Fuel cell thermal energy storage in phase-change material

Abstract

A cooling system for a fuel cell system 12 onboard a vehicle such as an aircraft has a primary ambient air heat exchanger coolant loop 18 for cooling the fuel cell system, and a secondary coolant loop

Description

TECHNICAL FIELD

[0001] The present disclosure relates to heat management of fuel cells. The disclosure has particular utility in the case of PEM (Proton Exchange Membrane) fuel cell systems such as hydrogen fuel cells onboard vehicles including aircraft and will be described in connection with such utility, although other utilities are contemplated. BACKGROUND AND SUMMARY

[0002] This section provides background information related to the present disclosure which is not necessarily prior art. This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all its features. A fuel cell is an electrochemical cell that converts chemical energy into electrical energy by spontaneous electrochemical reduction-oxidation (redox) reactions. Fuel cells include an anode and a cathode separated by an ionically conductive electrolyte. During operation, a fuel (e.g., hydrogen) is supplied to the anode and an oxidant (e.g., oxygen or air) is supplied to the cathode. The fuel is oxidized at the anode, producing positively charged ions (e.g., hydrogen ions) and electrons. The positively charged ions travel through the electrolyte from the anode to the cathode, while the electrons simultaneously travel from the anode to the cathode outside the cell via an external circuit, which produces an electric current. The oxidant supplied to the cathode is reduced by the electrons arriving from the external circuit and combines with the positively charged ions to form water. The reaction between oxygen and hydrogen is exothermic, generating heat that needs to be removed from the fuel cell.

[0003] Fuel cells may be used as power sources for electric motors of electric vehicles and hybrid electric vehicles, including aircraft. In such applications, fuel cells oftentimes are arranged in stacks of multiple cells and connected in a series or parallel arrangement to achieve a desired power and output voltage. Cooling systems for fuel cell-powered vehicles oftentimes use an airflow generated during movement of the vehicle as a heat transfer medium. For example, an ambient airflow may be directed from outside the vehicle through an air intake of the vehicle and through one or more heat exchangers disposed within the vehicle to cool a secondary fluid circulated through the fuel cell. An airflow generated in this manner is oftentimes referred to as ram air and, when ram air is used as a cooling medium in a vehicle, the vehicle may experience increased drag, which may reduce the energy efficiency of the vehicle.

[0004] In fuel cell-powered aircraft, the power output demanded of the fuel cells—and the amount of waste heat generated by the fuel cells—is greatest during take-off and climb. Maximum power output is only needed for 60-120 seconds on takeoff and initial climb to 304.8 meters (1,000 feet). After 304.8 meters (1,000 feet), power may be reduced by 20% for the normal climb portion of the flight, and the fuel cell powerplant typically operates at a higher efficiency, resulting in 30% lower heat output. Waste heat generated during operation of powered aircraft may be dissipated by positioning an air-cooled heat exchanger in an ambient airflow path through the aircraft. However, during takeoff roll (from zero velocity until takeoff speed), and also during initial climb, airflow across the heat exchanger surface is low, which makes the capacity of the heat rejection system lesser during that stage of flight. Moreover, directing ambient air to flow through the aircraft (instead of around the aircraft) when the aircraft is moving creates drag, with the amount of drag experienced by the aircraft being proportional to the volume of ambient air directed through the aircraft (and thus through the heat exchanger), the pressure drop across the heat exchanger. In practice, the volume of the air-cooled heat exchanger (and the volume of air directed through the heat exchanger) may be selected to accommodate the most demanding cooling requirements of the aircraft, which may occur when the aircraft is operating under high load conditions, e.g., during take-off and climb. However, sizing the heat exchanger in this way may cause the aircraft to experience an unnecessarily large amount of drag when the aircraft is operating under low load conditions, e.g., during cruise, when minimal waste heat dissipation is needed. In addition, when cooling requirements are low, the presence of an oversized heat exchanger onboard the aircraft adds unnecessary weight (which further increases induced drag) and bulk to the aircraft and additional components required to prevent overcooling of the fuel cells. Therefore, a supplemental cooling system covering at least 30% of heat output for 120 seconds would be of significant contribution to the efficiency of hydrogen fuel cell-powered airplanes.

