Vacuum solar thermal panel with a vacuum tight glass-metal sealing

The present invention relates to a vacuum solar collector and a method for producing said vacuum solar collector according to the preamble of the main claims.

[0002] Vacuum solar tube collectors are normally obtained by connecting multiple heat pipes in parallel, with heat absorber fins attached to them sealed in separate pumped glass pipes.

[0003] The disadvantage of this model lies in the significant dead space between the heat absorbers as well as in a significant part of the heat transfer fluid circuits outside of the vacuum insulation.

[0004] To overcome these limitations, flat vacuum solar collectors have been developed incorporating a flat vacuum tight cover with a transparent glass plate for visible solar radiation. Inside the vacuum shroud there are heat absorbers and inlet and outlet pipes from the shroud connected with heat absorbers. Solar radiation enters the sheath through the glass plate, is absorbed by heat absorbers and converted into heat, which is transferred to the conduit and to the thermal fluid flowing in the conduit. A high vacuum is maintained inside the sheath covering the heat absorbers and the connected ducts to prevent heat escaping into the external environment by convection.

[0005] US 4,332,241 and EP 1706678 disclose a vacuum solar collector comprising two parallel glass plates and a metal spacer frame for supporting the glass plates in an arrangement spaced apart. Parts of the surface of the glass plates have a metal coating to allow soldering to the metal spacer frame, thus providing a vacuum tight seal between the glass plates and the metal spacer frame. In addition, the spacer frame preferably includes deformable rods or bands made of lead or soft metal for soldering to the metal coating of glass plates to reduce the stress induced in the glass-metal gasket by differences in thermal expansion and pressure. GB 2259732 typically discloses a thermal insulation assembly with two parallel plates and a flexible peripheral seal, preferably made of silicone rubber or polysulphide, to allow the plates to move relative to each other due to the thermal expansion of the gas contained within the assembly.

[0006] Both of these technologies have severe internal limitations. Most soft metals (i.e. lead) are toxic and their use is becoming increasingly limited. Metallization of glass is based on surface coatings that can deteriorate much faster than bulk materials due to the fact that they extend only to a few layers of atoms. On the other hand, adhesives, silicone rubber or polysulphide allow gas to pass over time due to their organic components, thus preventing their use for long-term applications in high vacuum.

[0007] FR249956 discloses a vacuum solar collector comprising an upper and lower glass plate spaced apart and supported by metal or glass side walls whose edges are

EP 2 283 282 B1 sealed to the periphery of said panels using sintered pyroceramic paste.

[0008] US4493940 describes a solar collector in which the rigid side walls are sealed to the upper glass plate by means of sintered paste.

[0009] US4095428 describes a solar power plant comprising thermal collectors comprising an upper glass plate that transmits incident solar radiation to a steel substrate provided on its bottom side with a liquid passage for heat transfer fluid along the bottom surface of the substrate. A layer of fused silicon powder is placed on the upper surface of the steel substrate, held on the steel substrate by a thermally conductive soldering material. A rim formed of a thin steel strip connects the glass plate and the steel substrate, respectively, with a sintered glass paste solution and soldering.

[0010] JP59119137 describes a method of sealing a tubular metal body with a glass pipe collector. Furthermore, it discloses the production of a glass ring at the boundary of the tubular metal body prior to attaching said body to the glass tube.

[0011] GB1439444 describes a method of joining two pre-shaped glass members by resistive heating of a metal sealing element.

[0012] US4640700 discloses a method of attaching a plug pin to a CRT panel. GB2259732 discloses thermal insulation apparatus for obtaining windows with a flexible non-vacuum tight seal. US2005 / 181925 discloses a method of making a glass-metal connection to a solar energy collector tube.

[0013] The object of the present invention is to overcome the disadvantages of prior art solar collectors by providing a vacuum solar collector comprising a durable and reliable vacuum cover.

[0014] Another object of the present invention is to reduce the stress exerted on the glass-metal gasket due to atmospheric pressure and differences in thermal expansion of the collector components when they are heated during the firing cycle.

[0015] Another object of the invention is to provide a flat vacuum solar collector with two parallel plates.

[0016] Another object of the invention is to provide a flat vacuum solar collector with increased efficiency at temperatures above 200 ° C.

[0017] A further object of the invention is to provide a method for obtaining such a vacuum solar collector.

