Beam-controlled spectral-selective architecture for a radiative cooler
A passive cooler of the disclosure includes a thermal emitter having a substrate and a coating disposed on at least a portion of a first side of the substrate. The cooler has a beam guide made from a material having a high absorption to solar wavelengths and high reflectance at mid-infrared wavelengths. The beam guide is configured such that at least a portion of incident light is acted on by the beam guide before reaching the thermal emitter. In some embodiments, the beam guide has a graded optical index.
1. A passive cooler, comprising:
a thermal emitter having a substrate and a coating disposed on at least a portion of a first side of the substrate; and
a beam guide made from a material configured to absorb more than 40% of incident light at wavelengths <˜4 μm and to absorb less than 40% of incident light at wavelengths from ˜4˜13 μm, and wherein the beam guide is configured such that at least a portion of incident light is acted on by the beam guide before reaching the thermal emitter.
2. The passive cooler of claim 1 , wherein the beam guide has an optical index which is graded over at least a portion of a thickness of the beam guide.
3. The passive cooler of claim 2 , wherein the beam guide comprises a graded index film.
4. The passive cooler of claim 3 , wherein the graded index film comprises metal particles, and the density of metal particles varies over at least a portion of a thickness of the film.
5. The passive cooler of claim 4 , wherein the graded index film comprises SiO2, and the density of SiO2 varies over at least a portion of a thickness of the film.
6. The passive cooler of claim 5 , wherein the graded index film comprises two or more layers, and wherein at least one layer having a density of Ag and/or SiO 2 which is different than a corresponding density of at least one other layer.
7. The passive cooler of claim 1 , wherein the beam guide is configured to absorb more than 40%, 50%, 60%, 70%, or 80% (or any value therebetween) of incident light at wavelengths <˜4 μm.
8. The passive cooler of claim 1 , wherein the beam guide is configured to absorb less than 40%, 30%, 20%, 10%, or 5% (or any value therebetween) of incident light at wavelengths from ˜4˜13 μm.
9. The passive cooler of claim 1 , wherein the coating of the thermal emitter is configured to absorb less than 20% of incident sunlight at wavelengths <2 μm.
10. The passive cooler of claim 1 , wherein the substrate is a metal.
11. The passive cooler of claim 10 , wherein the substrate is aluminum.
12. The passive cooler of claim 1 , wherein the coating is polydimethylsiloxane (PDMS).
13. The passive cooler of claim 1 , further comprising a transparent cover spaced apart from the first side of the substrate.
14. The passive cooler of claim 1 , wherein a second side of the substrate has a coating.
15. The passive cooler of claim 14 , further comprising a transparent cover spaced apart from the first side and the second side of the thermal emitter.
16. The passive cooler of claim 1 , further comprising a housing having one or more insulative walls, wherein the insulative walls together with a second side of the thermal emitter define a chamber such that the second side of the substrate is exposed to the chamber.
17. The passive cooler of claim 1 , wherein the beam guide further comprises:
a tapered outer member having a spectrally-selectively inner surface configured to reflect mid-infrared light and absorb visible light and near-infrared light; and
a cooperating inner member having a spectrally-selective outer surface configured to cooperate with the outer member to guide incident mid-infrared light towards the thermal emitter.