A comparative analysis of opto-thermal figures of merit for high temperature solar thermal absorber coatings

Solar thermal absorber coatings play a key role in the thermal efficiency of receivers for applications in the field of Concentrated Solar Power (CSP). The development of stable spectral selective coatings with a high solar absorptance αsol and a low thermal emittance εth is often desired to reduce thermal losses. However, quantitative indicators describing selectivity and the trade-off between solar absorptance and thermal emittance is seldom discussed in the literature. In this review, relevant opto-thermal figures of merit are analyzed for the comparison of reference solar thermal absorber coatings, including real and ideal coatings, both black and spectral selective. The comparison is made for a temperature ranging from 25 to 1000 °C and for a concentration factor ranging from 20 to 1000, based on spectral data measured at room temperature from 0.25 to 20 μm. New figures of merit are introduced, i.e. a normalized selectivity ratio Si*, a trade-off factor Ztrade-off, a normalized solar reflectance index SRI* and a peak efficiency temperature Tpeak,opt. These metrics are derived from existing figures of merit and adapted for CSP. The set of figures of merit analyzed in this review offer a complementary perspective for the detailed characterization of any coating opto-thermal performance. For solar thermal absorber coatings, thermal efficiency ηthermal and peak efficiency temperature Tpeak,opt are respectively deemed more insightful than opto-thermal efficiency ηopt-th and maximum steady-state temperature TSST,max, when comparing the relative opto-thermal performance of two coating formulations.