What is Thermochromic Smart Glass?

Thermochromic glass differs from photochromic glass, which is triggered into a dark state by UV or visible light, rather than due to temperature increases from solar infrared.

Thermochromic and photochromic glass can both operate reversibly and both are considered passive technologies since they are not electrically-powered.

Benefits of Thermochromic Smart Glass

• Can be installed just as normal glass into a building facade
• Internal PVB lamination gives improved safety and noise insulation
• Glass changes between tinted and clear states within minutes of being triggered
• Maintains a view to the outside at all times
• Reduces glare and hence the resulting 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

Thermochromic Windows

Thermochromic materials used to date 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 unit (called an IGU).

The dynamic tinting results directly from radiant solar energy but could also be due to indirect thermal exchange, such as conduction or convection from adjacent building materials such as the glass itself, or from bricks, spacers or the metallic frame after they have been heated by the sun.

A triple-glazed unit typically comprises:

(i) a front sheet of glass

(ii) a gas space (filled with air, argon or krypton)

(iii) the thermochromic layer

(iv) another gas space and

(v) a back low-E coated sheet of glass.

The front piece of glass is needed as a wind barrier. The back piece of low-E coated glass keeps the heat which is retained in 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,
• then 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 a “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 the U-Value

The thermal transmittance of a building facade is measured by a variable called the U-Value and is ‘the rate of heat transfer through a structure divided by the difference in temperature across (and the area of) the structure’.

The calculations to define thermal transmittance for windows are standardised in ISO 10077 and ISO 15099. Typical U-Values for certain materials are given below:-

• Single glazed windows, allowing for frames: U-Value = 4.5 W/m²·K;
• Double glazed windows, allowing for frames: U-Value = 3.3 W/m²·K;
• Triple glazed windows, allowing for frames: U-Value = 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.

Air is also an excellent insulator and is often 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 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.

Conclusions

Thermochromic smart glass has clear benefits in reducing solar heat gain into buildings, reducing 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. URL
• “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