Full-year monitoring of the real-life performance of a thermochromic smart window in a tailored test building

Thermochromic smart windows switch between transmitting and blocking solar infrared radiation (IR) based on temperature. These properties enable passive modulation of solar heat gain, which significantly lowers energy consumption in buildings and increases comfort levels for occupants via a balanced indoor climate. In this study, we set up a tailored test building in Eindhoven (The Netherlands) to monitor the real-life performance of a thermochromic smart window. We produced full sized double-glazed windows comprising a laminated outboard made from thermochromic interlayers, installed them in the test building and monitored their light transmission, solar heat gain and glass surface temperature over a full year. This enabled us to study the thermochromic phase transition of a smart window under real-life conditions for the first time. We demonstrated that the temperature of the smart window's outer glass pane regulated the phase transition and that the overall performance matched well with lab data. A glass surface temperature of 30 °C, influenced by the outdoor temperature and solar irradiance (photothermal heating), led to phase transition to the IR blocking state. During summer the smart window transitioned to its IR blocking state around 13:00, whereas in winter glass surface temperatures remained below 30 °C, retaining an IR transparent state of the smart window. In spring and autumn photothermal heating on cold, but sunny days led to undesired transitions to the IR blocking state, where glass surface temperatures exceeded outdoor temperatures by up to 20 °C. We studied the effect of photothermal heating in detail to conclude that optimized behavior of the smart window can be achieved with a phase transition temperature around 35–40 °C.