GB2034021A

GB2034021A - Element for Absorbing Solar Radiation

Info

Publication number: GB2034021A
Application number: GB7926815A
Authority: GB
Original assignee: Rhone Poulenc Industries SA
Current assignee: Rhone Poulenc Industries SA
Priority date: 1978-08-02
Filing date: 1979-08-01
Publication date: 1980-05-29
Also published as: NL7905950A; GB2034021B; BE878040A; IL57940A0; DE2930942A1; ES483038A1; IT1122694B; ZA793896B; CH632833A5; IT7924892A0; GR69658B; IL57940A

Abstract

The element comprises: at least one tube (1) having at least one generally rectilinear zone, the said tube being suitable for the circulation of a heat-transfer fluid, and being made of an organic or organosilicon rubber or an organic thermoplastic material, and at least two flexible thermoconducting foils (2, 3) of continuous or discontinuous structure fixed in face-to-face contact with one another, at least one generally rectilinear zone of tube being enclosed between them. <IMAGE>

Description

SPECIFICATION Elements Which Absorb Solar Radiation The present invention relates to an element which absorbs solar radiation and which is intended to equip solar energy collectors.

Numerous solar energy collectors exist which are based on the greenhouse effect, associated with the black body effect. These collectors generally possess hollow plates or tubes which absorb the solar radiation and are placed inside transparent enclosures providing the greenhouse effect. Inside the plates or tubes, heat-transfer fluids circulate, which are intended to transfer the heat from the solar radiation towards cooler zones.

Such solar energy collectors are described, for example, in the French Patent Application No.

2,298,066. The collector described in this application possesses, inside an enclosure, an absorbing element consisting of a metal plate which has a rough surface or is painted black; a tubular metal coil is fixed to the face which is not exposed to the sun. Although such an absorbing element is usually very suitable, it exhibits disadvantages. In fact, it is heavy and rigid and hence bulky, which makes it difficult to install and to transport.

It has therefore been proposed to produce absorbing elements which are not rigid and which can be rolled up in order to facilitate transportation. Such absorbing elements are described in, for example, French Patent Application No. 2,307,233. This absorbing element consists of a flexible material which is shaped so as to form a circuit of channels for the circulation of the heat-transfer fluid. Such an absorbing element can easily be transported because it can be rolled up, but it exhibits the disadvantage that it provides poor resistance to frost or to a pressure of the heat-transfer fluid which is greater than atmospheric pressure; furthermore, the circulation of the heat-transfer fluid undergoes 1 800 changes in direction, and this creates significant pressure losses.

An object of the invention is to provide an element which absorbs solar radiation and can easily be transported and is relatively cheap and easy to position, whilst ensuring good absorption of the solar radiation.

An element which absorbs solar radiation has now been found which is characterised in that it possesses: at least one tube having at least one rectilinear zone, the said tube being suitable for the circulation of the heat-transfer fluid and being made of an organic or organsilicon rubber or an organic thermoplastic product, and at least two flexible thermoconducting foils of continuous or discontinuous structure, which are fixed in pairs, face to face, at least one rectilinear zone of tube being enclosed between them.

The tube used for the circulation of the heattransfer fluid should be made of a material which possesses good resistance to heat-ageing and to corrosion, whilst having adequate thermal conductivity. Furthermore, if the tube is used for conveying tap water, the material should be of a quality suitable for use with foodstuffs and should be resistant to scaling.

For example, the tube can be made of: an organic thermoplastic, such as low-density polyethylene, polyvinyl chloride, a polyamide or polyester, or, an elastomer, such as a natural or synthetic organic rubber or an organosilicon rubber.

The organic rubbers are preferably those which are resistant to heat, such as: butyl rubbers, ethylene propylene rubbers and polyurethane rubbers.

Organosilicon rubbers have good resistance to heat: accordingly those organosilicon rubbers which can easily be extruded and cured are preferred. Preferably, the elastomers used are obtained by hot curing, at above 1000C, compositions mainly containing diorganopolysiloxane gums, inorgnic fillers and organic peroxides. Such compositions are described in, for example, French Specifications Nos. 1,142,443,1,382,285, 1,486,530 and 2,017,663.

