Conversion efficiency is an important PV characteristic commonly used to compare PV technologies. Conversion efficiency provides a measure of how effectively a PV device converts sunlight into electricity. Conversion efficiency is calculated as the ratio of the peak power produced by a PV device in watts (Wp) to the power of the sunlight incident on that device in watts. Since the output of a PV device depends on a number of variables such as the spectral distribution of the sunlight and cell temperature, standard test conditions (STP) have been established for measuring PV conversion efficiency. PV conversion efficiency is measured in a laboratory with the PV device at 25 C and using a light source with an intensity of 1,000 watts per meter at the PV device and a spectral distribution that corresponds to air mass (AM) 1.5 global standard.

PV conversion efficiency is reported for both cells and panels. The conversion efficiency is greater for cells because of the inherent losses that result from assembling a number of cells into a panel. Since PV panels are installed on buildings, the conversion efficiency of panels should be considered when comparing PV technologies and not individual cell conversion efficiency.

Crystalline silicon PV

Crystalline silicon is the most common type of PV cell used in building applications today. Crystalline silicon has a conversion efficiency that ranges between 10 and 13 percent. In general, crystalline silicon PV panels are usually opaque and have either a dark blue or black antireflective coating. Some manufacturers will provide crystalline silicon PV panels in custom colors, but there is usually a minimum-order size, loss of efficiency, and price premium for this option. Additionally, crystalline silicon PV panels can be built with space between individual cells to allow light to pass through the panel for skylight and similar applications.

Crystalline PV cells can be further categorized as either monocrystalline cells or polycrystalline cells. The difference is in the manufacturing process. Monocrystalline silicon is grown from a silicon seed and results in a uniform crystalline structure. Polycrystalline silicon is produced from pouring molten silicon into a mold. As a result, polycrystalline silicon PV cells do not have a uniform crystal structure and their conversion efficiency is typically less than monocrystalline PV cells. However, the cost per square foot to manufacture polycrystalline silicon PV is less than monocrystalline silicon PV, which can offset the difference in conversion efficiency.

Thin-film PV

Thin-film PV cells are manufactured by depositing layers of semiconducting materials a few micrometers thick on a substrate such as glass or stainless steel. Thin-film PV cells are categorized based on the type of semiconducting material used. Amorphous silicon is the most common material used to produce thin film PV cells today. Amorphous silicon has no crystalline structure and has a PV panel conversion efficiency of between 5 and 8 percent. This conversion efficiency under STP is significantly less than crystalline silicon PV, but continuing research and development is improving thin-film PV conversion efficiency. In addition, thin-film PV conversion efficiency is better than crystalline silicon PV in low or diffuse light making it more efficient on cloudy days. Thin-film PV is also better in very hot desert regions because its cell temperature remains lower than crystalline silicon PV.

Even without significant improvements in conversion efficiency, thin-film PV has a number of advantages over crystalline silicon PV that make it preferable for a number of building applications. Thin-film PV is significantly less expensive to manufacture. Its PV cells require between 1 and 5 percent of the materials and energy required to manufacture crystalline silicon PV cells. More important, the manufacture of thin-film PV is much more automated and efficient than crystalline silicon PV. As a result, its lower conversion efficiency is offset by its lower cost per square foot.

Thin-film PV is also much better suited for building integrated PV (BIPV) applications. Thin-film PV can be deposited on vision glass and appear as an architectural tint as well as on spandrel glass, effectively turning the entire building curtain wall into a power generator. It can also be integrated with roofing materials on commercial flat roofs or shingles on sloped residential roofs. EC

This article is the result of a research project investigating the potential of the emerging PV market for the electrical contracting firm being sponsored by The Electrical Contracting Foundation Inc. The author would like to thank the foundation for its support.

GLAVINICH, director of Architectural Engineering and Construction Programs in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas, is a frequent instructor for NECA's Management Education Institute. He can be reached at 785.864.3435 or [email protected].