Producing electricity with solar photovoltaics (PV) emits no pollution, produces no greenhouse gases, and uses no finite fossil-fuel resources. The environmental benefits of PV are great. But just as we say that it takes money to make money, it also takes energy to save energy. The term “energy payback” captures this idea. How long does a solar panel 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 multi-crystalline-silicon PV modules, 3 years for current thin-film modules, 2 years for anticipated multi-crystalline 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, research has found that PV-systems fabrication and fossil-fuel energy production have similar energy payback periods (including costs for mining, transportation, refining, and construction).
Most solar cells and modules sold today are crystalline silicon. Both single-crystal and multi-crystalline 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 researchers reviewed previous energy analyses and did not include the energy that originally went into crystallizing microelectronics scrap. Their best estimates of electricity used to make near-future, frameless PV were 600 kWh/m2 for single-crystalsilicon modules and 420 kWh/m2 for multi-crystalline 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), they calculated a payback of about 4 years for current multi-crystallinesilicon PV systems. Projecting 10 years into the future, they assume a solar-grade silicon feedstock and 14% efficiency, dropping energy payback to about 2 years.
Other recent calculations support their figures. Based on a solar-grade feedstock, Japanese researchers calculated a multi-crystalline payback of about 2 years (adjusted for the U.S. solar resource). Energy payback was also calculated to be about 2 years for current multi-crystalline-silicon solar PV. For single-crystal silicon, they calculated a payback of 3 years when they did not charge for off-grade feedstock. A study of an actual manufacturing facility found that, for single-crystal-silicon modules, the actual energy payback time is 3.3 years. This includes the energy to make the aluminum frame and the energy to purify and crystallize the silicon.
Thin-film PV modules use very little semiconductor material. The major energy costs for manufacturing are the substrate on which the thin films are deposited, the film-deposition process, and facility operation. Because PV technologies all have similar energy requirements, we’ll use amorphous silicon as our representative technology.
The Dutch researchers estimated that it takes 120 kWh/m2 to make near-future, frameless, amorphous-silicon PV modules. They added another 120 kWh/m2 for a frame and support structure for a rooftop-mounted, grid-connected system. Assuming 6% conversion efficiency (standard conditions) and 1,700 kWh/m2 per year of available sunlight energy, they calculated a payback of about 3 years for current thin-film PV systems. Other studies calculated shorter paybacks for amorphous silicon, each ranging from 1 to 2 years.
Deleting the frame, reducing use of aluminum in the support structure, assuming a conservative increase to 9% efficiency, and factoring in other improvements, the Dutch projected the payback for thin-film solar PV would drop to just 1 year by 2009.
CuInSe2 and CdTe modules are already being sold in the 9%–12% efficiency range, so their energy payback may be less than a year, depending on design details, such as frames and mounting.
For an investment of 1 to 4 years-worth of energy output, rooftop PV systems can provide 30 years or more of clean energy. However, support structures for ground-mounted systems, which might be more advantageous for utility generation, would add about another year to the payback period.
An average U.S. household uses 830 kWh of electricity per month. On average, producing 1,000 kWh of electricity with solar power reduces emissions by nearly 8 pounds of sulfur dioxide, 5 pounds of nitrogen oxides, and more than 1,400 pounds of carbon dioxide. During its projected 28 years of clean energy production, a rooftop system with a 2-year energy payback and meeting half of a household’s electricity use would avoid conventional electrical-plant emissions of more than half a ton of sulfur dioxide, one-third a ton of nitrogen oxides, and 100 tons of carbon dioxide (see Figure 2). Solar PV is clearly a wise energy investment that affords impressive environmental benefits.