Soaring more than 1,200 feet into the sky, supertall skyscrapers are monuments of engineering prowess, from their structural design to their construction. Less well appreciated is the technology that makes them livable: How exactly do you heat and cool a tower that shoots a half mile into the sky?
In a standard high-rise, it’s mostly a matter of scale: With large enough pumps and heat exchangers, a single system with a cooling tower and chiller plant on the roof can service an entire building. But as a structure approaches supertall status, this one-building, one-system approach becomes too expensive, too inefficient, and too bulky.
“Supertalls are like cities standing vertically,” says Mehdi Jalayerian, an executive vice president for Environmental Systems Design, in Chicago, which consults on HVAC design for supertall towers. “The real challenge is how [do you] get environmental controls and amenities [to residents] as you get higher?”
The answer: Creatively.
Take the 121-story, 2,073-foot-tall Shanghai Tower, slated to become the tallest building in China and the second tallest in the world. Rather than think of the building as a single unit, Gensler opted to parcel the structure and install a hybrid cooling system. “There’s a certain threshold where it doesn’t make sense to put [the chiller plant] at the top so you bring it back to the ground, and you start thinking about the building in zones,” says lead project architect Ben Tranel, AIA, a principal based in San Francisco.
Gensler and engineering firm Cosentini Associates, in New York, divided the tower into nine zones, each 12 to 15 floors in height, and fitted it with two chiller plants, one in a sub-basement and the other in a mechanical space spanning the 82nd and 83rd floors. (Each of the nine zones also has its own ventilation system, water heaters, and electrical transformers.) By doing so, the designers eliminated the need for a massive chiller plant in the upper reaches of the building and reduced the load on the pumps at grade.
Zones also allow HVAC systems to operate with much-needed flexibility. Ambient air temperature changes with altitude, and for supertalls, the change can be substantial. At the Shanghai Tower, the air surrounding the upper floors will be as much as 6 F colder than that at grade.
Rearranging the HVAC system isn’t the only way Gensler will keep the tower cool. A cam-shaped exterior with a semi-fritted-glass curtainwall wraps the building, inside of which rise 21 air-conditioned atria, ranging from 10 to 14 floors tall, that feature restaurants and other amenities. The net effect is a blanket of chilled air that reduces the cooling load of the building core, where the hotel and offices are located, and does double duty as a passive cooling element. Thanks in large part to the double skin, the building uses 21 percent less energy than if it had a conventional HVAC system.
Besides air temperature, air pressure can significantly affect comfort inside supertalls. Cold outside air entering through the front door gets heated by a building’s HVAC system, rises through the interior via atria, elevator shafts, stairwells, and chases, and creates a pressure differential known as the stack effect. (In a warm climate, the effect is reversed, with hot air pouring into the upper floors and falling as it is cooled.)
At 10 stories, the stack effect is minimal. But at 120 stories or more, the pressure differential can be massive, creating uneven demand for heating and cooling and forcibly jamming elevator doors shut. The typical solution of using air pressure–preserving features, such as revolving doors, must be taken to an extreme in supertalls: Some structures have vestibules at every stairwell and elevator lobby.
Beyond the specialized technology and equipment, the key to comfort in a
supertall is a holistic design approach that embeds a heating and cooling strategy
at the center of the planning. “Supertalls have to be developed and integrated
as an integral design,” Jalayerian says. “The coordination and organization of
systems have to be hand in hand with the structure from the start.”
For example, Adrian Smith + Gordon Gill Architecture had only a general concept for the 167-story, 3,307-foot-tall Kingdom Tower, in Jeddah, Saudi Arabia, when they first started talking with Jalayerian about cooling strategies. One result of those conversations is the orientation of the tower, which, with wings running to the northeast and northwest, will reduce solar heat gain in the building, set to become the world’s tallest when it is completed in 2019.
All of which means there’s something deeply ironic about mega-structures. Even as fast-growing economies in the Middle East and Asia commission supertall skyscrapers as markers of their newfound wealth, and even as these towers push the laws of physics, their iconic shapes are, to a great extent, defined by something as banal as air conditioning.
Note: Image credits for the Shanghai Tower diagrams have been updated since first publication.