A Heat Exchanger and Uses Thereof

Abstract

A heat exchanger unit having a top and a bottom, the heat exchanger comprising a plurality of fins spaced apart from each other and having a predetermined length, thickness and height, with application for use with a photovoltaic solar panel.

Claims

1. A heat exchanger comprising a plurality of fins spaced apart from each other, the plurality of fins having a predetermined length (L.sub.fin), thickness (t.sub.fin) and height (h.sub.fin); wherein the plurality of fins are formed from a single sheet of material, and wherein where each fin of the plurality of fins are formed, an aperture having a predetermined width (W.sub.A) and a predetermined length (L.sub.A) is generated in the single sheet of material separating each fin of the plurality of fins.

2. The heat exchanger according to claim 1, wherein the plurality of fins have on open configuration through the heat exchanger.

3. The heat exchanger according to any one of claim 1 or 2, wherein the plurality of fins are segmented into distinct sections.

4. The heat exchanger according to any of the preceding claims, wherein the plurality of fins are arranged in a colinear or an offset manner relative to each other.

5. The heat exchanger according to any one of the preceding claims, wherein fin spacing, fin height or fin thickness change as a function of position on the single sheet of material.

6. The heat exchanger according to any one of the preceding claims, wherein the optimum fin spacing (S) is between about 1 mm and about 50 mm relative to each other.

7. The heat exchanger according to any one of the preceding claims, wherein the fin thickness (t.sub.fin) is between about 0.001 mm to about 5 mm; the fin height (h.sub.fin) is optionally between about 0.1 cm to about 10 cm; and the fin length (L.sub.fin) is optionally between about 1 mm to about 1500 mm.

8. The heat exchanger according to any one of the preceding claims, wherein the fins are composed of thermally conductive sheets or films of material selected from the group comprising a metallic film, a carbon-based film, or polymer films doped with conductive particles or a combination thereof.

9. The heat exchanger according to claim 8, wherein the metallic films are selected from films comprising aluminium, copper, stainless steel, tungsten, titanium, or combinations thereof.

10. The heat exchanger according to claim 9, wherein the carbon-based films are selected from films comprising graphite, pyrolytic graphite, synthetic graphite, graphene, carbon nanotubes, expanded graphite, graphite composites, carbon black, diamond, or combinations thereof.

11. The heat exchanger according to claim 9 or claim 10, wherein the conductive particles in the polymer films are selected from a diamond, carbon, transition metal nitrides such as AlN, transition metal oxides such as Al.sub.2O.sub.3, ceramics or combinations thereof.

12. The heat exchanger according to any one of the preceding claims, wherein the plurality of fins are flexible.

13. The heat exchanger according to any one of the preceding claims, wherein the plurality of fins are coated with an epoxy or polymer.

14. The heat exchanger according to claim 13 wherein the coating is selected from an elastomer, a phase change material, a thermoplastic, a copolymer or a combination thereof.

15. The heat exchanger according to any one of the preceding claims, wherein the single sheet of material is a sheet composed of a single material, or a laminate or composite of multiple sheets of the same or different material.

16. A heat exchanger comprising a plurality of fins spaced apart from each other and having a predetermined length (L.sub.fin), thickness (t.sub.fin) and height (h.sub.fin); and wherein each fin of the plurality of fins has a predetermined shape and is attached individually to a plate.

17. The heat exchanger according to claim 16, wherein the plurality of fins are composed of the same material, a composite of the same material, or a composite of different material.

18. The heat exchanger according to any one of the preceding claims, wherein air flow through the heat exchanger is by forced, natural, or passive convection.

19. The heat exchanger according to any one of the preceding claims, wherein the plurality of fins further comprises a support base.

20. The heat exchanger according to claim 19, wherein the support base is mounted individually to each fin.

21. The heat exchanger according to claim 19 or 20, wherein the support base is opaque, transparent, or a combination thereof.

