High Expectations in Many Industries for Innovative New Selective-Surface Coating,  Black CrystalÔ

News for The Solar Industry…

New Coating Process Promises to Rock Solar Industry
Solar Quantum leap

New Mexico & Florida is where the latest developments in the absorber coating for solar collectors are in progress. "Black Crystal™ Coating" was developed in a cooperative effort by Energy Laboratories, Inc. (ELI) a private company and Sandia National Laboratories (SNL) - one of the largest research institutes in the world. The goal of this effort was to create a product which could withstand the high temperature rigors of a cutting edge,  high temperature Super-Collector developed by ELI. The coating also needed to comply with economic, ecological and technical production demands of the 21st century solar industry while increasing competition to fossil fuels.

Revolutionary Coating Procedures

The result is dramatic. According to Rod Mahoney, Project Manager for SNL, Black Crystal™ has, in comparison to the widely used black chrome coating, demonstrated several improvements. Especially in production where the power costs can be reduced by an astonishing 96%. Better still are the improvements in pollution reduction. The new process greatly reduces the exposure of environmental pollution during production by generating no carcinogenic products and reduces the volume of water consumed up to 90% less than that of a conventional black chrome process. Further improvements have also been dedicated to the delicate outer coating. Black chrome was always protected by a paper or plastic film covering where as the Black Crystal™ is permanently sealed with a flexible crystalline coating thus eliminating the previous problems associated with the handling and the paper and plastic film coatings. Also improved is the thermal stability in air. The best black chrome may survive 200°C for short periods while Black Crystal™ is long term stable at over 320°C.
 

Environmentally friendly solar selective absorber material is the result of laboratory / industry collaboration

A new and environmentally friendly solar-selective absorber coating has been developed in Cooperative Research and Development Agreement between Sandia National Laboratories and Energy Laboratories, Inc., based in Jacksonville, Florida. (Sandia and the National Renewable Energy Laboratory operate jointly as Lun Lab, but the CRADA is legally with Sandia.) The material is produced using a new process that combines a crystallographic- metallic alloy and the application of a solution-derived-glass overcoat to improve optical properties and mechanical and thermal durability. This new high-performance solar-selective absorber material, called Black Crystal™ when on nickel-coated copper substrates, is designed for use in solar thermal water and air-heating systems.

The new coating process provides several advantages over existing products and methods. It eliminates chromium from the waste stream; it enables a dramatic reduction in the amount of energy required, from 30 to 80 times less than the black chrome process, with optical properties equivalent to black chrome. The ruggedized surface of the coating is resistant to burnishing and subsequent manufacturing operations. [Absorber components coated with solar-selective materials are important in thermal flat-plate collectors and concentrators because these materials significantly improve the thermal conversion efficiency over solar non-selective absorber heat losses, via radiation, as the operating temperatures increase. This new process should enable continued competition in the production of affordable solar-selective absorber coatings and further reduce the cost of this key component.

An additional feature of the Black Crystal coating is the potential to provide a low-cost high-temperature solar-selective absorber coating for parabolic concentrator receiver components, which operate at 370°C (700°F). Currently, no solar-selective coating is available that will remain optically stable in air for the thousands of hours required for power plant operation. Current estimates are that the new coating could reduce the cost of parabolic receivers by a factor of 3.

This new product provides a coating that addresses the environmental concerns of chromium bath solutions and has potential in solar thermal applications where glazings are not required and in higher temperature systems including parabolic concentrators. The new coating exhibits tailorable solar absorptance and thermal emittance values ranging from 92%-98% and 8%-25%, respectively, and is thermally stable at 325°C on nickel-coated copper substrates.

Black chrome is the solar-selective absorber material most widely used worldwide and is plated from chromic acid bath solutions and has potential in solar thermal applications where glazings are not required and in higher temperature systems including parabolic concentrators. The new coating exhibits tailorable solar absorptance and thermal emittance values ranging from 92%-98% and 8%-25%, respectively, and is thermally stable at 325°C on nickel-coated copper substrates.

Black chrome is the solar-selective absorber material most widely used worldwide and is plated from chromic acid bath solutions. The new process relies on simple, easily formulated less-hazardous solutions and low energy consumption to produce a coating with optical properties equivalent to (or better than) black chrome.

The new Black Crystal™ process consists of two parts. The first part of the process deposits a solar-selective coating at low energy consumption levels. This process rapidly grows a layer of tightly packed pinnacle structures that are comprised of a crystallographic-metallic alloy clustered in an optimal aspect ratio to enhance the absorption of electromagnetic energy within the solar wavelength region. The thermal emittance characteristics of this new coating are dependent upon both the coating thickness , the solar absorption characteristics are enhanced by not allowing the light rays that enter the overcoat / crystallographic-metallic alloy matrix to exit by total internal reflection because of the optical index difference at the matrix-air interface. Typically, a 4 to 6 percentage-point solar absorption increase is possible with only a slight increase in the thermal emittance.

