APPARATUS FOR THERMAL REGULATION OF A HIGH TEMPERATURE PEM FUEL CELL STACK

Abstract

The present invention provides fuel cell stacks comprising effective means to maintain the fuel cell stacks at a constant temperature using plates mated to at least one face of the stack and in contact with the edges of the repeat and non-repeat layers while making use of the phase change of working-fluids such as water or water-organic species mixtures for heat transfer. Also provided are processes for maintaining said fuel cell stacks at a constant temperature by adjusting the flow rate and pressure of the cooling fluid so that both liquid and vapor are present at the same time.

Claims

1. A fuel cell stack comprised of Repeating bipolar plates and MEAs and non-repeating layers, of end plates One or more plates mated to the at least one face of the stack and in contact with the edges of the repeat layers, said one or more plates being adapted to act as a constant temperature thermal reservoir.

2. The stack of claim 1 wherein the said plate incorporates a working-fluid flowing within a fluidic-circuit

3. The stack of claim 2 wherein the working-fluid water, or mixtures of water and propylene glycol, water and ethylene glycol, water and methanol, or water and ethanol.

4. The stack of claim 1 wherein the operating temperature of the stack is between 120C and 260C.

5. The stack of claim 3 wherein the pressure within the fluidic-circuit is adjusted to be the saturated vapor pressure of the working-fluid at the stack operating temperature.

6. The stack according to claim 3, wherein the steam quality of the mixture is adjusted to be between 5 percent and 95 percent.

7. The stack of claim 5 in which excess flow is introduced into the fluidic circuit to ensure the presence of both liquid and vapor phases.

8. The stack of claim 1 in which a thermally-conductive dielectric layer is disposed between the constant temperature thermal reservoir plate and the face of the stack

9. The stack of claim 7 in which a lubricant is disposed on the at least one face of the thermally-conductive dielectric layer.

10. The stack of claim one in which a lubricant is disposed onto the at least one face of the at least one thermally conductive dielectric layer.

Description

IN THE DRAWINGS

[0017] FIG. 1 is an isometric view of the fuel cell stack according to a first embodiment of the present disclosure.

[0018] FIG. 2 is an exploded isometric view of the fuel cell stack of FIG. 1.

[0019] FIG. 3 is a diagram of a piping-circuit for the working-fluid, which receives heat from and transfers heat to the fuel cell stack of FIG. 1

[0020] FIG. 4 is an isometric view of the thermal-mass with a fluidic-circuit.

[0021] FIG. 5 is an isometric cross-sectional view of the thermal-mass with fluidic-circuit.

[0022] FIG. 6 depicts a Boyd Corporation, Lytron, pressed tube cold plate.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The term “thermal-mass” as used herein denotes a monolithic plate which may receive and release thermal energy. In the present disclosure, the thermal-mass may freely exchange thermal energy with a fuel cell stack to which it is intimately mated.

[0024] The present disclosure is generally directed to the use of a thermal-mass incorporating a fluidic-circuit adapted to provide a constant temperature reservoir with which a fuel cell stack may exchange thermal energy. The pressure in the fluidic-circuit is adjusted so that the temperature of the two-phase flow corresponds to a specified saturation temperature. The presence of two-phases ensures that the thermal-mass and the exterior of the stack that it is mated to, are each maintained at a constant temperature.

[0025] Depicted in FIG. 1 according to a first embodiment of the present disclosure, the fuel cell stack, 1 is comprised of bipolar plates stacked in series with membrane electrode assemblies (MEAs) between the bipolar plates and with all of these components being between end plates situated at the opposite ends of the stack. The combination of bipolar plates and MEAs repeating and forming a face 9.

[0026] Referring to FIG. 2 at least one thermally-conductive, dielectric layer 8 is disposed on at least one face 9 of the fuel cell stack 1 a second thermally-conductive, dielectric layer 8 may be disposed on a second face 9 of the fuel cell stack. Additional thermally-conductive, dielectric layers 8 may be introduced onto additional faces of the fuel cell stack. In one suitable embodiment, the thermally-conductive, dielectric layer 8 is comprised of one of several commercially available materials which have a thru-the-plane thermal conductivity of between about 3 W/m*K and 15 W/m*K. Such materials are available as sheets of plastic or elastomer or from combinations of polymers and minerals.

[0027] At least one thermal-mass 2, incorporating a fluidic-circuit 3 is mated to the at least one face 9 of the fuel cell stack with the thermally-conductive dielectric layer 8 disposed between the thermal-mass 2 and the fuel cell stack, 1 for the purpose of routing thermal energy while preventing electrical contact. and a second thermal-mass 2 having a second fluidic-circuit 3 is mated to a second face of the fuel cell stack in like manner. In a related embodiment the fuel cell stack can have just one thermal-mass mated to just one face or multiple thermal-masses mated to multiple faces.

[0028] When thermal loads are present such as when heat is applied to the stack 1 during start-up or during other operation accompanied by a change in temperature such as shut-down, the stack 1 will expand or contract. Such thermal expansion or contraction varies among materials. During thermal expansion or contraction, when frictional forces are present between the thermally-conductive dielectric layer and an adjacent face of the stack 9 or between the thermally-conductive dielectric layer and the face of a thermal-mass 2 such forces and displacement can be great enough to deform, tear or otherwise damage the thermally-conductive dielectric layer. In one preferred embodiment a thermally-conductive dielectric grease is applied to the at least one face of the thermally-conductive dielectric layer 8 for the purpose of lubricating the interface between the thermally-conductive dielectric layer 8 and the face of the stack 9 or between the thermally-conductive dielectric layer 8 and the thermal-mass 2. In one preferred embodiment both faces of the thermally-conductive dielectric layer are coated with a lubricant. The purpose of the lubrication being to allow sliding to occur between the thermally-conductive layer and adjacent components so that it does not tear under the influence of frictional forces.

