Systems and methods for passive cooling and radiator for same
Disclosed herein are systems and methods for passively cooling water vapor to enable efficient condensation, and methods of making such systems. A passive cooler can include a thermally conductive substrate having a first side and a second side opposite the first side, a coating disposed on at least a portion of the first side of the substrate, and a housing having one or more insulative walls. The insulative walls may define a vapor flow channel from an inlet to an outlet of the housing such that the second side of the substrate is exposed to water vapor flowing through the vapor flow channel.
1. A passive cooler, comprising:
a thermally conductive substrate having a first side and a second side opposite the first side, wherein the first side has a solar reflectance of at least 95%;
a coating disposed on at least a portion of the first side of the substrate, the coating being transparent or substantially transparent to solar radiation and having emissivity of greater than 0.95 over a majority of the spectral band between 4 and 25 μm; and
a housing having one or more insulative walls, wherein the insulative walls together with the substrate define a vapor flow channel from an inlet of the housing to an outlet of the housing such that the second side of the substrate is exposed to vapor flowing through the channel.
2. The passive cooler of claim 1 , wherein the thermally conductive substrate has a conductive heat transfer coefficient of at least 3 Wm −2 K −1 .
3. The passive cooler of claim 1 , wherein the substrate comprises a reflective layer on the first side, wherein the reflective layer imparts solar reflectivity.
4. The passive cooler of claim 3 , wherein the reflective layer is silver.
5. The passive cooler of claim 1 , further comprising a fan configured to move vapor through the vapor flow channel of the housing.
6. The passive cooler of claim 1 , further comprising a transparent cover spaced apart from the first side of the substrate.
7. The passive cooler of claim 6 , wherein the coating is at least 50 μm thick.
8. The passive cooler of claim 1 , wherein the substrate is aluminum.
9. The passive cooler of claim 1 , wherein the substrate is glass.
10. The passive cooler of claim 1 , wherein the coating is a silicon-based organic polymer.
11. The passive cooler of claim 10 , wherein the silicon-based organic polymer is polydimethylsiloxane (“PDMS”).
12. A method of making a passive cooler, comprising:
forming a radiative cooling plate by providing a thermally conductive substrate having a first side and a second side, the first side having a solar reflectance of at least 95%, and applying a coating onto the first side, wherein the coating is transparent or substantially transparent to solar radiation and having emissivity of greater than 0.95 over a majority of the spectral band between 4 and 25 μm; and
placing the radiative cooling plate within an opening of an insulating housing such that an emission from the first side of the radiative cooling plate exits the insulating housing via the opening and the second side of the cooling plate is exposed to a vapor flow channel of the housing.
13. The method of claim 12 , further comprising covering the opening with a transparent cover spaced apart from the first side of the substrate.
14. The method of claim 12 , further comprising covering an external surface of the housing with a reflective layer.
15. The method of claim 14 , wherein the reflective layer is a foil tape.
16. The method of claim 15 , wherein the foil tape is aluminized foil tape.
17. The method of claim 12 , wherein the thermally-conductive substrate includes a reflective layer on the first side.
18. The method of claim 12 , wherein the coating is a silicon-based organic polymer.
19. The method of claim 18 , wherein the silicon-based organic polymer is polydimethylsiloxane (“PDMS”).
20. A method of condensing water, comprising:
providing a passive cooler according to claim 1 ; and
passing water vapor through the vapor flow channel of the passive cooler such that at least a portion of the water vapor condenses on the second side of the substrate to yield condensate.
21. The method of claim 20 , wherein the first side of the substrate is exposed to a moving air mass, such that thermal energy is emitted from the passive cooler via convection.