[0005] In our prior co-pending GB Patent Application Number GB 2215788.7 filed October 25, 2022 we describe a thermal management system, i.e., a cooling system for a fuel cell aboard a vehicle comprising fluid circuit configured to circulate a phase-change coolant material in a fuel cell cooling system to exploit the latent heat of vaporization of the coolant to reduce the required coolant flow rates, thus reducing the pump size, heat exchanger and other cooling system components thereby reducing the mass and parasitic power requirements of the cooling system. The two-phase coolant comprises a dielectric material which permits the coolant to be circulated directly through the fuel cell bipolar plate cooling channels.

[0006] In accordance with the present disclosure, there is provided a cooling system for a fuel cell system onboard a vehicle comprising a primary ambient air heat exchanger coolant loop for cooling the fuel cell system, and a secondary heat exchanger coolant loop comprising a fluid circuit configured to circulate a coolant in thermal contact with a phase-change material (PCM) and in thermal contact with the fuel cell to absorb heat from the fuel cell, wherein the secondary coolant loop includes a heat pump for cooling the PCM.

[0007] In one embodiment the cooling system for a fuel cell system is onboard a vehicle.

[0008] In one embodiment the secondary heat exchanger coolant loop includes an electrically driven heat pump.

[0009] In another embodiment the secondary heat exchanger coolant loop comprises an extension of the primary ambient air heat exchanger coolant loop. In such embodiment the secondary coolant loop may selectively be isolated from the primary ambient air coolant loop, and, the electrically powered heat pump is configured to cool the PCM to remove excess heat from the PCM when excess heat produced by the fuel cell is not completely dissipated by the primary ambient air heat exchanger coolant loop, and / or to add heat to the PCM to heat the fuel cell system during warmup.

[00010] In one aspect the heat pump is an electrically driven heat pump, and electricity for powering the heat pump is provided at least in part by the fuel cell onboard the vehicle.

[00011] In another aspect the heat pump is an electrically driven heat pump, and electricity for powering the heat pump is provided at least in part from an onboard battery, a ground based electrical energy cart, a ground based electrical energy grid or regenerative power produced, for example, by vehicle braking or in the case of a fuel cell powered aircraft propeller / motor braking during descent.

[00012] In yet another aspect the heat pump is a mechanically driven heat pump, which is mechanically driven at least in part by an onboard mechanical power service.

[00013] In still yet another aspect the heat pump is a thermally driven heat pump, which is thermally driven at least in part by an absorption refrigeration cycle, such as from hot exhaust from an onboard fuel cell or an onboard compressor outlet.

[00014] In another aspect the secondary heat exchanger coolant loop is independently connected to the fuel cell, separate from the primary ambient air heat exchanger coolant loop.

[00015] In still another aspect the secondary coolant loop is selectively connected to the same coolant loop used by the fuel cell primary ambient air heat exchanger, via valves.

[00016] In another aspect the heat pump is connected in series with the fuel cell and the primary ambient air heat exchanger coolant loop, wherein the primary ambient air heat exchanger coolant loop preferably is located immediately downstream of the fuel cell between the fuel cell and the heat pump.

[00017] In yet another aspect the heat pump is located between the fuel cell primary ambient air coolant loop and the secondary coolant loop.

[00018] In still yet another aspect a bypass valve is provided to allow heat from the fuel cell to be delivered to the heat pump without passing through the primary ambient air heat exchanger coolant loop.

[00019] In one aspect the primary ambient air coolant loop is sized below that required for maximum continuous power operation of the vehicle, and the secondary coolant loop is sized to supplement the primary ambient air coolant loop to match the maximum continuous power operation of the during peak vehicle operation, e.g., during take-off and climb in the case of a fuel cell powered aircraft.

[00020] In yet another aspect the cooling system includes a controller to control operation of the heat pump and valves and pumps to control flow of coolant through the fuel cell and the primary ambient air coolant loop and the secondary PCM coolant loop depending on cooling demands.

[00021] In various embodiments, the thermal connector to the PCM may be through independent heat exchangers, or through the fuel cell ambient air coolant heat exchanger(s).

[00022] In a particularly preferred aspect, the vehicle comprises a fuel cell powered aircraft.

[00023] The present disclosure also provides a method for cooling a fuel cell aboard a vehicle comprising providing a primary ambient air heat exchanger coolant loop and a secondary heat exchanger coolant loop comprising a fluid circuit configured to circulate a coolant in thermal contact with a phase-change material (PCM) and also in thermal contact with the fuel cell, and selectively circulating the coolant through the fluid circuit.

