Fuel cell power supply system and method with heat exchanger bypass

Abstract

A power system (20) includes a fuel cell system (28) with a fuel cell stack (32) and a charge air system (38) with an air compressor (40) and an exhaust turbine (42) for delivering pressurized air to and receiving exhaust gas from the fuel cell stack. The power system also includes a temperature control system (52) with a bypass line (54) and a diverter valve (56) configured to redirect the pressurized intake air or exhaust gas in a bypass path around the heat exchanger. The strategy improves the efficiency of the charge air system and can mitigate concerns such as turbine inlet icing when a temperature difference between a compressor outlet temperature and a turbine inlet temperature is less than a threshold temperature difference.

Description

Technical FieldThe present disclosure relates generally to a fuel cell power system, and more particularly to bypassing a heat exchanger in a charge air system for a fuel cell.Prior ArtFuel cell systems are used worldwide for power generation. Fuel cells generally produce electrical power through a controlled chemical reaction between a fuel and an oxidant, typically air, to generate electrical current. Fuel cells promising a reduction or even complete avoidance of certain types of emissions, in particular of the so-called greenhouse gases. Fuel cell applications have evolved in recent years from smaller scale power generation to higher power applications for the operation of power grids and heavy machinery. It has been found that conventional fuel systems may not be sufficient to provide an optimal power output range. In some cases, for example, it may be desirable for a fuel cell in a power generation or vehicle application to provide relatively low power output temporarily, but to be able to respond quickly to greater power demands. Highly varying power output requirements as well as high absolute power requirements may require sophisticated devices for the rapid and reliable adjustment of a flow of fuel and oxidizing reactants in a fuel cell.Pumps, compressors, exhaust turbines, and other equipment may in some cases be used to provide flexibility in oxidizer and fuel pressures and flow rates to meet varying load requirements as well as high absolute requirements. It is known to use a compressor and a coupled turbine, similar to a turbocharger application used in internal combustion engines, to provide improved efficiency in fuel cell systems. A known system employing a compressor and an exhaust turbine in a fuel cell system is described in U.S. Application No. 18 / 122,176 to Kruiswyk et al.SummaryIn one aspect, a power supply system includes a fuel cell system having a fuel cell stack, an intake line extending to the fuel cell stack, and an exhaust line extending from the fuel cell stack. The power supply system further includes a charge air system having an air compressor in the intake conduit, an exhaust turbine in the exhaust conduit, and a heat exchanger. The power supply system further includes a temperature control system having a bypass line and a diverter valve disposed at least partially in the bypass line. The intake conduit, the exhaust conduit, and the heat exchanger together define an air-to-exhaust heat exchange path for pressurized air and exhaust gas through the heat exchanger. The bypass line is fluidly connected to at least one of the intake line or the exhaust line and defines a bypass path around the heat exchanger for one of the pressurized air or the exhaust gas, and the diverter valve is movable between a closed position in which the bypass path is blocked and a second position in which the bypass path is opened.In another aspect, a method of operating a power supply system includes supplying exhaust gases from a fuel cell through an exhaust turbine to rotate the exhaust turbine, rotating a compressor based on the rotation of the exhaust turbine to pressurize the intake air for the fuel cell, and exchanging heat between the exhaust gases and the pressurized intake air. The method further comprises opening a bypass line to bypass pressurized intake air or exhaust gas around the heat exchanger, and reducing the exchange of heat between exhaust gas and pressurized intake air based on the opening of the bypass line.In yet another aspect, a charge air system for a fuel cell power supply system includes an intake conduit for supplying pressurized intake air from a compressor to a fuel cell stack, an exhaust conduit for supplying exhaust gas from the fuel cell stack to a turbine coupled to the compressor, and a bypass conduit including an inlet fluidly coupled to one of the intake conduit or the exhaust conduit and an outlet fluidly coupled to one of the intake conduit or the exhaust conduit, and a bypass path for pressurized air or exhaust gas around defining a heat exchanger coupled between the intake conduit and the exhaust conduit. The charge air system further includes a diverter valve disposed at least partially in the bypass line and movable between a closed position in which the bypass path is blocked and a second position in which the bypass path is open. The charge air system further includes a sensing mechanism configured to monitor a temperature difference between a compressor outlet temperature and a turbine inlet temperature, and a temperature control unit coupled to the sensing mechanism and in control communication with the diverter valve. The temperature control unit is configured to command setting of the diverter valve to the second position based on the temperature difference.Brief Description of the DrawingsFIG. 