Heating Mechanism for a Fuel Cell Stack

FIELD OF THE INVENTION

The present invention generally relates to a fuel cell stack and more particularly, but not exclusively to a heating mechanism for a fuel cell stack.

BACKGROUND OF THE INVENTION

Conventional petrol and diesel engine vehicles contribute significantly to emission of green house gases. Hence, in order to meet the stringent emission norms, fuel cells are being increasingly used. Polymer electrolyte membrane fuel cells or proton exchange membrane fuel cells which operate at relatively low temperatures are the most commonly used fuel cells in automotive applications. The operating temperature of these fuel cells is low as the membrane is sensitive to increase in temperature, and in order to prevent membrane degradation due to increase in temperatures beyond the operating temperature, vehicles employing these fuel cells are provided with air/liquid cooling. It is also essential to ensure that the membrane of these fuel cells is kept moist during the reaction between fuel and air to prevent membrane degradation. Therefore, air is usually humidified before being sent into the ~- fuel cell. This requires the installation of humidifiers in vehicles provided - with these fuel cells. Installation of a liquid cooling system and humidifiers in the vehicle not only leads to increase in weight of the vehicle but also leads to an increase in the cost of manufacturing

In order to eliminate the above mentioned problems, high temperature proton exchange member (HTPEM) fuel cells are being used increasingly. However, because of their high operating temperatures, which are typically above 130°C, fuel cells of the HTPEM type are required to be preheated in order that the electrochemical reaction be initiated. Various technical approaches for the preheating of high temperature fuel cells have been developed and are known in the art. In a known art, resistive heating is used to preheat the fuel cell stack by the insertion of resistive coil inside the high temperature fuel cell stack. However, good sealing of the fuel cell stack is required so as to prevent the resistive heating current from interfering with the fuel cell stack current. Moreover, good sealing is also required to prevent the high temperature fuel cells from leaking. Thus, use of resistive heating technique for preheating high temperature fuel cells, necessitates good sealing of the resistive coil placed inside the high temperature fuel cell, which can be a cumbersome process.

Other known arts involve the use of external heating elements or devices such as heating plates, heating pads, heating pumps and the like, which not only consume a longer time to heat the high temperature fuel ■ cells, but also increase ^e overall-cemplexity,- energy sofisu-mption and cost of the fuel cell system.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a simple and efficient heating mechanism to heat up a fuel cell stack and particularly a high temperature fuel cell stack, in order to initiate an electrochemical reaction inside the fuel cell stack. It is another object of the invention to provide a heating mechanism which aids in direct heating of a fuel cell stack. The present subject matter described herein therefore relates to a fuel cell stack heating system. The fuel cell stack heating system as per the present invention uses induction heating to preheat the fuel cell stack in order to initiate an electrochemical reaction inside the fuel cell stack. According to the present invention, the fuel cell stack heating system comprises a fuel cell stack, an insulation sheet disposed around the fuel cell stack, a primary coil wound around said insulation sheet disposed around the fuel cell stack, wherein the primary coil forms a part of a primary induction circuit, and a reflector housing surrounding said fuel cell stack. The primary induction circuit further comprises a capacitor and a terminal connecting two ends of the primary coil. An alternating voltage applied across the terminal causes current to flow through the primary coil.

Variation oTVurYeht~witfrlime~c^^^ field to be produced in the primary coil, which in turn causes an induced current to flow through the fuel cell stack. The fuel cell stack acts as a secondary circuit. Induced current flowing through the fuel cell stack generates heat inside the fuel cell stack. Additionally, heat dissipated by the primary coil is reflected back into the fuel cell stack by the reflector housing surrounding the fuel cell stack, thereby minimizing radiation loss. Thus, the fuel cell stack is heated not only by the induced current flowing through the fuel cell stack, but also by the radiative heat reflected by the reflector housing surrounding the fuel cell stack. Thereby, a fuel cell stack operating temperature in the range of 80°-160° can be easily attained. Further, rapid heating of the fuel cell stack is achieved by varying the alternating voltage applied to the primary coil, thereby enabling a quick initiation of the electrochemical reaction inside the fuel cell stack.

Moreover, induction heating at a high frequency also aids in reducing the probability of any electrochemical reaction getting hindered or enhanced because the time scales of alternating voltage is much shorter as compared to the time scale of the rate determining step of the electrochemical reaction taking place inside the fuel cell stack. Furthermore, the insulation sheet disposed around the fuel cell stack aids in ensuring that there is no interference of the current flowing through the primary coil with fuel cell stack current. As a result, a requirement-for gooel-sea^ing of-the-fuelceH stack can be done away-withr-- • -

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a fuel cell stack in accordance to the present invention.

Figure 2 illustrates a circuit diagram of the fuel cell stack heating system in accordance to the present invention.

