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TERMODYNAMICS Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/142/03/43/6493771/me-2020-mar2.pdf by guest on 24 September 2021 Termites have taught architects and engineers a lot about heating and cooling buildings—even if humans get it wrong sometimes. BY MARK WOLVERTON e humans like to congratulate ourselves for Pearce had grown up in Zimbabwe and was familiar with our ingenuity. Yet nature’s passive designs such chimneys. They rose six to 30 feet above termite nests often outperform our expensive, energy- and were strong enough for elephants to use to scratch W hungry technologies. And while engineers their flanks. and architects can improve their designs by mimicking the Below the chimneys, the termites farmed fungi for food. natural world, nature always has another lesson to teach. While the termites relied on soil’s thermal storage capacity That has certainly been the case for termites and air to help keep temperatures stable, Pearce realized that the conditioning. termites also had to breathe. The story begins in 1992, when Zimbabwean architect The nest has an elegant design. The hot air from the Mick Pearce received a commission to build Eastgate nest and fungus farm, which had higher concentrations of Centre, a two-building office complex and shopping mall carbon dioxide and methane, exited through the chimney in the country’s capital city of Harare. Pearce, however, by means of convection. As the hot air left, it pulled in fresh wanted to do more than just build a new building. He air from the surface through moist foraging side tunnels, wanted to eliminate the huge heating and cooling plants a which added water vapor to the stream. The chimney, 340,000 square feet development typically needed. warmed by the sun, heated the air exiting through it, Pearce already had a solution in mind. He began to for- adding an extra push to the convection cycle. mulate it while watching BBC’s natural history series, Life, “The termite nest was warmer than the outside, par- on TV one evening. ticularly at night,” Pearce said in an interview. “You get “I saw David Attenborough climbing inside the chimney air rising out of the chimney, and that pulls air in from the of a termite nest in Nigeria,” he said, and he realized that holes they go out to forage. That’s how they breathe.” evolution had already solved the problem. If termites could do it, Pearce thought, why not humans? MECHANICAL ENGINEERING | MARCH 2020 | P.43 Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/142/03/43/6493771/me-2020-mar2.pdf by guest on 24 September 2021 Termites may have brains the size of a grain of sand, but they can school engineers and architects about heating and cooling. Photo: Getty Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/142/03/43/6493771/me-2020-mar2.pdf by guest on 24 September 2021 Termite-inspired Eastgate Centre in Harare, Zimbabwe, uses convection cooling to remove hot air through chimneys located on the top of the building. Photo: David Brazier via Wikimedia Commons BUILDING EASTGATE million in energy costs during its first five years. Pearce had one thing going for him: Although Zimbabwe Of course, office temperatures were not as tightly con- is semitropical, Harare’s 4,800-foot elevation is cooler and trolled as conventional buildings. Most of the time, they provides an average temperature swing of about 10 degrees hover in the mid-70s °F range, but they may reach 80 °F to Celsius each day. Pearce figured he could pull in night air to 82 °F degrees at the end of the day for several weeks each cool concrete and masonry structures. They would act like year. This led Pearce and Ove Arup to improve the control the soil in a termite nest to store cooling. He could then system. chill incoming air by running it over those structures during “The trick is to coordinate the weather outside with the the day. controlled internal environment, taking into account lag In the Eastgate Centre that Pearce and multidisciplinary times,” he said. “It’s like tuning an instrument, like an organ engineering firm Arup Group designed, each floor had air in a church.” ducts running underneath it. These ducts contained con- Pearce was a pioneer of biomimicry, which draws inspi- crete blocks with protruding teeth, whose high surface area ration from the natural world for human design. He went enabled them to cool down and heat up efficiently. on to apply his approach to a more efficient office block in Banks of fans drove the cool air through the shopping area Melbourne, Australia, and build a tower and dining hall in and seven floors of offices. As the air warmed, it rose and China. exited through a central atrium connecting the two towers “I was convinced that by looking at animal architecture, and 48 brick chimneys that ran along their roofs. there are important clues for an architecture which inspires Pearce’s