"Smart" windows (electrochromic glass)
by Chris Woodford. Last updated: April 23, 2021.
Now you see it, now you don't! Do you ever have one of those days
when the Sun doesn't know whether it's coming or going, prompting you
to keep opening and closing your blinds so you
can read the words on your computer screen or stop your
furniture from fading? It won't be long before we consign that
particular problem to history, thanks to the arrival of
electrochromic glass ("smart" glass), which changes from light to
dark (clear to opaque) and back again, at the push of a button. It's
relatively simple, wonderfully convenient (no more faded
upholstery!) and has huge
environmental benefits. How exactly does
it work? Let's take a closer look!
Photo: Forget the curtains, forget the blinds! "Smart windows" made from electrochromic glass turn from clear to opaque and back again at the flick of a switch. Some are made of special glass; some are plastic films added on top
of ordinary glass.
What is electrochromic glass?
Glass is an amazing material and our buildings would be dark,
dingy, cold, and damp without it. But it has its drawbacks too. It
lets in light and heat even when you don't want it
to. On a blinding summer's day, the more heat ("solar gain") that
enters your building the more you'll need to use your
air-conditioning—a horrible waste of
energy that costs you money and
harms the environment. That's why most of the windows in homes and
offices are fitted with curtains or blinds. If you're into
interior design and remodeling, you might think furnishings like this are
neat and attractive—but in cold, practical, scientific terms they're
a nuisance. Let's be honest about this: curtains and blinds are a
technological kludge to make up for glass's big, built-in drawback:
it's transparent (or translucent) even when you don't want it to be.
Since the early 20th century, people have got used to the idea of
buildings that are increasingly automated. We have electric
clothes washing machines,
vacuum cleaners and much more. So why
not fit our homes with electric windows that can change from clear to
dark automatically? Smart windows (also referred to by the names
smart glass, switchable windows, and dynamic windows) do exactly that using a scientific idea called
electrochromism, in which materials change color (or switch from
transparent to opaque) when you apply an electrical voltage across
them. Typically smart windows start off a blueish color and gradually
(over a few minutes) turn transparent when the electric current passes through them.
Photo: Electrochromic glass changes color under electric control: Left: Here it's transparent and looks much like ordinary glass; Right: Apply a small voltage and it turns opaque (blueish and dark). Photos by Warren Gretz courtesy of
US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
How does electrochromic glass work?
There are quite a few different types of electrochromic glass: some merely darken (like
photochromic sunglasses, which turn darker in sunlight),
some darken and become translucent, while others become mirror-like and opaque. Each type is powered by a different technology and I'm going to describe only one of them in detail here:
the original technology, discovered by Dr Satyen K. Deb in 1969,
and based on the movement of lithium ions in transition metal oxides (such as tungsten oxide).
(Lithium, as you'll probably know, is best known as the chemical element inside rechargeable lithium-ion batteries.)
Ordinary windows are made from a single vertical pane of glass and
double-glazed windows have two glass panes separated by an air gap
to improve heat insulation
(to keep the heat and noise on one side or the other). More sophisticated windows (using
low-e heat-reflective glass) are coated with a thin layer of metallic chemicals so they keep your home warm in winter and cool in summer.
Electrochromic windows work a little bit like this, only the
metal-oxide coatings they use are much more sophisticated and
deposited by processes similar to those used in the manufacture of
integrated circuits (silicon computer chips).
Although we often talk about "electrochromic glass," a window like this can be made of either glass or plastic (technically called the "substrate," or base material) coated with multiple thin layers by a process known as sputtering (a precise way of adding thin films of one material onto another).
On its inside surface (facing into your home), the
window has a double-sandwich of five ultra-thin layers: a separator
in the middle, two electrodes (thin electrical contacts) on either side of the separator, and
then two transparent electrical contact layers on either side of the
electrodes. The basic working principle involves lithium
ions (positively charged lithium atoms—with missing electrons) that migrate back and forth between the two electrodes through the separator. Normally, when the window is clear, the lithium ions
reside in the innermost electrode (that's on the left in the diagram you
can see here), which is made of something like lithium cobalt oxide (LiCoO2). When a small voltage is applied to
the electrodes, the ions migrate through the separator to the
outermost electrode (the one on the right in this diagram).
When they "soak" into that layer (which is made of something like polycrystalline
tungsten oxide, WO3), they make it reflect light, effectively turning it opaque. They remain there all by themselves until the voltage is reversed, causing them to move
back so the window turns transparent once again. No power is needed to
maintain electrochromic windows in their clear or dark state—only to change
them from one state to the other.
Animation: How an electrochromic window works: Apply a voltage to the outer contacts (conductors) and lithium ions (shown here as blue circles) move from the innermost electrode to the outermost one (from left to right in this diagram). The window reflects more light and transmits less, causing it to appear opaque (dark). The layers are very thin coatings added to a weighty piece of glass or plastic known as the substrate (not shown here, for clarity).
So much for lithium-ion, what other technologies are available? Here are a few of them:
- Instead of placing a separator between the electrode layers, we can have an electrochromic material (a dye) that changes color when a current passes through it. That's similar to what happens in photochromic sunglasses, but
under precise electrical control. Chemical dyes that work electrochromically
which change reversibly between clear and blue or green.
- We can use nanocrystals (an example of nanotechnology,
which works on the atomic scale, roughly 1000 times smaller than what we call microscopic)
in a broadly analogous way to let more or less light pass through a smart window.
