The optimization of the poor heat transfer characteristics of fluids conventionally employed in solar devices are at present one of the main topics for system efficiency and compactness. In the ...present work we investigated the optical and thermal properties of nanofluids consisting in aqueous suspensions of single wall carbon nanohorns. The characteristics of these nanofluids were evaluated in view of their use as sunlight absorber fluids in a solar device. The observed nanoparticle-induced differences in optical properties appeared promising, leading to a considerably higher sunlight absorption. We found that the thermal conductivity of the nanofluids was higher than pure water. Both these effects, together with the possible chemical functionalization of carbon nanohorns, make this new kind of nanofluids very interesting for increasing the overall efficiency of the sunlight exploiting device.
The optical characterization is reported of a new fluid consisting of single-wall carbon nanohorns and ethylene glycol for solar energy applications. Carbon nanohorns play a significant role in ...enhancing sunlight absorption with respect to the pure base fluid. The obtained results are compared with those obtained for fluids suspending more conventional carbon forms, i.e. carbon-black particles. We found that nanohorn spectral features are far more favorable than those of amorphous carbon for the specific application. This result shows that carbon nanohorn-based nanofluids can be useful for increasing the efficiency and compactness of thermal solar devices, reducing both environmental impact and costs.
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► Single-wall carbon nanohorns (SWCNH) in glycol are optically characterized. ► SWCNH considerably enhance sunlight absorption properties of the base fluid. ► SWCNH-nanofluids are compared with more conventional carbon forms. ► SWCNH appear more promising for solar energy than amorphous carbon.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
It is well known that the efficiency of thermodynamic solar plants increases with working temperature. At present the main limit to temperature upscaling is the capability of the absorber to ...withstand high temperatures. The ideal solar absorber works at high temperatures, has a low thermal emissivity and a high absorptivity in the solar spectral range. This paper reports on the high-temperature emissivity characterization of ceramic zirconium carbide samples. The investigation has been performed for samples of different chemical compositions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
•Spectral normal emittance of SiC, TaB2 and ZrB2 ceramics is measured up to 850 °C temperature.•Surface features of samples affect the temperature–dependent emittance.•The size of surface features is ...key parameter.•Borides show a thermal emittance significantly lower than silicon carbide.
In this work, for the first time, we studied the temperature-dependent spectral emittance of highly refractory ceramics, e.g. silicon carbide (SiC) and two ultra-high temperature ceramics (tantalum diboride (TaB2), zirconium diboride (ZrB2)) to evaluate their potential for solar tower receivers applications. We measured the spectral normal emittance from 1 µm to 21 µm at temperatures up to 850 °C, in vacuum, by means of a novel experimental setup composed by an electric furnace connected to a FT-IR spectrometer. Comparison with previously published data collected with a different setup was also carried out to validate the results. In addition, the experimental spectral emittance was also compared with the values calculated from hemispherical reflectance at room temperature by means of Kirchhoff’s law. The results show that surface features play a fundamental role in the emittance of investigated ceramics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Porous graded materials (PGMs) consisting of ZrB2 are obtained by Spark Plasma Sintering (SPS) taking advantage of an asymmetric die configuration with changing cross section. Due to the temperature ...gradient established along the longitudinal direction, porosity changes up to about 26 vol.% were generated across the volume. The surface roughness of the resulting products increased as the holding temperature was reduced. Superior solar absorbance (α) and lower spectral selectivity (α/ε) properties were correspondingly obtained. PGMs displayed higher α and lower α/ε values with respect to highly dense samples produced using a constant cross section die. More important, these products exhibit similar or improved optical properties with respect to the reference SiC material. These findings are important in view of the development of a novel class of ceramics for solar energy applications, able to operate under higher temperature conditions while simultaneously satisfying optical, microstructural and mechanical needs.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
CSP (Concentrated Solar Power) plants technologies use the concentration of solar energy on a receiver to produce heat and then electricity by a thermodynamical process. A solar absorber material is ...used to convert the energy carried by light into heat. This type of material works at high temperatures (up to 1000 °C) under a highly concentrated solar flux (up to x1000 or more). Optical properties determine the performance of absorbers and it is thus necessary to measure their spectral absorptance and emittance. Solar absorptance is directly linked to the capacity of the absorber material to convert the solar flux into heat. Emittance drives the radiative thermal losses for the heated absorber and depends on the absorber temperature. The characterization of a material in operational conditions at high temperatures requires advanced apparatuses, and different measurement methods exist for the characterization of these two quantities of relevance regarding an absorber. A Round Robin Test (RRT) was conducted with the objective of comparing different new optical apparatuses and methods for measuring the emittance or luminance of various solar absorbers in air. Measurements were carried out directly at temperatures up to 560 °C while heating the samples, and also indirectly by hemispherical reflectance measurements at room temperature. In this paper, the Round Robin Test procedure to compare apparatuses is described, as well as the corresponding reflectance and emittance results on four types of materials. In addition, a discussion of some factors of influence over high temperature measurements in air and of the observed discrepancies among results from the evaluators is presented. The reliability of reflectance/emittance measurements is also demonstrated and statistics of deviations from the mean value are analysed. These allow us to infer information about measurement reproducibility. The reflectance spectra of all samples after high temperature measurements in air (up to 500 °C) do not show any significant changes.
