Airline passengers would never want their pilot to be blinded by reflected light as the landing aircraft is approaching the runway. The same concern would apply to air traffic controllers in their tower, directing traffic across an entire airport and in the sky around it. Vision is essential to safety, and unexpected glare can take that away.
While urban legends stress the dangers of laser pointers, solar photovoltaic (PV) arrays can unintentionally pose a more common, persistent and significant safety threat.
Solar energy production has a key role to play in a decarbonized energy economy, but one frequently overlooked aspect of these installations is the impact of the large flat pieces of glass in PV modules reflecting sunlight on their surroundings. One common misconception is that modules with antireflective coating would not have this issue. That coating’s primary purpose, however, is to improve module efficiency; it can actually worsen the glare impact on the surrounding area by dispersing the reflected light over a larger area, which in turn takes up more of an observer’s field of view.
As more solar projects are developed in increasingly urban environments, the overall issue of glare is gaining attention. The most notable codification of these concerns to date has been regarding the effects of reflected light on airport operations.
Mandating Analysis
Solar projects located on or within close proximity of airport property are subject to Federal Aviation Administration (FAA) regulations to mitigate any adverse impacts on pilots and air traffic control towers. Those regulations require a glare analysis, with results to be submitted to the FAA.
The regulations were inspired by an unfortunate situation that played out at Manchester-Boston Regional Airport in New Hampshire. After a project put solar panels atop an airport parking garage, authorities were surprised to find light being reflected into the air traffic control tower. The airport ultimately put tarps over the panels because they were preventing the controllers from doing their work safely. In hindsight, the problem seems obvious, but it simply had not occurred to anyone before then.
One catch to the FAA’s parameters: There is no precise definition of what project size or how close is close enough to call for the required study. Within five miles of an airport has emerged as a good rule of thumb to consider the impact of glare, though distance and the size of the installation are somewhat correlated. The bigger the array, the farther it can be from the airfield and still trigger the FAA-required glare analysis.
When a study is needed, there is one highly specialized, commercially available tool. That product by ForgeSolar utilizes the underlying Solar Glare Hazard Analysis Tool that the FAA requires and developed in conjunction with Sandia National Laboratories to assess glare. A properly trained glare specialist can typically run the analysis within a day and obtain preliminary results. If engaged early enough in a project, this can help guide design and technology decisions and avoid costly changes and rework.
Mitigating the Risks
In the event a glare study does identify significant impacts from PV glare, solar project developers do have options to mitigate the risk. The first is to select a new location for the arrays that is farther away from runways and airport traffic control towers. Naturally, this is not a popular choice.
A second option is to alter the choice of tracking technology. Typical utility-scale solar PV farms are built using single-axis tracking with backtracking, enabling the panels to rotate during the day and follow the sun through the sky while reducing row-to-row shading at dawn and dusk. Unfortunately, the increased production from backtracking algorithms, which are increasingly being utilized in single-axis tracking installations, also positions the modules to reflect more glare into the surrounding area with an increased incident angle of reflection during those hours. There are numerous tracking considerations and scenarios that factor into a project’s development, but the selection and control of the technology do offer some possibilities for reducing impacts, depending on the position of the solar PV farm in relation to the airport.
The third option is called suboptimal positioning. Fixed-tilt arrays in North America are generally faced due south, with the north edge tilted up to maximize solar exposure. By tilting the panels a few degrees east or west, it is possible to mitigate some glare at the cost of a portion of annual energy production. Similarly, panels with single-axis tracking that are tilted just a few degrees east or west can reduce glare at the price of some annual energy production.
By sacrificing perhaps 5%-10% of annual energy production with suboptimal positioning, it might be possible to achieve FAA compliance without changing the project location or tracking technology utilized. No one wants to sacrifice performance, but that may be preferable to accepting that a project cannot be built on the intended site. An optimization analysis can go deeper than the glare study, identifying at what point an installation would be compliant if some operational parameters were adjusted and what the anticipated impact on the annual energy production would be.
Withstanding the Glare
The potential of solar power is helping drive rapid growth in installations. As remote greenfield sites become harder to secure, these installations will increasingly encroach upon population centers. Airports have been among the first to discover the risks of reflected light, but they are not alone.
Other ground-level observers, such as residential developers or roadway planners, may raise objections to glare from solar panels. Solar project developers need to be aware of their options. Glare and optimization analyses can help in identifying and mitigating impacts, but finding acceptable and allowable parameters for surrounding-area impacts is heavily dependent upon the local authorities having jurisdiction. Unlike the FAA regulations, these localized scenarios are not uniform and are rarely codified as of yet.
Having an integrated engineer-procure-construct (EPC) partner working on a solar installation can position projects for success by coordinating permitting measures and identifying risks such as glare early in the process, when it is easier and less costly to make any necessary adjustments.
Interest in solar is high, but the changing marketplace is complicating the development of utility-scale solar farms. Having an integrated EPC contractor can help avoid common pitfalls in solar construction projects.