Patent Description:
The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as wheel loaders, excavators, dump-trucks, buses, marine vessels and passenger cars.

A fuel cell is an electrochemical cell which converts chemical energy into electricity. The fuel cell converts the chemical energy of a fuel, typically hydrogen, and an oxidizing agent, typically oxygen, into electricity. Accordingly, a fuel cell can be used as an alternative or as a complement to electric batteries. In recent years fuel cells have been considered for powering electric vehicles, such as pure electric vehicles and hybrid electric vehicles. Prior art documents <CIT>, <CIT> disclose fuel cell systems for vehicles.

Typically, a fuel cell system for a vehicle comprises a fuel cell stack comprising one or more fuel cells. In addition, the fuel cell system typically comprises a turbo and a humidifier. The turbo comprises a turbine and a compressor which are drivingly connected. During use of the fuel cell system, an inlet airflow to the fuel cell stack flows thereto via the compressor and the humidifier. The inlet airflow delivers the above-mentioned oxidizing agent to the fuel cell(s). An outlet airflow from the fuel cell stack flows therefrom via the humidifier and the turbine until it exits into an external environment. Some or all of the outlet airflow may bypass the humidifier at certain occasions. During use, the humidifier transfers water, or water and heat, from the outlet airflow to the inlet airflow.

A fuel cell system for a vehicle is often used in combination with an electrical energy storage system, such as a battery pack comprising lithium-ion cells. The requested power for the vehicle can be delivered in combination by the fuel cell system and the electrical energy storage system or by one of the systems, depending on different factors. For example, the distribution of power from each system may be optimized in dependence on energy efficiency.

Even though it is known to use a fuel cell system for powering a vehicle as e.g. mentioned in the above, there is still a strive to develop further improved fuel cell system technology.

In view of the above, an object of the invention is to provide a computer-implemented method for determining a degradation state of a turbo and/or a humidifier of a fuel cell system which at least partly alleviates one or more drawbacks of the prior art, or which at least provides a suitable alternative. Yet further objects of the invention are to provide a control unit, a fuel cell system, a vehicle, a computer program and/or a computer readable medium, which alleviate at least one or more drawbacks of the prior art, or which at least provide suitable alternatives.

According to a first aspect, the object is achieved by a computer-implemented method according to claim <NUM>.

Hence, there is provided a computer-implemented method for determining a degradation state of a turbo and/or a humidifier of a fuel cell system for a vehicle. The fuel cell system comprises a fuel cell stack, in addition to the turbo and the humidifier.

The turbo comprises a turbine and a compressor which are drivingly connected. During use of the fuel cell system, an inlet airflow to the fuel cell stack flows thereto via the compressor and the humidifier. The inlet airflow delivers an oxidizing agent to the fuel cell stack. An outlet airflow from the fuel cell stack flows therefrom via the humidifier and the turbine until it exits into an external environment. Some or all of the outlet airflow may bypass the humidifier at certain occasions. During use, the humidifier transfers water, or water and heat, from the outlet airflow to the inlet airflow.

By the provision of a method as disclosed herein, a degradation state of the turbo and/or the humidifier can be determined in an efficient manner. The present invention is based on a realization that it is advantageous to determine a degradation state of the turbo and/or the humidifier during use of the fuel cell system. For example, the determined degradation state(s) can be used for optimizing the use of the fuel cell system, such as for optimizing a power distribution between the fuel cell system and an electrical energy storage system of the vehicle. Additionally, or alternatively, the determined degradation state(s) can be used for determining a service need of the turbo and/or the humidifier, and/or if the turbo and/or the humidifier needs to be replaced.

Using the fuel cell system efficiency value and the fuel cell stack efficiency value for determining the degradation state of the turbo and/or the humidifier implies a fast, reliable and cost-efficient degradation determination.

