Symposium ES08—Materials Challenges in Surfaces and Coatings for Solar Thermal Technologies

Separation of O₂ from air to produce a high-purity stream of N₂ is considered via the aluminum-doped strontium ferrite perovskite (SrAl_yFe_1-yO₃) reduction/oxidation reactions in a two-step solar cycle. The cycle encompasses 1) the solar-driven thermal reduction of SrAl_yFe_1-yO₃; followed by 2) the re-oxidation with O₂ from air to produce high-purity N₂.
SrAl_yFe_1-yO₃ samples were synthesized and doped with controlled levels of aluminum on the B-site via the sol-gel method. Thermogravimetry was used to determine the non-stoichiometry at chemical equilibrium for temperatures between 673 and 1373, gas flows between 1 and 90% O₂–Ar, and aluminum dopant levels of between 0.0 and 0.2. A thermodynamic model based on the compound energy formalism was applied to thermogravimetric measurements to predict equilibrium non-stoichiometry as a function of temperature, O₂ partial pressure, and non-labile aluminum concentration. The predictions were used to calculate partial reaction enthalpies and standard entropies as functions of nonstoichiometry and aluminum concentration. Aluminum doping consistently increased in the enthalpy of reduction and, therefore, the oxygen affinity of the perovskite for all values of non-stoichiometry.
The thermodynamic characterization was used to inform a thermodynamic cycle analysis of an ideal air separation cycle. The analysis showed that, for a solar reactor operating at 1073 K, 99% purities were possible for oxide to air molar ratios of up to 0.8 to 1 in the air separator. The cycle efficiency showed a strongly-inverse relationship with N₂ purity. However, the addition of a recuperation stage significantly mitigated this decline. These findings will be used inform the study of more complex, concurrently or sequentially reducing B-site cations and dopants, as well as the tuning of compositions with other dopants and cycle operating conditions in future air separation cycles.

Recent developments in solar-thermal power generation aim as well to achieve higher temperatures to increase the efficiencies of the power cycles as to store the solar energy to enable baseload power generation from a transient energy source.
Thermochemical redox processes are an option to store large amounts of solar energy in a compact storage system. The enthalpy effects of these reversible chemical reactions can be exploited. Oxides of multivalent metals in particular, capable of being reduced and oxidized under air atmosphere with significant heat effects are perfect candidates for air-operated Concentrated Solar Power plants since in this case air can be used as both the heat transfer fluid and the reactant (O₂) and therefore can come to direct contact with the storage material (oxide).
Porous ceramic structures like honeycombs and foams are favorable for heat exchange applications, the idea of employing such structures either coated with or entirely made of a redox material like manganese oxide and cobalt oxide, as a hybrid sensible-thermochemical solar energy storage system in air-operated Concentrated Solar Power plants has been set forth and tested up to a prototype scale on DLR’s solar tower in Juelich, Germany. By cascading different redox materials heat can be stored even more efficiently than by single material systems,

Next-generation concentrating solar power (CSP) plants and future solar thermochemical processes require high-performance solar absorber coating with suitable photothermal properties as well as high-temperature stability. Traditional coating materials are often lacking some of the desirable properties or are difficult to be tailored to meet various stringent requirements. While various nanostructures, including particles and multilayers, have been studied, they are often not stable at high temperature. Over the last few years, we have been developing spinel metal oxide nanostructures with tailored photothermal properties for solar absorber coating applications. These oxides have high melting points and are chemically stoichiometric, so they are expected to be stable in air at high temperature. First, we studied nanoparticles of Cu(II) containing spinel oxide nanoparticles, including CuCr₂O₄, Cu_0.5Cr_1.1Mn_1.4O₄, CuFeMnO₄, etc. These particles were made from either bottom-up hydrothermal or top-down ceramic processing. Coatings made of these Cu spinel oxide nanoparticles exhibit higher solar absorptance and more importantly, better high-temperature stability in air over an extended period of testing time (2000 hours), compared to the state-of-the-art coating Pyromark. The influence of chemical compositions, particle size, coating conditions on the optical properties will be discussed. Thermal stabilities are studied via photothermal property measurement and structural analysis. Second, we explored new morphologies of metal oxide nanostructures, including nano-needles and nano-flowers. These unique structures can possess record-high solar absorptance (over 99%) even with very thin coating, owing to the strong light trapping effect. With suitable chemical compositions and surface passivation, the nanostructures can maintain their structural integrity at high temperature in air. The structures can be further tailored to show vastly different infrared properties. Finally, we showed that the recent high-entropy alloy concept can be applied to the spinel metal oxides to synthesize entropy-stabilized oxides, which may provide a new route to achieve unprecedented high-temperature stability for future solar-thermal coatings. Our studies demonstrate that nanostructures of spinel metal oxides offer tremendous opportunities for future high-temperature solar-thermal coating applications, by providing both high performance and excellent high-temperature stability.

