Broadband Solar Selective Absorber with Dallenbach-type Bilayer Structure Achieved Using Carbon Nanotube Membranes with Tailored Optical Spectra

TL;DR

This paper presents a novel solar absorber using a bilayer structure with a tailored carbon nanotube membrane, achieving high spectral selectivity and thermal performance for solar energy harvesting.

Contribution

It introduces a new design of a Dallenbach-type bilayer solar absorber using mixed chiral SWCNTs to tailor optical spectra, surpassing conventional materials.

Findings

Achieved solar absorptance of 0.84 and infrared emittance of 0.03.

Demonstrated high-temperature operation (~190°C) under nonconcentrated sunlight.

Outperformed blackbody-like absorbers in spectral selectivity.

Abstract

Spectrally selective absorbers that maximize solar absorption and minimize thermal radiation loss are crucial for efficient solar thermal energy harvesting. However, limitations imposed by the intrinsic properties of conventional materials hinder the fabrication of interference-based absorbers with desired optical properties. Herein, a high-performance solar absorber with a Dallenbach-type dielectric-metal tandem structure was fabricated using an ultrathin, subquarter-wavelength thickness single-walled carbon nanotube (SWCNT) membrane with tailored optical spectra as the absorbing layer. By mixing multiple SWCNT chiral structures, the optical response of the absorbing layer was tailored to approximate a theoretical complex refractive index spectrum of dielectrics; therefore enabling high solar absorptance, low infrared emittance, and a low angular dependence with a simple bilayer structure. The fabricated proof-of-concept 1 cm$^2$ absorber exhibited excellent spectral selectivity (solar absorptance/infrared emittance of 0.84/0.03), and an equilibrium temperature of $\approx 190^\circ$C ($270^\circ$C) under nonconcentrated ($2\times$ concentrated) sunlight, considerably outperformed a blackbody-like absorber. This study presents a high-performance solar absorber with a simple planar structure and proposes a concept of designing dielectric with the desired optical properties by mixing various types of structure-sorted SWCNTs.