RELATED APPLICATION

[0001] This application claims the benefit of U.S. Provisional Application No. 63/418,326, filed on October 21, 2022. The foregoing application is hereby incorporated by reference herein for all purposes.

BACKGROUND OF THE INVENTION

[0002] The present disclosure generally relates to fuel cells. More specifically, the disclosure relates to cooling subsystems of fuel cells.

[0003] Fuel cells are electrochemical devices that can be used in a wide range of applications, including transportation, material handling, stationary, and portable power applications. Fuel cells use fuel and air to generate electricity by electrochemical reactions and release reaction products as exhaust. For example, the byproducts generated by methanol fuel cells are water vapor and carbon dioxide. In addition to electricity, some energy in the fuels is released as heat.

[0004] A cooling subsystem containing a heat exchanger attached to a fuel cell stack can help to dissipate some of the waste heat to prevent overheating of the fuel cell system. Typically, during operation, working fluid flows through a heat exchanger. However, if the temperature of the surrounding environment drops below the freezing temperature of the working fluid for a sufficient amount of time, the working fluid within the cooling subsystem can freeze, and the expansion of the frozen fluid can cause damage to the fuel cell system.

[0005] Thus, it can be challenging to provide a fuel cell system with an adequate cooling subsystem, especially in locations that may have sub-freezing temperatures for extended periods. Therefore, it would be desirable to be able to provide a reliable cooling subsystem that is unlikely to cause damage to a fuel cell system in such an environment.

SUMMARY OF THE INVENTION

[0006] In accordance with an embodiment, a cooling subsystem is provided for a fuel cell system having a fuel cell stack comprising a plurality of membrane electrode assemblies and a plurality of bipolar plates. The cooling subsystem includes a heat exchanger and an insulated reservoir. The heat exchanger is mounted to a face of the fuel cell stack. The insulated reservoir is formed of a flexible material. The at least one heat exchanger is configured to drain working fluid gravimetrically downward to the insulated reservoir.

[0007] In accordance with another embodiment, a method is provided for cooling a fuel cell assembly. A fuel cell system is provided. The fuel cell system includes a fuel cell stack comprising a plurality of membrane electrode assemblies and a plurality of bipolar plates. A heat exchanger is mounted to a face of the fuel cell stack. Working fluid is flowed through the heat exchanger during operation of the fuel cell system, and then flowed from an outlet of the heat exchanger gravimetrically downward to an insulated reservoir formed of a flexible material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

[0009] Figure 1 is a perspective view of a fuel cell stack, with plate-and-tube type heatexchangers attached, in accordance with an embodiment.

[0010] Figure 2 shows an alternate design for a plate and tube type heat-exchanger that will allow for gravimetric draining of the working fluid.

[0011] Figure 3 is a side view of the plate of a plate-and-tube type heat exchanger.

[0012] Figure 4 shows a tube for a plate-and-tube type heat exchanger for gravimetric draining of a working fluid.

[0013] Figure 5 is a side view of a plate-and-tube type heat exchanger.

[0014] Figure 6 is a schematic diagram of a fuel cell system in accordance with an embodiment.

[0015] Figure 7 shows an embodiment of a heat exchanger plate having a channel cut into it.

[0016] Figure 8 shows a heat exchanger plate assembly with a cover plate and with fittings at the inlet and outlet, in accordance with another embodiment.

[0017] Figure 9 shows a Tesla-Valve geometry.

[0018] Figure 10 shows Tesla-Valve geometry as the channels of a heat exchanger plate.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] The present invention relates generally to fuel cell systems. . Like all powerplants, fuel cell systems must be cooled in order to operate. If fuel cell stacks are not cooled, then heat from inefficiencies in the fuel cell stack will cause temperatures to rise until damage occurs Since powerplants, such as fuel cells, often operate outdoors, such as in cars, trucks, busses and airplanes and in stationary applications to provide power for residences and for commercial spaces, a liquid coolant must be selected which will not freeze if the system is shut down when the environment temperature is low. Portable fuel cell systems can be placed in a backpack and worn by users to provide power to various electronic devices, such as radio and satellite communications gear, laptop computers, night vision goggles, and remote surveillance systems. Embodiments of fuel cell systems described herein can continue to generate and provide power at extreme temperatures. [0020] In automotive powerplants, the working fluid is typically a mixture of ethylene-glycol and water. A fifty percent ethylene-glycol solution has a freezing point of about -38° C. Commercial ethylene-glycol formulations contain corrosion inhibitors, which relax materials requirements in the cooling circuit of the powerplant. For applications where human or animal contact might occur, propylene-glycol may be used. Depending on the temperature requirements, other working fluids, such as Dowtherm™, may be used.

