Video: Insect architecture
IN THE heart of Africa's savannah lies a city that is a model of sustainable development. Its buttressed towers are built entirely from natural, biodegradable materials. Its inhabitants live and work in quarters that are air-conditioned and humidity-regulated, without consuming a single watt of electricity. Water comes from wells that dip deep into the earth, and food is cultivated self-sufficiently in gardens within its walls. This metropolis is not just eco-friendly: with its curved walls and graceful arches, it is rather beautiful too.
This is no human city, of course. It is a termite mound.
Unlike termites and other nest-building insects, we humans pay little attention to making buildings fit for their environments. "We can develop absurd architectural ideas without the punishment of natural selection," says architect Juhani Pallasmaa of the Helsinki University of Technology in Finland. As we wake up to climate change and resource depletion, though, interest in how insects manage their built environments is reawakening. It appears we have a lot to learn.
"The building mechanisms and the design principles that make the properties of insect nests possible aren't well understood," says Guy Théraulaz of the CNRS Research Centre on Animal Cognition in Toulouse, France. That's not for want of trying. Research into termite mounds kicked off in the 1960s, when Swiss entomologist Martin Lüscher made trailblazing studies of nests created by termites of the genus Macrotermes on the plains of southern Africa. It was he who suggested the chaotic-looking mounds were in fact exquisitely engineered eco-constructions.
Specifically, he proposed an intimate connection between how the mounds are built and what the termites eat. Macrotermes species live on cellulose, a constituent of plant matter that humans can't digest. In fact, neither can termites. They get round this by cultivating gardens of fungi that turn wood into digestible nutrients.
These fungus gardens must be well ventilated, and their temperature and humidity closely controlled - no mean feat in the tropical climates in which termites live. In Lüscher's picture, heat from the fungi's metabolism and the termites' bodies causes stagnant air laden with carbon dioxide to rise up a central chimney. From there it fans out through the porous walls of the mound, while new air is sucked in at the base.
So simple and appealing was this idea that it spawned at least one artificial imitation: the Eastgate Centre in Harare, Zimbabwe, designed by architect Mick Pearce. Opened in 1996, it boasts a termite-inspired ventilation and cooling system. Or at least it was thought to. It turns out, however, that few if any termite mounds work this way.
Keeping the temperature and humidity within termite mounds constant while at the same time getting rid of CO2 demands a very efficient process of gas exchange. A typical mound with about 2 million inhabitants needs to "breathe" about 1000 litres of fresh air each day. To investigate further what might drive such an exchange, Scott Turner, a termite expert at The State University of New York in Syracuse, and Rupert Soar of Freeform Engineering in Nottingham, UK, looked into the design principles of Macrotermes mounds in Namibia. They found that the mounds' walls are warmer than the central nest, which rules out the kind of buoyant outward flow of CO2-rich air proposed by Lüscher. Indeed, injecting a tracer gas into the mound showed little evidence of steady, convective air circulation.
Turner and Soar believe that termite mounds instead tap turbulence in the gusts of wind that hit them. A single breath of wind contains small eddies and currents that vary in speed and direction with different frequencies. The outer walls of the mounds are built to allow only eddies changing with low frequencies to penetrate deep within them. As the range of frequencies in the wind changes from gust to gust, the boundary between the stale air in the nest and the fresh air from outside moves about within the mounds' walls, allowing the two bodies of air to be exchanged. In essence, the mound functions as a giant lung.
This is very different to the way ventilation works in modern human buildings. Here, fresh air is blown in through vents to flush stale air out. Turner thinks there is something to be gleaned from the termites' approach. "We could turn the whole idea of the wall on its head," he says. We should not think of walls as barriers to stop the outside getting in, but rather design them as adaptive, porous interfaces that regulate the exchange of heat and air between the inside and outside. "Instead of opening a window to let fresh air in, it would be the wall that does it, but carefully filtered and managed the way termite mounds do it," he says.
Turner's ideas were among many discussed at a workshop on insect architecture organised by Théraulaz in Venice, Italy, last year. It aimed to pool understanding from a range of disciplines, from experts in insect behaviour to practising architects. "Some real points of contact began to emerge," says Turner. "There was a prevailing idea among the biologists that architects could learn much from us. I think the opposite is also true."
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Have your say
If there are winds blowing on the nest, then I think the most important effect will be from the average wind, not from the eddies. This average wind will push air into one side of the nest and air will exit from the leeward side.
But porous walls are not a good idea for us. We can do much better with counter-current heat exchange ventilation. Unfortunately it hasn't been used much in most countries.
Tue Feb 23 21:48:22 GMT 2010 by Rupert Soar
Measured airflows induced within the mound are highly oscillatrory (AC). They are neither steady-state pressure differential, circulatory or convective driven (all DC). The mound is an impedance device, intercepting broadband potential enegy within light eddy currents passing over its surface. It's like termites constructing a bottle, only when they construct the neck does its Helmholtz capability emerge, i.e. it hums, and it will not hum if you blow too hard over it. It is genuinely breathing, not with muscles but with sound. It is recordable and we have been playing these sounds to the public. Importantly, this is partial gas exchange, as it preserves temperature and humidity levels within the nest, and is the reason why all mammals use it and not stack effects or thermal recovery bulk airflow..
Thanks for the reply.
Can you explain why there isn't a "DC" component to the air circulation (or why it's so weak)?
I don't see how partial gas exchange preserves temperature and humidity. It seems to me that it simply mixes outside air with inside air, making the inside air more like the outside air.
Animals with a windpipe and nasal passages do effect a certain amount of heat exchange (and perhaps moisture exchange) with the flesh around the passages acting as a storage medium, transferring heat from the outgoing air to the incoming air. But in our homes we can do this more efficiently with counter-current heat exchangers.
Mon Feb 22 10:48:24 GMT 2010 by James Webley
It really is amazing to think that a colony of insects can work together to build structures like the 'magnetic' termite mounds in the Northern Territory, Australia, and all the time working on 'local rules'.
We could start with wind funnels/towers. In some cases, these can incorporate wind turbines. If the tower is designed to send out hot air, a tower top that swivels with the wind will help. Dynamic (computer-controlled) flexibility in configuration will produce returns faster. Dynamic window control for day-time lighting is already available.
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