What is Thermochromic Smart Glass?

Introduction

This article introduces thermochromic smart glass which tints (and/or reflects infrared) when heated by the sun. In contrast, photochromic glass darkens under ultraviolet or visible light.

Thermochromic and photochromic glass operate reversibly, and we consider both to be passive technologies, since they do not require electrical power. Consequently, this reduces cabling costs and electrical maintenance costs.

If you are looking for a high-level introduction to the subject, check out our article on the basics of smart glass.

Thermochromic glass, available on our Samples page

Benefits of Thermochromic Glass

• Can be installed just as normal glass into a building facade
• Internal PVB lamination improves safety and rejects UV
• Changes between tinted and clear states within minutes of being triggered
• Maintains a view to the outside at all times
• Reduces glare, discomfort and loss of productivity
• There are no wires to route, since it is not electrically-powered
• Reduces air-conditioning usage thanks to rejection of infrared (solar heat)

Thermochromic Windows

Thermochromic materials include silicate, borosilicate and phosphosilicate glasses as well as transition metal oxides, with the most common being vanadium dioxide.

The vanadium dioxide is either deposited as a thin film on a single glass lite, or laminated within a PVB (polyvinyl butyl) film and sandwiched into an insulated single, double or triple-glazed Insulated Glass Unit (IGU).

The dynamic tint results directly from radiant solar energy but could also be due to indirect thermal exchange, such as conduction or convection from adjacent building materials. Examples include bricks, spacers or metallic frames.

Let’s take a look at what a thermochromic double-glazed unit looks like:

Double-glazed insulating window schematic, Image Attribution: NcLean, CC BY-SA 3.0, via Wikimedia Commons

A double-glazed IGU comprises: 

1. outside facing glass
2. inner face of exterior glass (with thermochromic layer)
3. outer surface of interior glass (with low-e infrared rejection layer)
4. inner surface of interior glass
5. window frame
6. spacer for insulation
7. seals
8. internal reveal
9. exterior windowsill

The front piece of glass is needed as a wind barrier. The back piece of low-E coated glass reflects any heat retained by the thermochromic layer from re-radiating to the interior of the building.

Vanadium dioxide undergoes changes with temperature:-

• acting as a semiconductor at room temperature with notable transmittance in the infrared,
• as an absorbing dielectric at mid-range temperatures, and finally
• above 68ºC as a conductor which reflects infrared.

Doping vanadium dioxide with tungsten lowers the transition temperature to 29°C.

Thermochromism

The International Union of Pure and Applied Chemistry (IUPAC) defines thermochromism as follows:

“thermally-induced transformation of a molecular structure, which is reversible and produces a spectral change typically of visible colour (but not necessarily)”.

As we saw in our article on photochromic smart glass, the implications of this definition are two-fold:

(i) the stimulus could theoretically be any source of thermal energy (not just solar), so conduction and convection from nearby materials could also activate the thermochromic glass into a dark state;

(ii) the resulting ‘spectral change’ need not manifest itself in the visible light range but could be any change in optical properties.

Let’s unpack the first item by looking at how heat is transferred across a window:-

Thermal Transmittance and U-Value

The thermal transmittance of a building facade is measured by a variable called the U-Value and is defined as such: 

‘the rate of heat transfer through a structure divided by the difference in temperature across the structure and by the area of the structure’.

The calculations to define thermal transmittance for windows are standardised in ISO 10077 and ISO 15099. 

Typical U-Values are given below:-

• Single glazed windows: 4.5 W/m²·K
• Double glazed windows: 3.3 W/m²·K
• Triple glazed windows: 1.8 W/m²·K

The U-value includes heat flow due to conductive, convective and radiative heat transfer.

Thermal Transfer by Conduction

Conduction occurs in solids, liquids or gasses and requires physical contact between two or more materials having a temperature difference between them. 

The transfer of heat requires a transfer of kinetic energy through collisions between molecules, atoms and electrons and continues until the contacting bodies reach thermal equilibrium.

Examples of materials that conduct heat include metals and stone whereas wood, paper, air, and cloth are poor heat conductors.

Silver has the highest coefficient of heat conduction at 100, whereas other materials are ranked in relation to silver:-

• Copper: 92
• Iron: 11
• Water: 0.12
• Wood: 0.03

In contrast, a perfect vacuum cannot conduct heat since there is no matter to transfer the thermal energy and so is classified as having a coefficient of heat conduction of 0. 

Like a vacuum, air is also an excellent insulator and is sometimes used in the gap between panels of glass. Air prevents conduction but cannot prevent convection, since the latter relies on thermal transfer through a fluid (liquid or gas).

