GB2621415A

GB2621415A - Pumped two-phase fuel cell cooling

Info

Publication number: GB2621415A
Application number: GB2215788.7A
Authority: GB
Inventors: Legg Matthew
Original assignee: Zeroavia Ltd
Current assignee: Zeroavia Ltd
Priority date: 2022-10-25
Filing date: 2022-10-25
Publication date: 2024-02-14
Anticipated expiration: 2042-10-25
Also published as: GB2621415B; GB202215788D0

Abstract

A cooling system 10 for a fuel cell 12 onboard a vehicle, e.g. an aircraft, comprises a fluid circuit with a coolant channel 13 in contact with the fuel cell to absorb heat from the fuel cell, a heat exchanger 20 comprising an extension of the coolant channel and an airflow channel 48, a pump 24 for pressurizing and circulating the coolant through the system, and a throttling valve 26 for controlling pressure on the coolant as it circulates through the system. The coolant comprises a phase-change material and has a boiling point at pressure that is below the fuel cell operating temperature. The system also includes a controller for controlling operation of the pump and the throttling valve to at least partially evaporate the coolant. The coolant can be partially or completely evaporated so that it exits contact with the fuel cell as a two-phase mixture or as a super-heated vapour. The cooling system may also include a coolant buffer tank 32 for holding liquid coolant 18. Because this cooling system and method employs the latent heat of evaporation of a two-phase coolant, it reduces mass and parasitic power requirements of the cooling system.

Description

Intellectual Property Office Application No GI322157887 RTM Date:30 November 2022 The following terms are registered trade marks and should be read as such wherever they occur in this document:

NOVEC

FLUORINERT

Intellectual Property Office is an operating name of the Patent Office www.gov.uk/ipo

PUMPED TWO-PHASE FUEL CELL COOLING

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 he 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.

100% power output is only needed for 60-120 seconds on takeoff and initial climb to 1,000 feet. After 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 potentially 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 and the freestream dynamic pressure. 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 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 he of significant contribution to the efficiency of hydrogen fuel cell-powered airplanes.

[0005] In accordance with the present disclosure, we employ a two-phase coolant in a fuel cell cooling system to exploit the latent heat of vaporization of the coolant whereby 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. Additional benefits are a reduction in weight and size, and more packaging flexibility. 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 one aspect one or more parts of the cooling system is sized below that required for peak power operation of the vehicle, and the coolant and cooling system is selected to match the thermophysical properties to the fuel cells operating temperature for and employ heat of evaporation of the coolant during peak vehicle operation, i.e., during take off and climb in the case of a fuel cell powered aircraft.

[0007] In another aspect one or more parts of the cooling system is sized below that require for peak power operation of the vehicle, and coolant pressure is increased to increase boiling point of the two-phase coolant system.

[0008] In yet another aspect the cooling system includes a controller programmed to control operation of valves and pumps of the cooling system and flow of the coolant depending on cooling demands whereby to maintain coolant pressure in a desired range for evaporation/condensation to occur at a desired temperature, i.e., the design operating temperature of the fuel cell or fuel cell stack.

[0009] In yet another aspect, the coolant system includes a throttling valve and a controller configured to control operation of the throttling valve to match saturation temperature to target fuel cell set point or fuel cell operating temperature whereby to assure equal cooling across the evaporator and across the fuel cell stack.

wow In a particularly preferred aspect, the vehicle comprises a fuel cell powered aircraft.

[00011] 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.

[00012] According to aspect A of the present invention there is provided a cooling system for a fuel cell aboard a vehicle, wherein the cooling system comprises a fluid circuit configured to circulate a coolant comprising a phase-change material through a coolant channel in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the extension of the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.

[000131 Preferably the cooling system further includes one or more sensors upstream of the heat exchanger for detecting temperature and/or pressure of the coolant entering the heat exchanger.

[09014] Preferably the cooling system further includes a coolant buffer tank configured to hold liquid coolant between the outlet of the heat exchanger and the pump.

rooms] Preferably the air inlet to the heat exchanger is closable at least in part.

1000161 Preferably the air inlet to the heat exchanger is configured to close at least in part when the temperature of the coolant exiting the heat exchanger approaches ambient temperature.

moon] Preferably the coolant has a boiling point at 2 to 3 bar pressure that is 2 to 10°C below the fuel cell operating temperature.

r000181 Preferably the controller is configured to control operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation/condensation to occur at design operating temperatures of the fuel cell.

