GB1584764A

GB1584764A - Solar collector

Info

Publication number: GB1584764A
Application number: GB22520/77A
Authority: GB
Original assignee: BFG Glassgroup GIE
Current assignee: BFG Glassgroup GIE
Priority date: 1977-05-27
Filing date: 1977-05-27
Publication date: 1981-02-18
Also published as: FR2392336A1; BE867147A

Description

(54) SOLAR COLLECTOR (71) We, BFG GLASSGROUP, a Groupement d'Interet Economique, established under the laws of France (French Ordinance dated 23rd September 1967) of Rue Caumartin 43, Paris, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to solar collectors.

Various types of solar collectors are known and have the form of a box having a radiation transmitting cover sheet and containing an absorber.

It is an object of the present invention to provide an alternative construction of solar collector which can be manufactured easily using simple components.

According to the present invention, there is provided a solar collector comprising at least one radiation transmitting sheet, a second sheet, and one or more channel frame members which envelope the edges and marginal zones of the sheets, the sheets being permanently and moisture-tightly secured together at their margins to form a hollow panel, said panel having an inlet and an outlet for fluid medium and containing a radiation absorber spaced from said sheets, a thermally insulating layer between said absorber and said second sheet and means for conducting fluid medium between said inlet and said outlet along a path which is spaced and sealed from said radiation transmitting sheet or sheets and is in heat exchange relationship with said absorber.

Such a collector can be manufactured easily using widely available components of simple construction.

By virtue of the fact that the fluid medium flows along a path sealed from said radiation transmitting sheet or sheets, and because the panel is sealed against the ingress of atmo spheric moisture, the interior surface of the radiation transmitting sheet will remain pristine for a longer time so that the efficiency of the panel will remain at its original level for longer.

The use of one or more interconnecting channel frame members which envelope the edges and marginal zones of the sheet affords protection to the edges of the glass sheet. Said channel frame member or members can thus be used to provide the moisture tight sealing of said panel.

One or more of said sheets may have marginal lips and the sheets be directly sealed to one another, but it 'is preferable to seal flat sheets to one or more interconnecting members which ensure(s) the required spacing, since this simplifies manufacture.

In some preferred embodiments of the invention the or each said interconnecting member intervenes between the sheets. Said interconnecting member(s) is or are preferably of metal.

It is preferred that said sheets are moisturetightly sealed to said channel frame member(s) using a non-hardening adhesive material, such as a silicone mastic strip. In this way, the panel can be securely fixed, for example to the roof of a building, by anchoring the frame and any thermal expansion and contraction of the panel can be accommodated by the flexibility of the non-hardening adhesive material.

It is not necessary that this material should always provide a moisture tight seal, since the sheets may be sealed to one or more intervening interconnecting members.

Said second sheet may for example be a glass sheet, but in order to increase the robustness of the panel it is preferred that said second sheet should be of metal.

It is advantageous to use a said second - sheet of iron or steel which has been galvanised in order to provide the dual benefits of low cost and weather resistance.

Said sealing may be effected by soldering, but it is preferable to use an adhesive, There are many suitable adhesives which can be used, either alone or in combination.

It is preferred to use a polysulphide adhesive in combination with a butyl adhesive. Polysulphide adhesives adhere well to metals and glass, and butyl adhesives have the advantage of being substantially water impervious.

The insulating material used may be selected from a very wide range of substances. For example it may be of glass fibre. Advantageously however, said insulating material is a body of cellular material, preferably a foam plastics material, for example polyurethane.

The use of a body of insulating material facilitates assembly of the panel. As an alternative cellular material, expanded glass may be used.

The walls of an interconnected system of individual pipes may be used to absorb solar radiation directly, but preferably, there is a single said radiation absorber comprising a plate. For example, such a plate may be made of copper or steel and be provided with a network of tubes defining the fluid flow path.

Such an absorber plate can occupy a greater area of the panel and thus give a greater energy output for a given incident energy flux.

The absorber may be covered with a coating which enhances its radiation absorbing properties over one or more portions or the whole of the solar spectrum. Such a coating is preferably baked on so that it will last longer.

Advantageously, said plate forms one wall of an element having conduits therein for the circulation of said fluid medium. This simplifies construction.

It is especially suitable to use as absorber and circulation system a radiator of the plate type. As is well known, such radiators comprise two sheets of metal of which at least one is deformed so that when the sheets are assembled together, such deformations define one or more passages through which fluid may circulate. Such radiators may be obtained by deforming one or both sheets after they have been bonded together by making use of the known roll-bonding technique.

Such plates may for example be of steel or copper.

In order to enhance the ability of the collector to withstand stresses due to thermal expansion or contraction said means for conduction fluid medium between the fluid inlet and the fluid outlet may comprise one or more and preferably at least two expansion joints.

