GB2605140A

GB2605140A - Heat exchanger

Info

Publication number: GB2605140A
Application number: GB2103961.5A
Authority: GB
Inventors: Murrell Adrian; Underwood Steven; Povall Simon; Simmons Nicholas; Williams Christophe
Original assignee: Naked Energy Ltd
Current assignee: Naked Energy Ltd
Priority date: 2021-03-22
Filing date: 2021-03-22
Publication date: 2022-09-28
Also published as: GB202103961D0

Abstract

A heat exchanger has a plate 210 comprising at least two fins 220 extending from the surface of the plate to form at least one channel (230, fig 2) between said pair of fins. A pipe 250 is then placed in each channel, and the fins of each channel are bent towards each other around the respective pipe to at least partially enclose the pipe. The fins may be pressed such that the pipe has an oval or flattened shape. A semi-fluid material such as a paste or an adhesive may be applied between the pipe and the fins to improve contact and thermal transfer between the plate and the pipe. The plate may be formed of the aluminium and the pipe may be made of copper so that the pipe has a greater thermal conductivity and lower chemical reactivity.

Description

HEAT EXCHANGER

TECHNICAL FIELD

[0001] The present invention relates to heat exchangers, for example heat exchangers for use in solar collectors.

BACKGROUND

[0002] Photovoltaic thermal (PVT) collectors, also known as hybrid solar collectors, are power generation technologies that convert solar radiation into usable thermal and electrical energy. During the collection process, the surface temperature of photovoltaic (PV) cells tends to increase and become less uniform, causing thermal stresses to develop and in-turn risking PV cell degradation (e.g. due to micro-crack formation). The increase in surface temperature also has an adverse effect on both the PV and thermal efficiency (% of solar irradiation converted to electricity and heat) of the radiation collection system. To mitigate these effects, heat exchanger systems are integrated with PV cells in the PVT collector system and are operated (for example, via channels of cooling liquids) to maintain the PV cells at a uniform temperature, thereby reducing thermal stresses and maintaining optimum electrical and thermal efficiency of the PVT collector system.

[0003] Some known heat exchangers comprise a thermally conductive plate, onto which the PV cells are closely bonded, and conduits for circulating cooling fluid, usually liquid. They are designed such that localised heat generated at the PV cells is conducted to the circulating liquid. However, the overall thermal reliability of the PVT collector system depends on efficiently managing temperature changes (or temperature differences, delta) at interfaces between the PV cell the thermally conductive plate, and the cooling liquid.

[0004] Examples of solar collectors are shown in E P2898270A1 and EP3722698A1. SUMMARY [0005] Embodiments of the invention are not limited to solving such problems and may include solutions to other problems.

[0006] This summary is provided to introduce a selection of concepts in a simplified form that will he further described below in the "Detailed Description" section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to determine the scope of the claimed subject matter.

[0007] There is provided in the following a method of a heat exchanger, a heat exchanger and a solar collector comprising a heat exchanger.

[0008] A method of forming a heat exchanger described in the following generally comprises forming a plate comprising at least two fins extending from the surface of the plate to form at least one channel between a pair of fins. A pipe is then placed in each channel, and the fins of each channel are bent towards each other around the respective pipe to at least partially enclose the pipe. This may achieve good contact between the materials of the pipe and the channel.

[0009] The bending of the fins may be achieved in a pressing operation. Additionally or alternatively the fins may be pressed such that the pipe has an oval or flattened shape. This pressing may take place after the bending or the bending and pressing may be achieved in the same pressing operation. The pressure may be applied in a direction perpendicular to the plate, e.g. perpendicular to the surface on which the fins are formed.

[0010] In some embodiments it may not be necessary to use fins to locate the pipe or pipes and some benefit may be obtained from the pressing of the pipe alone. Therefore an alternative method of forming a heat exchanger described in the following comprises securing at least one pipe to the surface of a plate and compressing the or each pipe in a direction perpendicular to the plate so that the or each pipe has a flattened or oval shape.

