Earlier in March, the Airports Authority of India (AAI) announced that it will use 100 percent renewable energy at all its airports by 2024. AAI is India’s public sector airport developer and operator. Of the 137 airports it manages, the statutory body has, already commissioned in-house solar plants at 38 airports with a capacity of 40 megawatt peak. In 2015, the Cochin International Airport in Kerala became the first airport in the world to go completely solar by installing a 12 megawatt peak plant on 45 acres of land.

With solar panels becoming cheaper, airports saw an opportunity in harnessing the sun’s energy, while reducing their electricity bill, as well as carbon emissions. The annual electricity consumption of airports in India in 2019-2020 was 884 million units. The worldwide commercial aviation industry emits three percent of the total global greenhouse gases.

“Airports hold significant potential for solar power generation owing to availability of large, flat and shadow-free chunks of rooftops at terminals, hangars and car parks, along with buffer land around the runways. Typically, these lands are unsuitable for other activities due to regulatory requirements,” said Sustainable Green Airports Mission (SUGAM), a booklet released by the AAI in June.

However, glare from solar panels can pose a risk of accidents. The glass on the panels tends to reflect light which, even if it lasts a few minutes, can block a pilot’s vision during landing and take-off, and also the vision of the staff at the air traffic control (ATC) tower. The phenomenon is akin to how the vision of a person driving a car at sunrise or sunset in the direction of the sun is temporarily impaired.

Glare hazard was first highlighted in the United States in 2012, when air traffic controllers at the Manchester Boston Regional Airport complained that they could not see properly due to the reflection from solar panels. It is reported that solar PV panels worth $3.5 million had to be covered with tarp temporarily to avoid the glare. In 2013, pilots flying near the Ivanpah Solar Electric Generating System in California complained about glare from the facility. “Pilots described the glare as ‘blinding’, and at least one individual reported that the glare was ‘like looking into the sun’,” said a 2015 study by the Federal Aviation Administration (FAA) of the United States.

“The issue of glare can be crucial in India, given the high solar energy potential and the widespread adoption of solar installations,” said Sudhakar Kumarasamy, senior lecturer at the Faculty of Mechanical and Automotive Engineering Technology, Universiti Malaysia Pahang. “Geographical conditions such as clear skies and intense sunlight can exacerbate glare-related concerns, making effective glare analysis and mitigation strategies important.”

ATC needs clear vision, solar glare can hamper the vision

Even as solar panels are designed to absorb light, the top glass surface can reflect light, causing discomfort, retinal burn, veiling and after-image effects. An after-image is the one that continues to appear in the eye after a period of exposure to the original image, hampering vision. The veiling effect can be equated with masking of objects on the ground because of glare.

The problem for the ATC occurs mainly in the morning and evening, when the sun is at a lower elevation. When sunlight strikes the panel from a lower angle, the reflection will also have a lower angle. But when the sun moves higher up in the sky, the angle of reflection would be high too, making the reflection cross over the ATC, thereby not impacting it. For the aircrafts however, glare can happen at any time of the day, depending on their location, but is crucial during the critical phases of the flight- landing and take-off, when the aircraft is at a low height.

Studies show that if solar insolation (solar radiation on a given surface over a specific time period) of around seven watt per square metre enters the naked eye, it can cause an after-image that lasts from four to twelve seconds. Solar glare’s impact on the eye is categorised as green (low potential for an after-image), yellow (temporary potential to cause after image) and red (potential to cause retinal burning). Since PV panels do not focus reflected light, the possibility of a retinal burn is low. “A green glare is harmless for pilots as it causes a momentary glint,” said Gurpreet Singh Walia, founding director of Green Ops, Ahmedabad, a solar consulting firm that conducts glare analysis for airport solar projects. “But a yellow glare can disturb their concentration. However, both green and yellow glare are not safe at the ATC tower.”

An AAI official, who was posted at the Ahmedabad airport in 2016 when the airport went solar, said, on the condition of anonymity, that the ATC needs an absolute clear line of vision. “If they complain of glare, we immediately ground the module (removing the solar panels causing the glare).”

In India and the rest of the Northern hemisphere, panels usually face south to track the sun’s path and generate maximum electricity. “Generally, the potential for glare occurrence in south-facing modules is more likely to reflect sunlight in a way that aligns with the path of pilots, increasing the risk of glare-related issues, but the specific link between the two also depends on other factors,” explained Kumarasamy. Other factors affecting glare include the sun’s position, tilt angle, surface texture, colour and location of the PV modules.

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Regulatory scenario

Countries such as the USA, Germany, United Kingdom, Switzerland, France and Austria have their own guidelines for glare analysis. However, the interim 2013 policy by the US FAA on glare was widely followed, now superceded by a final policy that came in 2021. The 2013 policy prescribed using the Solar Glare Hazard Analysis Tool (SGHAT), a web-based simulation software to assess glint and glare for all hours of the day on all days of the year and even provide an estimation of power that can be produced from a solar plant in a particular location.

The tool is developed by Sandia National Laboratories and is available commercially in the form of Forge Solar. “Forge Solar is commonly used in India to conduct glare analysis,” informed Walia. The software simulates the presence of green, yellow and red glare when details such as ATC tower and runway coordinates, solar installation coordinates like height, tilt and azimuth angles and building height in case of roof-top projects are fed in. However, many commercial glare analysis simulation tools are available in the market now.

