EP2111520A2

EP2111520A2 - Liquid cooling of photovoltaic solar panels

Info

Publication number: EP2111520A2
Application number: EP08724917A
Authority: EP
Inventors: Guy Diemunsch
Original assignee: Aavid Thermalloy LLC
Current assignee: DIEMUNSCH, GUY
Priority date: 2007-01-31
Filing date: 2008-01-30
Publication date: 2009-10-28
Also published as: TW200845403A; EP2111520A4; WO2008094555A2; FR2911997B1; FR2911997A1; WO2008094555A3

Abstract

Photovoltaic solar panels are cooled below ambient air temperature with a positive energy balance using a cooling fluid flowing through a flat chamber of substantially uniform thickness provided against the back surface of each panel. The chamber thickness and other dimensions of the flat chamber are chosen so that, for a predetermined mass flow rate through the chamber in a given temperature range, a laminar flow regime is maintained. Obstacles which provide varying flow patterns may be provided in the chamber to disturb the boundary layer and improve heat transfer to the fluid. Since laminar flow minimizes the energy required to. move the fluid, there is very little parasitic drain of the power produced by the solar panel.

Description

LIQUID COOLING OF PHOTOVOLTAIC SOLAR PANELS

BACKGROUND OF THE INVENTION

1. Field of the invention

The invention concerns liquid cooling of photovoltaic solar panels. More particularly, the invention pertains to the cooling of solar panels generating low heat flux, particularly panels which do not operate with concentrated illumination.

2. Description of Related Art

Photovoltaic solar panels generate electricity proportionally to the light flux they receive. Their efficiency is reduced as the temperature increases. The ratio ΔP/P lies between -0.3 % and -0.5 % per Celsius degree. The ratio is linked to the material used to build the solar panel. When the light increases, the temperature also increases. Two physical phenomena cause this result. First, the ambient air is at a higher temperature (from 35 to 5O⁰C) in regions where the light flux reaches 1,000 Watts/m². Second, the solar panel stores heat until a balance is reached where the heat received equals the heat lost. The heat is lost by natural convection. The balance is stabilized when the temperature difference between the photovoltaic cell and the ambient air is greater than 20°C. Natural convection is limited by the position of the solar panel (horizontal at the equator) and is often reduced by the solar panel frame surrounding the cells . In summary, in sunny regions solar cells reach a temperature higher than 6O⁰C and lose between 10.5 % and 17.5 % of the power versus the power generated at 25⁰C.

As an example, for a solar cell with an efficiency of 16% at 25°C, the loss is at least 2.8 % which means an efficiency of 13.2 % at 60°C.

Current solutions achieve a balance between the heat received and the heat lost with a lower temperature difference by increasing the heat exchange surface on the back of the solar panel. This is accomplished by attaching a natural convection aluminum heat sink to the back of the solar cells. However, this solution only results in an increase in efficiency of 1.5 %, at most. The position of the solar panel is then defined by the orientation of the heat sink, where vertical, or substantially inclined, fin channels are desirable. The solution of an aluminum heat sink on the back of the solar cells does not cool efficiently when the solar panel is in a horizontal position. Also, the interface between the heat sink and the cells is difficult to manufacture because the surfaces of the solar panels are large. This solution, therefore, can only be used if the cost of the heat sink is lower than the cost of 1.5 % of efficiency .

Forced convection allows use of a heat sink with a plate set horizontally or to gain a few tenths of percent of efficiency with inclined solar panels. Horizontally- mounted panels are less limited by installation constraints but require energy for the forced convection. To reach temperatures lower than ambient air and/or to be able to make use of the accumulated heat, a cooling fluid such as water is used. The water can be obtained from a tap water supply network, from ground water, or from water which is cooled by passing through conduits in the ground. The criteria for using a cooling fluid is then:

(a) Energy needed to move the fluid << Energy saved with the solar panel, or

(b) Energy needed to move the fluid << Energy saved with the solar panel + additional thermal energy retrieved from the heated-up cooling fluid.

Different kinds of "cold plates" which use mainly water as the cooling fluid can be used. The flow state inside these cold plates is transient or turbulent. Because of that, the pressure drop is high so that the condition for equation (a) is not met. The high pressure also makes the condition of equation (b) difficult to reach.

