High Temperature Solar Thermal Applications Research Articles

High-temperature solar energy absorption enhancement of mixed-phase core–shell spherical Al-based composite particles

This study focuses on the development of heat transfer/thermal energy storage particles for direct irradiation solid particle solar receivers to improve the efficiency by enhancing solar absorption, thermal stability, thermal storage property, and mechanical strength. A novel dry-mix extrusion technique was adopted to synthesize Al-based core–shell composite particles. The research shows that particles coated with 5.5 wt% mixed-phase metallic compounds maintain over 90 % solar absorptivity after 300 h at 1200°C, 600-1200°C thermal cycling, and wear tests, with an absorptivity change negligible (±2.20 %) and a mass loss of less than 0.053 g. The cost of the particles is below 1.5€/kg with good flowability, compressive strength up to 398.6 MPa, and an energy density as high as 2675.3 kJ/(m3·K). Monte-Carlo finite element numerical results found that the absorption efficiency of a composite particle curtain with a thickness of 0.02 m and a volume fraction of 0.05 increased by 169.6 % over the original ones. The particles can operate for extended periods at higher temperatures or harsh conditions to improve energy conversion efficiency, extend service life, and reduce maintenance requirements. It demonstrates the potential application value in the field of thermal energy absorption and storage, providing highly promising candidates for future high-temperature solar thermal applications.

Thermal treatment of stainless steel substrates along with a thin silicon layer for high temperature solar thermal applications

Solar energy harvesting in solar thermal systems using different solar absorber coatings on collectors has been widely studied. Here, we incorporate a single layer Si (∼250 nm) onto one of the most common and cost-effective collector materials, i.e., stainless steel (SS304) to improve its optical performance by thermal treatment. During the heating cycle, the samples were heated at 900 °C for short durations and suddenly quenched to room temperature in air. A high solar absorptance of 0.92 along with an emittance of 0.37 were obtained after heat-treatment. The reflectance studies showed reduced uniform reflectance (<10%) over the entire UV–Vis-NIR region and high reflectance in the far IR region. The morphology, composition, and crystal structures were studied in detail. The Si layer and the quenching process aided in the formation of a highly irregular microstructure consisting islands of Cr-Fe-Mn-Si oxides and silicides (with trace amount of Ni) which in turn increases surface roughness. This irregular array of islands of varying sizes and varying gaps between them helps in increasing absorptance due to multiple reflections, scattering and diffraction effects. Similar studies carried out on SS202 substrates resulted in different surface morphology and hence different absorptance (0.84), suggesting that the surface morphology and absorptance are highly dependent on the substrate composition. The initial thermal stability data on heat-treated Si/SS304 suggests stability up to 700 °C in air for 1000 hrs. Our studies indicate that thermal treatment of Si/SS304 for a short duration is a plausible solution to develop cost effective high temperature solar thermal collectors.

Comprehensive thermal energy storage analysis of ceramic foam-enhanced molten salt in a shell-and-tube unit

Ceramic foam can be used to enhance the energy storage efficiency of molten salt for high-temperature solar thermal applications. However, its performance in a shell-and-tube unit is not fully understood. In this study, the energy storage performance of ceramic foam-enhanced molten salt in a shell-and-tube unit is investigated. The effects of ceramic foam configurations such as the filling height, porosity and outer diameter are studied. The results show that when the ceramic foam reaches the inner tube, the enhancement performance is remarkable, while it is insignificant in the case of below the inner tube. Although the total stored energy of the full foam-filled case is decreased by 13.6% compared to the no foam-filled case, the energy storage rate is improved by 54.9%. The variation of energy storage rate with porosity is generally regular. Ceramic foam with a small outer diameter has great enhancement performance; as the outer diameter increases, the difference in the enhancement gets less remarkable. The three factors are compared through a normalised effective porosity and the ceramic foam with varying outer diameters shows the best performance including the shortest melting time, the largest total stored energy and the highest energy storage rate. It is attributed that the ceramic foam surrounds the tube completely and concentrates around the tube, so its performance of conducting the heat of the tube is the best. The corrosion resistance of ceramic foams and the cost advantage make it a suitable thermal enhancer for molten salt in high-temperature solar thermal energy storage applications.