[0029] The fuel cell stack 1 produces thermal energy as a byproduct of its operation. This thermal energy must be removed if the stack is to continue to operate. The thermal energy may flow be removed by the thermal-mass 2 with said thermal energy elevating the temperature of a working-fluid 6 which enters the fluidic-circuit 3 in the thermal mass 2 through inlet 7 and which exits through outlet 10. The thermal energy is absorbed by the working-fluid 6 through the elevation of the temperature of the working-fluid 6 through a process called sensible heat transfer up until the working fluid 6 reaches to its boiling (vaporization) point. Thereafter additional thermal energy is absorbed by the working-fluid 6 which vaporizes and absorbs thermal energy from the stack 1 through a process termed latent heat transfer.

[0030] Referring to FIG. 3 the working-fluid 6 flows through a piping-circuit 11. In one embodiment a pressure regulator 12 is adjusted so that the pressure in the piping-circuit 11 corresponds to the vaporization saturation temperature of working-fluid 6 such as approximately 5.52 Bar for water to vaporize at 160C. In one embodiment the pressure regulator 12 is controlled via feedback from pressure transducer 16.

[0031] A prescribed working-fluid 6 temperature is achieved through assurance of the presence of both the liquid and gas phases. So long as the two phases are each present, the temperature will be constant but if one of the phases is absent then the temperature may not be constant. For example, it is possible for a working-fluid 6 to be at a temperature below the saturation temperature of the working-fluid 6 if only liquid is present. Such a liquid is termed “supercooled”. Also, for example it is possible for a working-fluid 6 to be at a temperature above the saturation temperature of the working-fluid if only a vapor is present. Such a vapor it termed “superheated.”

[0032] Referring to FIG. 3, one means to assure the presence of both the liquid and vapor phases (steam quality between zero and 100 percent, or as defined in chemical engineering between zero and one) at the saturation temperature, is to employ a piping circuit having a pump 14 delivering working-fluid 6 to the thermal-mass 2, which is receiving thermal energy from stack 1, at a rate which maintains the temperature at thermocouple 19 at the prescribed vapor saturation temperature with some additional flow being provided to ensure that the flow does not consist entirely of vapor at its saturation temperature. In one suitable embodiment, the steam quality is best maintained between 10 percent and 90 percent, more suitably between 20 and 80 percent even more suitably between 30 and 70 percent, and most suitably between 40 and 60 percent.

[0033] Pump 14 flow may be controlled via a signal from Mass flow meter 13 with the amount of flow supplied being a function of the output of thermocouples 19 which measures the temperature immediately downstream of thermal-mass 2 which is at a substantially identical temperature to working fluid 6 and (referring to FIG. 1) stack face 9 as measured by thermocouple 18. Flow in such a piping circuit is assured in the design direction only through a check valve 17. The flow is adjusted so that the temperature at thermocouple 19 is at the saturated vapor temperature of the working-fluid. Then additional flow is provided at a pre-set amount to ensure that the flow does not consist only of vapor. For example if 50 grams of water per minute are required to maintain thermocouple 19 at the saturated vapor pressure, then 55 grams of water may be provided to ensure that the flow consists of two phases and thus ensures a constant temperature.

[0034] In one preferred embodiment a condensing heat exchanger 15 is introduced in the piping-circuit to change the phase of the working-fluid to 100 percent liquid and which may also reduce the temperature of the working-fluid to below its saturation temperature. The working fluid may thus be used in a continuous loop.

[0035] The piping circuit may have some of the elements shown in FIG. 3 removed or additional components may be added such as storage tanks, accumulators, pressure relief valves and other process piping components without departing from the present disclosure.

[0036] The temperature of thermal-mass 2 may be maintained at a substantially uniform temperature throughout its volume by employing as the material of its construction one having a high thermal conductivity. For example 6063 aluminum alloy has a thermal conductivity of about 200 Watts*m.sup.−1*K.sup.−1 and copper 81100 alloy has a thermal conductivity of about 345 Watts*m.sup.−1*K.sup.−1.

[0037] Referring to FIG. 4, direct thermal contact between thermal-mass 2 and working-fluid 6 may be achieved through one embodiment in which fluidic-circuit 3 is integral to thermal-mass 2 such as in the case of channels being machine-cut into thermal-mass 2 or in the case of thermal mass 2 being fabricated through a casting or molding operation with the thermal circuit being produced through features in the die or mold. In one embodiment the thermal mass 2 consists of a base plate 4 and a cover plate 5. In one preferred embodiment the cover plate 5 may be affixed to base plate 4 through a welding process such as laser welding.

[0038] Referring to FIG. 6. In one embodiment the fluidic-circuit 3 in the thermal mass 2 may be a separate or discreet component that is intimately mated to thermal-mass 2 to minimize thermal contact resistance. The Lytron division of the Boyd Corporation in Pleasanton Calif. produces “pressed tube cold plates” of such configuration as shown in 20.