Bauxite-based proppants, commonly used in the fracking industry, are also employed in solid particle receiver types of Concentrated Solar Thermal (CST) technologies. However, fracking has been banned in many countries due to its environmental impact, leading to a decline in demand for proppants. As a result, the current proppant price of 1 €/kg is expected to rise due to reduced production capacities. Therefore, alternative particle types need to be developed for Concentrated Solar Thermal applications. Cost-effective particle production compared to traditional bauxite sintering has been explored. In this study, we conduct long-term isothermal aging tests at 1000 ◦C up to 4000 h to assess the optical degradation of novel cost-effective particles, which are composed almost entirely of recycled waste products. The performance of these novel particles is compared to traditional proppants. Additionally, we examine the durability of three types of coatings designed to enhance the solar absorptance of the particles from approximately 83 %–97.5 %. These coated particles are also subjected to climate chamber tests under controlled humidity and freezing conditions to determine if environmental factors will degrade the coatings.

F. Sutter, G. Alkan, N. Benameur, S. Marlin, G. S. Vicente, A. Morales, T. J. R Navarro, A. C. G. Alves, L. M. Arcos, D. Benítez , A. Fernandez-García, C. Oskay,
C. Grimme

Solar Energy Materials and Solar Cells
November 2024

Ni-based superalloys are commonly used in gas turbines because of their exceptional high-temperature mechanical properties. To secure a long service life, the materials must also have sufficient corrosion resistance. Therefore, diffusion coatings are widely used to enrich the surface in protective oxide scale-forming elements. For temperatures between 650 and 950 °C, where hot corrosion occurs, Cr-based coatings are advantageous. These are commonly applied via the laborious pack cementation process. Recently, a novel cost-effective Cr/Si slurry coating process has been developed which demonstrated resistance to oxidative high-temperature environments. Here, the protection of the slurry coatings against hot corrosion type I at 900 °C on the Ni-based superalloy Rene 80 is investigated and compared to coatings produced by pack cementation. Prior to the 300-h exposures in air containing 0.1% SO2 at 900 °C, 4 mg/cm2 of Na2SO4 was deposited on the material surfaces. The uncoated Rene 80 exhibited rapid dissolution of the initial oxide scale followed by catastrophic break away oxidation. In comparison, the slurry coatings showed significantly improved hot corrosion resistance compared to the uncoated alloy and a better protection than a Cr pack cementation coating. The Cr pack cemented Rene 80 showed improved hot corrosion resistance, but Cr depletion in the subsurface zone occurred with increasing exposure time, associated with the propagation of Al internal oxidation and increasing sulfidation. In contrast, the slurry coatings formed an external Cr2O3 scale coupled with an agglomeration of SiO2 underneath and a continuous Al2O3 subscale which offered a better diffusion barrier and leading to superior long-term protection against hot corrosion.

M. Kerbstadt, K. Ma,  E. M. H. White, A. J. Knowles, M. C. Galetz

High Temperature Corrosion of Materials Journal
July 24, 2024

Incorporating reactive elements (RE) into turbine coatings is a well-established surface treatment. However, suboptimal RE concentrations can lead to compromised strength, heightened brittleness, and reduced adhesion. In contrast, RE oxides offer advantages of avoiding these detrimental effects, counteracting corrosion phenomena
induced by V2O5 compounds and enhancing oxidation resistance. A notable challenge lies in optimizing RE oxide particle incorporation and understanding the influence of particles in coating microstructures. This study focuses on developing Ni–Al and Ni-Cr-Al type metal matrix composite (MMC) coatings on Inconel 617 (IN617), containing up to 11 vol% of Yttria (Y2O3) nanoparticles. Y2O3 nanoparticles and Ni were co-electrodeposited on IN617 followed by either pack aluminizing or a two-step chromizing and aluminizing process. An even distribution of Y2O3 nanoparticles was observed throughout the entire 100 μm coating thickness, leading to significant grain refinement in the sub-micron to nano range in both coating types. Y2O3-strengthened coatings were subjected to oxidation at 1100 ◦C and hot corrosion at 700 ◦C and were compared to their Y2O3-free counterparts. Present at grain boundaries, Y2O3 markedly enhanced the oxidation and corrosion resistance by reducing interdiffusion, improving the oxide scale adherence and binding V2O5, highlighting the potential of this method for advanced turbine blade coatings.

