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Knowledge transfer from biology to technology23. September 2004
Hornets – masters in the art of lightweight construction
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A hornet approaching the test stand.

Two hornet colonies are currently housed in test stands on an EMPA building rooftop in Dübendorf, under the watchful eye of staff from the Wood Laboratory. Seeking inspiration for new technical solutions, the researchers are investigating both the fascinating lightweight structures built by the social hymenopterans and their thermoregulation strategies.

The highly efficient thermoregulation system used by hornets enables them to maintain a steady brood temperature of around 29°C in the nest well into late autumn. In filigree lightweight structures built from chewed wood fibre and saliva, the social insects exploit a combination of physical principles to keep the nest interior at a constant high temperature – an intriguing phenomenon for materials scientists.

Research staff at EMPA are trying to determine the precise bionic potential of this accomplishment, i.e. whether such feats observed in the natural world might serve as prototypes for technical innovation, for instance, in the field of facade engineering.
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An inhabited, hornets‘ nest hanging from the test stand.

Temperature and moisture sensors inside the nest and light beams at its entrance allow them to record all movements and fluctuations for subsequent analysis.

 

Like all insects, hornets suffer high heat losses on account of their unfavourable body surface-to-volume ratio.

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Hornet hatching

However, a nest envelope incorporating air voids that slow heat transfer provides insulation and retains warmth for the brood. On hot days, by contrast, considerable quantities of surplus heat have to be extracted from the nest to prevent overheating. Here, the low thermal capacity of the lightweight nest construction – adopted so as to minimize the energy expended for building – offers little assistance. Moreover, the nest is continuously enlarged until the size of the colony peaks in late summer. The insects meet the different requirements through a combination of behavioural patterns, design principles and, not least, the appropriate choice of construction material.

Best-practice construction, cooperative behaviour and a suitable material

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A hornets‘ nest with the sheath cut open to show the internal structure.

The nest structure, comprising horizontal combs enclosed by a cellular envelope, allows the hornets to control the internal temperature with relative precision and reduce the heat lost by individual insects, especially on cool nights.

Conversely, when overheating threatens, the hornets exploit the vapour pressure gradient between nest interior and outdoor environment. The air in the nest is warmer and more humid and therefore contains more energy per unit weight than the outside air. The hornets sit at the nest entrance, using their wings as fans to increase the air change rate and thereby extract energy from the interior.

Also, the wood-based material used by the hornets is hygroscopic, i.e. moisture-absorbent. The nest fabric takes up moisture from the air at night, thereby releasing warmth, through the heat of condensation, to the nest interior. The reverse happens during the daytime, with heat being extracted by the evaporation process (cooling effect). The hornets are able to enhance this effect deliberately by increasing the moisture gain and ventilation rate.

Model for improving internal environments through breathing envelopes and climatic buffers

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Raoul Klingner investigating the structure of a hornets‘ nest.

The results of the EMPA project, backed by the Swiss National Science Foundation, will also feature in the dissertation being prepared by Raoul Klingner in collaboration with the Institute of Building Technology at the Swiss Federal Institute of Technology in Zurich. Both project and dissertation set out to gain a better understanding of the thermodynamic behaviour of natural structures made from wood.

The findings on these efficient mechanisms may conceivably inspire new applications in timber engineering. The options include both refinements to breathing multi-layered building envelopes and the better exploitation of the hygroscopic properties of wood through its use as a climatic buffer to moderate undesirable fluctuations in the internal environment.

Contact for content-related information

Raoul Klingner, Wood Laboratory, Tel. +41 44 823 46 60, E-mail: raoul.klingner@empa.ch

Dr. Klaus Richter, Wood Laboratory, Tel. +41 44 823 41 15, E-mail: klaus.richter@empa.ch

 

Editor

Martina Peter, Communications/Marketing Dept., Tel. + 41 44 823 49 87, martina.peter@empa.ch  

Links

www.empa.ch/wood: More about the Wood Laboratory at Empa.

Hornets' nests as a model for adaptive mechanisms and passive regulation: More about the project

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