[0021] The various working fluids present challenges, such as chemical incompatibility with polymers and a tendency to wick past seals and to leak. In fuel cells, membrane electrode assemblies (MEAs) are sandwiched between two bipolar plates. In high temperature PEM fuel cells, high temperature plastic films are typically used as gaskets in fuel cell stacks to form a seal between bipolar plates and MEAs. However, these gaskets do not always provide a reliable seal. If leaking coolant migrates past seals to the MEA, the leaked coolant can cause damage to the fuel cell, and it can become hazardous if there are overboard leaks to the environment.

[0022] The embodiments described herein allow for the use of any working fluid or mixture (including plain water, such as distilled water) as coolant even if the working fluid or mixture might freeze or solidify in low environmental temperatures. Water has the most favorable heattransfer characteristics of all common working fluids. Water can be readily employed as a coolant, using a phase-change strategy, which dramatically improves cooling efficiency. Thus, water is an inexpensive and readily available option for a coolant. Although plain water, such as distilled water, can be corrosive, stainless-steel alloys have been shown to be undamaged by water for long periods. Therefore, those portions of the fuel cell cooling circuit needing a high thermalconductivity material, such as in the system’s heat exchangers, can be formed of stainless steel, while other cooling circuit elements can be formed of polymer materials.

[0023] Under normal operating conditions, if water is employed as the working fluid, it will not freeze in a fuel cell system or other powerplant, so long as the powerplant is operating. Typically, the waste heat transferred to the water from the fuel cell stack elevates the temperature of the water sufficiently to keep it from freezing. However, if the fuel cell system is intentionally shut down or shuts down due to a fault while environmental temperatures are at or below the freezing point of water for a sufficient period, then the water can freeze. If the water within the cooling subsystem of a powerplant freezes, it will render the powerplant inoperable. Also, it will be noted that water expands during the freezing process and therefore any water freezing within the cooling subsystem of a powerplant has strong potential to burst or otherwise damage the tubes and other passages of the cooling circuit. Since for typical fuel cell stacks, the primary cooling circuit geometries are labyrinth channels interior to the fuel cell stack, if the water freezes, the stack, which is the most expensive component of the powerplant, could be damaged and even destroyed.

[0024] According to embodiments described herein, a fuel cell stack is provided with a cooling subsystem without interior labyrinth cooling channels. That is, the fuel cell stack is externally cooled by the cooling subsystem. In accordance with the embodiments, the working fluid of the cooling subsystem drains into a flexible reservoir. In some embodiments, if the working fluid is water, the water can drain harmlessly to the environment.

[0025] Figure 1 is a perspective view of a fuel cell assembly 100 in accordance with an embodiment. The fuel cell assembly 100 has a fuel cell stack 10 with heat-exchangers 1 attached to side faces of the fuel cell stack 10. The fuel cell stack 10 is constructed from alternating layers of bipolar plates and MEAs, where the edges of the bipolar plates comprise at least one surface for heat exchange to an external heat exchanger 1. In the embodiment of the fuel cell assembly 100 shown in Figure 1, two heat exchangers 1 are mated to two exterior side faces of the fuel cell stack 10. In the illustrated embodiment, each heat exchanger 1 is a plate-and-tube type heat exchanger, with the tubing arranged so that a working fluid may drain gravimetrically downward without pooling in sumps or collecting in labyrinth passages. As shown in Figure 1, each of the heat exchangers 1 has an inlet 16 and an outlet 17.

