09.05 Transparent insulating materials
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Introduction
These materials have very good insulating properties combined with good light transmission. On the whole these materials do not offer a good enough optical performance to be used for view windows, but are ideal for use where the desire is to increase the level of ambient light. TIMs are ideal for use in constructions such as Trombe walls.
Many transparent insulation materials are actually translucent, and they also act to diffuse direct light. TIMs have been classified by some authors using the following scheme:
A number of projects have been completed in which TIMs have been used on demonstration buildings. A useful summary of some of these projects is given in ETSU-OPET [1993].
Aerogels, xerogels and carbogels
Aerogels, xerogels and carbogels are silica-based materials which contain a significant volume of air-filled voids (Duer et al [1995]).
These compounds are produced by mixing an organic silicon compound with water, alcohol and catalysts into an ‘alcosol’. The alcosol gradually develops a silica structure containing small voids filled with solvent - the ‘alcogel’. The critical part of the manufacturing process is to remove the solvent without damaging the fragile silica structure, leaving behind a cellular material containing as much as 95% air-filled voids. The manufacture and properties of these materials are discussed in depth in books such as that by Brinker and Scherer [1990].
Aerogels are produced by drying out the alcohol solvent at high pressures (typically 90 atmospheres) and high temperatures (typically 280oC) in order to prevent collapse of the silica structure (collapse occurs due to high surface tension forces - if the alcohol is removed at a temperature and pressure above the critical point then the surface tension forces disappear). An alternative to this is first to replace the alcohol with CO2, which can then be evaporated safely at low pressure and low temperature (any temperature above 31oC) - the resulting material has been called a carbogel. Another alternative is to add other monomers to the alcogel, in order to strengthen the silica structure, which allows drying to take place at atmospheric pressure and temperatures below 100oC - this results in a xerogel, which is denser than an aerogel or carbogel.
Duer et al [1995] describe two aerogels (densities 90 and 150 kg/m3), a carbogel (density 173 kg/m3) and a xerogel (density 500 kg/m3), and the optical properties are discussed. It is generally found that the xerogel has slightly better optical properties, although slightly poorer thermal properties due to its higher density. However, the carbogel and aerogels are similar in performance.
Aerogels may be produced in monolithic form (slabs) or as granules, typically 2-6 mm diameter. The thermal conductivity of an aerogel is typically 0.02 W/mK or less (Field [1994]). Xerogels are believed to have a higher thermal conductivity, perhaps in the range 0.03-0.06 W/mK. This suggests that a panel comprising two 4 mm sheets of glass and a 16 mm aerogel-filled cavity will have a U-value of 1.0 W/m2K or less. To attain a U-value of 0.45 W/m2K, which is the current level required for walls in the UK, would only need an aerogel layer 41 mm thick. Lower thermal conductivity values can be obtained if air is pumped from the unit to create an evacuated aerogel (typically the absolute pressure should be reduced below about 10% of atmospheric pressure). Evacuating the unit (the aerogel is strong enough to withstand the forces due to evacuation of the unit) typically lowers the thermal conductivity down to 0.008 W/mK (Yannas [1994a]), which would allow a U-value of 0.45 W/m2K with just 16 mm of aerogel.
Aerogels produced by some processes are hydrophilic - they absorb ambient moisture and subsequently break down. However, aerogels can be produced in a form that repels water (hydrophobic) (Field [1994], Brinker and Scherer [1990]).
It is suggested that aerogels should not be subjected to repeated movement because the material is fragile and can be ground down into powder. However, with the development of hydrophobic forms of aerogel it is possible to vent the aerogel to atmosphere, thereby using pressure equalisation to limit deflections of the glass surfaces (although the thermal performance would not be as good as with evacuated units and the question of contamination of the aerogel with biological material would need to be considered).
Light transmitted through aerogels is filtered and appears yellow in colour (the reflected light appears blue). Furthermore there is a small amount of light diffusion and the view through an aerogel appears slightly cloudy.
