09.07 Variable glasses

Categories: Advanced Glazings

Introduction
Chromogenic glasses change their appearance (indicated by a change of colour) in response to some external stimulus.  A number of stimuli can be used, but the most common are light (or some other part of the electromagnetic spectrum), heat and electricity.

Chromogenic glazings are discussed in considerable detail by Lampert [1995].  This paper has 76 references, which cover many of the issues relating to the technical development of chromogenic glazings.  This paper also lists 70 organisations world-wide which are active in the development of electrochromic switchable glazings and 23 organisations world-wide which are developing switchable glazings of other types (this list is growing continuously).
 

Electrochromic glass
Electrochromic coatings may be applied to the surface of a pane of glass (the electrochromic coating must be protected by being sandwiched between two panes of glass, usually as laminated glass).

These multi-layered coatings (Lampert and Ma [1992 pp30] identify 16 different layer-structures for electrochromic coatings) respond to an electric potential by changing colour.  In simple terms such coated glasses may be switched between a high transmission state (bleached) and a low transmission state (coloured) state.  However, Selkowitz et al [1994] identify three idealised forms of electrochromic coating:

• coatings which switch from high transmission to low transmission across the whole spectrum of solar energy
• coatings which have a permanently low transmittance in the infra-red, but switch from high to low transmittance in the visible part of the spectrum
• coatings which have a permanently high transmittance in the visible, but switch from high to low transmittance in the infra-red part of the spectrum

Selkowitz et al [1994] also discuss the range of performance available from current electrochromic glazings; for a typical electrochromic glazing used in a double-glazed unit with a single low-e coating the shading coefficient varied from 0.67 (bleached) to 0.20 (coloured), whilst the visible light transmittance varied from 0.65 (bleached) to 0.08 (coloured).

It is important to note that in the coloured state a chromogenic glazing may absorb a considerable amount of solar radiation, with the result that the glazing temperature is very high.  This requires that the glazing is used as the outer pane of a multiple glazing unit.  However, there may still be a considerable problem with heat being radiated into the room from the coloured glazing, and so a low-e coating is often used on the adjacent pane of glass to reflect this radiated heat away from the room.

The time for the glazing to switch states depends on the direction of the switch, and times have been reported as low as 5 seconds and as high as several hundred seconds; this probably depends upon the size of the sample.  The potential at which electrochromic glazings switch can be as low as 2 V (Nagai et al [1994]), compared to perhaps 50 V for liquid crystals, and once switched the devices have a memory which means that the potential can be removed and the device will remain switched for several hours, or until a reverse potential is applied.

Potential benefits of electrochromic glazings in various USA climates are demonstrated by Warner et al [1992] and by Selkowitz. et al [1994].

Electrochromic glasses are presently very expensive, and are not yet available in sizes greater than about 1m by 1m because of difficulties in obtaining an even colour across the opacified glazing.  There is also an issue of durability, as repeated switching of the units, and switching with too high a potential, can cause the coating to degrade.

Although electrochromic glazings have a ‘memory’, so that the material remains in its switched state after a single application of a potential, there is little information as to how the performance varies as the memory fades.

The colour of electrochromic glazings is presently limited to blue, with the bleached state generally being transparent or slightly yellow.  Glazings which change from light blue to dark blue might be more aesthetically acceptable, but other colours are certain to be requested; photovoltaic cells were also available initially only in blue, but after repeated demands from clients and architects other colours are now available.

When coloured these glazings will absorb a significant amount of energy and become hot, which is recognised by Selkowitz. et al [1994], who indicate that the electrochromic glazing should always be on the outside of a glazing unit.  For this same reason it is unlikely that electrochromic glazings can be used other than in sealed units.  Since the electrochromic coating is usually protected by being laminated between two panes of glass these units will be heavier than double glazing units and, combined with the need to fit electrical cables, the installation process may be considerably more difficult.

Selkowitz. et al [1994] also note that it can be difficult for researchers to identify the voltage that can be applied across an electrochromic film.  Higher voltages result in faster switching, but can also lead to premature breakdown of the film.

When these issues have been resolved it is probable that electrochromic glazings will be heavily used.  Selkowitz. et al [1994] note particularly that because electrochromic windows must be controlled they offer guaranteed levels of performance that user-controlled systems cannot.  Furthermore, it is possible to use far larger areas of electrochromic glazing in a facade than is currently accepted for passive glazings.