Photothermal performance of three chromia-forming refractory alloys for high-temperature solar absorber applications

Spectral hemispherical emissivity is a crucial material characteristic that determines radiation heat transfer. In high-temperature solar thermal applications, it affects not only the efficiency of the solar energy absorption but also the heat losses caused by thermal radiation and the radiative heat transfer within the receiver. Due to the limitations of the working temperature of existing solar absorber coatings, the spectral hemispherical emissivity of the oxidized surface is a key performance indicator for evaluating the potential of a candidate refractory alloy for high-temperature (> 1000 °C) solar receiver/reactor designs. In this work, we systematically studied the photothermal performances of the oxidized surfaces of three widely used high-performance commercial chromia-forming alloys (Haynes 230, Hastelloy X, and SS 253MA) by analyzing the spectral hemispherical reflectance in the band 0.25–25 μm. The stability of the optical properties of the formed oxide layers have also been studied by exposing the three alloys at 1150 °C in air for three different exposure periods (10 h, 100 h, and 200 h). The results show that the solar absorptivity of all the samples is in the range of 0.800–0.855, with SS 253MA showing the best performances in offering both high and stable solar absorptivity in the range of 0.837–0.855. The evaluation of the photothermal performances suggest the potential of these three alloys in solar-thermal applications.

Reinforcement optical performance and thermal tolerance in a TiB2-HfB2-based double-layer spectral selective absorber via a pre-annealing strategy

As a vital component of existing and emerging solar-thermal technologies, spectrally selective absorbers (SSA) are currently limited to temperatures under 650 °C in vacuum and even lower in air due to the optical deterioration caused by temperature-related structural instability. Herein, refractory ceramic materials (HfB2 and TiB2) are used to construct a double-layer SSA by a combinatorial magnetron co-sputtering approach. Subsequently, optical properties of the as-deposited SSA are reinforced by a pre-annealing strategy, exhibiting a high solar absorptance of 93.2% and suppressed thermal emittance of 8.9%, as well as a direction-independent absorption. More significantly, the pre-annealing SSA embraces outstanding thermal robustness, retaining superior optical performance after annealing at 800 °C for 240 h. Solar-thermal efficiency reaches as high as 68.6% under the irradiation of 100 suns, which equals or surpasses state-of-the-art SSA, capable of those high-temperature solar-thermal applications. The high surface temperature of more than 100 °C is captured under 1.5 suns, promising great potential in solar interface evaporation-related applications. This work also provides the fundamental basis for designing SSA and expands the application scope of the refractory ceramic materials, opening the avenue to further exploration of a family of composite refractory ceramic.

Highly efficient and durable solar thermal energy harvesting via scalable hierarchical coatings inspired by stony corals

Stony coral morphology inspires ultra-stable sunlight absorber structure with highest reported absorptance for high-temperature solar thermal applications.

Thermal oxidation of stainless steel substrate with tunable spectral selectivity: Transition from a reflecting to a highly absorbing Cr–Fe spinel surface

The conventional methods of obtaining solar selective surfaces for high temperature solar thermal applications involve coating of the substrate by various methods such as physical vapor deposition, plasma spraying, anodization, etc. The present work is an attempt to enhance the optical properties of metals by heat treatment. The oxide layers formed by annealing of stainless steel 304 (SS 304) enhance the absorptance in the solar spectrum region. Influences of oxidation temperature and oxidation time span on the values of solar absorptance and thermal emittance have been studied. The annealing of SS 304 substrate was carried out in air at 600–900 °C. The time period of annealing plays a crucial role in the amount of oxides formed and thus is a variable parameter. The absorptance values obtained with isothermal oxidation at shorter and longer durations have been compared at a temperature of 900 °C. A cyclic loading approach is also employed to arrive at the optimal absorptance of the samples. It is further used to study the dependency of solar absorptance on the reaction kinetics with respect to varying oxidation time. Under the optimized annealing conditions, heat-treated SS 304 sample exhibited an absorptance of 0.920 and an emittance of 0.37. A plausible model for the high optical absorption in these oxidized surfaces with relatively low thermal emittance is rationalized. High temperature materials such as Inconel and Nimonic have also been subjected to isothermal annealing and the absorptance values were found to be 0.887 and 0.880, respectively.