22. The heat exchanger according to any one of the preceding claims, wherein the fins have a cross-sectional shape selected from a trapezoid, a sinusoid, a triangle, free-flowing, a square, a circle, a pentagon, a parallelogram, a kite, a crescent, a trefoil, a chevron, a cross, an equiangular shape, columnar, an oblong, an oval, a teardrop, a medallion, a star, a diamond, an L-shape.

23. The heat exchanger according to any one of the preceding claims, further comprising an airflow isolation means across the width of the plurality of fins.

24. The heat exchanger according to any one of the preceding claims, further comprising a base plate.

25. The heat exchanger according to claim 24, wherein the base plate is made from a metal selected from aluminium, stainless steel, titanium, copper, tungsten or alloys thereof.

26. The heat exchanger according to claim 24, wherein the base plate is made from a white or a transparent material.

27. The heat exchanger according to claim 26, wherein the white or transparent material is selected from glass, diamond, polymer, quartz, oxides and nitrides of transition metals such as aluminium nitride, aluminium oxide, Titanium Dioxide, and the like.

28. The heat exchanger according to any of the preceding claims, wherein one or more of the plurality of fins further comprise apertures, louvres, or dimples.

29. The heat exchanger according to any one of the preceding claims, wherein a plurality of the heat exchanger can be stacked one on top of the other.

30. The heat exchanger according to claim 29, wherein each heat exchanger of the plurality of stacked heat exchangers has a different fin thickness (t.sub.fin), fin height (h.sub.fin), or fin length (L.sub.fin), or a combination thereof.

31. The heat exchanger according to claim 29 or claim 30, wherein each heat exchanger of the plurality of stacked heat exchangers is made from a different material.

32. The heat exchanger according to any one of the preceding claims, wherein the plurality of fins are applied to a heat source as a singular unit, or as a series of units.

33. The heat exchanger according to claim 32, wherein the heat source is selected from a photovoltaic solar panel, a solar thermal collector, a PVT system, a heat pump, a radiator, an air conditioning unit, a battery unit, an electronic device, a transformer, or a chemical reactor.

34. The heat exchanger of any one of the preceding claims for use with a photovoltaic solar panel, a solar thermal collector, a PVT system, a heat pump, a radiator, an air conditioning unit, a battery unit, an electronic device, or a chemical reactor.

35. A heat exchanger for use with a photovoltaic solar panel unit having a top and a bottom, the heat exchanger comprising a plurality of fins spaced apart from each other and each fin of the plurality of fins having a predetermined length (L.sub.fin), thickness (t.sub.fin) and height (h.sub.fin), wherein the plurality of fins are formed from a single sheet of material and wherein an aperture having a predetermined width (W.sub.A) and a predetermined length (L.sub.A) is generated in the single sheet of material between each of the plurality of fins.

36. The heat exchanger according to any one of the preceding claims, wherein the plurality of fins are coated with a high emissivity thin film or paint.

37. A photovoltaic solar panel geometry comprising the heat exchanger of claim 1; wherein the photovoltaic solar panel is either bifacial or monofacial.

38. A method for making the heat exchanger of claim 1, the method comprising the steps of forging, extruding, stamping, punching, forming, die casting or machining the plurality of fins from the single sheet of material, folding back one or more of the plurality of fins from the surface of the sheet of material, wherein said folding back of the fin generates an aperture between each fin of the plurality of fins in the single sheet of material.

39. The method of claim 38, wherein each of the plurality of fins are folded back to between 1° to 90° relative to the surface of the sheet of material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0085] The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:—

[0086] FIG. 1(a)-(d) illustrates an embodiment of the invention with a segmented fin design. Fins are made from 0.5 mm thick sheet of material, aluminium for example, having a height of 2.5 cm and spaced apart by 3 cm. In FIG. 1 (a) there are 23 fins spread across a 15 cm by 15 cm test panel. In FIG. 1(b) the fins are shown on a typical 320Wp panel designed to be mounted in a portrait orientation. FIG. 1(c) shows a close-up of the fins adhered to back of a panel. FIG. 1(d): illustrates a basic sub-unit of the invention which is used to identify and label the key parameters. This shows two segments which are offset by a distance L.sub.off. The channel spacing in each segment is given by S, and each segment is separated by an interruption distance L.sub.int. The fin height h.sub.fin and fin length L.sub.fin are defined as shown in the figure. Fin thickness tin is also shown. Once the fin is cut out and folded back at an angle, an aperture is formed, allowing light to the back of the cell, with the aperture have a width W.sub.A and a length L.sub.A. This may be the same as the fin dimensions or larger depending on the cutting process.