The overcoat is uniformly applied in the liquid phase, using specialized techniques. The overcoat is then rapidly cured to the solid phase forming a durable glasslike layer that exhibits a uniform visual appearance and produces an absorber surface with high solar absorptance and low thermal emittance values, and a rugged surface that is resistant to burnishing and subsequent manufacturing operations. The overcoat process also provides additional thermal stability by decreasing the rate of oxidation of the as-deposited matrix. The optical properties for a Black Crystal™-coated sample, with and without the overcoat, as a function of thermal aging at 325°C in-air, in laboratory furnaces, are shown in the accompanying graphs.

Because of the inherent physical geometry of the coating surface enabling high absorptance, at normal and grazing incident angles, within the ultraviolet to near infrared electromagnetic spectrum and the resistance of the surface to abrasion, any application where these attributes are important is possible. Some examples include: photo and image processing technologies, optical/detector applications, and optical instrumentation.
Other possible solar thermal applications include heating systems that do not require glazings and higher temperature systems that include parabolic concentrators. Additional investigations are continuing for higher temperature applications which include parabolic concentrators; however, applications without glazings have not been addressed since more development would be needed to optimize the protective overcoat for these specialized exposure conditions.

The new solar-selective absorber coating process is having an immediate impact on the solar thermal collector industry and has significant potential to impact the solar thermal electric industry. Because of the positive attributes described, the new coating process is believed to have overcome three major barriers, manufacturability, product cost, and increased competition, to reducing the cost of the key component in a high performance solar thermal system: the solar-selective coating.

"Spectra Black"™ and "Spectra Shield"™

Black Crystal™ is composed of Spectra Black and Spectra Shield.
Spectra Black™ is a Crystallographic Bi-metallic Alloy Matrix which is constructed, using proprietary Nano-Technology, onto a substrate to create a crystalline radiation trap tailored to absorb radiation in the 190 to 2,700 nanometer wavelength range. This is then enhanced by...
Spectra Shield™ which is a Polycrystalline Silica Network fused into the Bi-metallic matrix to provide stability and resistance to environmental extremes (e. g. water, sand, dust, etc.).

Further duties of Spectra Shield™ are to encapsulate the basis-materials hermetically and long-term-solidly with a high grade of flexibility and stability of temperature (more than 1,200 °C). The basis for this coating is similar to the structure of aerogels, which serves as a closed heat-block. The basis-treatment which is used for Black Crystal™ originates from the current proven space-technology and has been used successfully for years.

Energy-requirements and amortization

Conventional coating-processes, for example electrolytic-coating, vacuum-coating, plasma-coating, etc., have an extremely high energy-requirement. The pay-back-time of your solar system using Black Crystal™ will be substantially shortened because of the low energy-consumption during the whole production. The energy-costs for producing Black Crystal™ as compared to black chrome is approximately 96% less.

By developing the new crystallographic matrix coating-process much attention has been paid to change course, that referring to system-prices, costs were reduced and efficiency was increased. This works now with Black Crystal™ and makes it possible to double the production-capacity in comparison to existing coating-systems while reducing system-prices for Black Crystal™ off at least one third (differs from the way of coating). Despite technical and ecological advantages of Black Crystal™ prices can be offered in market bounds like black chrome. Due to the fact that the energy-effort and the costs of machinery-building have been reduced, a further important step in direction "energetic amortization" of any kind of collector-building has been made.


Developed and Tested

Black Crystal™ is composed for stability at high temperatures. It is resistant under all weathers and environment conditions like for example dust, pollution or moisture. These special characteristics have already been proved in accelerated age-tests. Black Crystal™ has already shown at all ended tests a very high selectivity and stability which are, in this form, above all other high-power-absorber-coatings.

Black Crystal™ has been developed in a longtime research- and development-agreement (CRADA) in corporation of Energy Laboratories, Inc. of Jacksonville, Florida, USA and Sandia National Laboratories of Albuquerque, New Mexico, USA. As well ELI as Sandia are confirming the advantages opposite customary ways of coating and production especially the high stability in the high-temperature-area.
 

Advantages

For more information on this product please contact:

Mike Newman
Energy Laboratories, Inc.
Attention: Thin-films Division
5191 Shawland Road
Jacksonville,  FL 32254
904-693-4555 phone