Low-Emissivity Smartglass Windows

Smart glass can be enhanced by low-emissivity (low-e) coatings that improve energy efficiency, allowing smart building facades to achieve on-demand privacy, reduced glare and compliance with thermal efficiency codes such as Energy Star®.

Introduction

Smartglass windows, doors and skylights are making inroads in the commercial and residential property spaces, providing benefits such as on-demand privacy, improved visual comfort and even energy-generation through transparent photovoltaics.

When combined with low-e coatings, smartglass insulated glass units (IGUs) can enhance the energy efficiency of a building by reflecting solar radiation away during summer, and reflecting interior warmth back into the building during winter.

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) to allow control over privacy, dynamic tinting and improved 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 (heat) and 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 (known in physics as a ‘blackbody’). 

This 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, are classified as low-emissivity. Examples include aluminium foil (emissivity of 0.03) and silver (emissivity of 0.02).

Dark, opaque surfaces are generally classified 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, so 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?

Well, when a photon of solar radiation reaches a highly reflective surface, the photon interacts briefly with the electrons on the surface of the material without being absorbed and without causing a change in the energy state of the material. 

The photon ‘bounces off’ the surface with minimal change in its energy or momentum.

On the contrary, in the case of an emissive surface, the photon is absorbed by surface electrons which increase their energy state, climbing to a higher level (like climbing up a staircase), before ‘falling back down again’, whereupon they drop in energy state.

As a result of this, 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 the temperature of the material.

Thus, reflective materials can be seen to be the opposite of emissive materials because the latter absorb and re-emit photons.

How Low-e Glass Works

Sunlight incident on a building contains a range of wavelengths, which are illustrated below:-

UV, visible and infrared bands in sunlight

  • The shorter wavelengths are ultraviolet (UVC, UVB and UVA), which are high-energy and can damage interior furnishings (carpets, furniture, curtains, etc).
  • Then we have visible light which we want to maximise (up to our limits of visual comfort, of course).
  • Finally we have infrared, starting from about 700 nanometres (nm), which we perceive as heat.

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

In fact, when solar thermal energy is absorbed by glass, it can either be conducted away to nearby window components, shifted away by convection (air movement) or re-radiated as longer wavelength infrared.

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

Furthermore, when we add low-e glass to a double glazed unit, there is an impact on the temperatures of each pane of glass: 

  • the outer pane of glass is not so warm any more, since less heat is escaping from the building, and;
  • the inner pane retains interior warmth, maintaining its temperature

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

Let’s now look at the structure of an insulated glass unit (IGU) window:-

DGU Structure

Double-Glazed Unit (DGU) showing glass surfaces

Double-Glazed Unit (DGU) showing glass surfaces #1 to #4
Label
Description
#1
Outer facing glass (typ. thickness 3-10 mm)
#2
Inside surface of exterior pane
#3
Outer surface of interior pane
#4
Inside surface of interior pane
#5
Window frame
#6
Insulating gap filled with air, vacuum or gas (e.g. argon) and a ‘warm edge’ spacer (typ. insulating structural foam)
#7
Window seals
#8
Internal reveal
#9
Exterior windowsill

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

Smartglass DGU with Low-e

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

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, acting mainly in the UV and visible regions of the solar spectrum.

The low-e layer operates in the thermal region, reflecting inner warmth back into the building.

Low-e Coating Types

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 specific portions of the solar spectrum, such as shortwave infrared, longwave infrared or ultraviolet, and is designed to maximise the amount of 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 where a fluorinated tin dioxide thin film coating is deposited at high temperatures using chemical vapour deposition (CVD) and fused to the glass surface while still on the float line.

This creates a very durable bond which is designed to maximise solar heat gain, allowing heat in during winter and reflecting interior warmth back into the building.

Soft Coat

A soft coat is applied to glass by a magnetron sputter vacuum deposition process (MSVD), where multiple thin-film silver layers are applied ‘off-line’ (after the glass is formed) in a vacuum chamber at room temperature.

Silver films are unstable and must be applied to 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-e Glass

Passive low-e glass is 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-e Glass

Solar low-e glass is 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.

Especially when applied to outer glass layers, low-e coatings can result in some problems:-

  • 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-e layers into smartglass facades seems justifiably positive, given that their characteristics are complementary.

Whereas the smartglass layers (e.g. liquid crystal, electrochromic, electrophoretic or SPD), are designed to alter the visible light transmission for purposes of visual comfort and glare reduction, the low-e layer operates statically in the infrared region by reflecting solar heat out or reflecting interior heat back into the building.

There are also advances in thermochromic technology, such as the work being done by Brightlands Material Centre who have developed a technology that reflects solar thermal energy when the temperature reaches a set point. 

This dynamic (non-tinting) thermochromic layer can be adapted during manufacture for cool or hot climates.

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