HEAT EXCHANGER FOR THERMOELECTRIC APPLICATIONS

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to foam heat exchangers, and more particularly, to an apparatus and method for enhancing heat transfer in thermoelectric systems using foam heat exchangers.

2. Description of Related Art

The use of heat exchangers to dissipate heat in power electronics applications is well known. Heat exchangers or heat sinks are frequently metal radiators designed to remove heat from power electronics components, particularly, power transistor modules, by thermal conduction, convection or radiation. Without heat exchangers power electronics component would suffer from reduced performance and reliability.

Heat exchangers are often structured to have a maximum number of fins per unit volume radiating in a direction perpendicular to a heated surface. In particularly demanding applications, heat exchangers dissipate heat using forced convection to a cooling fluid over the heat exchangers to increase the heat dissipation of the exchanger. An even more efficient apparatus for dissipating heat is the use of foams, and in particular metal forms, which have a more effective surface area for heat transfer. Metal foams have recently been used to dissipate heat in power electronic applications; however, they have not been used in thermoelectric systems.

Accordingly, there exists a need for foam heat exchangers to be used with thermoelectric elements to build systems for a variety of heating and cooling

systems that reduce energy consumption and increase heat pumping capacity in such systems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide thermoelectric heating and cooling systems that use foam heat exchangers.

It is also an object of the present invention to provide thermoelectric heating and cooling systems that use metal foam heat exchangers.

It is another object of the present invention to provide thermoelectric heating and cooling systems that use foam heat exchangers to dissipate heat.

It is a yet another object of the present invention to provide thermoelectric heating and cooling systems having thermoelectric elements that use foam heat exchangers to reduce the energy consumption of the thermoelectric elements.

It is still yet another object of the present invention to provide thermoelectric heating and cooling systems having thermoelectric elements that use foam heat exchangers to increase the heat pumping capacity of the thermoelectric elements.

It is a further object of the present invention to provide a method for enhancing heat transfer of thermoelectric elements using foam heat exchangers.

A system for enhancing the efficiency of a thermoelectric heat pumping system including an array of thermoelectric elements having a temperature at a first surface of the array and a temperature at a second surface of the array opposite the first surface and at least one foam heat exchanger located adjacent one of the first surface and the second surface is provided. The fluid flowing

through the at least one foam heat exchanger reduces a difference between the temperature at a first surface of the array and the temperature at a second surface of the array thereby enhancing the efficiency of the system.

A method of enhancing the efficiency of a thermoelectric system having a thermoelectric array having a series of thermoelectric pairs arranged electrically in series is provided. The method provides for a first foam heat exchanger adjacent a first surface of the thermoelectric array and a second foam heat exchanger adjacent a second surface of the thermoelectric array opposite first surface; for generating a temperature at a first surface of the thermal array and a temperature at a second surface of the array that is different from the temperature at the first surface of the array; whereby fluid flowing through the first foam heat exchanger and the second foam heat exchanger reduces a temperature difference between the first surface and the second surface, thereby enhancing the efficiency of the thermoelectric system.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates a thermoelectric system having foam heat exchangers of the present invention;

Fig. 2 shows a table that compares the heat transfer coefficients of different foams used in the thermoelectric system of the present invention and the weight savings compared to a conventional heat exchanger;

Fig. 3 illustrates a thermoelectric system functioning in a heating mode and using foam heat exchangers of the present invention;

Fig. 4 illustrates a graph showing increased coefficient of performance of thermoelectric systems as the heat transfer coefficient of heat exchangers increase;

Fig. 5 illustrates a foam heat exchanger of the present invention shown in Fig. 3; and

Fig. 6 illustrates a foam heat exchanger according to a second embodiment of the heat exchanger of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Fig. 1 , a thermoelectric system 10 having a thermoelectric elements 15, is shown. Thermoelectric elements 15 are grouped in several P and N pairs or couples 20 that are arranged electrically in series. Electrical connectors 25 provide the connection between adjacent couples 20 and to a power source (not shown). Substrates 30 and 35 are ceramic substrates that provide insulation to system 10. Substrates 30 and 35 hold system 10 together mechanically and insulate couples 20 electrically. Substrate 30 has a surface 40 that is in contact a with foam heat exchanger 45. Similarly, substrate 35 has a surface 50 that is in contact with foam heat exchanger 55. Fans 60 and 65 are used to force fluid through heat exchangers 45 and 55, respectively. Although fans 60 and 65 are shown forcing air through heat exchangers 45 and 55, respectively, other types of mechanisms for removing other types of fluid could also be used. Surfaces 40 and 50 may be integral to heat exchanger 45 and 55, respectively, and form a base for connecting to surface 30 and 35 of thermoelectric array.

In Fig. 1 , foam heat exchangers 45 and 55 are located immediately adjacent to substrates 30 and 35 to maximize heat transfer from the surfaces 70 and 75 of thermoelectric elements 15. Foam heat exchangers 45 and 55 provide enhanced heat transfer area from surfaces 70 and 75, respectively.

