Abstract:
An automotive lighting assembly cooling system includes a heat pipe with an evaporation area proximate to a beat generating component, such as a Light Emitting Diode (LED), and a condensing area located remote from the evaporation area. Evaporation of fluid within the heat pipe transfers heat away from the heat generating component. The efficiency of the cooling system in one embodiment is increased by including fins associated with the condensing area and placing the fins in an area where air flow external to a moving vehicle assists in cooling the fins.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Generally, conventional automotive lighting systems utilize filament bulbs as a lighting source. However, filament bulbs have many drawbacks, including high consumption of electrical power, the generation of great amounts of heat, and readily breakable filaments. Recently, due to these drawbacks, light emitting semiconductor devices (LESDs), such as light emitting diodes (“LEDs”), have been adapted for use in certain automobile lighting systems.  
           [0002]    LEDs solve many of the problems associated with filament bulbs, because they emit light using a lower voltage and current than used by a filament bulb and are less prone to breakage. However, various other problems are associated with LEDs when used in automobile lighting systems. For example, typical LEDs produce significantly less light than filament bulbs. Accordingly, a number of LEDs must be used within a particular light assembly to generate the requisite amount of light. As the number of LEDs is increased, the amount of heat generated by the LEDs increases. Alternatively, LEDs which operate at a significantly higher power may be used. Of course, as the power of the LED increases, the amount of heat generated also increases.  
           [0003]    The increase in heat is a significant problem for LED systems. While the expected lifetime of an LED at room temperature may be tens of thousands of hours, that same LED when exposed to high temperatures may only last several thousand hours. Moreover, the amount of light emitted by an automotive lighting systems is a significant safety issue subject to various laws and regulations. Accordingly, an automotive lighting system must produce a stable luminous flux at all times throughout the lifetime of the lighting system. However, subjecting an LED to an increased temperature results in an immediate dimming of the LED at the time of exposure. Moreover, as an LED is exposed to increased temperatures over a period of time, the maximum achievable output of the LED is diminished even during uses at lower temperatures.  
           [0004]    Therefore, a need exists for an automotive lighting system that efficiently provides for dissipation or removal of excess heat generated by LEDs within the lighting system.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    The present invention comprises an automotive lighting system including an LED thermally coupled to a heat transfer condensing tube or heat pipe at the evaporation area of the heat pipe. Fins, which may be located exterior to the vehicle, are affixed to the heat pipe to assist in transfer of heat away from the heat pipe.  
           [0006]    In operation the heat pipe is filled with a fluid such as water or some other acceptable refrigerant. As the LED operates, heat is generated and transferred to the evaporation area through the shell of the heat pipe and then to the fluid. As the temperature of the fluid reaches its boiling point, additional heat is drawn from the heat pipe and some of the fluid changes to a vapor state, expanding throughout the void of the heat pipe.  
           [0007]    As the vapor expands in the void, it contacts the heat pipe at a condensation area which is located remote from the area at or near which the LED is mounted. Since the shell of the heat pipe is cooler at the condensation area than the evaporation area, heat is transferred from the vapor to the heat pipe at the condensing area. Fins may be placed external the heat pipe to assist in removing heat from the heat pipe, for example, by passing air over them. Accordingly, the condensing area is maintained at a temperature below the boiling point of the fluid. Thus, as the vapor contacts condensing area, heat is transferred from the vapor to the condensing area and out through the fins. This causes the vapor to condense into droplets of fluid which are directed to the area of the heat pipe near the LED.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a partial cutaway view of a heat pipe.  
         [0009]    [0009]FIG. 2 is a partial cutaway plan view of one embodiment of an LED condensation cooling system in accordance with the present invention.  
         [0010]    [0010]FIG. 3 is a sectional view of an alternate embodiment of an LED condensation cooling system in accordance with the present invention is shown.  
         [0011]    [0011]FIG. 4 is a side view of an automobile showing some exemplary locations for placement of the condensing area of a heat pipe. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    [0012]FIG. 1 is a partial cutaway view of a beat pipe. Heat pipe  100  comprises shell  110 , wick  120 , condensation area  130  and evaporation area  140 . Wick  120  is initially saturated with a fluid such as deionized water. The basic operation of heat pipe  100  is as follows. Condensation area  130  is placed so as to be thermally coupled to a heat generation site. Evaporation area  140  is placed in an area which is at a lower temperature relative to the heat generating site. Thus, as heat is generated, the heat is transferred to heat pipe  100  at condensing area  130 . As heat is transferred to condensing area  130 , the fluid within wick  120  near to condensing area  130  is heated. As the heated fluid reaches its vaporization temperature, the fluid is vaporized, thus absorbing more heat. The vaporized fluid expands into void  125  defined by shell  110 , transferring heat by convection away from condensation area  130 .  
         [0013]    The vaporized fluid expands within shell  110  to condensation area  140 . Condensation area  140  is cooler than evaporation area  130 , and is below the boiling point of the fluid. Therefore, as the vaporized fluid comes in contact with condensation area  140 , the vaporized fluid condenses and is absorbed into wick  120 . Condensation of the fluid releases heat of vaporization into condensation area  140 . Heat transferred from the fluid at condensation area  140  is passed through shell  110  to a heat sink external to heat pipe  100 . The external heat sink may comprise air within the body of a vehicle, air external to the vehicle, or even the vehicle itself.  
