THIN FILM SERVICES / APPLICATION AREA
 

infrared & low-thermal-signature coatings  
        example: spectrally selective solar absorbers


 MATERIALS / APPLICATION AREA:  Due to the very broad spectrum involved, infrared (IR) applications cover an exceedingly diverse range of thin film materials and applications, frequently involving unique requirements under uniquely harsh conditions.  Helicon's Thin Film System's history has included several thin film fabrication and analysis projects primarily in near-IR and mid-IR applications.  Helicon has provided extensive development work and consulting in thin film processes and architectures in infrared (IR) optical materials, ranging from various complex pyroelectric and thermoelectric material phases, to multi-function, missile radome coatings.  

EXAMPLE
 APPLICATION: Helicon has executed multiple thin film development programs in low-emissivity, spectrally selective solar absorbers.  In the various solar energy-generating apparatus that utilizes thermal energy, low-emissivity absorber coatings allow efficient conversion of sunlight to usable thermal energy, where radiant lo
sses, due to black-body emission, are minimized, thus markedly improving the attainable efficiency of the solar-thermal devices in which such
 
coatings are used. 
 

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This specific application/scenario
 
essentially entails the creation of spectrally selective coatings that behave as closely as possible to an ideal black-body
 
(epsilon = 1)
 
in the spectral region of highest solar
 
irradiance, 
whereas, this
 
same coating must behave spectrally as, essentially, a highly reflective metal 
(epsilon close to 0)
 
in the mid-to-far infrared regions (blue trace in adjacent figure),
 
the latter being the spectral region wherein essentially
 
all thermal 
emission occurs for practical temperatures of interest (e.g., 100C to 800C). 
 
This is typically accomplished through thin film 
layers comprising
 
graded, nano-structured cermets admittance-matched to air/vacuum
 
t
hrough
 
overlying dielectric layers.

Much commercial success has been realized in this area within the last decade, where success must be defined as a nexus between optical/thermal performance, cost performance, and a high environmental/thermal cycling reliability (as with all solar energy products).  Whereas the US has been relatively slow in commercially adopting the recent offerings in solar-thermal technology, these developments have revolutionized the solar-thermal industry in countries that rely heavily on solar-thermal energy, where module efficiency has improved dramatically, from roughly 60% to >90% in low-temperature applications, as a direct result of switching to these sub-micron multilayer coatings.  
 



 
 This performance advantage is provided for a fraction of the cost of a relatively low-efficiency PV absorber (e.g. copper indium selenide, cadmium telluride, etc). Put differently, such low-emissivity coatings are an extremely cost-effective means for enabling a tremendous efficiency gain; gains that would be competely unheard of in the solar PV arena.  
FIGURE 1: The blue line represents the typical idealized step-function reflectance spectra for the solar-selective coating as frequently portrayed in the research literature.  However, neither the step-function aspect nor the position represents a necessarily ideal performance; simply a didactic tool.  Ideal performance will be geography and operation dependent (this is the typically represented "AM1.5" solar spectra).  

Helicon remains enthusiastically engaged in the various thin film technologies applied in various high-temperature, mid-temperature and low-temperature solar-thermal devices.   
In the adjacent Fig. 2 is provided comparison of one example of the many reliability-tested, spectrally selective solar-absorber coatings we've developed at Helicon.  Comparison in the Figure is 
made to the best-known, top-tier German manufacturers' flagship commercial products (Alanod's "Sunselect" and Bluetec Gmbh, as labeled).  A "solar selective" commercial 
product from China is aso included.  It can be noted, by super-imposing the above solar irradiance, that our Helicon coating provides a performance level equivalent to the top German competitors.


 
      Helicon's coatings vs well-known commercial manufacturers:

 






 



FIGURE 3: Solar-selective coating deposited on 10cm copper strip at Helicon Thin Film Systems facility

 

FIGURE 2:Vis-NIR-Mid-IR, diffuse+direct reflectance of several spectrally selective absorber products these are generally graded nano-structured cermets admittance-matched to air through overlying dielectric layers.
  

 



In conclusion, it should be noted that the optical characteristic/coating designs in this application can be substantially altered, depending upon variations in application, color aesthetics, and geographical  solar conditions, etc ; also, it's worth noting that the absorption/reflection characteristic could be marginally improved in any of the solar-selective coatings displayed on this page by incorporating more layers into the coating design.  However, competitive coatings must rely on relatively few layers, if one is to simultaneously optimize environmental cycling, reliability, and cost targets. Additionally, higher-layer-number coatings to enhance the blue-end absorption performance will also tend to attenuate the desired optimum reflectance in the mid-to-far IR wavelengths.