They've mastered the art of preventing their homes from overheating in the hot African Sun, but termites have also worked out how to stop carbon dioxide from building up in their mounds. They rely on the Sun more than on wind, providing clues to architects looking for low-energy ways to keep buildings comfortable.
Many of our best technological ideas come from nature. Architects have wondered if the termite's ability to keep mounds at even temperatures is something we can learn from. After all, termite mounds are one of the most impressive constructions on Earth, relative to the size of the creatures that make them.
Yet almost all the research done on termite mound engineering has looked at Indian mounds, which occur mostly in shaded environments. These studies have reached conflicting conclusions on how the mounds manage to keep their temperatures so equitable.
The termite Macrotermes michaelseni, on the other hand, lives in African savannas where the Sun is scorching and wind speeds are much higher than in South Asian forests. Their mounds can be 3 meters (10 feet) high and contain millions of workers, all producing carbon dioxide and emitting their own heat.
MIT graduate student Samuel Ocko and his colleagues used a combination of thermometers and airflow monitors to observe how conditions changed in Macrotermes mounds over 35 days during autumn in Namibia. They reported their findings in the Journal of Experimental Biology.
External temperatures changed by 15 to 20ºC (27 to 36ºF), whilst inside the mounds, conditions varied by just 8º C (15º F) – something humans usually need energy-guzzling air conditioning to replicate. Airflow in the mounds was smooth, powered by convection from the temperature gradient between their centers and outside. Wind only had a secondary influence. Carbon dioxide levels stayed close to 5 percent over 24 hours, with airflow going upwards in the daytime and downwards at night.
The mounds have holes spanning up to 0.5 centimeters (0.2 inches) in diameter, allowing for gas exchange and creating a dazzling array of tunnels. They also tilt towards the equator.
The mounds of M. michaelseni showed only two major differences from their Indian equivalents. In Indian mounds, carbon dioxide levels vary significantly throughout the day, but temperatures inside stay even. In African mounds, there are big thermal gradients between the side facing the Sun and the center, and gas concentrations remain steady.
These complex structures may not be directly applicable to human buildings, but there are lessons to be learned about efficient passive temperature control. The mounds demonstrate that this can work even when buildings spend most of the day in direct sunlight.