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Using supercritical CO2 as a heat transfer fluid in microchannel receivers is a promising alternative for tower concentrating solar power plants. In this paper, the heat transfer and flow characteristics of supercritical CO2 in microchannels at high temperature are investigated by numerical simulations. The effects of microchannel structure, mass flow rate, heat flux, pressure, inlet temperature and radiation are analyzed and discussed. The results show that higher mass flow rate obtains poorer heat transfer performance with larger flow resistance of supercritical CO2 in microchannels at high temperature. The fluid and wall temperatures, average heat transfer coefficient and pressure drop all increase nearly linearly with the increases in heat flux and inlet temperature in the high-temperature region. Moreover, high pressure contributes to great hydraulic performance with approximate thermal performance. The effect of radiation on thermal performance is more pronounced than that on hydraulic performance. Furthermore, the optimized structures of inlet and outlet headers, as well as those of the multichannel in the microchannels, are proposed to obtain good temperature uniformity in the microchannels with relatively low pressure drop. The results given in the current study can be conducive to the design and application of microchannel receivers with supercritical CO2 as a heat transfer fluid in the third generation of concentrating solar power plants. [ABSTRACT FROM AUTHOR]

• Corrosion of three molten-salt mixtures were studied at 700 °C under argon. • Four alloys were tested and characterized by SEM, EDS and XRD. • C276, In625 and In718 are compatible with the NaKZn chloride-salt. • SS316 and In718 show low corrosion in the LiNaK carbonate-salt. • C276, In625 and In718 are compatible with the LiF-NaK carbonate-salt. Molten eutectic salt mixtures can be used as thermal energy storage materials or heat transfer fluids at concentrated solar power plants. The targeted operating temperature for the next generation concentrated solar power plants is 700 °C. Although chloride and carbonate eutectic salt mixtures with high thermally stable temperatures were reported to have great thermophysical properties, their corrosion to alloys at high temperature has not been thoroughly studied. Corrosion of the thermal energy storage materials and heat transfer fluids to alloys is critical, and oxygen in air is confirmed to accelerate the corrosion significantly. In the present study, three eutectic mixtures of NaCl-KCl-ZnCl 2 , Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 and LiF-Na 2 CO 3 -K 2 CO 3 were considered as high temperature thermal energy storage materials or heat transfer fluids. Corrosion of stainless steel 316, Hastelloy C276, Inconel 625 and 718 immersed in the three salt mixtures up to 21 days were experimentally investigated at 700 °C under argon atmosphere. The specimens after corrosion tests were characterized by scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The results show that SS316 is only compatible with the LiNaK carbonate-salt and the corresponding corrosion rate is lower than 95 μm/year. Both C276 and In625 have relatively good corrosion resistance for the NaKZn chloride-salt and LiF-NaK carbonate-salt, and the corrosion rates are lower than 78 μm/year. The highest corrosion rate over 1700 μm/year is detected for In625 immersed in the LiNaK carbonate-salt. In addition, In718 shows excellent corrosion resistance less than 10 μm/year immersed in all the three salt mixtures. [ABSTRACT FROM AUTHOR]

• Corrosion of three molten-salt HTFs were experimentally studied at 700 °C. • Five alloys were tested and characterized by SEM, EDS and XRD. • Corrosion of alloys in the NaKZn chloride-salt with air is not acceptable. • C276 shows lowest corrosion rate of 100 µm/year in the LiNaK carbonate-salt. • In718 is compatible with the LiF-NaK carbonate-salt. The state-of-the-art concentrated solar power tower technology requires novel heat transfer fluids which can operate at a temperature as high as 700 °C. Several molten eutectic salt mixtures are promising heat transfer fluid candidates because they are thermally stable at such high temperature and have good thermophysical properties. However, their corrosion properties to alloys are not investigated thoroughly in the literature. Molten salts usually have high corrosion to alloys, which is a critical issue since the piping and containing materials of the heat transfer fluid in a solar power plant are generally alloys. In this study, the corrosion of three eutectic mixtures including NaCl-KCl-ZnCl 2 , Li 2 CO 3 -Na 2 CO 3 -K 2 CO 3 and LiF-Na 2 CO 3 -K 2 CO 3 to different alloys were experimentally studied at isothermal condition of 700 °C in contact with air. Alloys tested were stainless steel 316 (SS316), Hastelloy C276 (C276), Inconel 625 (In625) and 718 (In718). Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques were employed to characterize the coupon surfaces after corrosion tests. Results show that all the alloys have corrosion rates higher than 450 μm/year immersed in the NaKZn chloride-salt. Therefore, the NaKZn chloride-salt is not applicable as a heat transfer fluid if air cannot be evacuated. C276 immersed in the LiNaK carbonate-salt and In718 immersed in the LiF-NaK carbonate-salt may form protective compact layers on the alloy surfaces to resist corrosion. However, the high cost of lithium salts is another major obstacle to commercialize heat transfer fluids. [ABSTRACT FROM AUTHOR]