SUMMARY OF THE INVENTION

The invention relates to a method and an apparatus for cooling a photovoltaic solar panel wherein a flat chamber of substantially uniform thickness is provided against the back surface of the solar panel, the chamber having an inlet and an outlet for moving fluid through the chamber, wherein a laminar flow of fluid is maintained in the chamber, while providing for good heat transfer between the panel and the fluid. The invention allows the cooling of solar panels with recuperation of heat using a minimum of energy to move the cooling fluid. A back plate is provided for connecting either directly to a back surface of a solar panel or to a thermal and mechanical interface which is connected to the back surface of the solar panel so that a flat chamber is formed between the solar panel and the back plate. The back plate includes an inlet for receiving a cooling fluid, e.g. water, and an outlet for removing the cooling fluid from the chamber. The movement of cooling fluid in the chamber results in a sheet-like flow of cooling fluid for removing heat from the solar panel.

Specifically, an apparatus is provided for cooling a solar panel of the type which receives direct, diffused or reflected solar radiation without any optical device to concentrate the solar radiation on the solar panel . The apparatus includes a plate having an inlet and an outlet and a flat surface bounded by a border having an edge extending from a first end at the flat surface to a second end at a height above the flat surface. The plate is connectable to the solar panel so that the second end of the edge faces a back surface of the solar panel and the flat surface is substantially parallel to the back surface of the solar panel. The plate edge and flat surface define a flat chamber for receiving cooling fluid from the inlet and directing the cooling fluid, in a sheet-like flow, along a flow path in the chamber toward the outlet. The sheet-like flow of cooling fluid provides thermal coupling with the solar panel when the plate is connected to the solar panel for transferring heat from the solar panel to the cooling fluid. As used herein the term "flat" chamber refers to a chamber of substantially uniform thickness between the back of the solar panel and the opposed flat surface, wherein the thickness is less than 20 mm, and preferably less than 10 mm.

In one embodiment, the chamber has a plurality of flow direction baffles for separating the fluid into channels and directing fluid flow along a flow path between the inlet and the outlet such that a circulation of the fluid occurs in the chamber in a substantially laminar sheet, part of which comprises a boundary layer with the back surface of the solar panel or with the thermal and mechanical interface.

To increase the heat transfer between the sheet of fluid and the solar panel, or the thermal and mechanical interface, the sheet of fluid is agitated or modified.

In one embodiment a multiplicity of ribs is included in the chamber, with the ribs oriented in the direction of the fluid flow to generate an un-stationary laminar flow pattern.

In another embodiment, the thickness of the sheet of cooling fluid is varied by disposing obstacles such as ribs on the back plate in the chamber in a direction perpendicular to the direction of fluid flow which generates instability in the boundary layer, i.e. instability of the layer of cooling fluid in contact with the back surface of the solar panel or in contact with the thermal and mechanical interface.

In yet another embodiment, a multiplicity of hemispherical elements are provided in the chamber and in flow path of the sheet of cooling fluid to vary the laminar flow pattern by generating Von Karman vortices .

In still another embodiment, a multiplicity of wings are provided in the chamber in the path of fluid flow to locally change the direction of the fluid flow and to generate instability of the boundary layer of the sheet- like laminar flow.

In a further embodiment, a multiplicity of rotating paddle wheels oriented transversely to the direction of fluid flow; the paddle wheels generate un-stationary flow patterns as well as an unstable boundary layer.

In each of these embodiments, flow velocity components transverse to the back of the panel are created, in order to disturb the boundary layer and increase heat transfer to the fluid.

Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, in which like elements are identified with like numerals:

Figure 1 shows, with an assembled overall view, an embodiment of the invention;

Figure 2 shows, with an exploded overall view, the embodiment of Figure 1;

Figure 3 shows, with a detailed view, a flat plate according to an embodiment of the invention with an attachment and guiding area for attaching the flat plate to a solar panel or to a thermal and mechanical interface;

Figure 4 shows, with an overall view, a chamber formed in the flat plate with flow direction baffles for guiding a sheet of cooling fluid in accordance with an embodiment of the invention;

Figure 5 shows, with an overall view, inlet and outlet fluid distribution in the chamber formed in the flat plate of Fig. 4;

Figure 6 shows, with a detailed view, flow distribution baffles separating the channels and inlet baffles for distributing fluid to the channels of the embodiment of Fig. 4;

Figure 7 shows, with an overall view, the chamber with ribs to destabilize the laminar liquid flow in the sheet of cooling liquid in accordance with another embodiment of the present invention; Figure 8 shows, with a detailed view, ribs in the channels to destabilize the flow of the sheet of cooling liquid;

Figure 9 shows, with an overall view, the chamber with profiled obstacles for varying the thickness of the sheet of cooling liquid in accordance with another embodiment;

Figure 10 shows, with a detailed cut view, the profiled obstacles of the embodiment of Fig. 9 forming a speed variation in the sheet of cooling liquid;

Figure 11 shows, with an overall view, the chamber with half-sphere-shaped obstructions to destabilize the laminar flow of the sheet of cooling liquid in accordance with another embodiment of the invention;

Figure 12 shows, with a detailed view, the half-sphere obstructions for destabilizing the laminar flow of liquid of the embodiment of Fig 11;

Figure 13 shows, with a detailed view, Von Karman vortices produced from the half-sphere obstructions of the embodiment of Figs. 11 and 12;

Figure 14 shows, with an overall view, the chamber with wing-shaped obstructions in accordance with still another embodiment of the invention;

Figure 15 shows, with a detailed view, the wing-shaped obstructions of the embodiment of Fig. 14;

Figure 16 shows, with a detailed view, the flow paths of the cooling liquid occurring in the liquid sheet produced in the embodiment of Figs. 14-15; Figure 17 shows, with an exploded overall view, another embodiment of the invention wherein paddle wheels are employed to obstruct the flow of the sheet of cooling liquid;

Figure 18 shows, with an exploded detailed view, the embodiment of Fig. 17;

Figure 19 shows, with an assembled detailed view, the embodiment of Fig. 17;

Figure 20 shows, with a section view, the liquid flow across a paddle wheel of the embodiment of Fig. 17;

Figure 21 shows, with an exploded perspective view, a combination of different obstructions in the fluid chamber; and

Figure 22 shows, with an exploded perspective view, the back plate interface with the thermal and mechanical interface and with a solar panel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the figures, a solar panel 1 is cooled by a thin ( < 8 mm ) sheet of cooling liquid which can be either directly in contact with the back of the solar panel or in contact with a thermal and mechanical interface 25 which is in direct contact with the solar panel (Fig. 22) . For each square meter of solar panel, the heat to be dissipated can reach 910 Watts. The heat flux is low (0.91 W/cm²). One of the most commonly used solar panels has the following planar dimensions: 1424 mm x 655 mm. The surface is thus 0.933 m². The cooling fluid can be a liquid, such as water, for example in an open loop arrangement. For the exemplary panel having planar dimensions of 1424 mm x 655 mm and a heat flux of 910 watts per square meter of panel, the required heat rejection is approximately 849 watts. For this amount of heat rejection to the flowing water the required mass flow rate of the water depends on the amount of water temperature rise (water exit temperature - water inlet temperature) the designer specifies. As an example, if the desired maximum temperature rise in the water is 5 degree C, the required mass flow rate is approximately 0.041 kg/sec.

The actual form of the mass flow calculation is well known to one skilled in the art; mass flow rate = heat rejection/ [ (specific heat of water) x (temperature rise of water) ] . Given a water density of 1 gram/cc this mass flow rate corresponds to a volumetric flow rate of approximately 2.45 liters per minute. The cross- sectional area of fluid flow is approximately the panel width (655 mm in this example) multiplied by the thickness of the sheet of fluid. Setting the fluid sheet thickness to 8 mm leads to a cross-sectional area of about 0.0052 m². Given this cross-sectional area and the volumetric flow rate of 2.45 liters per minute the flow velocity (a velocity whose direction is normal to the flow cross-sectional area defined above) is approximately 0.0078 meters per second. This flow velocity together with the flow cross-sectional dimensions and fluid properties establish the Reynolds Number (RN) of the flow. Reynolds Number is a dimensionless group commonly used to describe a flow condition in terms of the ratio of inertial to viscous forces on a flowing fluid. The specific form of the calculation is as follows:

