Guidance and information on using ForgeSolar analysis tools
These help pages include documentation and guidance on the ForgeSolar tools. This includes detailed descriptions of the editor, analysis methodology and results. ForgeSolar was built with the Solar Glare Hazard Analysis Tool technology (SGHAT), licensed from Sandia National Laboratories. Portions of the Help content is taken from the SGHAT User's and Technical Manuals which were originally written by Dr. Clifford K. Ho, Cianan Sims, Dr. Julius Yellowhair and Evan Bush.
With growing numbers of solar energy installations throughout the United States, glare from photovoltaic (PV) arrays has received increased attention as a real hazard for pilots, air-traffic control personnel, motorists, and others. The ForgeSolar suite of tools provide a quantified assessment of:
- when and where glare will occur throughout the year for a prescribed solar installation
- potential effects on the human eye at locations where glare occurs, and
- annual energy production from the PV system so that alternative designs can be compared to maximize energy production while mitigating the impacts of glare
ForgeSolar employs an interactive Google map where the user can quickly locate a site, draw an outline of the proposed PV array(s), and specify observer locations or paths. Latitude, longitude, and elevation are automatically queried from Google, providing necessary information for sun position and vector calculations. Additional information regarding the orientation and tilt of the PV panels, reflectance, environment, and ocular factors are entered by the user.
If glare is found, the tool calculates the retinal irradiance and subtended angle (size/distance) of the glare source to predict potential ocular hazards ranging from temporary after-image to retinal burn. The results are presented in a simple, easy-to-interpret plot that specifies when glare will occur throughout the year, with color codes indicating the potential ocular hazard. The tool can also predict relative energy production while evaluating alternative designs, layouts, and locations to identify configurations that maximize energy production while mitigating the impacts of glare.
ForgeSolar currently includes two tools for glare analysis, which are both accessed via the editor:
- GlareGauge - annual glare hazard analysis of PV arrays and receptors
- GlaReduce - optimization analysis of a single PV array over a range of module configurations (tilts and orientations)
For questions or feedback on Help content, please contact us.
Background and theory regarding solar glare and regulatory policies
About Glint & Glare
Glint is typically defined as a momentary flash of bright light, often caused by a reflection off a moving source. A typical example of glint is a momentary solar reflection from a moving car. Glare is defined as a continuous source of bright light. Glare is generally associated with stationary objects, which, due to the slow relative movement of the sun, reflect sunlight for a longer duration.
The difference between glint and glare is duration. Industry-standard glare analysis tools evaluate the occurrence of glare on a minute-by-minute basis; accordingly, they generally refer to solar hazards as 'glare'.The ocular impact of solar glare is quantified into three categories (Ho, 2011):
- Green - low potential to cause after-image (flash blindness)
- Yellow - potential to cause temporary after-image
- Red - potential to cause retinal burn (permanent eye damage)
These categories assume a typical blink response in the observer. Note that retinal burn is typically not possible for PV glare since PV modules do not focus reflected sunlight.
The ocular impact of glare is visualized with the Glare Hazard Plot. This chart displays the ocular impact as a function of glare subtended source angle and retinal irradiance. Each minute of glare is displayed on the chart as a small circle in its respective hazard zone. For convenience, a reference point is provided which illustrates the hazard from viewing the sun without filtering, i.e. staring at the sun. Each plot includes predicted glare for one PV array and one receptor.
Reflections from PV panels may impair observers. Studies have found that 7 W/m2 is enough to cause an after-image lasting 4 to 12 seconds (Ho, 2009). This represents a reflection of only 1-2% of typical solar irradiance (incoming sunlight) for a given location, which typically ranges between 800-1000 W/m2.
A key factor of reflectance is the position of PV modules relative to the sun. A panel that absorbs 90% of direct sunlight may reflect up to 60% when not directly facing the sun. This situation is common for low-tilt panels during sunset and sunrise (Yellowhair, 2015). The oft-repeated claim that PV panels reflect less than 5% of sunlight only holds true when the panels directly face the sun. For fixed-mount panels, this claim only applies during a few minutes of the day, at most.
