No A/C? No problem, if buildings copy networked tunnels of termite mounds

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 nests 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.

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 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.”

https://arstechnica.com/?p=1942139