[0087] FIG. 2(a)-(i) (a) and (i) illustrate isometric view of a segmented fin design made from single fins separately adhered to a panel; (b) illustrates an example of a single fin with triangular support; (c) illustrates an example of individual fins on a panel (white rectangle); (d) illustrates an example of a single fin with a rectangular base; (e) illustrates an example of fins mounted individually with a rectangular base; (f) illustrates an example of fins with a rectangular base made into continuous base plate; (g) illustrates fins mounted on a separate base plate; and (h) illustrates fins bent at the bottom to attach to a base plate.

[0088] FIG. 3(a)-(d) illustrates examples of fin materials and layer structures. (a) is a single material fin with base; (b) is a high thermal conductivity thin fin material with a support material; (c) is a high thermal conductivity fin material encapsulated in a support material; and (d) is a high thermal conductivity material included as particles in a support material.

[0089] FIG. 4(a)-(j) illustrate fin geometry alternates in profile attached to a panel (white rectangle) (a-e) shows back of panel attachments; (f-j) show both back and front of panel attachments. (a) is a single continuous fin; (b) is a plurality of segmented fins; (c) is a single fin with varying height; (d) is a plurality of segmented fin with varying height; (e) is a plurality of segmented fins with structures to isolate airflow across each section. (f(-(j) Same designs as shows in (a)-(e) but with the heat exchanger of the claimed invention on the front surface of the panel. The heat exchangers shown here could be made of highly reflective or transparent materials.

[0090] FIG. 5(a)-(h) show various embodiments of the heat exchanger of the claimed invention. FIGS. 5(a), 5(b), and 5(c) shows perspective, back and side views of a segmented design with colinear fins and four segments; FIGS. 5(d), (e), and (f) shows the same embodiment above with fin height increasing as a function of panel position. FIG. 5(g) shows an embodiment where the heat exchanger is applied as eight separate plates to a solar panel in portrait orientation. Each plate has eight segments and there are thirty-two segments in total. FIG. 5(h) shows the same concept applied to a solar panel mounted in landscape orientation. The fins in this case are orientated vertically i.e. in the direction of the flow.

[0091] FIG. 6(a) is an IR image of a 17 cm×17 cm single cell-sized panel mounted at 45°. The left-hand side of the image is a panel with no heat exchanger, while the right-hand side of the image is a panel with a 0.5 mm aluminium heat exchanger. FIG. 6(b) is a graph showing the scaling of cell temperature with heat source power (proportional to solar irradiation). The solid line is a finite element method (FEM) simulation, while the data points are experimental measurements from the test panel.

[0092] FIG. 7(a) shows the visible and IR images of an outdoor test using two identical 50 W panels. The right-hand panel is modified on its back surface with a heat exchanger of the claimed invention. FIG. 7(b) is a graph showing the temperatures over a 2-hour period measured at the back surface of a panel with the heat exchanger of the claimed invention. Max ΔT=3.7° C. FIG. 7(c) is a graph showing experimentally measured power curves using the PV test instrument shown in FIG. 9(a). Max power output is increased by 27% in this case. Background temperature was 10° C. and incident irradiance was 400 W/m.sup.2.

[0093] FIG. 8 illustrates the optimum fin spacing vs panel height for different fin temperatures calculated analytically.

[0094] FIG. 9 is a graph illustrating the solar cell temperature as a function of fin height for different materials and thicknesses of a heat exchanger of the claimed invention.

