Electrochromic Glass

Electrochromic glass changes tint when driven by an electrical signal, allowing control over heat and glare, thus facilitating Net Zero.

In a Nutshell

This article introduces electrochromic smart glass which can vary its opacity under application of a direct voltage. For a high-level primer on smart glass in general, please check out our article on the basics of smart glass.

Electrochromic glass changes its transmittance (i.e. how much light it passes) only when triggered by an electrical signal. This reversible change alters the state of the glass between transparent and opaque (or any state in between).

Below we see the general arrangement of an electrochromic cell:-

Cross-section of an electrochromic panel changing from transparent to opaque.

Electrochromic glass layers; Image attribution: Keyur.tithal, CC BY-SA 3.0, via Wikimedia Commons

Like a Battery, Sort of…

As you can see from the diagram, an electrochromic glass panel comprises a stack of layers.

This stack is normally a few microns thick (thousandths of a millimetre) and is created using physical vapour deposition, the same as in the semiconductor industry.

  • On the outer glass panels there are transparent conductive layers, normally of indium tin oxide (ITO). This converts the whole structure into something similar to a battery, with the ITO as the electrodes.
  • In the centre we find the ion storage layer, the electrolyte and the electrochromic layer.

When you apply a direct (DC) voltage to the structure, charged particles (normally lithium ions) migrate from the ion storage layer, through the electrolyte to the electrochromic layer (often tungsten oxide, which is transparent in its inactive state).

This causes the electrochromic layer to undergo electrochemical reduction-oxidation (i.e. redox) which results in absorption of light and this causes its colouration.

When you reverse the voltage, the lithium ions migrate back from the electrochromic (EC) layer, through the electrolyte, and back to the ion storage layer, returning the glass to its transparent state.

This change of state can take place in the order of minutes.

If you are looking for a deep-dive into the science, check out our article on What Is Electrochromism.

Why does Electrochromic Glass change Transmittance?

When we apply a voltage across the electrochromic stack, lithium ions ‘intercalate’ (i.e. insert themselves into) the electrochromic layer.

The inserted lithium ions reduce the ‘band gap’ of tungsten oxide to roughly 2 electron-Volts (eVolts). This means that incident photons with at least that energy will energise the electrons into a higher energy state.

Since visible light photons have this much energy, the electrochromic layer absorbs them, and the resulting light then lacks those wavelengths, i.e. it lacks visible light and thus appears tinted.

Electrochromic Glass Architecture

There are various architectures to electrochromic devices:

Hybrid Electrochromic

Hybrid electrochromic glass can retain its state for up to 4-5 days, thus exhibiting a memory function [3] and uses an inorganic electrochromic layer with an organic (polymer) electrolyte. 

Charge leakage limits retention though, returning the electrochromic glass eventually to its transparent state. 

The memory capability requires power only when it changes state. SPD and PDLC smart glass technologies, on the other hand, must be powered continuously as long they are maintained in their transparent states.

Solid State Electrochromic

Solid-State electrochromic glass does not have any memory capability unfortunately. However, it is resilient in cyclic tests under extreme temperature conditions and UV exposure, with a 20-30 year lifespan, ideal for building facades.

Why is Low Voltage Important?

If a product operates below 60Vdc it is considered a ‘Safety Extra-Low Voltage’ (SELV) device by the IET, as defined in European standard EN 60335. This is important for electrochromic glass installed in a building facade or a vehicle requiring the routing of cables.

The lower the voltage (and current), the lower the risk to safety and the cheaper the cables.

For a large smart glass installation you can imagine how lower-voltage technologies can reduce installation costs and operational maintenance costs.

Electrochromic Glass Applications

  • Architects and engineers who specify electrochromic glass should bear in mind that wiring regulations may differ in each country.
  • Vehicle OEMs must bear in mind the cost and weight of cabling. Also important is the voltage transformation needed from 48Vdc (aircraft) or 24Vdc (automotive) to the operating voltage of the glass (typically 3Vdc).
  • Consumer appliance OEMs must bear in mind the carbon footprint when mass manufacturing devices based on electrochromic glass.

Another factor: the extra-low voltage makes it feasible to directly power the electrochromic smart glass from photovoltaic (PV) solar panels on the building facade (or vehicle), facilitating an eco-friendly solution in every respect.

It’s The Environment, Stupid

One obvious benefit is a substantial reduction in air conditioning costs, thanks to the infrared (i.e. solar heat) rejection by electrochromic smart glass when installed in building facades (and vehicles).

The fact that the behaviour can be dynamically controlled allows owners, managers and users to tune the electrochromic smart glass facade to their needs, which can change over the course of a day and of course seasonally.

Switching Time

It is worth repeating that electrochromic smart glass takes in the order of minutes to change state, whereas SPD and PDLC smart glass technologies take seconds.

In architectural applications, a gradual change in tint can be an advantage since it allows our eyes to adjust. Also, buildings can benefit from a gradual change in the facade, avoiding step changes which may distract people, traffic or wildlife.

For transportation, this slow change could be a disadvantage or even a safety issue, since visibility depends on being able to see out of the vehicle. However, this factor is offset by the capability to control glare and heat at a low voltage and with low-voltage cabling.

Is Electrochromic Glass What I Need?

Well, this will depend on the cost vs benefits, both in the construction of the building, vehicle or consumer device, as well as the operational costs of running them.

Electrochromic smart glass needs to reduce substantially in cost in order to gain mass market approval.

The US Dynamic Glass Act, thanks to its 30% tax credits, will help of course, but this is just a temporary measure (until 1 Jan 2025).

More important is the low energy consumption and safe voltage, which could tip the scale for many applications.

Electrochromic Glass Manufacturers

Please note that a Google search for ‘electrochromic glass’ turns up many manufacturers who are not making electrochromic smart glass.

Here is a partial list of electrochromic glass manufacturers, enumerated in alphabetical order:-

Smartglass World – Marketplace

If you are looking for more electrochromic smart glass manufacturers, distributors or installers, look no further than our parameterised Search.

The screenshot shows that we have many companies listed already. You can filter further if you specify product attributes such as transmittance, electrical power or haze.

Smartglass World Marketplace search for electrochromic manufacturers, distributors and installers

Smartglass World Marketplace search for electrochromic manufacturers, distributors and installers

If you press the ‘Show Results’ button, this will list the specified companies. You can contact them by posting a request on our Marketplace.

References

[1] “Smart Switchable Glazings for the New Millennium” – Carl M. Lampert, Star Science

[2] “Colour and the Optical Properties of Materials”, Richard Tilley

[3] https://www.commercialwindows.org/electrochromic.php

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