Performance evaluation and durability studies of W/WAlSiN/SiON/SiO2 based spectrally selective solar absorber coating for high-temperature applications: A comprehensive study on thermal and solar accelerated ageing

We report a solar selective coating of W/WAlSiN/SiON/SiO2 fabricated using a Four-Cathode Reactive Unbalanced Direct Current (DC) magnetron sputtering system with high thermal stability, good durability and resistance to outdoor testing conditions. The coating also exhibits superior mechanical properties (Hardness ∼12 GPa). In addition, thermal shock tests at high temperatures and solar accelerated ageing measurements are investigated in-depth to analyze the performance of the solar absorber coating. The thermal shock tests of the samples at various temperatures in the range of 500–600 °C depict the excellent thermophysical resistance of the as-deposited samples. To test the thermal durability of the solar absorber coating, we applied 200 cycles of solar accelerated ageing on the samples using a solar accelerated ageing facility with concentrated flux density varying from 50 to 250 kW/m2. These cycles have been defined to replicate real high solar flux and temperature on the front side of the samples along with high cooling and heating rates, reproducing the abrupt variations of solar irradiation due to cloudy weather and subsequent thermal shocks for a given receiver. Overall, the durability tests of the solar absorber carried out under various conditions indicate a minimal change in the optical properties (Δα = 0.002 and Δε = 0), thus, making it a potential candidate for high-temperature solar thermal applications.

A New High-Temperature Durable Absorber Material Solution through a Spinel-Type High Solar Absorptivity Coating on Ti2AlC MAX Phase Material.

Enhancing the operating temperature of concentrating solar power systems is a promising way to obtain higher system efficiency and thus enhance their competitiveness. One major barrier is the unavailability of suitable solar absorber materials for operation at higher temperatures. In this work, we report on a new high-temperature absorber material by combining Ti2AlC MAX phase material and iron–cobalt–chromite spinel coating/paint. This durable material solution exhibits excellent performance, passing the thermal stability test in an open-air environment at a temperature of 1250 °C for 400 h and at 1300 °C for 200 h. The results show that the black spinel coating can offer a stable high solar absorptivity in the range of 0.877–0.894 throughout the 600 h test under high temperatures. These solar absorptivity values are even 1.6–3.3% higher than that for the sintered SiC ceramic that is a widely used solar absorber material. Divergence of solar absorptivity during these relatively long testing periods is less than 1.1%, indicating remarkable stability of the absorber material. Furthermore, considering the simple application process of the coating/painting utilizing a brush followed by curing at relatively low temperatures (room temperature, 95 and 260 °C in sequence), this absorber material shows the potential for large-scale, high-temperature solar thermal applications.

Optical characterisation of alumina–mullite materials for solar particle receiver applications

Alumina–mullite particles are used in high-temperature solar thermal applications such as solar particle receivers. In this study, optical properties of alumina–mullite materials with variable content of alumina and mullite are determined in the spectral range of 0.193–1.69 μm. Variable angle spectroscopic ellipsometry is performed for alumina–mullite thin films, which are fabricated by magnetron sputtering. The thin films are characterised by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy methods. The B-spline model is employed to generate ellipsometric parameters to fit the measured data and to obtain the optical properties. The investigated materials of variable content of alumina and mullite have a similar refractive index in the considered spectral range. The absorptive index of the alumina–mullite materials in the spectral range of 0.193–0.4 μm is higher than in the range 0.4–1.69 μm. The absorptive index decreases with increasing content of alumina in the spectral range of 0.193–0.4 μm. The material composed of similar proportions of alumina and mullite yields the highest absorptive index in the spectral range of 0.4–1.1 μm. The optical properties determined for the alumina–mullite materials are applied to obtain the radiative properties of spherical homogeneous particles. Mie theory is used to calculate absorption and scattering efficiency factors, as well as the scattering phase function. In addition, the scattering phase functions are obtained using the Henyey–Greenstein approximation and the transport approximation. The Monte Carlo ray-tracing method is employed to study the radiative transfer in a model one-dimensional particle curtain containing polydisperse particles exposed to high-flux solar irradiation. It is found that the overall reflectance, absorptance and transmittance of the particles only weakly depend on the optical properties of the materials investigated.