[0095] FIG. 10 illustrates (a) a schematic of a panel in a typical ground mount at 45°; and (b) a graph illustrating the panel cell temperature under 1 sun illumination (1000 W/m.sup.2) without a heat exchanger (red/top curve) and with the heat exchanger of the claimed invention (black/bottom curve).

[0096] FIG. 11 shows steps in manufacture of the invention. Fin cross section is first defined and punched (1). The fins are then folded back into position (2). Two or more plates can be overlapped to give higher fin density (4). Adhesive such as PSA can then be applied to the base or to the area where the exchanger will be adhered.

[0097] FIG. 12(a)-(c) shows (a) thermal images of the fins of a panel of the claimed invention in a staggered or stacked configuration. The thermal images show relatively uniform temperature distribution over fin length for a range of different designs. FIG. 12(b) shows bar charts illustrating the experimentally measured cell temperature drop of a range of test designs with segment number varying from 1 to 9, and fin thickness decreasing from 0.5 mm to 0.2 mm. FIG. 12(c) shows the percentage power output increase per cost of material for the same range of fin designs.

DETAILED DESCRIPTION OF THE DRAWINGS

[0098] The inventors propose using thermally conductive films such as metallic films, carbon-based films, or polymer films doped with conductive particles to create a novel heat exchanger. Combinations of these films could also be used. These materials can be manufactured on a large-scale using extrusion and/or roll-to-roll processing. Pyrolytic graphite film, for example, is very low density (1.9 g/cm.sup.3) and has extremely high thermal conductivity (1950 W/m/K), 9.5-times that of aluminium. Currently, it is primarily used as a heat spreader in electronic devices. Because of its high thermal conductivity, much thinner layers can be used as compared to aluminium which significantly reduces weight and material cost. The graphite used can be either synthetic or natural (mined from the ground). It is processed into a roll which allows for efficient transport. It can be purchased in large quantities from manufacturers.

[0099] The heat exchanger of the claimed invention can use an origami-inspired approach or a punched and formed approach to create novel, high-surface area heat exchanger designs from the film starter material. For example, the heat exchanger can be created from a single sheet of aluminium by punching a fin shape into the sheet of material, creating a design with a large surface area. The resulting holes from the creation of the punched fin allows light to reach the back of a solar panel accommodating the heat exchanger, and allows the heat exchanger of the claimed invention to be used with bifacial solar panels. When compared to heat exchangers formed from folding a continuous sheet of metal, the design of the claimed invention can achieve the same cooling using less material as the back of the solar panel is still used as an effective cooling surface. This heat exchanger can be attached directly to the back sheet of a solar panel or attached to a photovoltaic solar panel frame as shown in FIG. 1(b). FIG. 1(b) shows the fin orientation when the panel is intended to be mounted in a portrait orientation. FIG. 1(c) shows the case when the panel is intended to be mounted in a landscape orientation. In FIG. 1 (d), the specifics of the design of the claimed invention are shown on a basic subunit. The key distances which define the invention are labelled. This includes an offset distance (L.sub.off), a channel spacing (S), an interruption distance L.sub.int, a fin height h.sub.fin, a fin length L.sub.fin, and a fin thickness t.sub.fin. Once the fin is cut out and folded back, an aperture remains with a width W.sub.A and a length L.sub.A, allowing light into the back of the cell. This may be the same as the fin dimensions or larger depending on the cutting process.

[0100] The heat exchanger can be constructed from individual fins which are assembled as shown in FIG. 2(a). This consists of individually mounted fins as shown in FIG. 2(b) mounted as shown in FIG. 2(c). In this case, a triangular base mount is used. This may attach the fin directly to the back of the panel or to a baseplate. In FIG. 2(d) an alternative rectangular base mount is shown. They may be mounted separately as shown in FIG. 2(e) or combined into a single unit before application, such as in FIG. 2(f). Alternatively, the fins may be attached to a single baseplate as shown in FIG. 2(g). The fins may be folded to attach to the back of the panel, or the baseplate as shown in FIG. 2(h). These base mounts and baseplates can be made from the same material as the fin or from a different material. They can be transparent or opaque and made from metal, polymer, or glass, or a combination of these. They may be matched to the thermal expansion coefficient of the backplate, solar glass or other photovoltaic solar panel material.

