Spectrally Solar-Selective Nanotextured Stainless Steel Surfaces for Coating-Free Parabolic Trough Receivers

Abstract The urgent need to decarbonize medium- to high-temperature industrial process heat, combined with the high cost and limited lifespan of current parabolic trough collectors (PTCs), motivates the development of a simpler, more durable, and lower-cost alternative that retains commercial-grade performance. This work introduces a coating-free, vacuum-free spectrally selective solar absorber achieved by nanotexturing AISI 316 stainless steel with periodic submicron pyramidal arrays. Optical constants were rigorously obtained via multi-objective fitting of a Lorentz–Drude model to experimental data, enabling accurate 3D finite-difference time-domain (FDTD) wave-optics simulations over a broad wavelength range. The pyramidal geometry was optimized using particle swarm optimization coupled with FDTD and a one-dimensional steady-state thermal model. The optimized nanotextured surface (pyramid base 166 nm, height 755 nm) delivers solar absorptance αₛ = 0.98 and thermal emittance εₜₕ = 0.36 (at 400°C), absorbing 3–4% more incident solar energy than leading commercial cermet coatings (αₛ = 0.95–0.96, εₜₕ = 0.09–0.11) and compensating for the significantly larger emittance in IR. Under realistic PTC operating conditions (C = 82 suns, 1000 W/m2 incident flux, HTF temperature of 250°C), the monolithic nanotextured receiver achieves a thermal conversion efficiency of 78.6%, fully competitive with the best commercial vacuum-tube receivers (78.5–79.4%), while eliminating all components prone to degradation. By replacing complex coatings with purely geometric light trapping on a single structural alloy, this design would promisingly offer intrinsic high-temperature stability (>500°C), immunity to hydrogen-induced or oxidative degradation, and compatibility with scalable nanofabrication, paving the way for significantly simpler, lower-cost, and longer-lifetime parabolic trough systems for both power generation and solar industrial process heat.