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Karl E. Knapp & Theresa L. Jester
©2000 Karl E. Knapp & Theresa L. Jester
Critics of solar energy have been known to claim that it takes more energy to produce photovoltaic
(PV) modules than the modules will produce in their lifetime. We’ve conducted a detailed and scientific empirical study to look into this question. We found that the skeptics’ assertions are false. PVs recoup their production energy in two to four years, and go on to produce clean, renewable energy for twenty to thirty years or more!
Our study examined energy costs for two types of Siemens PV modules—single-crystalline silicon (SC-Si) and thin film copper indium diselenide (CIS). Crystalline silicon modules achieve an energy break-even in a little over three years. The energy payback time for thin film copper indium diselenide modules in full production is just under two years. Over their lifetime, these solar panels generate nine to seventeen times the energy required to produce them.
Our research was based on direct investigation of the energy requirements and net energy production of manufactured photovoltaic modules. Other studies employ production models with assumed process recipes, equipment sets, materials yields, and module efficiencies. None of them have used actual utility bills and accounting records.
By contrast, our study didn’t have to make any assumptions about yields. We just took energy requirements right off the utility bills and the materials requirements right off the bill of materials. This allowed us to include indirect materials as well, which as far as we can tell have never been included before. These include things like argon, nitrogen, etchants, cleaners, and so forth, all the way down to the cardboard box the modules get shipped in.
Energy Payback Time
Energy payback time is one standard of measurement adopted by several analysts to look at the energy sustainability of various technologies. It is defined as the time necessary for a photovoltaic panel to generate
the amount of energy used to produce it.
Two parameters determine the energy payback time for a PV module—how it is produced and how it is used. The energy needed to produce a product (specific energy) includes both the energy consumed directly by the manufacturer during processing, and the energy embodied in the incoming raw materials.
How a PV module is used is primarily a question of location and module efficiencey. Location determines the solar insolation, and combined with efficiency, determines the electrical output of the PV panel. But installation details are important too (fixed tilt or tracking, grid-connected or stand-alone, etc.), as are balance of system requirements such as mounting structure, inverter, and batteries. The module energy payback time is computed from this formula:
Specific Energy E EPBT = Energy Generation Rate = E
Figure 1 shows this relationship. The vertical axis shows specific energy and the horizontal axis shows the energy generation rate (with some representative estimates found in the literature indicated). Energy payback time can be expressed as the ratio of the total energy required to manufacture a photovoltaic module to the rate that the module converts the solar energy flowing from the sun at the installation site to electricity.
Figure 1: Specific Energy and Energy Generation Rate Relationship to EPBT E
42 Home Power #80 • December 2000 / January 2001
Constant Energy Payback
Alsema 1999 1,700 Kato 1997
Schaefer & Hagedorn 1992
Aulich et al 1985
• (Years) E
Production: Specific Energy E Energy Requirements KWHe / KWp
SC-Si SP75 CIS ST40
1,000 1,500 2,000
10 9 8 7 6 5 4 3 2 1
Location: Energy Generation Rate E Insolation KWH / m2 / year
Japan:1,424; Northern Europe: 1,000; U.S. Avg: 1,825; Boulder: 1,974; Phoenix 2,480; Detroit: 1,202
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