Module Reflectance Profiles
Sandia National Laboratories developed five generic PV module material reflectance profiles by analyzing over twenty PV module samples. These profiles are available in ForgeSolar and allow for customizing the material properties of the PV array during analysis.
The figure to the right illustrates the reflectance of each material profile as a function of incidence angle, where an angle of 0° implies the panels are directly facing the sun. For example, a high glancing angle near 90° for panels with 0° tilt (lying flat) occurs daily at sunrise and sunset.
Anti-reflective coatings (ARC) and surface texturing can reduce panel reflectivity, but this reduction is typically less than 8% (Yellowhair, 2015). In addition, greater surface texturing can increase the size of the subtended source angle (i.e. glare spot).
Guidance on conducting glare analyses and optimizations
- Project name
- Unique name to distinguish a particular project. For example, "LAX parking rooftop PV" or "Main street solar farm"
- Optional description for user convenience.
- Timezone offset
- Numerical +/- offset from UTC/GMT of the site location. For example, a site in New York, USA would utilize a timezone offset of -5. Options range from -12 to +14.
- Distance units
- Whether the distances, including heights and elevations, should be displayed in feet or meters.
- Site name
- Alphanumeric name describing this site and configuration
- Configuration description
- Optional description of this particular site and configuration
- Time interval (min)
- The time step, or sampling interval, for the annual glare hazard analysis. The sun position will be determined at each time step throughout the year. Regulatory authorities such as the FAA typically require a time step of 1 minute. Other values can be used to conduct faster analyses or "spot check" alternative configurations. The time interval must evenly divide 1440 (i.e. number of minutes in a day); suitable alternatives are 2, 4, 5, 10, 15, 20.
- Sun angle (mrad)
- The average subtended angle of the sun as viewed from earth is ~9.3 mrad or 0.5°.
- Peak DNI (W/m2 or Wh/m2)
- The maximum Direct Normal Irradiance at the given location at solar noon. DNI is the amount of solar radiation received in a collimated beam on a surface normal to the sun during a 60-minute period. On a clear sunny day at solar noon, a typical peak DNI is ~1,000 W/m2. More accurate values for a specific site location may be available from other data sources. The Typical Meteorological Year 3 (TMY3) data sets from the U.S. National Solar Radiation Database contain similar values for locations throughout the U.S.
- DNI varies?
- If checked, the peak DNI will be scaled at each time step according to the changing position of the sun and reduced DNI in the mornings and evenings. If unchecked, the DNI at every step will be set to the Peak DNI.
- Ocular transmission coefficient
- Coefficient accounting for radiation that is absorbed in the eye before reaching the retina. A value of 0.5 is typical (Ho, 2011; Sliney, 1973).
- Pupil diameter (m)
- Defines the diameter of the pupil of the observer receiving predicted glare. The size impacts the amount of light entering the eye and reaching the retina. Typical values range from 0.002 m for daylight- adjusted eyes to 0.008 m for nighttime vision (Ho, 2011; Sliney, 1973).
- Eye focal length (m)
- Distance between the nodal point (where rays intersect in the eye) and the retina. This value is used to determine the projected image size on the retina for a given subtended angle of the glare source. A typical eye focal length is 0.017 m (Ho, 2011; Sliney, 1973).
Components & Receptors
Information on creating and editing PV arrays, flight paths, routes and observation points
PV Array Component
Photovoltaic systems are represented by a contiguous planar polygon footprint and a set of customizable parameters. Each footprint comprises three or more vertices, defined by a latitude, longitude, elevation and height. Each distinct PV installation should be modeled with it's own PV array footprint in the editor. During analysis, sunlight is reflected over each PV array on a minute-by-minute basis according to the user-specified module tilt and orientation or axis tracking parameters if the system is not fixed-mount. The system then checks whether the resulting solar reflections intersect (impact) the receptors.
PV Array Footprint
PV arrays are simulated spatially with a contiguous planar convex polygon. This polygonal footprint comprises three or more vertices which are defined by a latitude, longitude, elevation and height. The footprint should encompass all planned PV modules in a given area. Non-contiguous PV systems, or those with substantial concavities, should be modeled with multiple PV array footprints.
