09.01 Glazing units
Open full view...Categories: Advanced Glazings
Multiple-sheet glazings
There are several reasons why glass may be used in a sandwich assembly (multiple glazing unit). Generally two or more panes of glass, separated by a gas-space, will have better thermal properties (the insulation value will be greater, although light transmission will be reduced further by each additional layer of glass, plastic or coating) and give better acoustic attenuation (which is particularly important for domestic dwellings which may be situated close to busy roads). However, some of the various coatings that can be applied to glass must be protected from attack by atmospheric moisture, cleaning agents or abrasion, and sealing them into multiple glazing units is the only solution. It is also important, where some solar control glasses are used, that the solar control glass is kept away from the building occupant - solar control glasses generally absorb excess solar radiation and can become very hot to the touch.
Double-glazed unit, coloured glass
The use of coloured glass in double glazing units is possible. Usually only the outer sheet of glass will be coloured, as this results in the lowest glass stresses (glass-cooling heat loss from the outer pane is always likely to be higher than from the inner pane). Furthermore the occupier of the building is protected from contact with the hot glass pane.
The use of coloured glass requires that the glass be toughened, and care must be taken to ensure that the glass is installed the right way round - it is however generally easy to tell which is the coloured pane of glass! The higher temperature of coloured glass when exposed to solar radiation may also have implications for the durability of the edge seal, and will also lead to greater expansion of the glass - the mounting of the glass is therefore important (there must be a reasonable clearance between the edges of the glazing unit and the glazing frame).
Double-glazed unit, suspended coated film between panes
Instead of coating one of the panes of glass it is possible to suspend a thin film of some coated polymer (usually polyester) between the panes of glass. This approach sub-divides the air-space, reducing convection currents within the glazing unit, and the suspended film may have coatings on both sides. The film must however be pierced at one edge or corner, to ensure that the temperature difference across the glazing unit does not generate a pressure difference across the film, which could lead to rupture of the film. This type of system also generally uses glazing edge spacers with better thermal properties, such as that shown in this image.
A typical suspended film is Southwall Technologies ‘Heat Mirror’, which had been used for more than 70 million square feet of glazing by 1993 (Southwall Technologies [1993]). A double glazing unit with two 6 mm panes of clear glass, a single Heat Mirror 88 suspended clear film and a 12 mm air-filled gas space has a centre-glass U-value of 1.76 W/m2K, and transmits 70% of daylight but only 44% of the total solar energy (shading coefficient 0.62). At the other end of the range using a Heat Mirror 33 clear film reduces the U-value slightly to 1.70 W/m2K, transmits 28% of daylight but only 13% of the total solar energy (shading coefficient 0.22). This demonstrates the range of performance that can be achieved simply by varying the coating on the plastic film.
This type of suspended film has been used for some time in the USA. There are obvious issues regarding the installation of such films, to ensure that the film does not wrinkle, and the glazing edge seal must not become soft (either as a result of age or high operating temperatures) or the tension in the film could pull the glazing edge spacer inwards.
Generally the polymer used for the suspended film can creep, and keeping the film taut is extremely important. It should be noted that a European manufacturer of suspended films withdrew the product after experiencing problems with the film-tensioning mechanism. Tensioning can be achieved either by pre-tensioning the film using the perimeter glazing spacer (which is an integral part of the suspended film system) or by heating the glazing unit after production so that the film shrinks slightly and pulls itself taut. However, at high temperatures expansion and creep of the film may lead to wrinkles, and it may be difficult to ensure that the film is initially flat at corners.
Increasing the number of films will subdivide the gas-space even further without a weight penalty, but as the number of films is increased so the light transmission decreases. More films may also be more difficult to tension properly.
Triple glazing
Triple glazing, and glazings with four or more panes of glass, may be used with any combination of glass type, surface coating and gas-fill described above. As a general rule however coloured glass will be outermost and surface coatings will be placed facing into the gas-spaces with one per gas-space. It is also possible that a sealed double glazed unit will be used with a separate sheet of glass for the third pane.
Where a double glazed unit is placed in parallel with a single pane of glass it is typical in Scandinavian practice that the extra pane of glass is on the outside of the unit. This stabilises the temperature range of the glazing unit (by eliminating the highs and lows of the annual and diurnal temperature variation) and reduces the likelihood of the edge seal coming into contact with water. The space between the glazing unit and the third pane is then vented to the outside, eliminating the risk of condensation in the space without compromising the thermal performance of the system. Conventional UK practice on the other hand is to add extra panes of glass to the room-side of windows!
Double-glazed unit, liquid-filled
McKee [1994] reports the development of a fluid-filled glazing system. The system comprises a triple glazing unit, but with the cavity next to the room-side glazing filled with a special fluid. The fluid is formulated to transmit light but trap infrared. The visual light transmission is reported as 70%, whilst the shading coefficient is less than 0.2.