[00024] In another aspect of the process, the heat pump is an electrically driven heat pump.

[00025] In a further aspect of the process, the electrically driven heat pump is electrically driven, at least in part, by electrically energy from the fuel cell.

[00026] According to aspect A of the present invention there is provided a cooling system for a fuel cell system comprising a primary ambient air heat exchanger coolant loop for cooling the fuel cell system, and a secondary coolant loop comprising a fluid circuit configured to circulate a coolant in thermal contact with a phase-change material (PCM) and also in thermal contact with the fuel cell to absorb heat from the fuel cell wherein the secondary coolant loop includes a heat pump for cooling the PCM.

[00027] Preferably the fuel cell system is onboard a vehicle.

[00028] Preferably the secondary coolant loop comprises an extension of the primary ambient air heat exchanger coolant loop.

[00029] Preferably the secondary coolant loop is configured to be selectively isolated from the primary ambient air coolant loop, and the heat pump is configured to cool the PCM to remove excess heat from the PCM.

[00030] Preferably the secondary coolant loop is configured to add heat to the PCM to heat the fuel cell system during warmup.

[00031] Preferably the heat pump comprises an electrically driven heat pump.

[00032] In one alternative electricity for powering the heat pump is provided at least in part by the fuel cell onboard the vehicle.

[00033] In another alternative electricity for powering the electrically driven heat pump is provided at least in part by one or more batteries onboard the vehicle.

[00034] In a further alternative electricity for powering the electrically driven heat pump is provided at least in part by a ground based electrical energy cart, or a ground based electrical energy grid.

[00035] In yet a further alternative electricity for powering the electrically driven heat pump is provided at least in part by regenerative power produced by vehicle braking or in the case of a fuel cell powered aircraft propeller / motor braking during descent.

[00036] Preferably the heat pump is mechanically driven at least in part by an onboard mechanical power service.

[00037] Preferably the heat pump comprises an absorption refrigeration cycle

[00038] In one alternative heat for the absorption refrigeration cycle is provided from hot exhaust from an onboard fuel cell.

[00039] In another alternative heat for the absorption refrigeration cycle is provided by an onboard air compressor.

[00040] Preferably the secondary coolant loop is independently connected to the fuel cell, separate from the primary ambient air heat exchanger coolant loop.

[00041] Preferably the secondary coolant loop is selectively connected via valves to the primary ambient air heat exchanger coolant loop.

[00042] Preferably the heat pump is connected in series with the fuel cell and the primary ambient air heat exchanger coolant loop, wherein the primary ambient air heat exchanger coolant loop is located downstream of the fuel cell between the fuel cell and the heat pump.

[00043] Preferably the heat pump is located between the fuel cell primary ambient air heat exchanger loop and the secondary coolant loop.

[00044] Preferably the cooling system further comprises a bypass valve configured to allow heat from the fuel cell to be delivered to the heat pump without passing through the primary ambient air heat exchanger coolant loop.

[00045] Preferably the primary ambient air heat exchanger loop is sized below that required for maximum continuous power operation of the vehicle, and the secondary coolant loop is sized to supplement the primary ambient air heat exchanger loop.

[00046] Preferably the cooling system includes a controller to control operation of the heat pump and valves and pumps to control flow of coolant through the fuel cell and the primary ambient air heat exchanger loop and the secondary coolant loop depending on cooling demands.

[00047] Preferably the fuel cell system is onboard a fuel cell powered aircraft.

[00048] According to aspect B of the present invention there is provided a method for cooling and fuel cell aboard a vehicle comprising: providing a cooling system for a fuel cell system according to aspect A of the present invention, and selectively circulating the coolant.

[00049] Preferably the heat pump is an electrically driven heat pump and including the step of powering the electrically driven heat pump, at least in part, by electrical energy from the fuel cell.

[00050] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[00051] Further features and advantages of the instant disclosure will be seen from the following detailed description taken in conjunction with the accompanying drawings. The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations and are not intended to limit the scope of the present disclosure.

