WO2009007905A2

WO2009007905A2 - Heat pipe

Info

Publication number: WO2009007905A2
Application number: PCT/IB2008/052721
Authority: WO
Inventors: Tim Dekker; Anthonie H. Bergman
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-07-11
Filing date: 2008-07-07
Publication date: 2009-01-15
Also published as: TW200928271A; WO2009007905A3

Abstract

A heat pipe, for cooling a light source, comprising a casing (201) enclosing a cavity (205) where a working fluid (210) is arranged for transporting heat performed by vaporization and condensation of the working fluid in said cavity, wherein the casing is shaped to at least partlyenclose a hollowness (207), wherein the casing comprises an inner casing surface (203), an outer casing surface (202), and a cavity wall (208), and wherein said casing (201) has at least 5 two open portions such that a passage between said portions, through said hollowness (207), is formed.

Description

Heat pipe

FIELD OF THE INVENTION

The invention relates to the field of thermal management of illumination devices, and more specific to the cooling of a light emitting diode using a heat pipe structure.

BACKGROUND OF THE INVENTION

Using light emitting diodes (LEDs) as lamps, in both industrial and consumer products are becoming increasingly popular. This is especially true for high power LEDs, which continues to replace traditional incandescent light bulbs in new automotive, industrial, backlight display, and architectural detail lighting systems. The reason for the growing popularity is that the LEDs excel in a wide range of performance and cost parameters, including spectral performance, long life, robustness, small form factor, instant light, and falling manufacturing cost.

However, the high power LEDs suffers from a high thermal load when used in traditional lighting applications. Important parameters of the LED such as efficiency, lifetime, and color are very sensitive to the temperature of the LED, thus making thermal management a key issue in LED lighting applications.

A popular way of conducting heat management, to reduce the thermal load, is to mount the LEDs on a metal-core printed circuit board (MCPCB), and equip the MCPCB with a heat sink. This type of cooling arrangement is often bulky since the heat sink needs to be quite large to provide the necessary cooling to the LED. By adding a fan blowing air at the heat sink a smaller heat sink can be used. However, the fan will consume extra power and will often add unwanted noise to the lighting arrangement.

Both heat sinks and fans are very bulky to use and many times the cooling arrangement constitutes more than half the total volume of the device. The large bulky structure hampers the design of elegant and sleek lighting applications.

A more effective and sleeker cooling arrangement involving a heat pipe is presented in patent application WO03/ 107440. A heat pipe comprising a sealed cylinder member of transparent or translucent material, having a wick disposed down the side of the and along the bottom thereof, partially filled with a transparent liquid which is placed under a partial vacuum to reduce its boiling point, is, together with a heat sink, added to a LED structure. The effective cooling of the heat pipe reduces the needed size of the heat sink and the translucency of the conducting material making up the heat pipe allows an optical transmission path of the light generated from the LED. However, not even the heat pipe arrangement of the cited patent application will solve the problem of getting rid of the bulky heat sink.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve or at least reduce the above given problems and to provide a more effective heat pipe reducing the need for cooling a light source, e.g. a LED, with an additional heat sink.

This object is achieved by a hollow double walled heat pipe structure arranged in accordance with claim 1. Preferred embodiments are defined by the dependent claims.

According to a first aspect of the present invention, there is provided a heat pipe comprising a casing enclosing a cavity where a working fluid is arranged for transporting heat by vaporization and condensation of the working fluid in said cavity, wherein the casing is shaped to at least partly enclose a hollowness, wherein the casing comprises an inner casing surface, an outer casing surface, and a cavity wall, and wherein said casing has at least two open portions such that a passage between said portions, through said hollowness, is formed.

The open portions that create a passage through the hollowness will create a 'chimney effect', which allows for an effective way of passive cooling, and thus reduce or eliminate the need for additional cooling using a heat sink. The casing can be shaped into any form that at least partially enclose a hollowness, and the two open portions, which together with the hollowness create a passage, can be located anywhere on the casing. For example, if the casing is shaped as a tube, enclosing a hollowness, and the two open portions are at each end of the tube, a hollow pipe is formed.

In an embodiment, the casing is shaped to enclose the hollowness.

The heat pipe may have at least one of its inner or outer casing surfaces made of a heat conducting material. This will allow differentiating the heat dissipation between the casing surfaces. For instance, one may want to dissipate most of the heat to the inner casing surface, which is additional cooled by the chimney effect.