Christoph Grimme, Kan Ma, Robin Kupec, Ceyhun Oskay, Emma M.H. White, Alexander J. Knowles, Mathias C. Galetz

Surface & Coatings Technology
May 8, 2024

Precise knowledge of particle optical properties is crucial to the advancement and success of directly-irradiated particle receiver technologies. This work presents the results of a Round Robin Test (RRT) that was conducted between seven laboratories, where each par-ticipant measured the absorptance and emittance for a set of five different particle types. This research was performed within the framework of the SolarPACES Task III group, and the re-sults helped establish a guideline for evaluating the optical properties of particles. The guide-line was published on the SolarPACES Task III website in May 2022.

Florian Sutter, Marco Montecchi, Angel Morales Sabio, Gema San Vicente, Aránzazu Fernández-García, Johannes Pernpeintner, Tomás Reche-Navarro, Simon Caron, Lucía Marínez-Arcos, Johannes Wette, Alejandro Calderón, Marc Majó, Inés Fernández, Patrick Davenport, Tucker Farrell, Clifford K. Ho.

SolarPACES 2022 Proceedings
March 14, 2024

The challenging development/modification of particles to meet the requirements of thermal stability, high absorptance and mechanical resistance with minimal economical cost is a key point to attain for the next particle receivers of concentrated solar power plants. The properties of the particles influence the performance of the system, since the concentrated solar radiation is absorbed and stored in these solid materials. In this work, resulting from the studies carried out within the HORIZON 2020 COMPASsCO2 project, the deposition of blackcoloured transition-metal oxides with spinel-like structure coatings on different particles is proven. Five different particles (state-of-the-art and innovative) developed by Saint-Gobain have been coated, studied and compared.
The coating deposition was adapted to the dip-coating methodology, ensuring high and good reproducibility. The composition of solid particles as well as the composition of the precursor solution, the curing methodology and the number of deposited spinel layers demonstrate that they have a great influence on the solar absorptance value and the abrasion resistance. Therefore, it is essential to find the perfect balance between all these different parameters in order to achieve the best performance of the particles. The addition of silica nanoparticles in the precursor solution provides rougher layers with enhanced absorptance, up to 0.980, on newly developed particles with excellent thermal stability and abrasion resistance

Meryem Farchado, Gema San Vicente, Naia Barandica, Florian Sutter, Goezde Alkan, Daniel Sanchez-Senoran, Angel Morales

Solar Energy Materials and Solar Cells Journal
December 26, 2023

The use of solid particles as direct heat absorbance and storage media promises enhanced storage densities in concentrated solar power (CSP) technologies. The long-term optical performance of those particles, which aim to be operational over years, is crucial. Dry powder coating with a deep black Cu-Mn-oxide pigment in a resonant acoustic mixer and subsequent sintering was employed to improve the long-term optical performance of hematite-rich spherical particles, which aimed to replace the state-of-the-art bauxite proppants. Due to the specific reactivity of the hematite particles, a new strategy using an Al-modified composition of the initial deep black pigment was required. The Al modification diminishes cation diffusion into hematite, allowing the formation of spinel-type Fe-Mn-Cu-Al-oxide coatings with favorable long-term temperature and optical stability. The effect of chemical composition of the coating layer on the coating process mechanism was discussed and the need for an elongated sintering time was noticed to ensure the termination of stable spinel phase formation. The structural and optical measurements revealed the enhancement of the properties of hematite absorber particles through this new modified coating process.

