301 Moved Permanently

---

nginx

Accompanying the expansion of solar energy is a rise in concerns about how local government planners and zoning enforcement officers can and should respond in order to protect the interests of the community from unintended consequences. One local concern involves worries that solar panels might create glare that would, at a minimum, be a nuisance to neighboring residential properties.

In Belmont, Mass., for example, the local planning board approved a solar project proposal for a local elementary school after months of delay. Neighbors had expressed concerns that the solar panels would deflect glare into their homes or around the neighborhood.

Under Massachusetts statutes, local governments are - at least in theory - substantially constrained in the restrictions they might place on solar facilities. A Massachusetts statute (Chapter 40A, Section 3) specifically provides that “[n]o zoning ordinance or bylaw shall prohibit or unreasonably regulate the installation of solar energy systems or the building of structures that facilitate the collection of solar energy, except where necessary to protect the public health, safety or welfare.”

This statute, however, might provide less protection than envisioned by its drafters. Belmont’s solar zoning bylaw (Section 3.3), for example, allows for rooftop solar PV units “by-right” in all zoning districts in the community, subject to “site plan review.” The Belmont bylaw (Section 4.3.8(c)(6)), however, provides that “solar collector panels shall be placed and arranged such that reflected solar radiation or glare shall not be directed onto adjacent buildings, properties or roadways.”

Under the Belmont zoning review process, the municipality’s planning board may not disapprove a proposed solar project. Under the local “site plan review,” the planning board is authorized to impose reasonable terms and conditions on the proposed use, but it does not have discretionary power to deny the use.

Despite this limitation, the planning board can effectively deny a project simply by repeatedly continuing the public hearing, thus never bringing the solar proposal to a vote. Under Belmont’s zoning bylaw (Section 7.3.3(c)), for example, the planning board must issue a final decision no more than 20 days subsequent to closing the public hearing unless the applicant agrees to a longer period. No restrictions exist, however, on when the planning board must close the public hearing.

Thus, developers might benefit from addressing solar glare issues in the initial zoning application stage of their project planning.

 

Be a good neighbor

There is a difference between “glint” and “glare.” Glare involves a generalized reflection of light from the sky surrounding the sun. Glint is a reflection of the direct light from the sun. Because popular vernacular does not recognize this technical distinction, the generic term “glare” is used in this article to encompass generically all reflected light from a solar PV installation.

In general, given that the whole concept of efficient solar power is to absorb as much light as possible while reflecting as little light as possible, standard solar panels produce less glare and reflection than does standard window glass. On a more technical level, solar panels use “high transmission, low iron glass” that absorbs more light, producing smaller amounts of glare and reflectance than normal glass does.

Certain design attributes of PV panels increase the absorption of light by, and reduce reflection from, solar PV panels. Initially, anti-reflective coatings on PV panels reduce the reflection of sunlight for PV panels.

PV panels use silicon to convert sunlight to electricity. Because silicon is naturally reflective, all PV panels are coated with anti-reflective materials that allow light to pass through the silicon and minimize reflection. In fact, what causes the dark appearance of PV panels (e.g., dark blue, black) is the fact that monocrystalline wafers, the main component of PV modules, are combined with anti-reflective coatings to maximize absorption.

In addition to the anti-reflective coatings, the surfaces of PV panels are roughened, a process called “stippling.” Stippled surfaces - also referred to as “dimpled” surfaces - diffuse reflection and, thus, eliminate glare.

In a dense urban setting, and with new buildings, it is particularly important to note that solar PV modules reflect less light than do modern rooftops. New construction today is usually designed with white rooftops, which reflect sunlight (and therefore heat), thus helping to control internal building energy costs. White “energy-efficient” roofs are designed to have a high albedo, and there are known glare issues with these roofs. PV panels may well reduce those glare impacts.

Installing solar PV panels on a white roof, in other words, will reduce, not increase, the potential for glare to occur. It would be reasonable to expect solar PV panels to create little, if any, glare to neighboring properties. Nevertheless, when a solar developer submits papers to local officials for a review and local zoning determination, a developer should reasonably submit, at the time of application, documentation to satisfy the following questions:

• Do the solar cells used in the modules have anti-reflective coatings?
• Is the surface of the solar modules smooth or stippled?
• Do the solar modules incorporate any type of “light trapping” technology?
• Is there documentation from the panel manufacturer of the albedo (reflectivity) of the panels?
• What is the color of the roof in the absence of the solar modules?

