Smart windows

perfection, brasilia april 2006 by seier+seier (cc) (from Flickr)
perfection, brasilia april 2006 by seier+seier (cc) (from Flickr)

Heard a story yesterday about Smart Windows on NPR .  They were talking about new smart glass technology which uses a nano-crystal film coating to window panes that can change heat transmission characteristics of the glass.

Apparently the nano-crystal film can electronically change their orientation to reflect or transmit heat. Thus their heat transmissivity could be changed by supplying a low-voltage current to the window coating.

Problems with todays windows

The problem with todays Low-E glass windows today is that they reflect heat year round. In summer that’s great, but if it’s winter or cold and there is abundant sunlight, this stinks.  With smart windows that can change their heat transmission, one can have the best of Low-E glass and dumb windows.

Integrating current smart windows could be problematic

The story went on to discuss that ideally the smart windows would somehow be tied into a building’s heating/cooling systems used to trigger the changes to the nano-crystal coating.  Seems like a good idea for a new building construction but not so good for current housing and commercial buildings due to the retrofit requirements.

Also, the fact that a building/house is heating mode doesn’t necessarily indicate that windows should transmit heat.  The only time they should really do that is when sunlight is hitting the window pain and it’s cold out.

Ideally a building with smart glass on all four sides would have windows to the east transmit heat on winter mornings but reflect heat the rest of the day, windows to the south transmit heat most of a winter daylight times, those to the west transmit during the afternoon, and windows to the north reflect heat all the time.  But any heat transmission would only if it was a sunny day.

Smart-er window design

For the current 130.6M houses and 4.7M commercial buildings already constructed in the USA it would be better if the smart window systems were isolated and separate from a buildings other systems and somehow more self-contained/passively-managed.

This could be done by including solar photo-voltaics tied to a thermocouple/thermoelectric device in the window that would trigger heat transmission only during sunlight and its cold outside. That way we could use the solar voltaics to power the transition to heat-transmitting as well.  The smart window would even be better if it somehow could be designed to require the solar power to keep it transmitting heat.  That way as the sunlight stops shining on the window, it starts reflecting heat.

TCO of smart-er windows

While any self-contained/passively managed smart-er window might cost more up front than a dumber smart window just connected to the buildings thermostat, it would probably be cheaper when all costs are accounted for.

  • With a non-self contained smart window, one needs an even smarter thermostat (driving a signal when in heating mode even though the furnace was not needed),  one has to run (low-voltage) wiring to plug into each and every smart window in a building and each window still requires some logic to transmit/transform the signal from the buildings thermostat to the window’s nano-crystal film.
  • With a self-contained smart window, one would need include additional control logic, a solar photovoltaic strip/cell and a thermocouple but it wouldn’t need to plug into any other building system.
Of course the nice thing about the self-contained, smarter window future changes to smart thermostats could be undertaken without impacting the smart windows (see my post on Smarter thermostats make for smarter grids).  And, the changes to current building codes to support all that additional wiring and plugs would not need to occur.

What do you think?