Comparison of the energy saving potential of adaptive and controllable smart windows: A state-of-the-art review and simulation studies of thermochromic, photochromic and electrochromic technologies

Windows play a huge role in today's buildings, allowing for outside view and providing occupants with daylight. However, windows are also often considered to be one of the weakest building components with respect to high thermal losses and are in addition often the reason for overheating and glare issues. In comparison to traditional static windows, dynamic solutions like adaptive and controllable smart windows have the ability to adjust their optical properties in response to changing boundary conditions and hence have the potential to improve the energy performance and the user comfort of buildings. The objective of this work is twofold: (1) To collect and present the state-of-the-art of commercially available smart windows from manufacturers, both adaptive and controllable products, i.e. thermochromic, photochromic and electrochromic smart windows. This collection provides the reader with valuable information about window properties such as the U-value, g-value, solar transmittance (Tsol) and visible solar transmittance (Tvis). However, it is currently difficult to obtain all the desired information about the products from the manufacturers' websites and other open channels. (2) To conduct building energy performance simulations on selected products from each technology. These products are also simulated using the same U-values as the reference window, and in addition, two theoretical cases have been simulated to investigate the theoretical potential of different smart windows. Here, the optical parameters take on fictitious values between 10 and 90% and between 0 and 100% transmittance, respectively. All cases are simulated at three different locations, i.e. Trondheim (Norway), Madrid (Spain) and Nairobi (Kenya), and are compared to a reference static window. In total, 63 cases are simulated using the simulation software package IDA Indoor Climate and Energy (IDA ICE). The results shows that the electrochromic window controlled by operative temperature has the highest potential in lowering the energy demand for all cases and locations. The study also highlights the importance of having the right control strategy and control levels for each specific case.