Optical and radiative characterisation of alumina–silica based ceramic materials for high-temperature solar thermal applications

Optical and radiative properties of alumina–silica based ceramic materials are determined in the spectral range of 0.2–2.5 µm. The effects of material thermal treatment on the material structure, composition as well as optical and radiative properties are investigated using the commercial CARBO HSP materials and in-house fabricated samples with sintering temperatures of 1400∘C and 1500∘C. The material structure and composition are characterised using scanning electron microscopy, X-ray micro computed tomography, powder X-ray diffraction and scanning transmission electron microscopy. Directional–hemispherical reflectance and transmittance of the samples are obtained using dispersive spectroscopy. A two-step inverse methodology consisting of an analytical solution based on the modified two-flux approximation and iterative Monte Carlo ray-tracing is developed and applied to infer the transport scattering albedo and the transport extinction coefficient, respectively. The structural and compositional investigation revealed that the presence of chemical reactions between Al2O3 and SiO2 to form mullite during the thermal treatment in the preparation process. The investigated samples are strongly absorptive in the spectral range of 0.2–0.4 µm, in which the transport scattering albedo is close to 0. Scattering is the dominant attenuation mechanism in the spectral range of 1.5–2.5 µm, in which the transport scattering albedo is above 0.8. The absorptive index of the in-house prepared samples sintered at 1400∘C is one to two orders of magnitude smaller than that of the commercial material in the spectral range of 0.2–2.5 µm. However, the absorptive index of the samples sintered at 1500∘C has similar values and is 25% smaller than that of the commercial material in the spectral ranges of 0.4–0.7 µm and 1.5–2.5 µm, respectively. The sample absorption increases with thermal treatment temperature because of higher extents of sintering.

Solution-Processed All-Ceramic Plasmonic Metamaterials for Efficient Solar-Thermal Conversion over 100-727°C.

Low-cost and large-area solar-thermal absorbers with superior spectral selectivity and excellent thermal stability are vital for efficient and large-scale solar-thermal conversion applications, such as space heating, desalination, ice mitigation, photothermal catalysis, and concentrating solar power. Few state-of-the-art selective absorbers are qualified for both low- (<200°C) and high-temperature (>600°C) applications due to insufficient spectral selectivity or thermal stability over a wide temperature range. Here, a high-performance plasmonic metamaterial selective absorber is developed by facile solution-based processes via assembling an ultrathin (≈120nm) titanium nitride (TiN) nanoparticle film on a TiN mirror. Enabled by the synergetic in-plane plasmon and out-of-plane Fabry-Pérot resonances, the all-ceramic plasmonic metamaterial simultaneously achieves high, full-spectrum solar absorption (95%), low mid-IR emission (3% at 100°C), and excellent stability over a temperature range of 100-727°C, even outperforming most vacuum-deposited absorbers at their specific operating temperatures. The competitive performance of the solution-processed absorber is accompanied by a significant cost reduction compared with vacuum-deposited absorbers. All these merits render it a cost-effective, universal solution to offering high efficiency (89-93%) for both low- and high-temperature solar-thermal applications.

Progress in heat transfer research for high-temperature solar thermal applications

High-temperature solar thermal energy systems make use of concentrated solar radiation to generate electricity, produce chemical fuels, and drive energy-intensive processing of materials. Heat transfer analyses are essential for system design and optimisation. This article reviews the progress, challenges and opportunities in heat transfer research as applied to high-temperature solar thermal and thermochemical energy systems. The topics discussed include fundamentals of concentrated solar energy collection, convective heat transfer in solar receivers, application of liquid metals as heat transfer media, and heat transfer in non-reacting and reacting two-phase solid–gas systems such as particle–gas flows and gas-saturated porous structures.

