Environmental Design + Construction - October 2008 - (Page 34) Cool Roofing Figure 1. A comparison of crystalline and thin-film technologies in bituminous applications, such as BUR (built-up roofs), SBS (styrene butadiene styrene) and APP (atactic polypropylene), and single-ply applications, including PVC (polyvinyl chloride), TPO (thermoplastic olefin) and EPDM (ethylene propylene diene monomer). Bituminous (BUR, SBS, APP) Crystalline Strengths • Bituminous roofs with rackmounted crystalline solar systems have proven very effective as substrates and require no special maintenance. • Crystalline systems are generally more efficient than thin-film. • Flashing details are critical for success. (Note: To maintain the manufacturer’s roof guarantee, approved roofing contractors need to be used.) • Ballasted systems need to be analyzed for potential creep. Thin-Film • Thin-film systems generally are lighter than crystalline systems. • Some manufacturers will allow direct adhesive application to many types of roofing systems. • Thin-film systems tend to be less expensive than crystalline. • Thin-film provides a wider spectrum of light and will produce energy over a longer time each day. • Thin-film is not as efficient per square foot. • Thin-film self-adhered solar systems should not be applied to granule-surfaced roofs due to surface irregularity. • If adhering directly to smooth-surface bitumen roofs, ensure that conduit runs are surface-mounted, and not installed in the insulation below the membrane. Installing conduit in the insulation can promote condensation and/or thermal shorts, reducing the insulations effectiveness and negating the positive effects of the solar power. Weaknesses with a dye sensitizer and use it to create electrical current in a nanocrystalline titanium dioxide semiconductor layer. The cells are encased in a carbon layer on the back and are contained in glass on both sides. A number of companies are developing this technology and they could be introduced commercially in the near future. These cells are expected to compete by significantly reducing the cost of PV cells. CONCENTRATING PHOTOVOLTAICS Concentrating photovoltaic systems use lenses or mirrors to concentrate sunlight onto highefficiency solar cells. These solar cells are typically more expensive than conventional cells used for flat-plate photovoltaic systems. However, the concentration of sunlight decreases the required cell area while also increasing the cell efficiency. This technology has potential for solar cell efficiencies greater than 40 percent. The high cost of advanced, highefficiency solar cells requires concentrated sunlight for the system to achieve a cost-effective comparison with other solar alternatives. Therefore tracking systems are usually installed to take advantage of every hour of available sunlight. Tracking system motors may require maintenance. The chart in Figure 1 compares crystalline and thin-film technologies in both bituminous and single-ply applications. SELECTING THE RIGHT TECHNOLOGY FOR THE PROJECT One of the most important considerations in selecting which solar technology to use is based on balancing the cost of the project with the energy it generates. That will vary based on project specifics such as its size and location. It is important to engage a knowledgeable manufacturer or solar integrator who can analyze your specific project and make recommendations based on the current technology available. Whether a crystalline or thinfilm system is selected, the steps to successfully installing a rooftop-mounted solar system are the same, but vary depending on membrane type and means of attachment. (See Figure 2.) Single Ply (PVC, TPO, EPDM) Crystalline Strengths • Mechanically attached single-ply roofs have proven very effective as substrates and require no special maintenance. • Crystalline systems are generally more efficient than thin-film. • In some instances, single-ply membranes will be subjected to extended shade, which could promote ponding conditions and moisture. Consult with the roofing manufacturer to determine how this might impact the roof guarantee. • Ballasted systems need to be analyzed for potential creep. Thin-Film • Thin-film systems generally are lighter than crystalline systems. • Some manufacturers will allow direct adhesive application to many types of roofing systems. • Thin-film systems tend to be less costly than crystalline. • Thin-film provides a wider spectrum of light and will produce energy over a longer time each day. • Thin-film is not as efficient per square foot. • Thin-film self-adhered solar systems should not be applied to ballasted single-ply roofs due to surface irregularity. • If adhering directly to smooth-surface single-ply roofs, ensure that the conduit runs are surface-mounted and not installed in the insulation below the membrane. Installing conduit in the insulation can promote condensation and/or thermal shorts, reducing the insulation’s effectiveness and negating the positive effects of solar power. Weaknesses Figure 2. Key steps to successfully installing a rooftopmounted solar system in ballasted and mechanically attached systems. Bituminous Ballasted • Ensure the frame is stable and does not creep. • Inspect roof for dry laps or mole runs prior to frame overlay. • Provide manufacturer-approved walk pads between the roof and frame. • Remove roof and insulation as required to installed approved base plates to roof and joist per engineering requirements • Flash all components using a two-part MBR flashing system. • Consult the roofing manufacturer for proper flashing methods and means. Single Ply Provide seam inspection prior to frame placement to ensure watertight laps. Provide manufacturer approved walk pads between the roof and frame. Mechanically Attached • Remove roof insulation as required to install approved base plates to roof and joist per engineering requirements. • Flash all components using the EPDM, TPO or PVC manufacturer’s standard boot details. • Consult the roofing manufacturer for required methods and means. including amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CIS) or copper indium gallium selenide (CIGS). These materials are good light absorbers and are fairly thin. They are known as thin-film because they are deposited in very thin layers on stainless steel, glass or a flexible substrate. The thickness of the film is less than 1 micron (a human hair is 50-100 microns). Like c-Si, thin-film PV cells are combined into modules and laminated to protect them from the elements. They are less expensive than c-Si, but only have a demonstrated efficiency of approx34 Cool Roofing imately 8 percent. The advantage of thin-film technology is that it can be applied over large areas, providing more oppor tunity to generate electricity in cloudy conditions. The shortage of crystalline silicon has led to increases in the use of thinfilm PV cells in the United States. Just a few years ago, they were virtually unheard of. Today, they make up approximately 30 percent of the market. PHOTOELECTROCHEMICAL PV CELLS Photoelectrochemical (PEC) PV cells are currently under development. Unlike other PV cells that use solid crystal layers to absorb light, PEC cells are liquid. They absorb light > Fall 2008
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