We already know that solar cells can be embedded in just about anything from wallpaper to stained glass, but a new solar window prototype from the National Renewable Energy Laboratory (NREL) is making headlines for its ability to responsively harness the power of the sun.

The window’s genius is that it turns opaque and generates energy only to the degree of sunshine it gets. Otherwise, it looks like normal clear glass. Existing solar windows use embedded, static sensors that can only capture a fraction of the sunlight hitting the glass’ surface. But the prototype’s thermochromic glass means that it can use the entire surface to make power—but only when it’s sunny out.

It basically acts like one of those fancy glasses lenses that transform into sunglasses when you walk outside. The window responds to heat, transforming from clear glass to fully tinted, energy-generating glass. The heat drives out methylamine molecules from the glass, causing it to darken and let through just 3 percent of light. When the glass cools off, it reabsorbs the molecules and goes clear, returning to a state where 68 percent of light can shine through. The prototype so far has achieved a solar power conversion efficiency of 11.3 percent.

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