•Round Robin Test conducted with nine partners.•Apparatuses comparison for measuring the thermal emittance of various solar absorbers.•Direct thermal measurements up to 560 °C while heating the samples.•Emittance calculation assuming Kirchhoff’s law of radiation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Parabolic trough collectors were investigated, applying the study results to a plant prototype installed in Florence (Italy) for residential supply. Ray tracing simulations examined optical ...characteristics and collection performance on the base of prototype geometry and functionality. The purpose was to control the realisation errors and overcome the difficulties arising in the development of the solar trough plant. The paper summarises the results of several studies analysing the interactions between collection efficiency, angular misalignments, mirror deformations, sun tracking and trough placement. The mirror deformations are reproduced introducing an original procedure. The unusual subject of imprecision in trough axis placement is discussed.
► Parabolic trough collectors were analysed, applying the results to a realised prototype. ► Energetic and angular analyses were performed using ray tracing simulations. ► An innovative procedure to reproduce rigid deformations of PTC was illustrated. ► We combined misalignments with mirror deformations, sun tracking, trough placement.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We report on the preparation, room temperature spectral reflectance and high-temperature thermal emittance characterization of different boride and carbide Ultra-High Temperature Ceramics (UHTCs). ...The investigated samples are compared with a reference material for solar absorber applications, i.e. silicon carbide. We show that spectral and thermal emittance properties of UHTCs are promising for novel solar receivers.
•A photovoltaic system based on DSCs associated with a solar concentrator is proposed.•The performances of the system were studied in indoor and outdoor condition.•Absence of cell damaging was ...observed up to 6 suns illumination.•Obtained results show that it is possible to use DSCs in solar concentrator systems.
The performances of a photovoltaic system based on dye-sensitized solar cells associated with a solar concentrator are investigated. An extensive indoor characterization is performed using both different wavelength bandwidths and concentration ratio. Starting from the acquired data, the system coupled with a solar tracking is also tested in outdoor condition. The obtained results show that it is possible to use this class of solar cells in solar concentrator systems, leading to a new class of devices with a high efficiency and a low cost.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Goethite (α-FeOOH) nanorods have been synthesized using two different methods, by aging at different temperatures of ferrihydrite suspensions obtained through co-precipitation, or by fast conversion ...of ferrihydrite through ultrasonic irradiation. The synthesis method and parameters influenced the size and shape of nanoparticles. Stable colloidal suspensions of nanorods have been prepared by purification of the precipitates and subsequent dispersion in deionised water by sonication. The synthesized nanopowders have been characterized by Powder X-ray Diffraction and Scanning Electron Microscopy and the colloidal suspensions by Dynamic Light Scattering and
ζ
-potential measurements. The magnetic field at nuclei level has been investigated by Mössbauer spectroscopy. Moreover, since a colloidal suspension of goethite is a mineral liquid crystal and due to its peculiar magnetic properties, polarized radiation transmittance of colloids in the UV-vis-NIR range has been determined under different magnetic field directions and intensities. It has been found that the transmittance in the NIR range can be tuned by changing magnetic field direction and strength.
Goethite (α-FeOOH) nanorods were synthesized and their size and shape were controlled by synthesis parameters. Stable colloidal suspensions were prepared and their transmittance in NIR range was tuned by modifying magnetic field direction and strength.
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IJS, KILJ, NUK, UL, UM, UPUK
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