[0011] In the case that fins are used, in this a ltemative method the securing may comprise forming at least two fins extending from the surface of the plate to form a channel to accommodate each pipe, placing a pipe in each respective channel, and bending the fins of each channel towards each other around the respective pipe to at least partially enclose the pipe.

[0012] There is disclosed here a heat exchanger comprising a plate comprising at least two fins extending from a surface of the plate to form at least one channel between a pair of fins and a pipe situated in each channel, wherein the fins of each channel are bent towards each other around the respective pipe to at least partially enclose the pipe.

[0013] There is also disclosed here a heat exchanger comprising a plate to which one or more pipes are secured, wherein the one or more pipes have a flattened or oval shape and are arranged with their short dimension perpendicular to the plate.

[0014] A solar collector may comprise an array of photovoltaic cells bonded to any of the heat exchangers as described here.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Some embodiments of the invention will be described, by way of example, with reference to the following drawings, in which: [0016] Figures la and lb are perspective and end views of a heat exchanger plate with channels, which may be formed as part of a method according to some embodiments of the invention.

[0017] Figures 2(a) to (d) are schematic diagrams illustrating steps of a method of forming a heat exchanger, according to some embodiments of the invention.

[0018] Figure 3 is a flowchart illustrating a method of forming a heat exchanger, according to some embodiments of the invention.

[0019] Figure 4 is a perspective view of a heat exchanger, according to some embodiments of the invention.

[0020] Figures 5(a) and 5(b) are schematic diagrams illustrating steps of a method of forming a heat exchanger, according to some embodiments of the invention.

[0021] Figure 6 is a perspective view of a heat exchanger according to some embodiments of the invention.

[0022] Figure 7 is a perspective view of a roller pressing machine.

[0023] Figure 8 is a perspective view of a heat exchanger according to some embodiments of the invention.

[0024] Figure 9 is a broken perspective view of a solar collector according to some embodiments of the invention.

[0025] Figure 10 is a perspective view of a parallel array of solar collectors according to some embodiments of the invention.

[0026] Figure 11 is an exploded isometric view of a solar collector according to some embodiments of the invention.

[0027] DETAILED DESCRIPTION

[0028] The invention will be understood from the following detailed description of embodiments, which are meant to be descriptive and not limiting. For the sake of brevity, some well-known features methods and systems procedures components and circuits are not described in detail.

[0029] This present invention is concerned with heat exchangers, in particular their design and fabrication, as well as their use in solar collectors.

[0030] Heat exchangers for PV cells typically comprise a heat exchanger plate, with a flat surface on to which the PV cells can be closely bonded, and conduits for circulating fluids. The heat exchanger plate must be thermally conductive (centre-to-edge) and have a good thermal conduction to the circulating fluid conduits. Designs of heat exchanger are therefore key to efficiency and reliability in not only PVT collectors but also other applications.

[0031] Heat exchangers are usually made of thermally conductive metals although other materials may be used. Stainless steel has a thermal conductivity of 16 W/mK, while for aluminium it is 220 W/mK and copper it is 390 W/mK. Aluminium is cheaper than copper and therefore a good material choice. Extrusion is a good fabrication method for making long thin aluminium heat exchanger profiles with water channels. However, water and aqueous glycol solutions can corrode aluminium when they circulate through it Although chemical corrosion inhibitors can be added to the circulating fluid, these can become diluted over time and corrosion risk then increases, leading to leaks and pressure loss.

[0032] Copper is a less chemically reactive than aluminium and copper pipes are well established as an optimum material for water pipes in plumbing and solar thermal systems. Therefore, a good combination used for some of the heat exchangers described here is to use copper pipes and an aluminium extrusion (plate). Other combinations of materials may be used. A challenge then is to bond these parts together in a way that achieves good thermal conduction and therefore low temperature differences.

