GB2261247A

GB2261247A - Multiple glazed panel soldered spacer joint

Info

Publication number: GB2261247A
Application number: GB9123526A
Authority: GB
Inventors: Michel Laurent; Xavier Dognies
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1991-11-06
Filing date: 1991-11-06
Publication date: 1993-05-12
Anticipated expiration: 2011-11-06
Also published as: GB9123526D0; NL9201921A; GB2261247B

Abstract

A multiple glazing panel comprises sheets of glass (1, 5) held in spaced relation by the webs (8) of at least one metal spacer member intervening between and soldered to the or each pair of successive glass sheets. The spacer members are soldered to the margins of the glass sheets via strips (3) of conductive enamel deposited on those margins. The evacuated inter-sheet space contains one or more additional spacer members (10) arranged to maintain the inter-sheet spacing against pressure differentials across the glass sheets. The multiple glazing panel affords very good thermal insulation and can be used in circumstances where a typical known panel would not be suitable. <IMAGE>

Description

MULTIPLE GLAZING PANEL ESPECIALLY FOR SOLAR COLLECTORS This invention relates to a multiple glazing panel comprising sheets of glass held in spaced relation by the webs of at least one metal spacer member intervening between and soldered to the or each pair of successive glass sheets.

Such glazing panels are well known and are typically used for glazing window openings of buildings and for other purposes where a greater degree of heat insulation is required than can be afforded by a single glass sheet.

Typically, such units are manufactured by metallizing the margins of glass sheets, usually with a layer of copper, and soldering spacer members to such metallized margins.

We have found, however, that such a manufacturing process results in the formation of a panel which has unsatisfactory properties when used in certain circumstances, particularly where a high degree of thermal insulation is required.

It is an object of this invention to provide a new form of multiple glazing panel which can afford very good thermal insulation, and which, at least in some embodiments, can be used in circumstances where a typical known panel would not be suitable.

According to this invention, there is provided a multiple glazing panel comprising sheets of glass held in spaced relation by the webs of at least one metal spacer member intervening between and soldered to the or each pair of successive glass sheets, characterised in that the spacer members are soldered to the margins of the glass sheets via strips of conductive enamel deposited on those margins, in that the or at least one inter-sheet space is evacuated, and in that such evacuated inter-sheet space contains one or more additional spacer members arranged to maintain the inter-sheet spacing against pressure differentials across the glass sheets.

Because such a panel has an evacuated inter-sheet space, thermal transfer across the panel due to gas convection currents is substantially avoided and accordingly the panel has good insulating properties. Furthermore, such good insulating properties are preserved because of the very good and long-lasting seal provided by soldering the spacer member(s) to the enamel strips on the glass sheets. We have found that applying an enamel strip onto a glass sheet does not entail the formation of micro-fissures in the glass such as tends to be provoked by spraying a metallizing coating onto a glass sheet as is done in the manufacture of soldered multiple glazing panels of classical construction. Also, such an enamel deposit tends to be less porous than a metallized coating.The advantage of using enamel strips, as opposed to a metallized strip, is that the bond between the enamel and the glass is stronger, and is better able to withstand stresses to which the panel will be subjected in use, with the result that the seal, and thus low gas pressure in an evacuated inter-sheet space, may be preserved for longer. Such stresses may be due for example to fluctuations in atmospheric pressure, to wind, and diurnal or other temperature variations to which the panel may be subjected.

Furthermore, such stresses on the spacer/sheet joints are to some extent relieved by the presence of the additional spacer members within the evacuated inter-sheet space. As stated, such additional spacers maintain the inter-sheet spacing, with the result that dishing flexure of the sheets due to evacuation of the space between them is substantially avoided as is that cause of stress on the joints between the metal spacer and the glass. Also, any additional temporary forces, for example due to gusts of wind to which the panel may be subjected, will not stress those joints so much as they would be stressed in the absence of the additional spacer(s). This is because any such stress will be transmitted through the additional spacer(s) so that the relative orientation of the two sheets of glass will remain substantially unaltered with respect to the marginal metal spacer.In a particular case, a wind gust might cause a panel to dish so that it was displaced through several minutes of arc at its margin. But both sheets would be similarly dished so that the marginal spacer holding them apart would be less stressed in flexure than if one sheet could be dished independently of the other, and stresses on the spacer joints would be reduced.