[000191 Preferably the coolant is circulated through cooling channels in the fuel cell.

[90029] Preferably the vehicle comprises a fuel cell powered aircraft.

[00021] According to aspect B of the present invention there is provided a method of cooling a fuel cell aboard a vehicle, comprising the steps of providing a cooling system according to aspect A of the present invention as set out above.

[000221 According to aspect C of the present invention there is provided a method of cooling a fuel cell aboard a vehicle, comprising the steps of: providing a cooling system comprising fluid circuit configured to circulate through a coolant channel a coolant comprising a phase-change material in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.

r000231 Preferably the method according to aspect B or aspect B further includes sensing the temperature of the coolant exiting the heat exchanger, and adjusting airflow through the heat exchanger based on the temperature of the coolant.

[000241 Preferably the method according to aspect B or aspect C wherein the coolant has a boiling point at 2 to 3 bar pressure that is 2 to 10°C below the fuel cell operating temperature.

[00025] Preferably the method according to aspect B or aspect C wherein the controller controls operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation/condensation to occur at design operating temperatures of the fuel cell.

[00026] Preferably the method according to aspect B or aspect C wherein the coolant is circulated through cooling channels in the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[00027] 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.

[00028] Fig. 1 is a flow diagram of a cooling system for a hydrogen fuel cell in

accordance with the present disclosure;

[00029] Figs. 2 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; [00030] Figs. 3 and 4 graphically illustrate pressure verses specific enthalpy of a two-phase coolant controlled in accordance with the present disclosure; and 1-000311 Fig. 5 is a block diagram showing operation and control of the cooling system 10

in accordance with the present disclosure.

DETAILED DESCRIPTION

[00032] Example embodiments will now he 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.

[00033] 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.

[000341 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.

[00035] 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 he 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.

[00036] 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.

[000371 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.

[000381 The present disclosure has particular applicability to proton-exchange membrane hydrogen fuel cells, or so-called hydrogen fuel cells, for powering aircraft, although the disclosure is not limited to hydrogen fuel cells for powering aircraft.

[000391 In our co-pending PCT Application Serial No. PCT/GB2022/051112, filed 29 April 2022 we describe a cooling system for a fuel cell onboard a vehicle including aircraft which in one aspect comprises a coolant circuit defining a coolant passageway and an auxiliary evaporative cooler. The coolant circuit is configured to circulate a coolant including a phase change material through the coolant passageway and through a portion of the fuel cell to absorb heat from the fuel cell. The auxiliary evaporative cooler comprises an inlet, an outlet, a coolant channel, an airflow channel, and a selectively permeable membrane that physically separates the coolant channel from the airflow channel. The inlet is configured to receive an airflow from an ambient environment. The outlet is in fluid communication with the inlet and with the ambient environment. The airflow channel is in fluid communication with the inlet and the outlet. The coolant channel is in fluid communication with the coolant circuit. The selectively permeable membrane is selectively permeable to the phase change material in the coolant. The auxiliary evaporative cooler is configured to evaporatively cool the coolant flowing through the coolant channel by promoting evaporation and transport of the phase change material from the coolant flowing through the coolant channel, through the selectively permeable membrane, and into the airflow flowing through the airflow channel.

[00040] Alternatively, as disclosed in our aforesaid PCT Application, the cooling system for a fuel cell onboard a vehicle comprises a plenum, a coolant circuit, an auxiliary evaporative cooler, and a thermal energy storage chamber. The plenum includes an inlet and an outlet in fluid communication with an ambient environment. The inlet of the plenum is configured to receive an airflow from the ambient environment. The coolant circuit defines a coolant passageway and is configured to circulate an aqueous coolant through the coolant passageway and through a portion of the fuel cell to absorb heat from the fuel cell. The auxiliary evaporative cooler comprises a coolant channel, an airflow channel, and a selectively permeable membrane that physically separates the coolant channel from the airflow channel. The coolant channel is in fluid communication with the coolant circuit. The airflow channel is in fluid communication with the inlet and the outlet of the plenum. The selectively permeable membrane is selectively permeable to water vapor. The thermal energy storage chamber is in fluid communication with the airflow channel of the auxiliary evaporative cooler. The auxiliary evaporative cooler is configured to evaporatively cool the aqueous coolant flowing through the coolant channel by promoting evaporation and transport of water vapor from the aqueous coolant flowing through the coolant channel, through the selectively permeable membrane, and into the airflow flowing through the airflow channel. The thermal energy storage chamber is configured to store thermal energy released from the aqueous coolant flowing through the coolant channel in the form of latent heat.