It is also possible to use the elastomers obtained by hot curing, at above 600 C, compositions mainly containing diorganopolysiloxane polymers containing a low proportion of vinyl orally radicals, generally 0.1 to 1% relative to all the radicals bonded to the silicon atoms, and, if desired, inorganic fillers, hydrogeno-organopolysiloxanes and a platinum catalyst. Such compositions are described in the French Patents Nos. 1,314,679, 1,360,908, 1,511,598,2,016,914and2,202,915.

The cross-section of the tube can have a simple geometric shape, for example an oval, elliptical or square shape; the cross-section of the tube is preferably circular.

The diameter of the tubes (the term "diameter" used in this context applies to the external diameter) can vary over a wide range of values.

However, if it is desired to manufacture absorbers which can be handled easily and are easy to install, it is advantageous to choose tubes having a diameter of 5 to 50 mm, preferably 7 to 40 mm; the thickness of the wall of the tubes, which thickness is related to this diameter, is usually 0.2 to 4 mm and preferably 0.3 to 3 mm.

If the heat-transfer fluid, which can be a gas, for example air, or a liquid, for example water, circulates in the tube at a pressure greater than atmospheric pressure, or if the liquid can freeze, the tube is preferably strengthened with an outer sheath consisting of a web of threads or fibres.

This web can consist of synthetic threads or fibres, for example made of polyamide or polyester, or of glass fibres or also of wires. The web can be formed either from single threads, such as bristles, or from threads or fibres which have been made into a strand beforehand. In order to form the outer web, the threads or fibres can be wound in a helix around the outside of the tube; they are preferably braided, knitted or woven.

An outer sheath formed by a web of wires advantageously covers, with its wires, from 25 to 85%, preferably from 30 to 75%, of the surface area of the tube. The wires can be made of copper, aluminium or steel of diameter suitably 0.05 to 0.4 mm, preferably 0.08 to 0.35 mm.

The outer sheath can be simultaneously manufactured and deposited on the outer surface of the tube, directly at the outlet of the extruders used to manufacture the tubes. Such a sheath can provide the tubes with a resistance to fluidcirculation pressures of more than 10 bars.

For convenience, reference will only be made to the "tube". Of course, the said tube can be sheathed or unsheathed and is intended to convey the heat-transfer fluid.

The thermoconducting foils are made of a material which is a good heat conductor. They can be made of a plastic which contains, for example, carbon black or metal particles as a filler. They are preferably made of metal; metals such as copper, steel and aluminium are very suitable.

Aluminium is preferably used.

The term "continuous structure" of the thermoconducting foils as used herein, is to be understood as meaning that the structure of the foils does not possess any holes; foils of continuous structure are therefore similar to thick films or thin sheets of metal.

The thickness of the thermoconducting foils of continuous structure must be sufficient to absorb and transfer large heat fluxes and also to withstand the stresses exerted on the absorbing elements during their manufacture and during their packaging and their positioning. Thicknesses of 0.02 to 0.3 mm are generally very suitable.

The term "discontinuous structure" of the thermoconducting folks, as used herein, is to be understood as meaning that the structure of the foils possesses holes; foils of discontinuous structure can therefore consist, for example, of fabrics obtained by weaving, knitting, braiding or twisting, The fabric can be produced with wires, for example made of copper, steel or aluminium, the diameter of which is suitably 0.1 to 0.4 mm, preferably 0.15 to 0.35 mm. The fabric advantageously possesses a close texture so that the surface area of the holes represents at most 60% of the total surface area of the foil.

The thickness of the thermoconducting foils of discontinuous structure is generally greater than that of the thermoconducting foils of continuous structure; in the zones where the wires intersect, it is suitably from 0.2 to 0.8 mm.

The thermoconducting foils are fixed in pairs, face to face, the tube being enclosed in such a way that, in an essentially diametral plane of the tube, the foils extend beyond the latter on either side.