In the most preferred embodiments of the invention, the or at least one said additional spacer member is in the form of a block of aerogel.

Aerogel is an oxide-based foam material which contains pores in up to 98W by volume. Aerogel is thus of low density so it does not add greatly to the weight of a panel, and it also has a very low thermal conductivity so that the panel has good insulating properties. Provided the aerogel is kept dry, blocks of that material can have ample structural strength in compression to act as spacer members for the purposes in view. The compression resistance of aerogel varies according to its density which may range from about 0.08 g/cm3 to about 0.4 g/cm3: for a density of 0.1 g/cm3 the resistance to compression is about 0.015 MPa, for a density of 0.2 g/cm3 the resistance to compression is about 0.15 MPa, and for a density of 0.3 g/cm3 the resistance to compression is about 0.3 MPa. Aerogel typically has a Young's modulus of the order of 106 to 107 N/m2.It is important, however, to protect the aerogel against humidity since its structure is highly vulnerable to attack by moisture. This is achieved by securing the spacer members to the margins of the glass sheets via strips of conductive enamel so as to seal the inter-sheet space containing the aerogel spacer member(s).

Aerogels may be formed from oxides of various materials, for example aluminium, zirconium, tin and tungsten, but perhaps the most common aerogel is formed from silica. Of course, an aerogel may incorporate a mixture of oxides, for example a silica aerogel may have additions of other oxides, for example of aluminium, tellurium, germanium or other materials so as to confer special desired properties on the aerogel to be formed.

Silica-based aerogels can be made in the form of blocks, for example up to 3 cm thick which have quite good transparency despite the foam structure of the aerogel. Of course the edges of the blocks would be visible in the finished panel. If it is desired to avoid that, then such inter-sheet space may be substantially filled with aerogel in the form of a single slab.

If it is envisaged that the panel will in use be subjected to rather high temperatures, then it will clearly be desirable to avoid the use of a thermally-degradable organic adhesive for holding a said additional spacer member in position within its inter-sheet space. Reliance may be had on a suitable sizing of such additional spacer so that it is simply held in place by the forces exerted on it by its neighbouring glass sheets due to the evacuation of the inter-sheet space. This also has the advantage of not unduly increasing intimacy of the contact between the additional spacer and the glass sheets such as might have a deleterious effect on the thermal insulation properties of the panel. If it is desired to form a bond between such an additional spacer and the glass, however, a said additional spacer member, e.g.

of silica-based aerogel, may be bonded to a glass sheet by the use of a solvent-free fusible adhesive material such as a powdered silicone resin. Alternatively, such aerogel may be formed in situ on a said glass sheet so that it is bonded directly thereto without the need for any organic adhesive: this could also help to reduce problems in handling of the aerogel prior to its incorporation into the panel.

Preferably, said conductive enamel is given a preliminary coating of solder. We have found that a consistent high-quality solder joint is more easily achieved, especially in series manufacture, when a said spacer member is soldered to a conductive enamel strip which has been given a preliminary coating of solder.

Of course secure attachment of the spacer members to the glass sheets by soldering will only last as long as the panel is not exposed to temperatures in excess of the melting point (solidus temperature) of the solder used. A particular use envisaged for panels in accordance with this invention is as insulating covers for solar collectors. A solar collector incorporating a panel according to this invention could reach a temperature of as much as 1800C. Solders conventionally used in the multiple glazing industry have a melting point of about 1700C. It is preferred that the solder used should have a melting point of at least 2000C. So-called silver solders may be used.