[0018] The present invention will become more fully understood and appreciated from the following detailed description considered in conjunction with the accompanying drawings, in which:

Fig. 1 shows a perspective view of a vacuum solar collector according to the invention;

Fig. 2 shows an exploded view of the collector assembly;

Fig. 3 shows a cross section of the perimeter wall of a vacuum solar collector;

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Fig. 4 shows an enlarged cross-section of the perimeter wall of a vacuum solar collector, showing a matched bulk glass-metal gasket having a peripheral belt edge embedded in the glass plate, according to a first embodiment of the invention;

Figures 5 and 6 show an enlarged cross-sectional view of the first and second embodiment of the vacuum solar collector perimeter wall, showing a compressed mass glass-metal gasket having a peripheral belt edge embedded in a sintered glass web connecting the perimeter belt to the glass plate, according to a second embodiment of the invention.

[0019] The vacuum solar collector according to the invention (Figs. 1 and 2) comprises a vacuum cover 30 defining a sealed volume, capable of withstanding atmospheric pressure after pumping, having at least a first plate 1 made of transparent glass for visible solar radiation. The conduit 13 for a thermal fluid with multiple heat absorbers 12 in good thermal contact with the conduit itself is enclosed in a vacuum jacket to avoid heat transfer to the environment by convection. Said conduit 13 enters and exits the vacuum shield 30 through outlet ports 20. Of course, there may be more than one such conduit 13.

[0020] The vacuum shield 30 may have first and second parallel plates 1 and 2, both made of glass, or a first plate 1 made of glass, and a second plate 2 made of metal, kept spaced apart by the housing 18 , placed inside the enclosure 30 between the plates 1 and 2, and the peripheral frame 3. Said housing 18 and the peripheral frame 3 can also support portions of the conduit 13 inside the enclosure 30 and heat absorbers 12 attached thereto.

[0021] As shown in Fig. 1, the solar collector is flat. The collector vacuum cover 30 includes the first and second plates 1 and 2 made of glass and a metal peripheral frame 3. It also includes two metal peripheral strips 4 and 5, each of which connects glass plates 1 and 2 to a metal perimeter frame 3. If the above plates are made of glass, then the solar collector has two active surfaces (two-sided flat collector), one of which collects solar radiation directly from the sun, and the other collects solar radiation reflected by a useful mirror (not shown in the Figure).

[0022] For the first plate 1 made of glass and the second plate 2 made of metal, the solar collector would be one-sided, i.e. it had only one side capable of collecting solar radiation. When the second plate is made of metal, the peripheral frame can be connected directly to the second plate by conventional metal-metal soldering, without the presence of an elastic peripheral strip, or, as shown in Figure 6, said peripheral frame 3A is in one piece with the other 2A metal plate.

[0023] The composition of the glass plate should be chosen to maximize transparency (transmission factor> 0.91).

[0024] Furthermore, those skilled in the art generally know that applying a coating to a glass plate, as it reduces the transparency of the glass, will also reduce the amount of solar energy that goes

To the collector, and thus the efficiency of the collector. According to the invention, the opposite can be achieved. In fact, by applying to the inside of glass panels a coating with low infrared emissions (1C, 2C), even reducing transmission in the visible part of the solar spectrum, the efficiency of the collector is increased at high temperature due to the dominant effect of reducing radiation losses due to the emission of infrared radiation by heat absorbers.

[0025] To obtain this result, the low-emission coating is selected in such a way that the reflection coefficient for the wavelengths between 4 and 6 microns (corresponding to a temperature range of 200-400 ° C) is greater than 0.9 and that the transmission for wavelengths between 0.25 and 1 micron it was more than 0.7. A collector operating for example at 265 ° C would have an infrared emission from a heat absorber with a peak at 5.4 microns.

[0026] It has been found that the coating according to the invention can increase the collector efficiency at 265 ° C by more than 30%.

[0027] It has also been found in accordance with the invention that the efficiency of the collector can be further increased by applying a second coating (1B, 2B) to the outer surface of the glass plates. This second coating is a commonly used anti-reflection coating.

[0028] In accordance with the invention, it has also been found that in the case of a one-sided collector, the efficiency can be further increased by applying a third coating (Fig. 6) to the inner surface of the metal plate to reduce its emission factor. This third coating is, for example, a commonly used galvanically deposited copper layer.

[0029] In addition, the glass should be heat toughened or layered to improve safety and reduce thickness. In the case of heat-tempered soda-lime float glass with high transparency, the thickness of the glass plates should be about 5 mm when housing 18 with supporting structures is considered at intervals of 120 - 160 mm.