Tailoring optical properties of surfaces in wide spectral ranges by multi-scale femtosecond-laser texturing: A case-study for TaB2 ceramics

The use of high power pulsed lasers is an effective tool for microstructuring material surfaces. It appears particularly useful when the material has some characteristics making difficult using other procedures (e.g. a high hardness). The present work reports on the femtosecond-laser treatment on tantalum diboride ultra-high temperature ceramics with different starting porosity fractions. The interaction with the laser beam creates a pattern with a complex multi-scale structure on the ceramic surface, whose characteristics depend on accumulated laser fluence and pristine porosity. Optical properties are significantly changed, allowing to separately optimize the interaction of the material with electromagnetic radiation spectrally located in different regions. As a case study, we apply the proposed strategy considering high-temperature solar thermal absorber applications, where the independent management of UV–Visible-Near IR radiation (sunlight) and Mid-IR (thermal radiation at the operating temperatures) are required. The correlation between the typical sizes of the realized multi-scale structures and the optical parameters (solar absorptance and thermal emittance in our example application) is discussed using an original predictive approach. The method here shown can be extended to every situation where materials are required to simultaneously interact with electromagnetic radiation in various spectral ranges. • Femtosecond laser-treatment creates a multi-scale surface pattern on ultra-hard TaB2 ceramics. • Local surface morphology and composition rely on accumulated laser fluence and pristine material porosity. • Optical properties are changed, with a remarkable increase of solar absorbance. • An original and simple method is described to correlate optical properties to the multi-scale surface characteristics. • The method is extremely useful to predictably optimize any surface interacting with light.

Analysis of high-flux solar irradiation distribution characteristic for solar thermochemical energy storage application

Designing equipment that meets the needs of efficient solar energy conversion and utilization is of great significance. In this paper, the design method of a solar simulator with Fresnel lens is studied from the perspective of concentrating solar energy research, high-temperature material testing, and optimal design method of reactor receiving surface. The Monte Carlo ray-tracing technique is applied to analyze the concentrating performance of a single xenon lamp-lens unit and hexagonal arrays of seven xenon lamp-lens units. For the single-disc concentrating unit, the Fresnel lens is analyzed and optimized for the reduction of the irradiance unevenness of the central spot, improvement in the peak irradiance, and the light convergence into different shapes to meet various needs. The numerical calculations are performed on the multi-disc simulator to obtain suitable installation conditions and reactor temperature distribution. A 42 kWel seven-disc Fresnel lens-based high-flux solar simulator is simulated. The solar simulator can provide a peak energy flow of 2.32 MW/m2 with a spot diameter of 46 mm. The temperature of the cavity receiver reached 964 K and 1024 K when using a parabolic mirror and ellipsoidal mirror, respectively under the illumination of 6 kW xenon lamp. The concentrating unit coupled with the solar reactor exhibited reasonable thermal energy storage efficiency, as well as electro-thermal efficiency. The designed concentrating system could be considered suitable for high-temperature solar thermal and thermochemical applications, especially where there is a strict requirement for the shape or uniformity of the spot.

Experimental study on forced convection heat transfer of KNO3–Ca(NO3)2 + SiO2 molten salt nanofluid in circular tube

Molten salt is a major heat storage medium for medium–high temperature solar thermal power applications. The heat storage density and heat transfer efficiency of molten salt are important factors of solar thermal power efficiency. Recently, many studies have reported that molten salt nanofluid, which is formed by adding nanoparticles in molten salt, can remarkably enhance the specific heat capacity of molten salt. However, research on the forced convection heat transfer of molten salt nanofluid is inadequate. In this study, the forced convection heat transfer experiments were carried out in a circular tube of a previously reported promising molten salt nanofluid composed of KNO3–Ca(NO3)2 + 1 wt% of 20 nm SiO2 nanoparticles (K-C-Snm). Results showed that compared with its base pure molten salt, the K-C-Snm molten salt nanofluid exhibited substantially better forced convection heat transfer performance under the same working condition. The Nusselt number and the convective heat transfer coefficient of K-C-Snm molten salt nanofluid were 16.3% and 39.9% higher than its base pure molten salt, respectively. The experimental data of K-C-Snm molten salt nanofluid largely differed from the classical correlation equations. Hence, a new empirical heat transfer correlation equation was set for K-C-Snm molten salt nanofluid, and the deviation between the experiment data and new correlation was within ±7%.

