Solar uses Great for Not so great for • DC uses (LED lights, DC shop • Energy hogging appliances motors, computers) (ranges, central AC systems) • Low to moderate power • Long distance transport of needs power • Remote, off grid needs • Long term high power use • Many AC needs (with an in low sunlight inverter Solar Cell Structure • Solar cells are a type of diode. • Most are made of silicon (Si) that have had impurities added to it to help build an electrical charge. • VERY thin and fragile. The Race for Efficiency • The drawback for solar has been cost per watt for the consumer. • Monocrystaline has been the standard for a while, slightly more efficient than polycrystaline. Highest Efficiencies • Crystalline Silicon • The highest efficiencies on silicon have been achieved on monocrystalline cells. The highest commercial efficiency (22%) is produced by SunPower, which uses expensive, high-quality silicon wafers. The University of New South Wales has achieved 25% efficiency on monocrystalline silicon in the lab, technology that has been commercialized through its partnership with Suntech Power. Crystalline silicon devices are approaching the theoretical limiting efficiency of 29% and achieve an energy payback period of 1-2 years. Solar power plants • The lucky sunny state of Arizona is about to become home to the world’s largest Solar Plant! Thanks to a just-announced contract between Abengoa Solar and Arizona Public Service Company (APS), the enormous solar plant called Solana will power up to 70,000 homes, and will be the first example in the country of a major utility getting the majority of its energy from solar. The 1900 acre plant will be completed by 2011. Thin-film solar cells In 2002, the highest reported efficiency for thin film solar cells based on CdTe is 18%, which was achieved by research at Sheffield Hallam University, although this has not been confirmed by an external test laboratory. The US national renewable energy research facility NREL achieved an efficiency of 19.9% for the solar cells based on copper indium gallium selenide thin films, also known as CIGS (also see CIGS solar cells). These CIGS films have been grown by physical vapour deposition in a three-stage co-evaporation process. In this process In, Ga and Se are evaporated in the first step; in the second step it is followed by Cu and Se co-evaporation and in the last step terminated by In, Ga and Se evaporation again. Thin film solar has approximately 15% marketshare; the other 85% is crystalline silicon. Most of the commercial production of thin film solar is CdTe with an efficiency of 11%. Typical Residential Application Connecting cells in series • Adds .55 volts with each cell • The amount of amps stays constant • The most common way residential panels are wired Connecting cells in parallel • Volts stay the same • Amps increase • Depending on your needs, you can certainly mix series and parallel wiring, especially if your panel is going to be big. • More common is to wire PANELS in parallel, rather than individual cells The most common solar layout: four rows of nine cells in series Typical 36-cell panel values • .55 v per cell • 21 v DC open current (this is ideal with no load) • Usually 17-18 volts on an average day • Ideal for charging 12v battery arrays Soldering on the positive (back) side Drawing a template. Field test your runs before committing them to the panel backing Adhesives, the secret ingredient. • Silicone II. Cheap, but may present long-term durability problems. • Ethylene-vinyl acetate. What we’re trying out now. Considered more durable in all-weather conditions. Used by some solar manufacturers. The air-free option. Junction box at the back of a panel. To the right are a pair of Schrader valves. We pumped air out of one valve while pumping in argon. The point of an air free environment is to remove moisture and increase efficiency. FINISHED! Field Test Half an amp reached. We’re happy! A multimeter is essential for solar, whether for design or use. Tips • If using screws, drill through all three layers at once, then number the spacers to sides for best fit-up. • Avoid scratching the surface if using plexi. • Test each cell before soldering. • If drilling, use a drill press. Tips (continued) • Sheet metal, glass, and plexi often come in 8’x4’ dimensions. Three equal pieces will make three 70 watt panels. Design for as little waste as possible to save money. • Templates make soldering easier, straighter, and for a more attractive finish. • The less solder used, the better. Most people use too much. Tips (continued) • A dull grey solder joint is a bad solder joint. • Order enough cells to make mistakes. • Some companies sell broken cells for beginner’s practice. • Silver-bearing solder is preferred by many panel builders. • Pony beads act as spacers between rows of cells and relieve stress on the center of the glass.
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