Capillary glass - ‘Okalux’
‘Okalux’ is a commercially-available (since 1965) advanced glazing comprising a slab of parallel hollow acrylic capillaries, which is installed fitted between two panes of glass in a sealed unit. Okalux is a translucent material which does not permit a view-through, but which acts to diffuse incident light by a process of multiple reflection - the acrylic ‘capillaries’ behave as short optic fibres with highly reflective walls. The capillaries are attached to a translucent woven fabric mat, and this is then installed between the panes of glass, which are sealed with a conventional edge sealant. Thicker insulating panels can be produced, with a thermally-broken aluminium edge (Kümpers and Link [1994]).
This type of advanced glazing material can be used to give more even light levels within a room, and also has better heat transfer properties than the air which it typically replaces in the double glazing unit.
The Okalux slab is fairly rigid, and appears to be used without any form of edge spacer - the Okalux material itself is in contact with both sheets of glass, and only needs to be sealed at the edges of the unit. Whilst this has obvious implications in terms of thermal performance (the edge of the unit does not compromise the overall performance) it should be noted that if the Okalux is in contact with both sheets of glass then it can be expected, over a period of time, to undergo a fairly strenuous cycle of movement due to wind-loading. The durability of the plastic capillary structure under cyclic loading is not clear and should be assessed - although this type of glazing has been used for some time it is not clear whether service units have been dismantled and checked for damage, nor is it clear that units have been performance-tested after several years in service.
It should also be noted that as a translucent material Okalux cannot be used to glaze a facade in its entirety, but if used in areas above head height it could be used to give a more even light level whilst cutting out direct solar radiation. The transmitted light from these devices is concentrated in a cone, and glare may be a problem unless suitable precautions are taken. Okalux is supplied with the capillaries layered between two sheets of a translucent woven mat, and this may reduce glare problems.
As this product is already available commercially it is possible that the technical issues raised above will be resolved.
Honeycomb materials
Honeycomb assemblies of hollow rods may also be used to redirect solar energy. Again solar energy is transmitted through the assembly by a sequence of multiple internal reflections.
Honeycombs also tend to be made of plastic materials with lower thermal conductivities than glass, such as polycarbonate or polymethylmethacrylate. The rods are hollow because air has much better insulating properties than solid materials and plastics have the advantage that they can be readily extruded into small hollow rods with uniform cross-sections.
As with capillary glass these structures are best protected between panes of glass. The risk of contamination with dirt and infestations by biological organisms is otherwise too great. The transmitted light from these devices is also concentrated in a cone, and glare may be a problem in some positions.
If plastic-based TIMs are used for solar collectors or as retrofit over existing opaque walls then care should be taken that the plastic can withstand the high temperatures that are likely to be generated (Peuportier [1994]). It is normal when fitting TIMs over an opaque wall to paint the wall surface black in order to maximise the absorption of energy. This may also have implications for the framing system, because thermal expansion of the material will be greater - the use of conventional glazing-rebate dimensions may not be sufficient in some applications.
Thin-wall polycarbonate sheet
Polycarbonate is also used in the form of thin-walled sheet, produced by an extrusion process. The cross-section of the sheet is divided into small cells which limit the occurrence of convection heat transfer. A number of cross-sections are commercially-available, and a selection is shown in this image.
This type of sheet has found a significant use in domestic conservatories in the UK, and a range of colour-tints are available. The strength of the plastic and the in-built edge seal makes the cost very favourable when compared to a double-glazed unit, whilst the thermal performance is very similar. However, light transmission through these products is distorted and a clear view is impossible. For overhead glazing these products are ideal.
The polycarbonate sheets are used directly, and there have been concerns raised about the compatibility of the polycarbonate with certain materials used to seal the sheets into framing systems. Durability of the plastics may also need to be considered carefully, and plastics are also more prone to scratching.
Glass blocks
Glass blocks may seem an unusual item to include here but they can be considered as transparent insulation. The light transmission through a glass block is distorted, and the thick walls of the glass block also provide a shading effect when the sun is high in the sky. In future the insulation content of glass blocks may be increased by replacing the air within the block by a material such as an aerogel.
Glass blocks are already used where light transmission is desired but the view should be limited. The addition of an aerogel to the glass blocks will allow these components to be used in greater amounts, and the strength of the glass will prevent mechanical damage to the aerogel. Glass blocks also have the advantage that they are self-supporting, doing away with the need for expensive framing systems.