[0033] In a copper pipe and aluminium extrusion heat exchanger for example, the total delta T (temperature difference) between the circulating fluid and the centre of the PV cells is made up of 4 main components of delta-T: Fluid-to-copper z Copper-to-aluminium Centre-to-edge (in aluminium plate) z Aluminium-to-PV (across binding layer) [0034] The fluid to pipe material delta-T, in this example the fluid-to-copper delta-T, depends on fluid dynamics. In large diameter pipes with slow fluid flow rate the fluid flows smoothly with little mixing across the radius. This laminar flow_ can result in a high delta-T between the fluid in the centre of the pipe and that at the edges that is in contact with the pipe wall. In smaller pipes and at higher flow rates the flow starts to become 'turbulent_ a nd there is greater mixing. This results in a lower delta-T between fluid and pipe. The delta-T using an 8 mm diameter cylindrical copper pipe would typically be 8-12.0 for laminar flow, while turbulent flow would reduce this to 1-2.C. Turbulence can be increased by reducing pipe diameter, increasing flow rate or adding features that deflect or disrupt the fluid flow direction.

[0035] Bonding between copper and aluminium, or other materials used for the plate and pipe, can be achieved by adhesive, braising (solder), welding or mechanical pressing. To achieve good thermal contact and low delta-T requires good metal-to-metal contact across a large enough area. For a solar collector the solution needs to be robust and low cost Adhesive has limited conductivity and can become brittle over time. Braising is time consuming and therefore costly in manufacture and can warp the aluminium plate. Welding requires expensive manufacturing hardware, can warp the plate and only achieves metal contact at small areas (points) on the metal.

[0036] In the methods described in the following, mechanical pressing is chosen. This is a low cost and attractive method but requires careful design to enable direct and uniform metal-to-metal contact [0037] Some of methods described in the following comprise forming a plate which comprises at least two fins extending from the surface of the plate to form at least one channel between a pair of fins. A pipe is then placed in each channel and the fins are bent towards each other around the respective pipe, for example by pressing, to at least partially enclose the pipe. The fins and enclosed pipe may then be further pressed so that the pipe has a flattened or oval shape. The initial pressing and the optional further pressing may be in a direction perpendicular to the plate. Both pressing operations may be performed in a single pressing stage, for example using an optimally shaped tool.

[0038] As will be explained further below, the initial bending of the fins around the pipe may ensure good metal to metal contact between the pipe and the plate. The further pressure on the fins and the enclosed pipe, to form the pipe into a compressed or oval shape, can further enhance the efficiency of the heat exchanger by increasing the turbulence of fluid in the pipe.

[0039] As noted in the foregoing some improvement in efficiency may also be achieved from the pressing of the pipe to an oval or flattened shape without the use of fins and there is also described here a method of forming a heat exchanger comprising: securing at least one pipe to the surface of a plate; and compressing the or each pipe in a direction perpendicular to the plate so that the or each pipe has a flattened or oval shape. The securing may be achieved for example by laser welding or any other technique known to those skilled in the art.

[0040] There is also described here a heat exchanger formed according to the described methods.

[0041] When the heat exchanger is in use, any localised heat generated at or around the PV cells is conducted via the plate to the pipe or pipes, where it is absorbed and removed by the circulating cooling fluid.

[0042] The materials of the pipe and the plate may be the same, for example they may both be formed of metal. Thus some of the methods described here could be used for the connection of a pipe to a plate of the same material in the formation of a heat exchanger.

[0043] Alternatively different materials may be used for the plate and the pipe. As noted above, the plate may be formed of aluminium and the pipe may be formed of copper. More generally, for the pipe a key criterion for the choice of material may be it being non-reactive to the coolant fluid, e.g. water.

For the plate a key criterion may be thermal conductivity, optionally low cost and easy to form with fins on the back, as explained further below. Therefore, the material of the pipe may have a lower chemical reactivity and/or greater thermal conductivity than the material of the plate.