Figure 3 illustrates a cross sectional view of the fuel cell stack in accordance to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter reference will now be made in detail to various embodiments of the present invention, examples of which are illustrated in the accompanying drawings and described below. While the invention will be described in conjunction with exemplary embodiments, it will be understood that present description is not intended to limit the invention to those exemplary embodiments. On the contrary, the invention is intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

The present invention provides a fuel cell stack heating system 100, which uses inductive heating for heating a fuel cell stack 10. As per one exemplified embodiment of the present invention, the fuel cell stack heating system 100 comprises a fuel cell stack 10, an insulation sheet 30 disposed around said fuel cell stack 10, a primary coil 60 wound around said insulation sheet 30 disposed around the fuel cell stack 10, wherein the primary coil 60 forms a part of a primary induction circuit 65, and a reflector housing 90 surrounding said fuel cell stack 10. Referring to Figure 1, description is given of the construction of the fuel cell stack 10. The fuel cell stack 10 comprises of at least two current collector plates 11, plurality of bipolar plates 12, and plurality of membrane electrode assemblies 14. In the present embodiment, each bipolar plate of the plurality of bipolar plates 12 is composed of graphite and is arranged alternatingly with each membrane electrode assembly of the plurality of membrane electrode assemblies 14. The plurality of bipolar plates 12 aid in regulating the flow of air and hydrogen into the fuel cell stack 10.

Further, an electrochemical reaction between reactants air and hydrogen takes place at the surfaces of the plurality of membrane electrode assemblies 14, wherein each membrane electrode assembly comprises of an electrolyte sandwiched between a cathode and an anode. Thus, chemical energy is converted to electrical energy. A gasket (not shown) is placed between each bipolar plate of the plurality of bipolar plates 12 and each membrane electrode assembly of the plurality of membrane electrode assemblies 14 to prevent cross mixing of hydrogen and oxygen inside the fuel cell stack 10. However, in order that the electrochemical reaction be initiated inside the fuel cell stack 10, the fuel cell stack 10 must be pre heated to its optimal operating temperature. The fuel cell stack heating system 100 (Shown in Figure 2) comprising the fuel cell stack 10, an insulating sheet 30 disposed over the fuel cell stack 10, a primary coil 60 wound around said insulation sheet 30 disposed over the fuel cell, stack 10, wherein the primary coil 60 forms a part of a primary induction circuit, and a reflector housing 90 surrounding said fuel cell stack 10; is used to pre heat the fuel cell stack 10 by means of induction heating.

The primary induction circuit includes the primary coil 60, a capacitor 80, and a terminal 50 connected to two ends of the primary coil 60. In order to heat the fuel cell stack 10, firstly an alternating voltage is applied across the terminal 50 connected to two ends of the primary coil 60, causing current to flow through the primary coil. Since the voltage supplied to the primary induction circuit is alternating voltage, time varying current flows through the primary coil 60. Further, the time varying current flowing through the primary coil 60 produces a time varying magnetic field across the primary coil 60, thereby causing an induced current to flow inside the fuel cell stack 10 as per Lenz's law, wherein the fuel cell stack 10 acts as the secondary coil. The choice of the alternating voltage to be supplied to the primary coil 60 depends on the requirement of the power in the fuel cell stack 10. —■•—•■--- The power requirement of the fuel cell stack can be computed from the below equation. Where 'P' is the power supplied to the fuel cell stack 10, 'h' is the heat transfer coefficient, 'm' is the mass of the fuel cell stack 10, Cp is the specific heat capacity, a 'is the Boltzmann constant, T is the temperature inside the fuel cell stack 10, 'A' is the surface area of the fuel cell stack 10 and * is the ambient temperature.

A unique value of the capacitor for a given applied voltage, frequency of the applied voltage and the resistance of the primary coil ensures maximum power transfer to the fuel cell stack 10. One of the ways of maximising the amount of induced current flowing inside the fuel cell stack 10 is by selecting an optimum value of capacitance of the capacitor 80 in the primary induction circuit. Thus, depending on the power requirement inside the fuel cell stack 10, for a given value of frequency of applied voltage and a given value of resistance of the primary coil 60, maximum power or maximum induced current can be obtained in the fuel cell stack by changing the value of capacitance each time. At a particular value of capacitance, maximum power or maximum induced current is obtained inside the fuel cell stack 10.
With the passage of induced current through the fuel cell stack 10, heat gets generated in the fuel cell stack 10. When the operating temperature of the fuel cell stack is attained, electrochemical reaction at the surfaces of the plurality of membrane electrode assemblies 14 gets initiated, causing fuel cell stack current to be produced. However, it is necessary to ensure that the induced current does not interfere with the fuel cell stack current.

