Approaching the Theoretical Maximum Performance of Highly Transparent Thermochromic Windows

Abstract

Thermochromic window coatings represent a promising technology to improve the energy efficiency of buildings in intermediate climates. With the technology approaching market introduction it is important to investigate its performance limits within smart windows and to identify existing development challenges. Here we analyze the theoretical maximum performance of thermochromic window coatings that modulate IR transmission whilst retaining high visible transparency. The set limitations lead to a theoretical maximum solar modulation of 39.1%. Within an insulated glazing unit (IGU), where at least 2 glass panes and a conventional low-e coating are required, this value is further reduced to 12.9%. We show that by carefully selecting a low-e coating with the highest compatibility to a thermochromic coated glass and by allowing 10% of modulation in the visible spectral range, the theoretical maximum can be increased to 23.1%, illustrating the importance to codesign and match both coatings within a smart window to reach optimum performance. Furthermore, we compared our current best-performing VO2:SiO2 composite coating within an IGU to the theoretical maximum. The analysis shows that with a solar modulation of 13.4%, the coating is currently at 59% of the theoretical maximum. Finally, we propose and discuss several strategies to proceed further toward the theoretical maximum.