Photovoltaic Systems (PV)

In 1839, Edmond Becquerel, a French scientist, discovered the process of using sunlight to produce an electric current in a solid material. Albert Einstein explained the photovoltaic mathematical principle in 1909 and scientists eventually learned that the photoelectric or photovoltaic (PV) effect caused certain materials to convert light energy into electrical energy at the atomic level. The photoelectric effect is the basic physical process by which a PV cell converts sunlight into electricity.

The energy of the absorbed light is transferred to electrons in the atoms of the PV cell. With their newfound energy, these electrons escape from their normal positions in the atoms of the semiconductor PV material and become part of the electrical flow, or current, in an electrical circuit, which then powers an external load, such as a light bulb.

Typical silicon PV solar cells are combined into modules that hold up to 40 cells; about ten of these modules are mounted in PV arrays that can measure up to several meters (10-20 feet) on a side. One or two such arrays would power a typical American house.

Crystalline Silicon Technology

Traditionally, most solar cells have been manufactured from crystalline silicon, which still accounts for over 85% of all solar cells manufactured today.

Silicon cells are made using sheets of silicon, or wafers, about one third of a millimeter thick. Multi-crystalline silicon cells convert between 10% - 15% of the sunlight into electricity, while more expensive mono-crystalline silicon cells have conversion efficiencies of between 14% - 22%.

Thin Film Technology

Often called the second-generation solar PV technology, thin-film products are less expensive and less complicated to produce than silicon products, but the trade-off is that they have lower electrical conversion efficiencies—between 7% -11%, compared to 14% - 22% for traditional multi-crystalline Silicon Technology. Nevertheless, thin-film products are now being manufactured at an over-all lower cost per watt than silicon products. The goal is to achieve 12-14% efficiencies resulting in costs of $0.05/kWh.

According to a report by Greentech Media Research and the Prometheus Institute, thin-film solar grew from 5.8 to 7.5 percent of worldwide PV production in 2006 and is expected to grow to 20 percent by 2010. The three competing thin film technologies at present are: Amorphous Silicon (a-Si); Cadmium Telluride (CdTe); and Copper Indium Gallium Diselenide (CIGS). Thin-film uses readily available raw materials and the finished products are suitable for a wider range of applications than silicon, e.g. roofing tiles and other BIPV materials.

Advantages of Thin-Film Technology:

• Simpler, more automated manufacturing processes
• Lower costs for finished product
• Requires little or no silicon, eliminating key dependency
• Malleability: can be applied to flexible substrates, e.g. steel foil, plastic, roof tiles
• Wider range of applications and markets, e.g. the low weight and flexibility of thin film panels allow them to be fully integrated into commercial and residential building materials (roofing tiles, windows, facades, etc. ), instead of just rigid glass panels