Solar-thermal energy conversion has been an intensively investigated recently due to its potential for full-spectrum solar utilization, for instance, in systems such as concentrating solar power (CSP). Typically featured with concentrated CSP plants are spectrally selective absorbers, which are usually made of surface coatings that are highly absorbing in the solar spectrum (0.28 to 2.5 µm) and less absorbing (or reflective) in the blackbody emission spectrum. For solar tower based CSP, the dominating factors in determining solar-thermal conversion efficiency is solar absorptance and thermal stability at the operational temperature in air for extended periods of time.

Developing solar absorber coatings that can absorb sunlight while enduring high temperatures is a challenging task, but they can be successfully synthesized with careful engineering. Our our team has recently designed an ultra-absorptive array of cobalt oxide nanoneedles. This cobalt oxide nanoneedle array is spectrally selective and its nanostructure increases the optical pathway of light, allowing the coating to possess over 99% solar absorptance; however, it is fragile, and degrades at high temperatures. Through altering the chemical composition and adding passivating layers, the nanoneedle maintains its distinctive shape, even after long-term high-temperature annealing. In this talk, we will also analyze the solar-to-thermal energy conversion efficiency before and after long-term thermal annealing.

Concentrating solar power (CSP), as a well-known high-temperature solar-thermal technology, is a promising approach to harvesting solar energy due to its efficient utilization of full-spectrum sunlight, and high availability in energy storage. To further pursue higher Carnot efficiency and greater cost reduction, next-generation CSP plants operating at higher temperatures (≥600°C) are widely accepted as a promising approach. A selective solar absorber, as a critical component in CSP systems, is required to maintain high sunlight absorption and low absorption (i.e., emission) beyond a cut-off wavelength in the infrared region to avoid thermal re-radiation even at elevated temperatures. To date, all the state-of-the-art selective absorbers such as cermets and photonic crystals with superior performance (i.e., high selectivity and great thermal stability) were manufactured with complicated and expensive high-vacuum micro-fabrication or nano-fabrication techniques, leading to high costs and challenges for largescale deployment. Therefore, the development of high-performance solution-processed selective absorbers is urgently demanded.
Titanium nitride (TiN), as an emerging plasmonic ceramic material, offers tunable plasmonic properties, as well as excellent thermal and chemical stability. TiN nanoparticles have been proven to exhibit stronger optical losses in the visible-near-infrared (NIR) range compared to traditional metallic plasmonic materials. In this work, we constructed a uniform and controllable sunlight absorption coating using well-dispersed colloidal TiN nanoparticles (20-80 nm). The absorption bandwidth, i.e., the cut-off wavelength, can be tuned through rationally controlling the concentration of the TiN colloid and the spin-coating speed. The excited plasmonic resonance of TiN nanoparticles resulted in highly selective absorption of sunlight around their resonance wavelengths. Due to the in-plane plasmonic coupling of close-packed TiN nanoparticles, the resonance wavelengths were able to red-shift to NIR range. As a result, selective sunlight absorption can be attained in the UV-visible-NIR range below the cut-off wavelength. An amorphous SiO₂ layer, prepared by spin-coating a perhydropolysilazane (PHPS) solution, acted as the protection and anti-reflection coating. Both full-spectrum sunlight absorption and strong IR reflection were achieved simultaneously when coated on various IR reflectors, including Au, stainless steel, Al, and TiN. Particularly, the absorbers with TiN reflectors exhibited a high solar absorptance of 93.5% and a low IR emittance of 21.1% at an elevated temperature of 1000 K, producing a solar-thermal energy conversion efficiency of 81.3% under 100 suns. Moreover, the absorber can maintain its superior performance at 1000 K under air and 1173 K under vacuum, respectively. To the best of our knowledge, this performance surpasses or equals those of state-of-the-art selective absorbers fabricated by high-vacuum micro and nano-fabrication techniques. The high efficiency and great thermal stability achieved in such low-cost solution-processed solar absorbers will benefit the development of next-generation CSP plants by providing large cost reductions in mass-production.