It will be noted that when used as a cover for a solar collector, a panel in accordance with this invention may undergo a diurnal temperature cycle which is much greater than that to which a conventional double-glazing panel in a window of a building is subjected during its working life. Indeed a panel according to this invention used in that way may be subjected to a diurnal thermal stress cycle which is greater than that to which a conventional double-glazing panel is subjected even during manufacture. In this context especially, the use of enamel strips deposited on the glass sheets affords particularly important benefits in terms of the useful life of the panel.

It will also be appreciated that the metal of the spacer members must be able to withstand the temperatures to which the panel will be exposed. Preferably, the metal of the or each said spacer member has a melting point greater than 2500C.

Preferably, the metal of the or each said spacer member comprises copper or a copper-based alloy. Such metals have adequate corrosion resistance, melting points which are satisfactory for the purposes in view, and good compressive strength so that they can be made thin enough to have enough flexibility to accommodate flexural stresses and so relieve their joints to the glass from stress.

Advantageously, the or each said spacer member comprises a web extending between a pair of substantially parallel flanges and is secured to the enamel strips by those flanges. This makes a greater area of spacer member available for bonding to the enamel, and thus promotes the reliable formation of a secure and lasting bond.

In some such preferred embodiments of the invention, the web and flanges of the or each such spacer member are made of different metals. This allows the metals used for forming the web and flanges to be selected with a view to the particular function which that part of the spacer member must perform.

In particular, it allows the metals to be selected so that the metal of which such a flange is made has a coefficient of thermal linear expansion which lies between those of the metal of the web and of the conductive enamel to which such flange is secured. This has the advantage of distributing any stresses due to differential thermal expansion-or contraction so that such stresses are more easily accommodated without rupturing the joint.

Preferably, the or each inter-sheet space of the panel includes a desiccant. This helps to keep the inter-sheet space dry, so reducing any risk of internal corrosion of the metal spacer member(s) and reducing any risk of condensation within the panel, and where aerogel additional spacers are used, it also helps to keep such aerogel dry.

In some preferred embodiments of the invention, at least one glass sheet face bears a coating which modifies the electro-magnetic radiation transmitting characteristics of that sheet. In such embodiments, the use of a conductive enamel strip has the further advantage over a metallized strip that the enamel can be deposited on top of the coating. A copper metallizing strip cannot be deposited on certain coatings which are especially in view because it will not adhere properly: the use of a conductive enamel strip saves having to remove such a coating over the strip area. A said coating may be used to confer any of several properties on the coated sheet as desired.

In some such embodiments, there is a said coating which is such as to reduce the emissivity of the coated face in respect of infra-red radiation having wavelengths greater than 3000 nm. This provides an enhanced greenhouse effect because little of the energy of the solar spectrum is emitted at such wavelengths while a very high proportion of the energy radiated by bodies which are not very hot is of such wavelengths.

Alternatively, or in addition, it is preferred that there is a said coating which is such as to reduce the reflectivity of the coated face in respect of total solar radiation. This enhances the proportion of total solar radiation which will be transmitted by the panel.

In some preferred embodiments of the invention, each said glass sheet is a sheet of glass which has an absorption in respect of total solar radiant energy of at most 5.5%, calculated for a sheet 4mm thick. This too enhances the proportion of total solar radiation which will be transmitted by the panel. This may be achieved by using a low-absorption glass of a special composition. Ordinary soda-lime glass has an absorption in respect of total solar radiant energy of about 10% for a sheet 4 mm in thickness.

The invention extends to a solar collector comprising a heat absorbing medium covered by a panel according to any of the claims hereof.

Preferred embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which: Figure 1 is a detail cross-section through an embodiment of panel according to this invention, and Figure 2 shows a detail cross-section through a solar collector incorporating a second embodiment of such a panel.

Figure 1 shows a finished panel intended for use for covering a solar collector, and it is shown the way up it would be installed in use.