[00052] Fig. 1 is a flow diagram of a cooling system for a fuel cell in accordance with one embodiment the present disclosure;

[00053] Fig. 2 is a flow diagram of a cooling system for a fuel cell in accordance with a second embodiment of the present disclosure;

[00054] Figs. 3A-3C are flow diagrams of a cooling system for a fuel cell in accordance with yet another embodiment of the present disclosure;

[00055] Fig. 4 is a flow diagram of a cooling system for a fuel cell in accordance with yet other embodiments of the present disclosure;

[00056] Figs. 5A and 5B are flow diagrams of a cooling system for a fuel cell in accordance with still yet another embodiment of the present disclosure;

[00057] Figs. 6 is a schematic depiction of an aircraft including a fuel cell and fuel cell cooling system onboard the aircraft in accordance with the present disclosure; and

[00058] Fig. 7 is a block diagram showing operation and control of the cooling system in accordance with the present disclosure. DETAILED DESCRIPTION

[00059] Example embodiments will now be described more fully with reference to the accompanying drawings. Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

[00060] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed.

[00061] When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[00062] Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

[00063] Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[00064] As used herein, the term “fuel cell” is intended to include an electrochemical cell that converts the chemical energy of a fuel (typically hydrogen) and an oxidizing agent (typically oxygen) into electricity through a pair of redox reactions. There are many types of fuel cells, but they all include an anode, a cathode, and an electrolyte that allows ions, usually positively charged hydrogen ions or protons, to move between two sides of the fuel cell. At the anode a catalyst causes the fuel to undergo oxidized reactions that generate ions, typically positively charged hydrogen ions, and electrons. The ions move from the anode to the cathode through the electrolyte. At the same time, electrons flow from the anode to the cathode through an external circuit, producing direct current electricity. At the cathode, another catalyst causes ions, electrons and oxygen to react, forming water in the case of a hydrogen fuel cell, and possibly other products. Fuel cells are classified by the type of electrolyte they use and by the difference in startup electrolyte they use.

[00065] As used herein the term “heat pump” any one or more mechanical or thermoelectric devices that create a heat flux including, for example, mechanical devices that produce a heat flux through a vapor compression cycle, a vapor absorption cycle, a gas cycle and a Carnot cycle as well as thermoelectric devices that create a transfer of heat through the Peltier Effect.

[00066] The present disclosure has particular applicability to proton-exchange membrane hydrogen fuel cells, or so-called hydrogen fuel cells, for powering vehicles such as aircraft, although the disclosure is not limited to hydrogen fuel cells for powering aircraft. The disclosure advantageously may be used in connection with other types of fuel cells including, but not limited to methanol and ethanol fuel cells, for both stationary and vehicular installations.

[00067] Fig. 1 depicts a process flow diagram of a cooling system 10 for a hydrogen fuel cell or fuel cell stack 12 in accordance with the present disclosure. The fuel cell or fuel cell stack 12 is located on board an aircraft 14 as shown in Fig. 6 and the aircraft is propelled at least in part by an electric motor 16. While the fuel cell or fuel cell stack 12 is illustrated as being located in the wings 13 of the aircraft, the fuel cell or fuel cell stack also may be disposed in the fuselage 14, engine, nacelle or any other part of the aircraft.

[00068] The cooling system 10 includes a primary ambient air heat exchanger coolant loop 18 including a heat exchanger 20 configured to extract heat from a fuel cell or fuel cell stack coolant. The fuel cell stack assembly is configured for transferring waste heat generated by fuel cell or fuel cell stack 12 to a cooling medium which is circulated through fuel cell 12 and heat exchanger 20 via conduits 24, 26 and pump 28. Valve 32 and / or pump 28 which is included in line 26, valve 32 and pump 28 may be under control of controller 100, also illustrated in Fig. 7.

[00069] An auxiliary or secondary heat exchanger coolant loop 36 in thermal contact with a phase change material (PCM) 60 is independently connected to the fuel cell 12 via valve 50, conduits 44,46 and pump 48 which circulates a fluid coolant through the fuel cell 12 and in thermal contact with the PCM 60 in heat pump 54. Valve 52 and pump 48 also are under control of controller 100 illustrated in Fig. 7. Secondary heat exchanger coolant loop 36 optionally includes heat pump 54 which is electrically driven by electricity from fuel cell 12, and also is controlled by controller 100.

