Solar thermal energy conversion in high-temperature hierarchical solar absorbers

Abstract

Solar absorber coatings for high-temperature concentrated solar thermal (CST) systems are crucial for efficient solar-thermal energy conversion. This research focuses on hierarchical coatings, investigating light-matter interactions across nano-, micro-, and macro-length scales to understand how surface morphology and material properties influence energy conversion. It also evaluates the accuracy of Monte Carlo ray tracing (MCRT) and finite-difference time-domain (FDTD) modelling methods for multi-length scale structures, identifying the most suitable approach for various applications. Finally, experimental characterisation assesses the optical performance and durability of coatings under dynamic ageing, providing insights into their long-term stability and optimisation for CST systems.

A comparison of MCRT and FDTD methods demonstrates their applicability to modelling light-matter interactions in hierarchical solar absorber coatings. MCRT effectively simulates macro-scale morphologies with characteristic lengths exceeding 20 micro meters, while FDTD captures detailed wavelength-dependent behaviour at nano- and micro-scales. These results highlight the importance of aligning modelling techniques with specific length scales to achieve accurate predictions of radiative properties.

At the nano-scale, a scalable nanolayer architecture comprising monodispersed nanospheres embedded in a binding matrix demonstrates remarkable solar absorptance of up to 97.64% and excellent stability after 1000h of ageing at 900C. Both theoretical and experimental results confirm that this nanolayer enhances absorptance by more than 40%, even under extreme conditions. At the micro-scale, simulations show that deeper micropores with smaller diameters and moderate to high surface coverage significantly improve light-trapping effectiveness, achieving over 70% effectiveness on arbitrary materials. At the macro-scale, surface coverage emerges as the dominant factor in enhancing absorptance, with macro-scale protrusions delivering improvements in light trapping, regardless of configuration or diameter.

Additionally, the development of a titania-based coral-structured coating highlights excellent optical performance and durability under high-temperature conditions. This coating effectively inhibits titanium cation diffusion, ensuring long-term stability and maintaining high solar absorptance even after extended ageing. By evaluating changes in optical properties before and after ageing, this study provides key insights into the time-dependent durability of hierarchical solar absorbers, offering a reliable and scalable solution for CST applications. Show more