Rugate filters for evacuated thermal collectors: IR mirror for improvement of energy conversion efficiency

Solar thermal absorbers lose an increasing amount of heat at higher operating temperatures, limiting their efficiencies. An infrared mirror characterized by a high transmittance in the visible region of the spectrum while having high reflectance in the mid-IR could be crucial for a partial recovery of heat radiated by absorbers operating at high temperatures. Thanks to the cold-side external photon recycling mechanism, an IR mirror applied to an evacuated collector could lead to increase in efficiency of up to 60%. Here, we demonstrate the feasibility of such a mirror, which in principle is developed for high-vacuum flat solar thermal collectors, but is easily adaptable to other applications. The mirror presented has a very simple design, based on a discrete rugate filter scheme, and does not require multi-cathode deposition tools, which further simplifies the manufacturing process.