 

Be accommodating

Assuming, solely for the sake of analysis, that solar panels in a dense residential neighborhood might generate enough glare of sufficient magnitude to be a possible nuisance to abutters, a second inquiry should assess the extent to which, if at all, that glare might be visible to a third-party observer given the elevation and direction of the glare. Even if glare occurs, in other words, unless it is directed toward, and seen by, an abutting property (referred to later as a “receptor” or “observer”), the glare will not pose problems.

Before considering the mathematics of sunlight reflectivity, it is important to understand several fundamental limitations concerning the extent to which glare might be visible to abutting properties.

First, for glare to appear, the observer must be able to see the tops of the PV panels. For this to occur, at a minimum, the receptor must be able to see the top of the roof on which the solar PV modules are located. To the extent that the solar panels are installed at some level above the roof, the receptor would need to be at a height sufficient to look down at the tops of the solar modules.

In an urban neighborhood, for example, if solar panels are placed on the top of a flat two-story roof, and no gradient exists between the solar installation and the receptor, persons at ground level, as well as people living in two-story abutting properties, will not be affected by any glare. Moreover, people living in buildings having more stories than the building upon which the solar modules are installed will also not be affected by solar glare unless the observers live in a story that is physically higher than the solar installation.

Second, for glare to appear, the altitude of the sun must be above the tops of the solar PV panels. Unless the sun is above the panels, sunlight cannot strike the panels to be reflected. While this may seem self-evident, this observation has particular significance for rooftop solar PV units in an urban setting.

For rooftop solar PV panels in a dense urban setting in particular, the sun will not likely be high enough in the sky for sunlight to generate glare in the early morning hours. To create glare in these early morning hours, the sun must not only be above the horizon, but must also be above the top of the roof and above the tops of surrounding buildings.

In this respect, solar installations in dense urban neighborhoods will differ from solar panels installed at airports. (For more information on this, see “Glare Factor: Solar Installations And Airports,” June 2013.)

It is important to understand how high the sun is required to be for light to reach the tops of solar PV panels. Because light travels in a straight line, the angle at which sunlight strikes a solar PV panel - the angle of incidence - will be equal to the angle at which the sunlight reflects from the PV panel - the angle of reflectance. Accordingly, the higher the sun is in the sky, the more any reflection is directed back into the sky (upward) rather than being directed horizontally toward neighboring properties.

Third, glare is insignificant when the location of the sun in the sky is close to the glinting object seen by an observer. The closer the observed angle between the sun and the object from which glare is reflected comes to 0°, the more the glare, if any, will be masked by the direct light from the sun. If glare occurs only in the early morning or late afternoon, in other words, it is likely to exist only when an observer would need to be looking directly into the sun to be exposed to the glare in any event. In these circumstances, glare will not be a problem.

Finally, glare is avoided when vegetation or other impediments stand between the observer and the solar panels. These impediments do not shade the solar panels. Rather, they act as sight barriers between potential observers and any solar panels. Neither must these sight barriers be next to the solar PV panels. They need only be between the solar PV panels and a potential observer. A home, for example, may be in the general neighborhood of a building with solar PV panels. That house, however, is at no risk of exposure to solar glare if other homes stand between it and the solar panels.

Sight barriers may be permanent solid structures, such as buildings. This type of barrier provides screening from any potential glare irrespective of season. In a dense urban neighborhood, in other words, glare is not a “neighborhood” problem. Glare, if any, will affect only immediate abutters. A home two buildings away from a solar installation will be screened by the intervening structures.

Sight barriers also may be vegetative. Unlike buildings, vegetation is seasonally important. If glare will occur in the fall or winter seasons, seasonal vegetation may not be available to serve as a sight barrier. In contrast, if the glare will occur in the spring or summer seasons, seasonal vegetation would prevent glare, if any, from reaching potential observers.