RN = pVl/μ where p = density of fluid V = velocity of fluid 1 = characteristic length μ = dynamic viscosity of fluid

In the instant example the characteristic length is the hydraulic diameter of the flow cross-section. Hydraulic diameter (D_h) is calculated in the following manner:

Dh = (4 x cross-sectional flow area) /perimeter of the flow cross-section

Given the aforementioned dimensions (655 mm x 8 mm flow cross-section) D_h is approximately 15.8 mm. Based on a water temperature of 20 degrees C, the approximate dynamic viscosity is 1.00 x 10^~3 kg/ (meter-second) , and density is approximately 1.0 gram/cc. The resulting RN for this example is approximately 122. With this RN the flow regime will be in the form of a substantially laminar sheet. The dynamic pressure drop generated by this flow is lower than 20 Pa for one square meter of solar cell .

As an alternate example, if the desired temperature rise of the fluid was reduced from 5 to 3 degrees C, the required mass flow rate to satisfy the required heat rejection of 849 watts would increase to about 0.068 kg/sec. Given the same fluid density as the previous example the resulting velocity and volumetric flow are approximately 0.013 meters per second and 4.1 liters per minute respectively. In this case, the resulting Reynolds Number is 204.

The photovoltaic panels would typically be arranged in a row to receive water in parallel, so that the temperature range and gradient is substantially the same for each panel. Working with a row of eight panels requiring 2.5 1/min for each panel yields a total of 20

1/min. A suitable pump for this capacity would be a spherical motor pump, such as the Ecocirc solar DC pump made by Laing GmbH.

The energy to operate a circulation pump for moving the cooling fluid inside the open loop arrangement is mainly related to the static pressure drop as the water is elevated to the solar panel. For a closed loop arrangement where the cooling fluid is circulated through conduits from the output back to the input, the static pressure drop is substantially less than the open loop arrangement, and practically negligible. The dynamic pressure drop in both arrangements is also comparatively negligible. For horizontal panels the circulating energy will be very low.

In accordance with an embodiment of the invention the laminar flow of liquid is produced by using a plate 2 having a flat surface bounded by a shelf 6 having an edge 4. The plate 2 is covered by a cover member for forming a chamber 40 for the cooling liquid. The cover member can be the back surface of the solar panel 1 or an intermediate plate such as the thermal and mechanical interface 25 which contacts the back surface of the solar panel. The chamber 40 defines a thickness of the sheet of cooling liquid. A lip 5 of the plate 2 is dimensioned to seat around the solar panel 1 (in one embodiment) or the thermal and mechanical interface 25 (in another embodiment) and will assist in the assembly.

An input 3 and an output 9 of the fluid are provided to and from an inlet port 30 and an outlet port 34 in the plate 2 as shown in Figs 2 and 5. The inlet port 30 and the outlet port 34 can be arranged to follow the width of the solar panel. Natural convection can be used to augment the function of the pump by locating the inlet lower than the outlet so that buoyancy forces in the fluid are in the same direction as the flow created by the pump .

The plate 2 with its lip 5 can be easily formed by injection molding. The plate can be mechanically attached to the solar panel 1 with pinching, gluing, riveting, the use of clips or in any manner known in the art. The shelf 6 between the edge 4 and lip 5 is used to support the joint or the glue. As an alternative to attaching the plate 2 directly to the back of the solar panel, the plate 2 can be mechanically attached to a thermal and mechanical interface 25 (Fig. 22) .

Soldering, brazing or welding is feasible for attaching the plate 2 to the thermal and mechanical interface 25 when those parts are made of metal. The shelf 6 will be used to support the joint or the glue or the soldering or the brazing.

To generate uniform cooling over the back surface of the solar panel 1, when the solar panel 1 is in any working position, i.e. horizontal or angled, the sheet of liquid will be divided, such as, for example, into parallel flow channels 7 in the direction of the fluid flow as show in Figures 4 and 5. It is preferred that the width of each channel 7 is a multiple of the size of the solar cells used to make up the solar panel 1. An inlet manifold 8 is provided for uniformly distributing the liquid to the channels 7.