The term "width of thermoconducting foil associated with the tube", as used herein, is to be understood as meaning that width or that part of the width of the thermoconducting foil, measured perpendicular to the axis of the tube, which participates in the exchange of heat with the rectilinear zone of the said tube, on either side of which zone the width is located. This term will be specified in the description of the various embodiments of the absorbing element.

Those skilled in the are will be able to determine the ratio of the width of thermoconducting foil associated with the tube, to the external diameter of the tube, as a function of the thickness of the thermoconducting foils, the diameter of the tube and the amounts of heat to be transferred. In general, the ratio of the width of thermoconducting foil associated with the tube, to the diameter of the tube, is from 2.5 to 10 and preferably from 3.5 to 8.

The face-to-face fixing of the thermoconducting foils of continuous structure can be achieved, for example, by glueing under the action of heat. This technique involves the use of foils which have been coated beforehand, on one face, with a heat-sealing varnish based on customary organic compounds, such as polyethylene, epoxy or acrylic or polyimide resins, or polyamide-imides, or also on silicone.

The coated faces of the foils are placed against one another and the glueing is effected by the application, on either side of the zone in which the tube is housed, of, for example, a roller heated to the heat-sealing temperature of the varnish. This temperature is generally in the range of 100 2200 C.

The two foils can, of course, be fixed by other processes. For example, the foils can be fixed by riveting or also by welding.

The face-to-face fixing of the thermoconducting foils of discontinuous structure is preferably achieved by welding. Thus, employing thermoconducting foils formed of galvanised wires made of copper, aluminium or steel, it is easy, on passing the assembly of tube and foils between two rollers heated to, say 200400 C, and having circular grooves, effectively to fix the two foils by welding on either side of the tube, the latter being located, together with that part of the foil which surrounds it, in the circular grooves in the rollers. If the wires made of copper, aluminium or steel are not galvanised, the foils can be spot-welded using an electric welding set.

In order to produce an absorbing element according to the invention, which possesses a rectilinear zone of tube, enclosed between two thermoconducing foils of braided fabric, it is preferable, instead of introducing the tube between the two thermoconducting foils, to produce, around the tube, a circular braid of which the diameter is substantially greater than that of the tube, for example 1.6 to 6.4 times greater. In order to carry out this production continuously, a multi-thread winder can be used which will produce and lay the braid as the tube moves along.

This winder can comprise several tens of bobbins which generally unwind spindles which group together several threads.

The circular tube/braid assembly is then introduced between the heated rollers, with circular grooves, which were used above. These rollers crush the braid and spread it, in an analogous shape to that of the foils, on either side of the tube. If the copper, aluminium or steel wires of the braid are galvanised, the rollers both effect the crushing of the braid and the face-toface fixing, by welding, of the foils resulting therefrom. If the wires are not galvanised, the rollers only effect the crushing of the braid and it is necesary, as above, to weld the foils subsequently with the aid, for example, of an electric welding set.

Preferably, the outer face of the thermoconducting foil exposed to the sun's rays has a deep colour and a dull appearance. For this purpose, it can be coated with a paint, for example a black, grey, green, red or blue paint, intended to absorb the sun's rays.

The outer face of the absorbing element which is not exposed to the sun can be covered with a layer of a cellular material. The purpose of this layer is to ensure good thermal insulation with respect to the surface which which this face will be in contact after the installation of the absorbing element.

The thickness of the cellular material is not critical; however, in order to avoid a substantial increase in the cost of the absorbing element, it is advisable to use a thickness of 0.3 mm to 40 mm.

By way of indication, the cellular material can be a phenolic foam, polyvinyl chloride foam or polyurethane foam or made of glass wool or rock wool.

For convenience, reference has been made above to two thermoconducting foils, but, of course, absorbing elements possessing one thermoconducting foil exposed to the sun and one thermoconducting foil which is not exposed to the sun, which foils are obtained, for example, by folding a single thermoconducting foil of which the surface area is equal to the sum of the surface areas of the two thermoconducting foils, form part of the invention, and, even in this case, reference will be made to two thermoconducting foils.

