Absorber Tube Coating Structure for High-Temperature Solar Selectivity
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Solution Overview
Problem
Existing absorber coatings for solar power generation face challenges in achieving high absorptance and low emissivity while maintaining durability at elevated operating temperatures, as the layers tend to degrade due to thermal stress and diffusion processes, leading to reduced absorptance and reflectivity.
Innovation Solution
The absorber coating incorporates a cermet material for the second barrier layer, composed of aluminium oxide and molybdenum, with an adhesion-enhancing layer and a sandwich structure that includes a third barrier layer, effectively preventing diffusion and enhancing adhesion, allowing for higher operating temperatures without compromising absorption properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the operating temperature is increased to improve energy conversion efficiency, then the energy conversion yield increases, but the durability of the absorber coating decreases due to thermal stress and diffusion processes
Solution Approach 1:
The absorber coating is divided into multiple functional layers: a substrate layer, a cermet absorption layer with high solar absorptance, an infrared-reflective layer to reduce thermal emissivity, and a protective oxide layer. This segmentation allows each layer to be optimized for its specific function while working together to maintain overall coating durability at elevated temperatures up to 550°C
Solution Approach 2:
The invention employs composite materials throughout the coating structure, particularly the cermet layer combining metal and ceramic phases for simultaneous optical absorption and thermal stability, and the infrared-reflective layer using composite oxide structures. These composite materials provide the necessary mechanical strength and thermal resistance to withstand high operating temperatures without degradation
2Reliability
If a molybdenum infrared reflective layer is used to achieve low emissivity, then the reflectivity in infrared range improves, but the adhesion and structural stability deteriorate due to internal stresses
Solution Approach 1:
An intermediate protective oxide layer is introduced between the molybdenum infrared-reflective layer and the substrate or adjacent layers. This intermediary layer acts as a stress buffer that accommodates thermal expansion differences and prevents direct stress transmission, thereby maintaining adhesion and structural integrity while preserving the low emissivity property of the molybdenum layer
Solution Approach 2:
The coating structure implements local quality optimization by placing the molybdenum infrared-reflective layer only where infrared reflection is needed, while using different materials (cermet, protective oxides) in regions where adhesion and stress resistance are critical. This localized material selection allows the molybdenum layer to fulfill its optical function without compromising overall structural stability
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
This configuration achieves high absorption (>95.5%) and low emissivity (<10%), enabling efficient energy conversion and extending the lifespan of the coating, allowing for operating temperatures up to 550°C and reducing energy costs by eliminating the need for additional heat exchangers and enabling the use of less expensive heat carrier media like water.
Implementation Method 1
a first barrier layer (24a), which faces the absorber tube and consists of a thermally produced oxide
Implementation Method 2
an infrared reflective layer (21), which is arranged on the barrier layers and is reflective in the infrared range
Implementation Method 3
at least one absorption layer (22), which is arranged above the infrared reflective layer
Data Source
AI summary
The radiation-selective absorber coating for absorber tubes of parabolic trough collectors includes two or more barrier layers (24a, 24b); an infrared reflective layer (21) on the barrier layers (24a, 24b); at least one cermet absorption layer (22) above the infrared reflective layer (21) and an antireflection layer (23) above the at least one cermet absorption layer (22). The two or more barrier layers (24a, 24b) include a first barrier layer (24a) of thermally produced oxide and a second barrier layer (24b) arranged above it. The second barrier layer (24b) is a cermet material including at least one oxide compound and at least one metal. The oxide compound is aluminium oxide, silicon oxide, nickel oxide and/or chromium oxide. The metal is molybdenum, nickel, tungsten and/or vanadium. The invention also includes an absorber tube with the absorber coating on it.