The first reference efficiency may be indicative of a determined efficiency of the fuel cell system when it is new, i.e. not yet used, or at least not yet used for a long time, such as used for less than <NUM> operating hours. The second reference efficiency may be indicative of a determined efficiency of the fuel cell stack when it is new, i.e. not yet used, or at least not yet used for a long time, such as used for less than <NUM> operating hours.

Degradation of the turbo and/or the humidifier may also be denoted wear of the turbo and/or the humidifier.

The term relative air humidity, or relative humidity, is known in the art, and may be defined as moisture content, i.e. water vapor, of the air, and may be expressed as a percentage of the amount of moisture that can be retained by the air at a given temperature and pressure without condensation. For example, a relative air humidity value of <NUM> % indicates that saturation has been reached and that condensation will occur.

Optionally, the air humidity threshold corresponds to a relative air humidity of <NUM> % or more, such as substantially <NUM> % relative air humidity. This may in an embodiment be referred to a saturated, or substantially saturated, water content in the air flowing downstream the humidifier and upstream the fuel cell stack during use. Accordingly, when the relative air humidity value is equal to or above the air humidity threshold, it can be assumed that the humidifier is functional without any issues.

Optionally, the efficiency decrease of the fuel cell system and of the fuel cell stack, and/or the relative air humidity at the inlet of the fuel cell stack, is/are measured during stationary operating conditions of the fuel cell system. This implies a more reliable measurement. Still optionally, the measurement/s is/are performed during a minimum time period during the stationary operating conditions of the fuel cell system. This implies a further improved measurement. For example, the values obtained from the one or more respective measurements may correspond to one or more respective average values, median values, or any other value which is based on the result from the one or more respective measurements.

Optionally, when it is determined that there is a combined degradation state of the turbo and the humidifier, e.g. when the relative air humidity value is below the air humidity threshold, the method may further comprise using a humidifier model for estimating a level of degradation of the humidifier. A humidifier model to estimate the level of degradation may for example be a model which models the humidifier in accordance with the first law of thermodynamics for open systems. For example, a model may be provided which models the ability of the humidifier, e.g. the ability of a membrane of the humidifier, to transport water mass and heat from a humid side to a dry side thereof. As such, by way of example, the humidifier may be modelled as two parts, a first part being a volume with dry air and a second part being a volume with wet air, wherein water mass and heat is transported through the membrane from the second part to the first part. By fitting measured parameters in the model, values of water mass and heat transported may be obtained, and these values may be used for estimating a state of health of the humidifier, i.e. its level of degradation. For example, these values may be compared with a reference to thereby obtain a value indicative of the state of health of the humidifier. The measured parameters may for example be measured downstream and/or upstream the humidifier during use, and may be at least one of an airflow, waterflow, relative humidity, pressure, air temperature and water temperature. In addition, the model may include a parameter relating to a thickness of the membrane, which for example may affect the ability of the membrane to transport water mass and/or heat as a function of time.

Optionally, the stationary operating conditions are associated with one or more predetermined road segments for the vehicle. The one or more predetermined road segments may be road segments where stationary operating conditions can be expected. For example, a predetermined road segment may correspond to a road stretch without, or with only minor, varying inclinations, such as a portion of a highway where there are no or only minor inclinations.

Optionally, the efficiency decrease of the fuel cell stack is measured by use of polarization curves and/or by electrochemical impedance spectra of the fuel cell stack. This implies a reliable and robust measurement and/or reliable and robust measurement values.

Thereby, by updating operating constraints, the service life of the fuel cell system may be increased. For example, if the fuel cell system is used in combination with e.g. an electrical energy storage system, the operating constraints may be updated so that the electrical energy storage system is used more and/or in other operating ranges, and/or so that the fuel cell system is used less and/or in other operating ranges. As a result, service life of the combined system, i.e. the fuel cell system and the electrical energy storage system, may be increased.