[0026] Figure 2 shows an alternative plate-and-tube heat exchanger design, which may be mounted to at least one face of a fuel cell stack 10. In this embodiment, the working fluid will completely drain under the influence of gravity. It will be understood that although plate-and- tube type heat exchangers are described and have proven efficacy when applied in fuel cell systems, other types of heat exchangers can also be used. For example, any heat exchanger into which a working fluid may be pumped and from which a working fluid may drain under the influence of gravity, and which can be mated to at least one face of the fuel cell stack 10 is suitable. [0027] Figure 3 is a side view of the plate 8 of a plate-and-tube type heat exchanger 1. As shown in Figure 3, the plate 8 has multiple channels 11 formed therein. In the illustrated embodiment, the channels 11 are formed into the plate 8 in a zig zag fashion such that when tubing 9 is pressed into the channels, the tubing 9 is formed in a serpentine shape, as illustrated in Figure 1. The channels 11 are formed to accept tubing that is formed in a serpentine shape, as illustrated in Figure 1. Alternatively, a serpentine channel may be produced in the interior of a plate, such as is depicted in Figures 7 and 8. Figure 4 shows the tubing 9 of the plate-and-tube type heat exchanger 1. Figure 5 is a side view of a plate-and-tube type heat exchanger. As shown in Figure 5, the tubing 9 is pressed into channels 11, deforming the tubing 9 so that the tubing 9 makes intimate contact with the walls of channels 11. In one embodiment, the tubing 9 is a stainless- steel tube with a nominal outside diameter 14 of about inch and a wall thickness of about 0.035 inches, and is pressed into channels 11 having a depth 12 of about 0.180 inches and a width 13 of about 0.25 inches.

[0028] The tubing 9 may be installed in the channels 11 by using a hydraulic press, which can apply forces high enough to deform the tubing 9. In practice, the plate 8 with channels 11 is placed into a hydraulic press. The tubing 9 is placed on top of the channel(s) 11 and a thick plate placed on top of the tubing 9. The hydraulic press applies force to the thick plate, which pushes the tubing 9 into the channel(s) 11. It will be noted that the outside diameter 14 of the tubing 9 in suitable plate-and-tube heat exchangers may be of values greater or lesser than inch, but the inside diameter of the tubing 9 must not result in capillary forces greater than the gravimetric force of the working fluid, else the working fluid would not completely drain when that is required.

[0029] Referring to Figure 6, an embodiment of a fuel cell assembly 100 includes a fuel cell stack 10, with heat exchangers 1 mated to at least one of the faces of the fuel cell stack 10. As shown in Figure 6, the fuel cell stack and heat exchangers assembly 100 is located gravimetrically above heat exchanger 2. The purpose of heat exchanger 2 is to release the thermal energy acquired by heat exchanger 1 from fuel cell stack 10. A pressure regulator 3 is located downstream of heat exchanger 2.

[0030] As shown in Figure 6, an insulated container 5 contains a flexible insulated reservoir 4, a pump 6, pressure regulator 3, and a three-way valve 7. When the water or other working fluid enters the heat exchanger 2, the fluid condenses, thereby releasing the latent heat of evaporation. In both heat exchangers 1 and 2, there will also be sensible heat transfer, increasing the overall heat transfer. It will be understood that the insulated container does not have to be a container, rather the components (reservoir 4, pump 6, pressure regulator 3, valve 7, valve 19) can simply be insulated components. However, having more components inside a common insulation would slow the drop in temperature inside the insulation due to there being more thermal mass.

