Keeping indoor spaces comfortable takes a lot of power. About half the energy Americans use in their homes goes toward heating and cooling, accounting for a sizable chunk of both utility bills and greenhouse gas emissions. Although many buildings have walls packed with insulation to maintain an ideal temperature, others—especially old buildings—are shockingly energy inefficient.
The new devices take advantage of a natural phenomenon called radiative cooling, which makes outdoor temperatures drop at night and helps cool Earth as a whole. Everything around us, including our bodies and buildings, are constantly venting heat in the form of mid-infrared radiation: electromagnetic waves that are among those at a lower frequency than the light you can see with your eyes.
The new material starts in cooling mode. Beneath an incredibly thin electrical conductor lies a small reservoir of water with copper ions dissolved inside. In this state, the device naturally radiates heat, cooling the inside of the building. Then, when the conductor layer applies a small electric charge, the dissolved copper deposits on its surface, forming a thin layer over the reservoir. Because copper emits very little of the mid-infrared heat it absorbs, the device now traps heat.