Rugate filter window coating for improvement of vacuum solar thermal absorber efficiencies

Solar thermal absorbers experience heat losses at high temperatures, thus limiting their efficiencies. In this work we present an InfraRed (IR) window coating capable of recycling thermal radiation back to the absorber to reduce losses and improve efficiencies. This mechanism is what we call cold-side external photon recycling. Several designs have been investigated in a preliminary study, ranging from a simple alternation of two different optical materials, as in the case of a Distributed Bragg Reflector (DBR) design, to more complex structures like the rugate filters. The ideal design of a rugate filter makes use of a huge number of different optical materials and layers, useful to reach a continuous variation of the refractive index profile which guarantee a minimization of the undesired reflections. Here, we show that some of these advantages can still be obtained by using a limited number of materials coupled with a proper design, and we extend prior theoretical work to include a realistic set of materials for ease of fabrication and validation. The resulting IR mirror, based on a rugate-multilayer structure, presents favorable properties: the high transmissivity in the visible region of the spectrum allows the Solar radiation to reach the absorber, without altering its performance greatly; instead, a reflection window width of almost 1 μm in the mid-IR region allows for partial recovery of heat radiated by the absorber that otherwise would be lost, and the reduced number of materials can simplify fabrication. Finally, the design has been developed for the particular case of a high-vacuum flat solar thermal panel, but it could be applied to any solar thermal device with an emissive surface without altering the absorber performance greatly.