In this presentation I will review some recent developments in the field of black spinel-type oxides for solar selective absorber coatings. The interesting optical properties of ternary and quaternary spinel-type oxides allow for manifold interband transitions, yielding a dark or even black appearance of many spinel oxides, which show remarkable stability at elevated temperature in air and vacuum. In order to overcome the barrier of initial investment cost, wet-chemical derived solar absorber coatings might be especially attractive. By sol-gel dip-coating and subsequent thermal annealing, we produced multilayered mixed Cu-Co-Mn-Si oxide [1,2]. After optimization of the multilayer design, a solar absorptance of 0.95 and a thermal emissivity of 0.12 at 100°C have been achieved. For the energy-efficient thermal annealing of 2 m long receiver tubes, a special induction heating process has been developed. On highly infrared-reflecting substrates, spinel oxide based layer stacks can be very selective. For a three layer configuration composed of a CuMnO_x thin film and two consecutive antireflective coatings, FeMnCuO_x and SiO₂, on aluminum substrates, a solar absorptance of 0.957 and a thermal emittance values of 0.038 at 100 °C were achieved [3]. Aiming at operation temperatures around 550°C, CuCoMnO_x, CuCoMnO_x-SiO₂, and SiO₂ layers were deposited successively on a WC-Co coatings. The absorptance of the multilayer structure coating obtained through this way increased from 0.821 to 0.915 and the thermal emittance decreased from 0.434 to 0.290. Defects on the surface of WC-Co coating were gradually reduced, the multilayered film stacks were compact and the surface roughness was reduced [4]. Copper-oxide spinel coatings can alternatively be deposited by spray-coating. Black metal oxide nanoparticles comprising copper-cobalt oxides (Cu_xCo_3−xO₄) and copper-manganese oxides (Cu_xMn_3−xO₄) were synthesized for solar absorptive potential by hydrothermal syntheses. To extend the spectrally-selective absorbance capability, the coating surfaces are geometrically-textured using sacrificial polymer beads that are jointly implemented in the spray-coating process [5]. Also crystalline Cu_xNi_yCo_z-x-yO₄ films have interesting optical properties. By optimizing the concentration of the solution, withdrawal speed and annealing temperature, a solar absorptance of 0.91 and an emittance of 0.14 were achieved for a single layer coating with composite oxide formation [6]. The remarkable stability at elevated temperatures in air and vacuum makes such black spinel oxide coatings an interesting candidate for solar applications involving concentrated solar radiation, such as the generation of solar electricity (concentrated solar power CSP), industrial process heating and solar cooling.

Over the past seven years, the US Department of Energy has developed a framework where chloride based salt eutectics can serve as the heat transfer fluid and thermal energy storage medium in concentrating solar thermal power (CSP) plants at a cost competitive with traditional power plants in the United States. However, catastrophic corrosion remains the highest risk to this framework. A series of completed and ongoing research and development initiatives have significantly evolved the understanding of corrosion control in chloride salt systems. These have brought into focus viable corrosion mitigation strategies and system requirements for an operable power plant at the megawatt size scale. This work presents these integrated results and describes a path from present day understanding into the next generation of CSP plants.

The U.S. Department of Energy (DOE) launched the SunShot Initiative in 2011 to achieve high thermal-to-electric conversion efficiency and to make solar technologies cost-competitive with conventional electric power generation. The sun’s energy is captured and stored in the form of heat in concentrating solar power (CSP) technologies. By using low cost materials that are stable for decades, CSP with the help of thermal energy storage (TES) systems is able to deliver renewable energy while providing important capacity, reliability and stability attributes to the grid, thereby enabling increased penetration of variable renewable electricity technologies. Current CSP systems are integrated with conventional steam-Rankine power cycles. Today’s most advanced technologies are power tower with 2-tank molten-salt TES, delivering thermal energy at 565°C using molten nitrates. Next generation CSP (Gen3 CSP) are targeting higher efficiencies by integrating the power tower with the TES to a supercritical CO₂ (sCO₂) Brayton power cycle. To achieve this integration, Gen3 CSP needs to operate at temperatures above 550°C requiring high-temperature advanced fluids in the range of 550°C to 750°C. Because nitrates are unstable at temperatures above 620°C, new salts are required. These high-temperature salts, such as chlorides, need to compatible with containment materials. To meet the cost of production of electricity, cost targets must be met using alloys and materials with acceptable corrosion and mechanical strength. The selected heat transfer fluid (HTF) candidates based on cost are chlorides, but they introduce a set of technological and engineering challenges because of their very corrosive characteristics for typical materials. Corrosion mitigation approaches are been investigated to control material’s degradation below 20 μm/year. Corrosion in molten chlorides is being controlled in atmospheres with the absence of oxygen and water. Catastrophic mechanical failure will occur if these impurities are present because intergranular attack will occur. Current studies are focused on: 1) redox potential control using active-metals in atmospheres with no-oxygen/water; 2) surface treatments to allow passivation, diffusive coatings; 3) coatings such as nickel-based (NiCo)CrAl(Y,Ta,Hs,Si); and 4) alumina forming alloys (AFA). Untreated In800H and 310SS alloys corrode rapidly (~2,500 to 4,500 μm/year) in molten chlorides, but when coated the lowest corrosion rate of 190 μm/year is obtained. Because the coatings were pre-oxidized before molten salt immersion, metallographic characterization of showed that the formation of a uniform thin alumina scale. The presence of this layer presumably reduced the corrosion of the alloy in the molten chloride in an atmosphere containing oxygen and water as impurities. Electrochemical impedance spectroscopy tests and metallographic characterization showed that the best performing AFA in molten chlorides without controlled atmosphere was Inconel 702 pre-oxidized in zero air (ZA) at 1050 °C, due to the formation of protective, dense and continuous alumina layers. When using an argon atmosphere during corrosion evaluations, these layers were unstable. AFAs to contain molten chlorides in air are promising for Gen3CSP applications because protective alumina layers were stable and able to grow from 5 μm (before immersion) to 13 μm (after 185 h of immersion) in the oxygen-containing atmosphere. The use of these alloys could be commercial feasibility and cost-effective because of the possibility of using oxygen-containing atmospheres instead of keeping enclosed systems with inert atmospheres to protect alloys from corrosion in molten chlorides.

