(Nanowerk News) Among the approximately 2,000 known species of termites, several are ecosystem engineers. The mounds built by several genera, for example Amitermes, Macrotermes, Nasutitermes and Odontotermes, reach up to eight meters in height, making them some of the largest biological structures in the world. Natural selection has been working to improve the ‘design’ of their mounds for tens of millions of years. What might human architects and engineers learn if they went to termites and weighed their way?
In a new study on Borders in Materials (“Termite-inspired metamaterials for flow-active building envelopes”), researchers show how termite mounds can teach us to create a comfortable interior climate for our buildings that doesn’t have the carbon footprint of air conditioning.
“Here we show that the ‘egress complex’, the intricate network of interconnected tunnels found in termite mounds, can be used to drive the flow of air, heat and moisture in new ways in human architecture,” said Dr David Andréen, a senior lecturer in Lund University’s BioDigital Matter research group, and first author of the study.
Termites from Namibia
Andréen and co-author Dr Rupert Soar, a professor in the School of Architecture, Design and the Built Environment at Nottingham Trent University, studied the termite mounds of Macrotermes michaelseni from Namibia. Colonies of this species can consist of more than a million individuals. At the heart of the mound lies a garden of symbiotic mushrooms, which termites cultivate for food.
The researchers focused on the complex of exits: a dense, lattice-like network of tunnels, between 3mm and 5mm wide, that connect the wider channels inside to the outside. During the rainy season (November to April) as the mound grows, it extends onto a north-facing surface, directly exposed to the midday sun. Out of season, termite workers block the escape tunnels. This complex is thought to allow evaporation of excess moisture, while maintaining adequate ventilation. But how does it work?
Andréen and Soar explored how the complex layout of the exit allows for oscillating or pulse-like flow. They based their experiment on a scanned and 3D-printed copy of a fragment of the exit complex collected in February 2005 from the wild. These fragments are 4 cm thick with a volume of 1.4 liters, 16% of which are tunnels.
They simulated wind with a speaker driving oscillations of an air-CO2 mixture through the fragments, while tracking mass transfer with sensors. They found that airflow was greatest at oscillatory frequencies between 30Hz and 40Hz; medium at frequencies between 10Hz and 20 Hz; and at least at frequencies between 50Hz and 120 Hz.
Turbulence aids ventilation
The researchers concluded that the tunnels in the complex interact with winds blowing over the mounds in a way that increases air mass transfer for ventilation. The oscillation of the wind at a certain frequency generates turbulence within, the effect of which carries breathing gases and excess moisture away from the heart of the mound.
“When ventilating a building, you want to maintain the delicate balance of temperature and humidity that is created inside, without hindering the movement of stale air out and fresh air in. Most HVAC systems struggle with this. Here we have a structured interface that allows exchange of respiratory gases, driven only by the difference in concentration between one side and the other. The conditions inside are maintained,” explained Soar.
The authors then simulated the complex egress with a series of 2D models, increasing in complexity from a straight tunnel to a grid. They used an electric motor to drive an oscillating body of water (made visible with dye) through the tunnel, and filmed the mass flow. They found, to their surprise, that the motor only needed to move air back and forth a few millimeters (according to weak wind oscillations) for the tides to permeate the entire complex. Importantly, the necessary turbulence only appears if the layout is lattice-like enough.
The building lives and breathes
The authors concluded that the egress complex could allow wind-powered termite mound ventilation in weak winds.
“We envision the walls of future buildings, fabricated with new technologies such as powder bed printers, will contain a network similar to an egress complex. This will make it possible to move air, through embedded sensors and actuators that only require a small amount of energy,” said Andréen.
Soar concludes: “Construction-scale 3D printing will only be possible if we can design structures as complex as they are in nature. Egress complexes are examples of complex structures that can simultaneously solve many problems: maintaining comfort within our homes, while regulating the flow of respiratory gases and moisture through the building envelope.”
“We are on the verge of a transition towards nature-like construction: for the first time, it is possible to design buildings that truly live and breathe.”