  This work experimentally studies silicon-cored tungsten nanowire selective
metamaterial absorber to enhance solar-thermal energy harvesting, simply
fabricated by conformally coating a thin tungsten layer onto a commercial
silicon nanowire stamp. Scanning electron microscopy is used to characterize
the morphology change before and after the tungsten deposition, while optical
spectroscopy is carried out to measure the spectral absorptance (or emittance)
in the broad wavelength range from solar spectrum to infrared. It is shown that
the tungsten nanowire absorber exhibits almost the same total solar absorptance
around 0.85 as the silicon nanowire but with greatly reduced total emittance
down to 0.18, which could significantly suppress the infrared emission heat
loss. A lab-scale solar-thermal test apparatus is used to measure the
solar-thermal efficiency of the tungsten nanowire absorbers from 1 up to 20
suns, experimentally demonstrating the improved performance due to excellent
spectral selectivity over the silicon nanowire and black absorbers. With
detailed heat transfer analysis, it is found that the tungsten nanowire
absorber achieves an experimental efficiency of 41% at temperature 203degC
during the solar-thermal test with stagnation temperature 273degC under 6.3
suns. It is projected to reach 74% efficiency at same temperature 203degC with
stagnation temperature of 430degC for practical application without parasitic
radiative losses from side and bottom surfaces, greatly outperforming the
silicon nanowire and black absorbers. The results would facilitate the
development of novel metamaterial selective absorbers at low cost for
highly-efficient solar thermal energy systems.
