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Technology - Selective Surface - Technical Data
Technical Data and Details
Quartz Encapsulated Bimetallic Alloy
Alternate Energy Technologies, LLC (AET) and Thermafin Holding, LLC has introduced a new solar-selective absorber coating that is technically a Quartz Encapsulated Bimetallic Alloy (QEBA). Crystal Clear Solar-Selective Coating is a major departure from either metal oxides (eg. Black Nickel or Black Chrome) or CERMETS. QEBA possess a layer of precise dimension that interferes as an anti-reflective and it resembles metal oxides in that it possesses a branched, treelike structure. The interference layer is produced without the use of chemical or vacuum deposition and the branched structures are grown out of a metal alloy with no oxides. Upon close examination the branched structure resemble billions of tiny long leaf pine trees (dendrite structure).
This multi-directional dendrite structure provides excellent absorptance at a wide variety of incident radiation angles. The minimal thickness of the bimetallic alloy allows for almost complete transparency in the infrared spectrum allowing the coating to exhibit excellent thermal emittance properties.
The Quartz Encapsulation layer serves a two-fold purpose. First, it provides an optical index transition layer to prevent light rays from leaving the bimetallic alloy structure after they have entered. This light trapping effect is accomplished by total-internal-reflection at the higher optical index quartz-to-air surface. Second, it provides encapsulation that protects the extremely fragile dendrite structure from damage during the manufacturing process.
Most solar absorber coatings in operation today have a disadvantage of short field lifetime. Scientists worldwide attempted to tackle this problem by developing accelerated age testing for ISO. QEBA has done extremely well when tested according to the ISO protocol. The following summarizes QEBA performance in the various areas of the ISO protocol.
Thermal Aging In Air as tested by an independent third party laboratory:
Temperature / Humidity / Freeze Cycling as Tested:
Surface toughness is a much-neglected area of coating tests in the USA and in the ISO realm. Since all strip coatings are processed into solar collector absorbers by various welding and manufacturing processes the toughness of the surface is very important. Coatings may pass the ISO/CD 12952.2 testing, but be of little value once they have been welded, bent, handled, protected, and fabricated into a finished absorber. In fact, research has shown that adhesives commonly used to apply films to CERMETS & Black Chrome to protect them from handling and manufacturing damage actually begin a chemical attack of the "protected" surface underneath. Uniform methodology for evaluating surface toughness is a task for the ISO, but until then physical burnishing is our best test.
Surface Toughness as done by an independent third party laboratory:
Thermal Aging Comparison with Other Marketed Coatings:
* - Coating flaked off the substrate
Production of QEBA costs very little. The energy required is extremely low and the quantity of raw materials used is financially appealing. The environmental impact of the coating process is minimal. As with the new generation of coatings and cermets, QEBA requires a clean and smooth substrate.
AET/Thermafin has involved itself closely in all aspects of substrate preparation to finished QEBA to assure a truly environmentally friendly product as they are the recipient of the Environmental Stewardship Award from the local utility/water supply municipality.
QEBA can be coated onto a variety of metallic and non-metallic substrates. The first commercial application of a QEBA coating is for medium-to-high temperature solar collectors. The trade name being used for this application is Crystal ClearTM.
AET/Thermafin has constructed a facility to perform substrate preparation and QEBA deposition. This facility currently has a capacity of 125,000 m² per year and has processes in place, or contracted, for recycle of all products not consumed during manufacture.
QEBA technology allows for the tailoring of absorptive and emissive properties somewhat independently. This opens the doorway to the design of solar collectors that operate in specific temperature ranges with appropriate selectivity.
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