Thermal Transfer by Convection

Thermal convection consists of a fluid (gas or liquid) expanding in volume as its temperature increases, and becoming lighter, then rising upwards, only to be replaced by a cooler denser fluid which is moving downwards. This cyclical movement forms a convection current.

Convection occurs near the interior and exterior surfaces of the IGU (insulated glazing unit), as well as within the cavity between the glazing layers, which is often filled with air or an inert gas like argon.

Convection near an interior glazing surface results from the cold glass chilling the adjacent air, and the resulting convection current can be felt as a draught, often mistakenly attributed to a leaky window. 

This begs the following questions:-

• Since anti-static and conductive coatings on glazing surfaces increase the electrical conductivity, and since electrical and thermal conductivity are closely related, would anti-static coatings also help to improve thermal conductivity?
• If so, could anti-static and conductive coatings reduce temperature differentials across glass, thereby reducing convection currents inside air cavities or near interior glazing surfaces?

Thermal Transfer by Radiation

Thermal radiation does not need any medium to transfer and can thus occur in gas, solids, liquids or even a vacuum. 

When an atom acquires energy, the motion of any electrically-charged particles such as protons and electrons results in charge-acceleration or dipole oscillation, which generates electromagnetic waves that emanate away from the object as infrared (thermal) radiation.

All objects which are above absolute zero emit some amount of thermal radiation. Our Sun is a good example of an object which transfers energy by radiation rather than conduction or convection.

Thermal radiation is the principal component responsible for activating thermochromic layers into energy states which absorb or reflect incident solar radiation.

Colour Rendering of Thermochromic Glass

In the case of thermochromic glass installed in medical facilities (like hospitals, clinics and dental surgeries), it is important to consider the colour rendering capacity of the glass facade.

Glass with a high colour rendering index (CRI>95) allows colours to be faithfully rendered, which permits red tissue to appear a true red.

Lower CRI would result in an inaccurate colour rendering, causing tissue to look off-colour. More on this topic in our article about colour rendering.

Thermochromic Glass and Net Zero

As pointed out in our article on Net Zero Buildings, built structures account for 30% of global energy consumption and 26% of global energy-related carbon emissions.

The ‘US Federal Sustainability Plan’ and the EU ‘Net-Zero Industry Act’ aim to foster climate neutrality through better energy efficiency, and by reducing global carbon emissions.

Thermochromic glass tints to a darker state when struck by sunlight, which reduces the heat gain into a building. This, in turn, reduces the air conditioning load, which has a direct impact on the building’s energy usage.

Indirectly, a lower air conditioning load reduces carbon emissions by reducing electricity drawn from (what are still primarily) carbon-based electricity plants.

The factors which dictate how much thermochromic glass will impact your Net Zero initiatives depend on:

• the geography (and orientation) of the building
• the design of the building (including the natural landscape)
• the building’s insulation, and
• the day-to-day operational usage.

As you can imagine, a museum is very different to a hospital, which is very different to a private residence, in terms of energy use.

We would recommend reaching out to manufacturers to investigate the variety of technologies available on the market today, and for this we have designed a bespoke parameterised search engine, especially for the smart glass sector.

Search for Thermochromic Glass Manufacturers

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

The screenshot below shows that we have many companies listed already, which can be filtered if you specify product attributes, such as minimum transmittance, U-value or haze:

Smartglass World Marketplace search for thermochromic smart glass companies

If you press the ‘Show Results’ button, this will produce a list of companies, which you can contact by posting a request on the Smartglass World Marketplace.

If you are short on time, just reach out to us through our Consultancy page, and we can do this work for you.

Outlook

Thermochromic smartglass reduces solar heat gain into buildings, controlling air conditioning costs and contributing to ‘net-zero’ construction.

Since the transmittance in the dark state is around the 10% mark, the view to the outside world is maintained, but glare is also reduced, improving productivity and speeding recovery times in hospitals.

References

• ISO 10077-2:2017 Thermal performance of windows, doors and shutters — Calculation of thermal transmittance — Part 2: Numerical method for frames, URL
• ISO 15099:2003 Thermal performance of windows, doors and shading devices — Detailed calculations, URL.
• What is a U-value? The NBS
• Nanoceramic VO2 thermochromic smart glass: A review on progress in solution processing, URL
• IUPAC. Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). Online version (2019-) created by S. J. Chalk. ISBN 0-9678550-9-8.
• “Fenestration for reducing building cooling needs” – C.G. Granqvist, in Eco-Efficient Materials for Mitigating Building Cooling Needs, 2015
• “Sunlight Responsive Thermochromic Window System” (2006), URL