Each channel 7 receives liquid from the inlet manifold 8, which receives liquid from the inlet port 30. The manifold 8 is defined between the edge 4 and a plurality of spaced-apart inlet baffles 32. The spaces between the inlet baffles provide the liquid to the channels 7. Liquid exits the channels 7 into an outlet manifold 36 which is defined between the edge 4 and a plurality of spaced-apart outlet baffles 38.

The channels 7 are separated by flow direction baffles 11 which extend between the inlet baffles 32 and the outlet baffles 38 and, preferably, are arranged perpendicular thereto. As best shown in Figs. 4 and 5, the ends of the flow direction baffles 11 intersect certain ones of the inlet and outlet baffles. In a preferred embodiment, the height of the flow direction baffles 11 and of the inlet and outlet baffles 32, 38 equal the height of the edge 4 forming the inner perimeter of shelf 6, such that the baffles will contact the back of the solar panel 1 (in one embodiment) or the thermal and mechanical interface 25 (in another embodiment) . The baffles will provide increased stiffness to the plate 2 and will also maintain the thickness of the sheet of liquid by providing support between the plate 2 and the solar panel 1 or the thermal and mechanical interface 25 in the event low pressure exists in the chamber 40 which could cause inward bowing of the solar panel 1, the thermal and mechanical interface 25 and/or the plate 2. The division of the liquid sheet into the channel 7 does not change the flow regime. The Reynolds Number remains close to 120.

The separation ribs 11 preferably have a narrow or rounded shape 10 at their tops to reduce the contact area with the solar panel or with the thermal or mechanical interface 25 and to increase the heat exchange surface between the panel and the fluid.

A purely laminar liquid flow reduces heat exchange because there is a thick boundary layer, which is defined as the layer of liquid directly adjacent the rear side of the solar panel 1 or the thermal and mechanical interface 25. This heat exchange is minimized when the solar panel is horizontal. The high thermal conductivity of water assists the heat exchange. In order to minimize the energy required to pump the water, it is desirable to increase the heat exchange while keeping the flow regime substantially laminar. Static or dynamic solutions generating movements in the laminar flow pattern or a varying of the thickness of the boundary layer meet these objectives.

The following solutions can be used alone or in combination with one or more of the other below listed solutions to obtain a desired cooling solution:

At least four static solutions can be used. Solution A in Figs. 7 and 8, and solution B in Figs. 9-10, generate two dimensional mixing, while solution C in Figs. 11-13 and solution D in Figs. 14-16 generate three dimensional mixing .

At least one dynamic solution E can be used as shown in Figs. 18-20.

A) Generating flow development length areas .

After a perturbation generating instability inside a laminar flow of liquid along the channels 7, which means a moving or un-stationary laminar flow pattern, there is an area where stable laminar flow is under development. This area is called the "flow development length." To create a constant and moving laminar flow pattern, a multiplicity of obstacles is placed in the chamber 40 in the flow path of the fluid. To minimize the pressure drop in the chamber 40, in one embodiment these obstacles are shown as spaced-apart ribs 14 oriented in the direction of the flow shown by arrow 12 and arranged in parallel rows as shown in Figures 7 and 8. The length of these ribs 14 will be, as an example, less than the flow development length. The space 13 between the ribs, in the direction of the flow, will be, as well, less than the flow development length for space 13.

B) Compression and expansion of the boundary layer.

This solution is realized by a multiplicity of obstacles 15 formed as successive ribs extending perpendicularly to the flow direction as shown in Fig. 9. These obstacles are shaped to reduce the local pressure drop and to at least double the local fluid speed 16 as shown in Fig. 10. Specifically, the obstacles 15 do not extend to the same height as the edge 4 or the inlet and outlet baffles 32, 38, so that the fluid can flow over the obstacles. This arrangement causes the thickness of the boundary layer to locally change in the location opposite the obstacles. This modification creates an unstable area inside the boundary layer. This unstable area is temporarily propagated after each obstacle 15, and then the fluid again becomes stable until it encounters the next obstacle 15. Ideally, an obstacle should be located at the position on the flat plate 2 where the boundary layer becomes stable after undergoing instability from a preceding obstacle. This solution also increases the stiffness of the plate across the direction of the flow.

