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.
Highlights
Solar energy plays a key role in the energy transition from fossil fuels to renewable energy [1]: several works showed that the adoption of energy-efficient and clean energy resources is crucial in reducing greenhouse gas emissions and pollution [2,3]
Our results show that a reduction in thermal emittance is essential to reach high operating and/or high stagnation temperatures in solar thermal unconcentrated applications
The stagnation temperature can be increased to about 400 ◦C without concentration
Summary
Solar energy plays a key role in the energy transition from fossil fuels to renewable energy [1]: several works showed that the adoption of energy-efficient and clean energy resources is crucial in reducing greenhouse gas emissions and pollution [2,3]. More than one fourth of the energy resources in the developed countries is nowadays used for heating and cooling [4,5], and industry represents a promising area of application [6]. Such a large fraction of energy can be provided by solar thermal collectors converting solar energy directly into heat with high efficiency. The core component of a solar thermal collector is the Selective Solar Absorber (SSA). An ideal SSA should perfectly absorb solar radiation (solar absorptance α = 1), whereas its thermal emission should be minimal (thermal emittance ε = 0). The Kirchhoff’s law of thermal radiation (Equation (1)) states that, for a given surface, in thermodynamic equilibrium, absorptivity and emissivity must be equal [7]
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