[0031] If the pump 6 is made from plastic or another material that does not rapidly conduct heat, then it can be outside the insulation. If the pump 6 were metal and outside the insulation, then if it became very cold and if the fluid in the flexible reservoir 4 were near freezing and the pump 6 started, the fluid might suddenly freeze when it came into contact with the cold metal pump 6. A cold plastic pump would be less likely to result in the working fluid suddenly freezing if it were outside the insulation compared to a cold metal pump. [0032] Other embodiments, such as when using ethylene-glycol/water mixtures which are vaporizing in heat-exchanger 1 can also realize substantial heat-transfer gains over sensible heat transfer strategies. Boiling and condensing are termed “latent heat transfer,” which transfers much more heat compared to simply changing the temperature of a working fluid. A system for heat transfer in which the temperature and pressure are controlled so that a working fluid absorbs heat and boils in heat exchanger 1 and releases heat by condensing in heat exchanger 2 has the potential to transfer a much greater amount of heat compared to a system in which a relatively cool working fluid flows through heat exchanger 1 and increases in temperature and absorbs thermal energy (without boiling), and then flows through heat exchanger 2 where the working fluid decreases in temperature and releases energy (remaining as a liquid the entire time and neither condensing or boiling). An advantage of the system disclosed herein is that the working fluid is boiling and condensing. Water can be advantageous, as other coolants are often toxic. Also, water has a very high heat of vaporization/condensation. So when water boils, it pulls a great deal of heat away and when it condenses, it releases that same amount of heat.

[0033] Referring to Figure 6, a normally-closed, three-way solenoid valve 7 is located within the thermally insulated container 5 and resides between the pressure regulator 3 and heat exchanger 2. The vent outlet of the valve 7 is connected to atmosphere and to the flexible insulated reservoir 4 at tube section 22. It will be noted that, in other embodiments, the valve 7 is another type of three-way valve, not a solenoid valve.

[0034] In normal operation, the working fluid flows out of heat exchanger 2, through valve 7, and then through pressure regulator 3 and finally into the reservoir 4. The pump 6 then recirculates the working fluid gravimetrically upward and into heat exchangers 1 and 2. It will be noted that the pump 6 is ideally inside the insulated container 5 in case a small amount of liquid remains inside it and freezes. However, if the pump 6 completely drains, it can be acceptable for the pump 6 and valve 19 to be external to the insulated container 5. It should be noted that if the pump 6 were external and exposed to very low temperature, and then energized to pump a working fluid out of the reservoir 4, and if that working fluid were near freezing, it is possible that the working fluid might instantly freeze when it made contact with the very cold pump 6.

[0035] If the system 200 is shut down intentionally or shuts down due to a fault, the normally- closed three-way solenoid valve 7 is de-energized and the flow of the working fluid is routed from heat exchanger 2 into tube section 22 (which is at atmospheric pressure). As shown in Figure 6, the tip 25 of the vent tube 22 is at atmospheric pressure and ensures that the interior of the flexible insulated reservoir 4 is also is at atmospheric pressure. Since tube section 22 is at atmospheric pressure, the working fluid in heat exchanger 1 and heat exchanger 2 will freely flow gravimetrically downward into reservoir 4. Working fluid that may be present in pressure regulator 3 and pump 6 will also flow gravimetrically downward, protecting these components from damage if the temperature drops below the freezing point of the working fluid. The system 200 can then remain idle until the temperature in the flexible insulated reservoir 4 approaches the freezing point of the working fluid. In one embodiment, a one-way valve, such as a duck-bill valve, is installed at the outlet of tube section 22 to allow atmospheric air to enter tube section 22 when the system is draining and to equalize the pressure in insulated reservoir 4 with the atmosphere. Such a valve prevents gross air exchange with the environment which serves to further minimize heat transfer.

[0036] Insulated container 5 includes provisions to minimize heat transfer at bulkheads 18. At bulkhead locations 18, tubing sections are formed of a material having alow thermal conductivity, such as polymer tubing. If a metal tube protrudes through the insulated container 5 from the inside to the outside, then the portion outside of the insulation will conduct excessive thermal energy out of the insulated container 5. It is desirable to maintain the temperature inside the insulated reservoir 4 for as long as possible so that the working fluid will remain as a liquid and not freeze. However, if the interior of the insulated container 5 is on the verge of freezing then the system may optionally pump all of the working fluid overboard and onto the ground. This is why water is desirable because it is non-toxic. If the tubing protruding from the interior of the insulated container 5 to the outside is plastic tubing, or other material that does not transfer thermal energy very quickly, then the amount of heat lost from the insulated container 5 is lessened and the period of time that liquids will not freeze inside it is extended.