1 is a schematic view of a machine according to an embodiment; FIG. 2 is a schematic view of a fuel cell system and a charge air system according to an embodiment; FIG. 3 is a schematic view of a fuel cell system and a charge air system according to another embodiment; FIG. 4 is a schematic illustration of a fuel cell system and a charge air system according to another embodiment; and FIG. 5 is a block diagram of a methodology and logic flow, according to one embodiment.DETAILED DESCRIPTIONReferring to FIG. 1, a machine 10 is shown according to an embodiment that includes a frame 12 and ground engaging drive members 14 coupled to the frame 12. In the illustrated embodiment, the machine 10 includes a truck having a cargo area 16 and an operator cabin 18. the machine 10 also includes a propulsion system 22, including, for example, an electric motor 24 and a transmission 26 configured to rotate the ground engaging elements 14. It should be appreciated that the machine 10 may include a variety of different types of machines, including a off-road truck, tractor, excavator, wheel loader, or a variety of other wheel or track machines for use on-road or road traffic. In still other applications, the machine 10 could include a stationary machine for power generation.The engine 10 may further include a power supply system 20, including a fuel cell system 28 having a fuel cell or fuel cell "stack" 32, a charge air system 38, a temperature control system 52, and a battery 30. the fuel cell system 28 may be configured to supply electric power to the electric motor 24 to operate it. Various other systems and subsystems onboard the machine 10 may also be electrically powered, and the power system 20 may include various electrical components for power supply, power supply, and conditioning, as known to those skilled in the art. As indicated above, the machine 10 and power system 20 could also be a stationary system for operating electric machines or supplying power to a local or regional power grid. Referring to FIG. 2, the fuel cell system 28 further includes an intake passage 34 extending to the fuel cell stack 32 and an exhaust passage 36 extending from the fuel cell stack 32. The intake conduit 34 conveys the air to be pressurized from an air inlet 48 to the fuel cell stack 32. the air provides a flow of oxidants to the fuel cell stack 32, and a separate fuel supply (not shown) may provide fuel to the fuel cell stack 32. The exhaust conduit 36 extends from the fuel cell stack 32 to an exhaust outlet 50. In one practical implementation strategy, the fuel cell stack 32 includes a proton exchange membrane (PEM) fuel cell stack, but the present disclosure is not limited thereby.The power supply system 20 also includes a charge air system 38 having an air compressor 40 in the intake passage 34 configured to pressurize the intake air to be supplied to the fuel cell stack 32. The charge air system 38 further includes an exhaust turbine 42 in the exhaust conduit 36 coupled to the air compressor 40 and configured to be rotated by an exhaust flow from the fuel cell stack 32 to rotate the air compressor 40. The compressor 40 and the exhaust turbine 42 coupled together may include a turbocharger. In one embodiment, an electric motor 39 may be coupled to compressor 38 and exhaust turbine 42 to provide electrically assisted turbocharger adjustment. Electrically assisted turbocharger adjustment may be used in each embodiment of the present disclosure. The charge air system 38 further includes a heat exchanger 44. the heat exchanger 44 enables the exchange of heat between pressurized intake air and exhaust gas. In one practical implementation, the heat exchanger 44 includes a primary surface recuperator having heat exchange surfaces 46 that are exposed to the flows of pressurized intake air and exhaust gas in a well known manner. In other embodiments, a different heat exchange strategy may be used. An aftercooler 66 may also be disposed in the intake passage 34 to cool the pressurized intake air to be supplied to the fuel cell stack 32. A humidifier 64 is also provided to moisten the pressurized intake air to be supplied to the fuel cell stack 32 using, for example, the heat energy of the exhaust