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.

Indoor testing of solar thermal absorbers requires an artificial light source with controlled emitted power and uniform illumination over the targeted sample. There are several devices described in literature which adopt different types of light sources and geometrical configurations, but they do not attain high uniformity, high power density, illumination of a large area and compactness simultaneously. The proposed solar simulator is composed of four identical high-power white LED arrays of 30 mm diameter each, placed on a supporting and cooling structure having a minimum volume of 26 cm × 26 cm × 8 cm. A numerical model has been developed, based on a ray tracing software, in order to define the optimal architecture. It has then been experimentally validated with measurements of the power density map, carried out with a 1% uncertainty pyranometer. Data shows that the built system is very stable over time and provides an illumination uniformity higher than 98% on a surface of 11 cm × 11 cm, which reduces to 95% on a surface of 15 cm × 15 cm. The power density can be adjusted within the 390–1360 W m⁻² range without affecting uniformity, and the system is easily scalable to illuminate larger surfaces. It represents a suitable solution for various laboratory purposes requiring stable, uniform and high intensity illumination.