In the present embodiment, the non interference of the induced current with the fuel cell stack current is ensured by maintaining the time period of the alternating voltage applied across the terminal 50 to be less than the time period of rate determining step of the electrochemical reaction taking place inside the fuel cell stack 10. In other words, it is ensured that the region of induced current and the region of electrochemical reaction are different. Therefore, the problem of leakage of the fuel cell stack 10 due to current interference is eliminated. Moreover, using a high frequency voltage across the primary induction circuit ensures that the distribution of induced current is concentrated within a narrow range called skin depth (shown in Figure 3) of the fuel cell stack 10. Further, the insulating sheet 30 disposed around the fuel cell stack 10 also aids in preventing interference of current flowing through the primary coil 60 with the fuel cell stack current. As a result, the requirement of good sealing for the fuel cell stack 10 can be done away with. Thus, by controlling the amount of induced current flowing inside the fuel cell stack 10, the amount of heat generated inside the fuel cell stack 10 can be controlled, depending upon the operating temperature range of the fuel cell stack 10. Thus, the present fuel cell stack heating system 100 is applicable not only to a high temperature fuel cell stack but also to a low temperature fuel cell stack.

Furthermore, a large number of turnings of the primary coil 60 and a high value of resistance of the primary coil 60 causes heat to be dissipated by the primary coil 60 as per power law l2R, where I is the current flowing through the primary coil 60 and R is the resistance of the primary coil 60. The heat thus dissipated by the primary coil 60 is reflected into the fuel cell stack 10 by means of the reflector housing 90 surrounding the fuel cell stack 10. Moreover, the reflector housing 90 also reflects the heat radiated by the fuel cell stack 10 back into the fuel cell stack 10. Thus, radiation loss is minimized by placing the reflector housing 90 outside the fuel cell stack 10, thereby ensuring that maximum amount of radiation is reflected back into the fuel stack 10. In the present embodiment, the reflector housing 90 surrounding the fuel cell stack 10 preferably comprises infrared reflector plates. Moreover, choice of the primary coil 60 with a low resistance also helps to ensure low wastage of heat and that all the power will be transferred directly to the fuel cell stack 10.

Thus, the fuel cell stack 10 gets preheated not only by the induced current flowing through the fuel cell stack but also by the radiative heat

• reflected by the refteetor housing 90 back-into the fuel cell stack 10. As a ~-w. result, a fuel cell stack operating temperature in the range of 80°-160° may be easily attained. Moreover, since the heating inside the fuel cell stack 10 is direct, therefore rapid heating of the fuel cell stack 10 is ensured.

Therefore, since the fuel cell stack heating system 100 causes direct and rapid heating of the fuel cell stack 10, the need for use of external heating elements such as heating plates, heating pads, heating pumps and the like which not only consume a longer time to heat the high temperature fuel cells, but also increase the overall complexity, energy consumption and cost of the fuel cell system can be eliminated. While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form, connection, and detail may be made therein without departing from the spirit and scope of the invention as defined in the appended claims:

WE CLAIM:

1. A fuel cell stack heating system (100), wherein said fuel cell stack heating system (100) comprises a fuel cell stack (10), an insulating sheet (30) disposed around the fuel cell stack (10), a primary coil (60) wound around said insulation sheet (30), and a reflector housing surrounding (90) said fuel cell stack (10), and wherein the fuel cell stack heating system (100) causes induction heating of said fuel cell stack (10).

2. The fuel cell stack heating system (100) as claimed in claim 1, wherein the fuel cell stack (10) comprises a plurality of bipolar plates (12), a plurality of membrane electrode assemblies (14) arranged alternatingly between the plurality of bipolar plates (12), and at least two current collector plates (11).

3. The fuel cell stack heating system as claimed in claim 1, wherein the primary coil (60) forms a part of a primary induction circuit, said primary induction circuit comprising a capacitor (80) and a terminal (50) connecting two ends of said primary coil (60).

4. The fuel cell stack heating system (100) as claimed in claim 3, wherein the primary coil (60) generates an induced current across the fuel cell stack (10) on application of an alternating voltage across the terminal (50) connecting two ends of said primary coil.

5. The fuel cell stack heating system (100) as claimed in claim 4, wherein the induced current generated across the fuel cell stack (10) heats the fuel cell stack (10) to an operating temperature in the range of 80°-160°.

6. The fuel cell stack heating system (100) as claimed in claim 5, wherein the heat generated in the fuel cell stack by the induced current is variable with varying capacitance of the capacitor (80) and varying frequency of the alternating voltage applied across the terminal (50) connecting two ends of said primary coil.

7. The fuel cell stack heating system (100) as claimed in claim 1, wherein the reflector housing (90) surrounding the fuel cell stack (10) comprises of infrared reflector plates, and reflects radiated heat back into the fuel cell stack (10).