[0101] The fins may be made from a single high thermal conductivity material as shown in FIG. 3(a) or may be made from a high thermal conductivity material adhered to a supporting material as shown in FIG. 3(b). They may be made from a continuous high thermal conductivity material encapsulated in a supporting material, as shown in FIG. 3(c), or a discontinuous high thermal conductivity material dispersed in a supporting material, as shown in FIG. 3(d).

[0102] In one embodiment, the heat exchanger structure can be created from a single film or material, or from multiple sections. These films may have the structures described in FIG. 3. By creating bends and folds in the design, different heat exchangers can be created by periodically adhering the film to the back of the panel or the baseplate. The thermal resistance of the heat exchanger can be tuned to match the solar panel design by choosing a material with a particular thermal conductivity κ, a film thickness t, a height of the film h, and a periodicity λ. Further, by using fins that are thin, these may be flexible and free-moving. For example, when the material used is aluminium or graphite, fins having a thickness of about 0.1 mm will result in flexible and free-moving fins. Free-moving structures have the advantage of moving with the wind, which would enhance heat transfer through forced convection and turbulence. The heat exchangers of the claimed invention do not need to be periodic or ordered. They may have sinusoidal, triangular, undulating, square, rectangular or fractal cross sections. When they are periodic, they can have multiple periodicities or vary depending on the position the heat exchanger of the claimed invention takes on the panel.

[0103] Side views of embodiments of the invention are shown in FIGS. 4(a)-(j). The fin structures may be continuous along the entire panel length or applied in sections as shown in FIG. 4(a) and FIG. 4(b), respectively. Examples are shown with 3-4 sections, however there may be any number of sections as the user sees fit. The fin height may vary as a function of panel position. An example of a linear variation is shown in FIG. 4(c), and in the segmented design in FIG. 4(d). An additional structure could be used to separate airflow from each of the sections, as indicated by the black line in FIG. 4(e). For some panels, such as bifacial panels, transparent base plates and fins, or highly reflective fins, are typically used. It would be possible to use these heat exchangers on the front surface of the panel also. The same concepts are shown both on front and back of panel in FIG. 4(f)-(j).

[0104] Photovoltaic solar panels are mounted at an angle to optimise solar irradiation. Shallow angles may cause issues for natural convection, with air getting trapped beneath the panel. The design may require modification of the top and bottom parts of the frame holding the panel to allow air to flow across the heatsink surface. The heatsinks could be segmented, or the length of the heat exchanger could be varied depending on position on the panel to optimise natural convection heat transfer, and to ensure uniform cooling. The heatsinks could be mounted as strips with gaps between these to allow airflow to move through. The heat exchanger may be long enough to protrude from the back of the panel surface, or it may be short enough to be completely hidden by the panel frame. Full back panel views of segmented fins are shown in FIG. 5. Simple parallel rectangular fins are shown. These fins may be made from any of the materials or layer structures outlined in FIG. 3. They may be constructed using the punched technique outlined in FIG. 2 or adhered individually as outlined in FIG. 4. They may have any number of different types of cross sections other than rectangular. FIG. 5(a)-(c) shows isometric, back and side views of an example of a 320Wp solar panel with simple rectangular fins. FIG. 5(d)-(f) shows isometric, back and side views of a segmented design with tapered fins. A simple linear, tapered design is shown, however other tapered designs could be used. In segmented designs, sections may have different fin height, periodicity and thickness. Sections may be made of different materials. Sections may be offset to disrupt airflow between each segment.

