Fuel cell system

Abstract

The fuel cell system (300) includes a fuel preheater (327), a fuel cell stack (302), and a cooling circuit (350) which is configured to implement a Rankine cycle and includes a condenser (351). The fuel preheater is configured to heat a flow of liquid hydrogen supplied to its input to supply a flow of gaseous hydrogen. The system further includes a transport means configured to transport the gaseous hydrogen to the fuel input (312) of the fuel cell stack, so that the gaseous hydrogen comes into thermal contact with the coolant fluid of the condenser. This allows the size and mass of the condenser to be reduced. By configuring the preheater to heat the coolant fluid within the cooling circuit, the efficiency of the Rankine cycle is increased.

Description

Technology Field

[0001] The present invention relates to a fuel cell system. Background Technology

[0002] Fuel cell systems using hydrogen as fuel are of great interest in transportation applications, including aviation, because they do not generate carbon dioxide during use. There is particular interest in storing hydrogen in liquid form before use in gaseous form, as the volumetric energy density of liquid hydrogen (~8.5 MJ / l) is higher than that of gaseous hydrogen (~2.7 MJ / l at 300 bar). It is known to vaporize liquid hydrogen using a preheater element to generate a flow of gaseous hydrogen (e.g., published patent applications CN112599815A, CN110597504A, JPS5026419U). In addition to vaporizing liquid hydrogen, fuel cell systems using liquid hydrogen must control the temperature of the generated gaseous hydrogen before feeding it into the fuel cell stack of the fuel cell system. It is known that waste heat from a fuel cell stack is utilized by heating gaseous hydrogen through heat exchange with a cooling fluid in a cooling loop associated with the stack before it is fed into the fuel cell stack (e.g., published patent applications KR20130070161A, CN112599815A, CN110957504A). Some types of fuel cell stacks operate at relatively low temperatures. For example, Polymer Electrolyte Membrane (PEM) stacks generally have operating temperatures below approximately 90°C. If a cooling loop for such a stack implements a Rankine cycle, a relatively low level of power is recovered with low efficiency. Furthermore, the condenser within the cooling loop required to implement the Rankine cycle can have a very large size and mass, making the system unattractive for transportation propulsion, particularly aircraft propulsion.The use of cold gaseous hydrogen, produced by evaporation from a liquid hydrogen reservoir, to condense the vaporized coolant fluid in a Rankine-type cooling circuit for a fuel cell stack is described in the paper "Exergy and exergoeconomic comparison between multiple novel combined systems based on proton exchange membrane fuel cells integrated with organic Rankine cycles, and hydrogen boil-off gas subsystem" (Marandi et al, Energy Conversion & Management 244 (2021), article 114532).

[0003] A first aspect of the present invention provides a fuel cell system comprising a fuel preheater, a fuel cell stack, and a cooling circuit arranged to cool the fuel cell stack, wherein the fuel preheater is arranged to heat a flow of liquid hydrogen supplied to its input and to supply a flow of gaseous hydrogen at its output, and the fuel cell system further comprises a transport means arranged to transport gaseous hydrogen from the output of the fuel preheater to the fuel input of the fuel cell stack, and the transport means and the cooling circuit are arranged so that a coolant fluid in the cooling circuit comes into thermal contact with gaseous hydrogen in the transport means during operation of the fuel cell system, and (a) the cooling circuit includes a condenser arranged to implement a Rankine cycle and condense the gaseous coolant in the cooling circuit, (b) the condenser and the transport means are arranged so that a coolant fluid in the condenser comes into thermal contact with gaseous hydrogen in the transport means during operation of the fuel cell system, and (c) the preheater is arranged to heat the coolant fluid in the cooling circuit. During the operation of the fuel cell system of the present invention, heat from the coolant fluid in the condenser is transferred to gaseous hydrogen while it is traveling from the preheater to the fuel cell stack, thereby helping to cool the fuel cell stack and also to heat the gaseous hydrogen to a temperature suitable for introduction into the fuel cell stack. The temperature of the gaseous hydrogen output by the preheater can be positioned so that when it enters the fuel cell stack (after heat exchange with the coolant fluid in the condenser), the temperature of the gaseous hydrogen is sufficiently high to allow for an efficient reaction within the stack.

