Advances in Photovoltaic (PV) materials and manufacturing processes are resulting in increased interest in the application of PV technology in buildings. To be directly competitive with conventional utility-delivered electric power, PV technologies need to have a low installation cost and a high solar conversion efficiency that is stable over the life of the installation.
Ongoing research and development (R&D) is improving traditional silicon-based PV performance as well as developing new exotic PV materials that could make existing PV technology obsolete in a few years. R&D advances coupled with rising energy costs, growing demand for electricity and heightened environmental awareness in the United States point to a potentially sunny future for PV.
Advancing PV technology will result in expanded PV use in the building market and create new opportunities for the electrical contractor.
Crystalline silicon PV
Crystalline silicon (c-Si) is the most common type of PV material used in building applications today. PV panels constructed using crystalline silicon have a conversion efficiency of about 10 to 13 percent and are usually rigid and opaque.
Crystalline silicon PV panels commonly have either blue or black antireflective coating. Manufacturers can produce variations on these colors for large orders at additional cost and reduced conversion efficiency. These panels are typically manufactured and installed as a unit that requires separate building supports and bracing.
Crystalline PV panels are usually installed on building roofs. However, crystalline PV cells have also been integrated into glass skylights by spacing the PV cells to allow light in and create an architecturally interesting pattern. These panels can also be installed as either fixed or adjustable sunshades on buildings.
Thin-film PV cells are manufactured by depositing layers of semiconducting materials a few micrometers thick on a glass or stainless steel substrate. These cells are considered the second generation (crystalline silicon technology represent the first generation). The most common thin-film PV material used today is amorphous silicon (a-Si).
Amorphous silicon has no crystalline structure and uses a fraction of the material needed to produce a crystalline silicon PV cell. It also has more automated and efficient manufacturing, which results in a lower cost per square foot. This offsets the current lower amorphous silicon PV conversion efficiency by between 5 and 8 percent, which is considerably less than crystalline silicon PV.
Current thin-film R&D efforts are aimed at increasing the conversion efficiency to equal or surpass that of crystalline silicon PV, as well as improving the stability of electric power output over the life of the cell.
There are several other thin-film PV materials under development that are not silicon based, promise improved conversion efficiencies and lower manufacturing costs. Two of the more promising compounds under development are cadmium telluride (CdTe) and copper indium gallium diselenide (CIGS).
While amorphous silicon is an environmentally friendly technology, there are environmental concerns about both CdTe and CIGS that are currently being debated in the industry. CdTe cells contain cadmium, which is a hazardous material and may require recycling. Similarly, CIGS can have traces of cadmium and selenium, which may also require recycling at the end of the installation's life.
Thin-film PV is better suited for building-integrated PV (BIPV) applications than crystalline silicon PV. Thin-film PV can be an integral part of a variety of building materials that make up the exterior skin of the building.
By incorporating thin-film PV into exterior building systems, the installation is more efficient because those systems do double duty. The need to install PV panels separately is eliminated. Not only do window, wall and roofing materials fulfill their traditional function of providing a boundary between the inside of the building and the outside environment, they also generate electricity.
Energy Conversion Devices Inc. (ECD Ovonics) has developed a proprietary manufacturing process for producing thin-film PV using a roll-to-roll manufacturing process similar to how newspapers are printed. Instead of a roll of newsprint, ECD Ovonics uses a roll of stainless steel that serves as a substrate for the deposit of amorphous silicon alloy. The resulting amorphous thin-film PV panel is both flexible and rugged and can be installed as part of the building's roof.
Organic and dye-sensitized PV
Organic and dye-sensitized PV cells are currently under development and are touted as the third generation of PV technology. These cells use carbon and other compounds instead of silicon and mimic plant photosynthesis in converting incident sunlight into electrical energy.
Organic and dye-sensitized PV cells can be integrated into plastic or other building materials and can be either transparent or have color makes them very architecturally attractive. Currently, these cells are less stable than silicon-based PV cells but R&D may change this in the near future.
Konarka is a new company that has developed a roll-to-roll process for manufacturing flexible PV using plastic as the substrate. Konarka's plastic PV material can be used to provide power to a number of handheld electronic devices such as cell phones and other personal electronics.
This technology could be used to produce PV wall and roofing materials for building exteriors, as well as interior PV wall coverings, ceiling tiles and other surface treatments that would harvest incident sunlight to produce electric power.
Impact of advancing PV technology
Advancing PV technology will provide more opportunities for integrating PV into residential and commercial buildings. The entire building exterior has the potential to become a power generator as ways are found for integrating PV into more and more building materials.
Thin-film PV is being integrated into vision glass that includes building windows, the glass that makes up skylights, facades and atria. The transmittance of vision glass incorporating thin-film PV can be varied to reduce incoming sunlight and glare just like traditional window tinting.
Opaque PV glass can also be produced for use as spandrel glass that, when coupled with PV vision glass, effectively turns the entire building envelope into an electric power generator.
The building roof has been the traditional place for locating PV. In the past, integrating PV into a building roof has usually meant installing PV panels that need their own separate support system. Additionally, there were not always architecturally pleasing.
For flat roofs on commercial buildings, manufacturers are experimenting with integrating PV into traditional roofing materials that can be installed directly by a roofing contractor in exactly the way that traditional single- and multiple-ply roofing materials are currently installed.
For sloped roofs, PV shingles are being produced using either crystalline silicon or thin-film PV technology.
These PV shingles are made to look like either asphalt or wood shake shingles. Thin-film PV panels and overlays are being produced for standing-seam metal roofing that can be supplied in a variety of colors to provide an aesthetically pleasing installation.
What is in it for ECs?
Advances in PV technology, as well as an increasing interest in the environment by building owners and the public, adds up to a potential new market for the electrical contractor.
The trend is toward integrating PV into traditional building materials in order to reduce the cost of the PV installation by eliminating the need for separate mounting and support systems. Other trades that have traditionally installed these systems, such as roofers and glaziers, will probably continue to install these integrated building materials.
However, the electrical contractor will need to interconnect the PV system and provide the balance of the system (BOS).
This can include inverters to convert the direct current (DC) generated by the PV system into alternating current (AC), interface the PV system with the building distribution system, provide energy storage devices such as batteries for stand-alone PV systems, or install protective and metering devices for utility-connected PV systems.
It is clear, advancing PV technology will provide new market opportunities for the electrical contractor. EC
GLAVINICH is an associate professor in the Department of Civil, Environmental and Architectural Engineering at The University of Kansas and is a frequent instructor for NECA’s Management Education Institute. He can be reached at 785.864.3435 or firstname.lastname@example.org.