Conventional air conditioners are energy hogs. According to the US Department of Energy, air conditioning in the United States accounts for 117 million metric tons of CO2 released into the atmosphere every year. Air conditioning is expensive, too—according to the Texas utility Austin Energy, households in warmer regions of the country can expect cooling their homes to account for 60 to 70 percent of their summer electric bills.
Though both residential and commercial structures cool somewhat at night through the natural radiation of thermal energy, this does not present a practical way of cooling during the day since even a small amount of solar radiation on a structure exposed to sunlight is enough to negate passive thermal radiation. The trick, then, is to obviate the absorption of solar radiation while simultaneously allowing passive thermal radiation to proceed. This had been accomplished previously, but the processes used very complicated and difficult-to-produce nanomaterials that required exotic and impractical fabrication techniques and could not be scaled to real-world application.
A team of engineers from the University of Colorado seems to have performed this trick. They have produced a metamaterial (a material engineered to have extraordinary properties) which has the ability to cool objects such as homes and buildings even under direct sunlight while consuming no energy or water, effectively acting as an air conditioning system. It is composed of a layer of visibly transparent polymers randomly embedded with tiny spheres of glass and then covered over by a thin layer of silver. Basically, incoming light of many different wavelengths gets caught up in and then reemitted by the spheres. The randomization of these spheres is part of what accounts for the wide range (96 percent) of reflectivity across the spectrum of sunlight. Incoming solar radiation is reflected back into the atmosphere while still providing a means of escape for infrared radiation. This metamaterial has demonstrated an average cooling power of more than 110 watts per square meter for a continuous 72 hours and more than 90 watts per square meter in direct noontime sunlight.
The glass-polymer hybrid is 50 micrometers thick, slightly thicker than aluminum foil, and can be manufactured economically in rolls, thus offering great opportunities for eco-friendly cooling of both commercial and residential buildings. It could also provide supplementary cooling to thermoelectric power plants, which currently rely on large amounts of water and electricity for that purpose.
The researchers plan to deploy a 700-square-meter prototype “cooling farm” in 2017. According to Ronggui Yang, a member of the University of Colorado team, “The key advantage of this technology is that it works 24/7 with no electricity or water usage. We’re excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture, and more.”