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It’s a Cool World


The invention of air conditioning has transformed the way we live. No longer do we have to sweat day and night in temperatures that we never quite adjust to. But unfortunately our comfort comes at an environmental cost. The HFC’s used in cooling and refrigerating are causing the ozone layer to degrade and this in turn causes an increase in global warming, but this is quite a minor problem compared to the amount of electricity used to keep ourselves cool.

Recent research, carried out by Dr Aaswath Raman and his colleagues at Stanford University may change that. This new idea allows buildings to radiate their heat into outer space without using pumps or compressors. The idea is surprisingly simple and mostlyrelies on natural science. Space is big, really really big, so big that you won’t believe quitehow mind staggeringly big it is (apologies to Douglas Adams). It’s also really quite nippy, at a mere 3°C above absolute zero. It is however, these conditions that make it an ideal heat sink. Earth radiates heat all the time, but thanks to the Sun giving us heat we never really notice. The trick that Aaswath has come up with is to develop a system that reflects the sunlight that we get from space back into space at a local level.
You can read the full article in Nature, but to summarize, he and his team have developed a material which reflects and radiates 97% of sunlight, at the most transparent wavelengths of the atmosphere (eight to 13 microns aka millionths of a meter). Using modern manufacturing techniques the material uses seven extremely thin (13 to 688 nanometers aka billionth of a meter) layers (four silicon dioxide and three hafnium dioxide) on a 200 nanometer thick layer of silver which acts as a mirror.These layers were then mounted on a silicon wafer to keep them flat and for test purposes they were installed in a specially designed box that minimizes the conductionof heat and they recorded that the wafer’s temperature was 4.9 degrees Centigrade cooler than its surroundings. This isolated unit was proof of concept, but if larger sheets of this material were placed directly on the roof of a building, the result would be to cool the entire surface area by radiating the heat away from the building. 
There are some cost issues with the materials used in this proof-of-concept and these need to be resolved before it can go into mass production andsince it also needs a direct-line-of-sight to the sky and outer space it’s only really suitable for roofs,but still it’s a start and someday, our homes and offices may have reduced electricity bills, whilst maintaining a livable environment.