gas.The power supply system 20 also includes a temperature control system 52 as mentioned above. The temperature control system 52 includes a bypass line 54 and a diverter valve 56 disposed at least partially within the bypass line 54. The intake passage 34, the exhaust passage 36, and the heat exchanger 44 together define an air-to-exhaust heat exchange path for pressurized air and exhaust gas through the heat exchanger 52. In one implementation, the thermal energy of the pressurized intake air may be transferred to exhaust gas via the heat exchanger 44, enabling extraction of the additional energy via the turbine 42 to pressurize additional intake air via the compressor 40. The bypass line 54 is fluidly connected to at least one of the intake line 34 or the exhaust line 36 and defines a bypass path around the heat exchanger 44 for one of the pressurized air for the exhaust gas. The bypass path may include one of an exhaust bypass path or an air bypass path. The diverter valve 56 may be movable between a closed position in which the bypass path is blocked and a second position in which the bypass path is open. In some embodiments, diverter valve 56 may be either fully open or fully closed; however, in some embodiments diverter valve 56 may have a range of open positions corresponding to a range of relative bypassed exhaust or air amounts. The air-to-exhaust heat exchange path may remain open while the bypass path is open such that a portion of the exhaust gases or air continues to pass through the heat exchanger 44. Embodiments in which the air-exhaust heat exchange path is closed while the bypass path is open are within the scope of the present disclosure.Diverter valve 56 may be electrically actuated, hydraulically actuated, or pneumatically actuated. It will therefore be appreciated that the bypass line 54 is closed in a first operating state and a full free flow of pressurized intake air and exhaust gas flows through the heat exchanger 44. In another operating state, the bypass line 54 may be opened by opening the diverter valve 56 such that at least a portion of the pressurized intake air or exhaust gas, as further explained herein, bypasses the heat exchanger 44.In the embodiment illustrated in FIG. 2, the bypass line 54 fluidly connects between a location of the intake line 34 upstream of the heat exchanger 44 and a location of the intake line 34 downstream of the heat exchanger 44. As seen in FIG. 2, the compressor 40 includes a compressor outlet 60. the bypass line 54 includes an inlet 55 at a location fluidly connected between the compressor outlet 60 and the heat exchanger 54, and an outlet 57 at a location fluidly connected between the heat exchanger 44 and the aftercooler 66. As indicated above, other settings are also contemplated in other embodiments.Referring to FIG. 3, a fuel cell system 128 is shown that includes a fuel cell stack 132 according to another embodiment. The fuel cell system 128 also includes a charge air system 138 including a compressor 140 and a turbine 142. A heat exchanger 144 is provided, which can be operated analogously to the embodiment of FIG. 2. An intake conduit 134 provides a supply of pressurized intake air from the compressor 140 to the fuel cell stack 132, and an exhaust conduit 136 conveys the exhaust gases from the fuel cell stack 132 to the turbine 142. An aftercooler is shown at 166 and a humidifier at 172. The fuel cell system 128 also includes a bypass line 154 and a diverter valve 156 in a temperature control system 152.The fuel cell system 128 may be very similar to the fuel cell system 28 of FIG. 2. In contrast, however, in fuel cell system 128, bypass line 154 fluidly connects between a location of exhaust line 136 upstream of heat exchanger 144 and a location of exhaust line 136 downstream of heat exchanger 144. It should be noted that the terms "upstream" and "downstream" are used herein with reference to an expected directional flow of fluids including pressurized intake air into and through a compressor and intake conduit, then through a fuel cell stack, and exhaust gas into and through an exhaust conduit and turbine. It is also understood that the fuel cell system 128 may be operated generally analogous to the fuel cell system 28 of FIG. 2. While in the embodiment of FIG. 2, the pressurized intake air bypasses the heat exchanger 44, in the embodiment of FIG. 3, the exhaust gas bypasses the heat exchanger 44.Referring to FIG. 4, a fuel cell system 228 according to another embodiment is shown including a fuel cell stack 232, an intake conduit 234 extending to the fuel cell stack 232, and an exhaust conduit 236 extending from the fuel cell stack 232. A humidifier is shown at 272, an aftercooler is shown at 266, and a heat exchanger is shown at 244. The fuel cell system 228 also includes a charge air system 238 having a compressor 240 and a turbine 242. The charge air system 238 also includes a temperature