[0037] The system 200 avoids damage from freezing when no power is available because of the flexible material for reservoir 4 and for tubing section 24, which connects the reservoir 4 with the pump 6. In the event of complete power loss, the reservoir 4 and the filled section of tube 24 can be allowed to freeze without the risk of damage because the flexible material allows for the expansion of the reservoir 4 and the tube section 24, which is caused by the expansion of the working fluid contained therein as it freezes. The flexible insulated reservoir 4 can be formed of a polymer that is compatible with the chemistry and temperature of the working fluid. An example of a common plastic tank material compatible with water at that about 50° C (or below) is polyethylene.

[0038] If the system 200 is shut down intentionally or due to a fault, the working fluid will completely drain from the cooling system into the flexible insulated reservoir 4. During cold weather, the insulated reservoir 4 will maintain the working fluid in the liquid state for an extended period, during which the powerplant may be restarted. By carefully insulating the reservoir and thermally isolating it from the environment and from the rest of the cooling subsystem, the working fluid can be expected to remain in the liquid phase for extended periods, such as between 12 and 24 hours.

[0039] In the case that water is the working fluid and if power is available to energize the pump 6 and the normally-closed solenoid valve 19, when the thermocouple 20 signals that the water temperature in the reservoir 4 approaches freezing, the water can be pumped out of the system 200. Once the water is pumped out of the system 200, the system 200 can remain in that state until the cooling subsystem is refilled with water at fill port 35.

[0040] If during cold weather, the working-fluid remains in the reservoir 4 for extended periods of time without being circulated through the operating powerplant, such as for longer than about 24 hours, and if no power is available, the working fluid can be allowed to freeze without the risk of associated system damage. In the case that the working fluid is allowed to freeze, the reservoir 4 may later be warmed using a heater 23, such as an electrically powered or a combustion powered heater to return the working-fluid to the liquid phase.

[0041] In one embodiment, the heat transfer from the fuel cell stack 10 to the working fluid is via phase change. The latent heat of evaporation transferred via the phase change of the working fluid from a liquid to a vapor is achieved when the pressure regulator 3 is adjusted so that the pressure between the pump 6 and pressure regulator 3 is at the saturation temperature of the working fluid most optimal for maintaining the fuel cell stack 10 in the preferred operating temperature range. Excess working fluid is supplied by the pump 6 so that a two-phase mixture of liquid and vapor are present in heat exchanger 1 during operation of the fuel cell. The presence of two phases is assurance that the temperature of the working fluid in heat exchanger 1 is constant and so long as two phases are present, heat exchanger 1 will remain at the saturation temperature corresponding to the internal pressure.

[0042] According to one embodiment, the plate 8 of the plate-and-tube heat exchanger 1 may be formed of an aluminum alloy with a thermal conductivity between about 150W/m*K and 250W/m*K. Alternatively, the plate 8 may be formed of another metal or material with suitable thermal conductivity. Referring to Figure 5, when an aluminum alloy plate with a thickness 21 of ^l A inch is used and when the average spacing 15 between channels 11 is about 1.6 inches, then the entire volume of the heat exchanger 1 will be at nearly a uniform temperature. [0043] According to an embodiment, the working fluid is water and the powerplant is a high temperature polymer electrolyte membrane (PEM) fuel cell stack 10 operating at about 160 °C and exchanging thermal energy with heat exchanger 1, which is maintained at about 150° C by adjusting pressure regulator 3 so that the pressure between the pressure regulator 3 and pump 6 is at about 4.8 bar. By flowing enough excess water from the pump 6, a two-phase mixture is assured in heat exchanger 1 and therefore a constant temperature thermal reservoir. In this embodiment, the fuel cell stack 10 operates at about 160° C and rejects heat to heat exchanger 1, which is a constant temperature thermal reservoir at 150° C due to the presence of the two-phase flow of water.