The fuel cell system may be adapted to be operated with adjustable operating dynamics and/or in an adjustable operating window defining operating constraints for the fuel cell system, wherein increasing the operating dynamics and/or the operating window is associated with an increased expected degradation of the fuel cell system and wherein reducing the operating dynamics and/or the operating window is associated with a reduced expected degradation of the fuel cell system.

By operating dynamics of a system is herein meant how the operation of the system is varied over time. For example, large and/or rapid variations of an operating parameter during use represent higher operating dynamics of the system compared to a situation with smaller and/or slower variations of the operating parameter. This may also be referred to as a slew rate of the system. By an operating window is herein meant a window, or range, within which an operating parameter is during use. By way of example, an operating parameter may refer to a power output from the system. As such, operating dynamics may be defined as power dynamics of the fuel cell system, e.g. how fast the fuel cell system can go from low power to high or full power. Other non-limiting examples of operating parameters are voltage level, ampere level and power throughput. As yet another non-limiting example, an operating parameter may relate to if a shutdown of the system is allowed or not. For example, too many shutdowns of the fuel cell system may result in higher degradation.

According to a second aspect, the object is achieved by a control unit according to claim <NUM>.

Hence, there is provided a control unit for determining a degradation state of a turbo and/or a humidifier of a fuel cell system for a vehicle, the fuel cell system comprising a fuel cell stack, in addition to the turbo and the humidifier, wherein the control unit is configured to perform the steps of the method according to any one of the embodiments of the first aspect.

Advantages and effects of the control unit are analogous to the advantages and effects of the method as disclosed herein. It shall also be noted that all embodiments of the control unit are combinable with all embodiments of the method, and vice versa.

The control unit may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. The control unit may comprise embedded hardware, sometimes with integrated software. Examples of physical relationships are: shared casing and components mounted on one or several circuit boards. Further, the control unit may be any kind of control unit, and it may also comprise more than one control unit, i.e. the control unit may be configured by two or more sub-control units, which may be provided close to each other or be separated from each other. In some embodiments, the control unit may be denoted a computer.

According to a third aspect, the object is achieved by a fuel cell system according to claim <NUM>.

Hence, there is provided a fuel cell system for a vehicle, the fuel cell system comprising a fuel cell stack, a turbo and a humidifier, and further comprising at least one first sensor for measuring efficiency decrease of the fuel cell system during use, at least one second sensor for measuring efficiency decrease of the fuel cell stack during use and at least one third sensor for measuring relative air humidity at an inlet of the fuel cell stack during use, wherein the fuel cell system further comprises a control unit according to any one of the embodiments of the second aspect.

Advantages and effects of the fuel cell system are analogous to the advantages and effects of the method as disclosed herein. It shall also be noted that all embodiments of the fuel cell system are combinable with all embodiments of the method and the control unit, and vice versa.

According to a fourth aspect, the object is achieved by a vehicle according to claim <NUM>.

Hence, there is provided a vehicle comprising a fuel cell system according to any one of the embodiments of the third aspect.

According to a fifth aspect, the object is achieved by a computer program according to claim <NUM>.

Hence, there is provided computer program comprising program code means for performing the steps of any of the embodiments of the first aspect when said program is run on the control unit according to the second aspect.

According to a sixth aspect, the object is achieved by a computer readable medium according to claim <NUM>.

Hence, there is provided a computer readable medium carrying a computer program comprising program code means for performing the steps of any of the embodiments of the first aspect when said program product is run on the control unit according to the second aspect.

Like reference characters throughout the drawings refer to the same, or similar, type of element unless expressed otherwise.

<FIG> depicts a schematic view of a fuel cell system <NUM> according to an example embodiment of the present invention. The fuel cell system <NUM> comprises a fuel cell stack <NUM>, a turbo <NUM> and a humidifier <NUM>. The turbo <NUM> comprises a compressor <NUM> and a turbine <NUM> which are drivingly connected, in this example drivingly connected by a rotatable axle <NUM>.