ForgeSolar will modify the vertex elevations if they do not initially reside on a single planar surface. For example, if a user attempts to model a non-planar footprint, such as multiple sides of a hill, the system would smooth the footprint and effectively flatten the hill. (In this example, a more accurate approach would be to model each hillside as a separate PV array.)
Note that ForgeSolar will convert the footprint polygon into a convex polygon during analysis by filling in any concavities. For example, a 'C'-shaped footprint would be modified into a half-circle. This adjustment is currently required by the glare-check algorithm during analysis.
Large PV array sites with many concavities should typically be modeled with multiple PV array footprints, instead of one large PV footprint. This can yield more accurate results which do not over-predict glare by over-estimating the size of the PV array after the required gap-filling.
PV footprint vertex coordinates, including elevation, are independent of the orientation and tilt. The vertices establish the tilt of the PV-array plane and do not influence the tilt or orientation of the individual panels themselves. For example, panels mounted flush on a 30° pitched roof will have PV-array vertices with different elevations to accommodate the pitched roof and resulting tilted PV-array plane (e.g., two vertices at 15 feet and two vertices at 10 feet). However, the panels should still be prescribed with a tilt of 30° (if they are flush mounted against the 30° pitched roof) and the appropriate orientation. A tilt of 0° indicates that the panels are parallel with the earth’s surface and facing upward, regardless of the prescribed vertex elevations.
ForgeSolar does not rigorously represent the detailed geometry of a system; detailed features such as gaps between modules, variable height of the PV array, and support structures may impact actual glare results. The PV array is simulated as a footprint filled with infinitesimally small panels reflecting sunlight in the trajectory of the tilt and orientation.
PV Array Parameters
General PV array parameters are described below. Module configuration, tracking and vertex parameters are described in subsequent sections.
- Descriptive alphanumeric name of this PV array
- Optional textual description of array
- Axis tracking
- Whether PV array modules are fixed-mount or utilize single- or dual-axis tracking
- Rated power (kW)
- Used to calculate the approximate maximum annual energy produced (kWh) from the system in the prescribed configuration (assuming clear sunny days). This is useful for comparing alternative configurations to determine which one has the maximum energy production. ForgeSolar system output calculations are approximate and should not supercede more accurate calculations conducted elsewhere.
- Module surface material
- The type of material comprising the PV modules. The reflectivities of the material choices have been characterized to generate scaled values for each time step. Refer to the Module Reflectance Profiles section for additional information.
- Reflectivity varies with incidence angle
- If checked, the reflectivity of the modules at each time step will be calculated as a function of module surface material and incidence angle between the panel normal and sun position.
- Specify the solar reflectance of the PV module. Although near-normal specular reflectance of PV glass (e.g., with antireflective coating) can be as low as ~1-2%, the reflectance can increase as the incidence angle of the sunlight increases (glancing angles); for example, at sunrise and sunset for low-tilt panels. Based on evaluation of several different PV modules, an average reflectance of 10% is provided as a default value. Only used if reflectivity does not vary with incidence angle.
- Slope error (mrad)
- Specifies the amount of scatter that occurs from the PV module. Mirror-like surfaces that produce specular reflections will have a slope error closer to zero, while rough surfaces that produce more scattered (diffuse) reflections have higher slope errors. Based on observed glare from different PV modules, an RMS slope error of ~10 mrad (which produces a total reflected beam spread of 0.13 rad or 7°) appears to be a reasonable value. Not used if correlate slope error to module surface type is checked.
- Correlate slope error with surface type
- If checked, the slope error value will be set per the table below, based on the selected material.
|PV Cover Type||Average RMS Slope Error (mrad)||Average Beam Spread (mrad)||Standard deviation of slope error||Standard deviation of beam error|
|Smooth glass without anti-reflection coating||6.55||87.9||4.43||53.3|
|Smooth glass with anti-reflection coating||8.43||110||2.58||30.9|
|Light textured glass without anti-reflection coating||9.70||126||2.78||33.3|
|Light textured glass with anti-reflection coating||9.16||119||3.17||38.0|
Fixed-mount PV panels are described by a tilt and orientation. These parameters are referred to as the module configuration of the PV array.