[0044] Figures 1a and lb show a perspective and end view of a heat exchanger plate 210 with channels 230, which may be formed as part of a method of forming a heat exchanger as described here.

[0045] The plate 210 may be manufactured using an extrusion technique, as it is known to be advantageous for manufacturing long thin profiles of aluminium. The channels 230 may be integrally formed with the plate 210 as part of the extrusion process. It is assumed that the detailed procedure for producing an extruded aluminium plate would be known to the person skilled in the art. Other standard industry techniques, such as additive manufacturing or moulding, would be known and available to the person skilled in the art for manufacturing the plate 210 and/or channels 230.

[0046] In the plate with channels shown in figure 1, each channel is formed between two parallel fins 220 extending from one of the major surfaces of the plate 210. The fins 220 may have a curved base at least on the side facing the channel where they meet the plate, to form a channel with a curved base. The base may have a flat section or may be semi-circular. In figure 1 multiple parallel channels 230 having the same dimensions are formed on the plate 210, in this example four channels. The channels are shown to be spaced and the spacing is shown to be larger than the width of the channels. However any geometry for the channels may be used as appropriate for the purpose of the heat exchanger.

[0047] A method of forming a heat exchanger will be further described with reference to figure 2 which shows schematically consecutive stages in the method, starting with a plate 210 in which channels 230 have been formed, for example as shown in figures 1a and 1b. The method is illustrated in figure 2 for one channel 230 but it will be appreciated that the same method may take place in parallel for each of the channels 230 shown in figure 1.

[0048] After the formation of the plate with fins as shown in figure 1, a pipe 250 is placed in each channel as shown in figure 2(a). The next stage in the method shown in figure 2 is to bend the fins 220 of each channel 230 towards each other around the pipe 250 to at least partially enclose the pipe 250 as shown in figure 2(b). This may be achieved in various ways, for example by applying pressure in a direction perpendicular to the plate 250 and using a suitably shaped pressure tool to cause the fins to bend.

[0049] The next stage is to apply further pressure to the bent fins 220 and thereby the pipe 250 until the pipe 250 has a flattened or oval shape. The reduction in height of the pipe 250 may be chosen by experimentation. For example the height of the pipe may be reduced to between 60% and 30% of its original height. Figures 2(c) and 2(d) schematically show a pipe which has been subjected to different amounts of pressure, with the pipe of figure 2(d) having been reduced in height by a greater amount than the pipe of figure 2(c). As noted elsewhere here the bending of the fins and the reduction of the height of the pipe may be achieved in a single pressing operation.

[0050] In the method shown in figure 2, the pipe 250 has an exterior diameter y that is smaller than the interior width x of the channel. Then, at the stage shown in figure 2(b), there is a space 231 between the fins and the pipe 250. The pipe 250 and channel 230 prior to bending the fins may be designed, for example by choice of suitable size and/or shape such that after bending and optionally pressing the fins 220, surface contact between the exterior surface of the pipe and the interior surface of the channel is increased. In the example shown in figure 2(d) the pipe width expands to x and fills the channel with good contact between the pipe and channel materials. At least 90% or possibly the whole of the outer surface of each pipe 250 may be in contact with the respective channel 230.

[0051] Figure 3 is a flowchart illustrating a method of forming a heat exchanger, for example as shown in figures 2(a) to (d).

[0052] The method of figure 3 begins with forming a thermally conducting plate comprising at least two fins extending from the surface of the plate to form at least one channel between a pair of fins, at operation 301. Next at operation 303 a pipe is placed in each channel. Then at operation 305 the fins of each channel are bent towards each other around the respective pipe to at least partially enclose the pipe (as shown in Figure 2(b)). In an optional operation 307, the fins and the pipe are pressed such that the pipe that has been partially enclosed by the fins has an oval or flattened shape (as shown in Figure 2(c) and (d))..

