Industrialised, Integrated, Intelligent sustainable Construction - I3con

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SUSTAINABLE CONSTRUCTION HANDBOOK 2 Figure 2. Two early models for mound ventilation. Left. Thermosiphon flow thought to occur in capped chimney mounds. Right. Induced flow thought to occur in open-chimney mounds. The earliest model for termite mound function was Martin Lüscher’s thermosiphon mechanism, in which the mound is a venue for metabolism-driven circulation of air [8]. Here, the colony’s production of heat (roughly 100 watts) imparts sufficient buoyancy to the nest air to loft it up into the mound and to drive it eventually to the mound’s porous surface. There, the spent air is refreshed, as heat, water vapor and respiratory gases are exchanged with the atmosphere across the mound’s porous walls. Gravity acting upon the increased density of the refreshed air then forces it downward into open spaces below the nest and eventually through the nest again. This mechanism was thought to operate in mounds with capped chimneys, those that have no obvious vents. The second model is known to biologists as induced flow [9-11], but it is probably better known to architects and engineers as the stack effect. This mechanism was thought to occur in open-chimney mounds [12]. Because the mound extends upward through the surface boundary layer, the large chimney vent is exposed to higher wind velocities than are openings closer to the ground. A Venturi flow then draws fresh air into the mound through the ground-level openings, then through the nest and finally out through the chimney. Unlike the thermosiphon model’s circulatory flow, induced flow is unidirectional. The similarities between the Eastgate Centre and termite mounds now become clear. The induced flow principle is implemented in the row of tall stacks that open into voluminous air spaces that permeate through the building (Figure 2). Meanwhile, heat from the building’s occupants and machinery, along with stored heat in the building’s thermal mass, helps drive a thermosiphon flow from offices and shops upward toward the rooftop stacks. In the climate of Harare, the combination provides for an impressive steadiness of interior temperature, accomplished without resorting to a costly and energy-hungry air-conditioning plant. This is where most of the building’s efficiencies accrue. 236 How the Eastgate Centre is not like a termite mound The design and function of the Eastgate Centre departs from termite mounds in some significant respects, however, and this makes for some interesting design anomalies. One of the more interesting involves temperature regulation. In the architectural literature, discussions of the Eastgate Centre are often accompanied by encomia to the impressive thermoregulatory abilities of the mound building termites. A few quotes make the point: “The Eastgate building is modeled on the self-cooling mounds of Macrotermes michaelseni, termites that maintain the temperature inside their nest to within one degree of 31 °C, day and night … “ 6 6 http://www.biomimicryinstitute.org/case-studies/case-studies/termite-inspired-air-conditioning.html
HANDBOOK 2 SUSTAINABLE CONSTRUCTION “Indeed, termites must live in a constant temperature of exactly 87 degrees (F) to survive.” 7 “Termites farm fungus deep inside their mounds. To do so, the internal temperature must remain at a steady 87 degrees F.” 8 “The fungus must be kept at exactly 87 degrees …” 9 There is just one problem: at least in the nest of Macrotermes michaelseni, there is no evidence that nest temperature is regulated. Indeed, there is good evidence that it is not. In the subterranean nest of Macrotermes michaelseni, for example, while temperatures are strongly damped through the day, they also closely track deep soil temperatures through the year (Figure 3). Consequently, the annual march of temperature in the nest ranges from about 14 o C in winter to more than 31 o C in the summer, a span of nearly 17 o C. Nor is there any evidence that mound ventilation affects nest temperature. In the nest of Odontotermes transvaalensis, which builds open-chimney mounds, eliminating ventilation altogether (by capping the open chimney) produces no discernible effect on nest temperature [13]. These observations have a straightforward explanation: when a nest is embedded in the capacious thermal sink of the deep soil, the nest energy balance (and hence its temperature) is strongly driven by this large thermal capacity. This produces the nest’s strongly damped diurnal temperatures, and the mound infrastructure and nest ventilation has virtually nothing to do with it. Figure 3. The annual march of temperature in the nest of a Macrotermes michaelseni colony in northern Namibia. For comparison, ground temperature 15 m away and at 1 m depth (about the depth of the nest) is also plotted. Another surprising feature involves the relationship between nest temperature and mound temperature. A core assumption of Martin Lüscher’s thermosiphon model is that buoyancy is imparted to nest air by waste heat from nest metabolism. There is, in fact, a considerable production of waste heat, estimated to range from about 80 watts, to as high as 250 watts, and this can elevate nest temperature by a few degrees above soil temperature [14]. However, for the thermosiphon to work, nest temperature must be warmer than mound temperature. Extensive measurements over the year of mound vs nest temperature for Macrotermes michaelseni reveal that nest temperature is most frequently cooler than mound temperature (Figure 4). In short, the nest air is most commonly stably stratified with respect to mound air, which means no thermosiphon flow is possible. Where the nest is warmer than the mound, the buoyant forces that result will be weak and unlikely to drive much flow [15]. 7 http://www.aia.org/aiarchitect/thisweek03/tw0131/0131tw5bestpract_termite.htm 8 http://database.biomimicry.org/item.php?table=product&id=1007 9 http://www.zpluspartners.com/zblog/archive/2004_01_24_zblogarchive.html 237
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Industrialised, Integrated, Intelligent sustainable Construction - I3con

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