This could be "a thermostat for the sun."
Back in 2011 at Greenbuild, I was so impressed with electrochromic glass that I gave it my Best of Show prize. At Greenbuild 2019 there is a new glass that changes tint on demand called Halio. It's faster than any other window, going from 99.5 percent black to clear in three minutes.
They developed the product for commercial uses, but I keep looking at this stuff and wondering why it isn't used in Passive House designs, where you want solar gain when it's cold, but want to block it when it's hot.
I noted 8 years ago that in Passive House design,
...solar heat gain is a major part of the energy equation and calculation; the sun is pretty much the furnace that heats the house. It is an unruly furnace; it is hard to turn off in the summertime and hard to control any time. On the south side of houses, tools for controlling sun range from louvres and brise soleil to grape vines. On the east and west sides, where the sun does not vary in altitude with the seasons as much, it is almost impossible to control.
I called electrochromic glass "a game-changer. It's not just about using more glass, but may be about completely rethinking the way we use glass in buildings, now that the solar furnace is almost as controllable as the gas or electric one."
I noted at the time that "I left the booth in a bit of a daze, thinking about its possibilities as a thermostat for the sun." Perhaps I get too excited. But it seems like such a good idea.