[000411 In accordance with the present disclosure, we control fuel cell cooling to remove excess waste heat generated by the fuel cell(s) or fuel cell stack across a full range of operating conditions including cruise, take-off and climb. To accomplish this we provide a multi-mode cooling system employing a two-phase coolant configured to operate below coolant boiling point during cruise, and configured to allow coolant temperature to rise so that two-phase cooling occurs thereby minimizing the need to increase pumping requirements at peak load. More particularly, we match the thermophysical properties of the coolant under operating conditions to the fuel cell cooling requirements at operating conditions by selection of an appropriate coolant, and controlling the cooling system to increase the boiling point of the coolant in a desired range for evaporation/condensation by altering pressure on the coolant. In this regard, we select a coolant that has a boiling point at 2 to 3 bar that is 2 to I0°C below the fuel cell operating temperature, and we control the pressure on the coolant to increase the pressure to raise the coolant boiling point and correspondingly the enthalpy of the system during peak cooling demand conditions, i.e., during takeoff and climb. The coolant should be a dielectric so that the coolant may be circulated directly through the fuel cell cooling channels. Various low boiling point dielectric fluids are available commercially for use as coolants in accordance with the present disclosure, including but not limited to NOVECTm and FLUORINERTTm, which are both available 3M Company. Other working fluids may include high temperature refrigerants such as R I 233zd, R I 243zf, R I 234ze(E) and R I 234ze(z).

[00042] More particularly, in accordance with the present disclosure we provide a cooling system for a fuel cell aboard a vehicle, wherein the cooling system comprises a fluid circuit configured to circulate a coolant comprising a phase-change material through a coolant channel in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the extension of the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.

[00043] In one aspect the system includes one or more sensors upstream of the heat exchanger for detecting temperature and/or pressure of the coolant entering the heat exchanger.

[00044] In another aspect the system includes a coolant buffer tank configured to hold liquid coolant between the outlet of the heat exchanger and the pump.

[00045] In another aspect the air inlet to the heat exchanger is closable at least in part.

[00046] In yet another aspect the air inlet to the heat exchanger is configured to close at least in part when the temperature of the coolant exiting the heat exchanger approaches ambient temperature.

[00047] In another aspect the coolant has a boiling point at 2 to 3 bar pressure that is 2 to I 0°C below the fuel cell operating temperature.

[000481 In a further aspect the controller is configured to control operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation and condensation to occur at design operating temperatures of the fuel cell and heat exchanger respectively.

[00049] In yet a further aspect the coolant is circulated through cooling channel in the fuel cell.

[00050] In a further aspect the vehicle comprise a fuel cell powered aircraft.

[00051] The present disclosure also provides a method of cooling a fuel cell aboard a vehicle, comprising the steps of: providing a cooling system comprising fluid circuit configured to circulate through a coolant channel a coolant comprising a phase-change material in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve, and controlling operation of the pump and throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.

[00052] In one aspect the method further includes sensing the temperature of the coolant exiting the heat exchanger, and adjusting airflow through the heat exchanger based on the temperature of the coolant.

[00053] In another aspect of the method the coolant has a boiling point at 2 to 3 bar pressure that is 2 to 10°C below the fuel cell operating temperature.

[00054] In a further aspect of the method the controller controls operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation/condensation to occur at design operating temperatures of the fuel cell.

[00055] In yet another aspect of the method, the coolant is circulated through cooling channels in the fuel cell.

[00056] 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 hydrogen fuel cell or fuel cell stack 12 is located on board an aircraft 14 as shown in Fig. 2 that is powered 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 of the aircraft 14, the fuel cell or fuel cell stack also may be disposed in the fuselage, engine nacelle, or any other part of the aircraft. The cooling system 10 includes a heat exchanger system as shown in Fig. 1, configured to extract heat from the fuel cell or fuel cell stack 12, for example, by contact with the fuel cell of fuel cell stack, e.g., through cooling channels shown in phantom at 13 in the fuel cell or fuel cell stack 12 transferring waste heat generated by the fuel cell or fuel cell stack 12 to a cooling medium, that includes coolant 18, an air-cooled heat exchanger 20, conduits 22, pump 24 and valve 26 under control of a controller 28 for circulating the coolant 18 through a heat exchanger 20 in contact with the fuel cell or fuel cell stack 12 to absorb heat from the fuel cell or fuel cell stack 12, and through air heat exchanger 20 into a coolant buffer tank 32. The cooling system is configured, sized and controlled to accommodate cooling requirements of the fuel cell or fuel cell stack 12 during cruise as well as peak power operation of the vehicle, i.e., take-off and climb, as will be described below. In other embodiments, cooling channels 13 may be disposed within a heat exchanger which is thermally coupled to fuel cell stack 12.