The use of solar energy for power generation provides an efficient sustainable energy solution. Among a number of technologies developed for power generation using solar energy, concentrating solar power (CSP) is encouraging because of the capability of thermal energy storage that makes it possible for the 24-hour energy production. Although the use of molten salts as heat transfer fluid and thermal storage in CSP has various advantages, they have a major disadvantage as they make the component systems highly susceptible to corrosion. Different approaches have been adopted to suppress hot corrosion including the use of high alloy steels and the use of high purity molten salts; both contribute to a substantial increase in construction and operating costs.
In this study, we investigate the use of protective coatings to enhance the corrosion resistance of the component systems against molten salts at an elevated temperature. Nickel aluminide coatings are deposited using highly scalable plasma spray technique with few different deposition parameters to obtain a variety of stoichiometries and morphologies. A low value of porosity is anticipated for corrosion resistance along with good adhesion with substrate, minimum unmelt-particles and adequate lamellar structure for optimum coatings.
Optimized coatings deposited on SS347 and SS304 stainless steel substrate were exposed to a mixture of NaNO3 / KNO3 molten salts at 600°C for up to 2500 hours. The uncoated substrates were also included in the test for comparison. After the test, the samples were characterised by electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction to evaluate their corrosion behaviour. In this presentation, the corrosion test results and the potential of nickel aluminide coatings to suppress hot corrosion by molten salts in an environment that simulates that of concentrating solar power plants will be presented and discussed.

Hydrogen embrittlement of metallic components is a leading cause for corrosion failure in high-temperature heat-exchange systems, whose components operate in extreme environments. Hydrogen is a byproduct of corrosion in water- or H₂S-containing environments commonly associated with heat transfer media employed in high-temperature heat exchange systems. To mitigate surface corrosion, protons should ideally be discharged from the solid-liquid interface into the liquid phase as gaseous H₂. Though some atomic and/or ionic H is absorbed by the solid as point defects, ultimately inducing embrittlement and making the metal increasingly susceptible to fracture. For H uptake to occur, it must penetrate a synthetic or native surface layer (e.g. oxide, sulfide, carbonate), which involves interfacial and bulk solid-state ionic processes, including surface adsorption and absorption, as well as bulk diffusion. Hence, there is a significant opportunity to elucidate these processes, and to engineer solid ionic barrier coatings that mitigate H uptake and embrittlement in high-temperature thermal energy systems and metal components employed elsewhere.

Here we aim to develop a solid ionic H barrier coating by employing design and engineering approaches to (i) minimize the H point defect solubility and mobility in the coating material, and (ii) elucidate the role of interfaces in corrosion barriers during the H blocking process. We employ a doping strategy for a model non-stoichiometric oxide monoclinic ZrO₂ following a recently-developed theoretical framework based on density functional theory and statistical thermodynamics that predicts point defect concentrations in metal oxides, including H point defects [1]. We measured electrical conductivity and H solubility in doped monoclinic ZrO₂ compositions that had not be reported prior, and these experiments provided quantitative and qualitative validation of the theoretical modeling framework [2]. Temperature programmed water desorption was employed to quantify H solubility in these oxides and to elucidate the nature of H defects. Doping was found to effectively modulate hydrogen solubility in ZrO₂, and we modified the oxide microstructure and performed solubility measurements under varied oxygen partial pressure to elucidate the H-containing defects, and to illustrate the influence of porosity and grain boundaries on H pick-up. Additionally, we electrochemically investigated the H flux through oxide thin film coatings grown on steel, with the intention of investigating the impact of solid-solid interfaces and microstructure on H barrier coating performance.

Acknowledgements
We gratefully acknowledge the MIT Energy Initiative and Equinor Inc. for financial support.

References
[1] M. Youssef, M. Yang, B. Yildiz. Phys. Rev. Applied, 5 (2016) 014008-16.
[2] W.J. Bowman, J. Yang, B. Yildiz. (Submitted)

