Climate control needs to cut out the AC, says S.Ananthanarayanan.
Rising CO2 in the air and rising temperatures have driven up the use of air conditioning in homes and workplaces, and hence the consumption of electricity. As much of our electricity comes from burning coal, the increased load leads to more CO2 in the air and, in turn, greater need for air conditioning. Home owners and architects would welcome a way out of the spiral.
The last few decades have hence seen several groups working on methods of passive cooling, or cooling things down without the conventional refrigeration units. One approach has been with surfaces that reflect, in place of absorbing heat from sunlight or the surroundings. Another approach is with special surfaces that radiate in such a way that heat travels not just out from the warm object, but passes through the atmosphere as well.
Xiangyu Li, Joseph Peoples, Peiyan Yao, and Xiulin Ruan, from Purdue University, Indiana, USA, in a paper in the journal, Applied Materials and Interfaces, of the American Chemical Society, describe a material that can reflect away the heat that falls on it, as well cool by radiating in that special way.
It is well known that wearing white or painting buildings white would reflect heat and help things stay cooler. Buildings even use a complex, multilayer covering, to promote reflection, or use a reflective metal layer. But these are expensive procedures and are not even possible for many applications. Single layer paints have been used, but these often need thick coats and are only partially effective. The best high reflection paints, the paper says, are only 91% reflective.
While warm objects do cool by radiation of heat, most of the heat that is sent out is absorbed by the surrounding air and the ambient temperature goes up, which sends the heat right back, and reduces the cooling that radiation brings about. There is, however, a special band of wavelengths of radiation which does not heat the surroundings and is able to pass through the atmosphere, and away to outer space. Radiation at this band would thus cause real, net cooling, of the object and of the earth as well. This radiation band, known as the sky window, is in the infra-red region, in the wavelength range from 8 micrometers to 13 micrometers, a band of wavelengths at which none of the components of the atmosphere can absorb radiation.
A first material that favoured radiation of heat in this wavelength band was silicon, the material of electronics and solar cells. A group at Stanford University created a composite film, made of silicon dioxide and hafnium dioxide. Silicon dioxide emits strongly at 9 micrometer and hafnium dioxide at 9-13 micrometers. The film, when laid over an object, would thus warm by the heat of the object and radiate at these wavelengths. Combined with an arrangement for reflectivity of 97%, the Stanford group reported cooling by 4-5°C, of an object placed in open sunlight. The system also works with a comparatively cheaper material, titanium dioxide, in place of hafnium dioxide.
What the group at Purdue University has now reported is a far simpler arrangement, an ‘ultra-white’ paint, which results in high reflectivity as well as emission in the special band of frequencies. A reason why existing high reflectivity paints, which contain a suspension or a film, of titanium dioxide particles, for selective emission, have not resulted in cooling of more than 2°C, is mainly that titanium dioxide particles absorb ultra violet light, the paper says. The way materials absorb light when photons of light fall on them is that photons knock about the electrons of atoms in the material, and pass on their energy to the material. The quest has hence been for materials where electrons are more firmly bound and photons of ultra violet light cannot knock them loose. Using such materials, however, leads to less reflectivity and what we gain by the better materials gets set off by their effect, the paper says.
The arrangements the Purdue team report are a film with nanoparticles of an alternative material, barium sulphate, and an acrylic paint that contains barium sulphate. With barium sulphate, the paper says, it takes higher energy for electrons to be separated from atoms. The result is that photons of the ultra violet are not absorbed, while they are still reflected by the film or the paint. By choosing a proper distribution of particle sizes, the team has achieved reflectance of 97.6%, the paper says.
Another property of barium sulphate particles is that their internal, mechanical vibrations are strong at the frequency that corresponds to the wavelength of 9 micrometers. This is within the sky window of 9-13 micrometers. The particles in a film or paint hence convert a large fraction of heat energy in an object to emission at 9 micrometers and send it out, to pass through the atmosphere and away from the earth!
During trials conducted, the temperature of a sample dropped 10.5 °C below the ambient temperature during the nights and stayed 4.5−10 °C below the ambient temperature during daytime. Whereas the temperature of a control test with commercial paint increased 6.8 °C above the ambient temperature at 2−3 PM. Over all, the barium dioxide film achieved an average cooling power of 117 W over every square meter. The cooling capacity of a one tonne AC unit is about 3.5 kW. In comparison, a film-covered rooftop with an area of 50 square meters could provide 570 W of cooling. The cooling by a conventional AC unit is during the time it is on and working, whereas the cooling film would work continuously. And the cooling could be twice as much, as the walls would also be painted. Most of the area under the roof could hence be fairly well cooled, with no power consumed.
Three of the authors of the paper are in the process of obtaining a patent for the technology. The material may hence soon be commercially available.
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