The heat pipe may have the outer casing surface made of a non-heat conductible material so it can be designed to match or blend in with its surroundings. The heat pipe may have the cavity of the heat pipe partially filled with a working fluid.

The heat pipe may have the cavity wall covered by a wick comprising a material capable of exerting a capillary force on the working fluid, which more effectively transports the condensed vapor or create a liquid film on the surface. This will cause the condensation to spread on the surface and provide optical clarity and a better cooling effect.

The heat pipe may have a transparent casing material and a working fluid, which makes the heat pipe structure transparent in its design. This will allow light from the light source to pass through the heat pipe structure without being blocked. The working fluid in the heat pipe may be water, purified deionized water, alcohol, or mercury. These fluids have good heat transportation properties.

The heat pipe may have at least one of its outer or inner casing surfaces equipped with cooling flanges to more effectively dissipate heat from the casing.

The heat pipe may have at least one of its outer or inner casing surfaces equipped with at least one air vent, capable of venting heated air originating from the inner casing surface to the outer casing surface, or the reverse to more effectively dissipate heat from the casing.

The heat pipe may have a light source that is a light emitting diode. According to a second aspect of the present invention, there is provided a use of a heat pipe for cooling a light source.

According to a third aspect of the present invention, there is provided a lighting device comprising a light source and a heat pipe, wherein said heat pipe is arranged to cool the light source.

According to a fourth aspect of the present invention, there is provided a method of cooling a light source by a heat pipe where the heat pipe comprising a casing enclosing a cavity where a working fluid is arranged for transporting heat by vaporization and condensation of the working fluid in said cavity, wherein the casing is shaped to at least partly enclose a hollowness, wherein the casing comprises an inner casing surface, an outer casing surface, and a cavity wall, and wherein said casing has at least two open portions such that a passage between said portions, through said hollowness, is formed, wherein the method comprises the step of transporting heat by vaporization and condensation of the working fluid in said cavity. It is noted that the invention relates to all possible combinations of features recited in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non- limiting detailed description of currently preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein: Figure 1 illustrates an ordinary heat pipe structure.

Figure 2 illustrates a double walled heat pipe structure according to an embodiment of the present invention.

Figure 3 shows a cross section of a double walled heat pipe structure in an upright and a downright position according to an embodiment of the present invention. Figure 4 illustrates a rectangular double walled heat pipe structure according to an embodiment of the present invention.

Figure 5 illustrates a double walled heat pipe structure with a slit according to an embodiment of the present invention.

Figure 6 illustrates a double walled heat pipe structure with cooling flanges on the outer and inner casing surfaces according to an embodiment of the present invention. Figure 7 illustrates a double walled heat pipe structure with air vent holes according to an embodiment of the present invention.

All the figures are highly schematic, not necessarily to scale, and they show only parts, which are necessary in order to elucidate the invention, other parts being omitted or merely suggested.

DETAILED DESCRIPTION OF EMBODIMENTS

In recent years heat pipe structures have been introduced into the field of cooling semiconductor devices. Figure 1 illustrates an ordinary heat pipe structure consisting of a casing 104, shaped in the form of a sealed hollowed tube or a cavity 108, which usually is made of a material with good heat conducting properties such as copper, aluminium, or glass.

The heat pipe cavity holds a coolant 102, also known as a "working fluid", which ordinary at least partially fills the heat pipe. The working fluid, which may be placed under a partial vacuum so as to lower its boiling point, may either be ordinary or purified deionized water, ethanol, mercury, or some other suitable fluid.

The internal or cavity wall 106 of the heat pipe casing 104 is often covered by a wick or a similar material which exerts a capillary force on the liquid phase fluid. The wick material often consists of some kind of hydrophilic coating, sintered metal powder, a series of grooves, or any other coating, material, or structure capable of effectively soaking up and transport the liquid fluid.

