected system. Assuming a
conversion efficiency of 6% and 1,700 kWh/m2 per year of sunlight, the payback
would be three years for current thin-film PV systems with frames, and one to
two-year paybacks for amorphous silicon.
“With assumed life expectancies of 30 years, and taking into account the
fossil-fuel-based energy used in manufacture, 87% to 97% of the energy that PV
systems generate won’t be plagued by pollution, greenhouse gases, and
depletion of resources,” it states.
Most solar cells and modules sold in the U.S. now are crystalline silicon, and
the purification and crystallizing of both single-crystal and multicrystalline
silicon is the most energy-intensive part of the manufacturing process for solar
cells. Energy is also needed to cut the silicon into wafers, process the wafers
into cells, assemble the cells into modules (including encapsulation), as well
as the overhead energy needed for the manufacturing facility.
The PV industry recrystallizes several types of off-grade silicon from the
microelectronics industry, and estimates for the energy used to purify and
crystallize silicon vary widely, making it difficult to calculate energy
payback. The electricity used to make frameless PV is 600 kWh/m2 for
single-crystal silicon modules and 420 kWh/m2 for multicrystalline silicon
which, assuming 12% conversion efficiency and 1,700 kWh/m2 per year of sunlight
(the U.S. average is 1,800), the payback would be four years for current
multicrystalline-silicon PV systems and, assuming solar-grade silicon feedstock
and 14% efficiency within a decade, the payback would drop to two years.
Elimination of the aluminum frame with thin-film solar would account for much of
the energy savings, and the need for ground-mounted support structures such as
concrete foundations and heavy framing produces the same energy payback as a
roof-mounted PV system.
“PV is clearly a wise energy investment that affords impressive environmental
benefits,” the fact sheet concludes. During its 28 remaining years of assumed
operation, a PV system that supplies half a home’s electricity would eliminate
half a ton of sulfur dioxide, one-third of a ton of nitrogen-oxides and 100 tons
of carbon dioxide, equivalent to avoiding the emission from operating two cars
for those 28 years.
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