designers also used several other tricks inspired the design team to make buildings that follow natural pro- by nature. Office windows, for example, were shaded by cesses, cycles, and systems,” he said. overhangs and plants to keep sunlight from heating the Yet despite his successes, Pearce did not learn everything building too quickly. The exterior featured jagged gabbles termites had to teach him. and facades that act like cactus spikes, whose high surface areas disperse heat rapidly at night. BEYOND BIOMIMICRY On many levels, it worked spectacularly well. Eastgate Since Eastgate, scientists have learned much more about cost 10 percent less to build than a similarly sized building how mound-building termites (and other animals) work with air conditioning. The building also used 35 percent less their thermodynamic magic. This has led to a new approach energy than comparable buildings in Harare and saved $3.5 that physiologist J. Scott Turner and architect Rupert C. Soar MECHANICAL ENGINEERING | MARCH 2020 | P.45 call “beyond biomimicry.” They want to create “buildings a large central atrium that could draw air passively through that are not simply inspired by life,” they wrote in a 2008 the building. What is often not mentioned is the clever use paper, “but that are, in a sense, as alive as their inhabitants.” of large heat storage blocks that could damp temperatures New research techniques that reveal the unseen intrica- through the hot day, while using high-powered fans to cies of natural structures make this goal possible. ‘empty’ those heat stores during the cool night. Termites One example involves the use of 3D X-ray tomographic do not have the latter. Termites do not use this to maintain imaging. Kamaljit Singh of the Department of Earth Science temperature.” Downloaded from http://asmedigitalcollection.asme.org/memagazineselect/article-pdf/142/03/43/6493771/me-2020-mar2.pdf by guest on 24 September 2021 at Imperial College London and his colleagues used the So if the Eastgate model isn’t quite right, what is? technique to image sections of termite nests from Senegal Scientists are discovering that termite heating and cooling and Guinea. This provided unprecedented details of the nest is far more subtle and complex—especially because different wall microstructure. termite species use different techniques, depending on their While both nests were built by the same species, Trinervi- individual environments and traits. termes geminatus, they used very different construction The species that inspired Mick Pearce and his Eastgate materials, depending on the local environment. Yet both sets Centre design farms fungus, and the termites must remove of walls showed similar—and surprising—features. the carbon dioxide the fungus generates so they do not All the walls contained an intricate network of micro- suffocate. That problem is solved by the large chimney pores, with smaller pores deep inside the nest transitioning structures. to larger pores near on outer walls. This network facilitates Other species, such as the one studied by Singh, build the exchange of carbon dioxide with the outside and drains closed nests with no ventilation to the outside world. rainwater from the nest by capillary action. Instead of chimneys, they use the micropore structure to let When the researchers used their 3D tomographic images the nest “breathe” like a lung. to simulate these processes, they found they operated much like gaseous diffusion within human lungs. “BREATHING” MOUNDS The work demonstrates that Pearce’s Eastgate design, as While it’s been known for some time that termites had successful as it is, got it wrong. developed some sort of mechanisms for controlling the “Eastgate Centre was built on a wrong conception of internal environment of their nests, researchers disagreed how termites regulate the on the details. temperature of their nests,” Termites obviously Natural ventilation for Chimneys direct hot air out of the said Singh’s co-author, Guy building, hot air could be used for used convective process high-rise buildings energy production if, for example, Theraulaz of the University (termite model) vertical axis wind turbines or sterling to circulate heat and air, engines are mounted on the chimneys. of Toulouse. “Still, Eastgate but what drove it? Did the works.” - warm air wind create differences in “It is appropriate to speak - cool air Vegetation, reduces air pressure between the sunlight heating. of [Eastgate] as ‘termite- nest interior and exterior, inspired’ design,” added inducing a Venturi flow of Turner, who teaches at the air? Was it an internal meta- SUNY College of Environ- bolic process? mental Science and Forestry. It was challenging to “The architect, Mick Pearce, pin down the mechanism drew more on inspiration without complete or direct rather than knowledge of measurements.

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