Example 2: IR multilayers

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 through 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.