The durability of solar mirrors is a critical factor for the deployment of concentrating solar power (CSP) plants [1,2]. Accelerated aging models currently applied in the polymer, electronic and photovoltaic fields have recently been reviewed [3], and the issues of their application to solar mirrors have been discussed. Lifetime prediction of solar mirror requires determining the kinetic laws of the degradation related to the stress factors level and the associate models. Temperature and humidity has been identified to be major stress factors for solar mirrors [4-6]. Accelerated aging in temperature and temperature with humidity at different levels were performed to assess the dependent parameters of models selected from the literature. Results from three different mirrors technology will be presented and analyzed to extract kinetics parameters for models. These parameters include the apparent activation energy for the Arrhenius temperature law, the Peck and Eyring coefficients for humidity. The experimental values were then assessed for specular reflectance loss of solar mirrors. Finally, using these parameters, acceleration factors were calculated for solar mirrors. An effective temperature taking into account the Arrhenius degradation law was used rather than the commonly used mean temperature. The relevance and utility of this effective temperature compared to a simple mean have already been discussed in the literature [7-9]. The problem of coupling all previous laws together is also addressed. Finally, a calculation of acceleration factor related to different CSP sites around for standard damp heat test (85°C, 85%RH) will be presented and discussed.
References
[1] International Energy Agency (IEA), Technology roadmap - CSP, 2010.
[2] M. Mehos, C. Turchi, J. Vidal, M. Wagner, Z. Ma, C. Ho, W. Kolb, C. Andraka, and A. Kruizenga, “Concentrating Solar Power Gen3 Demonstration Roadmap,” tech. rep., National Renewable Energy Laboratory and Sandia National Laboratories, 2017.
[3] C. Avenel, O. Raccurt, J.-L. Gardette, and S. Therias, Review of accelerated aging test modeling and its application to solar mirrors, Solar Energy Materials and Solar Cells, Volume 186, November 2018, Pages 29–41
[4] M. Thomas, M. Lind, C. Buckwalter, J. Daniel and J. Hartman, Heliostat mirror survey and analysis, tech. rep.(tech. rep.) 1979, Pacific Northwest Laboratory.
[5] R. Girard, C. Delord, A. Disdier and O. Raccurt, Critical constraints responsible to solar glass mirror degradation, Energy Procedia 69, 2015, 1519–1528.
[6] A. Czanderna, K. Masterson and T.M. Thomas, Silver/glass mirrors for solar thermal systems, tech. rep.(tech. rep.) 1985, Solar Energy Research Institute (SERI).
[7] T. J. McMahon, “Accelerated testing and failure of thin-film PV modules,” Progress In Photovoltaics: Research And Applications, vol. 12, pp. 235–248, 2004.
[8] S. Kurtz, K. Whitfield, D. Miller, J. Joyce, J. Wohlgemuth, M. Kempe, N. Dhere, N. Bosco, and T. Zgonena, “Evaluation of high-temperature exposure of rack-mounted photovoltaic modules,” in 34th IEEE Photovoltaic Specialists Conference (PVSC), pp. 002399–002404, 2009.
[9] M. Tencer, J. S. Moss, and T. Zapach, “Arrhenius average temperature: the effective temperature for non-fatigue wearout and long term reliability in variable thermal conditions and climates,” IEEE Transactions on Components and Packaging Technologies, vol. 27, no. 3, pp. 602–607, 2004.

Mitigation of the soil accumulation is imperative to decrease the maintenance cost while increasing the long-term energy efficiency and energy generation of solar power installations. Here we describe the formation of atomically bonded, optical-quality, nanostructured porous thin glass film coatings, utilizing metastable spinodal phase separation in a low-alkali borosilicate glass system. The coatings, which are optically imperceptible, simultaneously provides graded-refractive-index antireflectivity and superhydrophobic anti-soiling capabilities, with excellent mechanical durability. In particular, through its inherent antireflective characteristics, these nanostructured surfaces are found to promote a general and an invaluable 3–7% relative increase in current output of multiple direct/indirect bandgap photovoltaic cells. Besides, the coatings maintain optical transparency and superhydrophobic attributes when subjected to simulated sand/dust-storm conditions (based on NOAA reported conditions in dessert environments). The present concept represents a fundamental basis for development of advanced coated optical quality products that can significantly improve the performance of CSP components.

Solar thermal energy to electric power conversion can be a very cost effective and scalable approach to harnessing the abundant solar energy. Solid state thermoelectric devices are reliable, scalable, and can be made cost-effectively for low $/Watt scenarios. In contrast to solar PV systems, solar-thermoelectric devices can offer high electric power density (W/unit area of active semiconductor), can be made to operate even when sunlight is not available using thermal storage systems, and particularly, in addition to providing electric power, can also provide hot-water from the heat that is not converted to electric power. As a low-cost route to solar thermo-electric conversion demonstration, we will report on our early studies in developing a solar thermo electric generator (STEG) system. We used a a 1 m² Fresnel lens to collect direct solar light over a large area and directed it onto a ~0.01 m² area thermoelectric generator modules (TEG), equivalent to a x100 concentration. The planar Fresnel lens was not coated with any anti-reflection coating in these early studies, but are planned. The TEG modules consisted of ~300^○C-capable Bi₂Te₃-based modules, arranged in a linear 1.5" x 4" array. Since all the heat impinging on the TEGs is not converted to electric power, we developed a water-based heat removal system on the cold-side of the TEG modules. The thermal management system consisted of a circulating water loop and a heat-rejection finned heat-sink with a fan. The testing of the STEG module was conducted outdoors. Without any high-emissive (i.e., absorptive) coating on the TEG hot-side, the STEGs produced a voltage of 2.14 V. Even with a modest improvement in emissive/absorptive coating, for the same Fresnel lens configuration, we noted a significant increase in the voltage output to 5.72 V. The x2.7 increase voltage output, with essentially the same internal electrical resistance of the TEG module, would translate into x7.2 more power output. Under extremely sunny conditions, we have observed that the voltage output could go up to 8 V. Clearly, the influence of the heat-absorption coatings on the TEG modules indicates the need for further optimization as well as anti-reflective coating on the front-end planar Fresnel lens light focuser. With further optimization of Fresnel lens geometries, absorptive coatings, and thermal resistance of the TEG devices, higher temperatures can be obtained on the hot-side and therefore higher Carnot efficiencies can be obtained. For these applications, Novus is developing scalable, large-area high-temperature TEG modules based on half Heusler materials. We believe the STEG approach is an attractive way to harness the abundant solar energy in a scalable and cost-effective way.

