Selective Solar Absorber Coating for Low Laser Reflection
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Solution Overview
Problem
Existing solar collector materials face challenges with high infrared reflection leading to thermal radiation losses and inefficient heat transfer due to the high reflectivity of aluminum surfaces at laser wavelengths, requiring powerful lasers and low productivity in welding processes.
Innovation Solution
A composite material with an optically effective multilayer system on the sun-facing side and an optically active layer on the rear side, featuring a mirror layer, absorber layer, and protective layers to reduce reflection and enhance heat transfer, while maintaining low thermal radiation losses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a metallic carrier material with high infrared reflection is used, then thermal radiation losses are reduced, but the reflection at laser wavelengths remains high requiring powerful lasers and low productivity
Solution Approach 1:
The patent divides the coating system into functionally distinct segments: a lower optically inactive metallic layer (Al, Cu, or stainless steel) providing high infrared reflection, and an upper optically active interference layer (TiO2, SiO2, ZrO2) controlling laser wavelength reflection. This segmentation allows each layer to independently fulfill its specific function without compromising the other.
Solution Approach 2:
The patent applies local quality by making the optical properties position-dependent: the lower metallic layer maintains uniform high infrared reflection across the surface, while the upper interference layer is designed with specific thickness and refractive index to selectively reduce reflection at laser wavelengths (800-1064 nm) while preserving infrared reflection. This local differentiation resolves the contradiction between the two opposing requirements.
2Reliability
If the carrier material is made of aluminum or aluminum alloy, then heat transfer efficiency is improved, but laser welding becomes less effective due to high reflectivity
Solution Approach 1:
The patent introduces an optically active interference coating as an intermediary layer between the aluminum carrier material and the laser beam. This intermediary layer mediates the interaction by selectively absorbing or reflecting laser wavelengths while being transparent to the aluminum's thermal conduction properties, thus preserving heat transfer efficiency while enabling effective laser welding.
Solution Approach 2:
The patent changes the optical parameters (refractive index, thickness) of the coating layers to create destructive interference at laser wavelengths, reducing reflection from aluminum's typical 90%+ to manageable levels. The aluminum carrier material's thickness and composition can also be adjusted to optimize both thermal conduction and compatibility with the coating system.
3Use of energy by moving object
If a multilayer system with at least five layers is applied, then solar radiation absorption is enhanced, but device complexity increases
Solution Approach 1:
The patent employs composite materials combining metallic layers (Al, Cu, stainless steel) with optically active dielectric layers (TiO2, SiO2, ZrO2, HfO2). This composite structure integrates the high infrared reflection of metals with the tunable optical interference properties of dielectrics, achieving superior solar absorption while managing the complexity through systematic layer design rather than random multilayering.
Solution Approach 2:
The patent addresses complexity by transitioning from a single-layer approach to a multilayer dimensional structure, where each layer contributes a specific optical function. The systematic arrangement of layers with varying refractive indices and thicknesses creates constructive interference for solar absorption and destructive interference for laser reflection, transforming a complex problem into a manageable layered architecture.
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 composite material achieves high solar radiation absorption and reduced infrared reflection, improving heat transfer efficiency and productivity in solar collectors by minimizing laser energy loss and maintaining thermal radiation efficiency.
Implementation Method 1
an optically effective multilayer system made up of at least five layers... specifically influences the reflection... the optical parameters of this optically effective layer are selected
Implementation Method 2
absorb the incident solar radiation (300 - 2500nm) as best as possible and convert it into heat
Implementation Method 3
minimize losses through infrared radiation, the reflection of a material used there must be as high as possible... in the wavelength range between 2.5 μm and 50 μm
Implementation Method 4
the optically active layer on the rear side... specifically influences the reflection of the uncoated carrier material in such a way that the joining of heat transfer tubes to the composite material with the laser becomes more effective
Data Source
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AI summary
The invention relates to a composite material for use as a selective solar absorber having a carrier layer (1), wherein at least the following layers are present on a side (B) of the carrier layer: a reflection layer (3), an absorber layer (5), and a dielectric and/or oxidic antireflection layer (7), wherein a layer (4) made of a nitride, a carbide, or a carbonitride of a metal or a mixture of at least two metals from the subgroups IV, V, or VI is present between the absorber layer (5) and the reflection layer (3), and an optically active layer (6) made of a metal compound having stoichiometric composition is present between the absorber layer (5) and the dielectric antireflection layer (7). The invention further relates to a composite material for use as a selective solar absorber, having a carrier layer (1) made of aluminum or an aluminum alloy, wherein an optically active layer (8) is present on one side (A) of the carrier layer, reducing the reflection of the uncoated carrier material by at least 5% for a specific wavelength ? in the wavelength range of 200 nm to 10µm, preferably between 200 and 2500 nm, for irradiation at a specific index angle, and reducing the reflection of the uncoated carrier material by no greater than 20%, preferably no greater than 5%, in the wavelength range of 2.5 µm to 50 µm.