The geometric shape of the thermoconducting foils is not critical; thermoconducting foils of simple geometric shape, for example of square or rectangular shape, are preferably chosen.

The invention will now be illustrated, merely by way of example with reference to the accompanying drawings, in which: Figure 1 is a top view of a first embodiment of an absorbing element according to the present invention.

Figure 2 is a view in section, through a plane AA perpendicular to the thermoconducting foils and to the axis of the tube, of the absorbing element according to Figure 1.

Figure 3 is a top view of a particular arrangement of the absorbing element according to Figure 1.

Figure 4 is a top view of a second embodiment of an absorbing element according to the invention.

Figure 5 is a top view of a third embodiment of an absorbing element according to the invention.

Figure 6 is a view in section, through a plane BB perpendicular to the thermoconducting foils and to the axes of the rectilinear zones of the tube, of an absorbing element according to the second or third embodiments, which comprises a layer of cellular material on the outer face of the thermoconducting foil which is not exposed to the sun.

Figure 7 is a top view of a fourth embodiment of an absorbing element according to the invention.

Figures 8 and 9 are top views of a fifth and sixth embodiment of the absorbing element according to the invention.

The absorbing element according to the invention, of which a first embodiment is shown in Figures 1 and 2, comprises an essentially rectilinear tube (1), and two thermoconducting foils (2, 3) which are fixed face to face and enclose the tube in such a way that, preferably in an essentially diametral plane of the tube, they extend beyond the tube on either side.

The width of the thermoconducting foils associated with the tube is not arbitrary; it is chosen so that the ratio of the width of thermoconducting foil associated with the tube, to the external diameter of the tube, is from 2.5 to 10 and preferably from 3.5 to 8. Thus, tubes having an external diameter of 20 mm are associated with thermoconducting foils having a width of 50 to 200 mm.

The thermoconducting foils of the absorbing element are generally spread out flat on the floor of the transparent enclosures which provide the greenhouse effect. However, they can easily assume a different shape (for example a parabolic or semi-cylindrical shape) in order to ensure a better concentration of the heat flux in the direction of the heat-transfer tube.

The thermoconducting foils can also be kept inclined, relative to the floor of the enclosure, by any suitable means, if the floor of the encosure is horizontal. This arrangement favours the absorption of the solar radiation.

The absorbing element shown in Figure 1 can be produced by enclosing the tube (1) between two thermoconducting foils having a length such that the resulting element can be used directly in a transparent enclosure. During positioning, several absorbing elements are advantageously joined in series by means of suitable connectors which are mounted at the ends of the tube (1) and arranged in a zig-zag fashion, analogously to the arrangement shown in Figure 3.

The thermoconducting foils of the absorbing elements are generally located in the same plane or in parallel planes.

Several absorbing elements joined in series are advantageously produced from a single tube which is enclosed at preferably regular intervals between two thermoconducting foils; this embodiment avoids the use of connecting means between two contiguous absorbing elements.

The arrangement according to Figure 3 can also be produced from a very long absorbing element consisting of a tube enclosed between two thermoconducting foils in the form of strips, essentially along the median axis of the said strips. The particular arrangement shown in Figure 3 then involves a juxtaposition of segments of absorbing elements, joined to one another in series by bends (6) which are bare parts of the tube.

A very long absorbing element can be produced, for example, by directing the tube, manufactured by extrusion, from the outlet of the extruder and, after sheathing if desired, between the two thermoconducting foils which move at the same speed as the speed of advance of the tube. A guide device with rollers makes it possible correctly to apply the two foils to one another, one being placed above the tube and the other below.

The arrangement shown in Figure 3 can then be achieved in accordance with the process described below: Parts of the tube are bared, at chosen intervals, which are identical or different, along the absorbing element, by removing (for example by cutting) the thermoconducting foils until the tube has been exposed. The length of each bared part of the tube is at least equal to the width of the thermoconducting foils.

