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Song X.,Beihang University | Dong G.,Beihang University | Gao F.,Beihang University | Diao X.,Beihang University | And 3 more authors.
Energy | Year: 2014

Effect of solar incidence angle was analyzed in order to accurately simulate the heat flux distribution around the absorber tube outer surface. Helical screw-tape inserts was proposed to homogenize the absorber tube temperature distribution and improve the thermal efficiency. Three dimensional periodical models of flow and heat transfer were established and solved with the heat flux of different transversal angle (β). The results show that β affects the flux distribution more greatly than longitudinal angle (ψ). Transversal angle (β) of 11.567mrad increases the relative change of heat loss (Qloss) as inlet temperature rises, and also increases the maximum temperature on absorber tube (Tmax) and the maximum circumferential temperature difference (δT). But its effect reduces as Reynolds number rises. Within the range of studied Re, the helical screw-tape inserts of given geometrical parameters greatly reduce the Qloss, Tmax and δT, which indicates that helical screw-tape inserts is a feasible way to enhance the heat transfer inside the receiver. © 2014 Elsevier Ltd. Source


Zheng L.,Beihang University | Zheng L.,CAMDA New Energy Equipment Co. | Gao F.,Beihang University | Zhao S.,Uppsala University | And 3 more authors.
Applied Surface Science | Year: 2013

In order to optimize and prepare high performance Mo-SiO2 cermet solar selective absorbing coating, a series of Mo-SiO2 cermet films with different metal volume fraction were deposited on optical glass using mid-frequency (MF) and radio frequency (RF) co-sputtering. The reflectance (R) and transmittance (T) in the wavelength range of 250-2500 nm have been simulated using SCOUT software with different dielectric function models. The optical constants, film thickness, metal volume fraction and other parameters have been deduced from the modeling. The fitted optical constants were then used to simulate and optimize the Mo-SiO2 solar selective coating and samples were prepared based on the optimized parameters. The Maxwell Garnett (MG) and Bruggeman (BR) effective-medium theory have been added in the dielectric function models to describe low metal volume fraction cermet layer (LMVF) and high metal volume fraction cermet layer (HMVF), separately. The optical spectra (R and T) of all single films were in a good agreement with the fitted spectra by dielectric function models. The experimental measured reflectance of the solar selective coating was also in rather good agreement with the optimized result. The solar absorptance of theoretically optimized selective coating was 0.945, while the absorptance of the experimental coating was 0.95. The thermal emittance of 0.15 (at 400°C) was obtained. ©2013 Elsevier B.V. All rights reserved. Source


Zheng L.,Beihang University | Zheng L.,CAMDA New Energy Equipment Co. | Zhou F.,CAMDA New Energy Equipment Co. | Zhou Z.,CAMDA New Energy Equipment Co. | And 4 more authors.
Solar Energy | Year: 2015

In this paper, the angular solar absorptance of Mo-SiO2 double cermet solar selective absorber coating was simulated based on the deduced optical constants and calculated from measured reflectance spectra in the wavelength range 0.25-2.5μm for incidence angles between 0° and 75° by interval of 15°. It was found that Mo-SiO2 double cermet solar selective absorber with angular absorptance α=0.945 from 0° up to 60° is a wide-angle absorptance coating. The effect of thermal annealing on optical properties, microstructure and morphology of Mo-SiO2 double cermet solar selective coating was investigated. The annealing in vacuum at 600°C results in grain refinement of Mo particle, denser cermet layer and photo-thermal conversion efficiency increasing of 0.6%. Annealing at 800°C leads to grain growth of polycrystalline structure Mo, inter-diffusion in cermet layer and the photo-thermal conversion efficiency decreasing of 1.3%. © 2015 Elsevier Ltd. Source

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