Pollution-eating roof tiles could improve air quality

Analysis: ETFE roof may be good choice for Tampa Bay Rays stadium

May 28, 2015

Planners of a new stadium for the Tampa Bay Rays could benefit by looking into some of the stadium roofing technologies used around the world, Stephen Nohlgren writes. A roof made from ethylene tetrafluoroethylene is one option for creating an open-air stadium feel while protecting guests from humidity and the elements. Tiny dots in the roof can help filter sunlight and keep the stadium cooler, he notes. Minor-league baseball can tolerate muggy Florida's open-air stadiums. If rain or lightning wipes out $1 Tuesday, who cares if average attendance slips from 1,300 to 900? Major-league stakes are higher. Forget nostalgic notions about baseball under summer skies. Fans forking out $40 for a ticket and a beer want protection and comfort.

As a result, the Miami Marlins and Tampa Bay Rays play in roofed stadiums with air conditioning, a situation that is neither gratifying nor cheap. St. Petersburg built a cavernous throwback to the domed stadiums of the 1970s. Tropicana Field has climate control but at the price of spartan insularity and an artificial turf that wears down players' knees. Miami opted for a retractable roof, which allows for natural grass and keeps fans comfortable. But that added more than $100 million to construction costs for a roof that stays open an average of 10 games in an 81-game home season. What if there was another way? From a cutting-edge arena in Singapore to a luxury commercial development in Miami to the snowy reaches of Minnesota, architects are experimenting with light and wind to create comfy "micro climates'' inside large public spaces.

Some are cutting costs to boot. These innovations could benefit the Tampa Bay area one day. For all our political angst over locating a new baseball stadium, a tougher issue is to come: how to foot the bill. Less-expensive building techniques that please our senses could work to everyone's benefit, no matter how our protracted stadium debate turns out. Seeing the light Minneapolis has a catchy slogan these days: "Clear is the new retractable.'' The city decided to replace its aging domed stadium used by the NFL's Vikings after snow caused the roof to collapse. The new venue, now under construction, features a fixed, sloping roof that wards off the elements while letting light pass through. The roof is made from ethylene tetrafluoroethylene, an intriguing polymer known as ETFE. It is stronger than glass but 100 times lighter. It stretches like a rubber band under pressure, such as from wind or piles of snow. Most important, manufacturers can embed ETFE with tiny dots that filter sunlight and lower temperatures.

The result shades fans but still lets them see sky, clouds and outside surroundings. "It's kind of a Minnesota version of an open-air stadium,'' says Michele Kelm-Helgen, chairwoman of the Minnesota Sports Facilities Authority. "We looked at retractable roofs, but we were concerned that given our climate, we may not have cause to open the roof that much." A glass wall at one end of the roof has 100-foot pivoting doors that let in fresh air and breeze. "Even in the wintertime, you feel like you are outside, but you will be warm,'' Kelm-Helgen says.

Bruce Wright, a University of Minnesota professor who specializes in lightweight building materials, compared the effect with that of a city bus covered with a thin sheet of advertising wrap. "People cannot see in, and you would think that inside, it would be like being in a tunnel," Wright says. "But from the inside, you can see out very clearly. It blocks the sun but does not block your view.'' ETFE construction typically features at least two membranes separated by air pockets a foot deep or more that create an insulating pillow effect. Sometimes a middle layer is added, Wright says, with air pressure valves that move it up or down, allowing computers to alter the degree of shade as the sun moves across the sky. Light patterns reverse at night. Passersby can peer into the stadium for an iconic view of a huge community gathering. Fans see a dark roof but can still glimpse the downtown skyline through the glass wall.

Keeping cool

When the Rays proposed a new stadium on St. Petersburg's waterfront seven years ago, the site was too hemmed in for a retractable roof. Shade and protection would have come from a "sail," a moveable canopy that stretched out to a huge tower beyond the outfield. The project tanked for many reasons, including skepticism that sea breezes could keep fans comfortable. Now a $1 billion development just south of the Miami River is taking the sail concept a step further. Brickell City Centre, three city blocks from Biscayne Bay, is marketing jet-set luxury. Offices, condos, restaurants and a shopping mall will be connected by a "Climate Ribbon,'' a covering of undulating fabric and glass that will shade patrons, channel rainwater into cisterns and create a mini wind tunnel underneath. Strategically placed angles and louvers will capture and funnel even the slightest breeze off the water, says Chris Gondolfo, vice president of Swire Properties, the developer. "It's like older homes with a chimney,'' Gondolfo says. "There is a constant flushing of airflow through something. It's something like a 6- to 8-knot breeze even on the stillest day.'' Part of the Brickell pitch is outdoor dining in August. "People want that experience of being outside and not having the mechanical comfort associated with air conditioning,'' says Michael Soligo, CEO of RWDI, the wind consultant on the Climate Ribbon. A stadium with a clear roof could also channel wind, says Soligo, who has worked on dozens of stadium projects, including the sail. "ETFE is a good material. If you understand where the air comes in and where to force it out and how to keep the air moving, you are not going to create a greenhouse,'' says Soligo. "If people really want an open-air feeling, you can do a lot for them.'' Singapore, about 85 miles from the equator, has humidity you can slurp. Yet one end of its new 55,000-seat National Stadium remains open to the city even when its retractable ETFE roof closes for games. Rather than cooling the entire stadium, designers installed air-conditioning vents under each seat. A digital ticketing system turns them on only when people are sitting in the seats. Cool air flows down to the playing field so competitors don't get heat stroke. As air eventually warms, it rises and escapes through vents in the roof. Energy costs are 60 percent below those of conventional methods.

Dollars and cents

Major League Baseball's two newest stadiums offer a sense of how much a retractable roof adds to construction costs. Target Field, an open-air stadium in Minneapolis for the Minnesota Twins that opened in 2010, cost around $400 million, not counting land or infrastructure. Two years later, Marlins Park came in at $520 million. The stadiums, both designed by Populous, are similar in size and configuration. The main difference is the retractable roof. When Tampa Bay area residents speculate about the cost of a new Rays stadium, the Marlins' $520 million is often tossed around as a minimum starting point. A lighter, fixed roof might bring that cost down, says Mike Wekesser, lead designer on Target Field and now sports design director of the architectural firm AECOM. Retractable-roof designs create lots of weight with the roof itself, the trusses the roof moves on and the mechanisms that propel it, requiring heftier supporting columns and foundations. "Miami is an enclosed stadium with a hard deck and (is) air-conditioned. It is designed that way,'' Wekesser says. "You could bring down the cost — I don't know by how much — by using lighter material. You would bring down the tonnage of steel, and in Tampa Bay, concrete. It is one of the biggest costs in any stadium.'' Brickell City Centre's Climate Ribbon will cost $30 million and cover 150,000 square feet, which makes it about half as big as the Marlins Park roof for one-quarter the cost. Wekesser cautions against simple comparisons. Baseball stadiums have quirky shapes with long trusses to support any roof, he says. Some interior spaces, such as luxury suites, always need AC. Admitting enough light to support natural grass could conflict with providing enough shade for fans. And in Florida, hurricane standards come into play. Still, "I think you can find a way to put a skin on a new ballpark that does not have to be retractable,'' Wekesser says. "The roof could be more like a canopy or umbrella, with side ventilation so air could pass through. The edges could be opaque enough to get shade. It could have that open-air feel of old-time baseball. "Kind of like sunglasses, but very light sunglasses.''


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