Low-Emissivity Smart Glass Windows

Introduction

Smartglass windows, doors and skylights are making inroads in the commercial and residential property spaces. 

They provide benefits such as on-demand privacy, improved visual comfort and on-site energy-generation through transparent photovoltaics.

When combined with low-emissivity (low-e) coatings, smartglass insulated glass units (IGUs) can further enhance the energy efficiency of a building. 

Low-emissivity coatings work by reflecting solar radiation away during summer and reflecting interior warmth back into the building during winter.

How they work depends on which surface carries the low-e coating.

This article is based on a recent Smartglass World project for a luxury residential refurbishment in New Zealand through our client, Viridian Glass. 

The double-glazed units we provided for this project combine dyed liquid crystal (DLC) smartglass with a low-e ‘soft coat’ (more on that later). This allows dynamic control over privacy and glare, with year-round energy efficiency.

But first, let’s break down some core concepts.

What is Emissivity?

The emissivity of a surface measures its capability to emit thermal radiation (i.e. heat). 

This depends on the material composition, the surface texture and the temperature. It is a surface property.

Strictly speaking, emissivity is the ratio of the energy radiated from a material compared to the energy that would be radiated at the same temperature by an ideal emitter. In physics, we call an ideal emitter a ‘blackbody’.

This ratio gives us a dimensionless number between 0 and 1:

• An emissivity of 0 indicates that the material is a poor emitter
• An emissivity of 1 indicates that the material is a perfect emitter

Highly-reflective surfaces, such as clean polished metals, classify as low-emissivity. Examples include aluminium foil (emissivity of 0.03) and silver (emissivity of 0.02).

Dark or opaque surfaces generally classify as high-emissivity. Examples include anodised aluminium (emissivity of 0.9), brick (0.9), concrete (0.91) and glass (0.95).

Don’t forget, we are talking about thermal radiation. 

A glass window should aim to be an excellent transmitter of visible light and an excellent reflector of thermal radiation.

Emissivity vs Reflectivity

But how can a material be considered reflective and have a low emissivity? It almost seems counter-intuitive, right?

Reflective Surfaces

When a photon of light reaches a highly reflective surface, it interacts with the electrons on the surface without being absorbed. It does not cause a change in the energy state of the material.

In fact, the photon ‘bounces off’ the surface with minimal change in its own energy or momentum.

Emissive Surfaces

In the case of an emissive surface, the photon is absorbed by surface electrons. 

They increase their energy state, climbing to a higher level (like climbing up a staircase), before ‘falling back down again’, whereupon they lose energy.

As a result of this released energy, a new photon is emitted, normally of a different wavelength.

The interaction results in a change to both the incoming photon and the interacting material, causing an increase in temperature.

Thus, reflective materials can be seen to be the opposite of emissive materials.

How Low-Emissivity Windows Work

Below we illustrate the range of wavelengths present in sunlight:

• The shorter wavelengths are ultraviolet (UVC, UVB and UVA), which are high-energy and can damage interior furnishings (e.g. carpets, furniture, curtains).
• Then we have visible light which we ideally want to maximise in a building (up to our limits of visual comfort, of course).
• Finally we have infrared, which we perceive as heat.

When sunlight enters a window facade, it heats the glass and the interior furnishings, whereupon it reradiates as longer-wavelength infrared. 

In fact, when glass absorbs solar thermal energy, the heat either conducts to nearby window components, or transfers by convection or reradiates as longer wavelength infrared.

Low-e coatings on interior glass surfaces reflect longwave infrared from furnishings back inside, maintaining warmth in colder seasons.

If we add low-e coatings to the outward-looking pane of glass in a double glazed unit, there is an impact on the temperature: 

• heat reflects away from the building, resulting in a cooler outer pane of glass
• the inner pane retains its moderate interior temperature

This results in a lower temperature differential between the outer and inner glass panes, minimising cold draughts due to convection, and reducing condensation within the window.

DGU Structure

The outer-facing side is on the left (where the sun shines). The bottom of the image shows the window sill (in brown). 

The glass panes appear in blue with an intermediate air gap (normally containing an inert gas such as argon).

The green structure is the window frame, often made of metal, plastic or wood.

Double-Glazed Unit (DGU) showing glass surfaces #1 to #4

Now we will look at how this translates to a smartglass DGU with a low-emissivity layer.

Low-Emissivity Smartglass DGU

Here we describe the smartglass DGUs supplied for the above-mentioned project in New Zealand. 

This shows a dyed liquid crystal (DLC) smart film layer, sandwiched between glass surfaces #2 and #3 and a low-e layer adhered to glass surface #5.

Dyed Liquid Crystal smartglass with low-e soft coat

The DLC layer provides electrically-controlled tinting to reduce glare and improve visual comfort when the sun shines.

The low-e layer on the interior surface of the interior pane reflects inner warmth back into the building, which is ideal for cooler climates.

Low-Emissivity Window Coatings

Low-e glass has a microscopically thin, transparent coating applied to it, either during manufacture or during post-processing.

The coating can be designed to reflect shortwave infrared, longwave infrared or ultraviolet, maximising the visible light transmitted.

Low-e materials include thin film silver and ceramics, either as single, double, triple or even quadruple layers, often just a few nanometres (nm) in thickness.

There are two principal low-e coating techniques, called ‘hard coat’ and ‘soft coat’.

Hard Coat

A hard coat is applied to glass by a pyrolytic process. 

A fluorinated tin dioxide thin film is deposited at high temperature using chemical vapour deposition (CVD), which fuses to the glass while still on the float line.

This creates a very durable bond, hence the term ‘hard coat’.

Soft Coat

A soft coat is applied to glass ‘off-line’ (after the glass is formed).

Normally, a deposition process applies multiple thin-film silver layers in a vacuum chamber at room temperature.

However, silver films are unstable and must be applied on an interior surface (typically surface #3 in a DGU) to protect their reflective properties.

There are two resulting types of low-e glass which can be manufactured with either a hard coat or a soft coat.

One is called Passive low-e glass, the other Solar low-e glass:-

Passive Low-Emissivity Glass

Passive low-emissivity windows are designed for cold climates. 

They passively heat the building by allowing energy from the sun to enter the home. This reduces heating costs.

Passive low-e glass results from applying a low-e coating to surface #3 or #4 in a DGU (i.e. furthest from the sun).

When a building interior is heated by the sun, the shortwave infrared is absorbed and later re-radiated as longwave infrared. 

When this longwave infrared tries to exit the window, the low-e coating reflects the heat back inside.

Solar Low-Emissivity Glass

Solar low-emissivity windows are designed to keep buildings cool in hot climates and results from applying a low-e coating to surface #2 (closest to the sun).

This reflects solar infrared away from the building and reduces air conditioning costs.

When applied to outer glass layers, low-e coatings can result in some issues:-

• The high reflectivity of the IGU can increase sunlight glare on nearby traffic or towards adjacent buildings or wildlife.
• The metals used in the low-e coatings can inhibit radio frequency (e.g. mobile phone) signals within the building.

Outlook

The outlook for combining low-emissivity layers on smart glass windows seems promising, given that their characteristics are complementary.

Whereas the smart glass improves visual comfort, the low-e layer reflects heat out in summer or reflects interior warmth back inside in winter (depending on its location in the IGU).

Finally, there are advances in thermochromic (switchable) technologies, such as the work conducted by Brightlands Material Centre. 

They have developed a technology that reflects solar thermal energy when the temperature reaches a set point. Their dynamic (but non-tinting) thermochromic layer is adaptable to cool or hot climates.

We can expect to see more such dynamic technologies in future that adapt to changing conditions.

Watch this space.