All you wanted to know about a Solar Glare Assessment

What is a Solar Glare Assessment?

A Solar Glare Assessment (also Glint and Glare Study) is the report of an expert about the expected Glaring Hazard of a Solar Power Plant. This report consists of the calculation of the glare duration, the time of glare and the viewing angles (e.g. between reflecting area and driving direction).

A Solar Glare Assessment verifies, if the dazzling effect could be dangerous for traffic or an unreasonable nuisance for neighbours. It can also contain recommendations for glare reducing provisions. The solar glare assessment alone is not binding; it comes into effect by the requirements of the relevant building authority.

Who needs a Solar Glare Assessment?

A Solar Glare Assessment often is required by the building authority, the road authority, airport or railroad authorities. The designer of PV-plants gets covered by a solar glare assessment thus avoiding later law suits from neighbours or traffic authorities. Before construction the costs for adapting the plans (e.g. direction of PV-panels) are low in comparison with a later re-construction of the plant.

After construction of a solar park a glint and glare study can be required (by a court, community or authority), in order to calculate the dosage of light immission over the course of a full year. Since the measurement of glare always is an instantaneous value only, the glare calculation delivers the more significant results.

Dazzling by Photovoltaics - Dazzling by Solar Heating - What is the difference?

Photovoltaic Parks and Solar Thermal Plants have some things in common regarding their dazzling effect, such as large glass areas, the orientation towards the south and the place of installations on roofs or open areas. Solar Thermal Plants are typically mounted with a higher angle, but in principle the elevation angle of both technologies can be chosen arbitrarily.

There is, however, a difference in light scattering caused by the different surface structure of the glass used and the form of the glass. PV-panels often are prismatic (their surface is structured with pyramid patterns) and therefore cause a wider scattering of the light rays than flat glass. On large area collectors the glass in the middle can sag somewhat, thus causing reflected light to concentrate. This happens more often by solar thermal collectors, which usually have larger surfaces than PV-panels.

Glaring from glass facades - Glaring from Solar Plants

Glass facades exhibit a dazzling effect, which in principle will cause disability glare exactly like solar plants. They too, reflect part of the light and therefore have the same potential to pose a glaring hazard as solar power plants. The composition of the glass is no decisive factor in the reflection. Anti-Reflective-Coatings can reduce reflections somewhat, but typically this effect is not strong enough to avert disability glare.

In contrast to photovoltaic plants and solar thermal plants, glass facades are typically vertical. This fact can reduce the possible duration of glaring, which has to be verified in each case for a specific point of interest. The composition and tinting of the glass does not matter in this case, because light is reflected on the first interface with air (i.e. the upper-most surface of the glass).

Unreasonable Glaring - Which limits are there?

Since dazzling from sun reflection are always assumed to cause disability glare - i.e. you have to avert your gaze - the strength of glaring (i.e. the luminous density) is not a decisive factor for the assessment of solar glare. The most important parameter therefore is the duration of the exposure of glaring on a human. In lab experiments medical scientists have agreed, that the limit for risk of health is given by an exposure of 1 hour per day and 60 hours per year.

The limit for an unreasonable nuisance is defined in several national guidelines. In Germany the Light-Guideline (Licht-Leitlinie) issued by the Ministry of Environment defines the limits for glare duration at 30 minutes per day and 30 hours per year. In Austria the same values were determined in guideline OVE R11-3.

How is Glaring measured?

The luminous density is the relevant parameter for the dazzling effect. It is measured in Candela per Square-Meter [cd/m²]. The sun, depending on its position (midday or morning/evening) has a luminous density between 6 million cd/m² and 1.6 billion cd/m². The luminous density of the sun and its reflections therefore is much larger, than all other known luminous densities (blue sky 8000 cd/m², artificially lit surfaces with several 100 cd/m², computer monitors 500 cd/m²).

Even if only a few % of the sunlight are reflected by the glass, the luminous density of sun reflections is so large, that it will cause disability glare (i.e. beyond the limit the eye can adapt to), which will cause damage of the retina during longer exposure.

What data is needed for a Glare Calculation?

Besides the planning data of the installation (location, direction, size), the data of sun positions, ground topography and horizon at the specific location are needed. The gathering of this information is the first step of any Solar Glare Assessment. In the case that specific authorities' requests or building requirements are given, the points of interest (immission-points) for the calculation are to be determined.

How to proceed if the assessment diagnoses a glaring hazard?

If a dangerous glare or unreasonable nuisance is diagnosed, the planning of the installation is to be adapted in order to reduce or totally remove this effect. Glaring hazards are to be completely removed from critical traffic zones and glare durations for neighbours have to be reduced below the thresholds of significance.

The project designer has several degrees of freedom to accomplish this. On one hand, the location, the layout as well as the angles of elevation and azimuth can be adapted. On the other hand, the construction of shades (such as non-transparent fences) can help to interrupt the line of vision between installation and point of interest and thus protect areas from any immission.