[00057] Referring again to Fig. 1, the cooling system incorporates a first sensor 40 in line 42 connecting pump 24 with fuel cell or fuel cell stack 12. Sensor 40 is followed downstream of fuel cell or fuel cell stack 12 by throttling valve 26. Sensor 40, throttling valve 26 and pump 24 are all connected to control 28. In operation subcooled liquid coolant is flowed through the fuel cell or fuel cell stack 12 to cool the stack. Controller 28 receives temperature and pressure data of the coolant in line 42 from sensor 40, and is configured to regulate pump 24 and valve 26 to control the evaporation temperature of the coolant by altering the pressure and accordingly the boiling point of the coolant flowing though the fuel cell or fuel cell stack 12. Saturated vaporized coolant exits the fuel cell or fuel cell stack 12, and is passed via line 46 to air cooled condenser 20 in which heat is extracted from the hot vaporized coolant, and the coolant condensed to liquid which is then passed via line 22 to coolant buffer tank 32.

[00058] Controller 28 also receives temperature readings from sensor 44 upstream of the fuel cell or fuel cell stack 12, and is configured to regulate throttling valve 26 to match the saturation temperature of the coolant, to target fuel cell or fuel cell operating stack set point temperature. In so doing we can assure substantially equal cooling across the fuel cell or fuel cell stack 12. That is to say, the system is arranged so that the fuel cell stack cooling channels act as an evaporator receiving subcooled liquid coolant from the pump 24. Heat from the fuel cell or fuel cell stack 12 will warm the coolant to its boiling point and partially or completely evaporate the coolant so that it exits the stack cooling channels as either a two-phase mixture or a superheated vapor. The now hot coolant is passed to the heat exchanger 20 which may be cooled by air or other means where it condenses back to a subcooled liquid. From the heat exchanger 20 the coolant is circulated to the coolant buffer tank 32 which serves as a vapor separator and coolant buffer to ensure no vapor enters the pump. The use of a two-phase coolant exploits the latent heat of vaporization to reduce the required coolant flow rates, pump size, tube and coolant volume. A further benefit is that evaporation through the fuel cell stack will provide better temperature uniformity and higher possible convective heat transfer coefficient making the heat transfer from/to coolant more efficient.

[00059] Also, by controlling coolant pressure, we can close the air inlet 48 of the heat exchanger 20 during cruise to reduce drag. Controller 28 receives temperature from temperature sensor 40 located between pump 24 and cooling channels 13. As temperature of the coolant approaches ambient temperature, we can close the air inlet 28 of the heat exchanger and/or reduce mass flow of the coolant pump 24 to reduce drag and/or power consumption respectively.

[00960] The below Table 1 illustrates the advantages of the use of a two-phase cooling system in accordance with the present disclosure over a single phase system. Estimation of cooling flow and pumping requirements for single and two-phase systems.

re Ct emoe IC outlet temper e heat sapssaty Power reduction two phase ( ok flow me flow RI to differenVa

TABLE 1

[00061] Fig. 5 is a block diagram showing operation and control of cooling system 10 in accordance with the present disclosure. The controller 28 is configured to receive input including outside air temperature, fuel cell power requirements, fuel cell temperature, and coolant temperature and pressure from sensors 42 and 44, and controls pump 24 and throttling valve 26 to increase pressure on the coolant to increase the coolant boiling point to maintain the fuel cell stack at a desired operating temperature. Also, controller 28 may control the air inlet 48 of the heat exchanger 20 to reduce drag, e.g., during cruise, when cooling demands are reduced, or when temperature of the coolant approaches ambient temperature.