[00070] Referring to Fig. 2 in another embodiment, the secondary heat exchanger loop 36a, includes a heat pump 54 powered by electric power from fuel cell 12 and also a second ambient air heat exchanger 56 for cooling the PCM 60 (via a fluid coolant).

[00071] Referring to Fig. 3A, in yet another embodiment, the PCM coolant is circulated through to the primary ambient air heat exchanger coolant loop 18 used for the fuel cell 12 and additionally is cooled by heat pump 54. The PCM 60 is isolated by valves 62, and / or 64 when not in use. As before, the heat pump 54 is powered by electricity from fuel cell 12, and pump 28 and valves 62, 64 are controlled by the controller 100. In other embodiments, the coolant can also be selectively pumped through heat exchangers by operation of pumps, with or without valves.

[00072] Alternatively, as shown in Fig. 3B, the PCM coolant can be circulated through the same ambient air heat exchanger coolant loop 18 used by the fuel cell 12 and ambient air heat exchanger 20. When excess heat is produced by the fuel cell 12 and is not completely dissipated by the ambient air heat exchanger 20, the PCM can absorb excess heat. As before, the PCM is in turn cooled via a heat pump 54, which is powered by the fuel cell.

[00073] Alternatively, as shown in Fig. 3C, the PCM also can be used to adjust or modify the temperature of the fuel cell system, for example, to warm the fuel cell at startup, by running the heat pump 54 in reverse. The PCM 60 also may be bypassed by valves 70, 72. Also shown is pump 28 to drive the coolant flow through the system 18. Not shown is a coolant reservoir typically at the outlet of the ambient air heat exchanger 20. Controller 100 can operate pump 28, and or valves 32, 72, 70, and heat pump 54. In this and other embodiments, the controller may be programmed to alter the system behavior to suit various operating conditions.

[00074] Referring to Fig. 4 in yet another embodiment, a separate ambient air heat exchanger 20A is provided dedicated for cooling heat pump 54. Alternatively the two ambient air heat exchangers 20, 20A can be combined into one component, and PCM coolant flow from the heat pump 54 is added in series with heat from the fuel cell 12, and combined in the ambient air heat exchanger 20. Preferably, the ambient air heat exchanger 20 is located immediately downstream from the fuel cell 12. As illustrated in the previous figures, valves are provided configured to bypass any of the components as may be required based on operating conditions. Also, air flow through the heat exchangers may be modulated, e.g., via baffles (not shown) to reduce aerodynamic drag when less than full cooling capacity is required.

[00075] Referring to Fig. 5A, in yet another embodiment, a heat pump 120 is disposed between the fuel cell coolant loop 122, 124 and the ambient air heat exchanger 20. This allows heat to be pumped in either direction between the phase change material and the coolant loop, while also allowing the fuel cell and phase change media to operate at different temperatures. Thus, during startup, the fuel cell 12 can be heated while also chilling the PCM 60. As before electric power for the heat pump may be supplied by the fuel cell. Alternatively, as shown in Fig. 5B, a bypass valve 126 and conduit 128 may be added to allow heat from the fuel cell 12 to be delivered to the heat pump without passing through the ambient air exchanger 20. This operating mode may be used when heating the phase change material. In place of proportional valves, pumps 28a, 28b also may be operated independently to deliver fractions of the PCM 60 coolant to various coolant paths.

[00076] Various changes may be made without departing from the spirit and scope of the disclosure. For example, referring to Fig. 1, electrical power for the heat pump 54 may be supplied at least in part from an auxiliary power supply such as an onboard battery, a ground base electricity energy cart, a ground based electrical energy grid, or regenerative power produced by vehicle braking (in the case of a land based vehicle) or propeller / motor braking during descent (in the case of a fuel cell powered aircraft). Alternatively, the heat pump may be mechanically driven by an onboard mechanical power service, or by an absorption refrigeration cycle, for example, from hot exhaust from the fuel cell or an onboard compressor. An advantage of an absorption refrigeration cycle is that it is light weight, and has no moving parts. Also, excess hydrogen fuel could be used to generate electricity specifically for cooling the PCM, i.e., permitting further time shifting of thermal management requirements. Plus, the evaporation of liquid H2 fuel absorbs heat, which also can be captured to further chill the PCM.