[0030] Fig. 2 shows an exploded view of the collector assembly in which its elements are separated for better identification.

[0031] Figs. 3, 4 and 5 show a cross section of the circumferential wall of the vacuum cover, said circumferential wall comprising a circumferential frame 3, supporting the two plates 1 (or 101 in Fig. 5) and 2 in spaced apart, and two peripheral belts 4 (or 104 in Fig. 5) and 5 connected to the peripheral frame 3 using a vacuum-tight metal-metal gasket 6 (or 116 in Fig. 5) and 7 and attached to the first and second glass plates, 1, 101 and 2, by means of a vacuum-tight glass-metal gasket 8 (108 in Fig. 5) and 9.

[0032] Fig. 6 shows a one-sided solar collector comprising a glass plate 1A supported by a frame 21 constituting a single metal part performing the functions of both a metal peripheral frame 3A and a bottom plate 2A, and a housing 18A for supporting the glass plate 1A and the parts of conduit 13 connected thereto and heat absorber 12 (not shown in Figure 6).

[0033] The peripheral frame is connected to the glass plate by a metal perimeter belt 5A as described above. Preferably, an alloy having adjustable extension 4 is used for circumferential belt 4 and 5

It has a 48% Ni (48% Ni / Fe alloy) content due to the close correspondence between the thermal expansion coefficient values ​​for it and for soda-lime glass.

[0034] In the context of the present term, "vacuum-tight" should be understood as follows: a gasket or component is generally considered vacuum-tight if, when testing on a leak detector, a helium peak mass spectrometer shows a leak rate less than ₁₀^-10 cm³ s^-1.

[0035] By the expression "mass glass-metal gasket" 8, 9, 108 is meant a vacuum-tight gasket between the glass plate 1, 2 or 101 and the metal peripheral belt 4, 5 or 104 containing glass 14 or 114 (Fig. 5) covering the edge 16, 116 (Fig. 5) of the metal peripheral belt 4, 5 or 104. The vacuum-tight glass-metal mass gasket 8, 9, 108 is obtained by melting and then solidifying the glass 14, 114 including the edge 16, 116 (Fig. 5) peripheral belt 4, 5, 104, in order to obtain the glass 14, 114 sticking directly to the perimeter belt.

[0036] The vacuum-tight mass glass-metal gasket can be of two types, according to the first or second embodiment of the invention, respectively:

a) it can be a matched glass-metal gasket 8 having a rim 16 of peripheral belt 4 embedded in the glass material 14 due to local melting and subsequent solidification of the glass plates 1, 2 (Fig. 4);

b) it may be a squeeze glass-metal gasket 108 having a perimeter 116 of the peripheral belt 104 embedded in the glass material 114, due to the melting of the sintered glass material connecting the perimeter belt 104 to the glass plate 101 (Fig. 5).

In both embodiments, the glass material 14, 114 adheres directly to the metal peripheral belt 4, 104. In the first embodiment, the glass material 14 forms part of the first plate 1, which is always made of glass, while in the second embodiment the glass material 114 is a slightly added sintered glass forming a meniscus at the edge of the metal peripheral belt 104.

[0038] When the glass plate is heated, it first becomes soft at a certain temperature, and then melts at a higher temperature, becoming liquid or melted.

[0039] In the fitted glass-metal gasket 8 (Fig. 4), embracing the edge of the peripheral belt 4 and the adhesion of the glass material 14 to the metal perimeter belt 4 axis is achieved by means of local melting of the glass plate 1. Local melting of the glass is obtained by heating the perimeter belt together with the glass plate at a temperature close to, but not exceeding, the glass softening point (for soda-lime glass around 720 ° C). Then the circumferential belt 4 is heated further (e.g. induction) above the glass softening temperature (about 800 ° C), inserted into glass plate 1 by about 1-2 mm and finally retracted, allowing molten glass to form a meniscus on both sides of the perimeter belt. Finally, the glass plate 1 is quickly cooled to produce the required stress level according to normal quenching principles.