C) Generating Von Karman three dimensional vortices.

Von Karman vortices are un-stationary vortices created by cylindrical or spherical obstacles inside a laminar flow. The principle is to provide a multiplicity of hemispherical obstacle bumps 17, as shown in Figures 11- 13, which preferably occupy the entire thickness of the liquid sheet, i.e. the bumps have a height equal to the height of the edge 4 and the inlet and outlet baffles 32, 38 in order to provide structural support of the solar panel 1. These bumps generate vortices 18 in a clockwise direction and vortices 19 rotating counter clockwise with respect to the direction of fluid flow. These vortices do not stay behind the obstacle 17 and are propagated along the direction of fluid flow until the flow stabilizes. Successive downstream bumps are preferably positioned where the vortices cease.

D) Creation of wing end vortices. Instead of the bumps 17 of Figures 11-13, upstanding arcs or wings 20 can be provided as shown in Figures 14- 16. At the end of each wing there is one or more vortices 21 created by the interaction of two flows: one deviated by the wing and the other one not deviated. The height of the wings is less than the thickness of the liquid sheet. The distance between the tops of the wings and either the solar panel or the thermal and mechanical interface 25 should be larger than the thickness of the boundary layer to obtain an area where the flow is not deviated by the wing. As indicated by arrow 42 in Fig. 15, the wings are concave in the direction facing the fluid flow.

To improve the efficiency, the orientation of the wing will reduce the effect of natural convection when the flow is perpendicular to gravity.

E) Dynamic mixing solution, paddles.

In the embodiment shown in Figures 17-20, a multiplicity of rotating paddle wheels 22 are introduced inside the sheet of cooling liquid, perpendicularly to the flow direction. The diameter of the paddle wheels 22 will be roughly two times the thickness of the liquid sheet. Half of each paddle wheel is guided by a semi-cylinder trough 23 formed in the plate 2. A mechanical play will be set to allow easy rotation of the paddle wheel.

The paddle wheels will be maintained in place by sandwiching between the plate 2 and either the solar panel 1 or the interface 25. The attachment can be made as well by clipping inside the trough 23. In this latter case, the guide will be slightly deeper than half of the diameter of the paddle wheel.

The paddle wheels each have at least two paddles 48, the number of paddles being determined by the desired heat exchange coefficient. The width of each paddle is approximately half the liquid sheet thickness. The paddle wheels 22 with the paddles 48 are arranged in the troughs 23 such that the paddles are at the same level as the external diameter of the wheel 22 to approach, without contact, the surface of the solar panel or the thermal or mechanical interface 25.

The paddle pushed by the fluid 12 causes rotation 24 as shown in Figure 20. The paddle wheel 22 moves closely to the solar panel 1 or the thermal and mechanical interface 25 and mixes the boundary layer. At the same time, the paddle 48 changes the local speed of the fluid and destabilizes the laminar flow.

It should be apparent from the foregoing that the cooling fluid increases in temperature as it flows from the inlet 30 to the outlet 34 and as the solar panel is cooled. The heated water can be used, as is common with so-called hybrid solar panels, for domestic use if the water is sufficiently sanitary, or for heating of a structure such as a house.

Thus, while there have been shown and described and pointed out fundamental novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices described and illustrated, and in their operation, and of the methods described may be made by those skilled in the art without departing from the spirit of the present invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.

Claims

CLAIMSWhat is claimed is:

1. Apparatus for cooling a solar panel of the type which receives direct, diffused or reflected solar radiation without any optical device to concentrate the solar radiation on the solar panel, the apparatus comprising : a plate having a shelf surrounding a flat surface with a first end provided with an inlet and a second end provided with an outlet, the solar panel being fixable to the shelf to form a flat chamber of substantially constant thickness for receiving cooling fluid from the inlet and directing the cooling fluid in a sheet-like flow along a flow path in the chamber toward the outlet, whereby heat is transferred from the solar panel to the cooling fluid, wherein the thickness and other dimensions of the flat chamber are chosen so that, for a predetermined mass flow rate of a given fluid through the chamber in a given temperature range, a laminar flow with a Reynolds number of less than 2000 will be achieved.