[0053] The amount of pressure and reduction in the height of the pipe may vary with different methods and applications for the heat exchanger. The method has been tested in which pressure has been applied until the structure is 30% 60% of its original diameter, for example the maximum depth of the pipe measured perpendicular to the plate is 30% 60% of the original maximum depth.

[0054] Figure 4 shows a perspective view of a pipe in a channel formed on an extruded aluminium plate that has undergone operations as described with reference to figure 3. This is typical of what might be achieved with pipes of 8mm external diameter pressed into 10mm internal diameter channels. It is notable that the fins do not meet so that the pipe is only partially enclosed. However the fin height may be chosen such that the whole of the outer cylindrical surface of the pipe is covered by the fins after bending. It can be seen that the fins 220 are in close contact with the pipe 250.

[0055] In tests, the 8mm diameter for the pipe was chosen since this is a standard diameter for copper pipe. The width of lOmm for the channel was chosen to allow a suitable amount of height reduction (squashing) of the pipe to give good fluid flow characteristics. If the internal pipe diameter in the small dimension (vertical as illustrated) is squashed down to 3-5 mm that makes the fluid flow more turbulent and reduces the water-to-copper delta T. If this dimension were reduced further to below 2 mm, that would increase the likelihood of the squashed pipe becoming blocked by small solid particles that are often contained in the circulating thermal fluid. It will therefore be appreciated that the choice of diameters and amount of reduction in pipe height will vary according to the intended application of the heat exchanger.

[0056] The pipes and channels may be designed or chosen such that the pressing step is not required to achieve this outer surface contact For example, the exterior diameter of the pipe 250 may match the interior width of the channel 230 and the base of the channel 230 may have a curvature matching the curvature of the pipe 250. An alternative method is illustrated with reference to figures 5(a) and 5(b) in which the pressing operation 307 is omitted.

[0057] The method illustrated in figures 5(a) and 5(b) is similar to that of figures 2(a) and 2(b) in that after the formation of the plate with fins as shown in figure 1, a pipe 250 is placed in each channel as shown in figure 5(a). Here however it will be noted that there is already close contact between the outer surface of the pipe 250 and the inner surface of the channel 230. The next stage in the method is to bend the fins 220 of each channel 230 towards each other around the pipe 250 to at least partially enclose the pipe 250 as shown in figure 5(b). Again this may be achieved in various ways, for example by applying pressure in a direction perpendicular to the plate 250 and using a suitably shaped pressure tool to cause the fins to bend. The bending of the fins 250 increases the contact area between the inner surface of the channel 230 and the outer surface of the pipe 250.

[0058] Figure 6 shows a perspective view of a pipe in a channel formed on an extruded aluminium plate that has undergone operations as described with reference to figures 5(a) and 5(b). This is typical of what may be achieved with a lOmm external diameter pipe in a lOmm external diameter channel where the fins have been bent to partially enclose the channel.

[0059] The fins should be thin enough to be flexible and able to be easily mechanically formed around the copper pipe but thick enough to conduct heat efficiently. With the above choice of pipe and channel dimensions, fins of 10 mm high and 1 mm thick were found to satisfy these requirements. In other words a fin height equal to the width of the channel may be suitable in general.

[0060] As noted above, it is useful for the extrusion to have a curved (circular or oval) shape at the base of the fins so that the copper pipe squashes into uniform and close contact with the channels, e.g. the extrusion.

[0061] The pressing step may be carried out using industry standard roller press or brake press machines. A roller pressing machine is shown in figure 7. As noted above, the pressing step to squash the pipe may be performed after the fins are bent around the pipe or may be performed at the same instance when the fins are bent around the pipe, for example using an optimally shaped tool.

