Scientists at the University of the Basque Country have developed nanomaterials that absorb up to 99.5% of incoming sunlight, potentially transforming the efficiency of concentrated solar power plants.
The copper cobaltate nanoneedles, enhanced with zinc oxide coating, demonstrate superior thermal stability compared to existing carbon nanotube technology while maintaining exceptional light absorption properties under extreme operating conditions.
The breakthrough addresses a fundamental engineering challenge in concentrated solar power systems: creating materials that can simultaneously trap nearly all incoming light while surviving the intense heat and environmental stress of continuous operation.
Traditional carbon nanotubes, though capable of absorbing approximately 99% of sunlight, deteriorate rapidly when exposed to high temperatures and humidity levels typical in solar tower environments. This degradation has limited their practical application in commercial solar installations, forcing engineers to apply protective coatings that reduce their optical performance.
Dr Iñigo González de Arrieta’s team at the Thermophysical Properties of Materials research group, working with colleagues at the University of California, San Diego, tested the nanoneedles under conditions simulating actual solar tower operation. The materials maintained their structural integrity and optical properties even when subjected to temperatures exceeding those that would destroy carbon nanotube arrays. This stability eliminates the need for protective coatings that typically compromise absorption efficiency in other ultrablack materials.
Concentrated solar power plants operate on a fundamentally different principle than photovoltaic panels. Instead of converting light directly into electricity through semiconductor junctions, CSP systems use hundreds or thousands of mirrors called heliostats to focus sunlight onto a central receiver tower. The concentrated solar flux at the receiver can reach intensities more than 1,000 times greater than normal sunlight, generating temperatures sufficient to heat molten salts to over 565°C.
The receiver’s surface material must possess specific engineering properties to function effectively in this environment. Beyond absorbing virtually all incident light across the solar spectrum, it must conduct heat efficiently to the working fluid, resist oxidation at elevated temperatures, and maintain dimensional stability through thousands of thermal cycles. The nanoneedle structure addresses these requirements through its unique geometry and material composition.
What Is a Solar Tower and How Does It Work?
The exceptional light-trapping ability of copper cobaltate nanoneedles stems from their carefully engineered microstructure. Each nanoneedle measures approximately 10-20 micrometres in length with diameters in the hundreds of nanometers range, creating a forest-like array on the receiver surface. When sunlight enters this forest, it undergoes multiple reflections between adjacent needles before eventually being absorbed. This geometry creates what engineers call a ” graded refractive index”-the effective optical properties gradually transition from air to the bulk material, minimising reflection at the interface.
The zinc oxide coating serves multiple engineering functions beyond its primary protective role. At the nanoscale, zinc oxide forms a conformal layer that preserves the needle geometry while adding chemical stability. The coating’s wide bandgap semiconductor properties also contribute to the thermal management system. While zinc oxide remains transparent to visible and near-infrared light, which carries most of the solar energy, it can be engineered to emit thermal radiation at specific wavelengths, helping to regulate the receiver’s operating temperature.
From a materials science perspective, copper cobaltate belongs to a class of compounds called spinels, which exhibit remarkable thermal and chemical stability. The crystal structure consists of copper and cobalt atoms occupying specific sites within an oxygen lattice, creating strong ionic-covalent bonds that resist decomposition even at temperatures approaching 1000°C. This inherent stability contrasts sharply with carbon nanotubes, which begin oxidising in air at temperatures as low as 400°C.
The manufacturing process for these nanoneedles likely employs hydrothermal synthesis or chemical vapour deposition techniques familiar to engineering students. In hydrothermal synthesis, precursor solutions containing copper and cobalt salts react under controlled temperature and pressure conditions, with careful pH adjustment directing crystal growth along preferred orientations. The resulting nanoneedles grow perpendicular to the substrate surface, self-organising into the dense arrays necessary for optimal light absorption.
Engineers must also consider thermal expansion coefficients when designing these systems. The nanoneedle array must maintain adhesion to the underlying substrate through temperature swings of several hundred degrees. The thermal expansion coefficient of copper cobaltate closely matches that of common substrate materials, such as stainless steel or specialised ceramics, thereby reducing thermal stress that could cause delamination or cracking during operation.
The heat transfer characteristics of the nanoneedle forest present interesting engineering trade-offs. While the high surface area enhances light absorption, it could impede heat conduction to the working fluid. Engineers address this by optimising needle density and ensuring good thermal contact between the needle bases and the substrate. The zinc oxide coating may actually improve thermal conductivity compared to bare copper cobaltate, though specific measurements would be needed to confirm this benefit.
The collaboration between European and American research institutions reflects the global engineering effort required to advance solar technology.
Testing took place at one of Europe’s few high-temperature optical measurement facilities, which is capable of simulating solar tower conditions. These specialised laboratories use concentrated xenon arc lamps or laser systems to generate the high photon flux densities encountered in actual CSP plants while maintaining precise temperature control and atmospheric conditions.
Transitioning from laboratory demonstrations to commercial deployment necessitates addressing several key engineering considerations. The nanoneedle coating process must scale economically to cover the large surface areas of industrial receivers, which can exceed 100 square meters in utility-scale installations. Manufacturing consistency becomes critical, as variations in needle density or orientation could create hot spots that accelerate material degradation.
Integration with existing CSP infrastructure presents additional challenges. Current receiver designs optimise fluid flow patterns and heat exchanger geometries for specific absorber materials. Implementing nanoneedle coatings may require redesigning these components to exploit the improved absorption characteristics fully. Engineers must also develop quality control methods to detect and repair coating damage during the decades-long operational lifetime expected of solar towers.
The economic implications extend beyond simple efficiency improvements. If nanoneedle coatings enable higher operating temperatures, engineers could specify more efficient power conversion cycles, potentially incorporating supercritical carbon dioxide turbines instead of conventional steam systems. Higher temperatures also enhance thermal storage density in molten salt systems, thereby reducing tank volumes and associated costs for overnight energy storage.
Spain’s existing CSP infrastructure, currently providing about 5% of national electricity generation, offers an ideal testing ground for these advanced materials. The country’s solar towers could serve as demonstration sites, allowing engineers to validate long-term performance under actual operating conditions. Data from these installations would inform further material optimisation and guide deployment strategies in other markets.
Future research directions may explore alternative coating materials that enhance electrical or thermal conductivity while maintaining the exceptional optical properties. Engineers might investigate gradient coatings, where the composition varies through the thickness, optimising different layers for specific functions. Advanced manufacturing techniques, such as atomic layer deposition, could enable precise control over coating properties at the nanoscale.