The upper limit to the length of each bared part of the tube depends on the desired geometry of the arrangement: however, it is preferable, in order to reduce the surface areas to be removed from the thermoconducting foils, not to exceed five times the width of these foils. The bared parts of the tube are then preferably coated, for example with a paint having a deep dull colour.

Therefore, in this first step, an assembly is obtained which is formed of several segments of absorbing elements (analogous to absorbing elements as shown in Figure 1), which are joined to one another by the bared parts of the tube. The length of each segment and the number of segments also depend on the desired geometry of the arrangement.

In the second step, this assembly is bent in a zig-zag fashion, at the locations of the bared parts of the tube, in order to form bends, whilst arranging the segments side by side so that the thermoconducting foils of the segments of absorbing elements are essentially in the same plane or in parallel planes which are inclined relative to the floor of the transparent enclosure.

Inlet and oulet tubes (4) and (5) for the heattransfer fluid can be pieces of tube, attached, for example, by welding or glueing to the tube (1); preferably, they are bared parts of the tube (1).

The absorbing element according to the second embodiment shown in Figure 4 comprises, between two thermoconducting foils, several rectilinear zones of the tube (1), joined in series and in a zig-zag fashion by bends (6) which are not enclosed between the two thermoconducting foils. Connecting tubes (4) and (5) are provided forthe circulation of the heattransfer fluid.

The absorbing element according to the third embodiment shown in Figure 5 is analogous to the absorbing element shown in Figure 4, but the bends (6) are covered by the thermoconducting foil (2) exposed to the sun and are not covered by the thermoconducting foil (3) which is not exposed to the sun. The connecting tubes can be formed by baring the tube (1) by cutting the thermoconducting foils at (7) and (8).

Figure 6 is a view in section, through a plane BB perpendicular to the thermoconducting foils, of an absorbing element according to the second and third embodiments, which element is provided, on the face which is not exposed to the sun, with a layer of cellular material (9).

In the second and third embodiments of the absorbing element according to the invention, the rectilinear zones of the tube are preferably parallel to one another and uniformly distributed between the thermoconducting foils.

The zone of thermoconducting foil delimited by two contiguous rectilinear zones of the tube is associated, in halves, with the two rectilinear zones of the tube which delimit it.

The distance between two contiguous rectilinear zones of the two is chosen so that the ratio of that part of the width of the thermoconducting foil which participates in the exchange of heat with a rectilinear zone of the said tube, to the external diameter of the said tube, is from 2.5 to 10 and preferably from 3.5 to 8 Although, in the above description, reference has been made to the "rectilinear zones" of the tube, this does not exclude the production of absorbing elements using tubes comprising, for example, transverse undulations. An absorbing element, according to the invention, which comprises such a tube is shown, in top view, in Figure 7.

Thus, the term "rectilinear zone" denotes portions of the effectively rectilinear tube as well as zones of the tube of general rectilinear appearance.

It is to be understood that two adjacent rectilinear zones can form part of the same tube (Figure 4) or two different tubes (Figure 8).

The absorbing elements according to the invention can also possess several tubes enclosed between the thermoconducting foils. Two embodiments of such absorbing elements are shown in Figures 8 and 9.

The absorbing element according to the invention, of which a fifth embodiment is shown in Figure 8, comprises a plurality of essentially parallel tubes (1), enclosed between two thermoconducting foils; each tube comprises connecting tubes (4, 5). Such absorbing elements are advantageously joined in series, the homologous tubes of two adjacent absorbing elements themselves being joined in series.

The absorbing element according to the invention, of which a sixth embodiment is shown in Figure 9, comprises two tubes (1) enclosed between two thermoconducting foils; the two tubes are arranged between the thermoconducting foils in a manner analogous to that of the tube of the absorbing element according to the second embodiment.

Of course, the various embodiments, described above, of the absorbing element according to the invention are not the only possible embodiments as one skilled in the art will appreciate. For example, the rectilinear zones of the tube can be enclosed between two thermoconducting foils such that the thermoconducting foil not exposed to the sun can be in the form of a plurality.of segments of a strip, the rectilinear zones of the tube being enclosed substantially along the median axis of the strip. The width of the thermoconducting foil associated with the tube will be in this case the width of the thermoconducting foil exposed to the sun associated with the tube.

