Radiative cooling with solar spectrum reflection
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Solution Overview
Problem
Daytime radiative cooling is challenging due to solar radiation heating, which complicates the achievement of cooling below ambient air temperatures, especially in buildings exposed to direct sunlight, as existing technologies struggle to effectively manage solar radiation absorption and thermal emission.
Innovation Solution
A radiative cooling apparatus comprising a multi-layer stack of materials with a solar spectrum reflecting portion and a thermally-emissive portion, integrated along a depth dimension, which suppresses solar radiation absorption and enhances mid-infrared emission, allowing for cooling even under direct sunlight by reflecting solar wavelengths and emitting thermally-generated radiation in the atmospheric transparency window.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a device is exposed to the sky for radiative cooling, then cooling below ambient air temperature can be achieved at night, but daytime cooling is difficult due to heating by the sun
Solution Approach 1:
The device spectrum is segmented into two distinct portions: a solar spectrum portion (0.3-2.5 micrometers) that is reflected to minimize heating, and a thermal emission portion (8-13 micrometers atmospheric window) that is emitted for cooling. This spectral segmentation allows the device to simultaneously reject solar heating and emit thermal radiation for cooling.
Solution Approach 2:
The device exhibits different optical properties at different wavelengths: high reflectivity in the solar spectrum range and high emissivity in the thermal emission range. This local quality variation across the spectrum enables the device to reflect harmful solar radiation while efficiently emitting cooling radiation through the atmospheric transparency window.
2Temperature
If radiative cooling is implemented during daytime, then cooling demand can be met during peak hours, but solar radiation absorption prevents achieving temperatures below ambient
Solution Approach 1:
The device converts the harmful solar radiation that would normally heat the building into a beneficial effect by reflecting most solar wavelengths while selectively emitting thermal radiation in the atmospheric window. The solar reflection property prevents heating, and the thermal emission property actively cools the building during daytime hours.
3Temperature
If a structure is exposed to direct sunlight, then daytime cooling is more useful, but achieving cooling below ambient temperature becomes significantly more challenging
Solution Approach 1:
The device changes its optical parameters across different wavelength ranges: maintaining high reflectivity (R > 0.9) across the solar spectrum (0.3-2.5 micrometers) to reject solar heating, while achieving high emissivity (E > 0.9) in the thermal emission range (8-13 micrometers) to maximize radiative cooling through the atmospheric transparency window.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The apparatus achieves a temperature drop of 4-5 degrees Celsius below ambient air temperature even under direct sunlight, providing effective daytime cooling and reducing energy demands by minimizing heat absorption and maximizing thermal emission, thus offering a passive and energy-efficient cooling solution for buildings and structures.
Implementation Method 1
a solar spectrum reflecting portion configured and arranged to suppress light modes, thereby inhibiting coupling of the incoming electromagnetic radiation, of at least some wavelengths in the solar spectrum
Implementation Method 2
suppress light modes, thereby inhibiting coupling of the incoming electromagnetic radiation
Implementation Method 3
a thermally-emissive portion that includes a portion of the different material that are arranged in the depth dimension, and configured and arranged to facilitate, simultaneously with the inhibiting coupling of the incoming electromagnetic radiation, thermally-generated electromagnetic emissions from the object
Implementation Method 4
radiatively emit to outer space through a transparency window in the atmosphere between 8-13 micrometer wavelength range
Implementation Method 5
a structure facilitates far-field radiation at particular wavelengths while blocking radiation at solar wavelengths
Implementation Method 6
The plurality of different material can include multiple alternating layers of material that can reflect up to 97% of incident sunlight while emitting strongly and selectively in the atmospheric transparency window
Data Source
AI summary
Various aspects as described herein are directed to a radiative cooling apparatuses and methods for cooling an object. As consistent with one or more embodiments, a radiative cooling apparatus includes an arrangement of a plurality of different material located at different depths along a depth dimension relative to the object. The plurality of different material includes a solar spectrum reflecting portion configured and arranged to suppress light modes, thereby inhibiting coupling of the incoming electromagnetic radiation, of at least some wavelengths in the solar spectrum, to the object at a range of angles of incidence relative to the depth dimension. Further, the plurality of material includes a thermally-emissive arrangement configured and arranged to facilitate, simultaneously with the inhibiting coupling of the incoming electromagnetic radiation, the thermally-generated electromagnetic emissions from the object at the range of angles of incidence and in mid-IR wavelengths.