Hemispherical and near-specular solar reflectance are today the main parameters used in solar mirror characterization. Unfortunately they can not properly describe the mirror effectiveness in redirecting the Sun radiation towards a receiver in CSP applications because the reflectance concept is developed by assuming the impinging radiation travelling as a plane wave, like that emitted by an infinitely far point light source. Besides that, the measurement of the near-specular reflectance is so much difficult that slowed down the progress of the SolarPACES reflectance guidelines, still stopped at the 2013 version. The present paper launches a new idea in the ongoing discussion on solar-mirror qualification to work around these difficulties: solar mirrors could be better characterised by the new parameter named Sun Conic Reflectance (SRC), which represents the amount of solar radiation reflected at a point of the mirror, and intercepted by the receiver with acceptance-angle 2φR. The standard measurement of SCR requires the setting of two conditions about the solar radiation: spectrum and divergence. The first is ASTM G173-03, for the second here was chosen that of a perfect clear-sky day, when the Sun-disk surface is uniformly radiating, and viewed from the Earth under 9.46 mrad, but any other could be chosen if it can experimentally implementable. For φ < 4.73 = 9.46/2 mrad, SCR and near-specular solar reflectance behave very differently, because the first represents the solar radiation intercepted by the receiver, while the second refers to the reflection of impinging plane-waves, with solar spectrum, towards directions deviating from the specular one of less than φ. When φ ≥ 4.73 mrad, the difference is much lower, and becomes null when φ is large enough. The second advantage of SCR is that it can be directly measured by means a simple experimental set-up which schema is shown and discussed in the paper. The formulae for computing SCR from near-specular reflectance are also reported. © 2016 Author(s).

Proposal of a new parameter for the comprehensive qualification of solar mirrors for CSP applications

Montecchi, M.
2016

Abstract

Hemispherical and near-specular solar reflectance are today the main parameters used in solar mirror characterization. Unfortunately they can not properly describe the mirror effectiveness in redirecting the Sun radiation towards a receiver in CSP applications because the reflectance concept is developed by assuming the impinging radiation travelling as a plane wave, like that emitted by an infinitely far point light source. Besides that, the measurement of the near-specular reflectance is so much difficult that slowed down the progress of the SolarPACES reflectance guidelines, still stopped at the 2013 version. The present paper launches a new idea in the ongoing discussion on solar-mirror qualification to work around these difficulties: solar mirrors could be better characterised by the new parameter named Sun Conic Reflectance (SRC), which represents the amount of solar radiation reflected at a point of the mirror, and intercepted by the receiver with acceptance-angle 2φR. The standard measurement of SCR requires the setting of two conditions about the solar radiation: spectrum and divergence. The first is ASTM G173-03, for the second here was chosen that of a perfect clear-sky day, when the Sun-disk surface is uniformly radiating, and viewed from the Earth under 9.46 mrad, but any other could be chosen if it can experimentally implementable. For φ < 4.73 = 9.46/2 mrad, SCR and near-specular solar reflectance behave very differently, because the first represents the solar radiation intercepted by the receiver, while the second refers to the reflection of impinging plane-waves, with solar spectrum, towards directions deviating from the specular one of less than φ. When φ ≥ 4.73 mrad, the difference is much lower, and becomes null when φ is large enough. The second advantage of SCR is that it can be directly measured by means a simple experimental set-up which schema is shown and discussed in the paper. The formulae for computing SCR from near-specular reflectance are also reported. © 2016 Author(s).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/20.500.12079/6126
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