Countercurrent heat exchange in desert animals

Countercurrent exchange is a mechanism widely used by different species for a variety of purposes. In general, countercurrent exchange is a crossover of a certain property, f.ex. heat, between two bodies (be it fluids or gases), flowing in opposite directions.1

 

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Animals in extreme environments, both hot and cold, rely on the countercurrent exchange in order to regulate their body temperature and keep it independent of the ambient temperature. Many species use this mechanism in relation with their circulation system, especially in their extremities.

Desert animals use this exchange in their respiratory system, for better management of body heat and water. Having little or no access to the latter, they must rely on metabolic water. Maintaining proper body heat is crucial for survival during the cold desert nights. Thus minimising water and heat loss through breathing is the reason for countercurrent heat exchange in their nasal passages. As warm air has a higher capacity to carry water, breathing out warm air can cause substantial losses to the animal. Thus the animal captures the water and heat from its own breath, before it leaves the body.karolina-pajnowska-presentation3The countercurrent heat exchange takes place in the nasal turbinates – a complicated system of narrow, convoluted passages, lined with membrane. It is a common mechanism for many mammals, humans included, but if greatly differs in efficiency levels. In 1981, Knut Schmidt-Nielsen suggested that the system’s efficiency in desert animals results from the complexity of the nasal turbinates. Consequently, the non-desert species with simpler geometry of their nasal passages are not able to cool the expired air to sufficient temperature, and are therefore more prone to larger reduction in respiratory water loss.2

For desert animals, this system works especially good during the night, when the ambient temperature (Ta) is lower than the body temperature of the animal. Some species also use it while temporarily entering a colder environment, f.ex. an underground burrow.karolina-pajnowska-presentation4During the inspiration, the cold dry air passes over the the nasal mucosa, absorbing heat and moisture from warm tissues before entering the lungs. As a result, the nasal passages become colder by means of evaporation.karolina-pajnowska-presentation5When exhaled, the warm humid air passes over cooled nasal mucosa, giving away heat and water, which condenses on the mucosa. The water is then available for the next inspiration. In case of the kangaroo rat (Dipodomys merriami) the water conservation can reach 54% at Ta 30° and 83% at Ta 15°. In contrast, humans can only recover about 16% of water vapour at Ta between 16° and 35°.3

In short, the desert animals breathe out cold air, while the heat and water stays inside the body. This system works perfectly during cold nights in the desert, however, in the daytime, there is a need for a complimentary mechanism to get rid of excess heat, as opposed to storing it. Some species, such as dogs, use panting. Others, like camels, use different systems in the day, as they prioritise cooling over water conservation.

Desert environment, with significant temperature amplitudes, requires flexible and adaptive measures not only in animals, but also in architecture. Taking inspiration from elaborate respiratory mechanisms of desert species can result in an improved performance in building ventilation. The perfect solution would combine the performance goals for daytime and nighttime using the same building block system in the outer wall. In the daytime it would cool and humidify the incoming hot, dry air. During the nighttime it would protect the building against the cold air and store the heat and water to be reused in the daytime. Ideally, reusing the water would be possible due to minimal water loss in the block.

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In the nighttime, the cool dry air passing through the wall gets saturated with water, permanently resident in the building blocks, and the heat, harvested during the daytime. The block cools down by means of evaporation. The heated, humid air flows into the building. At the point of exiting the building, the water condenses on the previously cooled block. The heat is released and harvested just like the water. The air exiting the building is as cool and dry as it was before (ideally), or with minimal loss in both of these parameters.

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During the day, the warm dry air is cooled down with water resident in the pores of the block, while the heat is released into the wall, where it’s stored for the use in the night. The air flowing into the building is therefore cool and humid. On the way back the air gives away the water back to the building block. This means that the chemistry of the element needs to allow for water absorption, as the condensation is no longer possible in the daytime conditions.

Drawing inspiration from the nasal turbinates, it is the geometry of the design that can do part of the job. In order to allow the air to pass through the wall, the brick obviously has to be porous. A complicated, intricate system of spongy canals will increase the storage capacity for both heat and water, and provide a more generous absorption surface, thus elevating the efficacy of the system. karolina-pajnowska-presentation8The pattern derived from the bony nasal structures of desert animals can be evocative of complex arabic mosaics. Some of the traditional Moroccan geometries could be repurposed for creating internal structures in the building block.karolina-pajnowska-presentation9The geometry is however not enough to ensure the heat exchange.The choice of material is crucial, and it has to be one with significant capacity for water and heat storage. One that comes to mind is ceramic. In this case, a complimentary system of micro-pores resulting form the choice of ceramic material can support the perforated form of the building block. Porosity in ceramic can be achieved by means of adding special ingredients and a specific burning process. Ceramics are often used as insulators on industrial devices to prevent energy loss and retain heat.

 

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As for fabrication, ceramic allows for many different techniques. To create a system of tunnels, one could use the arabic patterns for projection onto the block’s surfaces along the X, Y and Z axis, and subsequently create holes according to the projection. However, the complexity level resulting from this process would not be enough.

What seems a better solution is using the additive fabrication methods. 3d printing allows for a very intricate system of openings. The path for printing could be derived from the arabic geometry, and translated into seemingly random layers.

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Some trials have been conducted quite successfully, and lead to creation of many great inventions, such as the Cool Brick.

As the designers describe it: “Comprised of 3-D printed porous ceramic bricks set in mortar, each brick absorbs water like a sponge and is designed as a three dimensional lattice that allows air to pass through the wall. As air moves through the 3-D printed brick, the water that is held in the micropores of the ceramic evaporates, bringing cool air into an interior environment, lowering the temperature using the principle of evaporative cooling. The bricks are modular and interlocking and can be stacked together to make a screen. The 3-D lattice creates a strong bond when set in mortar. The shape of the brick also creates a shaded surface on the wall to keep a large percentage of the wall’s surface cool and protected from the sun to improve the wall’s performance.”4

 

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The brick is very successful at cooling air, but it doesn’t store heat, and only works for cooling. It is not enough for desert environment, but it is a great starting point.

To conclude, the desired heat and water storage capacity in the brick have to be achieved by more than perforation, material porosity, and geometrical complexity. An additional layer of another material serving as inlay in the pores could be the next step of investigation. Hopefully this could lead to creation of a “breathing house”, which can cool itself down and heat itself up by means of natural ventilation, while re-using its water supplies with minimal losses along the process.

 

 

 

Sources:

  1. https://en.wikipedia.org/wiki/Countercurrent_exchange#Countercurrent_exchange_of_heat_in_organisms
  2. Schmidt-Nielsen K 1981. Counter-current systems in animals. Scientific American 244 118-128.
  3. http://www.open.edu/openlearn/ocw/mod/oucontent/view.php?printable=1&id=2477
  4. http://www.emergingobjects.com/2015/03/07/cool-brick/
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