TEC154 2010S, Class 35: Solary Energy (2) Overview: * Review * Photovoltaic Technology * Photovoltaic manufacturing * The Color Spectrum and Energy * ... Admin: * No readings for Friday. * EC for Convocation tomorrow. What we did on Monday * We talked about the science of photovoltaics * We have a semiconducting material (typically crystal silicon) * It is doped (made impure) in some regions with boron and in others with phosphorous * That doping leads to an electric field. * When a photon strikes, it can knock out an electron which then travels freely * It gets attracted to the positive side. * As it moves, it leaves behind a hole * If we wire up the two sides, we get a circuit +--------------+-----+ | | | +--* | | | | | | This is the cell | | | | | | <-- + | | | | e- --> | | | | | | | | <-- + | | | | e- ---> | | | | | *------+ | +--------------+-----+ | | | This is a wire +------------------------------+ * Issues: * If the hole (+) and electron (e-) are near enough to each other, the electron can move into the hole, rather than to the other side (recombination) * Light may be reflected, so we get no effect * The photon may get through (transmission) * The photon may not have the right energy to release the electron * We also talked about how to fabricate something like this * Because we're slicing, the slices of silicon have to be pretty thick 1/2 mm * Note the P (boron) layer is much thicker than the n (phosphorus) layer * 1 boron per 100K silicon atoms - 1/2 mm thick * 1 phosphorus per 1K silicon atoms - 1/1000 of a mm thick * The light absorbtion occurs near the boundary, so most of the silicon is wasted * We could save money if we could make this thinner The color spectrum * From blue (high energy) to red (low energy) * And beyond (in both directions) * The proper energy level for silicon-based photovoltaics is right at the end of the red spectrum * Most of the spectrum is wasted - the photons serve no purpose * We can change the absorbtion by using other materials * Note: The amount of light at each place in the spectrum depends on whether you're in space or on earth Comparing polycrystalline silicon to single-crystal silicon * At some points, the bonds between neighboring atoms can be "looser" * So, the wandering electrons and holes can end up in those spots * Polycrystalline materials are much cheaper to manufacturer * Right now, we don't know which is really more cost efficient - they are priced cost competitively for the same amount of generation - you need more polycrysalline material, but it's cheaper per unit area Single-crystal silicon * When sliced and polished, its reflective * Reflective is bad - We lose the photons * So we normally etch the surface chemically, which gives "pyramids" Photons reflect internally * When we make polycrystalline materials, the material is substantially thinner, etching may take off too much of the material. * Note: In polycrystalline materials Losses in a typical silicon photovoltaic device * Overenergetic photons: 32% loss * These are lost as heat * Underenergetic photons: 24% loss * So over 50% of the loss is fundamental physical losses * Other internal cell losses: 21% loss * By making it thinner, we we can improve this (it may be the best place to improve) * Front surface reflection: 3% loss * Can improve with anti-reflectives * Shading by front contact: 3% loss * Cell packing density: 2% loss * Has changed significantly in recent years Big issues: * How do you minimize cell losses? * How do you use less material? Making polycrystalline silicon * One technique: Melt, make a big block, saw it into small pieces * A firm that is now part of British petrolium: BP solar * "We're in the energy business ... we don't want our business to go down the drain." * Another technique: Pull a sheet of silicon out of molten silicon * Similar concept to forming bubbles of soapy water * Technology done by Mobil originally (since spun off) * These are more fragile, but very thin * So we have three technologies competing with each other in the U.S. marketplace Graph (with axis reversed) of band gaps * There are lots of alternatives to crystal silicon * Silicon is used because it can start aboarbing lower in the energy levelv * It's also inexpensive. * "CulnSe2 is one o fth emost abosrbing semiconductors known" * CdTe has taken off in the past few years * Much cheaper to manufacture How does the technology move into the marketplace? Costs: * In a good location with a good photovoltaic, we might be able to generate electricity for $0.15/kilowatt hour. * In less good locations, you can generate electricity for about $0.50/kilowatt hour. * In-home electricity is about $0.09/kilowatt hour. * So why use photovoltaics? * There isn't a wire to everywhere (you can't always connect to the grid) * A photovoltaic is certainly cost competitive to an oil-fueled generators where you need to fly in the fuel * A portable refrigerator (e.g., to carry vaccines) - photovoltaic fridges are preferable to kerosene-powered refridgerators * Useful cycle * Uses found, so there's a market * Market means that manufacturing increases * Increased manufacturing reduces cost * Cheaper cost means more uses * If you have to run a power line more than 1/4 or a 1/2 mile, photovoltaics are cost-competitive * As photovoltaics come down in price, the distance reduces * Electric fence powering * Electric pump * Takes advantage of problems with photovoltaics : E.g., that it doesn't work at night * Photovoltaic shingles (take advantage of space you already have) * We'd like to move to photovoltaic "plants" * Picture of one that has been dismantled * But there are a number planned * Primarily in Califiornia, where renewable fuels are mandated by 2020 Other approaches * Thin-film materials can be manufactured by spraying on layers * Focus light on a single photovoltaic * Mostly just in lab; not commercial DOE publication: Evolution of the use of technoogy (about 20 years old, so the last two estimates are probably optimistic by 10-15 years) * Space * Remote homes, Remote communications, Military * 80's Consumer Good * Remote communities (international), Muniicalp * Grid-connected buildings, Local grid support (substations) * Utility peak power (from 2005) * Utility bulk power (from 2010)