[0062] A semi-fluid material, for example in the form of a paste or grease or adhesive, may be applied between the copper pipe and the thermally conductive aluminium plate. The material may fill any voids where there is no direct contact between the surfaces of the pipe and the plate following the pressing step. This may improve the adhesion between the pipe and the plate and/or further reduce temperature delta across the pipe material and the thermally conductive aluminium plate. For solar collectors a vacuum grease may be required, such as is generally known in the art Also it is useful for the material to have low vapour pressure so that it does not evaporate over time, The material may be applied to the inside surface of the channel prior to the placement of the pipe and/or the outside surface of the pipe, either as a layer or in drops or lumps that spread during the forming method. The semi-fluid material may be thermally conductive although in some applications high thermal conductivity may not be essential.

[0063] Some methods as described here have been tested and the following results have been achieved: [0064] Temperature difference data comparing a roll-bonded aluminium heat plate and extrusion heat plates formed with 10 and 8 mm copper pipe pressed in: Delta-T (*C) Thermocouple data Aug20 Thermocouple data 70ct20 Aluminium 10 mm Aluminium 8 mm roll-bond copper roll-bond copper Water-to-pipe pipe-to-plate contact Centre-to-edge 2 4.8 2.5 1.2 0 3.6 0 0.3 1.3 3.3 1.2 2.5 TDTLKIXf Dpir Arr 0 ift, 3-ert [0065] The above data was recorded in lab testing. The roll-bond heat plate was the reference heat plate in which 6 channels were formed within the aluminium plate, resulting in very low delta-T of 3.5C. The extrusion with 10 mm copper pipe showed a much higher delta-T of 11.7C, largely due to the higher water-to-pipe (water-to-copper) and pipe-to-plate (copper-to-aluminium) values. However with the 8 mm copper pipe and an optimised pressing process, both these values reduced significantly, resulting in a total delta-T similar to the reference heat plate.

[0066] Thus methods described here may be used to achieve an efficient PVT heat exchanger with a low cost and robust manufacturing solution. An example of a heat exchanger is shown in figure 8. The heat exchanger 800 comprises a plate 210 to which pipes 250 are secured using fins 200 as described elsewhere here. In the heat exchanger 800 the pipes 250 are interconnected to form a continuous conduit for coolant from an inlet 801 to an outlet 802 using an appropriate number of connectors 803 between parallel lengths of pipe. One connector 803 is shown, outside the extent of the plate 210. In an alternative arrangement a continuous meandering pipe may be provided to avoid the need for connectors which may be wholly or partially within the extent of the plate 210. It is not essential for the pipes 250 to be straight or to be in a parallel arrangement on the plate 210 although this may be convenient for manufacturing. The inlet 801 and outlet 802 may be connected to a reservoir for cooling fluid as is known in the art.

[0067] Any of the methods and heat exchangers described here may be used to provide a solar collector in any manner known in the art. Figure 9 shows in broken perspective view one example of a solar collector in which a heat exchanger as described here may be incorporated.

[0068] The solar collector of figure 9 comprises a sealed elongate transparent tube 901 containing a solar energy collector assembly 902. Multiple tubes 901 each containing solar energy collectors may be provided in a parallel array 900 as shown in figure 10.

[0069] The solar collector is shown in exploded isometric view in figure 11 and comprises a heat exchanger 1101 which may comprise any of the heat exchangers described here. The heat exchanger 1101 is secured to a tabbed PV array chain assembly 1106 of the kind generally known in the art in a layered structure. In the illustrated example the layered structure comprises the heat exchanger 1106, ethylene vinyl "EVA" separation layer 1102, backing sheet 1104, a further EVA separation layer 1105 and the PV array 1106.

[0070] It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages.

[0071] Any reference to 'an' item refers to one or more of those items. The term 'comprising' is used herein to mean including the method steps or elements identified, but that such steps or elements do not comprise an exclusive list and a method or apparatus may contain additional steps or elements.

