Coral-inspired hierarchical structures for sunlight harvesting

Concentrating solar thermal (CST) is an efficient renewable energy technology with low-cost thermal energy storage. CST relies on wide-spectrum solar thermal absorbers that must withstand high temperatures (> 700°C) for many years, but state-of-the-art coatings have poor optical stability. Here, we show that the largely overlooked macro-scale morphology is key to enhancing both optical resilience and light trapping. Inspired by stony-coral morphology, we developed a hierarchical coating with three tuneable length-scale morphologies: nano- (~ 120 nm), micro- (~ 3 µm) and macro-scales (> 50 µm). Our coating exhibits outstanding, stable solar-weighted absorptance of > 97.75 ± 0.04% after ageing at 850°C for more than 2,000 hours. The scalability of our coating is demonstrated on a commercial solar thermal receiver, paving the way for more reliable high-performance solar thermal systems. Scleractinia, commonly known as stony corals (Fig. 1a), have evolved their morphology over millions of years to improve their chances of survival. A symbiotic relationship with algae, which need sunlight for photosynthesis, was an evolutionary milestone 240 million years ago that enabled corals to secure nutrients in otherwise infertile waters1 and thrive in all Earth’s oceans. Sunlight attenuation in seawater initially restricted coral colonies to shallow waters2,3. To thrive in deeper waters where light is more scarce, coral morphology4 has evolved to improve light trapping5 via multiple internal light reflections (Fig. 1b, c). We can then learn from stony-coral morphology in engineering and science where light trapping is needed, including sunlight harvesting using concentrating solar thermal (CST) systems6,7. Absorber coatings applied to solar receivers in CST plants have the function of converting concentrated sunlight in a wide-spectrum into thermal energy8 for many applications, including electric power generation (Fig. 1d) 9,10. Importantly, CST incorporates thermal energy storage, a more affordable, scalable, and durable alternative than other well-known storage technologies for long duration energy storage11. A key barrier to the wide adoption of CST, contributing to both increasing cost and reducing performance, is the poor durability of its light-absorbing coatings12. These coatings need to withstand high temperatures (> 700°C) and thousands of thermal cycles over many years of operation13. The best-known CST coatings are spinel-based coatings (Supplementary Note 1) such as Pyromark 2500® (henceforth referred to as Pyromark)14, which is widely considered the gold-standard in the CST industry. These coatings implement an organic binder15 that decomposes during a curing process to produce a nano-textured porous coating with spinel pigments, without macro-scale (> 50 µm) features. Solar-weighted absorptance, the key performance metric16, is typically reported after long-term isothermal exposure at high temperature, with the highest reported values being 94.6% after ageing for 2350 h at 850°C14, 97.2% after aging for 2,000 h at 800°C15, and 96.3% after aging for 3800 h at 770°C13. However, unstable optical performance is generally observed in CST coatings because the elevated temperatures re-arrange the material phases, alter the material composition13, and modify the nano-scale morphology via sintering and crystal grain growth17. Advanced light absorbers made of carbon nanotubes18 and graphene19 can absorb more than 99% of incoming light from every angle, but these coatings burn at the surface temperatures commonly found in conventional receivers20. Most coating research so far has focused on texturing the nano-scale morphology and improving the thermal stability of the materials13,15,21−23, while neglecting the micro- (~ 3 µm) and macro-scale (> 50 µm) geometries24 and the tuning of various length-scale morphologies in the coating to maximise light absorptance. Hierarchical structures have been shown to be a powerful tool to improve radiative cooling in clothing25, as well as mechanical rigidity and stability in sea sponges26. Here, we show that a hierarchical design with coral-inspired micro- and macro-scale features can produce high-temperature solar absorbers with enhanced light absorption and outstanding optical resilience, which we define as the capacity to retain stable optical properties despite material degradation.