Some naysayers argue that solar panels don't make sense because it takes so much energy to make them--mining, smelting or refining, processing, etc. Do they really save fossil fuel energy and greenhouse gas emissions over the long run? The simple answer is yes. They save a whole lot. But then the question is: what kinds of solar panels are better than others?
A recent life-cycle analysis published at the Institute of Science in Society (ISIS) showed that in a nice sunny place like Spain, PV panels reach energy payback (when they've saved as much fossil fuel as it took to make them) in about one to three years, depending on the type of panel. Interestingly, the new thin-film chemistry cadmium telluride (CdTe) fared best at 1.1 year despite having the lowest efficiency (9%), while monocrystalline silicon fared worst at 2.7 years despite having the highest efficiency (14%). This may seem counterintuitive, but the explanation is simple: it takes a lot less energy to make CdTe film. See the graph below, which shows greenhouse gas emissions from the manufacture of different kinds of PV panels (each kind scaled to the same lifetime power production). (The four bars on the right, "Case 3", is US data.)
If your climate is less sunny than Spain, there will obviously be a longer payback time; but most of the continental US gets as much solar radiation as the location in the study. (It got 1,700 kWh/m^2/year, which is 4.7 kWh/m^2/day. As the map below shows, basically everywhere in the continental US is this sunny or better, except for the Pacific Northwest, upper Midwest, and New England.)
Avoiding Cadmium emissions
You might balk at the thought of cadmium in PV panels--after all, isn't it a toxic heavy metal that's outlawed in RoHS? Well, yes. But the especially interesting thing this ISIS article says is that using cadmium solar cells actually reduces cadmium emissions to the environment. Cadmium is contained in the exhaust of burning fossil fuels, and so much exhaust is prevented by using the panels instead of fossil fuels, that it pays for itself ten times over. Of course, crystalline silicon PV panels will be even better, because they don't directly contain any cadmium (30x improvement over fossil fuels).
Death Knell for Crystalline PV
Though thin-film PV panels have been in the market for more than a decade, the new chemistries that are coming to maturity (mostly CdTe and copper indium gallium selenide, "CIGS") are fundamentally changing the game. They are still not as efficient, but they are good enough, and they are far cheaper (up to 1/3 the cost per watt), even before reaching true economies of scale from mass-manufacturing. Now we see that they are environmentally preferable as well, since they have faster energy payback times. In the years ahead, crystalline silicon will become relegated to satellites, mobile systems, and other applications where compactness is king, while thin-film PV will be the default style. Crystalline prices will stay high, but thin-film prices will continue to drop steadily until they hit grid parity (arguably they have already), at which point they will plummet because manufacturing will ramp up massively. CdTe and CIGS will ultimately be price-limited by the rarity of their ingredients; the next wave after them will be dye-sensitized solar cells, made from common titanium dioxide and organic dyes. It'll be probably ten years or more before those dominate, though, and in that much time, who knows what other chemistries will come along? In any case, keep an eye out for it: the future of PV is thin.
image credits: ISIS, NREL
front page image: flickr/kevinthoule, Creative Commons license.
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(Posted by Jeremy Faludi in Features at 8:51 AM)