Different types of electrochromic windows have different configurations, but most have several different layers. In one popular design, sold under the brand name of Halio, there are six separate surfaces. The electrochromic layer is sandwiched between two layers of PVB (polyvinyl butyral) polymer, with heat-toughened glass on either side of that. Then there's an argon insulating layer, a
low-e coating, and, finally, a layer of interior glass. Electrochromic units can also be customized in various ways, with thicker outer layers for security or weatherproofing, different low-e coatings, more or less insulation, and so on. Some can be controlled automatically by smartphone apps or wired to roof-top
pyranometers (sun sensors) so your windows darken automatically when
the sunlight is strong enough.
Stick-on electrochromic films
The smart windows we've looked at so far are generally installed as self-contained units: you
fit an entire window with its specially coated glass at great expense.
You can also get smart-window technology in a slightly cheaper form: manufacturers
such as Sonte and Smart Tint® make thin, self-adhesive and stick-on electrochromic film you can apply to your existing windows and
switch on and off with simple smartphone apps.
Electrochromic films use technology similar to an LCD display, which
uses liquid crystals, under precise electronic control, to change how much light can get through.
When the current is switched on, the crystals line up like opening blinds, allowing light to stream
straight through; switched off, the crystals orient themselves randomly, scattering any light passing through
in random directions, so making the windows turn opaque.
The performance is impressive. According to Smart Tint,
its films transmit about 98 percent of light when they're clear and switch in less than a second to
their opaque state, when the light they let through drops to about a third;
they've been tested to switch back and forth over 3 million times.
Animation: How an electrochromic film works: The film contains liquid crystals (blue). When the current is switched off, the crystals point in random directions and scatter incoming light, turning the film opaque. When the current switches on, the crystals
align like opening blinds, letting virtually all the light through.
What's good and bad about electrochromic windows?
Smart windows might sound like a gimmick, but they have a huge
environmental benefit. In their darkened state, they reflect back
virtually all (about 98 percent of) the light falling on them, so
they can dramatically reduce the need for air-conditioning (both the
huge cost of installing it and the day-to-day cost of running it).
(View Glass, one manufacturer, estimates electrochromic glass can cut
peak energy use for cooling and lighting by around 20 percent.)
Since they're electrically operated, they can easily be controlled by a smart-home system
or a sunlight sensor, whether there are people inside the building or not.
According to scientists at the US Department of Energy's National
Renewable Energy Laboratory (NREL), windows like this could save up
to 8 percent of a building's total energy consumption; they use only tiny
amounts of electricity to switch from dark to light (100 windows use
about as much energy as a single
incandescent lamp) so make a
huge net energy saving overall. Other benefits of smart windows include privacy at the
flick of a switch (no more fumbling around with clumsy, dusty
curtains and blinds), convenience (automatically darkening windows
can save your upholstery and pictures from fading), and improved
security (electrically operated curtains are notoriously unreliable).
Photo: Hot stuff! This thermal (infrared) image
shows how hot a car gets when you park it in direct sunlight: colors indicate temperatures with red and yellow hottest and blue coldest. Electrochromic glass fitted to a car might help to solve this problem. You'd simply flick a switch to darken the
windows when you parked and the vehicle would be nice and cool when you came back! Photo courtesy
of US Department of Energy.
It's a given that glass printed with electrodes and fancy metal
coatings is going to be several times more expensive to install than
ordinary glass: a single large smart window typically comes in at
around $500–1000 dollars (about $500–1000 per square meter or $50–100) per square foot).
There are also questions about how durable the materials are, with current
windows degrading in performance after only 10–20 years (a much
shorter life than most homeowners would expect from traditional
glazing). Another drawback of current windows is the time they take
to change from clear to opaque and back again. Some technologies can take several minutes
(Halio quotes three minutes for its glass to fully darken from clear),
though stick-on electrochromic films are much faster, changing from clear to opaque and back in less
than a second.
How will smart windows improve in future?
Another possibility might be to combine electrochromic windows
and solar cells so that instead of uselessly reflecting away
sunlight, darkened smart windows could soak up that energy and store it
for later. It's easy to imagine windows that capture some of the
solar energy falling on them during the day and store it in batteries
that can power lights inside your home at night, though, of course, a
window can't be 100 percent transparent and working as a 100 percent efficient
solar panel at the same time. The incoming energy is either transmitted through the
glass or absorbed and stored, but not both. A window that doubled as a solar
cell would likely involve compromise from both sides: it'd be a relatively dark
window even when clear and much less efficient at capturing energy than a
really good solar cell.
One thing we can be sure of is seeing much more of electrochromic technology in future!
Do moving lithium ions sound a little bit familiar?
If you know a little bit about technology, the idea of an electronic sandwich that works by shuttling
lithium ions back and forth between layers might just ring a bell: it's exactly the same principle
we use in rechargeable lithium-ion batteries (the ones in laptops,
cellphones, and most electric cars)!
Photo: A lithium-ion battery works in a very similar way to an electrochromic window.
In a battery, we use an electric current to move the lithium ions from one layer to another, so
storing up energy; when the ions move back again, they release the stored energy, usually over a period of
several hours, powering your laptop, cellphone, or other portable device.
When it comes to batteries, we're looking to store as much energy as possible, for as long as possible,
which means a lot of lithium ions and a very chunky device.
On the other hand, when we're interested in making electrochromic windows, we're much more interested in optics.
Which layer the lithium ions are in determines how much light passes through but, either way, the layers
need to be extremely thin or the device won't function as a window at all.
Relatively few ions move in electrochromic windows, compared to lithium-ion batteries: windows need
to dark or lighten in seconds or minutes, not the three to four hours that a laptop battery takes to charge!
The very strong similarity between lithium-ion batteries and electrochromic windows is no coincidence;
if you check out Floyd Arntz et al's 1992 patent
Methods for Manufacturing Solid State Ionic Devices, the very first sentence
gives the game away, noting that their invention is a "device usable as an electrochromic window and/or as a rechargeable battery."
According to these authors, broadly the same manufacturing methods can be used in both cases.