control system 252 having a bypass line 254 and a diverter valve 256. The fuel cell system 228 may operate similar to the previous embodiments, but has certain differences. Compressor 240 includes compressor outlet 260, and turbine 252 includes turbine inlet 262. The bypass line 254 fluidly connects the intake line 234 to the exhaust line 236. Also in the illustrated embodiment, the bypass line 254 fluidly connects a location of the intake line 234 that is fluidly between the compressor outlet 260 and the heat exchanger 244 to a location of the exhaust line 236 that is fluidly between the turbine inlet 262 and the heat exchanger 244. It may therefore be understood that, instead of bypassing the heat exchanger 244 with intake air or exhaust gas and returning the bypassed intake air or exhaust gas to an intake line and an exhaust line, respectively, in the embodiment of FIG. 4, the heat exchanger 244 is bypassed by supplying the pressurized intake air more or less directly from the compressor 240 to the turbine 242 to extract the energy of the compressed air mixed with the exhaust gas in the turbine 242 to operate the compressor 240.Referring again to FIG. 2, features of the fuel cell system 28, particularly the temperature control system 52, are additionally shown that may be similar to the features used in other embodiments. Accordingly, the present description and explanation of the temperature control system 52 may be analogously understood as applicable to the embodiment of FIGS. 3 and 4. The temperature control system 52 includes a sensing mechanism configured to monitor a temperature difference between a compressor outlet temperature and a turbine inlet temperature. In particular, the temperature control system 52 may include a compressor outlet temperature sensor 68 and a turbine inlet temperature sensor 69, which are electrically connected to a temperature control unit 70. A "sensing mechanism" as used herein includes one or more temperature sensors that directly or indirectly sense the temperature of the pressurized intake air and / or exhaust gas, and includes so-called virtual sensors. The temperature control system 52 may also include a humidity sensor 72 coupled to the humidifier 64. With the temperature control unit 70 coupled to the sensors 68 and 69 and in control communication with the diverter valve 56, the temperature control unit 70 may be configured to command the diverter valve 56 to be adjusted to the second position based on the monitored temperature difference, thereby opening the bypass line 54. In one embodiment, diverter valve 56 is adjusted to an open position where the temperature difference is less than a threshold temperature difference. In other embodiments, the humidity of the pressurized intake air sensed by the humidity sensor 72 may also be taken into account in calculations or lookup tables to determine when to open or close the diverter valve 56. For example, a relatively higher humidity could indicate a perceived increased risk of icing the turbine inlet and, in conjunction with a temperature difference or absolute temperatures, justifies opening the diverter valve 56. Other applications taking into account temperature differences, absolute air or exhaust temperatures, ambient temperatures, humidity, and various other factors will be apparent to those skilled in the art.It has been discovered that certain operating conditions may result in the charge air system 38 having relatively lower efficiency and even being counterproductive under certain circumstances. For example, the compressor 40 may be operated at a relatively low speed when the load of the fuel cell system 28 is relatively low if there is a relatively low exhaust gas flow. The compressor outlet temperature may be at least temporarily close to or even below the turbine inlet temperature. Accordingly, operation of the heat exchanger 44 may result in the pressurized intake air actually cooling the exhaust gas and reducing the amount of energy that may be extracted from the exhaust gas via the turbine 42. For example, according to the present disclosure, when the compressor outlet temperature is within a threshold temperature difference from the turbine inlet temperature, the diverter valve 56 may be actuated to open the bypass line 54 and thus reduce heat exchange between the pressurized intake air and the exhaust gas. In operation of the embodiments of FIGS. 2 and 3, generally analogous operations may be performed. The embodiment of FIG. 4 may also be operated generally analogously, except that in this case, the pressurized intake air from the compressor 240 may be supplied to the turbine 242 to maintain a minimum compressor speed without increasing the oxidant supply to the fuel cell stack 32. As indicated above, at least in the embodiment of FIG. 4, an open state range of diverter valve 256 could be used to vary a relative amount of pressurized intake air bypassing heat