Metallic Waveguide Array for Directional Radiative Cooling
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Solution Overview
Problem
Satellites in space face challenges in cooling due to the inability to use conduction or convection, relying solely on radiative cooling, which can be ineffective when a body like the Sun or Earth subtends a portion of the cooling panel, causing the satellite to absorb heat instead of emitting it effectively.
Innovation Solution
Employing a metallic waveguide array tuned to the infrared region with specific spectral and spatial control, allowing for the rejection of incoming radiation while emitting waste heat in a controlled direction, using an aspect ratio that provides spectral selectivity and directionality without incurring significant ohmic losses, and orienting the waveguide axis obliquely to avoid absorption of radiation from other bodies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a flat black panel is used to radiate waste heat into deep space, then the satellite can be cooled effectively, but the panel will absorb incident thermal radiation from celestial bodies like the sun, earth, or moon, causing the satellite to heat up instead of cool
Solution Approach 1:
The patent applies local quality by making different parts of the cooling panel have different optical properties. Specifically, the panel uses selective surfaces that are highly emissive in the infrared wavelength range (8-14 μm) where the satellite emits heat, but highly reflective in the visible and near-infrared ranges where sunlight and terrestrial radiation occur. This allows the same surface to simultaneously emit thermal radiation effectively while reflecting harmful incident radiation from celestial bodies.
Solution Approach 2:
The patent employs color changes in the optical sense by using materials with wavelength-selective emissivity. The cooling panel appears different at different wavelengths: it is effectively 'black' in the infrared range (high emissivity for heat emission) but 'white' or reflective in the visible spectrum (low absorption of sunlight). This wavelength-dependent optical property resolution allows the panel to emit heat while rejecting solar and terrestrial radiation.
2Temperature
If the waveguide depth is increased to establish directionality, then spectral selectivity and directional control are improved, but manufacturing difficulty increases and ohmic losses become significant
Solution Approach 1:
The patent applies parameter changes by optimizing the waveguide dimensions to specific ranges that balance directional control with manufacturing feasibility and loss minimization. The waveguide depth is set to approximately 10-100 micrometers, and the width is optimized to provide the desired directional emission pattern while maintaining acceptable ohmic losses. By carefully selecting these geometric parameters, the system achieves sufficient directionality without incurring excessive manufacturing complexity or energy losses.
Solution Approach 2:
The patent uses partial action by implementing waveguides with depths that provide adequate directionality for the application but are not maximally deep. This partial implementation achieves the necessary directional control for radiative cooling while avoiding the excessive manufacturing complexity and ohmic losses that would result from much deeper waveguide structures. The design accepts a compromise that provides sufficient directional emission without pushing to the theoretical maximum depth.
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 solution effectively directs thermal radiation away from the satellite, preventing absorption of unwanted radiation and enhancing cooling efficiency by emitting waste heat towards deep space while rejecting radiation from other celestial bodies, thus maintaining thermal equilibrium.
Implementation Method 1
employing a metallic waveguide that is arranged to be tuned to an infrared (IR) region of interest in order provide both spectral and spatial control over the emitted/absorbed thermal radiation
Implementation Method 2
One of the principles of radiative cooling is Kirchoff's Law, which states that in thermal equilibrium the processes of thermal absorption and thermal emission are equivalent
Implementation Method 3
it is possible to reject, e.g., reflect, particular incoming wavelengths of radiation while simultaneously emitting other wavelengths of radiation
Implementation Method 4
a panel with an array, e.g., a one- or two-dimensional array, of such IR waveguides directs thermal radiation from a body in a specific direction
Implementation Method 5
the waveguide axis may be obliquely oriented with respect to the substrate normal... radiation from that body will not be absorbed because it is not in line with the waveguides of the array
Data Source
AI summary
A metallic waveguide tuned to an infrared region of interest provides spectral and spatial control over emitted/absorbed thermal radiation. The ratio of the depth of the waveguide to the smallest lateral dimension thereof is such that that the lateral dimension provides spectral selectivity and that the waveguide is deep enough for a fixed lateral dimension to establish directionality but is not so deep that it incurs severe ohmic losses. A panel with an array of such waveguides directs thermal radiation from a body in a specific direction and with a spectral response that is the result of the physical dimensions of the individual waveguides that make up the waveguide array and the arrangement of the waveguides in the array. The waveguide axis may be obliquely oriented with respect to the substrate normal so as to impart non-normal directionality to the emitted radiation with respect to the substrate normal surface.