Remember when I talked about evaporative cooling a few weeks ago?  Well, here’s a new wrinkle on that technology.  One that Drs. Jeffrey Grossman and Nicola Ferris, along with their post-doc, Dr. Zhengmao Lu (and two grad students- Elise Strobach and Ningxen Chen) of MIT have been developing.  (It was reported in this article: Passive Sub-Ambient Cooling from a Transparent Evaporation-Insulation Bilayer [Joule])

These folks noticed that camels pose an anomaly.  These animals need to keep cool in the desert- yet they are ‘clothed’ in a thick layer of insulating fur.  Huh?

Actually, many denizens of the desert also don thick coats to keep cool.  It seems that those coats prevent the loss of moisture- but lets the body sweat away, yielding evaporative cooling galore.  (A shaved camel loses 50% more moisture than does an unshaved one!)

So, they set out to develop a hydrogel/aerogel-based evaporative cooling device, one that mimics the perspiration system of the camel.  The use of a transparent bilayer relies upon water evaporating from the hydrogel, passing through the porous aerogel  (of ultralow thermal conductivity).  They’ve attempted to optimize the temperature drop from ambient conditions and the effective cooling time.

Hydrogels are capable of absorbing water and then releasing that water via evaporation, yielding the passive cooling effect- without using a joule of power.  The trick is to extend this effect long enough to be of utility.  Hence, the addition of an aerogel (light, porous insulating material) on top of the hydrogel- to yield said performance. It’s evaporation/insulation coupling mimics the camel’s physiology, in that the hydrogel is akin to the sweat gland (letting water evaporate and yielding a cooling effect), with the aerogel mimicking the camel’s fur (insulating the camel from the ambient heat, but still allowing for the escape of water vapor).  But, don’t think this is a massive structure- the bilayer is about 10 mm in thickness.  (Each layer is 5 mm thick.)

The aerogel is basically sand (silica)- but the processing necessary to produce it escalates the costs.  (Technologists are diligently attempting to reduce the production costs.)  The hydrogel is about 97% water (which eventually dissipates due to evaporation).

The cooling effect of their device was about 7 C (12.5 F) below the ambient levels; and with the addition of the aerogel, the effect could be extended by a factor of 5- to about 250 hours.  (That’s more than 10 days!)   (By the way, the hydrogel alone lowered the temperature by 8 C- but lost a degree of cooling for that 20% ‘shelf-life’ differential.)

The goal is to use this to cool buildings, keep perishable food for longer periods and to assist in the transport of vaccines to remote locations.

Happy 4th day of Chanuka-  The Festival of Lights.