Seeking a More Effective Natural Light Future for Underground Transit
A team of WSP USA lighting experts explore ways to introduce core sunlighting systems into one of the last frontiers for the natural lighting technology.
Tuesday, March 21, 2023
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Darkness, unhealthy conditions and a general disconnect from life above ground have long been associated with underground transportation.
Natural light, conversely, is associated with several health benefits, including increased energy and vitamin D levels, lowering blood pressure, killing bacteria and uplifting your mood.
The history of developments to maximize the benefits and sustainable use of core sunlighting systems to deliver natural lighting solutions within buildings — especially when conventional methods aren’t possible — stretches back more than five decades. This served as inspiration for a team of lighting experts at WSP USA who wanted to investigate the potential for bringing the benefits of this design approach to transit station platforms located deep underground.
The work was part of a WSP-funded Research and Innovation Fellowship aimed at identifying a Future Ready® initiative that could benefit from the research.
Cutaway 3D sectional study of a path to a platform.
Benefits and Challenges of Natural Light
Natural light is especially beneficial for transit workers who spend most of their daylight hours working in underground transit. In addition to that, sunlight is also free, so daylight harvesting is a sustainable resource that can reduce the cost and need for electric lighting.
While efforts have been made in recent years to introduce natural light into designs for newer stations, success was largely limited to structures only one-story deep. For this research project, the WSP team sought to build on existing knowledge by exploring new techniques for bringing daylight deeper below grade.
“Maximizing the use of natural light resources can be one of the most difficult challenges in light design,” said Jeffrey Berg, WSP’s lead researcher and lighting designer for the team.
While original experimenting with sunlighting was done mostly with mirrors, the use of films has made more sunlighting solutions possible.
“The problem was that it’s not only difficult to diffuse light, but also to get it to turn around corners and get it evenly distributed,” Berg said. “It’s also difficult to control glare with mirrors only. The solutions that work best seem to be hybrid systems that use films to diffuse the light and help distribute it.”
A three-dimensional view of the core sunlighting model at the platform level.
Seeking a Viable Solution
While there are basic daylighting strategies that aren’t very costly, a project like this is different, said Elliot Glassman, computational modeling and daylighting specialist in WSP’s built ecology group.
“You start to get into more costly solutions when there are specific constraints,” Glassman said. “In a transit station, you're underground, so it's not like you could have access to sidelighting from a window,” noting that it’s typically easier to design for new construction than it is to retrofit an existing structure for ideal daylight performance.
In order to understand all of the challenges that need to be addressed with an underground station that is largely obstructed by sunlight, the team studied an existing high-volume transit station, utilizing models, CADD files, report graphics, architectural plans and onsite visits to investigate potential paths for collector light tubes from the street level or above, and down to the platform level, about 50 feet underground.
The most time-consuming part of the study was the modeling and collection of key data, which included a sensitivity analysis that examined a variety of sunlight collectors at key dates and times throughout the year. The team also analyzed the built form that their solution would take at every single point along the path to the transit platform.
One additional need for the work to be useful is fire resistance.
The proposed path of the vertical duct, in the design, extends about 80 feet from above the roof of the station, then through the passenger concourses and to the platforms. Because it passes through key structural elements and spaces, a fire-rated separation system — like fire-rated glass or a gypsum board wrap system — that enables light to continue to pass through the tube while maintaining required fire separation, is recommended.
The effect of core sunlighting alone is represented by this platform rendering.
The Promise of New and Existing Solutions
Notably, the research on this project included a specific focus on developing ways to connect existing lighting products, which could potentially lead to new solutions or, even, enhancements to options that already exist. There were also some modeling limitations to overcome.
In their simulation of the path of beamed sunlight to a platform, the team modeled a sunlight collector with flat rectangular mirrors to direct the natural light down a mirrored tube and, from there, along a path to the transit platform level. The sunlight would then be distributed to a light pipe, which can evenly distribute illumination with a special optical film or combination of films.
While light pipes are a proven technology, they are designed around an artificial light source — not sunlight. So, the next challenge was to devise a custom-built optical coupling that would be able to direct beamed sunlight — at a suitable angle and beam spread — that would work within the light pipe’s optics. It was at this stage that the team discovered their envisioned collector could be replaced, instead, with a standard product.
Takeaways and Opportunities
“I was a little naive about what it would take to distribute sunlight,” Berg said. “I thought it would be simple to distribute it horizontally through that light pipe but, in fact, it's pretty difficult. The good news is we've inspired product development for creation of the missing piece and that’s very encouraging.”
While it’s still in development, the team believes this will be something that makes their sunlighting solution for underground transit a reality, enabling its use within transit projects worldwide.
“This was a great opportunity to really dig deep into something we might not have had the luxury to do otherwise,” Glassman said, adding that this work has already created both short- and long-term benefits for WSP.
“In addition to Jeff's desire to see this become a product, it has also increased our familiarity with modeling the light tube systems, and that has contributed immediate impact on some of our other projects that we are exploring this daylighting strategy on,” Glassman added.
Berg and Glassman were grateful for the opportunity to pursue this innovation through WSP’s fellowship program.
“It’s something I don’t know that I would have done, even practicing independently,” Berg said. “You have to set aside the time and the budget and, I think the company and our supervisors were great about making this possible.”
“It was something we started off thinking would be relatively straightforward and then we delved into the complexity of it,” Glassman added. “But, because of that complexity, we really learned a lot.”
On top of that, the results of their research are propelling the market to make product improvements.