Passive Cooling with Smart Materials

Passive cooling in buildings can be achieved with solar-blocking smart glass and radiative coatings applied to exterior surfaces.

In a Nutshell

If we want to keep our buildings cool in warmer climates, we basically have two options:

  • Stop heat getting into the building
  • Remove the retained heat out of the building

Passive glazing technologies such as thermochromic, thermotropic and photochromic glass can block solar heat and light from entering window glazings.

They are not electrically-driven but rather respond automatically to increases in temperature or light.

Some of these technologies darken visibly, whereas some simply reflect infrared (without changing tint) when they reach a temperature set point.

On the non-glazed parts of the building, such as roofs and walls, ‘passive radiative cooling’ coatings and paints are now available which reradiate retained heat in the 8-13 micrometer infrared waveband. 

This waveband corresponds to the ‘infrared atmospheric window’, which allows these wavelengths to exit the atmosphere straight into space, without being absorbed by carbon dioxide, ozone or water vapor.

Together, these technologies can passively cool the glazed and unglazed parts of a building envelope, with further applications across transportation, textiles and industry.

Solar Radiation

Solar energy reaching the Earth is composed of x-rays, gamma rays, ultraviolet, visible light, infrared and radio waves. 

The heat we perceive corresponds to infrared wavelengths and comprises approximately 49% of all solar energy. Visible light, on the other hand, comprises 42%.

The graph below shows the solar intensities of down-going solar radiation (from the sun), compared to up-going thermal radiation (leaving Earth into space).

Passive Cooling with radiative atmospherics

Attribution: https://en.wikipedia.org/wiki/File:Atmospheric_Transmission.svg 

The graph depicts how much energy is transmitted by the atmosphere and shows a dip around 10 micrometers, corresponding to water vapor and other gasses.

This is the infrared atmospheric window where infrared is not absorbed or scattered by the Earth’s atmosphere. It is transmitted straight out to space.

In this waveband, the atmosphere thus acts as a heat sink.

Why Passive Cooling?

In order to reach Net Zero carbon emissions in the building sector, we need to reduce our energy consumption, so active (i.e. electrically-driven) smart glass may be counter-productive.

Hence, we only consider passive smart glass in this article. 

However, we must remember that an important part of passive cooling design is user comfort, and that passive technologies lack this user-driven control. 

Instead, they are triggered automatically by nature.

Overview of Passive Cooling

There are many ways to achieve passive cooling in building design:-

Better Insulation

Reduce heat entering the building with insulation placed on the roof, windows, walls and flooring. The ‘Passive House’ movement takes this to the next level. This also stops warmth leaving the building in winter.

Zoning

Zoning sleeping areas in parts of the building which avoid rising heat, and limiting thermal mass in these areas.

Garden Shading

Exterior shading through landscape design with strategically-positioned deciduous trees, such as maples, oaks and beech. These trees have leaves in summer that reduce sunlight impact on a building and lack leaves in winter which lets sunlight reach the building.

Air Movement

Stimulating convective air movement with ventilation fans, high ceiling windows and roof vents.

Earth Coupling

Earth coupling of thermal mass elements, such as concrete floor slabs and walls, which conduct interior heat down to the ground. This works only where deeper earth temperatures are lower than ambient temperatures.

Interior Furnishings

Choosing interior furnishing materials that do not retain warmth. Examples include cotton or linen for bedding and blankets, and green garden walls.

Passive Cooling with Dynamic Glass

With 21st century architecture focussed on floor-to-ceiling glazing, it makes sense to use dynamic (smart) glass to reduce the solar infrared that causes temperatures to rise.

Smart glass allows us to maximize daylighting while also controlling solar heat and light. This minimizes air conditioning and artificial lighting costs.

We now look at three passive smart glass technologies that can dynamically block infrared solar heat:

Thermochromic Smart Glass

As already pointed out in our earlier article, thermochromic glass changes its properties at higher temperatures.

Some technologies tint, whereas others simply reflect infrared (without tinting).

For those that tint, the changes in visible light transmittance (VLT) can typically be from:

  • 75% VLT (surface temperature of 20ºC), to 
  • 60% VLT (surface temperature of 80ºC).

