A Selective Solar Absorber for Unconcentrated Solar Thermal Panels

Abstract

A new Selective Solar Absorber, designed to improve the Sun-to-thermal conversion efficiency at mid temperatures in high vacuum flat thermal collectors, is presented. Efficiency has been evaluated by using analytical formulas and a numerical thermal model. Both results have been experimentally validated using a commercial absorber in a custom experimental set-up. The optimization procedure aimed at obtaining Selective Solar Absorber is presented and discussed in the case of a metal dielectric multilayer based on Cr2O3 and Ti. The importance of adopting a real spectral emissivity curve to estimate high thermal efficiency at high temperatures in a selective solar absorber is outlined. Optimized absorber multilayers can be 10% more efficient than the commercial alternative at 250 °C operating temperatures, reaching 400 °C stagnation temperature without Sun concentration confirming that high vacuum flat thermal collectors can give important contribution to the energy transition from fossil fuels to renewable energy for efficient heat production.

<strong></strong>Characterization and Thermal Aging Tests of Cr Based Multilayer for Unconcentrated Solar Thermal Applications

“…For standard ISO/CD 12592, 2 the is a combination of optical properties (eq.8), but the standard test was based on load profiles for absorbers used for DHW system, in which, due to the low operating temperature the weight of emittance is one half of absorptance. For mid-high temperature applications, the relative importance of the absorber thermal emittance respect to light absorptance is determined by the absorber efficiency that changes in a significant way with temperature [13,14], implying that the standard is not adequate to assess the performances of SSA for application different than DHW. As pointed out by Cao [10], the relative weight depends by the operating temperature and this would result in a temperature dependent .…”

“…The total thickness of the multilayer is about 150 nm, and the single layer thicknesses were obtained to maximize the absorber efficiency at 1000 W/m 2 and 573 K operating temperature as reported in [18]. The characterization and the ageing test were performed in the MTB system [13,14]. It consists of a stainless-steel vessel closed on its upper side by an extra-clear glass where the absorber is suspended by supports of negligible thermal conductivity.…”

“…In this work, the absorptance ( ) and spectral averaged emissivity ( ̅ ( )) of a recently developed SSA [13] at its working temperature were estimated, by means of the Mini Test Box (MTB) system [13,14]. It is an home-made calorimetric emissometer which reproduces a flat plate collector under high vacuum conditions and equipped with a calibrated LED array system [15] as artificial light source.…”

Effect of Electrodeposition Time on Absorptance, Roughness and Thermal Stability of Black Chromium Absorbing Surfaces

“…The direct conversion of solar energy into heat (solar thermal) can be achieved through the use of solar collectors. A large fraction of energy resources is nowadays used for heating and this energy demand can be supplied by solar thermal collectors 4 .…”

Enhancing the solar-to-thermal energy conversion in high vacuum flat plate solar collectors

“…Among all the possible designs investigated [6] (cermets, nano and metamaterials, photonic crystals, etc.) selective absorbers based on multilayer allow controlling the thermal emission while ensuring excellent thermal stability and high solar absorption [7]. Indeed, the low emissivity substrate is responsible for the low emittance and the absorption of the solar radiation is guaranteed from the multiple reflections at the interfaces between the layers composing the multilayer.…”