A first glass sheet 1 carries on its upper face, the face which would be directed towards the sun in use, an optional coating 2 designed to reduce the total energy reflectivity of that face in respect of solar radiation. Such a coating may for example be a magnesium fluoride coating deposited to a 1/4 wavelength thickness, or it may be a multi-layer coating.

Around the margin of its uncoated face, there is deposited a layer of a conductive enamel 3, which is coated with a tinning layer 4 of a solder having a melting point in excess of 2000C. A suitable solder alloy with a melting point of 2210C contains 96% tin and 4% silver by weight.

Suitable enamel compositions are known per se, for example as used for forming the conductive elements of heatable windows for road vehicles. One suitable composition for the enamel is sold by Degussa AG of Frankfurt, Germany under their designation "SP 627". That conductive enamel contains about 65% silver.

A second sheet of glass 5 bearing an optional coating 6 of doped tin oxide which reduces the emissivity of the coated face in respect of infra-red radiation having wavelengths greater than 3000 nm also carries a marginal strip of conductive enamel 3 which is again tinned with hard solder 4. That second enamel strip 3 is also deposited onto the non-coated face of its respective sheet.

A metal spacer member is laid over the tinned enamel strip on the second glass sheet 5 and is spot-soldered along its length to hold it in place. As illustrated, the spacer member consists of two flanges 7 which are attached to a web 8 by metal joints 9. Such joints 9 may be made by welding, brazing or hard soldering. The web 8 is suitably of copper which may be plated with a tin/lead alloy to increase its corrosion resistance. The flanges 7 are suitably formed of a metal which has a coefficient of linear thermal expansion which is intermediate the respective coefficients of linear thermal expansion of copper and the enamel used.

The open-topped box formed by the second sheet 5 and the spacer member is then substantially filled with one or more blocks of aerogel 10 which will act as additional spacer means after assembly of the panel, and the first sheet 1 is then laid up to the second sheet 5 so that the tinned enamel strip is in contact with the exposed flange 7 of the spacer member, and the margins of the assembly are heated to melt the solder 4 to bond the panel together. This heating is conveniently done by inducing eddy currents in the conductive enamel, the spacer member and the solder layers 4.

Greater detail about a suitable soldering technique is to be found in British Patent Specification No 2 122 057.

After bonding and cooling of the panel, its interior space is evacuated to an internal pressure of between 10 Pa and 10 kPa. Any tendency for the glass sheets of the panel to become dished due to pressure differences across them is resisted by the aerogel 10 which is a tight fit within the inter-sheet space. The tightness of the fit also serves to hold the aerogel 10 stationary within the inter-sheet space.

In a specific practical embodiment, the aerogel layer 10 has a thickness of 2 cm. Such a panel can have total solar energy transmissivity of at least 70% and a K coefficient of thermal transmissivity of less than 0.6 W.m-2.K-1.

In a variant, a perforated tube containing desiccant is secured to a spacer web 8 within the inter-sheet space along one margin of the panel.

In a second variant, the aerogel is formed in situ on one of the glass sheets, again as a tight fit so that it does not move about within the eventual inter-sheet space.

In a third variant, the glass used is a soda-lime glass which has an iron content of less than 0.04 % calculated as Fe203 so that it has an absorption of solar radiation of about 2.2 %, calculated for a sheet 4mm thick.

In a fourth variant, the sheets of glass are thermally tempered, for example by subjecting them to a suitable cooling schedule after firing of the enamel strips 3.

In a fifth variant, an optional protective element is provided around the edges of the panel. Such a protective element may be formed by reactive injection moulding to fill the channel of the spacer member 7, 8 and to provide lips extending over the margins of the glass sheets 1, 5 around their peripheries.

Figure 2 shows a second embodiment of panel according to the invention mounted above a solar collector.