“SGHAT’s efficiency in real-time situations can be influenced by factors like the accuracy of input data, the complexity of the airport landscape environment and the computational resources available,” said Kumarasamy who compared glare assessment techniques in one of his research papers. “The simulation may not capture every dynamic condition accurately and might require simplifications that could affect its real-time applicability. The limitations of SGHAT include the assumption of clear skies, a simplified representation of glare-sensitive areas and the inability to model certain complex reflections accurately.”

India does not have a glare assessment policy of its own, but the Directorate General of Civil Aviation (DGCA) requires airport operators to conduct glare analysis before installing solar projects at airports. “Glare issues within the operational area/close vicinity of the airports are regulated by DGCA for safe aircraft operation,” J.B. Singh, general manager, Corporate Communications, AAI, told Mongabay-India, in an email response. “Accordingly, before taking up the solar projects in airport premises, the DGCA approval is taken.”

Air side or operational area is the part of the airport beyond the gates – runways, taxiways and ATC, where visibility is critical. Cochin International Airport that has the biggest airport solar facility in India, does not have any panels in their operational area. “Shoulders (areas around the runways) are required to be free of any obstruction as the aircraft can go off the runway, which is why even drains along the runways are covered,” a pilot, who frequently flies to Cochin airport, said, on conditions of anonymity. “Ideally, no solar panels should be placed there.”

Another AAI official posted at Leh, who did not wish to be named, said that to be on the safe side, they prefer installing solar panels on roof-top or parking areas rather than operational areas. “But at airports that have paucity of land, the operational area is used for solar.”  The Delhi airport was the first to install solar in its operational area and has not faced any problem till date, said an email response from Delhi Airport Infrastructure Limited, the company that manages Delhi Airport. Kolkata airport too has a 15 megawatt peakground mounted solar plant in 67.5 acres of its operational area.

Ratheesh R. Nair, a solar feasibility expert, said that the mandate from DGCA is to assess glare over ATC towers and pilots because those are the places, where the naked eye vision is required. “With the coming of digital ATCs and ‘Instrumental’ landing systems, naked eye vision from both these places will be rendered redundant. There will be no ATC towers and monitoring can happen from anywhere. Glare will not be an issue then.” said Nair.

Ground reality

Besides a comprehensive glare analysis, common measures to mitigate glare include tweaking panel orientation and tilt (to minimise glare angles towards crucial lines of sight), anti-reflective coating (ARC) on glass or textured glass and physical barriers between solar panels and operational areas to block the line of sight.

A study by Seoul National University of Science and Technology, South Korea, found that if solar panels face in the opposite direction from the runways and potential flight paths, they would reflect the sun’s rays outside the aircraft route. “An airport is an almost open space without shading, the sacrifice corresponding to the annual energy generation (even if panels are not southwards) for solar glare free conditions is less than 6%,” the study says.

According to Swapnil Walunj, head of marketing and strategy at Borosil Renewables Ltd., a glass manufacturing company, nearly 98 percent of the solar panels in the market have an anti-reflective coating, but it does not serve anti-glare properties. In 2019, Borosil launched ‘Selene’ an anti-glare solar glass, suitable for PV installations near airports. “As compared to regular solar glass (which is generally coated with anti-reflective coating), the anti-glare glass has a specially designed texture that diffuses the reflected light. The resultant image or reflection is not sharp, but incoherent, unlike the glare from a smooth surface. Adding texture to a solar panel diffuses sunlight and is an economical method to deal with glare from the panels,” Mr. Walunj shared. The product, however, did not make a strong headway in the Indian market.

“Except for an 8 MW plant at Karnataka’s Hubbali airport, anti-glare glass has not been used anywhere in India. We knew ‘Selene’ would have a niche market, but went ahead with this product looking at the safety perspective. We export it now,” he said.

“Not mandating the use of anti-glare glass can be a safety hazard. France has a clearly defined standard from the perspective of glare, for the use of solar modules near airports,” he added.

Even as it is the airport operator’s responsibility to conduct glare analysis, sources at AAI, who did not wish to be named, confirmed that the job is handed over to the solar Engineering, Procurement and Construction Agency (EPC) contractor designated to install solar at the airport. “It is in the EPC’s scope of work to conduct the glare analysis and then have it approved by AAI’s operations department,” said an AAI official, who was posted at Chennai airport when it went solar in 2016.

On the other hand, for EPC contractors, glare analysis is a mere approval to get the project started. “Glare analysis is a theoretical exercise and not really important in my opinion,” said an Ahmedabad-based solar developer. Meanwhile, Walia insists that glare hazards need to be studied at the feasibility stage and not at the stage of execution, which is when the EPC comes in.

Mongabay-India reached out to Vikram Solar (a solar panel manufacturing company that has installed plants at Cochin, Kolkata and four other Indian airports), to understand their experience in installing solar panels at airports and their glare analysis, but they declined to comment.

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Banner image: Solar panels at the Narita Airport in Japan. Representative image. Airports that harness solar energy face a challenge as glare from solar panels can cause flash blindness and obstruct view for the pilots. Photo by Takashi M/Flickr.

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