Windows are a basic necessity in a building to let in light and dissipate heat, but you don’t always want to have both functions at the same time. Now, engineers at North Carolina State University (NCSU) have developed a new material that allows windows to easily switch between three modes.
This new dynamic window can switch between a normal clear mode (which lets in light and heat), a mode that blocks heat but remains transparent, and a tinted mode (blocks some light but not heat). In this way, users can enjoy the sun protection effect all year round.
The key to it all lies in a small material called tungsten oxide, which is often found in dynamic glass windows based on the electrochromic principle. Normally, tungsten oxide is transparent and when you apply an electrical signal it darkens and blocks light, making it very convenient for tinting on demand windows.
But in new research, UNC researchers have discovered a whole new hidden skill: When water is added, it turns into hydrated tungsten oxide, which has additional settings when used in electrochromic windows.
When closed, it remains transparent to light and heat, perfect for those monotonous winter days when people need as much light and heat as possible. When some electrons and lithium ions are injected into the material, it first goes through a phase where it blocks infrared light (which senses heat) while remaining transparent to visible light wavelengths. Finally, as more electrons enter the material, it transitions to a dark phase in which it blocks both visible and infrared light, making it perfect for summer use.
Exactly why tungsten oxide hydrate works the way it does is still uncertain, but scientists at the University of North Carolina have a hypothesis.
"The presence of water in the crystal structure makes the structure less dense, so it is less susceptible to deformation when lithium ions and electrons are injected into the material," said Jenelle Fortunato, first author of the study. "Our hypothesis is that because tungsten oxide hydrate can accommodate more lithium ions than ordinary tungsten oxide before deformation, two modes are created. One is a 'cold' mode, where the injection of lithium ions and electrons affects the optical properties but the structure has not yet changed, and this mode absorbs infrared light. Then, after the structure has changed, a 'dark' mode emerges that blocks both visible and infrared light."
While there is no shortage of dynamic window designers on the market, so many modes are not usually available in one system. Even if it does, it usually requires a bulkier device. In this case, because only one material is required, the glass thickness and energy requirements are about the same as regular tungsten oxide windows.
Delia Milliron, co-corresponding author of the study, said: "The discovery of dual-band (infrared and visible) light control technology in a single material already well known to the smart window community may accelerate the development of commercial products with enhanced functionality."
The research was published in the journal ACS Photonics.