[0105] Sections may be made of different materials. Sections may be offset to disrupt airflow between each segment. Good adhesion between the heat exchanger and the photovoltaic solar panel and the fin structure and base are very important to ensure good thermal contact. There are several commercially available glues and epoxies that can be used such as water-based pressure sensitive adhesive (PSA) or modified silicone adhesive. For PSAs an elastomer functions as the primary base material, which can be natural rubber, vinyl ethers, acrylics, butyl rubber, styrene block copolymers, silicones and nitriles. Modified silicones consist of polyether backbone and silane terminal functionality. They can be prepared from high molecular weight polypropylene oxide, end capped with allyl groups, followed by hydrolysation to produce a polyether end-capped with methyldimethoxysilane groups. The heatsink may be attached without using glue, just by applying pressure against the frame. Bars could be used to evenly apply the pressure. An alternative would be to mechanically fix the heat exchanger to the backplate using various securing means such as a nail, a screw, a tack, snap-fit configuration, adhesive, and the like. A combination of both techniques could be used. The contact area should be controlled in order to tune the thermal resistance of the heat exchanger and ensure uniform cooling.

[0106] To demonstrate the performance of the heat exchanger of the claimed invention, the inventors created a test rig where heater pads were inserted in a standard solar panel geometry, which allowed precise control of the heat source while monitoring cell temperature, surface glass temperature, and the heat flux through the back surface of the panel. FIG. 6(a) shows IR thermal images of the front of the test bed. The panel on the left has no heat exchanger, whereas the panel on the right has an aluminium heat exchanger with a fin thickness of 0.5 mm. To test performance under different conditions, the current in the heater pad is controlled such that the heat source density changes from 100 W/m.sup.2 to 800 W/m.sup.2. FIG. 6(b) shows the resulting solar cell temperature measured experimentally (data points) and predicted using numerical simulations (solid line).

[0107] The inventors also compared the performance of two identical monocrystalline 50 W solar panels with and without the heat exchanger of the claimed invention. The panels were placed outdoors in realistic weather conditions at the beginning of March in Ireland (see FIG. 7(a)-(c)). Maximum solar irradiance was measured at 600 W/m.sup.2 and the background temperature was 10° C. The backside of the panel with no technology reached 30° C., whereas the backside temperature with the technology remained below 25° C. Even with a small change in temperature, a significant increase of 27% in output power from the panel with the heat exchanger of the claimed invention was observed.

[0108] The inventors performed extensive analytical and numerical studies to optimise the design. FIG. 8 illustrates the relationship between the optimum fin spacing and the panel height at various temperatures, calculated using analytical theory. The optimum spacing depends on aspect ratio and the expected fin operating temperature.

[0109] The optimum fin height and thickness are calculated using numerical FEM simulations. In FIG. 9, it is shown that for the aluminium foil (a thickness of 0.05 mm) there is a reduced benefit in increasing the fin height past 50 mm. This is because the thermal resistance is large and the thermal gradient along the fin height is significant. Increasing the height of the fin beyond this has minimal effect because the temperature difference between the fin and the air is too low to exchange a reasonable amount of heat. For the thicker aluminium sheet (0.5 mm) there are benefits out to 10 cm, however this will add additional cost and weight to the device.

[0110] FIG. 10 shows simulated results for a full-scale panel demonstrating a 24° C. temperature drop with the applied technology. This would correspond to ˜12% increase in output efficiency and could significantly increase panel lifetime.

[0111] FIG. 11 shows a proposed manufacturing process for the heat exchanger of the invention. The cross-section of the fin is first defined using punching, cutting, laser scribing or similar mechanical processes. The fin is then released from the sheet by folding the fin back to between 1° and 90° relative to the sheet surface. Individual plates can be overlapped to increase the fin density (see FIG. 12(a) “stacked”). When stacking individual plates one on top of the other, the plurality of fins of a second plate fit through the apertures created by the plurality of fins of a first plate. The stacked plates can be moved relative to each other to create a specific spacing between the fins of those plates as desired. In general, the plurality of fins of the first plate are placed in the centre of the apertures of the second plate. This effectively decreases the spacing of the fins on the stacked plates by half and provides an improved fin density regime for cooling. An adhesive is applied to the back of the heat exchanger to permit the user to adhere the heat exchanger to a device. The process of using a single sheet of material uses much less material than folding a single sheet and opens apertures which allow light to the back of the panel.