[0004] delete

[0005] Compared to cases where a cooling circuit implementing a Rankine cycle includes a condenser, the size and weight of the condenser can be reduced, and all heat from the condenser is dissipated into the surrounding air. A preheater is positioned to heat the coolant fluid within the cooling circuit, thereby increasing the efficiency of heat conversion into mechanical work by the Rankine cycle.

[0006] The preheater may be positioned to burn a portion of the liquid hydrogen flow supplied to the input of the preheater to vaporize the remainder of the liquid hydrogen flow or to generate heat for vaporization and heating. Alternatively, the preheater may be positioned to burn at least a portion of the hydrogen output from the fuel cell stack to vaporize the liquid hydrogen flow supplied to the input of the preheater or to generate heat for vaporization and heating.

[0007] The fuel cell system may include a supercharger, and the supercharger includes a compressor that is driven by a turbine during the operation of the fuel cell system and is also arranged to supply compressed air to the input of the fuel cell stack, and the fuel cell system is arranged so that combustion products generated by a preheater are supplied to the turbine.

[0008] A second aspect of the present invention provides a propulsion system comprising a fuel cell system according to the first aspect of the present invention and an electric thruster arranged to receive power from the fuel cell system and use this power to supply propulsion thrust.

[0009] A third aspect of the present invention provides an aircraft comprising a propulsion system according to a second aspect of the present invention. Brief explanation of the drawing

[0010] Embodiments of the present invention are described below with reference to the accompanying drawings merely as examples: FIG. 1 illustrates a fuel cell system of the prior art. FIGS. 2 and 5 illustrate fuel cell systems that are not examples of the present invention but are useful for understanding the present invention. FIGS. 3, 6 and 7 each illustrate first to third exemplary fuel cell systems of the present invention. FIG. 4 illustrates the operation of a Rankine cycle performed by a cooling device included in the system of FIG. 3. FIG. 8 illustrates an exemplary propulsion system of the present invention. Specific details for implementing the invention

[0011] FIG. 1 schematically illustrates a fuel cell system (100) of the prior art. The fuel cell system (100) comprises a polymer electrolyte (proton exchange) membrane (PEM) fuel cell stack (102) having a cathode side (104) and an anode side (106) and a cooling circuit (150). The cooling circuit comprises the PEM fuel cell stack (102), a coolant / air heat exchanger (153), and a pump (154) arranged to pump a coolant fluid around the cooling circuit (150). When the fuel cell system (100) is in operation, air and fuel are supplied to the inputs (108, 112) of the cathode side (104) and the anode side (106) of the PEM fuel cell stack (102), respectively. A cathode exhaust gas containing air and water vapor is output from the cathode output (110) of the PEM fuel cell stack (102), and unreacted hydrogen is output from the anode output (114). DC electric output power is generated at the electric output (120). The PEM fuel cell stack (102) has a coolant input (116) and a coolant outlet (118). Waste heat from the PEM fuel cell stack (100) is removed by a coolant fluid (e.g., water) in the coolant circuit (150) and transferred to a coolant / air heat exchanger (153). Due to the relatively low operating temperature (less than about 90°C) of the PEM fuel cell stack (102), the heat exchanger (153) has significant weight and size and may exhibit significant drag when mounted on the outside of an aircraft containing the system (100) in aviation applications.

[0012] Referring to FIG. 2, the fuel cell system (200) comprises a proton exchange (polymer electrolyte membrane) (PEM) fuel cell stack (202), a supercharger (217) including a compressor (224) which is configured to be driven by a turbine (222) and also driven by an electric motor (246), a humidifier (226), a fuel preheater (227) (e.g., a catalytic burner), and a heat exchanger (262). The fuel cell system (200) further comprises a cooling circuit (250) including a pump (254) configured to pump a coolant fluid (in this case, water) around the cooling circuit (250), a deionizing device (255), a distributor (252), and a coolant / air heat exchanger (253). The PEM fuel cell stack (202) has a cathode side (204) and an anode side (206), each having an input section (208, 212) and an output section (210, 214). The PEM fuel cell stack (202) has a coolant input section (216) and a coolant output section (218). A cooling branch (260) is arranged in parallel with the PEM fuel cell stack (202) and includes a heat exchanger (262).

