Evaluating Glare from Roof-Mounted PV Arrays
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As more homeowners choose to install PV systems, jurisdictional authorities and other stakeholder groups are increasingly scrutinizing the potential impacts of rooftop PV arrays. One recurring concern is that the reflected sunlight associated with roof-mounted PV may adversely affect residential neighborhoods.
Reflected sunlight causes glare. However, the surface reflectance of different objects or building materials varies. For example, fresh snow reflects 80%–95% of the light striking its surface, whereas black asphalt reflects only 5%–10%. The reflectivity of a PV array, as with any other surface, is a function of its albedo, which is a measure of the fraction of the sun’s radiation reflected from a surface. The albedo of different materials ranges from 0 (no reflection) to 1 (100% reflection). You can compare the glare potential of PV arrays to that of other materials by comparing albedo values. For example, Figure 1 is based on data published by the Federal Aviation Administration (FAA) in the report “Technical Guidance for Evaluating Selected Solar Technologies on Airports.”
Generally speaking, the albedo of glass is closer to that of snow than it is to that of asphalt. However, PV modules do not use conventional glass, since they cannot generate power from reflected sunlight. PV modules must absorb as much light as possible, while reflecting as little light as possible. Manufacturers typically accomplish this by using low-iron, high-transmission glass that is treated with an anti-reflective coating. In some cases, they texture the surface of the glass to increase its light-trapping properties. Texturing and the use of anti-reflective coatings also minimize the surface reflectance of individual PV cells.
As a result, the surface reflectance of solar modules and arrays is considerably less than that of other common surfaces. As shown in Figure 1, solar arrays reflect less sunlight than concrete, vegetation, bare soil and even wood shingles. FAA guidelines note that, depending on the angle of the sun, PV modules with anti-reflective coatings may reflect as little as 2% of the incoming sunlight.
Based on these data, you would not expect PV arrays to generate much glare, if any, toward neighboring properties. However, the FAA report also notes that light reflected off smooth, polished surfaces—such as PV arrays—is more concentrated than light reflected off rough surfaces such as roads, trees or choppy water. Therefore, the FAA requires that PV installations in the vicinity of airports undergo a geometric glare analysis. To facilitate this process, Sandia National Laboratories’ website (sandia.gov/phlux) hosts a “Solar Glare Hazard Analysis Tool” and a suite of related tools.
Because large-scale PV systems are installed at or near airports in many cities—including Boston, Denver, Indianapolis, Oakland, San Jose and San Francisco—glare hazard analyses generally indicate that PV arrays present no hazard to air navigation. While FAA standards apply to PV arrays at or near airports only, concerns about glare and reflectance associated with PV arrays also arise in other settings. For example, project developers occasionally conduct glare studies for large-scale PV power plants, perhaps due to proximity to a prominent road or landmark.