AskNature

The Challenge

Cooling systems within households and automobiles require lots of energy to function properly. In residential buildings alone, cooling accounts for 6% of total U.S. energy consumption. Not only is this rate of energy consumption expensive, but it also has detrimental environmental impacts, as the production of electrical energy in the U.S. is largely dependent on fossil fuels. In recent years, interest in developing other methods of cooling has increased. One promising technique has gained recent attention within the field: passive radiative cooling.

To understand radiative cooling, we must first understand Kirchhoff’s law of thermal radiation, which states that materials with a temperature above 0 K (zero degrees Kelvin, when particles no longer move) continuously absorb and emit electromagnetic waves. Essentially, everything around us (including ourselves) is immersed in a bath of electromagnetic radiation, and heat is exchanged among objects at different temperatures by absorbing and emitting electromagnetic waves. Earth maintains this state by radiating heat to outer space, which is very cold. As a result, ambient temperature decreases at night.This occurs because Earth’s average surface temperature (about 300 K) is substantially higher than that of outer space (about 3 K). As the sun passes below the horizon, t solar heating decreases in areas on Earth experiencing night, and temperatures continue to drop in those areas until the sun rises again. In passive radiative cooling, a material achieves a cooling effect through high thermal radiation and high reflectivity. If a material is able to emit more thermal energy than it is absorbing from the sun, it achieves electricity-free cooling. The key to passive radiative cooling is manipulating a material’s reflectivity and thermal radiation, and many researchers have developed approaches to do just that. However, these photonic approaches are not scalable, as they require high precision manufacturing techniques, which are costly and have low yield.

Innovation Details

The photonic cooling film is made of polydimethylsiloxane (PDMS), a widely used, flexible polymer. The polymer contains randomly distributed spherical ceramic particles, which create a textured surface of pyramid-like structures. Due to these encapsulated particles and the photonic architecture of the film, the material exhibits a high reflectance of solar light as well as absorption selectivity, which creates a passive cooling effect. Moreover, the film is manufactured using a facile microstamping method, which is promising for scaled-up production. Radiative cooling technology is practically applicable to many energy-efficient thermal management systems, including cooling systems for buildings, automobiles, electronics, and even clothing. The scalability of the film could make the wide-ranging and widespread use of passive radiative cooling a reality.