The fuel cell system <NUM> may further comprise at least one first sensor (not shown) for measuring an efficiency decrease of the fuel cell system <NUM> during use, at least one second sensor (not shown) for measuring an efficiency decrease of the fuel cell stack <NUM> during use and at least one third sensor <NUM> for measuring relative air humidity at an inlet of the fuel cell stack <NUM> during use. The at least one third sensor <NUM> may be adapted to measure a relative air humidity at any position provided downstream the humidifier <NUM> and upstream the fuel cell stack <NUM>. The at least one first sensor for measuring an efficiency decrease of the fuel cell system <NUM> during use may for example be at least one of a sensor which measures a power output from the fuel cell system <NUM> and a sensor which measures a fuel flow to the fuel cell stack <NUM>. The power output may for example be measured by use of voltage and/or current sensors. By way of example, the efficiency decrease of the fuel cell system <NUM> may be obtained by comparing the measured power output from the fuel cell system <NUM> with the measured fuel flow to the fuel cell stack <NUM>. The at least one second sensor for measuring an efficiency decrease of the fuel cell stack <NUM> may for example be at least one of a sensor which measures electrical current generated by the fuel cell stack <NUM> and a sensor which measures voltage level of the fuel cell stack <NUM>.

Measured values from the at least one second sensor may be used for obtaining the efficiency value of the fuel cell stack <NUM>.

As shown in <FIG>, the fuel cell system <NUM> may further comprise a fuel tank <NUM>, such as a hydrogen fuel tank. Fuel may flow in a fuel path F1 from the fuel tank <NUM> and into the fuel cell stack <NUM> during use. As further depicted by a flow arrow F2, excess fuel may be recirculated out from the fuel cell stack <NUM> and back into the fuel cell stack <NUM> again.

Fuel is arranged to enter at an anode side of each fuel cell of the fuel cell stack <NUM>. Air is arranged to enter at a cathode side of each fuel cell of the fuel cell stack <NUM>. The air is entered via an air path A. The air path A at least partly pass the compressor <NUM>, the humidifier <NUM> and the fuel cell stack <NUM> in subsequent order. Thereafter, the air path A at least partly passes the humidifier <NUM> and the turbine <NUM>. As shown, at least a portion of the airflow A may selectively bypass the humidifier <NUM> by use of bypass circuit <NUM> comprising a bypass valve <NUM>. As shown, the bypass circuit <NUM> and the bypass valve <NUM> may be arranged downstream the fuel cell stack <NUM>.

The humidifier <NUM> is arranged to transfer water, or water and heat, from air that has passed the fuel cell stack <NUM> to air which will enter the fuel cell stack <NUM>.

The fuel cell system <NUM> may as further shown comprise a control unit <NUM>. The control unit <NUM> may be configured to control the operation of the fuel cell system <NUM>. The control unit <NUM> may additionally or alternatively be configured to perform a method according to an example embodiment of the present invention. For example, the control unit <NUM> may be arranged to be in communicative contact with the at least one third sensor <NUM>. Additionally, or alternatively, the control unit <NUM> may be configured to control opening and closing of the bypass valve <NUM>.

<FIG> depicts a vehicle <NUM> according to an example embodiment of the present invention. The vehicle <NUM> is in this example a truck, more particularly a towing truck for towing one or more trailers (not shown). It shall however be understood that the invention is not limited to only this type of vehicle, but may be used in any other vehicle, such as a bus, a wheel loader, an excavator, a dump-truck, a passenger car and a marine vessel.

The vehicle <NUM> comprises a fuel cell system <NUM>, such as the fuel cell system <NUM> as shown in <FIG>. The vehicle <NUM> may as shown also comprise a control unit <NUM> as also e.g. shown in <FIG>. Accordingly, the control unit <NUM> may be an onboard control unit. Additionally, or alternatively, the control unit may be an off-board control unit, such as a remote server. As such, according to an example embodiment, the vehicle <NUM> may be adapted to communicate with an off-board control unit.

