This document from 2007 looks into providing a solution to the aesthetic concerns regarding façade integration of solar thermal collectors. These architectural concerns are caused through the black and irregular surfaces of solar absorbers as well as the piping which remain visible through the front glass.
The solution to overcome this, as described in the document, is to hide the system behind non-transparent glass but without blocking the solar radiation. In particular the project looks into developing selective filters which reflect only a small part of the solar spectrum in the visible range while letting the rest of the radiation heat the absorber. The impact on the efficiency of the system is kept to a minimum i.e. less than 10%.
The project concludes that this new development will provide a solution to the growing demand of solar energy in buildings.
Source: Ecole Polytechnique Fédérale de Lausanne