- Module orientation/azimuth (°)
- The azimuthal facing or direction toward which the PV panels are positioned. Orientation is measured clockwise from true north. Panels which face north, which is typical in the southern hemisphere, have an orientation of 0°. Panels which face south, which is typical in the northern hemisphere, have an orientation of 180°. If a known orientation is based on magnetic north, the location-specific declination must be used to determine the orientation from true north.
- Module tilt (°)
- The elevation angle of the panels, measured up from flat ground. Panels lying flat on the ground (facing up) have a tilt of 0°. Tilt values between 0° and 40° are typical.
Tracking System Parameters
Single-axis module tracking systems are described by a unique set of parameters. These angular inputs model the tracking axis, rotation range and backtracking behavior. Dual-axis module tracking systems are assumed to track the sun at all times.
- Tilt of tracking axis (°)
- Tilt above flat ground of axis over which panels rotate (e.g. torque tube). System on flat, level ground would have axis tilt of 0°.
- Orientation of tracking axis (°)
- Azimuthal angle of axis over which panels rotate. Angle represents the facing of the axis and system. For example. typical tracking system in northern hemisphere has tracking axis oriented north-south with an orientation of 180°, allowing panels to rotate east-west with potential south-facing tilt. Typical tracking system in southern hemisphere runs south-north with axis orientation of 0°, yielding east-west rotation with potential north-facing tilt.
- Offset angle of module (°)
- Additional tilt angle of PV module elevated above tracking axis/torque tube.
- Maximum tracking angle (°)
- Maximum angle of rotation of tracking system in one direction. For example, a typical system with a 120° range of rotation has a max tracking angle of 60° (east/west).
- Resting angle (°)
- Angle of rotation of panels when sun is outside tracking range. Used to model backtracking. Panels will revert to the position described by this rotation angle at all times when the sun is outside the rotation range. Setting this equal to the maximum tracking angle implies the panels do not backtrack.
- Latitude (°)
- North-south measurement of location relative to the equator, with range of [-90° to 90°]. Latitude is measured in decimal degrees and assumes the WGS84 datum.
- Longitude (°)
- Measurement of east-west position relative to Prime Meridian, with range of [-180°, 180°]. Longitude is measured in decimal degrees and assumes the WGS84 datum.
- Elevation/altitude (ft or m)
- Elevation above mean sea level at specified location. ForgeSolar automatically queries the Google Elevation services for an approximate value.
- Height above ground (ft or m)
- User-specified height above ground of point. The height of a rooftop PV system should measure from the ground to the PV panel centroid above the roof. A ground-mount system would have a height measured to the PV panel centroid.
- Total elevation (ft or m)
- Sum of the elevation and height above ground. The system will automatically calculate the height or total elevation when the other is provided. During analysis, the total elevation determines the Cartesian Z value of the point. For more accuracy, the user should perform analyses using minimum and maximum values for the vertex heights, based on the PV panel dimensions, to bound the height of the plane containing the solar array. Doing so will expand the range of observed solar glare when compared to results using a single height value.
The Route receptor is a generic multi-line representation which can simulate observers traveling along continuous paths such as roads, railways, helicopter paths, and multi-segment flight tracks.
Routes can be created quickly in the Map Editor:
- Activate the Route drawing mode by clicking the Route button above the map.
- Click once on a location in the map to begin drawing a route
- Click once to add a vertex to the route. Repeat as many times as is necessary; routes can include many line segments
- Double-click on final position to end the Route
- To add an additional vertex to the Route after it has already been completed, click and drag one of the "ghost" points within the polyline in the map.
- Descriptive alphanumeric label of receptor
- Is route one-way?
- If checked, the system will assume observers travel along the route in the direction it was drawn (i.e. order of increasing vertex #). Together with the view angle parameter, this will filter out glare appearing behind the path of travel. If unchecked (default), the system will assume observers travel in both directions.