Water, with or without an anti-freeze additive, may be used as thermal transfer fluid.

Any anti-freeze and/or anti-corrosion additive used should of course comply with legal requirements for example regarding toxicity.

Said radiation transmitting sheet or sheets may be made from any transparent glass, for example ordinary soda-lime glass.

Preferably the glass sheet or at least the outer glass sheet if there is more than one is tempered, and preferably chemically tempered.

This increases its resistance to breakage.

Preferably, there is a said radiation trans mitting sheet which is of glass and bears a coating which increases the reflectivity of the sheet in respect of radiation having wavelengths greater than 3000 nm. In this way the proportion of infra-red radiation emitted by the ab sorber which is transmitted back through the glass sheet to the outside of the panel is re duced, giving an augmented greenhouse effect.

This coating is preferably on a sheet face which faces the absorber.

Advantageously there is a said radiation transmitting sheet which is of glass and bears a coating which increases its transmissivity in respect of radiation having wavelengths between 400 nm and 2000 nm. In fact, only a very small proportion of energy is dissipated by the sun at wavelengths greater than 2000 nm, and the adoption of this feature will re duce the proportion of solar radiation which is reflected by the glass and thus increase the efficiency of the heating panel.

In some preferred embodiments of the in vention there is a said radiation transmitting sheet which is of glass of a type which has an inherently higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.

One way of increasing the transmissivity of glass in respect of solar radiation is to re duce its content of ferrous oxide. Ferrous oxide has a deleterious effect on this property.

The ferrous oxide content can be reduced in two main ways, firstly by ensuring that any iron present is in its trivalent (ferric oxide) form, and secondly by using a glass of a special composition in which very little iron is present.

The first of these ways may be accomplished by melting the glass in an oxidizing atmosphere or by incorporating, in the batch, an easily decomposable oxide such as an oxide of manganese and/or cerium. The total weight of manganese oxide and/or cerium oxide may be between 0.1 and 1.0% of the weight of the batch, and preferably the batch contains between 0.1 and 0.5% (by weight) of man ganese oxide calculated in the form MnO2.

This helps to ensure that substantially all iron oxide present in the glass is in the form of ferric oxide. Preferably there is less than 0.12% by weight ferric oxide in the glass, for example one suitable glass composition includes by weight: 0.27% MnO2; 0.1% Fe,O3; 0.06% CeO2.

One suitable special compositions for a low iron glass is as folows (patrs by weight): SiO2 72 Na2O + K20 14 CaO 8 MgO 4 Altos 1.7 Fe2O3 0.01 SO3 0.3 Advantageously said panel comprises two radiation transmitting glass sheets of which the inner sheet bears a said coating which increases its reflectivity in respect of radiation having wavelengths greater than 3000 nm and the outer sheet has a higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.

Preferably the panel contains a desiccant.

This avoids the usual need to flush the panel with dry gas after assembly to expel moisture.

Advantageously the desiccant is located between the absorber(s) and the insulating material.

Preferably, the desiccant used is a molecular sieve or silica gel.

The invention will now be described with reference to the accompanying diagrammatic drawings of which Figures 1 and 2 are crosssectional views of preferred embodiment of solar collector in accordance with the invention.

In Figure 1, a fluid circulating system 1 having inlet and outlet conduits 2, 3 and a generally flat blackened energy absorbing surface 4 is mounted in a bed of insulating material 5 within a sealed hollow panel indicated at 6.

The hollow panel 6 comprises a corrosion resistant, metal sheet 7 and a glass cover sheet 8, and these are held together by a channel frame component 9 having flanges 10 which overlap marginal zones of the metal and glass sheets 7, 8.

The required spacing between these sheets is ensured by a spacer 11 which intervenes between the sheet margins. The spacer 11 is secured to the margins of the metal and glass sheets 7, 8 by bodies of adhesive respectively indicated at 12, 13 to ensure moisture-tightness of the panel.

Bodies of desiccant 14 are located between the water circulating system 1 and the insulating material 5.

If desired, an optional anti-reflection coating 15 indicated in dotted lines may be applied to the outer surface of the glass sheet 8. This will increase the transmissivity of the glass sheet. Alternatively or in addition, a far infrared (wavelength greater than 3000 nm) reflecting coating may be applied to the inner surface of the sheet 8. Such a coating is indicated in dotted lines at 16, and it will operate to reflect back into the absorbing surface 4 infra-red radiation which the absorber will emit, thus increasing the efficency of the unit.

