Solar cells and energy payback
Producing electricity with photovoltaics (PV) emits no pollution,
produces no greenhouse gases, and uses no finite fossilfuel
resources. The environmental benefits of PV are great. But just as
we say that it takes money to make money, italso takes energy to
save energy. The term “energy payback”captures this idea. How long
does a PV system have to operate to recover the energy—and
associated generation of pollution and CO2—that went into making the
system, in the first place?
Energy payback estimates for rooftop PV systems are 4, 3, 2, and 1
years: 4 years for systems using current multicrystalline-silicon PV
modules, 3 years for current thin-film modules, 2 years for
anticipated multicrystalline modules, and 1 year for anticipated
thin-film modules (see Figure 1). With energy paybacks of 1 to 4
years and assumed life expectancies of 30 years, 87% to 97% of the
energy that PV systems generate won’t be plagued by pollution,
greenhouse gases, and depletion of resources.
Based on models and real data, the idea that PV cannot pay back its
energy investment is simply a myth. Indeed, researchers Dones and
Frischknecht found that PV-systems fabrication and fossilfuel energy
production have similar energy payback periods (including costs for
mining, transportation, refining, and construction).
What is the Energy Payback for Crystalline-Silicon PV Systems?
Most solar cells and modules sold today are crystalline silicon.
Both single-crystal and multicrystalline silicon use large wafers of
purified silicon. Purifying and crystallizing the silicon are the
most energy-intensive parts of the solar-cell manufacturing process.
Other aspects of silicon-cell and module processing that add to the
energy input include: cutting the silicon into wafers, processing
the wafers into cells, assembling the cells into modules (including
encapsulation), and overhead energy use for the manufacturing
facilities.
Today’s PV industry generally recrystallizes any of several types of
“off-grade” silicon from the microelectronics industry, and
estimates for the energy used to purify and crystallize silicon vary
widely. Because of these factors, energy payback calculations are
not straightforward. Until the PV industry begins to make its own
silicon, which it could do in the near future, calculating payback
for crystalline PV requires that we make certain assumptions.
To calculate payback, Dutch researcher Alsema reviewed previous energy analyses and did not include the energy that originally went into crystallizing microelectronics scrap. His best estimates of electricity used to make nearfuture, frameless PV were 600 kWh/m2 for single-crystalsilicon modules and 420 kWh/m2 for multicrystalline silicon. Assuming 12% conversion efficiency (standard conditions) and 1,700 kWh/m2 per year of available sunlight energy (the U.S. average is 1,800), Alsema calculated a payback of about 4 years for current multicrystallinesilicon PV systems. Projecting 10 years into the future, he assumes a solar-grade silicon feedstock and 14% efficiency, dropping energy payback to about 2 years.