The absorbing elements according to the present invention which are described above are intended principally as elements which absorb solar radiation. However, they can also be used as heat exchangers. For this purpose, it suffices to send, into the tube, a fluid heated to a temperature which is at least 200C greater than that of the atmosphere which it is desired to heat.

The calorific power provided by these exchangers is of the order of the calorific power of customary radiators made of cast iron or sheet steel. For this use of the absorbing elements, it is not essential, for satisfactory operation, to paint the surface of the thermoconducting foils a deep dull colour.

The elements which absorb solar radiation, according to the invention, exhibit numerous advantages.

In fact, since they are not rigid, it is easy to transport them in the form of rolls to the sites where they are to be used, to unroll them and to cut them to the desired length. The connecting of the tube of the absorbing element to the circuit for heat-transfer fluid is easy to carry out because it suffices to bare the ends of the tube and to provide them with suitable connectors.

By virtue of the choice of the materials used for their construction, the absorbing elements according to the invention need only be subjected to small thermal stresses and they have good resistance to frost and corrosion. Moreover, the choice of the materials which can be used enables one to select materials for the tube having good resistance to scaling and those suitable for use with foodstuffs without difficulty.

The absorbing elements according to the invention can thus easily be arranged in transparent enclosures providing the greenhouse effect.

Moreover, such absorbing elements are easy to manufacture and, if appropriate, to assemble, and the low price of the raw materials which can be used for their manufacture, and also the ease with which they can be positioned, constitute great advantages to the users.

The following Examples further illustrate the present invention.

Example 1 The Example describes the manufacture of an absorbing element according to the invention, which is provided with thermoconducting foils of continuous structure.

A tube made of crude silicone elastomer, having an internal diameter of 10 mm and an external diameter of 12 mm, is manufactured by extrusion at a speed of 8 m/minute.

The crude silicone elastomer used to manufacture the tube is prepared by mixing the following constituents: 100 parts of an a,w-bis-(trimethylsiloxy)- dimethyl-polysiloxane gum having a viscosity of 45 million cP at 250C and containing 0.3% of methylvinylsiloxy units, 60 parts of a pyrogenic silica having a specific surface area of 200 m2/g and treated with octamethylcyclotetrasiloxane, 3 parts of an a,co- dihydroxydimethylpolysiloxane oil having a viscosity of 50 cP at 250C, and 2 parts of a paste containing 50% by weight of 1 ,4dichlornbenzoyl peroxide in a silicone oil.

At the outlet of the extruder, the tube circulates for 30 seconds in an oven heated to 2800 C, the effect of which is to crosslink the elastomer, and the tube then passes through a multi-thread winder dispensing glass fibres, each of which consists of 32 individual stands.

The tube leaves the winder provided with a braid of glass fibres; it is then directed between two strips of annealed aluminium; each strip has a width of 6 cm and a thickness of 0.12 cm and possesses one face coated with a heat-sealing acrylic varnish melting at about 150eC.

The two strips are then applied face to face, using a guide device with rollers, whilst enclosing the sheathed tube, the faces in contact being those which are covered with the acrylic varnish.

The strips are glued by passing the preformed absorbing element under two metal rollers, heated to 1 800C and spaced 1.6 cm apart, which press on the zone of the strips which is located on either side of the housing of the sheathed tube.

Finally, one of the two faces of the absorbing element is coated with a dull black paint which resists solar radiation.

Example 2 This Example describes the preparation, using an absorbing element according to Example 1, of an arrangement according to Figure 3 and the use of this arrangement for collecting solar radiation.

Over the- entire length of a very long absorbing element, and at regular intervals, the sheathed tube is exposed over a length of 14 cm by suitably cutting the aluminium strips. In this way, an assembly containing 12 segments of absorbing element each having a length of 1 50 cm, is obtained, which segments are joined to one another by the bared parts of the sheathed tube.

These parts are painted with the paint used in Example 1.

