The second law of thermodynamics is a wall that is stopping us from doing marvellous things, like perpetual motion ;-). One of its tenets is that heat always goes from higher temperature to lower temperature. This means that you cannot cool a body unless you place it in an ambient that is colder.
To do that you need to use power and this is what we do with our air conditioning systems, and overall the world uses 15% of the electricity produced to power air conditioning systems. This cost money (in producing electricity) and contribute 10% to global greenhouse emissions.
All indicators point towards an increase of cooling demand, both because more and more people like, and can afford, to remain cool in Summer heat and because the Earth is warming up (in a perverse way the more the Earth warms up the more use we make of air conditioning, the more power we need and the more greenhouse emissions are produced that in turns contribute to the Earth warming… not good!).
You cannot fool the second law, but you may take a grander view. And this is what researchers at Stanford have been doing. Yes, they say, you cannot cool something below the ambient temperature without using power but … what do you mean by ambient?
If you measure the temperature just outside your window it may be some 40 degrees C hot (if you are in Vegas, as the researchers were when experimenting) but if you take a grander view and you look up at the sky the temperature up there is more than freezing cold!
Now, just imagine that you can irradiate the heat using some radiation that is not blocked by the surrounding air but goes straight up to the space above the Earth, then the second law will no longer stay in the way because what you are doing is transferring heat from a body that is 40C hot to a space that is -270C cold.
This is what our planet does every single day. It receives short wave radiations from the Sun (that warm it up) and convert these into long way (infrared) radiation that are sent in the interplanetary cold.
Hence there is a trick! Use a radiation that can go through the atmosphere and you are done.
The Stanford researchers have created panels that are able to do just that and they are reporting their findings in an article published on Nature Energy.
The panels are covered with super thin metal oxide layers that are able to reflect the incoming heat from the Sun (up to 97%) and convert the heat of water flowing into pipes under the surface into radiation that can be dispersed into space (radiative cooling).
Through fields experiments carried out in the hot Las Vegas Summer, they have shown that these panes can cool water to 3 to 5 degrees C below ambient temperature.
Panels covering the roof of a two story building can service, with basically no electricity, its air conditioning. This opens up the possibility to dramatically reduce power usage for air conditioning in the near future.
Another proof that we really need to think … out of the box!