         [0014]    Wicking is used to accelerate the transfer of fluid condensed at condensation area  130  to evaporation area  140 . This transfer results from capillary action within wick  120  The condensation of fluid at condensation area  130  and evaporation of fluid at evaporation area  140  results in comparatively more fluid within wick  120  at condensation area  130  compared to evaporation area  140 . Thus, fluid flows within wick  120  from condensation area  130  to evaporation area  140 . Wicking is useful in that transfer of fluid is not dependent on the orientation of heat pipe  100 .  
         [0015]    With reference to FIG. 2, a partial cutaway plan view of one embodiment of an LED condensation cooling system in accordance with the present invention is shown. LED panel  200  is designed to be located within the light assembly of a vehicle. LED panel  200  includes a number of individual LEDs  202  and is selected from material with desired thermal conductivity such that heat generated by LEDs  202  may be transferred through LED panel  200 . The number of LEDs is a design choice. Heat pipe  204  is thermally coupled to LED panel  200  at evaporation area  206 . Thermal coupling may be realized in any acceptable manner. By way of example, but not of limitation, thermal coupling may be achieved by placing heat pipe  204  in direct contact with LED panel  200  or by bonding heat pipe  204  to LED panel  200  with a thermally conducting epoxy.  
         [0016]    Heat pipe  204  includes fins  208  which protrude from shell  210 . Fins  208  increase the effective surface area of shell  210  in the proximity of condensation area  212 . Accordingly, the transfer of heat out of heat pipe  204  is increased. Heat transfer may also be increased by forcing air past shell  210  and/or fins  212 . Heat transfer into heat pipe  210  may also be increased by the use of fins. For example, by locating fins within evaporation area  206 , the surface area may be increased, resulting in increased transfer of heat. Use of fins within the evaporation area of a heat pipe is particularly useful when using heat pipes designed to have a reservoir of fluid in the condensation area.  
         [0017]    Referring now to FIG. 3, another embodiment of the present invention is shown. LED panel  300  is designed to be located within light assembly  302 . Heat pipe  304  is thermally coupled to LED panel  300  at evaporation area  306 . LED panel  300  and heat pipe  304  in this embodiment are an integrated unit. Heat pipe  304  includes fins  308  which protrude from shell  310  at condensation area  311 . Fins  308  in this embodiment are hollow, and wick  310  extends into fins  308  to increase flow of fluid away from condensation area  311  and toward evaporation area  306 .  
         [0018]    As stated above, increased heat transfer may be realized by increasing the flow of air across the evaporation area and/or the fins of the heat pipe. This flow of air may be realized in a variety of embodiments. For example, the evaporating area and/or fins may have a dedicated fan or the vehicle&#39;s engine compartment fan may be used to direct air flow across the evaporating area and/or fins.  
         [0019]    Alternatively, the evaporating area and/or fins may be mounted so as to make use of the air flow around the vehicle as the vehicle moves. FIG. 4 shows exemplary locations  400 ,  402 ,  404  and  406  on vehicle  408  which may be particularly advantageous in practicing this aspect of the invention. According to one embodiment, evaporating area and/or fins may be mounted so as to allow for thermal coupling through the outer surface of the vehicle. Other embodiments include mounting a heat pipe so that the evaporating area and/or fins of the heat pipe are directly exposed to air flow around vehicle  408  while being flush with the outer surface of the vehicle.  
         [0020]    While the present invention has been described in detail with reference to certain exemplary embodiments thereof, such are offered by way of non-limiting example of the invention, as other versions are possible. By way of example, but not of limitation, the present invention is flexible enough to allow for a variety of design choices in addition to those set forth above. For example, the evaporating area and/or fins of the heat pipe may be designed to extend beyond the outer surface of the vehicle, such as in the form of a styling feature. Additionally, the evaporating area and/or fins of the heat pipe may be camouflaged as trim pieces. Moreover, fins may be included either inside or outside of the shell at the evaporation area and/or the condensation area to provide for additional heat transfer. These and other modifications are within the scope of the present invention.  
         [0021]    Furthermore, in practicing the present invention, the heat pipe may be constructed of any suitable material or materials. Suitable materials may include, but are not limited to, aluminum, stainless steel, or copper. Design considerations in selection of the material may include heat conductivity, cost, aesthetics, and compatibility with other materials either in the light assembly or the vehicle.  
         [0022]    Additionally, while certain embodiments discussed above include use of a wicking material, a number of alternative embodiments exist. For example, wicking may be provided by artery wicks, sintered material, slab and tunnel wicks, wrapped screens or the formation of microgrooves with the walls of the heat pipe. Alternatively, the heat pipe may be fabricated with smooth walls, with the condensation area located above the evaporation area such that as the vapor condenses into fluid state, the fluid flows to the evaporation area due to gravity.  
         [0023]    Moreover, the fluid and pressure may be selected so as to provide the desired cooling capability in consideration of the fluid&#39;s latent heat of vaporization and thermal conductivity. Accordingly, the fluid may be deionized water, anhydrous ammonia, Freon, or oil or any other suitable fluid and the pressure within the heat pipe may be modified upward or downward so as to effect the desired temperature of condensation and boiling point. It is anticipated that a variety of other modifications and changes will be apparent to those having ordinary skill in the art and that such modifications and changes are intended to be encompassed within the spirit and scope of the invention as defined by the following claims.