The segments of absorbing element are arranged as shown in Figure 3 and then installed in the bottom of a parallelepipedal box, made of laminated polyester, which has a length of 1 70 cm, a width of 70 cm and a height of 3 cm; this box is covered with a 5 mm thick glass plate.

The box is placed, at an inclination of 450, on a terrace exposed to the south, and a stream of water is caused to circulate in the tube under a pressure of 1 bar.

The stream of water passes through and heats a tank containing 70 litres of water.

The solar energy received is measured using a solarimeter and the energy collected is measured using a heat counter.

The experiments took place every day from 10 a.m. to 6 p.m.

The results of the measurements indicate that, for an insolation providing the collector (formed by the absorbing element and the box) with a mean daily power in the region of 300 to 600 Watts/m2, the efficiency of the collector varies from 40 to 60%.

Example 3 This Example describes the manufacture of an absorbing element according to the invention, which is provided with thermoconducting foils of discontinuous structure.

A tube made of silicone elastomer, having an internal diameter of 10 mm and an external diameter of 12 mm, identical to the tube used in Example 1, is introduced at a speed of 8 m/minute into a first multi-thread winder comprising 24 bobbins which each unwind a spindle which groups together four wires made of galvanised copper, the diameter of the wire being 0.2 mm.

The tube leaves the first winder sheathed with a braid having a diameter of slightly more than 12 mm, the wires of which cover about 45% of the external surface area of the tube. Still at a speed of 8 m/minute, the sheathed tube is introduced into a second multi-thread winder comprising 36 bobbins which each unwind a spindle which groups together 11 wires made of galvanised copper, the diameter of the wire being 0.3 mm.

The tube leaves the second multi-thread winder sheathed with the braid of 12 mm diameter and surrounded by a circular braid of 40 mm diameter. The assembly then passes between two metal rollers, with circular grooves, which are placed one above the other and are heated to 3000C; this device flattens the circular braid of 40 mm diameter, on either side of the tube. An absorbing element according to the invention has thus been produced, which comprises a tube made of silicone elastomer, which tube is sheathed with a braid of copper wires and provided with two thermoconducting foils of braided gauze, the foils each having a width of approximately 6 cm and being joined on their edges, and the tube being intercalated and centred between the two foils.

In order to improve its absorption properties, the absorbing element is finally coated, on one face, with a dull black paint which resists solar radiation.

Example 4 This Example describes the preparation, using an absorbing element according to Example 3, of an arrangement according to Figure 3 and the use of this arrangement for collecting solar radiation.

The thermoconducting foils of braided gauze are cut, at regular intervals, over the entire length of a very long absorbing element, in order to expose the sheathed tube over a length of 14 cm for each interval; an assembly of 12 segments, each having a length of 1 50 cm, is obtained, which segments are joined to one another by the exposed parts of the sheathed tube: these parts are painted with the paint used in Example 1.

These 12 segments are placed side by side (according to the arrangement shown in Figure 3) in the bottom of a box, made of laminated polyester, having dimensions of 170x70x3 cm, the painted face of the segments being turned towards the top of the box. The box is closed with a plate made of polyester (polypropylene maleophthalate), having, on its outer surface, a layer of polyvinylidene fluoride which stops the ultraviolet radiation; the collector thus obtained is used under the conditions indicated in Example 1 for heating a tank containing 70 litres of water.

The results of the measurements are similar to those found in Example 2 for an absorbing element provided with thermoconducting foils of continuous structure.

Example 5 This Example describes the use of an arrangement according to Example 2 as a heat exchanger. The thermoconducting foils do not have a face painted black.

The arrangement has a useful surface area of about 1 m2 and is arranged on a metal frame having dimensions of 155x75 cm; the assembly is positioned vertically on the floor of a workshop having dimensions of 25x 1 5x5 m, the temperature of which is of the order of 200C. The absorbing element arrangement is then operated as a heat exchanger, a stream of hot water being sent into its tube, made of silicone elastomer, at a flow rate of 120 litres/hour and at an inlet temperature of 800C.

