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Abraham Kribus – One of the best experts on this subject based on the ideXlab platform.

  • Dense Wire Mesh as a High-Efficiency Solar Volumetric Absorber
    Volume 1: Aerospace Heat Transfer; Computational Heat Transfer; Education; Environmental Heat Transfer; Fire and Combustion Systems; Gas Turbine Heat , 2017
    Co-Authors: Maya Livshits, Lior Avivi, Abraham Kribus

    Abstract:

    Heating a gas to over 1,000°C with concentrated sunlight can enable advanced high-performance applications such as solar-driven combined cycles and solar thermo-chemical processes. Solar receivers using volumetric porous Absorbers are intended to produce the ‘volumetric effect’ leading to reduced heat loss and high Absorber Efficiency. However, experiments on volumetric Absorbers have not shown this effect, and the Absorbers’ Efficiency is usually in the range of 70–80% rather than the desirable range of over 90%. Several porous structure geometries, including the well-known ceramic honeycomb and ceramic foam, were investigated with a numerical model. The results show that even optimal configurations still fall short of the desired range of Absorber Efficiency. A new candidate structure, a dense wire mesh, was investigated and compared to the conventional Absorbers. The volumetric convection coefficient was also measured experimentally to provide validation of the single report found in the literature for this structure. An attractive solution with high Efficiency of 90% was found for a dense wire mesh with pore diameter of 1 mm and porosity of 0.83. This geometry seems then a promising candidate for future volumetric Absorbers.

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  • Experimental study of ceramic foams used as high temperature volumetric solar Absorber
    Solar Energy, 2016
    Co-Authors: Sylvie Cloutier, Cyril Caliot, Abraham Kribus, Y. Gray, Gilles Flamant

    Abstract:

    Abstract Volumetric Absorbers appear to be a promising technology in order to heat air above 1000 °C to feed combined-cycles (Brayton and Rankine thermodynamic cycles in cascade) for solar thermal electricity production. Thus, the choice of the Absorber characteristics (material, geometrical parameters) is the key parameter to reach a maximum solar-to-thermal energy conversion Efficiency. In this study, a test bench was developed and the solar-to-thermal Efficiency of reticulate porous ceramics with open pores was characterized. Improvements with current state-of-the-art were made by the use of a homogenizer, ensuring spatially homogenized concentrated solar flux irradiation. Fluxmetry and calorimetry measurements were realized to evaluate the flux map incident on the tested Absorber samples. A co-current position for the solar incoming power and the air flow rate was chosen for future comparisons with numerical predictions of atmospheric volumetric receivers. Several foam samples currently available in the industry were tested (Silicon Carbide, SiC) covering a wide range of porosity (72–92%) and pores per inch (5–20). A new selective material was also investigated (Zirconium Diboride, ZrB2). Experimental results for random foams were compared to the SiC honeycomb Absorber considered to be the reference volumetric receiver. In the literature, two trends were proposed to produce the best performances: (1) the use of foams with large pore diameters and high porosity and (2) the use of small pore diameters with low porosity. This study showed the second trend resulted in the best Absorber Efficiency. In addition, the use of selective materials was found promising for atmospheric air receiver, provided the solar absorptivity and the durability could be controlled.

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  • Parametric Study of Volumetric Absorber Performance
    Energy Procedia, 2014
    Co-Authors: Abraham Kribus, Y. Gray, M. Grijnevich, Cyril Caliot

    Abstract:

    Abstract This study investigates the possible performance of volumetric Absorbers as a function of geometric and material properties, aiming to identify the best Absorber design parameters and the highest Efficiency that may be expected. A simplified model is used with non-equilibrium heat transfer correlations and the two-flux approximation of one-dimensional radiative transfer. The radiative flux model has been validated against a detailed Monte-Carlo simulation. This model is simple enough for fast computation and parametric study. The results show that considerable gains in volumetric Absorber Efficiency can be achieved by careful selection of the Absorber properties. In addition to porosity and pore size, two additional overlooked properties are also significant: the thermal conductivity of the Absorber material, and the optical selectivity of the Absorber surface. Under certain conditions, reducing the thermal conductivity leads to a significant decrease in emission loss, almost reaching the ideal volumetric Absorber at local thermal equilibrium. This implies that the common preference for Absorber materials such as SiC should be reconsidered. Spectral selectivity of the Absorber material can also produce a significant increase in Efficiency, but this is valid only when the selectivity is close to ideal.

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Cyril Caliot – One of the best experts on this subject based on the ideXlab platform.

  • Experimental study of ceramic foams used as high temperature volumetric solar Absorber
    Solar Energy, 2016
    Co-Authors: Sylvie Cloutier, Cyril Caliot, Abraham Kribus, Y. Gray, Gilles Flamant

    Abstract:

    Abstract Volumetric Absorbers appear to be a promising technology in order to heat air above 1000 °C to feed combined-cycles (Brayton and Rankine thermodynamic cycles in cascade) for solar thermal electricity production. Thus, the choice of the Absorber characteristics (material, geometrical parameters) is the key parameter to reach a maximum solar-to-thermal energy conversion Efficiency. In this study, a test bench was developed and the solar-to-thermal Efficiency of reticulate porous ceramics with open pores was characterized. Improvements with current state-of-the-art were made by the use of a homogenizer, ensuring spatially homogenized concentrated solar flux irradiation. Fluxmetry and calorimetry measurements were realized to evaluate the flux map incident on the tested Absorber samples. A co-current position for the solar incoming power and the air flow rate was chosen for future comparisons with numerical predictions of atmospheric volumetric receivers. Several foam samples currently available in the industry were tested (Silicon Carbide, SiC) covering a wide range of porosity (72–92%) and pores per inch (5–20). A new selective material was also investigated (Zirconium Diboride, ZrB2). Experimental results for random foams were compared to the SiC honeycomb Absorber considered to be the reference volumetric receiver. In the literature, two trends were proposed to produce the best performances: (1) the use of foams with large pore diameters and high porosity and (2) the use of small pore diameters with low porosity. This study showed the second trend resulted in the best Absorber Efficiency. In addition, the use of selective materials was found promising for atmospheric air receiver, provided the solar absorptivity and the durability could be controlled.