[0112] FIG. 12(a) shows a thermal image of a specific design, and the stacked version of that design with higher fin density. The relatively uniform fin temperature shows that they are acting with high fin efficiency and well contacted to the heat source. FIG. 12(b) shows the experimentally measured temperature reduction at cell level for different numbers of segments from 1 to 9 and thicknesses ranging from 0.5 mm to 0.2 mm. This shows that there is limited change in the temperature performance when reducing thickness to 0.2 mm. FIG. 12(c) shows that the cost per % power output improvement decreases with thinner fin thickness. This shows an advantage to going to thinner fins.

Materials and Methods

Solar Panel Construction

[0113] A typical layout of a photovoltaic solar panel is a layer of glass, a first encapsulant layer, a solar cell, a second encapsulant layer, and a back sheet. The glass is low iron tempered glass and usually between 2.8-4 mm thick. It provides the main structural support for the solar cells, which are extremely thin and brittle. The encapsulant is usually a form of ethylene-vinyl acetate (EVA) optimized to withstand prolonged UV exposure. The back sheet can be made of polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), or propriety materials such as DuPont™ Tedlar® (a polyvinyl fluoride film). These materials must protect the cells from moisture ingress and temperature cycling.

Heat Exchanger Materials

[0114] The proposed heat exchanger can be made from any material with high thermal conductivity (>50 W/m/K). In particular, the inventors have demonstrated the heat exchanger of the claimed invention using aluminium and expanded graphite. The high thermal conductivity material may be supported or encapsulated by a low thermal conductivity material in geometries as depicted in FIG. 3. Metals including but not limited to aluminium, copper, stainless steel, titanium and alloys containing these elements. Carbon-based materials including but not limited to graphite, pyrolytic graphite, expanded graphite, diamond, carbon filler, carbon nanotubes, graphene are of particular interest due to low density, high thermal conductivity and high IR emissivity. Other high thermal conductivity materials such as aluminium nitride, silicon nitride and boron nitride are of interest.

Heat Exchanger Testing

[0115] To reproduce the effects of heating due to solar irradiation of a solar panel in a quantitative and reproducible fashion, the inventors devised a simple rig with a silicone heating mat in place of a silicon solar cell. A glass front panel and a polycarbonate back panel were cut to area 17 cm×17 cm. The glass was 2.8 mm thick and the PC panel was 1.5 mm thick. A heating mat was used with an input voltage of 12V, and maximum power dissipation of 15 W. This permitted the inventors to get up to 800 W/m.sup.2 as a heat source density, which is enough to represent solar panels under standard test conditions. A thermal image of the front surface (glass) is shown of the device without and with the heat exchanger on left and right-hand sides respectively in FIG. 6(a). The image is taken with a heat source density of 800 W/m.sup.2. The current was controlled to sweep the heat source density from 100 W/m.sup.2 to 800 W/m.sup.2 and the cell temperature was monitored with and embedded thermocouple for the cases with and without the heat exchanger. The results are shown in FIG. 6(b). The experimental measurements are shown as data points, whereas the solid lines represent the prediction of numerical models performed using the 3D finite element method. In extreme environments, panels could experience peak heat source densities of 1920 W/m.sup.2. A higher power heating mat is required to simulate these conditions.

[0116] As a further demonstration a standard 50 W photovoltaic solar panel was fitted with a heat exchanger made using a 0.5 mm aluminium sheet. Individual fins were adhered to the back of the solar panel without the use of a back plate to reduce cost. FIG. 7(a) shows visible and thermal images of the device of the claimed invention in outdoor conditions under 600 W/m.sup.2 solar irradiance. The panel on the right is fitted with an aluminium heat exchanger of the claimed invention with 0.5 mm thickness. FIG. 7(b) shows the temperature recorded at the back of the panel without (black) and with (red) the heat exchanger of the claimed invention with background temperature (blue).

[0117] FIG. 7(c) shows the power output curve measured using industry standard instrumentation with (black) and without (red) the heat exchanger. This demonstrated a 27% increase in output power under the conditions given.