In a glass-metal compression gasket 108 (Fig. 5), embracing the edge of the metal peripheral belt 104 and adhering to the metal peripheral belt 104 is achieved by melting sintered glass having a much lower softening temperature compared to the temperature

Softening the glass plate, and by connecting the peripheral belt 104 to the glass plate 101, once it has solidified. The glass frit, either amorphous or crystalline, can be selected to have a coefficient of thermal expansion (λ) slightly lower than the values ​​for the components to be sealed, in order to achieve some additional compression of the seal. For soda-lime glass and 48% Ni / Fe alloy, both of which have coefficient of thermal expansion λ = 90 * 10^-7 K^-1, a glass frit having λ about 75 * 10 may be used^-7 K^-1. A compressed glass-metal gasket 104 is obtained by first applying a thick paste, obtained by mixing a glass frit powder with a binder and a suitable solvent (e.g. nitrocellulose and amyl acetate) as a continuous web about 2 mm high and wide on top of the glass plate, then by inserting a perimeter belt into said glass frit paste band and then firing the completed assembly, held together by a useful supporting structure, in a useful furnace.

[0040] In particular, it has been found to be advantageous to dry the glass frit web before inserting the perimeter belt, then placing the glass plate with the dried web upside down and touching the top edge of the perimeter belt, holding it together by a useful support structure, and then firing the completed assembly in an oven . In this way, when the glass frit web reaches its melting point, it flows down along both sides of the peripheral belt, creating such a perfectly symmetrical and homogeneous meniscus for the glass-metal gasket, as shown in Figure 5. Firing the frits should follow a normal cycle thermal, as described in the material safety data sheets (for a typical glass frit, the firing temperature will reach 450 ° C for about 30 minutes). The supporting structure should be made so as to compensate for the difference in thermal expansion in relation to the other components of the assembly.

[0041] This can be achieved, for example, by making a support structure or firing cage from a suitable steel (e.g. AISI430) and dimensioning it so that it reaches the desired dimensions at the melting temperature of the frit and that at this temperature it stretches the metal peripheral belt 5, 5A, 104, and placed such a belt in a desired place relative to the glass plate 2, while keeping it in contact with the molten frit.

[0042] According to a further aspect of the invention, the mass glass-metal gasket can be advantageously improved if at least a metal component embedded in the glass frit is oxidized before forming such a mass glass-metal gasket; wherein said oxidation is preferably such that an approximately uniform and durable oxide layer 4A, 5B having an approximately regular thickness is obtained, and preferably also such as to improve the bond strength of glass to metal by at least 10% when measured in a test shear.

[0043] Said pre-oxidation of the metal component is preferably carried out by heating in an oven at a temperature suitable for growing a uniform oxide layer on the surface of the metal component. Of course, other treatments can be envisaged, including for example heating in an oxygen enriched atmosphere.

[0044] A squeezed bulk glass-metal gasket may also be obtained in a less advantageous manner by first placing the peripheral belt 104 (Fig. 5) on the surface of the glass plate 1, and then adding a glass frit paste web 114 on top of the glass plate 1 on both sides

On the sides of the peripheral belt 104. Said glass frit web in turn melts and solidifies again to form a glass-metal gasket. The latter technique is more complicated than the technique previously described herein, and the presence of the glass frit ribbon on only one side of the perimeter belt could make the glass-metal gasket more fragile.

[0045] Both matched and squeezed glass-metal gaskets 8 and 108 can be strengthened by means of a closure with a useful epoxy resin on one or both sides of the peripheral belt. The epoxy resin for the vacuum side should be selected so that it has very low degassing and good high temperature stability, so that it will later withstand the gas removal cycle by heating during pumping (ie the epoxy resin known under the trade name "Torr Seal by Varian" may be used ).

[0046] The thickness of the peripheral belt 4, 104 should preferably be selected in the range of 0.1 - 1 mm to avoid gaps at atmospheric pressure, while reducing heat conduction during welding, to prevent heating of the glass-metal gasket, which in turn could cause loss of vacuum tightness by such a seal.

[0047] It should be noted that according to the invention the metal peripheral belt is continuous, i.e. welded to form a continuous belt), vacuum-tight, and includes at least one elastic deformable portion that is capable of elastic deformation at least so that the avoidance of mass glass-metal gasket being damaged and ceasing to be vacuum-tight . when it undergoes the process of pumping said cover and the heat treatment of the collector and potential mutual displacements of the glass plate and the metal peripheral belt connected to it.

[0048] It should be noted that the term "heat treatments" of the collector refers to both a glass-metal sealing method and other treatments such as heat treatment to remove gas by heating while pumping the collector at a temperature of over 200 ° C to reduce pressure internal, which also limits the cleaning of the collector due to pyrolysis.