The solar transmittance drops accordingly from 54% to 42% when struck by solar heat.

Materials include silicate or borosilicate glass doped with vanadium dioxide, which is either deposited as a thin film or laminated within PVB.

The image below shows thermochromic smart glass samples that you can purchase in our Samples Shop

Thermochromic smartglass sample
The image confirms a blue tint when the smart glass increases in temperature due to sunlight.

The graph below shows how transmittance decreases in the 500-780 nm (including the near infrared) waveband as temperature increases. 

Thermochromic smart glass spectral chart

Thermotropic Smart Glass

Thermotropic smart glass exhibits a different mechanism at higher temperatures. This glass changes from transparent to translucent where it scatters solar radiation.

The scattering effect reduces visual glare and heat gain, minimizing air conditioning costs in buildings, as well providing visual comfort for building and vehicle occupants.

Thermotropic materials use additives manufactured by microencapsulation of thermoresponsive compounds. 

These are dispersed into transparent matrix polymers, such as ethylene vinyl acetate (EVA) and polyvinyl butyral (PVB). 

  • Below the ‘switching temperature’, the refractive indices of the additive and the polymer match and this allows light to pass unimpeded.
  • At higher temperatures, the refractive indices move further apart, leading to scattering of solar radiation.

This type of smart glass is not the same as PDLC liquid crystal glass which is active (i.e. powered electrically).

Photochromic Smart Glass

Photochromic glass changes tint when exposed to increased light levels (as opposed to temperature). 

This is explained in detail in our earlier article.

Photochromic materials are currently available only as a film, which we must affix adhesively to the interior of a glass window, since they do not lend themselves to lamination.

The image below shows the visual darkness of such films when exposed to sunlight.

Photochromic film adapts to changes in sunlight
You can view technical details of this film and purchase a sample in our Samples Shop.

Coatings for Radiative Cooling

Standard white paints reflect sunlight and can help to reduce heat gain on building exteriors. 

However, as mentioned above, newer materials are now available which also radiate energy to space via the infrared atmospheric window (8 – 13 micrometers).

We call this concept ‘radiative sky cooling’ or ‘passive radiative cooling’.

Many people will be surprised to hear that this idea dates back to courtyard architectures of ancient Iran which used wind towers, domed roofs and air vents to achieve the same effect.

These ancient self-cooling structures were capable of passively cooling water and even making ice, simply by controlling the temperature and thus the density of air.

How does Radiative Cooling work?

Water vapor in the atmosphere absorbs wavelengths around 6 micrometers (μm) and above 20 μm, whereas carbon dioxide absorbs wavelengths around 15 μm. 

This leaves an atmospheric transmission window from 8 to 13 μm, which does not absorb or scatter radiation, thus appearing transparent to these infrared wavelengths.

The atmospheric transmission varies with geography due to different concentrations of water vapor and the subsequent impact on relative humidity.

Suppliers

Several organizations now offer radiative cooling paints or coatings, in various stages of commercial readiness:

These coatings and paints find applications in building exteriors, roads, transportation, textiles and solar cells.

Benefits include:

  • Eco-friendly materials
  • Water-soluble 
  • Simple to apply (spray, roller or brush) 
  • Good adhesion
  • Summer daytime cooling power of up to 110 W/m2
  • Applicable for aluminum, steel, and flexible tent canvases
  • Reduce temperatures by 5-8 ºC ( 8-12ºF) below ambient air temperatures, and by 10-15 ºC (18-25 ºF) below an uncoated surface
  • Operate in direct sunlight

These characteristics are merely representative and serve to give an idea of the capabilities of these materials.

Outlook

The outlook for passive cooling technologies is bright.

Government policies to meet Net Zero carbon emissions require us to reduce energy usage, and buildings are one of the worst offenders.

By blocking infrared with passive smart glass windows, and reradiating heat with passive cooling paints on non-glazed building exteriors, we could make a substantial contribution to achieving these goals.

We look forward to seeing future advances in glazing technologies that allow radiative coatings to achieve the same effect, transparently, in smart windows.

Need vendor-neutral advice choosing smart glass for your next project?