Friday, August 29, 2008

Common Solar Electricity Terms: A Primer

The following terms should help you understand more about this blog's content. For more detail on each, Wikipedia is a good source. Also, most makers of panels and inverters provide technical data of their products online.

AC/DC: Alternating Current/Direct Current. Tesla developed alternating current in the 1880s, selling the patent to George Westinghouse. AC systems overcame the limitations of the direct current systems (used by Thomas Edison) by distributing electricity efficiently over long distances.

Amorphous Silicon Cells: Silicon that does not have a crystalline structure and which is not conductive. Such cells are often called "thin-film." Uni-Solar, Nano-Solar and First Solar all make their own types of amorphous silicon photovoltaics.

Array: The sum of all solar panels linked together in a system.

Cost per Watt: Used in the solar industry to determine solar panel costs. This is a changing value related to panel technology (thin-film usually costs less than crystal silicon modules) and the market availability. Thus, a 200WAC panel priced at $4/Watt would be $800 per panel to the solar integrator, usually more to a self-installer.

Crystalline Cells: High-purity, semiconductor-grade silicon (only a few parts per million of impurities) is melted in a crucible, usually made of quartz. Dopants such as boron or phosphorus can be added to the molten silicon changing it into n-type or p-type (negative/positive) extrinsic silicon. A seed crystal, mounted on a rod, is dipped into the molten silicon and pulled upwards while rotating at the same time. By precisely controlling temperature, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt. Like slicing bread, wafers are carefully sliced from the cylinder and polished for use as semi-conductors or solar cells.

Framed modules or Panels: Most photovoltaic cells come as framed modules and produce DC power. Also known as panels, they are composed of a small group of five to six-inch silicon wafers mounted on thin metal to which is attached electrical leads and framed in either silver or black anodized aluminum. Other forms of cells similar roof tiles and shingles; another is a laminate for bonding to steel seam roofs and single-membrane roofing materials.

Grid-tied: All systems featured on this blog are tied into the area power grid (AC) and this likely entails net metering. Whatever surplus energy a PV system produces goes back into the grid and the ratepayer's meter will run in reverse thereby deducting kilowatt hours from the bill. Credit is usually given at the wholesale rate per kilowatt hour.

Inverter: All photovoltaic panels produce DC power which can be stored in batteries. An inverter changes DC power to AC for connection to the area electric utility. Large systems can require many inverters to handle/convert the DC load.

Kilowatt Hour: The kilowatt-hour (symbolized kWh) is a unit of energy equivalent to one kilowatt (1000 watts or 1 kW) of power expended for one hour (1 h) of time. The kilowatt-hour is not a standard unit in any formal system, but it is commonly used in electrical applications.

Off-Grid or Stand-Alone system: A PV system that is not connected to a power grid. The DC power generated by PV cells is stored in batteries sufficient to meet the demand of a specific situation.

Photovoltaic (PV): Converting light into electricity. Photo means "light" voltaic means "electric". Often referred to as "PV" for short. Other terms are "solar power" or "solar electric".


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