[0049] The above-mentioned at least one part capable of deforming the resilient 10, 110 metal peripheral belt 4, 5; 104 is preferably at least capable of elastic deformation such that it allows said belt to be lengthened by 0.1-0.3 mm relative to an axis perpendicular to the glass plate.

[0050] According to a further aspect of the invention, a metal peripheral belt 4; 104 includes an intermediate part given between the first and second parts of said metal belt, said first part connecting the first glass plate 1; 101 and is a vacuum-tight mass glass-metal gasket 8, 108, said second part of said metal belt 4; 104 connects the peripheral frame 3, and is a vacuum-tight metal-metal seal 6, 116; at least one elastic deformable part is given in said intermediate part.

[0051] According to a further aspect of the invention, at least one portion capable of deforming elastically 10, 110 preferably comprises at least one non-straight portion or at least one at least partially curved portion or at least one notch 10, 110. This

Crimping 10, 110 preferably has a semi-circular form, has a radius between 2 and 4 mm, and extends over the entire length of the peripheral belt.

[0052] When both plates 1 and 2 are made of glass, the perimeter belt is attached to them by means of a mass vacuum-tight glass-metal gasket. If the first plate 1 is made of glass and the second plate 2 is made of metal, a vacuum-tight metalmetal gasket, obtained, for example, by conventional soldering, welding or brazing, can be supplied directly to the peripheral connection of the frame to the metal plate, or as previously described , the perimeter frame and bottom metal plate can be made as a single metal part.

[0053] The solar collector vacuum cover according to the invention also includes a conduit for pump 19, typically made of a copper tube, connected to a vacuum pump (not shown). After pumping off the vacuum shroud, the pump inlet 19 can be sealed by clamping, which is a typical method used in refrigeration systems.

[0054] There should also be an outlet culvert, typically made of a stainless steel tube or bellows 20, leading the heat absorber duct 13 outside the vacuum shroud 30, through the peripheral frame 3, while minimizing heat transfer to them.

[0055] Inside the vacuum shield, a getter (ion-sorption) pump of the known type may also be present in order to continuously pump off any residual gases except for noble gases.

The invention also relates to a method for producing a vacuum solar collector comprising a vacuum envelope defining a sealed volume, capable of withstanding atmospheric pressure after pumping, and having at least a first plate 1, 2, 101 made of glass, a metal perimeter belt 4, 5, 104 , and a vacuum-tight mass glass-metal gasket between the glass plate 1, 2, 101 and the metal peripheral belt 4, 5, 104.

[0057] According to the present invention, the glass material 14, 114 is positioned close to the edge 16, 116 of the peripheral belt 4, 104. Said glass material may form part of said first plate 1 or a slightly added glass frit material 104. Said glass material 14, 114 is heated above its melting point and in turn cooled below said temperature in order to achieve the adhesion of the glass material to the perimeter belt and to connect it to the glass plate 1, while also covering the edge of the perimeter belt. This can be obtained in two ways: said glass material, placed close to the perimeter belt edge, molten and in turn solidified again, it can come from a glass plate or it can come from a glass frit paste band, which, when the perimeter belt is placed on its surface on the glass plate surface 101, is placed on both sides of the peripheral belt 104.

[0058] When the glass forming the mass glass-metal gasket comes from a glass plate 1 (matching glass-metal gasket), the method can be described by the following steps:

- the glass plate 1 is heated to a temperature close to, but not exceeding, its softening point;

- the peripheral belt 4 is heated to a temperature above the softening point of the glass plate 1;

- the edge 16 of the circumferential strip 4 is pressed against the surface of the glass plate 1 to achieve

Local melting and insertion of the edge into the glass plate 1, such that the edge 16 of the peripheral strip 4 is enclosed by the glass 14 of the glass plate 1;

- the peripheral belt 4 retracts from the glass plate 1 to form a meniscus on both sides of the edge 16 of the peripheral belt 4;

- the glass plate 1 and the peripheral belt 4 are cooled to a temperature below the softening point of the glass plate, obtaining a vacuum-tight glass-metal gasket between the glass plate 1 and the metal peripheral belt 4.