[0072] The figures illustrate exemplary methods. While the methods are shown and described as being a series of acts that are performed in a particular sequence, it is to be understood and appreciated that the methods are not limited by the order of the sequence unless otherwise stated. For example, some acts can occur in a different order than what is described herein. In addition, an act can occur concurrently with another act Further, in some instances, not all acts may be required to implement a method described herein.

[0073] It will be understood that the above description of an embodiment is given by way of example only and that various modifications may be made by those skilled in the art. What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable modification and alteration of the above devices or methods for purposes of describing the aforementioned aspect, but one of ordinary skill in the art can recognize that many further modifications and permutations of various aspects are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the scope of the appended claims.

Claims

Claims: 1. A method of forming a heat exchanger, the method comprising: forming a plate with at least two fins extending from the surface of the plate to form at least one channel between a pair of fins; placing a pipe in each channel; and bending the fins of each channel towards each other around the respective pipe to at least partially enclose the pipe.
2. The method of claim 1, further comprising pressing the fins to compress the pipe to an oval or flattened shape.
3. The method of claim 1 or claim 2 comprising applying semi-fluid material to the inside surface of the at least one channel and/or the outside surface of the pipe prior to the placement of the pipe.
4. A method of forming a heat exchanger, the method comprising: securing at least one pipe to the surface of a plate; and compressing the or each pipe in a direction perpendicular to the plate so that the or each pipe has a flattened or oval shape.
5. The method of claim 4 wherein the securing of the at least one pipe comprises: forming at least two fins extending from the surface of the plate to form a channel to accommodate each pipe, placing a pipe in each respective channel, and bending the fins of each channel towards each other around the respective pipe to at least partially enclose the pipe.
6. The method of claims wherein the compressing of the pipe is performed by compressing the fins to compress the pipe to the flattened or oval shape.
7. The method of claim 2 or claim 6 wherein the bending and the pressing of the fins are performed in the same pressing operation.
8. The method of any of claims 2 to 7, wherein the pressing reduces the depth of the pipe in the direction perpendicular to the plate to 60% to 30% of its original diameter.
9. The method of any of claims 1, 2, 3 or 5 to 8 wherein the bending of the fins increases surface contact between the outer surface of each pipe and the interior surface of the channel.
10. The method of any of claims 2 to 9 wherein each pipe and channel are shaped prior to bending the fins such that the pressing increases surface contact between the outer surface of each pipe and the interior surface of the channel.
11. The method of any of claims 1, 2, 3 and 5 to 10 wherein each pipe and channel are shaped prior to bending the fins such that after bending and optionally pressing the fins, the at least 80% of the outer surface of each pipe is in contact with the respective channel.
12. The method of any of claims 1, 2, 3 and 5 toll wherein each channel has a curved base.
13. The method of any of claims 1, 2, 3 and 5 to 12 wherein, prior to the placing, the exterior diameter of each pipe is smaller than the interior width of the respective channel.
14. The method of any of claims 1, 2, 3 and 5 to 13 wherein the plate comprising the fins is formed by extrusion.
15. A heat exchanger comprising: a plate comprising at least two fins extending from a surface of the plate to form at least one channel between a pair of fins; and a pipe situated in each channel; wherein the fins of each channel are bent towards each other around the respective pipe thereby at least partially enclosing the pipe.
16. The heat exchanger of claim 15 comprising semi-fluid material between the respective surfaces of the at least one channel and the pipe.
17. The heat exchanger of claim 15 or claim 16 wherein the plate and the fins are formed of the same material.
18. The heat exchanger of claim 15, 16 or 17 wherein at least 80% of the outer surface of each pipe is in contact with the interior surface of the respective channel.
19. A heat exchanger comprising: a plate to which one or more pipes are secured, wherein the one or more pipes have a flattened or oval shape and are arranged with their short dimension perpendicular to the plate.