[0013] When the fuel cell system (200) is in operation, air enters through the air intake (213), passes through the air filter (215), and is delivered to the compressor (224) of the supercharger (217). The compressed air output from the compressor (224) passes through the heat exchanger (258) to cool the compressed air, passes through the humidifier (226), and is supplied to the input section (208) of the cathode side (204) of the PEM fuel cell stack (202). The cathode exhaust gas containing air and water vapor exits the cathode side (204) from the output section (210), the water vapor of the cathode exhaust gas is recovered by the humidifier (226), and the remaining cathode exhaust gas is delivered to the turbine (222) of the supercharger (217). A flow of liquid hydrogen (LH2) fuel from a fuel storage (not shown) is input into the system (200) at the fuel input section (280) and delivered to the fuel preheater (227). Air is input into the preheater (227) through the air input section (282). A portion of the liquid hydrogen flow is combusted in the preheater (227) to generate heat, and this heat converts the remainder of the flow into gaseous hydrogen (GH2), which is supplied to the input section (212) of the anode side (206) of the PEM fuel cell stack (202) via the ejector (230). The gaseous hydrogen output from the preheater (227) is transported to the ejector (230) by a transport means (228) along a path passing through the coolant / air heat exchanger (253), so that the gaseous hydrogen in the transport means (228) comes into thermal contact with the coolant fluid in the heat exchanger (253). Accordingly, heat is transferred from the coolant fluid in the heat exchanger (253) to the gaseous hydrogen in the transport means (228) which is transferred from the preheater (227) to the ejector (230). The heat exchanger (253) is smaller and lighter than the heat exchanger (153) of the fuel cell system (100) of FIG. 1, because part of the cooling of the coolant fluid in the heat exchanger (253) is provided by the gaseous hydrogen from the preheater (227) (i.e., the gaseous hydrogen in the transport means (328)) during the operation of the fuel cell system (200).Combustion products (CP) from the preheater (227) are supplied to the turbine (222) of the supercharger (217). Hydrogen exits the anode side (206) of the PEM fuel stack (102) from the output section (210) and is recirculated to the input section (212) by the ejector (230) via the water trap (232). The water trap (232) has an output section (219) in which hydrogen in the anode (206) can be purged from the PEM fuel cell stack (202).

[0014] DC power is provided from the electrical output section (220) of the PEM fuel cell stack (202) and is provided to the low-voltage bus (244) via the DC / DC converter (241) and inverter (242). The low-voltage bus (244) supplies power to the electric motor (246). For example, power can be supplied to an electric propeller (not shown) through the electrical output section (248) of the DC / DC converter (241).

[0015] The preheater (227) is positioned so that the temperature of the gaseous hydrogen entering the transport means (228) is sufficient to obtain an appropriate reaction rate within the fuel cell stack (202), even when minimal additional heat is transferred from the cooling fluid in the cooling circuit (250) to the gaseous hydrogen, such as during the startup of the system (200) or when the electrical output power of the stack (202) is low.

[0016] FIG. 3 illustrates a first exemplary fuel cell system (300) of the present invention, wherein the system (300) comprises a PEM fuel cell stack (302). The system (300) is similar to the fuel cell system (200) of FIG. 2. The components of the system (300) are indicated by reference numerals that differ by 100 from the corresponding components of FIG. 2. The fuel cell system (300) comprises a PEM fuel cell (302), a heater (357), a turbine (358) arranged to drive an electric generator (359), a condenser (351), a deionization device (355), and a cooling circuit (350) comprising a pump (354) arranged to pump a coolant fluid (in this case, water) around the cooling circuit (350). The cooling branch (360) is arranged in parallel with the PEM fuel cell stack (202) and comprises heat exchangers (362, 364). Cooling water enters the PEM fuel cell stack (302) from the liquid-form coolant input section (316), and is vaporized in whole or in part by waste heat from the PEM fuel cell stack (302) to produce steam or steam and water at the coolant output section (318), which is input to the heater (357) along with water from the cooling branch (360). The heater (357) vaporizes the input water and supplies steam to the turbine (358). The steam drives the turbine (358) and is condensed in the condenser (351). Thus, the cooling circuit (350) converts waste heat from the PEM fuel cell stack (302) into useful work using the Rankine cycle.