Foam heat exchangers 45 and 55 are made from highly conductive materials such as aluminum, copper or graphite. Exchangers made from such materials are not only highly conductive, but because they are formed as a foam, they have a very high porosity and surface area to further enhance their heat transfer capacity. Traditional heat exchangers used in thermoelectric applications have fins to dissipate heat. In comparison to foam heat exchanges, finned heat exchangers have a very limited surface area. Furthermore, traditional heat exchangers are relatively heavy compared to foam heat exchangers 45 and 55 of the present invention. Reducing the weight and/or volume and increasing the heat transfer capacity of heat exchangers is of great concern when both small and large heating and cooling thermoelectric systems are used.

Referring to Table 1 in Fig. 2, the heat transfer coefficients, maximum temperature and weight savings of a traditional heat sink compared to three foam heat exchangers of differing porosities, is shown. Comparing Foam A having a porosity of 10 PPI (pores per inch), the coefficient of heat transfer is over eighty- seven (87) times greater that that of the traditional heat sink. By doubling the porosity of the foam heat exchanger to 20 PPI, the coefficient of heat transfer of the Foam B is increased to one hundred and thirty (130) times that of the convention heat sink. Again doubling the porosity of the foam heat exchanger to 40 PPI, the coefficient of heat transfer of the Foam C is increased to one hundred and eighty-eight (188) times that of the convention heat sink. Not only is there a tremendous increase in heat transfer capacity, but the weight savings of the foam heat exchangers is also significant. The substantial weight savings reduces the overall weight of the thermoelectric refrigeration or heating system is which these exchangers would be used. Further, by reducing the maximum temperature of the system, the overall temperature difference across the thermoelectric array is decreased significantly. The coefficient of performance (COP) of thermoelectric systems is defined as the heating or cooling capacity divided by the power

consumed. The COP is inversely proportional to the maximum temperature difference across the array.

Referring to Fig. 3, a first embodiment of the present invention having a thermoelectric system 90 using foam heat exchangers 95 and 100 configured in a heating mode, is shown. A DC voltage from a power source 105 is applied across thermoelectric elements 120 and a current 110 flows in the direction shown. Pairs 115 (P and N pairs) of thermoelectric elements 120 absorb heat from a surface 125 and release heat to a surface 130 at the opposite side of device 120. Surface 125 where the heat energy is absorbed becomes cold and the opposite surface 130 where the heat energy is released becomes hot. This "heat pumping" phenomenon, known as the Peltier effect, is commonly used in thermoelectric refrigeration or heating. In this embodiment, fan 135 forces air through heat exchanger 100 which absorbs heat and is cooled. Fan 140 forces air through heat exchanger 95 to transport heat away from surface 130 to be heated. Power source 105 used in this configuration can be a battery, a fuel cell or any other similar device used to supply current. Thermoelectric system 90 can be converted from a heating mode to a cooling by reversing the polarity of DC poser supply 105.

Foam heat exchangers 95 and 100 provide substantial heat transfer capacity across surfaces 130 and 125, respectively, compared to traditional heat sinks to increase the efficiency of system 90. By having a high heat transfer coefficient foam heat exchangers 95 and 100, a lower the temperature difference between the opposing surfaces of thermoelectric elements 120, is achieved. This low temperature difference increases the performance of the overall system 90 by consuming less energy. Thus the overall system, whether it is configured as a heating or a cooling system, has a very high performance.

Fig. 4 shows the relationship between performance of the system and the coefficient of heat transfer using the foam heat exchangers of a typical

thermoelectric system. Coefficient of performance is defined as heating or cooling capacity divided by the power consumed by the system.

Referring to Figs. 5 a second configuration of a foam heat exchanger system 150 is shown. Foam heat exchanger system 150 has a thermoelectric array 155 having a series of thermoelectric pairs 160 arranged in series. Thermoelectric device array 155 has surfaces 165 and 170. System 150 is arranged to have a single foam heat exchanger 175 to dissipate heat from surface 170. Depending upon the application, a second foam heat exchanger may not be required. Alternatively, a traditional heat exchanger may be used in place of a foam heat exchanger depending upon the application and placed adjacent surface 165. Different configurations of placing foam heat exchangers can be used to maximize heat transfer and depending upon the application. Similarly, a single system can include several thermoelectric array, each having one or more foam heat exchangers.

In Fig. 6 a third embodiment of a foam heat exchanger system 180, is shown. System 180 is arranged similar to the system of Fig. 5, except that the heat exchanger is a combination foam and fin heat exchanger 185. System 80 has an array 190 of thermoelectric elements 195. Elements 195 have surfaces 200 and 205. In the embodiment of Fig. 5, a second foam heat exchanger may not be required. Alternatively, a traditional heat exchanger may be used in place of a foam heat exchanger depending upon the application. Additionally, different configurations of placing foam heat exchangers can be used to maximize heat transfer and depending upon the application.

While the instant disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope thereof. In addition, many modifications may be made to adapt a particular situation or material to the

teachings of the disclosure without departing from the scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment(s) disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.