Transparent conductive tantalum doped tin oxide as selectively solar-transmitting coating for high temperature solar thermal applications

The transparent conductive oxide (TCO) SnO2:Ta is developed as a selectively solar-transmitting coating for concentrated solar power (CSP) absorbers. Upon covering with an antireflective layer, a calculated absorptivity of 95% and an emissivity of 30% are achieved for the model configuration of SnO2:Ta on top of a perfect black body (BB). High-temperature stability of the developed TCO up to 1073 K is shown in situ by spectroscopic ellipsometry and Rutherford backscattering spectrometry. The universality of the concept is demonstrated by transforming silicon and glassy carbon from non-selective into solar-selective absorbers by depositing the TCO on top of them. Finally, the energy conversion efficiencies of SnO2:Ta on top of a BB and an ideal non-selective BB absorber are extensively compared as a function of solar concentration factor C and absorber temperature TH. Equal CSP efficiencies can be achieved by the TCO on BB configuration with approximately 50% lower solar concentration. This improvement could be used to reduce the number of mirrors in a solar plant, and thus, the levelized costs of electricity for CSP technology.

High-temperature stable refractory nanoneedles with over 99% solar absorptance

Solar absorber coatings have widely been investigated for solar-thermal technologies including concentrated solar power and solar thermochemistry. While various nanostructures such as nanowires and nanotubes have been commonly used for high solar absorptance owing to their potent light trapping effect, the high temperature stability of these nanostructures has yet to be established due to either coarsening of nanostructures or oxidation of certain materials in air (e.g., Si and C). In this work, we developed a nanostructured solar absorber from a family of high-temperature refractory spinel oxides, Co3O4 and CuCo2O4, with ultra-high solar absorptance over 99%. Once coated with a thin layer of HfO2 or SiO2 through atomic layer deposition, the Co3O4 and CuCo2O4 nanoneedles preserve their high aspect ratio and sharp tips, allowing the solar absorbers to maintain the superior absorptivity and excellent thermal stability at an elevated temperature for an extended period (650 °C and 800 °C for 100 h for passivated Co3O4 and Cu1Co2O4, respectively). These results suggest that solar absorbers made from refractory spinel oxide nanoneedles can be used for high-temperature solar thermal applications with ultrahigh absorptance.

Structure, optical simulation and thermal stability of the HfB2-based high-temperature solar selective absorbing coatings.

Transition metal borides are a kind of potential materials for high-temperature solar thermal applications. In this work, a novel SS/HfB2/Al2O3 tandem absorber was prepared, which exhibited high solar spectrum selectivity (α/ε) of 0.920/0.109. The optical constants of the coating were obtained using spectroscopic ellipsometry, and the dispersion model of the HfB2 layer was modeled with the Tauc–Lorentz dispersion formula. In addition, the reflectance spectrum simulated by the CODE software corroborated well with the experimental results. The thermal stability test indicated that the HfB2/Al2O3 solar absorber coating was thermally stable in vacuum at 600 °C for 2 h. When extending the annealing time to 100 h, the coating could maintain high spectral selectivity after aging at 500 °C irrespective of whether in air or vacuum. All these results indicate that the coating has good solar selectivity and is a promising candidate for high-temperature solar thermal applications.

Design and characterization of a high-flux non-coaxial concentrating solar simulator

To improve the spatial uniformity of the concentrating solar simulators, a concept of non-coaxial deflection angle was introduced to the typical ellipsoidal reflectors. Based on this idea, a new-type 42 kWe high-flux non-coaxial concentrating solar simulator was designed and built. The Monte Carlo ray-tracing technique was applied to optimize the value of the non-coaxial deflection angle and simulate the flux distribution of this new-type solar simulator. A flux mapping system based on the indirect method was used to characterize the solar simulator optically. The relative deviation for the measured and the simulated results of this new-type solar simulator, as well as the simulated results of a conventional concentrating solar simulator, were compared and analyzed. The results show that the spatial nonuniformity of this new-type solar simulator over a circular target of 50 mm in diameter, improved to 7.2% from 40.3% of a conventional one. This non-coaxial concentrating solar simulator is considered very suitable for high-temperature solar thermal, thermochemical and high-concentration photovoltaic applications, especially where there are strict requirements for spatial uniformity.