RACA Journal February 2023 | Page 15

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International News

NEW SYSTEM COULD COOL BUILDINGS UP TO 10 ° C

- WITHOUT ELECTRICITY

As the world ’ s climate continues to heat up , the global demand for air conditioning is now skyrocketing .
MIT
Graphic representation of the passive cooling and material factors .

In 2019 , the need for cooling drew in 8.5 % of the world ’ s total electricity consumption , equating to some 1 billion tons of

CO 2 emissions .
As more air conditioning units draw ever more power each year , we now appear trapped in a cycle , only accelerating the problem of global heating further .
COOLING WITHOUT POWER One possible way to break this cycle could lie with passive cooling . This kind of technology absorbs heat from the surrounding environment and then exploits physical effects including insulation , evaporation and radiation to transfer this heat away from the system being cooled – all without any added power .
There is still a long way to go before passive cooling systems can be rolled out on commercial scales , though . Not only do existing designs have a limited cooling performance , they also tend to use large amounts of water and their efficiency is limited and dependent on environmental conditions , such as heat and humidity .
THREE COOLING LAYERS A team of researchers in Massachusetts has recently made important steps towards overcoming these challenges . Within a flat , three-layered panel , Zhengmao Lu and colleagues at MIT combined several passive cooling techniques - each counteracting the shortcomings of the others .
The panel ’ s top layer features a highly insulating aerogel : an ultra-light , sponge-like material , featuring sparse networks of cross-linked polymers , where a vast majority of the volume is taken up by empty space . This structure makes aerogels highly insulating to heat , while allowing gases and other kinds of radiation to readily pass through .
Underneath the aerogel , Lu ’ s team incorporated a hydrogel : a material featuring a similar network of insoluble polymers , this time immersed in water . This layer is insulated by the aerogel above , but as the heat energy that does make it through the top layer is absorbed , the water it contains is partly evaporated into vapor – which rises up through the aerogel .
In addition , the hydrogel converts some of the heat it absorbs into infrared radiation . Since both the aerogel and Earth ’ s atmosphere are transparent to this radiation , that energy is then released back into outer space , without heating up the air outside .
Finally , the researchers placed a reflective , mirror-like material beneath the hydrogel . This layer reflects back any heat that manages to pass through the top two layers – ensuring that as much heat as possible is absorbed by the hydrogel .
OUTPERFORMING PAST DESIGNS A key advantage of this design is that it combines the unique benefits of insulation , evaporation and radiation .
As the aerogel ’ s strong insulation cools the hydrogel beneath , this second layer can convert heat into water vapor and infrared

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