[0044] In this embodiment, water is vaporizing in the heat exchanger 1 , and latent heat transfer is the dominant mode. The operating mode may be relied upon to transfer about 2200 Joules per gram of flowing water. The flow rate of water in such an operating mode may be about five times lower compared to relying on sensible heat transfer from flowing a coolant that is not vaporizing in the heat exchanger. Therefore, a relatively small amount of water is required within the cooling sub-system compared to a traditional design and if the system were drained and needed to be refilled, a relatively minimal amount of water would be required.

[0045] Referring to Figure 7, according to an alternative embodiment, a heat exchanger plate

26 has channels that are machined, photo-etched, or produced with another fabrication method. Rather than having tubing pressed into the channels such as the plate 8 shown in Figure 5, a heat exchanger assembly 300, shown in Figure 8, incorporates a cover plate 28 that is welded or otherwise affixed to the heat exchanger plate 26 so that fluid is routed through channel 27 and which is prevented from bypassing (if the cover plate 28 were affixed to the plate 26 with channel

27 only at the perimeter, then working fluid might flow under the cover plate 28 without passing through the channels 27, thereby bypassing some regions) by welding, bonding, or other methods. Fluid fittings 29 are introduced at the entrance and outlet of channel 27, as shown in Figure 8.

[0046] In one embodiment, the channel 27 has a Tesla-Valve geometry 30, as shown in Figure 9. The Tesla-Valve allows fluid to freely flow in direction 32 and resists flow in direction 31. In the fluidic circuit shown in Figure 6, one or more of the heat exchangers 1, 2 may incorporate a plate 33 with Tesla-Valve channels 30, as shown in Figure 10. The Tesla-Valve channels 30 serve to resist backflow of the working fluid towards the pump 6. Such backflow might result when the working fluid vaporizes in the heat exchanger 1 or 2. The fluid need not be completely prevented from back-flow but rather restricted. The restriction serves to lessen the magnitude of pressure spikes propagating back toward the pump 6, allowing for smoother operation. In another embodiment, the Tesla-Valve geometry is introduced separately in the fluidic circuit and not as a feature in a heat exchanger plate.

[0047] Embodiments described herein provide a fuel cell system 200 having a cooling subsystem 100 through which a working fluid is pumped. The working fluid receives thermal energy via phase change while it is inside a heat exchanger 1 mated to an externally-cooled fuel cell stack 10. In some embodiments, the thermal energy is rejected to the environment in a separate heat exchanger 2.

[0048] According to embodiments, an insulated flexible reservoir 4 is located near the gravimetric bottom of the cooling subsystem 100. The insulated flexible reservoir 4 receives working fluid being circulated through the heat-receiving and heat-rejecting heat exchangers 1 and 2, and through other system elements and is the supply from which the working fluid is pumped. Because it is formed of a flexible material, the insulated flexible reservoir 4 can expand and is not damaged if the working fluid inside it freezes.

[0049] When the fuel cell system 200 is purposefully shut down or if it shuts down due to a fault, the working fluid will drain, under the influence of gravity into the insulated flexible reservoir 4. The insulated flexible reservoir 4 will maintain the working fluid in the liquid phase for an extended period. If the working fluid in the insulated flexible reservoir 4 approaches the working fluid’s freezing temperature and if no power is available, then the working fluid can be allowed to freeze without any resulting system damage.

[0050] If water is the working fluid, and if the system 200 is shut down or shuts down due to a fault and if water drains into the insulated flexible reservoir 4 and if power is available, then the system pump 6 can be energized to harmlessly transfer the water out of the system 200 to the environment if the water temperature approaches its freezing point. After the system 200 is drained, it may be simply refilled with water at the gravimetric top of the cooling subsystem through a fill port 35.

[0051] In other embodiments, the working fluid may be a glycol- water mixture. The percentage of glycol added in those cases can be to modify the boiling point, as a corrosion inhibitor, or for reasons other than to prevent freezing. The invention includes the use of any working-fluid which may freeze or solidify at low environmental temperatures.

[0052] In view of the foregoing, it should be apparent that the present embodiments are illustrative and not restrictive and the invention is not limited to the details given herein but may be modified within the scope and equivalents of the appended claims.