In a specific practical example, the energy absorbing surface 4 of the circulating system measured 1 m by-2 m and the hollow panel 6 was 7 cm thick. The glass cover sheet 8 was uncoated and of ordinary soda-lime glass 3 to 4 mm thick. This sheet was spaced 2.5 cm from the energy absorbing surface 4. The walls of the circulating system 1 were made thin, 0.6 mm to 0.8 mm, so as to keep the thermal inertia of the system low. The desiccant 14 used was silica gel, and the insulating material was a 3 cm thick panel of polyurethane foam. The metal sheet 7 was a sheet of galvanised mild sheet 0.7 mm thick.

The adhesive 12, 13 used -was a polysulphide THIOKOL (Trade Mark) type, which was protected from the ingress of moisture by a layer of butyl (not shown).

This collector when compared with the previously known collectors has the advantage of a more favourable cost-effectiveness, and it will retain its efficiency for long periods of time.

In a variant embodiment, in order to increase the transmissivity of the glass sheet 8 in respect of solar radiation, a thin coating 15 was applied to what was to form the outer surface of that sheet when installed in the panel. The coating material used was magnesium fluoride. An alternative and very inexpensive -way to increase the transmissivity of the glass is to etch its surface.

In another variant, the interior surface of the glass cover sheet 8 was provided with a coat ing 16 to reflect far infrared radiation. SnO2 doped with fluorine ions was deposited on the glass sheet to a thickness of between 100 and 500 nm by the well known pyrolysis method.

Another very suitable material for forming such a coating is indium oxide.

In a further variant, the cover sheet 8 was of a glass having a transmissivity for solar radiation which is inherently higher than that of ordinary soda-lime glass.

Figure 2 shows an alternative embodiment of solar collector which comprises a sealed hollow panel held in a frame. Many elements of the panel are as was described with reference to Figure 1, and these parts have accordingly'been allocated corresponding reference numerals.

The panel 6 of Figure 1 is modified into a panel 17 by the addition of a second cover sheet 18 of glass held spaced from the first cover sheet 8 by an intervening spacer 19 connected to the cover sheets 8 and 18 by bodies of adhesive material 20, 21 to ensure moisture-tightness of the joints. The second, i.e. outer, cover sheet 18 is made of a sheet of a glass having a low content of ferrous oxide so as to impart to it a transmissivity for solar radiation which is inherently higher than that of ordinary soda lime glass, and it will be noted that the inner cover sheet 8 is provided with a coating 16 of SnO2 as described with reference to Figure 1.

In one specific example which has been constructed the outer cover sheet 18 was made of a glass having the following composition by weight: SiO2 72 Na2O + K20 14 CaO 8 MgO 4 Awl203 1.7 Foe203 0.01 SO, 0.3 It will further be seen from Figure 2 that expansion joints 22, 23 are provided within the panel 17 respectively in the inlet and outlet conduits 2, 3 to accommodate thermal expansion and contraction of the fluid circulating system 1.

The panel 17 is located within a frame 24 by strips 25, 26 of non-hardening mastic material so that thermal expansion and contraction of the panel as a whole can be accommodated.

The frame 24 is provided with flanges 27 whereby it can be anchored for example to the roof of a building.

The expression "ordinary soda-lime glass" is used herein to denote ordinary commercially available window glass. It will be appreciated that the exact composition of such glass varies according to where, and possibly when, it was made, but these variations have remarkably little effect on its optical properties.

The composition of such glass is as follows (proportions by weight): SiO2 68 to 74 % Na2O 12 to 16 % CaO 7 to 14 % K2O 0 to 1 % Al2O8 0.3 to 3 % MgO 0 to 4.5 % ("Les Industries Verrieres - Dunod, Paris 1966).

In addition, such glass contains impurities including iron oxide often in divalent form.

WHAT WE CLAIM IS:- 1. A solar collector comprising at least one radiation transmitting sheet, a second sheet, and one or more channel frame members which envelope the edges and marginal zones of the sheets, the sheets being permanently and moisture-tightly secured together at their margins to form a hollow panel, said panel having an inlet and an outlet for fluid medium and containing a radiation absorber spaced from said sheets, thermally insulating layer between said absorber and said second sheet and means for conducting fluid medium between said inlet and said outlet along a path which is spaced and sealed from said radiation transmitting sheet or sheets and is in heat exchange relationship with said absorber.

2. A collector according to claim 1, characterised in that said sheets are flat and sealed to one or more interconnecting members which ensure the required spacing.

3. A collector according to claim 2, characterised in that the or each said interconnecting member intervenes between the sheets.

4. A collector according to claim 2 or 3, characterised in that said interconnecting member(s) is or are of metal.

5. A collector according to any preceding claim, characterised in that said sheets are moisture-tightly sealed to said channel frame member(s) using a non-hardening adhesive material.