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  • Parametric Study of Volumetric Absorber Performance
    Energy Procedia, 2014
    Co-Authors: Abraham Kribus, Y. Gray, M. Grijnevich, Cyril Caliot

    Abstract:

    Abstract This study investigates the possible performance of volumetric Absorbers as a function of geometric and material properties, aiming to identify the best Absorber design parameters and the highest Efficiency that may be expected. A simplified model is used with non-equilibrium heat transfer correlations and the two-flux approximation of one-dimensional radiative transfer. The radiative flux model has been validated against a detailed Monte-Carlo simulation. This model is simple enough for fast computation and parametric study. The results show that considerable gains in volumetric Absorber Efficiency can be achieved by careful selection of the Absorber properties. In addition to porosity and pore size, two additional overlooked properties are also significant: the thermal conductivity of the Absorber material, and the optical selectivity of the Absorber surface. Under certain conditions, reducing the thermal conductivity leads to a significant decrease in emission loss, almost reaching the ideal volumetric Absorber at local thermal equilibrium. This implies that the common preference for Absorber materials such as SiC should be reconsidered. Spectral selectivity of the Absorber material can also produce a significant increase in Efficiency, but this is valid only when the selectivity is close to ideal.

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  • The promise and challenge of solar volumetric Absorbers
    Solar Energy, 2014
    Co-Authors: Abraham Kribus, Y. Gray, M. Grijnevich, Gur Mittelman, Sébastien Mey-cloutier, Cyril Caliot

    Abstract:

    Abstract This study investigates the potential performance of volumetric Absorbers as a function of geometric and material properties, aiming to identify the best Absorber design parameters and the highest Efficiency that may be expected. A simplified one-dimensional model was used to represent a planar slab of ceramic foam Absorber, with local thermal non-equilibrium and effective volumetric properties. Three approaches for modeling the radiative transfer were considered, and the S 4 discrete ordinates model was selected based on validation against a detailed Monte-Carlo simulation. The boundary conditions were investigated in detail. This model is simple enough for fast computation and parametric study, yet reasonably realistic to represent real Absorbers. The results reveal several guidelines to improve the Absorber performance. Optimization of geometry (porosity and characteristic pore diameter) is insufficient to reach high Efficiency. A significant increase in convection heat transfer is required, beyond the normal behavior of ceramic foams. A reduction in the thermal conductivity of the Absorber material is also needed to maintain the desired temperature distribution. Finally, spectral selectivity of the Absorber material can also help to further increase the Absorber Efficiency, in contrast to the common opinion that it is effective only at low temperatures. With a combination of these measures, Absorber efficiencies may be increased for example from around 70% to 90% for air heating to 1000 °C under incident flux of 800 kW/m 2 .

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Timothy N. Lambert – One of the best experts on this subject based on the ideXlab platform.

  • Characterization of Pyromark 2500 Paint for High-Temperature Solar Receivers
    Journal of Solar Energy Engineering, 2013
    Co-Authors: A. Roderick Mahoney, Andrea Ambrosini, Marlene Bencomo, Aaron Christopher. Hall, Timothy N. Lambert

    Abstract:

    Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The radiative properties, aging, and selective Absorber Efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective Absorber coatings currently being developed. The solar absorptance ranged from ∼0.97 at near-normal incidence angles to ∼0.8 at glancing (80°) incidence angles, and the thermal emittance ranged from ∼0.8 at 100 °C to ∼0.9 at 1000 °C. After thermal aging at temperatures of ∼750 °C or higher, the solar absorptance decreased by several percentage points within a few days. It was postulated that the substrate may have contributed to a change in the crystal structure of the original coating at elevated temperatures.

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  • Characterization of Pyromark 2500 for High-Temperature Solar Receivers
    ASME 2012 6th International Conference on Energy Sustainability Parts A and B, 2012
    Co-Authors: A. Roderick Mahoney, Andrea Ambrosini, Marlene Bencomo, Aaron Christopher. Hall, Timothy N. Lambert

    Abstract:

    Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The cost, application, curing methods, radiative properties, and Absorber Efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective Absorber coatings currently being developed. The directional solar absorptance was calculated from directional spectral absorptance data, and values for pristine samples of Pyromark 2500 were as high as 0.96–0.97 at near normal incidence angles. At higher irradiance angles (>40°–60°), the solar absorptance decreased. The total hemispherical emittance of Pyromark 2500 was calculated from spectral directional emittance data measured at room temperature and 600°C. The total hemispherical emittance values ranged from ∼0.80–0.89 at surface temperatures ranging from 100°C – 1,000°C. The aging and degradation of Pyromark 2500 with exposure at elevated temperatures were also examined. Previous tests showed that solar receiver panels had to be repainted after three years due to a decrease in solar absorptance to 0.88 at the Solar One central receiver pilot plant. Laboratory studies also showed that exposure of Pyromark 2500 at high temperatures (750°C and higher) resulted in significant decreases in solar absorptance within a few days. However, at 650°C and below, the solar absorptance did not decrease appreciably after several thousand hours of testing. Finally, the Absorber Efficiency of Pyromark 2500 was determined as a function of temperature and irradiance using the calculated solar absorptance and emittance values presented in this paper.© 2012 ASME

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