By Jennifer Ouellette, Ars Technica
“For the first time, it may be possible to design a true living, breathing building.”
The mounds that certain species of termites build above their nests have long been considered to be a kind of built-in natural climate control—an approach that has intrigued architects and engineers keen to design greener, more energy-efficient buildings mimicking those principles. There have been decades of research devoted to modeling just how these mounds function. A new paper published in the journal Frontiers in Materials offers new evidence favoring an integrated-system model in which the mound, the nest, and its tunnels function together much like a lung.
Perhaps the most famous example of the influence of termite mounds in architecture is the Eastgate Building in Harare, Zimbabwe. It is the country’s largest commercial and shopping complex, and yet it uses less than 10 percent of the energy consumed by a conventional building of its size because there is no central air conditioning and only a minimal heating system. Architect Mick Pearce famously based his design in the 1990s on the cooling and heating principles used in the region’s termite mounds, which serve as fungus farms for the termites. Fungus is their primary food source.
Conditions have to be just right for the fungus to flourish. So the termites must maintain a constant temperature of 87° F in an environment where the outdoor temperatures range from 35° F at night to 104° F during the day. Biologists have long suggested that they do this by constructing a series of heating and cooling vents throughout their mounds, which can be opened and closed during the day to keep the temperature inside constant. The Eastgate Building relies on a similar system of well-placed vents and solar panels.
The passive cooling design of the Eastgate Center in Zimbabwe, with its distinctive chimneys, was inspired by termite mounds. David Brazier / CC BY-SA 3.0
There are different types of termite mounds, depending on the species, which makes identifying universal principles a bit tricky. For instance, in 2019, scientists at Imperial College London studied the mounds of a different type of African termite common to Senegal and Guinea. This species doesn’t farm fungus, so their mounds lack the distinctive chimneys and window-like openings of the Zimbabwe termite mounds that inspired Pearce’s design for the Eastgate Building. There are no visible openings at all. Instead, there are pores, the natural result of how the mounds are made: by stacking pellets of sand mixed with termite spit and soil. It’s these pores that help the structure “breathe’ and also dry out faster after heavy rains.
In the case of the Zimbabwe termite mounds, the precise mechanism has long been a matter of debate. Is it a form of induced flow (aka the “stack effect“), the fact that heat from the colony’s inhabitants drives air up and out through the mound’s vents (thermosiphon flow), or a combination? Or perhaps a different kind of model is needed.
Physiologist Scott Turner of SUNY-Syracuse and Rupert Soar of Nottingham Trent University co-authored a 2008 paper arguing that Pearce had relied upon erroneous assumptions when he designed the Eastgate Building. Specifically, there is no solid evidence that termites regulate the temperature of their nests. Pearce’s design was a success nonetheless, but Turner and Soar envisioned “buildings that are not simply inspired by life—biomimetic buildings—but that are, in a sense, as alive as their inhabitants and the living nature in which they are embedded.”
This latest paper by Soar and David Andréen of Lund University in Sweden explores an alternative hypothesis first proposed by Turner in 2001. In this scenario, the termite mound is one component in a larger integrated system that incorporates the mound, the underground nest and the complex lattice-like network of excavated tunnels known as the “egress complex,” which could act as a driver for selective airflows. Turner envisioned this system as a functional analog of a lung, letting in oxygen and letting carbon dioxide escape. In practical terms, it’s a multiphase gas exchanger.
The termites are also able to achieve faster evaporation of excess water after it rains by transporting and depositing the water around the egress tunnels. Those tunnels are ventilated most strongly by winds, speeding up evaporation without disrupting the oxygen/CO2 balance inside the nest.
Soar and Andréen wanted to demonstrate that the egress complex could be used to promote flows of air, heat, and moisture in architectural design. “When ventilating a building, you want to preserve the delicate balance of temperature and humidity created inside, without impeding the movement of stale air outwards and fresh air inwards,’ said Soar. “Most HVAC systems struggle with this. Here we have a structured interface that allows the exchange of respiratory gasses, simply driven by differences in concentration between one side and the other. Conditions inside are thus maintained.”
The duo focused on the termite species Macrotermes michaelseni, which locate their tunnels mostly on the north-facing aspect of the mound’s spire, extending part way down the basal cone. That egress complex is the only opening between the mound and the external environment. It usually emerges at certain times of the year, notably during the growing season when termites are most active, and wetter weather increases the humidity of the surface clay.
Soar and Andréen collected samples of the egress complex structure from a termite mound at the Omatjenne Research Station in Namibia and made a CT scan of its mesh structure. Their analysis revealed that most nodes in this network had, on average, three to four connected edges, though a few had as many as seven connected edges. The channels were smooth and curved, with a circular cross-section. Then they conducted two sets of experiments.
For the first experiment, the team created a 3D replica of their egress complex fragment to avoid damaging the original sample. They simulated wind using a speaker that drove an oscillating mixture of oxygen and CO2 through the network, tracking the mass transfer with a sensor. The results showed that the greatest airflow was achieved at oscillation frequencies between 30–40 Hz. There was moderate airflow at 10–20 Hz and low airflow at 50–120 Hz. This suggests that the tunnels interact with wind blowing on the mound in such a way that enhances the mass transfer of air for ventilation.
Turbulence proved to be the key, and only certain frequencies of oscillation seemed to produce that turbulence. To find out more, Soar and Andréen conducted a second experiment using a series of 2D models out of transparent acrylic plastic, each with a different geometry. One featured straight tunnels, one had a simpler lattice network structure, and the third had a deeper lattice network structure with a larger number of nodes, typical of an actual egress complex.
This time they used an electromotor to drive oscillating water spiked with a fluorescent dye through the tunnels, filming the flow. The result: Only a few millimeters of air movement (akin to weak wind oscillations) was needed to get the ebb and flow to move through the entire complex. And that all-important turbulence only developed if the structure was sufficiently lattice-like.
The authors envision using these principles to create walls for buildings that contain built-in networks similar to the egress complex, embedded with sensors and actuators to move air around as needed using small amounts of energy. It would make for a very energy-efficient building envelope (which separates the exterior from the interior), enabling regulation of both the interior climate, as well as the micro-climate of the envelope itself, reducing, for example, microbial growth. The authors suggest emerging technologies like powder-based printing methods would be well-suited to creating such intricate materials.
“Construction-scale 3D printing will only be possible when we can design structures as complex as in nature,” said Soar. “The egress complex is an example of a complicated structure that could solve multiple problems simultaneously: keeping comfort inside our homes, while regulating the flow of respiratory gasses and moisture through the building envelope. We are on the brink of the transition towards nature-like construction: for the first time, it may be possible to design a true living, breathing building.”