Gözde Alkan, Peter Mechnich  and Johannes Pernpeintner

Coatings Journal
November 10, 2023

Chromium alloys are being considered for next-generation concentrated solar power applications operating > 800 °C. Cr offers advantages in melting point, cost, and oxidation resistance. However, improvements in mechanical performance are needed. Here, Cr-based body-centred-cubic (bcc) alloys of the type Cr(Fe)-NiAl are investigated, leading to ‘bcc-superalloys’ comprising a bcc-Cr(Fe) matrix (β) strengthened by ordered-bcc NiAl intermetallic precipitates (β’), with iron additions to tailor the precipitate volume fraction and mechanical properties at high temperatures. Computational design using CALculation of PHAse Diagram (CALPHAD) predicts that Fe increases the solubility of Ni and Al, increasing precipitate volume fraction, which is validated experimentally. Nano-scale, highly-coherent B2-NiAl precipitates with lattice misfit ∼ 0.1% are formed in the Cr(Fe) matrix. The Cr(Fe)-NiAl A2-B2 alloys show remarkably low coarsening rate (∼102 nm3/h at 1000 °C), outperforming ferritic-superalloys, cobalt- and nickel-based superalloys. Low interfacial energies of ∼ 40/20 mJ/m2 at 1000/1200 °C are determined based on the coarsening kinetics. The low coarsening rates are principally attributed to the low solubility of Ni and Al in the Cr matrix. The alloys show high compressive yield strength of ∼320 MPa at 1000 °C. The Fe-modified alloy exhibits resistance to age softening, related to the low coarsening rate as well as the relatively stable Orowan strengthening as a function of precipitate radius. Microstructure tailoring with Fe additions offers a new design route to improve the balance of properties in “Cr-superalloys”, accelerating their development as a new class of high-temperature materials.

Kan Ma, Thomas Blackburn, Johan P. Magnussen, Michael Kerbstadt, Pedro A. Ferreir´os, Tatu Pinomaa, Christina Hofer, David G. Hopkinson, Sarah J. Day, Paul A.J. Bagot, Michael P. Moody, Mathias C. Galetz, Alexander J. Knowles.

Acta Materialia Journal
July 26, 2023

The accumulation of soiling on photovoltaic modules, and on the mirrors of concentrating solar power systems cause non-negligible energy loss with economic consequences. These can be mitigated, or even prevented, through appropriate actions if the magnitude of soiling is known. Particle counting analysis is a common procedure to characterize soiling as it can be easily performed on micrographs of glass coupons or solar devices that have been exposed to the environment. Soiling studies have typically assumed that particle counting produces invariant and inter-institution consistent results. However, particle size distribution analysis is affected by the operator and the utilized methodology. The results of a round-robin study are presented in this work to explore and elucidate the uncertainty related to particle counting and its effect on the characterization of the soiling of glass surfaces used in solar energy conversion systems. An international group of soiling experts analysed the same 8 micrographs using the same open-source ImageJ software package. The variation in the particle analyses results were investigated to identify specimen characteristics with the lowest coefficient of variation and the least uncertainty among the various operators. The mean particle diameter showed the lowest variance among the investigated characteristics , whereas the number of particles exhibited the largest variation. Additional parameters, such as the fractional area coverage and the distribution’s skewness, yielded an intermediate variance. These results can provide useful information on optical and microscope-based soiling monitoring and characterization.

Greg P. Smestad, Cody Anderson, Michael E. Cholette, Pavan Fuke, Ahmed Amine Hachicha, Anil Kottantharayil, Klemens Ilse, Mounia Karim, Muhammad Zahid Khan, Herbert Merkle, David C. Miller, Jimmy M. Newkirk, Giovanni Picotti, Florian Wiesinger, Guido Willers, Leonardo Micheli.