Key points in the solar cycle to consider in relation to glint include the following: longest day (worst case scenario: greatest glint); shortest day (best case scenario: least glint); autumn equinox (sun rises at north 90° east and sets at north 90° west); spring equinox (sun rises at north 90° east and sets at north 90° west).

Sight barriers have daily implications as well as seasonal implications. Reflections that might be seen as glare by neighbors will be horizontal. Horizontal reflections, however, occur only twice a day: to the west in the morning when the sun is in the east, and to the east in the afternoon/evening when the sun is in the west. The usefulness of sight barriers as a screen against potential solar glare depends on their location vis a vis the solar PV panels at the time of day that glare might occur.

When a solar developer submits papers to local zoning officials, it might be beneficial for the developer to also submit, at the time of application, documentation of the following:

• The extent to which there exist immediately abutting buildings having a height that would provide sight-lines that include a view of the top of the solar panels; and
• The sight barriers that exist between buildings having a height that would provide sight-lines that include the top of the solar panels, distinguished by whether these sight barriers are useful at screening potential glare at the time of day when the glare is likely to occur.

 

Be prepared for math

Basic principles of light and mathematics can be used to document the potential of solar glare occurring to the detriment of properties that abut an urban rooftop solar PV system. Two basic concepts need to be understood to assess the glare potential: (1) the vertical nature of glare (i.e., “elevation” or “altitude” - terms that are used interchangeably) and (2) the horizontal nature of glare (i.e., “azimuth”).

An assessment based on each of these concepts leads to the conclusion that glare from rooftop solar PV modules is unlikely to be a problem in an urban neighborhood. The elevation of glare, if any, from rooftop solar PV modules is likely to be above neighboring properties. But you will be expected to submit your proof!

The basic concept to understand in any discussion of glare elevation is that the angle of incidence is always equal to the angle of reflectance. The empirical inquiry is then whether the potential observer is within the altitude of reflection given the distance of the observer from the solar PV panel. At any angle of reflectance, as a potential observer is further and further away from the solar PV panel, the elevation of the reflected sunlight (i.e., any glare) is more likely to be above the observer and, thus, not seen. In contrast, at a high angle of reflectance, the elevation of reflected sunlight will likely be above the observer - even at short distances.

Given the basic principle of light reflectivity, evaluating the angle of reflectance from a solar PV panel must begin with a determination of the altitude of the sun (in degrees) relative to the ground. The “solar altitude” is the angle of the sun in degrees above or below the horizon. Of course, the operative surface when calculating light reflectivity is not the horizon, but the angle at which the solar panel is mounted relative to the horizon.

The minimum possible elevation for any solar reflection occurs when the angle of the sun in respect to a solar PV panel is 0°. A rooftop solar PV panel with a slope of 22°, for example, produces an angle of reflection of 136°. Because the altitude of reflection is 180° minus the angle of reflection, the minimum altitude of reflection will be 44° for a solar PV panel with a slope of 22°. The sum of the angle of reflection and the altitude of reflection will be 180° - a straight line. Accordingly, the altitude of reflection will be 180° minus the angle of reflection.

As the angle of the sun in relation to the solar PV panel increases, the angle of reflection will decrease and the altitude of reflection will increase. The altitude of the sun differs based on a number of different factors: the time of day, the season of the year, and the latitude at which the solar PV panel is located.

As the angle of the sun in relation to the solar PV panel increases, the angle of reflection will decrease and the altitude of reflection will increase. Given a solar altitude of 30° and a PV panel slope of 22°, the angle of reflection would be 106° and the altitude of reflection would be 74°. With a solar altitude of 60°, the angle of reflection would be 76° and the altitude of reflection would be 104° from the same panels.

In the example of the proposed solar installation in Belmont, it was possible to show that the altitude of the reflection was above any reasonable concern.

In the spring and fall, at an observer distance of only 20 feet from the solar panels, the noontime elevation of glare, if any, would be nearly 80 feet. By the time the distance of an observer from the solar panels reaches 35 feet, the noontime elevation of any glare would be more than 135 feet in the air.