In use, one part of the heat pipe, for instance the lower end 110 in Figure 1, is placed against the thermal active region of the object that is going to be cooled, which in this case is the LED. The working fluid 102 is, during operation, heated by the heat produced by the active region of the LED until it is vaporized, at which point it rises up through the vapor cavity 106 to the upper cooler part of the heat pipe 112. In this manner the heat is effectively transported away from the active region as latent heat within the working fluid vapor. The vapor is effectively cooled in the upper parts of the heat pipe 112 where it condenses. The condensed vapor is transported back to the hotter end of the heat pipe 110 by the force of gravity and by the capillary action exerted on the condensed vapor in the wick. Thus, a cooling cycle is completed. Heat transferred to the casing by the heating in the lower part 110 and by the condensation process in the upper part 112, is radiated to the surrounding ambient air, cooling the heat pipe casing 104. Figure 2 illustrates a heat pipe case structure (201) according to a currently preferred embodiment of the present invention. The tube like heat pipe case structure 201 is a double walled heat pipe structure having two casing surfaces exposed to the ambient air, an outer casing surface 202 and an inner casing surface 203, and the two open portions, one at the top of the casing structure and one at the bottom of the casing structure, form together with the hollowness a hole throughout the structure 207 . A cross section 204 shows that the double walled heat pipe structure, comprising a casing, a vapor cavity 205, a working fluid 210 partially placed in a vacuum, internal walls 208 with or without a wick 206, an upper cooler part 214, and a lower hotter part 212, has the same basic construction and functionality as the heat pipe presented in Figure 1 with the exception of the hollow double walled arrangement.

The advantage of the double walled heat pipe structure shown in Figure 2 is that the casing surface area, which makes contact with the ambient air, is roughly doubled in comparison to the casing surface area of an ordinary heat pipe such as the one shown in Figure 1. Due to the larger surface area the heat dissipation rate will increase thus cooling the LED connected to the hollow double walled heat pipe more efficiently.

A further advantage with the hollow double walled heat pipe structure is that the hole in the middle can create a 'chimney effect', which allows for an effective way of passive cooling. Figure 3 shows a cross section of how the airflow travels a in a hollow double walled heat pipe structure, in an upright position 301 and in a horizontal position 302. In both positions 301 and 302 the lower point of the heat pipe structures is the hottest while the upper point is the coolest. The heated ambient air inside the hollowness will heat up and travel upwards throughout the hole at the top of the heat pipe structure. This will effectively draw new cool air into the hole in the lower part of the double walled heat pipe structure and thereby cool the hottest part of the structure. Thus, the hollow double walled heat pipe structure combines the effect of a larger heat dissipation rate, due to the larger surface area, with a air cooling effect, due to the chimney effect of the hollow heat pipe structure, comparable to a heat sink with an fan moving air throughout it. Further advantages includes that the total size of the heat pipe cooling system can be reduced considerably in size, and thus completely getting rid of the bulky heat sink which is hampering the design of elegant and sleek lighting applications including LEDs. Furthermore, the hollow double walled heat pipe structure can be made in many shapes such as cones, bowls, cubes, rectangles, etc. as long as the shapes includes a hollowness to provide the added surface area and the chimney effect. For example, figure 4 shows an embodiment of the invention where the hollow double walled heat pipe is in the shape of a rectangle 400, with a rectangle hole 401 throughout the structure. The hollow double walled heat pipe structure can easily be adapted to suit a wide range of lighting applications needing cooling, not only those including LEDs. The hollowness of the double walled heat pipe structure does not have to be completely enclosed to provide the advantages discussed above. Figure 5 shows an embodiment of the invention where the hollow double walled heat pipe has a slit 500 running along the side of the heat pipe. The heat pipe structure still provides the advantage of having a large surface area and also the same chimney effect as the heat pipe structure discussed in Figure 2.

Another advantage of the hollow double walled heat pipe structure is that the casing surface can be made from any material, heat conductive or not, so it can be designed to match and to blend in the surrounding environment or to look esthetical pleasing. Ordinary at least one of the inner or outer casing surfaces should be made from a heat conducting material. The outer casing surface can for instance be made in a material that matches the design of the lamp. This enables it to be fully incorporated into the overall design of the LED and the lighting application.

In another embodiment the casing material of the hollow double walled heat pipe structure can be completely made of a light (including the UV-, IR-, and the visible light spectrum) transparent material. By using glass together with a clear and transparent working fluid such as water, a completely transparent heat pipe structure can be made, thus effectively making the heat pipe structure more or less invisible. In this case the transparent heat pipe can be placed closer to the warm active region in the LED without the risk of blocking the light. To enhance the transparency of the heat pipe, the inner wall or wick can be matched with the working fluid and coated with a hydrophilic coating or anti-fog coating, which causes the condensation of the working fluid to spread on the surface and thus provide optical clarity. For example in one embodiment a thin and highly transparent liquid film is formed on the surface instead of light obstructing droplets. In another embodiment the droplets formed on the surface is quickly transported away, by capillary farces in the material, down to the hotter part of the heat pipe to once again be vaporized.