With reference to <FIG>, a flowchart of a method according to example embodiments of the invention is shown. The method is used for determining a degradation state of a turbo <NUM> and/or a humidifier <NUM> of a fuel cell system <NUM> for a vehicle <NUM>, e.g. the fuel cell system <NUM> as shown in <FIG>.

As shown by boxes with dashed lines, the method may further comprise:.

The air humidity threshold may correspond to a relative air humidity of <NUM> % or more, such as substantially <NUM> % relative air humidity.

The efficiency decrease of the fuel cell system <NUM> and of the fuel cell stack <NUM>, and/or the relative air humidity at the inlet of the fuel cell stack <NUM>, may be measured during stationary operating conditions of the fuel cell system <NUM>. For example, the measurement/s may be performed during a minimum time period during the stationary operating conditions of the fuel cell system <NUM>, such as a time period of <NUM>-<NUM> minutes.

The stationary operating conditions may be associated with one or more predetermined road segments for the vehicle <NUM>. The one or more predetermined road segments may be road segments where stationary operating conditions can be expected. For example, a predetermined road segment may correspond to a road stretch without, or with only minor, varying inclinations, such as a portion of a highway where there are no or only minor inclinations.

By way of example, when it is determined that there is a combined degradation state of the turbo <NUM> and the humidifier <NUM>, e.g. when the relative air humidity value is below the air humidity threshold, the method may further comprise using a humidifier model for estimating a level of degradation of the humidifier <NUM>. A humidifier model to estimate the level of degradation may for example be a model which models the humidifier <NUM> in accordance with the first law of thermodynamics for open systems. For example, a model may be provided which models the ability of the humidifier <NUM>, e.g. the ability of a membrane (not shown) of the humidifier <NUM>, to transport water mass and heat from a humid side to a dry side thereof. As such, by way of example, the humidifier <NUM> may be modelled as two parts, a first part being a volume with dry air and a second part being a volume with wet air, wherein water mass and heat is transported through the membrane from the second part to the first part. By fitting measured parameters in the model, values of water mass and heat transported may be obtained, and these values may be used for estimating the degradation state of the humidifier. For example, these values may be compared with a reference to thereby obtain a value indicative of the state of health of the humidifier. The reference may for example correspond to a situation when the humidifier <NUM> is new and not yet used in operation. The measured parameters may for example be measured downstream and/or upstream the humidifier <NUM> during use, and may be at least one of an airflow, waterflow, relative humidity, pressure, air temperature and water temperature. In addition, the model may include a parameter relating to a thickness of the membrane, which for example may affect the ability of the membrane to transport water mass and/or heat as a function of time. By way of example, the model used for the humidifier <NUM> for obtaining the values for heat and water mass transferred per time unit may be based on the humidifier model as described in the following Article: "<NPL>. It shall however be noted that this is just an example of how to model a humidifier, and the method is not limited to only this example. In general, any model which can provide values for heat and water mass transported from the humid side to the dry side of the humidifier <NUM> may be used for estimating the degradation state of the humidifier <NUM>.

The efficiency decrease of the fuel cell stack <NUM> may be measured by use of polarization curves and/or by electrochemical impedance spectra of the fuel cell stack <NUM>.

<FIG> shows a graph with polarization curves C1, C2 for the fuel cell stack <NUM>. In this example, the y-axis represents fuel cell voltage level (volt) and the x-axis represents fuel cell current level (ampere). Accordingly, the graph represents fuel cell voltage as a function of fuel cell current. The example graph includes two polarization curves, C1 and C2. The polarization curve C1 relates to the fuel cell stack <NUM> at a time T0. The polarization curve C1 is in this example representing the second reference efficiency as mentioned in the above. For example, the polarization curve C1 may relate to the fuel cell stack <NUM> when it is new, i.e. a new non-used fuel cell stack <NUM>. The polarization curve C2, on the other hand, may relate to the fuel cell stack <NUM> when it has been used for a number of operating hours, e.g. T0 + n hours, where n is a positive integer. A difference between the polarization curves C1 and C2 represents a level of degradation of the fuel cell stack <NUM>, indicated by the downwardly directed arrow in the graph. Accordingly, the level of degradation of the fuel cell stack <NUM> may be obtained by use of this graph. For example, the level of degradation of the fuel cell stack <NUM> may be expressed in percentage.

