PV reflectivity safe for airports

Glare analysis performed via simulated environment for proposed PV array near airport.

07/06/2011


In April 2010, a 5-MW photovoltaic (PV) array was proposed at a 70-acre site located 1 mile south of Newquay Airport in Cornwall, England. The PV array will have a direct grid connection and export energy to the local utility, Western Power Distribution, and is slated for completion in March 2012. However, airport officials expressed concern about the possibility of sunlight reflecting off the PV panels and creating glare conditions for pilots and air traffic controllers in the tower.

The city council of Cornwall contracted Parsons Brinckerhoff (PB) to conduct a glare analysis.

Creating a geometric model

The PB team analyzed glare conditions quantitatively by generating a simulated environment with 3-D CAD and BIM software. Because the PV system had not yet been designed by a third party, PB’s first task was to estimate the physical size and orientation of the array. The PV designer had decided on a 21-degree tilt-angle fixed toward the south. Given this tilt-angle, PB estimated the physical layout necessary to generate 5 MW and situated it within the proposed boundary.

One significant challenge was simulating sunlight reflections based on the sun’s daily path and its angle changes throughout the year, then incorporating the view from a moving aircraft. The analysis was simplified by selecting static viewer perspectives for the pilot along each runway approach from a specified distance. A third viewer perspective was added for the control tower. Using energy simulation software, the sun’s path through the sky was simulated for one day per month for each month. The project aimed to detect if and when direct glare would be observed from any of the three perspectives. PB generated animations for each perspective for all 12 months to demonstrate the sun’s path and reveal glare occurrences.

To detect glare in the simulations, the PV panels were modeled as mirrors with 100% reflectivity. This was an exercise purely in the geometry between the sun, the PV array, and the viewer perspectives. The model and animations revealed that direct glare would be encountered at one of the viewer perspectives, the approach to runway 30, in the evening during August (see Figure 1). Because the aircraft flies much faster than the sun travels through the sky, the pilot would fly through this glare and experience it only for an instant.

Figure 1: This screenshot shows direct glare from the PV array observed on a Runway 30 approach on an August evening. Courtesy: Parsons Brickerhoff

Analyzing PV reflectivity

Another important factor in determining the effect of glare from PV arrays is to determine the intensity of light reflected from PV panels. While the mirrors used in the computer model had 100% reflectivity, PV panels are observed to reflect very little light.

The Technical Guidance for Evaluating Selected Solar Technologies on Airports report stated that several PV installations at or near airports had not created glare conditions and that PV material only reflected approximately 2% of incident sunlight, which is less than bare soil. Soon after the FAA guidance was published, it was adopted by the U.K. Civil Aviation Authority (CAA). Due to the FAA’s reliable data, PB felt comfortable concluding that even if the geometry did reveal a direct glare occurrence, it would be so minimal that it would not cause a problem.

The short-lived glare and the minimal intensity of light reflected off PV material led the PB team to conclude that glare would not have a negative effect on airport operations.

If glare or reflection from a PV array is a concern, the following items should be included in the project requirements:

  • Rolled glass for diffuse reflection
  • Anti-reflective coating
  • Product demonstration or mock-up arranged with stakeholders.

EXCLUSIVE: Watch this glare simulation video.

Roseman is a supervising electrical engineer and certified project manager with Parsons Brinckerhoff. He is experienced in lighting and power distribution for various types of buildings, parking structures, and streetscapes, as well as aviation lighting. He has been with PB for 12 years.

Click to view a glare simulation clip. Courtesy: Bruce Ross Associates Inc.