Awhile back we were discussing whether or not a photovoltaic module returns more energy than it took to manufacture. I posed the question to the PV Users list and it initiated a discussion that is still going. I found this recent posting especially interesting and hope at least a few ecopathers do, also. A bibliography on this topic follows. Doug Fields ------------------------------------------------------------------- This subject keeps coming up. Sometimes I think that the last couple generations have had so many disappointments in having our beliefs shattered, from believing in Santa Claus, on down. People start to think, "Whatever we do, it is a losing battle, entropy wins, goodbye Planet Earth." But no, PV panels really do make a net gain in energy, and it doesn't take all that long to recoup the energy expended. It does, however, depending, not as you might think-- on how you measure the total inputs--but rather, on which solar cell technology we are discussing. We recently had an interesting discussion of this a couple months ago at a BP Solar sponsored conference in New Orleans about 3 months ago, and we had a lot of people from various aspects of the industry, including Richard Perez of Home Power magazine, David Katz of Alternative Energy, Dr. Jean Posbic, Sr. Technical Director for BP Solar, Gerry Braun, Thin Films Dept., BP Solar, and about 40 or 50 of the who's who in the industry. OK, enough credentials. Taking worst case scenario, old method of making silicon ingots from scratch, in an electric induction furnace, starting from raw silica sand to liquefy, purify, cast, and then cool the ingots gradually, slicing them with a diamond saw (all of which is how Siemens still makes their cells), it takes maybe 6 years at the most to re-coup the kilowatt hours that went into production. Remember, the production process itself is done on a mass scale, so the costs are spread out by thousands of kilowatts of PV panels made every year. The PV manufacturers themselves are always trying to save steps, save money, and above all, save electricity. There are a lot of factories out there whose years production is somewhere on the order of a couple of megawatts per year. Next, it is no longer necessary to start from raw silica oxide sand. A PV cell only requires 1 part per million purity. Computer chips have to be 1 part per billion impurities. There is now so much computer chip scrap silicon on the market, that it has become a major source for PV production. There are a lot of new technologies which have brought the energy cost of making PV modules down to about 3 years. Some of these include the extruded ribbon technology, which has many variations. The most efficient of this particular method is the production line of ASE, Americas, in Billerica, MA. They extrude six ribbons in one continuos six-sided hollow tube. There is no waste, as they cut the tubes first into 3-inch high six-sided rings (for lack of a better term), with a laser. Then they cut the edges of these so that they end up with six - 3"square thin cells, with no waste. It is all computer controlled and robotized. The thin film people have really got it down, now, too. Then there are the people like Amonix, who use only a tiny piece of silicon chip, the size of your little fingernail, and they make it precisely like a computer chip, with pins integral to the back of the chip. Think of what a Pentium chip looks like, only about 1/4 the size. Then, they put a huge Fresnel lens on the front and a heat sink on the back and focus 300+ suns on that little chip. You don't hear about this technology, as it isn't available for the home market: it is only reserved for huge arrays, as they use a sophisticated tracker with high precision accuracy. They are not to be confused with Midway Labs out of Chicago, who are now out of business. Amonix has put up some 3 and 5-megawatt power stations, feeding the grid. Finally, a physicist down in Brazil did and interesting calculation a couple of years ago, which was reported in Renewable Energy magazine, which is a European journal. He compared the energy it takes to derive, refine, and produce power from something like 5 grams of plutonium to the energy produced by 5 grams of silicon, using the same criteria, and figuring in all the aspects of manufacture, ganging cells into modules, etc. He came up with a figure, if I remember correctly, of around 7 years for the silicon, and something like 120 years for the plutonium. However, the waste disposal of the plutonium was not even entered into the equation, as it wouldn't have been fair to the silicon, as silicone isn't poisonous! - -- Robert Warren --------------------------------------------------------------------- A number of people on the list requested that I follow through on my offer, several weeks ago now, to post a bibliography of papers regarding the embodied energy in PV modules. My list is not a complete account of everything ever published on the subject, just the papers I've cobbled together in spare moments from the resources available at the Georgia Tech library (you'll probably need a good university library to find these). With that disclaimer, here they are: K. Kato, A. Murata, and K. Sakuta, "Energy pay-back time and life-cycle CO2 emissions of residential PV power system with silicon PV module," Progress in Photovoltaics, v. 6, pp. 105-115, 1998. K. Kato, A. Murata, and K. Sakuta, "An evaluation on the life cycle of photovoltaic energy system considering production energy of off-grade silicon," Solar Energy Materials and Solar Cells, v. 47, pp. 95-100, 1997. A. Inaba, K. Yamada, and H. Komiyama, "An energy evaluation for solar photovoltaic energy systems," 28th Intersociety Energy Conversion Engineering Conference, pp. 2.481-2.484, 1993. W. Palz and H. Zibetta, "Energy pay-back time of photovoltaic modules," International Journal of Solar Energy, v. 10, pp. 211-216, 1991. H.A. Aulich, F.W. Schulze, and B. Strake, "Energy pay-back time for crystalline silicon photovoltaic modules using new technologies," 18th IEEE Photovoltaic Specialists Conference, pp. 1213-1217, 1985. The two papers by Kato, et al. are very similar, but seem to be very high quality work. I hope these will be of interest. I highly recommend that anybody interested in this subject also dig up the references at the end of the paper posted to the list by Richard Corkish on December 21. ___________________________________ Alan Ristow University Center of Excellence for Photovoltaics Georgia Institute of Technology Atlanta, Georgia
