Windows are pretty basic necessities for letting in light and heat, but you don’t always want both at once. Now engineers at North Carolina State University (NCSU) have developed a new material that allows windows to easily switch between three modes.

The new dynamic windows would be able to switch between a normal transparent mode that lets in light and heat, a mode that blocks heat but remains transparent to light, and a tinted mode that blocks some light but not heat. That should keep users covered in all seasons.

The key to it all is a little material called tungsten oxide, which often shows up in dynamic windows that work on the principle of electrochromism. Normally transparent, tungsten oxide becomes darker and blocks light when you apply an electrical signal, making it handy for windows that tint on demand.

But in the new study, the NCSU researchers discovered a brand new trick hiding up its sleeve. Adding water turns it into tungsten oxide hydrate, and when this is used in electrochromic windows it gives it an extra setting.

When switched off, it remains transparent to light and heat – ideal for those drab winter days when you need as much of both as possible. When some electrons and lithium ions are injected into the material, it first goes through a phase where it blocks infrared light (felt as heat) while remaining transparent to visible wavelengths of light. And finally, as more electrons pass into the material it transitions into a dark phase where it blocks both visible and infrared light, perfect for summer.

Exactly why tungsten oxide hydrate works like this remains uncertain, but the NCSU scientists have a hypothesis.

“The presence of water in the crystalline structure makes the structure less dense, so the structure is more resistant to deformation when lithium ions and electrons are injected into the material,” says Jenelle Fortunato, first author of the study. “Our hypothesis is that, because the tungsten oxide hydrate can accommodate more lithium ions than regular tungsten oxide before deforming, you get two modes. There’s a ‘cool’ mode – when injection of lithium ions and electrons affects the optical properties, but structural change hasn’t occurred yet – which absorbs infrared light. And then, after the structural change occurs, there’s a ‘dark’ mode that blocks both visible and infrared light.”

While there’s no shortage of dynamic windows in development, you don’t often get this many modes in one system. When you do, they usually require bulkier setups. In this case, since only one material is required, it should keep the thickness of the glass and energy requirements to more or less the same as regular old tungsten oxide windows.

“The discovery of dual-band (infrared and visible) light control in a single material that’s already well-known to the smart windows community may accelerate development of commercial products with enhanced features,” said Delia Milliron, co-corresponding author of the study.

The research was published in the journal ACS Photonics.