[00077] A feature and advantage of the use of a PCM and heat pump in accordance with the present disclosure is that it permits reduction of the size and mass of the necessary heat exchanger for cooling the fuel cells as compared to conventional ambient air heat exchangers. Also, the disclosure makes use of excess electric power (or excess mechanical energy or thermal energy) to chill the PCM when excess power is available, in essence time-shifting the cooling of the fuel cell, which in turn increases the payload or range of a fuel cell powered vehicle.

[00078] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. Various changes and advantages may be made in the above disclosure without departing from the spirit and scope thereof.

Claims

1. A cooling system for a fuel cell system comprising a primary ambient air heat exchanger coolant loop for cooling the fuel cell system, and a secondary coolant loop comprising a fluid circuit configured to circulate a coolant in thermal contact with a phase-change material (PCM) and also in thermal contact with the fuel cell to absorb heat from the fuel cell, wherein the secondary coolant loop includes a heat pump for cooling the PCM.

2. The cooling system of claim 1, wherein the fuel cell system is onboard a vehicle.

3. The cooling system of claim 1 or claim 2, wherein the secondary coolant loop comprisesan extension of the primary ambient air heat exchanger coolant loop.

4. The cooling system of claim 3, wherein the secondary coolant loop is configured to be selectively isolated from the primary ambient air coolant loop, and the heat pump is configured to cool the PCM to remove excess heat from the PCM.

5. The cooling system of claim 4, wherein the secondary coolant loop is configured to add heat to the PCM to heat the fuel cell system during warmup.

6. The cooling system of any of claims 2- 5, wherein the heat pump comprises an electrically driven heat pump.

7. The cooling system of claim 6, wherein electricity for powering the electrically driven heatpump is provided at least in part by the fuel cell onboard the vehicle.

8. The cooling system of claim 6, wherein electricity for powering the electrically driven heatpump is provided at least in part by one or more batteries onboard the vehicle.

9. The cooling system of claim 6, wherein the electricity for powering the electrically driven heat pump is provided at least in part by a ground based electrical energy cart, or a ground based electrical energy grid.

10. The cooling system of claim 6, wherein the electricity for powering the electrically driven heat pump is provided at least in part by regenerative power produced by vehicle braking or in the case of a fuel cell powered aircraft propeller / motor braking during descent.

11. The cooling system of any of claims 2-5, wherein the heat pump is mechanically driven at least in part by an onboard mechanical power service.

12. The cooling system of any of claims 2-5, wherein the heat pump comprises an absorption refrigeration cycle.

13. The cooling system of claim 12, wherein heat for the absorption refrigeration cycle is provided from hot exhaust from an onboard fuel cell.

14. The cooling system of claim 12, wherein heat for the absorption refrigeration cycle is provided by an onboard air compressor.

15. The cooling system of claim 1 or claim 2, wherein the secondary coolant loop is independently connected to the fuel cell, separate from the primary ambient air heat exchanger coolant loop.

16. The cooling system of any of claims 1 -15, wherein the secondary coolant loop is selectively connected via valves to the primary ambient air heat exchanger coolant loop.

17. The cooling system of any of claims 2-16, wherein the heat pump is connected in series with the fuel cell and the primary ambient air heat exchanger coolant loop, wherein the primary ambient air heat exchanger coolant loop is located downstream of the fuel cell between the fuel cell and the heat pump.

18. The cooling system of any of claims 2-16, wherein the heat pump is located between the fuel cell primary ambient air heat exchanger loop and the secondary coolant loop.

19. The cooling system of 18, further comprising a bypass valve configured to allow heat from the fuel cell to be delivered to the heat pump without passing through the primary ambient air heat exchanger coolant loop.

20. The cooling system of any of claims 2-19, wherein the primary ambient air heat exchanger loop is sized below that required for maximum continuous power operation of the vehicle, and the secondary coolant loop is sized to supplement the primary ambient air heat exchanger loop.

21. The cooling system of any of claims 2-20, wherein the cooling system includes a controller to control operation of the heat pump and valves and pumps to control flow of coolant through the fuel cell and the primary ambient air heat exchanger loop and the secondary coolant loop depending on cooling demands.

22. The cooling system of any of claims 1-21, wherein the fuel cell system is onboard a fuel cell powered aircraft.

23. A method for cooling a fuel cell aboard a vehicle comprising:providing a cooling system for a fuel cell system as claimed in any of claims 1 -22, and selectively circulating the coolant.

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