- View angle (°)
- Defines the left and right field-of-view of observers traveling along the Route. A view angle of 180° implies the observer sees glare in all directions. A view angle of 50° (default) implies the observer has a field-of-view of 50° to their left and right, i.e. a total FOV of 100°. This default is based on FAA research which determined that the impact of glare that appears beyond 50° is mitigated (Rogers, 2015).
Additional resources and research on solar glare
Ho, C. K., Ghanbari, C. M., and Diver, R. B., 2011, Methodology to Assess Potential Glint and Glare Hazards From Concentrating Solar Power Plants: Analytical Models and Experimental Validation, ASME J. Sol. Energy Eng., 133. (Download)
Federal Aviation Administration (2013). Interim Policy, FAA Review of Solar Energy System Projects on Federally Obligated Airports. Federal Register: 63276-63279 (link)
Rogers, J. A., et al. (2015). Evaluation of Glare as a Hazard for General Aviation Pilots on Final Approach, Federal Aviation Administration ( link )
Ho, C. K. and Sims, C. A., 2013, Solar Glare Hazard Analysis Tool (SGHAT) User's Manual v. 3.0. (Download)
Ho, C. K., Sims, C. A., Yellowhair, J. E. and Bush, H. E., 2014, Solar Glare Hazard Analysis Tool (SGHAT) Technical Reference Manual, SAND2014-18360 O, Sandia National Laboratories, Albuquerque, NM. (Download)
Overview presentation of the Solar Glare Hazard Analysis Tool (SGHAT) (Download)
Ho, C. K., April 2013, Relieving a Glaring Problem, Solar Today Magazine (Download)
Barrett, S., June 2013, Glare Factor: Solar Installations And Airports, Solar Industry, Volume 6, Number 5. (link)
Ho, C. K., 2012, Glare Impacts from Solar Power Plants near Airports, in Proceedings of ACC/AAAE Airport Planning, Design and Construction Symposium, Denver, Colorado, Feb. 29 - Mar. 2. (Download)
Ho, C. K., 2011, Observations and Assessments of Glare from Heliostats and Trough Collectors: Helicopter Flyover and Drive-By Sightings, in proceedings of SolarPACES 2011, Granada, Spain, Sept. 20-23. (Download)
Ho, C. K., 2011, Summary of Impact Analyses of Renewable Energy Technologies on Aviation and Airports, Presentation to Federal Aviation Administration, Feb. 16. (Download)
Ho, C. K., Ghanbari, C. M., and Diver, R. B., 2010, Methodology to Assess Potential Glint and Glare Hazards From Concentrating Solar Power Plants: Analytical Models and Experimental Validation, SAND2010-2581C, in proceedings of the 4th International Conference on Energy Sustainability, Phoenix, AZ, May 17-22. (Download)
Ho, C. K. and Khalsa, S. S., 2010, Hazard Analysis and Web-Based Tool for Evaluating Glint and Glare from Solar Collector Systems, in proceedings of SolarPACES 2010, Perpignan, France, Sept. 21-24. (Paper) (Presentation)
Ho, C. K., Ghanbari, C. M., and Diver, R. B., 2009, Hazard Analyses of Glint and Glare From Concentrating Solar Power Plants, SAND2009-4131C, in proceedings of SolarPACES 2009, Berlin, Germany, Sept. 15-18. (Download)
Yellowhair, J. and C.K. Ho. Assessment of Photovoltaic Surface Texturing on Transmittance Effects and Glint/Glare Impacts. ASME 2015 9th International Conference on Energy Sustainability collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum. 2015. American Society of Mechanical Engineers. (Presentation)
Sliney, D.H. and B.C. Freasier, 1973, Evaluation of Optical Radiation Hazards, Applied Optics, 12(1), p. 1-24.
Miscellaneous links and information
ForgeSolar includes GlareGauge, the leading solar glare analysis tool used globally every day. ForgeSolar is based on the Solar Glare Hazard Analysis Tool ("SGHAT") licensed from Sandia National Laboratories. Our tools meet the FAA standards for glare analysis.