<FIG> shows a graph representing electrochemical impedance spectra of the fuel cell stack <NUM>. Electrochemical impedance spectroscopy is e.g. known for characterizing a condition of an electrochemical system, such as a fuel cell. Electrochemical impedance spectroscopy may for example be performed by providing a sinusoidal pulse to the system, such as a voltage pulse, and measuring a current pulse from the system, or vice versa. In this example, the y-axis represents imaginary impedance and the x-axis represents real impedance of the fuel cell stack <NUM>. The impedance is in this example expressed in Ohm. Accordingly, the graph represents imaginary impedance as a function of real impedance of the fuel cell stack <NUM>. In the graph, two curves C3, C4 are plotted. The curve C3 relates to the fuel cell stack <NUM> at the time T0 as mentioned in the above. In this example, the curve C3 is representing the second reference efficiency as mentioned in the above. The curve C4 relates to the fuel cell stack <NUM> when it has been used for T0 + n hours, where n is a positive integer. Hence, a difference between the curves C3 and C4 represents a level of degradation of the fuel cell stack <NUM>, indicated by the upwardly directed arrow in the graph. Accordingly, the level of degradation of the fuel cell stack <NUM> may be obtained by use of this graph. For example, the level of degradation of the fuel cell stack <NUM> may also in this example be expressed in percentage.

For example, the polarization curves and/or the electrochemical impedance spectroscopy may be measured in a DC/DC converter (not shown) which electrically connects the fuel cell system <NUM> to at least one electric motor (not shown) of the vehicle <NUM>. The at least one electric motor is typically used for propulsion of the vehicle <NUM>.

The fuel cell system efficiency value which corresponds to the measured efficiency decrease of the fuel cell system <NUM> during use with respect to the first reference efficiency may for example be represented as indicated in <FIG>. Accordingly, as shown, fuel cell system efficiency may be plotted as a function of fuel cell system power. The curve C5 represents the fuel cell system <NUM> at the time T0 as mentioned in the above and the curve C6 represents the fuel cell system <NUM> at the time T0 + n hours as also mentioned in the above. Accordingly, the level of degradation of the fuel cell system <NUM> may be obtained by use of this graph. For example, the level of degradation of the fuel cell system <NUM> may also in this example be expressed in percentage.

Additionally, or alternatively, the method may further comprise:.

The above mentioned method may be implemented in the control unit <NUM> as a computer program which comprises program code means for performing the steps of the method.

Claim 1:
A computer-implemented method for determining a degradation state of a turbo (<NUM>) and/or a humidifier (<NUM>) of a fuel cell system (<NUM>) for a vehicle (<NUM>), the fuel cell system (<NUM>) comprising a fuel cell stack (<NUM>), in addition to the turbo (<NUM>) and the humidifier (<NUM>), wherein the method comprises:
- obtaining (S1) a fuel cell system efficiency value which corresponds to a measured efficiency decrease of the fuel cell system (<NUM>) during use with respect to a first reference efficiency,
- obtaining (S2) a fuel cell stack efficiency value which corresponds to a measured efficiency decrease of the fuel cell stack (<NUM>) during use with respect to a second reference efficiency, and
- determining (S3) the degradation state of the turbo (<NUM>) and/or the humidifier (<NUM>) based on a difference between the fuel cell system efficiency value and the fuel cell stack efficiency value.