2. The apparatus of claim 1 comprising an inlet manifold extending across said chamber at said first end and an outlet manifold extending across said chamber at said second end, said inlet manifold comprising a plurality of spaced-apart inlet baffles, and said outlet manifold comprising a plurality of spaced apart outlet baffles.

3. The apparatus of claim 2 further comprising at least one flow direction baffle extending between said manifolds to form a plurality of longitudinal flow channels .

4. The apparatus of claim 3, further comprising a plurality of spaced-apart obstacles positioned on the flat surface in the flow channels.

5. The apparatus of claim 4 wherein the obstacles comprise spaced-apart ribs arranged in parallel rows, each rib having a length less than a flow development length of the sheet-like flow of cooling fluid and being separated from adjacent ribs by a distance less than the flow development length, the ribs being oriented in a direction parallel to the flow direction and contacting the back surface of the solar panel.

6. The apparatus of claim 4 wherein the obstacles comprise ribs extending across the chamber transversely to the flow path, each rib having a height less than the thickness of the chamber, the ribs being separated from adjacent ribs by a distance less than a flow development length of the sheet-like flow of cooling fluid.

7. The apparatus of claim 4 wherein the obstacles comprise hemispherical bumps having a height equal to the height of the shelf, said bumps forming Von Karman vortices in the sheet-like flow.

8. The apparatus of claim 4 wherein the obstacles comprise wings having a concave surface facing the flow, the wings having a height less than the height of the shelf.

9. The apparatus of claim 4 wherein the plate comprises troughs in the flat surface perpendicular to the flow channels, the obstacles comprising paddle wheels mounted in the troughs, the paddle wheels being rotated by the sheet-like flow of cooling.

10. The apparatus of claim 9 wherein the paddle wheels each have at least two paddles, each paddle having a width which is approximately half the thickness of the chamber.

11. The apparatus of claim 1, further comprising a thermal and mechanical interface member mechanically connected to the plate for providing a thermal interface between the plate and the solar panel.

12. The apparatus of claim 11, further comprising a lip surrounding the shelf.

13. A method of cooling a photovoltaic solar panel having a back surface, the method comprising: providing a flat chamber of substantially uniform thickness against the back surface of the solar panel, the chamber having a flat surface spaced from the back surface by said thickness, a first end provided with an inlet, and a second end provided with an outlet; moving a fluid along a flow path through said chamber from said inlet to said outlet; and maintaining a laminar flow of said fluid in said chamber by controlling the flow so that the Reynolds number is less than 2000.

14. The method of claim 13 wherein the flow is controlled so that the Reynolds number is less than 1000.

15. The method of claim 14 wherein the flow is controlled so that the Reynolds number is less than 120.

16. The method of claim 13 wherein the chamber has a thickness of less than 20.0 mm.

17. The method of claim 16 wherein the chamber has a thickness of less than 8.0 mm.

18. The method of claim 13 wherein the fluid is water.

19. The method of claim 18 wherein the flow is controlled so that the fluid has a mass flow rate through the chamber in the range of 40-60 g/sec per square meter of cross section of the solar panel.

20. The method of claim 13 wherein the flow is controlled so that the fluid exhibits a dynamic pressure drop of less than 20 Pa between said inlet and said outlet.

21. The method of claim 13 further comprising providing baffles in said chamber which generate a substantially uniform rate of flow across the width of said chamber.

22. The method of claim 13 further comprising placing obstacles in said flow path, wherein said obstacles generate flow velocity components toward said back surface .

23. The method of claim 22 wherein said obstacles comprise ribs on said flat surface, said ribs extending transversely to said flow.

24. The method of claim 22 wherein said obstacles comprise one of cylindrical and hemispherical obstacles which generate Von Karman vortices .

25. The method of claim 24 wherein said obstacles are hemispherical knobs, each knob having an apex which contacts said back surface.

26. The method of claim 22 wherein said obstacles comprise wings which generate wing end vortices .