[00062] Figs. 3 and 4 graphically illustrate take-off and cruise conditions of a fuel cell powered aircraft equipped with a two-phase coolant controlled in accordance with the present disclosure at locations 1, 2 and 3. Figure 3 illustrates NOVECTm649 as coolant @ 1800kW for take-off: - Evaporator inlet -62°C, 2.8 Bar - Evaporator outlet = FC temperature (-85°C) - Condenser inlet -85°C, 2.5 Bar Condenser outlet = saturation temperature -3°C - Ambient temperature 30°C - 303K ambient - 358K fuel cell -2.5kW pumping power Novec 649, Q = 1800 kW, mdot = 17.5 kg/s [00063] Figure 4 illustrates take-off and cruise conditions at locations 1, 2 and 3 for NOVECTm649 as a coolant @ 900kW 18,000 ft for cruise: - Evaporator inlet -54°C, 1.85 Bar - Evaporator outlet = FC temperature (-70°C) - Condenser inlet -54°C, 1.7 Bar - Condenser outlet = saturation temperature -3°C - Ambient temperature -20°C - 253K ambient - 343K fuel cell -0.6kW pumping power Novec 649, Q = 900 kW, mdot = 9.0 kg/s [00064] As will be seen from the foregoing, the use of a two-phase coolant in accordance with the present disclosure exploits the latent heat of vaporization to reduce the required coolant flow rates, pump size, tube and coolant volume of the cooling system for an onboard fuel cell powered vehicle. A further benefit is that evaporation through the fuel cell stack will provide better temperature uniformity and higher possible convective heat transfer coefficient making the heat transfer from/to coolant more efficient.

[00065] Other benefits include reduction in mass and parasitic power of cooling system components, and improved temperature uniformity in fuel cell stack and heat rejection equipment (e.g. radiators), and improved temperature range (e.g. MT/HT PEM).

[000661 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

What is Claimed: A cooling system for a fuel cell aboard a vehicle, wherein the cooling system comprises a fluid circuit configured to circulate a coolant comprising a phase-change material through a coolant channel in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the extension of the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.
2. The cooling system of claim 1, further including one or more sensors upstream of the heat exchanger for detecting temperature and/or pressure of the coolant entering the heat exchanger.
3. The cooling system of claim 1 or claim 2, further including a coolant buffer tank configured to hold liquid coolant between the outlet of the heat exchanger and the pump.
4. The cooling system of any of claims I to 3, wherein the air inlet to the heat exchanger is closable at least in part.
5. The cooling system of claim 4, wherein the air inlet to the heat exchanger is configured to close at least in part when the temperature of the coolant exiting the heat exchanger approaches ambient temperature.
6. The cooling system of any preceding claim, wherein the coolant has a boiling point at 2 to 3 bar pressure that is 2 to 10°C below the fuel cell operating temperature.
7. The cooling system of any preceding claim, wherein the controller is configured to control operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation/condensation to occur at design operating temperatures of the fuel cell.
8. The cooling system of any preceding claim, wherein the coolant is circulated through cooling channels in the fuel cell.
9. The cooling system of any preceding claim, wherein the vehicle comprises a fuel cell powered aircraft.
10. A method of cooling a fuel cell aboard a vehicle, comprising the steps of: providing a cooling system as claimed in any of claims I to 9.
11. A method of cooling a fuel cell aboard a vehicle, comprising the steps of: providing a cooling system comprising fluid circuit configured to circulate through a coolant channel a coolant comprising a phase-change material in contact with the fuel cell to absorb heat from the fuel cell; a heat exchanger comprising an extension of the coolant channel and an airflow channel connecting an air inlet and an air outlet, wherein the coolant channel is separated from the air flow channel; a pump configured for pressurizing and circulating the coolant through the system; a throttling valve for controlling pressure on the coolant as the coolant is circulated through the system, wherein the coolant has a boiling point at pressure that is below the fuel cell operating temperature, and a controller configured to control operation of the pump and the throttling valve to partially or completely evaporate the coolant so that the coolant exits contact with the fuel cell as a two-phase mixture or as a super-heated vapor.
12. The method of claim 10 or claim 11, further including sensing the temperature of the coolant exiting the heat exchanger, and adjusting airflow through the heat exchanger based on the temperature of the coolant.
13. The method of any of claims 10 to 12, wherein the coolant has a boiling point at 2 to 3 bar pressure that is 2 to 10°C below the fuel cell operating temperature.
14. The method of any of claims 10 to 13, wherein the controller controls operation of the throttling valve and pump to maintain the coolant in a desired range for evaporation/condensation to occur at design operating temperatures of the fuel cell.
15. The method of any of claims 10 to 14, wherein the coolant is circulated through cooling channels in the fuel cell.