6. A collector according to any preceding claim, characterised in that said panel is sealed using an adhesive.

7. A collector according to any preceding claim, characterised in that said thermally insulating layer is a body of cellular material.

8. A collector according to any preceding claim, characterised in that there is a single said radiation absorber comprising a plate.

9. A collector according to claim 8, characterised in that said plate forms one wall of an element having conduits therein for the circulation of said fluid medium.

10. A collector according to claim 9, characterised in that said element is a radiator of the plate type.

11. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass and bears a coating which increases the reflectivity of the sheet in respect of radiation having wavelengths greater than 3000 nm.

12. A collector according to claim 11, charatcerised in that said coating is on a sheet face which faces the absorber.

13. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass and bears a coating which increases its transmissivity in respect of radiation having wavelengths between 400 nm and 2000 nm.

14. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass of a type which has an inherently higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.

15. A collector according to claim 11 or 12 and claim 13 or 14, characterised in that said panel comprises two radiation transmitting glass sheets of which the inner sheet bears a said coating which increases its reflectivity in respect of radiation having wavelengths greater than 3000 nm and the outer sheet has a higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.

16. A collector according to any preceding claim, characterised in that said panel incor

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Claims

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expansion joints 22, 23 are provided within the panel 17 respectively in the inlet and outlet conduits 2, 3 to accommodate thermal expansion and contraction of the fluid circulating system 1.

The panel 17 is located within a frame 24 by strips 25, 26 of non-hardening mastic material so that thermal expansion and contraction of the panel as a whole can be accommodated.

The frame 24 is provided with flanges 27 whereby it can be anchored for example to the roof of a building.

The expression "ordinary soda-lime glass" is used herein to denote ordinary commercially available window glass. It will be appreciated that the exact composition of such glass varies according to where, and possibly when, it was made, but these variations have remarkably little effect on its optical properties.

The composition of such glass is as follows (proportions by weight): SiO2 68 to 74 % Na2O 12 to 16 % CaO 7 to 14 % K2O 0 to 1 % Al2O8 0.3 to 3 % MgO 0 to 4.5 % ("Les Industries Verrieres - Dunod, Paris

1966).

In addition, such glass contains impurities including iron oxide often in divalent form.

WHAT WE CLAIM IS:- 1. A solar collector comprising at least one radiation transmitting sheet, a second sheet, and one or more channel frame members which envelope the edges and marginal zones of the sheets, the sheets being permanently and moisture-tightly secured together at their margins to form a hollow panel, said panel having an inlet and an outlet for fluid medium and containing a radiation absorber spaced from said sheets, thermally insulating layer between said absorber and said second sheet and means for conducting fluid medium between said inlet and said outlet along a path which is spaced and sealed from said radiation transmitting sheet or sheets and is in heat exchange relationship with said absorber.
2. A collector according to claim 1, characterised in that said sheets are flat and sealed to one or more interconnecting members which ensure the required spacing.
3. A collector according to claim 2, characterised in that the or each said interconnecting member intervenes between the sheets.
4. A collector according to claim 2 or 3, characterised in that said interconnecting member(s) is or are of metal.
5. A collector according to any preceding claim, characterised in that said sheets are moisture-tightly sealed to said channel frame member(s) using a non-hardening adhesive material.
6. A collector according to any preceding claim, characterised in that said panel is sealed using an adhesive.
7. A collector according to any preceding claim, characterised in that said thermally insulating layer is a body of cellular material.
8. A collector according to any preceding claim, characterised in that there is a single said radiation absorber comprising a plate.
9. A collector according to claim 8, characterised in that said plate forms one wall of an element having conduits therein for the circulation of said fluid medium.
10. A collector according to claim 9, characterised in that said element is a radiator of the plate type.
11. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass and bears a coating which increases the reflectivity of the sheet in respect of radiation having wavelengths greater than 3000 nm.
12. A collector according to claim 11, charatcerised in that said coating is on a sheet face which faces the absorber.
13. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass and bears a coating which increases its transmissivity in respect of radiation having wavelengths between 400 nm and 2000 nm.
14. A collector according to any preceding claim, characterised in that there is a said radiation transmitting sheet which is of glass of a type which has an inherently higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.
15. A collector according to claim 11 or 12 and claim 13 or 14, characterised in that said panel comprises two radiation transmitting glass sheets of which the inner sheet bears a said coating which increases its reflectivity in respect of radiation having wavelengths greater than 3000 nm and the outer sheet has a higher total energy transmissivity in the solar spectrum than ordinary soda-lime glass.
16. A collector according to any preceding claim, characterised in that said panel incor

porates a desiccant located between the absorber and the insulating material.
17. A solar collector substantially as herein described with reference to either of the accompanying drawings.