Heliac has developed a concentrator concept largely based on modified, but well-known industrial processes. The solar concentrator itself consists of a Fresnel-like lens made in a thin polymer film which is glued onto planar glass for geometrical stability. The lens with focal distance of 2 meters focuses the light onto a heat-exchanger type flat receiver made by the welding of two corrugated steel plates, forming channels where a circulating liquid will be heated by the incident sunlight. Both components are fixed on a dual axis solar tracker, each holding 8 lenses and receivers. Advantages of the system are many; a flat lens greatly reduces the wind cross section (when positioned in horizontal mode), reducing the strength requirements for the whole structure, including the foundation. Flat lenses also make cleaning easier, and simplifies logistics compared to e.g. parabolic mirrors. Furthermore, the two-dimensional focusing reduces the precision requirements from tens of millidegrees to degrees, allowing for use of low-cost gear-motors. Also, the acceptance angle of the sunlight is much higher than on mirror-based systems, as the result of combined two-dimensional focusing and a refractive lens. We have measured an acceptance angle of approximately 7 degrees, which is a large advantage if there is atmospheric spreading of the light, either due to cloud cover or smoggy conditions. All these advantages sum up to a much lower cost, where expected sales price is in the range of 350€/kWp(thermal) to 450€/kWp(thermal) depending on plant size. Installation is fast, and the system is modular. First plant for district heating (95C) are being brought in operation Q1 2019. Next plant will feature a higher operating temperature (350C) and for that partners and solutions to implement higher temperature operation are sought.

The optimisation of solar selective coatings (SSC) for solar receivers plays a critical role in the efficiency of a concentrated solar power (CSP) plants. In this work two different SSC has been developed: one of them to operate in vacuum for parabolic through collectors (PTC) and the other to operate in air for linear Fresnel collectors. The final achieved multilayer stack designs were obtained after a combination of PVD parameters optimisation, characterisation and optical simulation of the individual layers. Multi-layered coating for evacuated receivers: SiO₂/AlNW/Mo/W deposited using magnetron sputtering showed excellent optical selective properties α = 94.0% and ε₄₀₀ = 4.8% and long-time thermal stability at the operation temperature of CSP plants. SSC has been scaled up to 4 m long tubes and its efficiency is going to be tested in real plant. Chromium oxynitrides, AlCrO/AlCrON/AlCrN deposited by magnetron sputtering were selected to develop solar selective coating to operate in air due to its exceptional thermal stability and oxidation resistance at high temperatures. Again, excellent optical selective properties, total solar absorptance above 90% and thermal emittance below 10% were measured. Thermal stability of the coatings was tested by means of an accelerated ageing test based on Arrhenius calculations.

The extending market of the concentrated solar power plants requires the use of high-temperature materials for solar surface receivers that would ideally heat an air coolant beyond 1300 K. The currently used Ni-based alloys present operating working points between 970 and 1070 K as severe damages due to oxidation would occur beyond these temperatures and therefore a complementary heating of the coolant using fossil source or biomass is required. Ceramics like SiC are resistant to oxidation at higher temperatures (1470 to 1770 K), but are sensible to cracking that would quickly break the solar modulus. The collaborative project “Multilayer Multifunctional Advanced Coatings for Concentrated Solar Power plants” (2MAC-CSP) aims at developing a process to coat a high-temperature alloy with a ceramic coating (AlN or SiC/AlN stacking) to combine the properties of the substrate (creep resistance, machinability) and of the coating (slow oxidation kinetics, high solar absorptivity).
Coatings were deposited on Plansee TZM^TM (Mo-based) and Sandvik Kanthal APMT^TM (Fe-Cr-Al based) alloys using the High Temperature Chemical Vapor Deposition (HT-CVD) process developed at SIMaP-CNRS laboratory in Grenoble, France and optimized in collaboration with the Sil’tronix ST company based in Archamps, France. The unique solar facilities available at PROMES-CNRS in Odeillo, France, were used to perform high temperature cyclic oxidation in air with thermal shocks (temperature rates during heating and cooling are up to 100 K s^-1) using the “REacteur Hautes Pression et Température Solaire” (REHPTS), and high temperature optical measurements using the “Moyen d’Essai et de DIagnostic en Ambiance Spatiale Extrême” (MEDIASE). Room temperature optical measurements were also performed to investigate the consequences of the high-temperature treatments on the optical properties of the materials. Structural characterizations were performed using Scanning Electron Microscopy coupled with Energy-Dispersive X-ray Spectroscopy (SEM/EDS), X-Ray Diffraction (XRD) and micro-Raman analyses in order to identify surface evolution and damages due to high temperature oxidation. The results of short high temperature oxidation (up to 80 minutes) performed in solar furnaces were compared to the ones of longer oxidation tests (up to 1,500 h) performed in resistive furnaces.
The first results showed the difficulties to protect TZM^TM from fast oxidation: smokes of gaseous molybdenum oxide appeared from 670 K due to grain boundary diffusion through the coating. Tests on aluminide and silicide coatings are going on in order to add an additional barrier to Mo and O₂ diffusion.
Kanthal APMT^TM supports oxidizing atmospheres at 1400 K, due to the formation of a protective layer mainly made of alumina. AlN and SiC/AlN coatings slightly changed the oxidation resistance and improved the solar absorptivity of the substrate. After oxidation and the built-up of an oxide layer, this improvement was maintained. Nevertheless, the coatings cracked after a few fast thermal shocks from 1400 to 300 K. According to our simulations, this is the result of high stress levels (2.5 GPa) that go beyond the failure limit of the coating (1.8 GPa).
High temperature oxidation resistance and optical properties of metallic alloys were improved by the different coatings. However, the high temperatures reached (> 1300 K) led to a rapid diffusion of oxygen and alloying elements of the refractory alloy and the fast thermal shocks led to high stress levels not compatible with metal-ceramic systems with large difference in thermal expansion coefficients.