Solar Energy Materials and Solar Cells Journal
July 4, 2023

The performance of advanced materials for extreme environments is underpinned by their microstructure, such as the size and distribution of nano- to micro-sized reinforcing phase(s). Chromium-based superalloys are a recently proposed alternative to conventional face-centred-cubic superalloys for high-temperature applications, e.g., Concentrated Solar Power. Their development requires the determination of precipitate volume fraction and size distribution using Electron Microscopy (EM), as these properties are crucial for the thermal stability and mechanical properties of chromium superalloys. Traditional approaches to EM image processing utilise filtering with a fixed contrast threshold, leads to weak robustness to background noise and poor generalisability to different materials. It also requires an enormous amount of time for manual object measurements on large datasets. Efficient and accurate object detection and segmentation are therefore highly desired to accelerate the development of novel materials like chromium-based superalloys. To address these bottlenecks, based on YOLOv5 and SegFormer structures, this study proposes an end-to-end, two-stage deep learning scheme, DT-SegNet, to perform object detection and segmentation for EM images. The proposed approach can thus benefit from the training efficiency of CNNs at the detection stage (i.e., a small number of training images required) and the accuracy of the ViT at the segmentation stage. Extensive numerical experiments demonstrate that the proposed DT-SegNet significantly outperforms the state-of-the-art segmentation tools offered by Weka and ilastik regarding a large number of metrics, including accuracy, precision, recall and F1-score. This model forms a useful tool to aid alloy development microstructure examinations, and offers significant advantages to address the large datasets associated with highthroughput alloy development approaches.

Zeyu Xia, Kan Ma, Sibo Cheng, Thomas Blackburn, Ziling Peng, Kewei Zhu, Weihang Zhang, Dunhui Xiao, Alexander J Knowles and
Rossella Arcucci

PCCP Journal
May 26, 2023

A deep-black Cu, Mn, Fe- pigment with a spinel structure was employed to coat standard proppants in order to improve long term solar absorptance. The coating process was performed by high-energy, high-speed mixing of proppants and small quantities of spinel powders in a resonant acoustic mixer. A continuous powder coating is achieved by electrostatic attraction between the proppant surface and the coating particles. Consolidation and strong attachment of the coating is achieved by the subsequent sintering beyond the spinel melting temperature. Chemical reaction and bonding between spinel coating and proppant lead to the incorporation of Al, Mg and Ti into the spinel structure. Coated bauxite proppants exhibit a significantly improved, long-term stable solar absorption accompanied by a promising abrasion resistance. The presented coating methodology is considered to be scalable to industrial production.

Gözde Alkan, Peter Mechnich and Johannes Pernpeintner
Coatings Journal
May 31th, 2022

Particles can be used in CSP applications as solid heat transfer and storage medium, heating them directly e.g. in a falling particle receiver. Since the particles interact directly with solar radiation, it is relevant to measure their optical properties accurately. This guideline details the measurement process to determine solar absorptance and thermal emittance of the particle layer.

F. Sutter, J. Pernpeintner, S. Caron (DLR), A. Morales, G. San Vicente, A. Fernández-García (CIEMAT), M. Montecchi (ENEA), A. Calderón, M. Majó, I. Fernández (Universitat Barcelona), P. Davenport, T. Farrell (NREL) and C. Ho (SANDIA)

SolarPACES 2022
May, 2022

This third paper focuses more specifically on chromium-based alloys and their potential for application in future CST systems. The development of advanced materials is critical to the next generation of CST technologies, particularly for heat exchangers operating in extreme environments, not only for electricity generation but also for the provision of process heat at high temperature. Chromium-based alloys, including nickel aluminide (Cr-NiAl in the following) and silicide (Cr-Si) variants, present promising solutions due to their high-temperature stability, oxidation resistance, and improved mechanical properties.

The focus of this second informative paper is on the evaluation of metal alloys for heat exchanger tubes, and the investigation of their properties both as bulk materials and coatings.

This article is a first of a series of informative papers aiming at disseminating the main achievements of COMPASsCO₂ project to a wide audience, in order to increase awareness on sustainable energy technologies and highlight the efforts of both research and industry for the transition to a carbon neutral energy mix in Europe. It summarises the activities conducted in the first 18 months of implementation (November 2020 – April 2022), and the main accomplishments, with a specific focus on the development and testing of novel materials.