In the summer, at an observer distance of only 20 feet from the solar panels, the noontime elevation of solar glare would be nearly 130 feet. By the time the distance of an observer from the solar panels reaches 35 feet, the noontime elevation of any glare would be nearly 225 feet in the air.

In the winter, at an observer distance of 20 to 35 feet from solar panels, the noontime elevation of solar glare, if any, would be between 25 feet and 45 feet in the air.

In interpreting elevation figures, it is, of course, important to remember that the elevation of the glare is not from ground level, but rather from the level of the solar panels. If the solar panels being reviewed are rooftop units, the elevation of the glare needs to be increased, at each distance level, by the height of the building on which the solar panels are located. If the solar PV panels are on top of a flat-roof two-story building - in other words, an additional 30 to 35 feet should be added to each elevation.

When a solar developer submits papers to local zoning officials, it may be beneficial to submit, at the time of application, documentation of the following:

• The altitude of reflection for the solar modules in the early morning, at noon, and in the late afternoon/early evening for the spring equinox, summer solstice, fall equinox and winter solstice;
• An identification of illustrative neighboring properties along with the distance of those properties from the closest solar module;
• An identification of characteristics, both of the property on which the solar modules are located (e.g., parking lots, green space) and of the immediately surrounding neighborhood (e.g., streets, alleys) that increase the distance from the solar modules to the neighboring properties; and
• The elevation of reflection on the above dates and times for the illustrative neighboring properties plus or minus some reasonable band of impact assessment.

Determining the azimuth of the sun is important for purposes of assessing the potential glare from a solar installation. At any given site, the sun not only moves across the sky every day, but its path in the sky changes during various times of the year. This, in turn, alters the destination of resultant reflections.

Glare has little nuisance effect when it is at moderate angles (45° or less) from the observer’s focus of view. Because PV panels reflect light in a specific direction, the PV reflection when viewed from a substantial angle of azimuth is much less intense than when viewed directly. Moving just 30% off of direct reflection reduces the intensity of sunlight reflection by 80%. Glint has virtually no effect when the angle of azimuth is close to or greater than 90° from the observer’s focus of view.

The azimuth of the sun is important, also, to determine the direction of the direct sunlight reflection from a solar PV panel. Reflections facing south at low to moderate angles of inclination near horizontal reflections are confined to both of the following: (a) just north of due east to approaching southeast and (b) just north of due west to approaching southwest. Reflections at any given vertical angle happen twice a day in opposite directions. Reflections to the west occur in the morning (with the sun in the east); reflections to the east occur in the afternoon/evening (with the sun in the west). In contrast, during the day, reflections are skyward.

The dual consideration of the elevation and the directionality of reflection are interrelated tasks to undertake. The first step is to determine the elevation of reflection which might have a glare impact on neighboring buildings. The second step is to determine the direction of reflection. For glare to pose a nuisance value to a neighboring building, that building must be both at the elevation of the glare and in the direction of the glare at the same time. A consideration of one or the other of these steps might render the other moot. For example, if no buildings are within the elevation of glare, calculating the direction of glare becomes unnecessary.

When a solar developer submits papers to local zoning officials, it might be beneficial for the developer to also submit, at the time of application, documentation of the following:

• The azimuth of the sun at sunrise, noon and sunset for the latitude of the solar PV panels and the critical dates and times identified above;
• A listing of buildings within the direction of reflection, given the azimuth of the sun at those times and on those dates; and
• A determination of whether the buildings in the direction of reflection will also be at the altitude of reflection at those times and on those dates.

There is little possibility that solar PV panels will generate glare. Although concern with glare is an understandable one, an objective consideration of the factors that affect the creation of glare leads to the conclusion that solar PV panels installed in an urban neighborhood will not result in glare problems to neighbors.

At the same time, these neighbors will be coming to the town meetings to hear about your project. It is a good idea to come equipped to explain and show how solar power will give them no cause for complaint. R

 

Roger D. Colton is a co-founder of the economics and financing practice of Fisher, Sheehan and Colton in Belmont, Mass. Colton is also co-chair of the Belmont Energy Committee and a member of the Massachusetts Municipal Energy Group. He can be reached by email at roger@fsconline.com.