There are several ways of further increase the heat dissipation capacity of the hollow double walled heat pipe presented in Figure 2. The inner or the outer, or both, casing surfaces can be equipped with cooling flanges of various sizes and shapes. This will increase the heat dissipation surface thus increase the heat dissipation capacity. Figure 6 shows an embodiment of the invention where the hollow double walled heat pipe structure is equipped with cooling fins on the outer surface 600 and on the inner surface 601. Also, fans can be placed at one or both ends of the heat pipe to increase the air flow around the whole heat pipe structure (both outer and inner casing surfaces) or just in the hollowness of the heat pipe (the inner casing surface only).

The shape and size of the hollow double walled heat pipe structure can also be changed and adapted to a given situation to provide an optimal heat dissipation capacity. For instance, air-venting holes can be placed throughout the heat pipe to let hot air to escape from the inner casing surface out past the outer casing surface or the reverse. Figure 7 shows an embodiment of the invention where the hollow double walled heat pipe structure is equipped with additional air vent holes between the inner and the outer casing surfaces where excessive heat can escape. The arrows indicate the airflow in the heat pipe structure in its upright position and in its horizontal position. The hollow double walled heat pipe structure can also be combined with ordinary MCPCB-, heat sink-, and fan solutions to provide an increased heat dissipation capacity.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. The described embodiments are therefore not intended to limit the scope of the invention, as defined by the appended claims.

Claims

CLAIMS:

1. A heat pipe comprising: a casing (201) enclosing a cavity (205) where a working fluid (210) is arranged for transporting heat by vaporization and condensation of the working fluid in said cavity, wherein the casing is shaped to at least partly enclose a hollowness (207), wherein the casing comprises an inner casing surface (203), an outer casing surface (202), and a cavity wall (208), and wherein said casing (201) has at least two open portions such that a passage between said portions, through said hollowness (207), is formed.

2. The heat pipe according to claim 1, wherein at least one of the inner or outer casing surfaces is made of a heat conducting material.

3. The heat pipe according to claim 1 or 2, wherein the working fluid partially fills the cavity of the heat pipe.

4. The heat pipe according to claim 1 or 3, wherein the outer casing surface is made of a non-heat conductible material.

5. The heat pipe according to any of the previous claims, wherein the cavity wall is covered by a wick (206) comprising a material capable of exerting a capillary force on the working fluid.

6. The heat pipe according to any of the previous claims, wherein the casing material and the working fluid are transparent.

7. The heat pipe according to any of the previous claims, wherein the casing material and the working fluid are matched to counteract transparency-reducing condensate to form.

8. The heat pipe according to any of the previous claims, wherein the working fluid comprises at least one of the following: water, purified deionized water, alcohol, or mercury.

9. The heat pipe according to any of the previous claims, wherein the at least one of the outer or inner casing surfaces are equipped with cooling flanges (600, 601).

10. The heat pipe according to any of the previous claims, wherein the at least one of the outer or inner casing surfaces are equipped with at least one air vent (700), capable of venting heated air originating from the inner casing surface to the outer casing surface, or the reverse.

11. The heat pipe according to any of the previous claims, wherein the light source is a light emitting diode.

12. The heat pipe according to any of the previous claims, wherein the casing is shaped to enclose the hollowness.

13. Use of a heat pipe according to any of the previous claims, for cooling a light source.

14. A lighting device comprising a light source and a heat pipe according to any of the previous claims, wherein said heat pipe is arranged to cool the light source.

15. The lighting device according to claim 14, wherein the light source is a light emitting diode.

16. Method of cooling a light source by a heat pipe, the heat pipe comprising: a casing (201) enclosing a cavity (205) where a working fluid (210) is arranged for transporting heat by vaporization and condensation of the working fluid in said cavity, wherein the casing is shaped to at least partly enclose a hollowness (207), wherein the casing comprises an inner casing surface (203), an outer casing surface (202), and a cavity wall (208), and wherein said casing (201) has at least two open portions such that a passage between said portions, through said hollowness (207), is formed, wherein the method comprises the step of transporting heat by vaporization and condensation of the working fluid in said cavity.