[0017] When the fuel cell system (300) is in operation, the flow of air and liquid hydrogen (LH2) fuel is input from the fuel and air input sections (380, 382), respectively, to a preheater (327) (e.g., a catalytic burner). A portion of the liquid hydrogen fuel flow is combusted by the preheater (327) to convert the remaining flow into gaseous hydrogen (GH2), which is then transported by a transport means (328) through a heat exchanger (364) and an ejector (330) along the path to the input section (312) of the anode (306), thereby causing the gaseous hydrogen in the transport means (328) to come into thermal contact with the coolant fluid in the condenser (351). The size and weight of the condenser (351) are reduced compared to the size and weight of the heat exchanger (153) of the fuel cell system (100) of FIG. 1, so that the fuel cell system (300) is more suitable, for example, for aviation applications. Combustion products (CP) from the preheater (351) are transferred to a heater (357) in the cooling circuit (350) to provide heating of steam or steam and water within the cooling circuit (350) between the PEM fuel cell stack (302) and the turbine (358), so that additional work can be extracted by the turbine (358) with greater thermodynamic efficiency than would be possible without the heater (357). The combustion products are then discharged from the fuel cell system (300) via the turbine (322) of the supercharger (317).

[0018] The fuel cell system (300) further includes low-voltage and high-voltage DC / DC converters (343, 345) and a low-voltage bus (344), respectively.

[0019] FIG. 4 illustrates a plot (10) of temperature (T) versus entropy (S) for the cooling water in the cooling circuit (350) of the fuel cell system (300) of FIG. 3. The cooling circuit (350) performs a Rankine cycle to convert waste heat from the PEM fuel cell stack (302) into useful work extracted by the turbine (358). The location in the cooling circuit (350) immediately before the coolant fluid input (316) and the heater (357) is indicated as 1 and 2, respectively, in FIG. 3 and FIG. 4, and the location immediately before and after the turbine (158) is indicated as 3 and 4, respectively. The work extracted by the turbine (358) is indicated by the area enclosed by the plot (10), which has a portion (10a) representing the change in temperature (T) and entropy (S) of the coolant fluid (steam) across the turbine (358). In the absence of the heater (357), the plot (10) has a portion (10b) representing the change in temperature (T) and entropy (S) of the coolant fluid (steam) across the turbine (358). Therefore, the area (5) in FIG. 4 represents the amount of additional work extracted by the turbine (358) due to the presence of the heater (357) within the cooling loop (350) during one cycle of the cooling loop (350). The liquid / gas phase separation line for the coolant fluid is indicated by 12 in FIG. 4. The isobaric line is indicated by 14.

[0020] Sequential by Rankine cycle ( w net Thermodynamic efficiency from which ) is extracted ( or th ) is as follows.

[0021]

[0022] Here Q in is the sum of the heat input into the coolant fluid by the PEM fuel cell stack (102) and the heat input into the coolant fluid by the heater (357), Q out is heat output from the condenser (351). Pure heat ( w net ) is the difference between the work extracted by the turbine (358) and the work performed by the pump (354). h i Thermodynamic efficiency when is the enthalpy at position i where i=1, 2, 3, or 4 ( or th ) is calculated as follows.

[0023]

[0024] Therefore, the thermodynamic efficiency of work extracted by the turbine (358) is also increased by the presence of the heater (357) (in addition to an increase in the absolute amount of work extracted), because the enthalpy (h3) of the coolant fluid at position 3 (immediately before the turbine (358)) is greater than h3 when there is no heater (357), so the value of h3-h2 is increased by the heater (357).

