Concentrating optics enable solar thermal energy to be harvested at high temperature (<100oC). As the temperature of the
receiver increases, radiative losses can become dominant. In many concentrating systems, the receiver is coated with a
selectively absorbing surface (TiNOx, Black Chrome, etc.) to obtain higher efficiency. Commercial absorber coatings are
well-developed to be highly absorbing for short (solar) wavelengths, but highly reflective at long (thermal emission)
wavelengths. If a solar system requires an analogous transparent, non-absorbing optic – i.e. a cover material which is
highly transparent at short wavelengths, but highly reflective at long wavelengths – the technology is simply not
available.
Low-e glass technology represents a commercially viable option for this sector, but it has only been optimized for visible
light transmission. Optically thin metal hole-arrays are another feasible solution, but are often difficult to fabricate. This
study investigates combinations of thin film coatings of transparent conductive oxides and nanoparticles as a potential
low cost solution for selective solar covers. This paper experimentally compares readily available materials deposited on
various substrates and ranks them via an ‘efficiency factor for selectivity’, which represents the efficiency of radiative
exchange in a solar collector. Out of the materials studied, indium tin oxide and thin films of ZnS-Ag-ZnS represent the
most feasible solutions for concentrated solar systems. Overall, this study provides an engineering design approach and
guide for creating scalable, selective, transparent optics which could potentially be imbedded within conventional low-e
glass production techniques.
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