In Figure 2, the panel comprises two sheets 11, 12 of thermally tempered, low-iron, glass, each having a thickness of 4 mm, which are bonded together by a spacer member 13 soldered to enamel strips 14 previously deposited on the margins of the glass sheets. The second glass sheet 12 bears an optional low emissivity tin oxide coating 15 onto which the associated enamel strip is deposited. In this embodiment, the enamel is formed from a paste containing, by weight, 50% silver, 3% glass frit (lead borosilicate), 41W volatiles (perpineol and dibutylphthalate), and 6% ethylcellulose.

As illustrated, the spacer 13 is a unitary member of channel section. In a variant of this or the previous embodiment, the spacer member simply consists of a flat web extending between the sheets of glass.

Before assembly of the panel, a plurality of spaced blocks of aerogel such as that shown at 16 are distributed over the surface of the second glass sheet. The aerogel used for forming the blocks has a density of about 0.2 g/cm3 giving it a resistance to compression of about 0.15 MPa, and the size and spacing of the blocks is selected having regard to that and to anticipated stresses in the finished panel.

After soldering and cooling this panel is also evacuated and the resulting pressure differential across the glass sheets 11, 12 presses those sheets against the aerogel blocks which are thus held in place and act as spacers resisting further dishing of the sheets. Deformation of the sheets due to such pressure differentials may be held to a minimum by appropriate sizing of the aerogel blocks 16 and the spacer 13. The thickness of the aerogel blocks is adapted to the width of the web of the spacer, or vice versa, so that the aerogel blocks are a close fit between the glass sheets.

The finished panel is located in a frame 17 by which it is held in spaced relationship above the energy absorbing surface 18 of a solar collector.

Packing strips of rock wool or glass wool may be interposed between the frame 17 and the panel it order to protect the edges of the panel from chipping and in order to accommodate differences in thermal expansion if desired.

Claims

1. A multiple glazing panel comprising sheets of glass held in spaced relation by the webs of at least one metal spacer member intervening between and soldered to the or each pair of successive glass sheets, characterised in that the spacer members are soldered to the margins of the glass sheets via strips of conductive enamel deposited on those margins, in that the or at least one inter-sheet space is evacuated, and in that such evacuated inter-sheet space contains one or more additional spacer members arranged to maintain the inter-sheet spacing against pressure differentials across the glass sheets.

2. A panel according to claim 1, wherein the or at least one said additional spacer member is in the form of a block of aerogel.

3. A panel according to any claim 1 or 2, wherein said conductive enamel is given a preliminary coating of solder.

4. A panel according to any preceding claim, wherein the solder used has a melting point of at least 2000C.

5. A panel according to any preceding claim, wherein the metal of the or each said spacer member has a melting point greater than 2500C.

6. A panel according to any preceding claim, wherein the metal of the or each said spacer member comprises copper or a copper-based alloy.

7. A panel according to any preceding claim, wherein the or each said spacer member comprises a web extending between a pair of substantially parallel flanges and is secured to the enamel strips by those flanges.

8. A panel according to claim 7, wherein the web and the flanges of the or each such spacer member are made of different metals.

9. A panel according to claim 8, wherein the metal of which such a flange is made has a coefficient of thermal linear expansion which lies between those of the metal of the web and of the conductive enamel to which such flange is secured.

10. A panel according to any preceding claim, wherein the or each inter-sheet space of the panel includes a desiccant.

11. A panel according to any preceding claim, wherein at least one glass sheet face bears a coating which modifies the electro-magnetic radiation transmitting characteristics of that sheet.

12. A panel according to claim 11, wherein there is a said coating which is such as to reduce the emissivity of the coated face in respect of infra-red radiation having wavelengths greater than 3000 nm.

13. A panel according to claim 11 or 12, wherein there is a said coating which is such as to reduce the reflectivity of the coated face in respect of total solar radiation.

14. A panel according to any preceding claim, wherein each said glass sheet is a sheet of glass which has an absorption in respect of total solar radiant energy of at most 5.5%, calculated for a sheet 4mm thick.

15. A solar collector comprising a heat absorbing medium covered by a panel according to any preceding claim.