The development of Generation 3 (GEN3) concentrated solar power (CSP) plants implies an increase of the temperature of the heat they deliver to the power cycle in order to increase the plant efficiency and lower the costs. In particular, current central tower systems operate at maximum temperatures of 550 ^oC mainly due to the severe degradation that the state of the art absorber paints (i.e. Pyromark®) suffer at higher temperatures. In previous works [1,2] aluminum titanium oxynitrides Al_yTi_1-y(O_xN_1-x) were shown to be excellent candidate materials for solar selective coatings (SSC). These results confirmed that the designed SSCs based on these materials withstand breakdown at 600 ^oC in air after 900 hours of thermal cycling.
In this paper, we discuss the high temperature (up to 800 ^oC) stability in air of a solar absorber coating based on Al_yTi_1-y(O_xN_1-x) deposited by cathodic vacuum arc (CVA) at higher working pressure (P = 2.1 Pa) than those discussed in [1] and [2]. The composition, morphology and microstructure of the films were characterized by ion beam analysis, scanning and transmission electron microscopy and X-ray diffraction. The optical properties were determined by ellipsometry and spectrophotometry (UV-Vis-NIR, FTIR). The microstructural and morphological characterization shows the formation of a solid solution of AlTiN crystalline nanoparticles embedded in an amorphous Al₂(O, N)₃ matrix. This particular microstructure provides the coating with a high absorption coefficient within the whole wavelength range of interest (0.3 to 25 um) as modeled by spectroscopic ellipsometry. Hence, this single layer absorber shows a solar absorptance, α, of 92% and an emissivity, ε_RT, of 70%. The addition of an antireflective Al₂O₃ layer and post-deposition thermal treatments improved the optical properties of the absorber to better values (α=96% and ε_RT=60%) than those of Pyromark®. The thermal stability in air of the absorber was firstly analyzed by cyclic heating tests, showing no degradation after 300h of cycles in air at 700 ^oC. Subsequently, the samples were tested in a solar furnace at 650 ^oC and 800 ^oC for 12 hours at environmental conditions. Therefore, this oxynitride nanocomposite absorber coating presents the best thermal in-air stability studied so far by our group.
[1] I. Heras et al., Sol. Energy Mat. Solar Cells, 176, 81-92 (2018)
[2] R. Escobar-Galindo et al., Sol. Energy Mat. Solar Cells, 185, 183-191 (2018)

Solar thermal technologies, converting the abundant solar energy into heat and electricity, have attracted extensive attention in recent years. It is desirable to increase the operating temperature of concentrated solar power plants for higher conversion efficiency, which requires the design of high-temperature components and, in particular, the solar absorber coatings. For efficient photo-thermal conversion, the solar absorber surface must have high solar absorptance (α ≥ 0.98) and a low thermal emittance (ε ≤ 0.05) at the operational temperature (T ≥ 500°C). For this purpose, we propose spectrally selective SiC-metal (Si-M) based nanocomposite absorbers with low infrared emissivity above 500°C. The as-fabricated ultrathin nanocomposite absorber with sandwich-like configuration, consisting of SiC/SiC-M/SiC on reflective layer coated on stainless steel substrate, has exhibited an absorptance of 80-85% in the wavelength range of 280-2000 nm. The surface plasmon polaritons of metal inclusion within the SiC matrix of ultrathin nanocomposite layer have contributed to the broadband light absorption, while the top most layer of SiC acts as anti-reflection layer. The bottom reflective layer of tungsten coated on stainless steel substrate reduces the infrared emittance and improve the thermal stability of the absorber. With the exceptional thermal shock resistant and anti-corrosive properties of SiC as well as the rational design of metal inclusions in the SiC matrix, the proposed absorber can achieve near-perfect absorption in the solar spectrum and also get truncated with a sharp slope in the infrared region for mid to high temperature solar thermal applications.