Issues of high cost will gradually be resolved, more so now that electrochromic glazings are being offered in the automotive industry.  However, the use of electrochromics will be tempered by the need to determine the optimum switching strategy - at what point is the glazing changed from bleached to coloured - because this affects the energy balance of the building.  Electrochromics also have the advantage that, as switchable products, they can also be used for privacy.

Nagai et al [1994] state that Asahi Glass in Japan are aiming to develop a glazing system ‘by 1996’ which has durability under sunshine of at least 10 years, can operate for 100,000 cycles and be available in sizes of at least 40x60 cm.  Indeed, a device has already been cycled 100,000 times at 60^oC without loss of stability.  However, the limited size and durability may discourage the use of these products in precisely those areas where they will be most attractive - tall office buildings - because those buildings are often prestigious (the owner of such a building does not want to replace the glazing after 10 years, which is a highly visible process and sure to be commented on by competitors).
 

Photochromic glass
Photochromic glass (or photochromic plastic) changes its colour in response to light levels.  Such glass automatically darkens when the light is strong (it is usually the UV component of the light that causes the response - Lampert [1995]).  Photochromic materials are already used in sunglasses.

Lampert and Ma [1992 pp40] suggest that a typical photochromic glazing developed for sunglasses has the properties

visible light transmittance:     89% uncoloured     26% coloured

and takes 3-4 minutes to fade from coloured to 57% visible light transmittance at room temperature.  It is suggested that the total solar energy transmittance varies from 85% to 50%.

It is suggested by Lampert [1995] that the durability of photochromic glass under continuous colour/bleach cycling and resistance to chemical attack are excellent, and that photochromic glass is probably the most chemically stable of the chromogenic materials.  However, the limited rate of colour change may be a problem.

It should also be noted that the behaviour of the photochromic glazing is in-built and the architect and occupier are unable to define the control strategy beyond the initial selection of the glazing.  This may cause the building occupiers to feel that they have no control over their working or living environment.
 

Thermochromic glass
Coatings can also be produced which change their colour in response to changes in the ambient temperature.  These thermochromic glazings can be made to change their colour over about 5^oC ambient temperature change.  Materials can also be produced which physically change phase at some temperature - these are known as thermotropic materials (Lampert [1995]).  Lampert also discusses the thermotropic ‘hydrogels’, which are used in layers typically 1 mm thick, and which respond to changes in ambient temperature.

Thermochromic and thermotropic glazings may be used to provide some form of over-temperature protection for other advanced glazing systems (Fraunhofer ISE [1995] pp31, Wilson et al [1995]).  The temperature at which the colour change occurs can be determined during the manufacturing process.

It is suggested by Lampert [1995] that the hydrogels may suffer from problems with UV stability, limited cyclic lifetime and inhomogeneity during switching.  However, an interesting possibility is the addition of a transparent conductor next to the thermochromic/thermotropic material so that the transition can be induced by electrical heating - this at least gives the occupier some control over the glazing.
 

Liquid crystals
Liquid crystals work on the basis either of a molecule changing its orientation in the presence of an electric field, or of some needle-like suspended particle changing its orientation.  Liquid crystals are opaque until a potential is applied (the molecules/particles are randomly oriented) and require a continuously-applied potential for the liquid crystal to switch to and stay in the bleached state.

It is indicated by Lampert [1995] that the largest chromogenic glazings yet produced are based on liquid  crystals - sizes of 1 m by 2.5 m have been produced using NCAP (nematic curvilinear aligned phase) technology (Lampert and Ma [1992] pp37).  Lampert and Ma [1992 pp36-39] discuss liquid crystal devices in some depth.  They identify typical performance values for a polymer dispersed liquid crystal (PDLC) device as
 

Suspended particle (electrophoretic) devices use multi-layered films in which needle-shaped particles of polyiodides or paraphathite, typically 1mm long, are suspended in an organic fluid or gel laminated between two parallel conductors.  An electric field causes the particles to align themselves with the field (Lampert and Ma [1992 pp39]).  It is indicated that the switching voltage depends on the thickness of the device, and may be between 0-20 V, or even up to 150 V AC.

It is suggested by Lampert [1995] that liquid crystal glazings typically operate at around 60-100 V AC (although this may become lower in future, perhaps 20 V or less).  The power consumption is suggested as less than 20 W/m², but power must be continuously applied.  It is also indicated that in the ‘clear’ state these glazings are hazy, and that UV stability may be poor.

The development of suspended particle devices has been delayed by problems of long-term stability, cyclic durability and particle settling, amongst others, as indicated by Lampert and Ma [1992 pp39].