[0025] FIG. 5 illustrates a fuel cell system (400). The system (400) is similar to the fuel cell system (200) of FIG. 2. The parts of the system (400) are indicated by reference symbols that differ by 200 from the corresponding parts of FIG. 2. The fuel cell system (400) includes a cooling circuit (450) that includes a coolant / air heat exchanger (453). When the fuel cell system (400) is in operation, the flow of liquid hydrogen (LH2) fuel is input to a preheater or burner (427) (e.g., a catalytic burner) that burns gaseous hydrogen output from the anode side (406) of the PEM fuel cell stack (402) using air output from the cathode side (404) of the PEM fuel cell stack (402). The heat generated by the burner (427) converts the flow of liquid hydrogen into a flow of gaseous hydrogen, which is then supplied to the input (412) of the anode (406) by a transport means (428) via a path where the gaseous hydrogen comes into thermal contact with the coolant fluid in the coolant / air heat exchanger (453) contained in the cooling circuit (450). (In addition to vaporizing the liquid hydrogen input to the burner, the generated gaseous hydrogen can be significantly heated.) The gaseous hydrogen provides cooling to the coolant fluid in the heat exchanger (453), and thus can be smaller and lighter than the heat exchanger (153) of the fuel cell system (100) of FIG. 1. The combustion product (CP) output from the burner (426) is supplied to the turbine (422) of the supercharger (417). Hydrogen is continuously purged from the anode side (406) to enable stable operation of the PEM fuel cell stack (402). The fuel cell system (400) further includes low-voltage and high-voltage DC / DC converters (443, 445) and a low-voltage bus (444), respectively.

[0026] FIG. 6 illustrates a second exemplary fuel cell system (500) of the present invention. The system (500) is similar to the fuel cell system (300) of FIG. 3. The parts of the system (500) are indicated by reference symbols that differ by 200 from those indicated on the corresponding parts of FIG. 3. When the fuel cell system (500) is in operation, a flow of liquid hydrogen (LH2) fuel is input into a heater (557), and a portion of this flow is combusted to heat steam and / or liquid water in a cooling circuit (550), thereby increasing the amount of useful work extracted by the turbine (558) and the thermodynamic efficiency of the extraction compared to the case where the heater (557) is not present. The remainder of the liquid hydrogen flow input into the heater (557) is simultaneously heated to generate a flow of gaseous hydrogen, which is input to the anode side (506) of the PEM fuel cell stack (502) by the transport means (528) via the condenser (551) and the ejector (512) in a path where the gaseous hydrogen from the heater (557) in the transport means (528) comes into thermal contact with the coolant in the condenser (551). This allows the size and weight of the condenser (551) to be reduced compared to a conventional Rankine array condenser because the coolant fluid in the condenser (551) is cooled by the flow of gaseous hydrogen output from the heater (557). The combustion products from the heater (557) are supplied to the turbine (522) of the supercharger (517). The fuel cell system (500) further includes low-voltage and high-voltage DC / DC converters (543, 545) and a low-voltage bus (544), respectively. The heater (557) serves both as a preheater to vaporize liquid hydrogen input from the system fuel input (580) (and may also heat the generated gaseous hydrogen) and as a means to heat the coolant fluid within the cooling circuit (550). Depending on the operating state of the fuel cell stack (502), the coolant fluid exiting the stack (502) from the coolant output (518) may be in liquid form, gaseous form (i.e., steam), or a combination of liquid water and steam.

[0027] FIG. 7 illustrates a third exemplary fuel cell system (600) of the present invention similar to the fuel cell system (500) of FIG. 6. The parts of the fuel cell system (600) are indicated by reference numerals that differ by 100 from the corresponding parts shown in FIG. 6. When the fuel cell system (600) is in operation, exhaust gas from the cathode side (604) of the PEM fuel cell stack (602) is output from the stack output section (610), transferred to a humidifier (626) to remove water vapor, and then input into a heater (657). Gaseous hydrogen fuel is supplied to the stack input section (612). Unreacted gaseous hydrogen fuel is output from the stack output section (614) and input into the heater (657) via a water trap (632). Air and gaseous hydrogen fuels output from the PEM fuel cell stack (602) are combusted in a heater (657) (which may be a catalytic burner) to heat the coolant fluid of the cooling circuit (650) and also convert liquid hydrogen (LH2) from the fuel input (680) into gaseous hydrogen, which is then transferred via a transport means (628) to the stack input (612) on the anode side (606) of the PEM fuel cell stack (602) so that the gaseous hydrogen comes into thermal contact with the cooling fluid of the condenser (651). In addition to vaporizing the liquid hydrogen input from the fuel input (680), the generated gaseous hydrogen may also be heated. The gaseous hydrogen cools the coolant fluid of the condenser (651), making it smaller in size and weight than the condenser of a conventional system and smaller than the coolant / air heat exchanger (153) of the fuel cell system (100) of FIG. 1. Gaseous hydrogen in the transport means (628) is heated by heat from the condenser (651) before being input into the PEM fuel cell stack (602). The combustion product (CP) is supplied to the turbine (622) of the supercharger (617).