By measuring the temperature difference between the inlet and the outlet for the stream of water, it is found that the exchanger having a surface area of 1 m2 uniformly provides a calorific power of 450 Watts to the air in the workshop.

Claims

1. An element suitable for absorbing solar radiation which comprises: at least one tube having at least one generally rectilinear zone, the said tube being suitable for the circulation of a heat-transfer fluid, and being made of an organic or organosilicon rubber or an organic thermoplastic material, and at least two flexible thermoconducting foils of continuous or discontinuous structure fixed in face-to-face contact with one another, at least one generally rectilinear zone of tube being enclosed between them.

2. An element according to claim 1 in which the ratio of the width or of that part of the width of the thermoconducting foil associated with a rectilinear zone or tube to the exterior diameter of said tube is from 2.5 to 10.

3. An element according to claim 1 or 2 in which the tube as a diameter from 5 to 50 mm.

4. An element according to claim 3 in which the tube has a diameter of 7 to 40 mm.

5. An element according to any one of claims 1 to 4 in which the tube is formed from an organic thermoplastic material which is low density polyethylene, polyvinyl chloride, a polyamide or a polyester.

6. An element according to any one of claims 1 to 4 in which the tube is formed of an organic rubber which is a butyl rubber, ethylenepropylene or polyurethane.

7. An element according to any one of claims 1 to 4 in which the tube is formed of an organosilicon rubber from heat hardening compositions containing a diorganopolysiloxane gum, mineral filler and organic peroxide.

8. An element according to any one of claims 1 to 7 in which the thermoconducting foils are fixed by heat sealing, welding or rivetting.

9. An element according to any one of claims 1 to 8 in which the thermoconducting foils are metal foils.

10. An element according to claim 9 in which the metal foils are of continuous structure and are sheets of thickness from 0.02 to 0.3 mm.

11. An element according to claim 9 in which the metal foils are of discontinuous structure and are webs of metal wires of diameter from 0.1 to 0.4 mm.

12. An element according to any one of claims 9 to 11 in which the metal foils are of aluminium.

13. An element according to any one of claims 1 to 12 in which the exterior surface of one of the thermoconducting foils has a dark colour and a matt appearance.

14. An element according to any one of claims 1 to 13 in which the face of one of the thermoconducting foils is covered with a layer of cellular material.

1 5. An element according to any one of claims 1 to 14 in which the tube possesses an exterior braiding made of a bundle of synthetic fibres or glass fibres, as reinforcement.

16. An element according to any one of claims 1 to 14 in which the tube possesses on its exterior a bundle of metal wires of average diameter from 0.05 to 0.4 mm.

17. An element according to any one of claims 1 to 1 6 in which at least two generally rectilinear zones of tube are enclosed between the two thermoconducting foils said rectilinear zones being connected to one another.

1 8. An element according to any one of claims 1 to 1 6 which comprises a single rectilinear zone enclosed between two thermoconducting foils.

19. An element according to any one of claims 1 to 16 in which a rectilinear zone of tube is enclosed between two thermoconducting foils in the form of strips substantially along the median axis of said strips.

20. An element according to claim 19 which comprises segments of element connected to one another in series by parts of the tube which are not enclosed by the strips.

21. A plurality of absorbing elements according to claim 18 connected in series in zig-zag relationship such that the thermoconducting foils are substantially in the same plane or in parallel planes.

22. An element according to claim 1 substantially as described in any one of Examples 1 to4.

23. An element according to claim 1 substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

24. An element according to claim 1 substantially as hereinbefore described with reference to Figure 4 or 5 of the accompanying drawings.

25. An element according to claim 1 substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

26. An element according to claim 1 substantially as hereinbefore described with reference to Figure 9 of the accompanying drawings.

27. Process for the preparation of an element as defined in claim 20, which comprises: baring at regular or irregular intervals part of the tube along the length of an element as defined in claim 19, each bared part having a length at least as wide as the said strips, folding in a zig-zag arrangement the resulting element such that the strips of the segments are substantially in the same plane or in parallel planes.

28. Process according to claim 27 substantially as hereinbefore described.