[0059] When the glass forming the mass glass-metal gasket comes from a glass frit paste band (a compressed glass-metal gasket), the method can be described by the following steps:

providing a glass frit paste comprising a glass frit material, said glass frit paste being obtained by mixing together a powder of the glass frit material, solvent and binder;

the frit glass paste is placed on top of the surface of the glass plate 101 to form a continuous web;

then there are two possible ways to perform the method:

- the ribbon is dried, then the glass plate 101 with the dried ribbon is placed upside down on the peripheral belt 104, touching the edge 116 of the peripheral belt 104, held in place by a useful supporting structure

- the edge 116 of the peripheral belt 104 is inserted into the glass frit paste web also in contact with the surface of the glass plate 101;

- the glass frit paste is heated and melted to form a meniscus of molten glass frit between the side of the peripheral belt 104 and the surface of the glass plate 101;

- the glass frit is cooled and solidified to obtain a vacuum-tight mass-glass-metal gasket between the glass plate 101 and the metal peripheral belt 104.

[0060] The method of producing a matched glass-metal gasket is advantageous when the thermal stress of the glass plate is required because it can be applied during stressing at virtually no cost, and the method of producing a compressed glass-metal gasket should be used when no thermal stress is anticipated glass plate (i.e. in the case of laminated glass), because it requires a much lower temperature.

[0061] In both cases (matched or squeezed glass-metal gasket) the glass-metal gasket can then be reinforced with a suitable epoxy resin on one or both sides of the peripheral belt as described above.

[0062] One of the advantages of the present invention is that it gives a solar collector with a vacuum-tight casing having a glass-metal gasket very simple to implement, yet very reliable.

[0063] The circumferential belt is made deformable by the presence of crimping. This makes it possible to reduce the stress caused in the glass-metal gasket by the pressure difference between the inside and outside of the casing when it is pumped out and by the differences in the thermal expansion of the collector components during the heat treatment of the collector, in particular gas removal by heating during the pumping cycle at temperatures above 200 ° C.

[0064] A further advantage is that the cover does not use toxic or hazardous materials.

[0065] Finally, it should be noted that many of the inventions described herein can be contained independently of one another in a vacuum solar collector. This applies in particular to the following inventions:

a1) a vacuum solar collector comprising a glass-metal gasket realized according to claim 1, and preferably comprising one or more features described in the appended claims 2-6, a2) a method of producing a vacuum solar collector according to claim 21, and preferably comprising one or more of claims 22 -25, b) a vacuum solar collector comprising the features of the preamble of claim 1, characterized in that that the metal peripheral belt is a continuous and vacuum-tight belt, and includes at least one elastically deformable portion that is capable of elastic deformation at least so that the mass glass-metal gasket is damaged and becomes non-leakproof, when it is subjected to the process of pumping the shell and heat treatment of the collector and potential mutual displacements of the glass plate and the metal peripheral belt connected to it. It should be noted that this belt preferably also includes one or more of the features described in the appended claims 10-13.

c1) a vacuum solar collector containing on the inside of its glass plates (1, 2) an infrared mirror coating and / or on the outside of the glass plate an anti-reflective coating, and / or on the inside of the bottom metal plate and an coating of the infrared mirror. It should be noted that these coatings preferably also contain the features described in the characterizing part of the appended claims 14-16.

c2) a method of increasing the efficiency of a vacuum solar collector, which is characterized by the application of anti-reflection coatings and / or an infrared mirror on external and / or internal surfaces, respectively glass and / or on the inside of the lower metal plate (1, 2, 1A, 2A). It should be noted that this method preferably also includes the steps described in the characterizing part of the appended claims 26-28.

d) a one-sided vacuum solar collector according to the preamble of claim 1, and further comprising a single piece metal frame including both a peripheral frame 3 and a bottom metal plate of the collector,

e) supporting structure or firing cage for the vacuum solar collector according to the preamble of claim 1, preferably made of suitable steel (e.g. AISI430) and dimensioned in such a way that it reaches the desired dimensions at the frit melting point and that at this temperature it extends it has a metal peripheral belt and places such a belt in the desired position relative to the glass plate, while keeping it in contact with the molten frit,

f) a vacuum solar collector having a vacuum-tight glass-metal gasket made using a glass-based frit in which at least a metal component embedded in the frit is oxidized before producing such a mass-glass-metal gasket; at

Wherein said oxidation is preferably such as to produce an approximately homogeneous and durable oxide layer having approximately regular thickness, and preferably also such as to improve the bond strength of glass to metal by at least 10% when measured in a test shear.

[0066] Said pre-oxidation of the metal component is preferably carried out by heating in an oven at a temperature suitable for growing a uniform oxide layer on the surface of the metal component (for a 48% Ni / Fe alloy typically 10 min at 560 ° C). It should further be noted that other treatments may be envisaged, including heating in an oxygen enriched atmosphere.

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