Surface-pitted TiN nanoparticles for direct absorption solar collectors

Abstract Direct absorption solar collectors use nanofluids to absorb and convert solar radiation. Despite the limitations of the photothermal properties of these nanofluids within the absorption spectra range, modifying the surface structure of the nanoparticles can broaden their absorption spectrum, thereby significantly improving the solar thermal conversion efficiency. This paper utilizes the finite element method to investigate the influence of surface pits on the photothermal properties of plasmonic nanoparticles, considering both material composition and surface micro-nano structures. Based on the findings, a novel TiN nanoparticle is proposed to enhance photothermal performance. This nanoparticle exhibits the lowest average reflectance (0.0145) in the 300–1100 nm wavelength range and the highest light absorption intensity across the solar spectrum, enabling highly efficient solar energy conversion. It not only reduces material costs but also effectively broadens the light absorption spectrum of spherical plasmonic nanoparticles. The distributions of the electric field, magnetic field, and energy field of the nanoparticles indicate that the combination of the “lightning rod” effect and surface plasmon resonance (SPR) significantly enhances both the electric and magnetic fields, thereby increasing the localized heating effect and improving the photothermal performance. Additionally, the number and size of the pits have a significant impact on the absorption efficiency ( η abs ) of TiN nanoparticles. When the surface of the nanoparticles has 38 pits, η abs can reach 90%, with the minimum optical penetration depth ( h ) of the nanofluid being 7 mm and the minimum volume fraction ( f v ) being 6.95 × 10 −6 . This study demonstrates that nanoparticles with micro-nano structures have immense potential in solar thermal applications, particularly in the field of direct absorption solar collectors.