Radiative cooling, a passive cooling technique, has shown great potentials in recent years to lower the power consumption of air conditioning. With the ever-increasing cooling power being reported, the theoretical cooling limit of such a technique is still unclear. In this work, we proposed a theoretical limit imposing an upper bound for the attainable cooling power. To approach this limit, we exploited the localized surface plasmon resonance (LSPR) of self-doped In 2 O 3 nanoparticles, which enhance the emissivity in both primary and secondary atmospheric windows. The measured cooling power of poly(methyl methacrylate) (PMMA) films containing 4.5% In 2 O 3 nanoparticles is very close to the limit with the closest value only about 0.4 W/m 2 below the limit. Hopefully, this work may help the researchers better evaluating the performance of their device in the future and pave the way for achieving even higher radiative cooling powers during the daytime operations with the help of LSPR.
Daytime radiative cooling has become a promising passive cooling technique in recent years, showing great potential in building-related applications to reduce the power consumption of air conditioning. Currently used mid-infrared emitters mainly rely on phonons in crystals, molecular vibrations in polymers (which have limited tunability) and other sophisticated photonic and plasmonic structures (which rely on expensive techniques). In this work, we propose t he use of doped semiconductor nanoparticles as selective emitters, whose infrared optical properties can be readily tuned using the doping concentrations. The theoretical simulation shows that wide bandgap semiconductors with a bandgap greater than 3.5 eV can suppress solar absorption effectively. The plasma frequency f p should be approximately 2400 cm −1 to have enhanced emissivity in the atmospheric window for nanoparticles embedded in a medium with electric permittivity of 2.3. The complex refractive index of the metamaterial is calculated with the effective medium theory and the spectral and angular emissivity is estimated with the help of optical theories. A metamaterial containing 5% (volume fraction) doped ZnO nanoparticles is predicted to exhibit a cooling power greater than 100 W m −2 and a temperature drop of more than 10 • C under a wide range of ambient temperatures.
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