exchanger 244. Such a strategy may be used to maintain a compressor outlet temperature above a turbine inlet temperature or some other threshold, if desired.Industrial applicabilityReferring now to the drawings in general, but also now to FIG. 5, a flowchart is shown illustrating an example methodology and logic flow, according to an embodiment. At a block 310, the exhaust gas is supplied from a fuel cell or fuel cell stack through an exhaust turbine. From block 310, the flowchart 300 proceeds to a block 320 to rotate the compressor to pressurize the intake air for the fuel cell or fuel cell stack. At a block 330, heat is exchanged between the exhaust gas and the pressurized intake air.From block 330, the flowchart 300 proceeds to a block 340 to monitor a temperature difference between a turbine inlet and a compressor outlet, as explained herein. From block 340, the flowchart 300 proceeds to block 350 to open the bypass line based on the temperature difference to bypass the intake air or exhaust gas around the heat exchanger. From block 350, the flowchart 300 proceeds to block 360 to reduce the exchange of heat between exhaust gas and pressurized intake air, as explained herein.It should be appreciated that in the embodiment of FIG. 2, the diverted pressurized intake air is recirculated to the intake passage 34. In the case of the embodiment of FIG. 3, the diverted exhaust gas is recirculated to the exhaust conduit 136. In the embodiment of FIG. 4, the diverted intake air is supplied to the exhaust conduit 236. As also discussed above, exchanging heat between exhaust gas and pressurized intake air during operation of a fuel cell may occur at a higher operating load, and opening a bypass line may include opening the bypass line based on a decrease or an expected decrease in the load. For example, a fuel cell operating load request may be monitored and when a decrease in the requested operating load is detected, the bypass line may be opened. In some cases, a requested operating load may be reduced before the operating load is actually reduced, such as by predicting a future load request based on environmental conditions. Thus, a bypass line can be opened in expectation of reducing the operating load of the fuel cell.A fuel cell system, a charge air system, and a temperature control system as discussed herein may be all parts of a power supply system. Moreover, to understand the present description, a temperature control system may be part of a fuel cell system or part of a charge air system, or other combinations without limitation. A power supply system, fuel cell system, charge air system, or temperature control system may be sold and provided as a first equipment product (OEM) or as a retrofit product, including, without limitation, upgrade or retrofitting.The present description is for illustrative purposes only and should not be construed as limiting the scope of the present disclosure in any way. It will therefore be appreciated by those skilled in the art that various modifications could be made to the embodiments disclosed herein without departing from the intended and appropriate spirit and scope of the present disclosure. Other aspects, features and advantages will become apparent upon examination of the accompanying drawings and appended claims. As used herein, the articles "a / a / an" are intended to include one or more elements and may be used interchangeably with "one or more.". When only one item is intended, the term "a" or similar language is used. Also, the terms "comprises", "comprising", or the like are intended to be open terms. Furthermore, the term "based on" is intended to mean "based at least in part on" unless expressly stated otherwise.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedUS 18 / 122,176

[0003]

Claims

A power supply system (20) comprising: a fuel cell system (28, 128, 228) comprising a fuel cell stack (32, 132, 232), an intake line (34, 134, 234) extending to the fuel cell stack, and an exhaust line (36, 136, 236) extending from the fuel cell stack; a charge air system (38, 138, 238) comprising an air compressor (40, 140, 240) in the intake line, an exhaust turbine (42, 142, 242) in the exhaust line, and a heat exchanger (44, 144, 244); a temperature control system (52, 152, 252) comprising a bypass line (54, 154, 254) and a diverter valve (54, 156, 256) positioned at least partially in the bypass line; the intake line, the exhaust line, and the heat exchanger together define an air-to-exhaust heat exchange path for pressurized air and exhaust gas through the heat exchanger; and the bypass line is fluidly connected to at least one of the intake line and the exhaust line and defines a bypass path around the pressurized air or the exhaust gas heat exchanger, and the bypass valve is movable between a closed position where the bypass path is blocked and a second position where the bypass path is open.The system of claim 1, wherein