The importance of finding more effective and cleaner ways of producing energy other than fossil fuels is growing fast in order to reduce CO₂ emissions that contribute to the greenhouse effect. Concentrating solar power (CSP) uses reflectors to redirect and concentrate the solar radiation onto a receiver, where it is transformed into heat. Downstream heat exchangers and gas turbines are the final responsibles of transforming the heat into electricity. Current CSP plants are operated at a maximum of 550 °C, but an increase of the operation temperature to 800 °C, in combination with solar-selective coatings, would significantly improve their efficiency by more than 10 %. In order to approach this issue, a transparent conductive oxide (TCO) thin film based on SnO₂ doped with Ta was developed and optimized by reactive magnetron sputtering [1]. This material was shown resisting 800 °C in vacuum for 4 hours, transmitting incident sunlight and blocking infrared emission from the underlaying blackbody absorber. As such, it exhibits all properties required of a solar-selective transmitter for high-temperature CSP usage. In this work, thermal stability tests were carried out in air using an electric furnace for a total of 12 hours at 650 °C and 800 °C, respectively, for laboratory samples and industrially produced coatings. These results will be shown and discussed. In addition, the coatings were tested under environmental conditions at 650 °C and 800 °C in a solar furnace, again for a total of 12 hours each, providing information about their behaviour and performance in a situation much closer to the final application. Spectrophotometry, Rutherford backscattering spectrometry and conductivity measurements, among other techniques, were used to track the evolution of the properties and performance of SnO₂:Ta thin films under these conditions.

[1] F. Lungwitz et al., submitted to Sol. Energy Mat. Solar Cells (2018)

This work was supported by the EU H2020 RISE project “Framework of Innovation for Engineering of New Durable Solar Surfaces” (FRIENDS², GA-645725).

Concentrating solar power (CSP) technology has been one of the current renewable electricity generation methods, which has attracted much attention in recent years due to its integration with cost-effective thermal energy storage systems. In order to further reduce the cost of CSP technology, it is imperative to operate the CSP plants at higher temperatures (e.g., >800^oC) with enhanced efficiency (e.g., solar-to-thermal figure-of-merit (FOM) as close to the theoretical upper limit of ~95% as possible), with reduced thermal emission loss. It is also highly desirable to retain or store as much of the absorbed solar energy as possible. In standard CSP tower structures, the metal alloy substrates are considered as geometrically flat, which represents a good IR reflector. However, with specifically designed surface structures such as controlled microcavity or nanocavity array, even the metallic surface can be converted to an excellent solar absorber, especially when combined with a small amount of sunlight-absorbing ceramic coating like the black oxide.
Such unconventional metal surface structures have been prepared which demonstrated very high solar absorptivity values close to 99%, yet exhibited desirably reduced emissivity at high operating temperatures around ~800^oC. These light-trapping micro or nano cavity structures can be fabricated by industrially inexpensive methods. Effects of various structural configurations on light-absorption and emissivity control, and some practical implications will be discussed. Other aspects of solar absorption and desired retention/storage of solar energy will also be discussed.

We present a scalable top-down fabrication method based on nanoimprint lithography (NIL) to be used in the manufacturing of selective solar absorbers. By using a deposition mask, nanodisk arrays are fabricated to create plasmonic metamaterial absorber structures.

The deposition mask is a two-layer system. The underlying lift-off layer aids the subsequent mask lift-off, whereas the NIL fabricated top layer is defining the location and shape where the nanodisks are grown during metal deposition. The chosen pattern layout is not optimized for a solar absorber application, however, due to the versatility of this process, the absorber features could be tailored by choosing an optimized nanopattern for the nanoimprint step.

As proof-of-concept we show the fabrication of elliptical tungsten nanodisks on top of metallic substrates with 100 nm oxide layers, creating a metal nanoparticles-insulator-metal reflector structure. Two different material deposition methods are compared: direct current sputtering and high-power impulse magnetron sputtering. We characterize the fabricated nanodisks by AFM and SEM imaging and comment on the selective absorptivity and thermal stability compared to unstructured absorbers.

Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived, designed, and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the meso (tube shape and geometry) and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Modeling results showed that fractal-like structures and geometries can increase the effective solar absorptance by 5 – 20% and the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. Meso-scale prototypes were fabricated using additive manufacturing techniques, and a macro-scale bladed receiver design was fabricated using Inconel 625 tubes. On-sun tests were performed using the solar furnace and solar tower at the National Solar Thermal Test facility. The test results demonstrated enhanced solar absorptance and thermal efficiency of the fractal-like designs.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.