Mitigating the Impacts of Glint and Glare
The National Renewable Energy Laboratory, supporting the Department of the Navy Renewable Energy Program Office, has developed an innovative glint/glare analysis and visualization methodology to understand and mitigate the possible impacts of light reflecting off solar photovoltaic arrays.
By Michael Hillesheim, Alicen Kandt, and Steven Phillips
The National Renewable Energy Laboratory, supporting the Department of the Navy Renewable Energy Program Office, has developed a glint/glare analysis and visualization methodology to understand and mitigate possible impacts of light reflecting off solar photovoltaic arrays. Above, solar photovoltaic panels
installed on the roof of Hangar 1122 at Naval Air Station Jacksonville, Fla. U.S. NAVY PHOTO BY CLARK PIERCE
The Department of the Navy Renewable Energy Program Office has been tasked by the Secretary of the Navy with bringing 1-GW of renewable energy generation into procurement by the end of 2015. Some of this new, renewable energy generation will come from large-scale, greater than 10-MW, solar photovoltaic (PV) assets deployed on Department of Navy-owned land.
In pursuit of meeting its renewable energy goals, the Department of the Navy has encountered concerns related to siting solar PV systems near installations that support aviation operations. A primary concern is the potential for glint and glare impacting pilot site lines—either at the base or nearby public airports. Glint is a momentary direct reflection of light whereas glare is an indirect reflection of light that can be both larger and of longer duration.
The Renewable Energy Program Office enlisted the National Renewable Energy Laboratory (NREL) to conduct analyses at installations where glint/glare had the potential to impact aviation operations. As part of this effort, NREL developed an innovative visualization methodology that presents results in an easy-to-understand format. Visualization allows stakeholders to make informed decisions about the potential mission impacts caused by installing solar PV systems.
Glint/glare can be caused by many natural and manmade features, including lakes and ponds, buildings, vehicles and snow. Generally, the reflection from PV systems is low intensity, similar to the impact from a body of water. The Federal Aviation Administration has provided interim guidance stating that glint/glare analysis must be completed for all PV systems proposed to be installed within its controlled boundaries. The guidance states PV systems can cause no glare to the air traffic control tower and only glare with a low potential for after-image during the last 2-mi of the standard landing approach (straight-in, 3⁰ glide slope). After-image is what appears in one’s vision after the exposure to the original image has ceased.
The Federal Aviation Administration also requests that the Solar Glare Hazard Analysis Tool (SGHAT) be used to complete any analysis. To constrain the analysis, SGHAT requires information about local flight operations and design parameters of the PV array. These include location, shape, and orientation/tilt. While the Department of Defense has issued guidance recommending the use of SGHAT for solar PV impact analysis on its air operations, it does not specify allowable risk thresholds.
Flight operations are modeled using either observation points or flight-path traces. An observation point assumes a 360° unobstructed view from a vantage point defined by a specified longitude, latitude and altitude. An example of an observation point is an air traffic control tower. Flight paths are a straight line of observation points that restrict the view angle and downward azimuth. Glare occurring behind and/or below the aircraft is not considered in the output. SGHAT also assumes perfect, clear-day conditions, no line-of-sight obstructions, and no use of ocular aids, such as sunglasses, to reduce irradiance to the retina.
Department of the Navy flight operations are not the same as flight operations for commercial air traffic. Flight patterns are much more complex, including more overhead routes. Landing approaches are often non-linear and generally steeper (up to 10°). SGHAT is capable of varying the steepness of descent, but is unable to analyze curved flight patterns and landing approaches. To address these limitations, NREL developed a novel approach to assessing glint/glare while still working within the capability of the tool.
To develop the new approach, NREL worked closely with air operations and air wing personnel at Naval Air Station Meridian, Miss. Aviators and air traffic controllers provided detailed information about flight patterns, landing approaches, altitudes, cockpit visibility, and line-of-site. Based on their input, it was determined that non-linear flight paths could be replicated by a series of observation points in lieu of analyzing segments of flight paths. Both observation points and the minimum and maximum prescribed altitudes would be used to mimic the flight pattern and landing approaches. By doing this, researchers were confident that the results would be comprehensive, and, in all cases, conservative.
Proposed PV system design parameters were assessed to identify the potential impact the arrays could have on the numerous flight patterns and landing approaches for each runway and the control tower. Preliminary SGHAT runs showed that in order to ensure a complete and defensible set of results, more observation points were needed to fill in the gaps. Results also indicated that the average altitude for each flight segment was adequate to ensure robust results. In total, about 300 SGHAT runs were completed, which provided a thorough understanding of potential impacts from the proposed PV systems.
(Top) Location of observation points (represented by lettered dots) used for the left-hand, four-plane carrier break pattern (with and without flaps) landing on Runway 01L at Naval Air Station Meridian, Miss. Blue outline represents proposed photovoltaic system sites. (Bottom) Results of the Solar Glare Hazard Analysis Tool analysis for both the left- and right-hand four-plane carrier break patterns (with and without flaps) landing on Runway 01L. IMAGES BY BILLY ROBERTS, NREL
TRANSLATING DATA INTO DECISIONS
SGHAT results can be difficult to interpret by personnel unfamiliar with the tool. NREL conceived a new way to display the information in an easy-to-understand format. The novel method plotted the outputs along the flight paths, showing pilots where they could expect to encounter glare. This information, in turn, enabled the aviators and base operations personnel to be able to provide constructive feedback concerning system placement and design.
The visual depiction of the analysis proved to be a valuable tool in demonstrating that the base’s proposed PV array could be completed and remain compatible with flight operations.
MEETING MISSION GOALS
NREL has developed a new methodology for analyzing and visualizing complex flight patterns with respect to glint/glare impacts from solar PV systems. The new approach provides conservative, defensible results that can be used as an aid in determining the development pathway for a PV system.
In addition, the visualization methodology packages the SGHAT outputs into an image that is easy to understand and interpret, permitting all stakeholders to feel knowledgeable about the possible glint/glare impacts that a solar PV system could have on flight operations. For the Department of the Navy, leveraging renewable energy is a more than just a goal, it is a strategy. But developing it cannot come at the expense of safety and mission continuity.
[This work was supported by the U.S. Department of the Navy under Contract No. DE-AC36-08-GO28308 with the National Renewable Energy Laboratory.]