[0028] The coolant fluid in the cooling circuit (650) is water. The cooling circuit (650) is arranged so that the water exists in a steam form between the heater (657) and the condenser (651), and in a liquid form between the condenser (651) and the coolant input (616) of the PEM fuel cell stack (602). The liquid water entering the PEM fuel cell stack (602) from the coolant input (616) is heated by waste heat from the stack (602) and is converted in whole or in part into steam. All remaining liquid water output to the coolant output (618) is converted into steam by the heater (657) together with the water from the cooling branch (660). The steam coming from the heater (657) is delivered to the turbine (658) included in the cooling circuit (650).

[0029] FIG. 8 illustrates an exemplary propulsion system (700) of the present invention comprising the fuel cell system (300) of FIG. 3 and an electric thruster (790). The electric thruster (790) comprises an inverter (792) and an electric motor (794) arranged to drive a propeller or fan (796). When the propulsion system (700) is in operation, power generated at the electric output (348) of the fuel cell system (300) is supplied to the inverter (792) of the electric thruster (790). In another exemplary propulsion system of the present invention, the fuel cell system (300) is replaced with one of the fuel cell systems (500, 600) of FIG. 6 and 7. The propulsion system (700) may be included in an aircraft.

[0030] Any of the preheaters (227, 327, 427, 557, 657) of the fuel cell system (200, 300, 400, 500, 600) can each adjust the rate at which hydrogen is burned within it, thereby allowing the temperature of the gaseous hydrogen leaving the preheater to be adjusted.

[0031] delete

Claims

Claim 1 A fuel cell system (300; 500; 600) comprising a fuel preheater (327; 557; 657), a fuel cell stack (302; 502; 602), and a cooling circuit (350; 550; 650) arranged to cool the fuel cell stack, wherein the fuel preheater is arranged to heat a flow of liquid hydrogen supplied to its input and supply a flow of gaseous hydrogen at its output, and the fuel cell system further comprises a transport means (328; 528; 628) arranged to transport gaseous hydrogen from the output of the fuel preheater to the fuel input (312; 512; 612) of the fuel cell stack, and wherein the transport means and the cooling circuit are arranged such that, during the operation of the fuel cell system, a coolant fluid in the cooling circuit comes into thermal contact with the gaseous hydrogen in the transport means, and (a) the cooling circuit implements a Rankine cycle and is arranged to condense the gaseous coolant in the cooling circuit A fuel cell system (300; 500; 600) comprising a condenser (351; 551; 651), (b) said condenser and a transport means (328; 528; 628) arranged so that during operation of the fuel cell system, a coolant fluid in the condenser comes into thermal contact with gaseous hydrogen in the transport means, and (c) said preheater (327; 557; 657) arranged to heat a coolant fluid in a cooling circuit. Claim 2 In claim 1, the preheater (327; 557) is configured to burn a portion of the liquid hydrogen flow supplied to its input to generate heat to vaporize or vaporize and heat the remainder of the liquid hydrogen flow, in a fuel cell system (300; 500). Claim 3 In claim 1, the preheater (657) is configured to burn at least a portion of the hydrogen output from the fuel cell stack (602) in order to generate heat to vaporize or vaporize and heat the flow of liquid hydrogen supplied to the input of the preheater, in a fuel cell system (600). Claim 4 In claim 2 or 3, the fuel cell system further comprises a supercharger (317; 517; 617), the supercharger comprises a compressor (324; 524; 624) arranged to be driven by a turbine (322; 522; 622) when the fuel cell system is operating, the compressor is arranged to supply compressed air to an input section (308; 508; 608) of a fuel cell stack (302; 502; 602), and the fuel cell system is arranged to supply combustion products generated by a preheater to the turbine, the fuel cell system (300; 500; 600). Claim 5 A propulsion system (700) comprising a fuel cell system and an electric thruster (790) according to any one of claims 1 to 3, wherein the electric thruster (790) is configured to receive power from the fuel cell system and supply propulsion thrust using said power. Claim 6 An aircraft including a propulsion system pursuant to Paragraph 5. Claim 7 delete Claim 8 delete Claim 9 delete

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