Optimum Design

[0118] The important parameters for the natural convection heat exchanger design are the fin height h.sub.fin, the fin thickness t.sub.fin and the fin spacing S. The fins define air channels of thickness (fin spacing) S, and the fluid properties, temperature difference and aspect ratio of the problem determine the optimum fin spacing S.sub.opt. In natural convection there is no external driving force and one must rely on buoyancy to drive the airflow. As such, the balance between buoyancy force and viscous drag is critical and determines the steady state fluid flow. There are several dimensionless parameters which can be used to characterize different regimes of fluid flow. The optimum spacing S.sub.opt can be found analytically for flat plate geometry and is well known from the academic literature (Thermally Optimum Spacing of Vertical, Natural Convection Cooled, Parallel Plates, A. Bar-Choen, and W. M. Rohsenow, Transactions of ASME, 116/Vol. 106, FEBRUARY 1984).

[0119] In one embodiment of the present invention, an optimum spacing of 0.8 cm for the 17 cm testbed was preferred, increasing to 1.4 cm for a standard 320 W panel of height 1.5 m. A simple fin-type heat exchanger was chosen to allow simple analytical expression to understand scaling. Fin length, spacing and thickness could be changed as a function of position on a photovoltaic solar panel in order to ensure uniform temperature of the cells in the photovoltaic solar panel, if this is deemed important.

[0120] The optimum fin spacing derived above assumes the fins are uniform temperature and are of negligible thickness, so they effectively have very high thermal conductivity. Realistic fin performance will depend on its thermal resistance, and there will be an optimum thickness and length. The optimum fin thickness and length for a specific fin material can be determined using finite element method numerical simulations. These simulations performed by the inventors include heat transfer due to conduction, convection and radiation and a full analysis of the fluid flow.

[0121] The optimum fin length h.sub.fin depends on the temperature difference, but also the fin thickness and the material thermal conductivity. Using a high thermal conductivity material or thicker fin allows similar performance to be achieved at a shorter fin length. The inventors used two thicknesses of 0.05 mm and 0.5 mm to demonstrate this for fins made from aluminium and graphite foil, a synthetic high thermal conductive material, as shown in FIG. 9. The thinner sample of 0.05 mm is representative of aluminium foil, which has a thickness in this range. The solar panel surface temperature is shown to decrease appreciably up to 50 mm, after which there are little gains. The thicker fin (0.5 mm) represents thin sheet metal which is more rigid. This shows a significantly lower temperature and gains are continued to be observed out to 100 mm. In this case, the cost and weight of the aluminium become a factor and the optimum length will need to be decided as a trade-off between cost, weight and performance.

[0122] The use of a synthetic graphite foil with a thermal conductivity of 1500 W/m/K is almost 8-times more thermally conductive than aluminium and has a lower density. The simulations show a slight improvement with the 0.5 mm thickness case as compared to 0.5 mm of aluminium. However, the more exciting result is that the 0.05 mm graphite film fin still outperforms the 0.5 mm aluminium fin. This graphite film fin is likely to be more expensive than aluminium, however it seems that one could use significantly less material, which would allow a user to save on cost and weight.

[0123] Although many different cooling technologies have been proposed in the past, they have not proven economically viable for large scale photovoltaic solar panel farms. The heat exchanger of the claimed invention can be retrofitted to existing solar panels in order to increase electricity output and prolong the lifetime of the solar panels themselves. The heat exchanger of the claimed invention can be added post-production to existing back sheet solar panels, or the heat exchanger itself could serve the function of the back sheet and be integrated at the panel manufacturing stage. The cost of photovoltaic solar panels has decreased by a factor of 10 in the last 5 years. In some arid regions, with high solar irradiance, such as parts of India, solar power generation has become cheaper than fossil fuels. The use of the heat exchanger of the claimed invention could lower the cost of production even further allowing solar panel farms globally to compete with other renewable and non-renewable power generation technologies.

[0124] In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms “include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa.

[0125] The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.