By circulating the fluid summer heat can be recovered using a heat exchanger or winter heat can be added and used to warm the building. The fluid can be dyed, and a photochromic dye can be added so that the glazing reacts to ambient light levels.
There are obvious concerns regarding the sealing of these units, particularly if a network of pipes is required. Replacing a damaged unit may cause problems if the system needs to be drained, and there must be concerns about possible expansion and contraction of the fluid. The amount of energy that is absorbed by a coloured liquid will be high, and positioning the liquid on the room side of the glazing could lead to problems with thermal radiation from the glass. Although circulation of the fluid will reduce the risk of overheating there is the question of what will happen if the circulating system fails in some way (presumably the liquid will get very hot).
The weight of the unit is also significant, and special framing sections may need to be developed. The possible development of biological matter within the system must be considered; whilst chemicals can be added to the liquid to inhibit the development of algae and similar organisms there is then a need to consider what will happen if a problem occurs say five years after installation - the building owner may call in a local engineer to fix the problem, who could then inadvertently refill the system with the wrong liquid.
Multiple glazing with ‘electric glass’
The coatings that are usually applied to glass have a significant metal content. As a result the coating is electrically conductive, but because the coating is very thin it has a high electrical resistance. If a voltage is applied across the coating, through electrodes running along the top and bottom of the glass, then the low-e coating becomes hot. If the low-e coating is on the room-side of a gas-space the insulating properties of the gas-space ensure that most of the heat is transmitted into the room. Electric glass is already being produced in Finland under the trade-name EGlas.
‘Electric glass’ is already used in applications where condensation is undesirable (the glass only has to be heated to a fraction of a degree above the adjacent air temperature and condensation cannot occur). Electric glass is also useful for avoiding the formation of ice and for clearing snow from a glass surface.
The obvious drawback with electrical glass is the need to provide an electrical sub-system. However, this will become more straightforward as the relevant technology is already being developed for photovoltaics. Photovoltaics could even be combined with electric glass to provide a system that completely eliminates glazing heat loss whilst also heating the room and eliminating cold-radiation and cold down-draughts from the glass. The elimination of cold down-draughts would then allow simple changes to room layout, such as moving radiators and other heating systems away from their usual location below the window to a location against an internal wall, where all of the heat can be retained within the building.
Evacuated double-glazed units
The most effective alternative to the units described above is to remove the air from the glazing unit altogether. Conduction and convection heat transfer cannot occur through a vacuum, and so the only mechanism for heat transfer would be by infrared radiation. A surface coating can then be used to significantly reduce the radiation component of the heat transfer.
Simko et al [1995] describe the manufacture and performance of evacuated glazings. A typical evacuated glazing might comprise two panes of 4 mm glass, with a 0.15 mm space, held apart by 0.25 mm diameter spacers on 25 mm centres. It is suggested that the spacers are difficult to see with the unaided eye, and that the optical clarity of such a unit is nearly as good as a conventional double glazing unit.
The small gap between the sheets means that a hard vacuum is required, and the edge seal must be strong enough to withstand this. Typically the edge seal is formed from sintered glass, which requires that the units are baked at high temperatures (about 500oC - Simko et al [1995]) during production. The air is pumped out of the units through a special tube, which is then sealed over with solder glass. The general layout of such a unit is shown in this image. However, the units are still very fragile, and early indications have been that even transporting the units can lead to fracture.
Simko et al [1995] indicate that a centre-glass U-value of 0.9 W/m2K can be routinely achieved using two hard low-emissivity coatings (soft low-e coatings cannot be used because the process of forming the edge seal occurs at a temperature that would destroy a soft coating). In this condition about 40% of the heat transfer through the unit is due to conduction through the support pillars, whilst the remaining 60% is due to radiation heat transfer. The optical properties of the unit should be similar to those obtained for an air-filled unit.
The spacers that are used to separate the panes of glass may be visible under some lighting conditions (it has been suggested that the optical clarity of evacuated glazings are ‘nearly as good’ as ordinary double glazing units!). The spacers are also the limiting factor in determining the performance of these units - sufficient spacers must be used to prevent local distortions of the glass (this causes higher thermal stresses in the glass and distorts reflections).
In terms of thermal stresses Simko et al [1995] indicate that unit have survived glass-to-glass temperature differences of 70oC, although it is uncertain whether units would survive this temperature differential superimposed on fluctuating wind-loads.
Ideally this type of glazing would last longer in small units (wherever small panes of glass have been used traditionally - for example Georgian sash windows in the UK). However, the nature of the edge seal - sintered glass - means that the heat transfer path at the edge of the glazing is very significant when compared to the centre-glazing heat transfer; this means that the average U-value of a small sample is entirely dominated by the edge performance. Although it has been suggested that increasing the bite of the edge gasket would help to shield the poorer-performing glazing edge this would also reduce the vision area of the glazed system, in contrast with most architects’ ever-growing desire for more glass and less frame.