Requirements for glare intensity modelling
Glare intensity modelling is a common planning requirement for solar photovoltaic developments. Increasingly, it is becoming a requirement for other development types as well, for example shiny buildings or even outdoor statues.
The starting point for this requirement was, most likely, airport concerns over solar panels near their runways and air traffic control towers. In the USA, there was even a relatively high profile case of panels that were built and found to cause unacceptable glare in the adjacent air traffic control tower[1].
Ways of calculating acceptable glare levels
There are lots of mechanisms that could reasonably be factored into a process for determining an ‘acceptable’ amount of glare. These include:
- Maximum or average intensity (Watts/square metre) at a potential observer’s location – or more accurately the intensity at the retina.
- Angle between the source of glare and the direction an observer is likely to be facing, this is particularly relevant for drivers of vehicles or trains.
- Angular width of the glare source – a reflection that takes up 50% of an observer’s field of view is more noticeable than a glare source that takes up 1% of an observer’s field of view.
Other factors pertaining to the context of any glare could be incorporated as well, for example:
- Time of day at which glare occurs, e.g. glare at 5 AM towards an office that does not open until 7AM could be considered a non-issue.
- Duration of glare – effects that last for a few minutes per year could be considered less hazardous than effects that last for hours per day.
- Relative position of the Sun – if an observer looking towards glare from a panel is also looking towards the Sun, the relative impact of the glare can be diminished.
All of the above are defensible as grounds for assessing impact significance, and none of them are bullet-proof. Notwithstanding this, Sandia Laboratories developed the ‘Solar Glare Hazard Analysis Tool (SGHAT), which became the gold standard of glare assessment for solar panels in the USA and across the Atlantic.
SGHAT used to be freely available, however it is now restricted to government and military users. The SGHAT modelling approach remains the most recognisable and is frequently requested by stakeholders, particularly in the aviation world.
SGHAT approach
The calculation methodology that goes into the SGHAT model is not discussed at length here. What is most important is the metric for deciding what is acceptable and what is not. The model was built specifically for solar developments near airports, with standards set for pilots on final approach within two miles of the runway threshold and personnel in an Air Traffic Control tower.
The two quantities that determine a pass/fail are:
- Retinal Irradiance – which is the intensity of the light in Watts per square centimetre that arrives at the observer’s retina.
- Subtended Source Angle – which is the angular size of the reflection.
These two parameters are combined. A reflection that has a large angular size has a lower acceptable intensity than a reflection with a small angular size. Figure 1 shows how the results are evaluated.
Figure 1: SGHAT glare approach
The calculation considers the direction an observer is facing (designed for approaching pilots – the model assumes that an air traffic controller could be staring straight at the reflector).
There are three zones within the SGHAT approach. Reflections below the green line have a ‘low potential for a temporary after-image’. This is considered acceptable for pilots on final approach.
Reflections between the green line and the red line have the ‘potential to cause a temporary after-image’, while reflections above the red line have the potential to cause ‘permanent retinal damage’. In practice, flat reflectors do not produce reflections above the red line within almost any realistic scenarios.
Current approach
SGHAT itself has been taken away, but all is not lost. The SGHAT methodology is still in use and is available from Pager Power. In addition, it is worthwhile assessing the context of reflections beyond the parameters within the SGHAT model, some of which have been discussed within this article.
This is because other factors can affect the real-world impact, and can improve the likelihood of new developments coexisting safely with existing infrastructure.
If you have any queries or upcoming projects, please feel free to contact the Pager Power team to discuss, or take a look at our glint and glare guidance document.
References
[1] Kandt, A and Romero, R, 2014, Siting Solar Photovoltaics at Airports (Preprint), National Renewable Energy Laboratory