20. The heat exchanger of claim 19 wherein each pipe has a flattened or oval shape.
21. The method of any of claims 1 to 14 or the heat exchanger of any of claims 15 to 20, wherein the plate is manufactured from a first material and the pipe is manufactured from a second material.
22. The method or the heat exchanger of claim 21, wherein the thermal conductivity of the second material is greater than the thermal conductivity of the first material.
23. The method or the heat exchanger of claim 22, wherein the chemical reactivity of the first material is greater than the chemical reactivity of the second material.
24. The method of any of claims 1 to 14 or the heat exchanger of any of claims 15 to 22 wherein the materials of the plate and the pipe are metals.
25. Use of a heat exchanger according to any of claims 15 to 24 for cooling photovoltaic cells in a solar collector.
26. A solar collector comprising an array of photovoltaic cells secured to a heat exchanger as claimed in any of claims 15 to 24.Amendments to the claims have been filed as fololows: -Claims: 1. A method of forming a heat exchanger, the method comprising: forming a plate with at least two fins extending from the surface of the plate to form at least one channel between a pair of fins; placing a pipe in each channel; bending the fins of each channel towards each other around the respective pipe to at least partially enclose the pipe; and pressing the fins to compress the pipe to an oval or flattened shape.2. The method of claim 1 or claim 2 comprising applying semi-fluid material to the inside surface of the at least one channel and/or the outside surface of the pipe prior to the placement of the pipe.3. The method of claim 2 wherein the bending and the pressing of the fins are performed in the same pressing operation.4. The method of any preceding claim wherein the pressing reduces the depth of the pipe in the direction perpendicular to the plate to 60% to 30% of its original diameter.5. The method of any preceding claim wherein the bending of the fins increases surface contact a) 20 between the outer surface of each pipe and the interior surface of the channel.6. The method of any preceding claim wherein each pipe and channel are shaped prior to bending Othe fins such that the pressing increases surface contact between the outer surface of each pipe and the interior surface of the channel.7. The method of any preceding claim wherein each pipe and channel are shaped prior to bending the fins such that after bending and pressing the fins, the at least 80% of the outer surface of each pipe is in contact with the respective channel.8. The method of any preceding claim wherein each channel has a curved base.9. The method of any preceding claim wherein, prior to the placing, the exterior diameter of each pipe is smaller than the interior width of the respective channel.10. The method of any preceding claim wherein the plate comprising the fins is formed by extrusion.11. A heat exchanger comprising: a plate comprising at least two fins extending from a surface of the plate to form at least one channel between a pair of fins; and a pipe situated in each channel; wherein the fins of each channel are bent towards each other around the respective pipe thereby at least partially enclosing the pipe; and the fins are pressed to compress the pipe to an oval or flattened shape. 12. 13. 14. 15. 16. C\J C\ICO 17.LO 18. 19. 20.The heat exchanger of claim 11 comprising semi-fluid material between the respective surfaces of the at least one channel and the pipe.The heat exchanger of claim 11 or claim 12 wherein the plate and the fins are formed of the same material.The heat exchanger of claim 12, 13 or 14 wherein at least 80% of the outer surface of each pipe is in contact with the interior surface of the respective channel.The method of any of claims 1 to 10 or the heat exchanger of any of claims 11 to 14, wherein the plate is manufactured from a first material and the pipe is manufactured from a second material.The method or the heat exchanger of claim 15, wherein the thermal conductivity of the second material is greater than the thermal conductivity of the first material.The method or the heat exchanger of claim 16, wherein the chemical reactivity of the first material is greater than the chemical reactivity of the second material.The method of any of claims 1 to 10 or the heat exchanger of any of claims 11 to 16 wherein the materials of the plate and the pipe are metals.Use of a heat exchanger according to any of claims 11 to 18 for cooling photovoltaic cells in a solar collector.A solar collector comprising an array of photovoltaic cells secured to a heat exchanger as claimed in any of claims 11 to 18.