the heat exchanger (44, 144, 244) comprises a primary surface recuperator.The system of claim 1 or 2, wherein the bypass line (254) fluidly connects the intake line to the exhaust line; and wherein the bypass line fluidly connects between a location of the intake line, fluidly between a compressor outlet (260) and the heat exchanger, and a location of the exhaust line, fluidly between a turbine inlet (266) and the heat exchanger.The system of claim 1 or 2, wherein the bypass line (54) fluidly connects between a location of the intake line upstream of the heat exchanger and a location of the intake line downstream of the heat exchanger; the system further comprises an aftercooler (66), and the location of the intake line downstream of the heat exchanger is fluidly between the heat exchanger and the aftercooler.The system of claim 1 or 2, wherein the bypass line (154) fluidly connects between a location of the exhaust line upstream of the heat exchanger and a location of the exhaust line downstream of the heat exchanger.The system of any of claims 1 to 5, further comprising an electric motor (39) configured to rotate the compressor.The system of any of claims 1 to 6, wherein the temperature control system further comprises a sensing mechanism (68, 69) configured to monitor a temperature difference between a compressor outlet temperature and a turbine inlet temperature; and wherein the temperature control system further comprises a temperature control unit (70) coupled to the sensing mechanism and in control communication with the diverter valve, and the temperature control unit is configured to command the adjustment of the diverter valve to the second position based on the temperature difference.A method of operating a power supply system (20), comprising: supplying exhaust gases from a fuel cell (32, 132, 232) through an exhaust turbine (42, 142, 242) to rotate the exhaust turbine; rotating a compressor (40, 140, 240) based on the rotation of the exhaust turbine to pressurize the intake air for the fuel cell; exchanging heat between the exhaust gas and the pressurized intake air; opening a bypass line (54, 154, 254) to bypass pressurized intake air or exhaust gases around a heat exchanger (44, 144, 244); and reducing the exchange of heat between the exhaust gas and the pressurized intake air based on opening the bypass line.The method of claim 8, further comprising recirculating bypassed pressurized intake air or exhaust gas to an intake conduit (34, 134) or an exhaust conduit (36, 136).The method of claim 8, further comprising supplying bypassed pressurized intake air from an intake conduit (234) to an exhaust conduit (236) that supplies the exhaust gas to the turbine.The method of any of claims 8 to 10, wherein opening the bypass line comprises opening a diverter valve (56, 156) to open a bypass path through the bypass line while an air-exhaust heat exchange path through the heat exchanger remains open; wherein opening the bypass line comprises opening the bypass line based on a temperature difference between a compressor outlet temperature and a turbine inlet temperature; and wherein the temperature difference is less than a threshold temperature difference.The method of any of claims 8 to 11, wherein exchanging heat between the exhaust gas and the pressurized intake air occurs during operation of the fuel cell at a higher operating load and opening the bypass line comprises opening the bypass valve based on a reduction or an expected reduction in the operating load.A charge air system (38, 138, 238) for a fuel cell power supply system (20), comprising: an intake conduit (34, 134, 234) for supplying pressurized intake air from a compressor (40, 140, 240) to a fuel cell stack (32, 132, 232); an exhaust conduit (36, 136, 236) for supplying exhaust gases from the fuel cell stack to a turbine (42, 242, 342) coupled to the compressor; a bypass conduit (54, 154, 254) including an inlet (55) fluidly coupled to one of the intake conduit or the exhaust conduit and an outlet (57) fluidly coupled to one of the intake conduit or the exhaust conduit and defining a bypass path for pressurized air or exhaust gas around a heat exchanger (44, 144, 244) coupled between the intake conduit and the exhaust conduit; a diverter valve (56, 156, 256) disposed at least partially in the bypass line and movable between a closed position in which the bypass path is blocked and a second position in which the bypass path is open; a sensing mechanism (68, 69) configured to monitor a temperature difference between a compressor outlet temperature and a turbine inlet temperature; and a temperature control unit (70) coupled to the sensing mechanism and in control communication with the diverter valve, and the temperature control unit configured to command the diverter valve to be adjusted to the second position based on the temperature difference.

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