WO2020169647A1

WO2020169647A1 - On-board integrated diagnostic system and method for fuel cells

Info

Publication number: WO2020169647A1
Application number: PCT/EP2020/054317
Authority: WO
Inventors: Qi Tian; Florian UHRIG
Original assignee: Vitesco Technologies GmbH
Priority date: 2019-02-20
Filing date: 2020-02-19
Publication date: 2020-08-27
Also published as: DE102019202276A1

Abstract

An electrical system for operating an electric motor with a DC electrical energy source and comprising a converter and a signal-monitoring device, is presented. The electric motor is electrically coupled to the converter, and the converter is electrically coupled to the energy source. The converter is adapted to provide a signal to the energy source, which signal is monitored by the signal-monitoring device.

Description

On-board Integrated Diagnostic System and Method for Fuel Cells

The invention relates to a system and method for monitoring and diagnosing the current state of fuel cells, particularly in a vehicle application. The concept may find application in a configuration for vehicles with an electric drive motor and a fuel cell to provide electrical energy.

Fuel cells such as are used in vehicles benefit from an optimal operating state for chemical reactions under varying environment conditions, such as varying pressure, temperature, load and humidity. The evolution of the state of a fuel cell under the influence of these conditions may be difficult to model; nevertheless, accurate information concerning the state of the chemical reactions can be of great value in optimizing system performance. With knowledge of the current state of the fuel-cell operation, it is possible to adapt system parameters, detect problem operation earlier and more reliably, and even extend the operating life of a fuel-cell system.

Electrified vehicles including hybrid-electric vehicles (FIEVs) and battery electric vehicles (BEVs) typically comprise one or more DC electrical power sources, such as a fuel cell and a a traction (or high-voltage) battery, which provide power to an electric drive or traction motor or machine for propulsion. DC-DC converters are used to convert voltage levels between the elements of the system, while power inverter converters (or“inverters”) are used to convert direct current (DC) power to alternating current (AC) power.

An electrified vehicle may use a fuel cell as power source to provide electrical power, either as the sole source of electrical power to the motor and to an optional battery, or in addition to an external DC or AC network supply with which the optional battery may be charged.

In such a configuration, the battery and the fuel cell are DC elements, and the drive motor may be an AC element, and converters and inverters may be used to transfer the electrical energy between the different elements.

Fuel cells and a stack of fuel cells may be measured with statistical methods, for example electrical impedance spectroscopy (EIS), current interruption (Cl), and cell-voltage-monitoring (CVM). The characteristics may be measured in the laboratory under varying constraints. Then models for system control fuel-cell diagnostics, etc. may be developed. This process is static and must be performed “off-line”. Because of the complex measurement equipment, many methods such as EIS may be difficult to apply in real use (“on-line”), and in real time.

Therefore, there are always differences between the actual status of the fuel cells and the models and data which have been measured previously; this is a disadvantage for system control and performance. In addition, laboratory measurements are always expensive and require longer lead times.

Therefore, it may be advantageous to perform measurement and characterization of the fuel cells during normal use. It may be desirable to have methods to do observation and measurement in real-time. It may be advantageous to use the existing electrical components of the fuel-cell system to the extent possible, while limiting additional hardware.

SUMMARY OF THE INVENTION

An embodiment of the invention comprises a DC-DC or DC-AC converter between a DC electrical energy source and an electrical machine or motor. The electrical machine or motor may receive electricity via an inverter. A particularly

advantageous implementation may comprise a 3-phase inverter, which on the AC side is connected to an electrical machine (e-machine) or motor, and on the DC side is electrically connected to a fuel cell (either directly or via a DC-DC converter) and optionally is connected to a battery. The converter may additionally be configured to superimpose signals on the electrical current flow from the fuel-cell, or may be configured to interrupt some or all of the current flow between the fuel-cell and the other components, and is configured to measure these same signals.

In a feature of the invention, the method may be carried out on-line on an electricity-producing cell which is in an active state delivering power to a load.

In another feature the electricity-producing cell may be a polymer electrolyte membrane (PEM) fuel-cell, or proton exchange membrane, with the same abbreviation.

In a further feature, specific hardware may be provided which enables the flow of current from the fuel-cell to be interrupted, either partially or completely. In another feature, specific hardware may be provided which measures the return of the signals coming from the fuel-cell.

In further features, the converter may be combined with or integrated in the inverter, and the inverter may be configured to superimpose signals on the electrical current provided to the fuel-cell, or may even be configured to interrupt some or all of the electrical flow between the fuel-cell and the other components, and to measure these same signals.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a topology with a DC-DC converter and an inverter coupled to an AC motor.

Fig. 2 shows topology used for Electrical Impedance Spectroscopy.

Fig. 3 shows a topology for Current Interrupt monitoring.

A fuel-cell vehicle includes a fuel cell stack which transforms hydrogen from a fuel tank and oxygen from ambient air into electricity, which is in turn used to power an electric motor or charge a battery. The fuel cells may advantageously be polymer electrolyte membrane (PEM) fuel cells. PEM cells can often operate at temperatures less than 100°C.

A possible basic electrical topology of a fuel-cell vehicle is shown schematically in Figure 1 . Figure 1 shows a topology with a fuel-cell system 210, DC-DC Converter 220 and DC-AC Inverter 230. The DC side of the inverter is connected to, and gets power from, the fuel cell system FC. The AC side of the inverter drives an electric motor or e-machine EM. The motor in turn drives the wheels. In an alternative topology, the fuel cell and the DC side of the inverter may operate at a common DC voltage, which is typically determined by an appropriate operating voltage for the fuel cell system. The AC voltage for the electric or traction motor EM is determined by the inverter as appropriate for the motor and the drive which is desired from the motor. Note that the converter function may be integrated into the DC-AC inverter, in which case no separate DC-DC Converter 220 is needed. The DC side of the inverter may also get power from, and be connected to, a high-voltage DC battery (not shown). The voltage for the fuel cell may independent of the battery voltage, due to the operation of a DC-DC converter. The converter may also be coupled to additional DC electrical energy storage and/or additional DC electrical energy sources. Different topologies can be envisioned, whereby either each DC element (Fuel Cell, battery, etc.) is connected via a DC-DC converter to the DC-AC inverter 230, or where the DC-AC inverter is also a converter, and either a battery or a Fuel Cell is connected directly to the DC-AC inverter.

The electrical switches used to provide, e.g., converter and inverter functionality in various embodiments may be MOSFET’s or IGBT’s, or other semiconductor devices, or other forms of electrical switches.

Turning to Figure 2, a topology is shown for Electrical Impedance Spectroscopy (EIS). In this topology, a small-signal current signal 351 with a selected frequency or frequency pattern is sent from the converter 320 as a current signal. This may be done at the converter, for example using the electrical switches in the converter. Alternatively, the signal may be superimposed on current coming from the energy source, for example using an external signal source. A response 352 is received and monitored at the signal monitor of the converter, with feedback from the fuel cell or fuel-cell stack 310. This feedback may be represented as a voltage response from the fuel cell or fuel-cell stack.

The signal may be a current stimulus signal with a certain frequency, or a set of frequencies, or some other characteristic where the response 352 can be monitored, recognized and compared with the signal 351. The sending of the signal 351 may be started by a measurement request 353 to the converter, or it may be done at regular intervals, or according to a different plan or signal. An evaluated signal with measurement results from the converter may be provided as 354.

The converter may be configured as a step-down (buck) converter, or a step-up (boost) converter, or a boost-buck converter. The electrical switches or transistors of the converter may be used to also superimpose a signal on the current flow. The transistors may be of the type MOS or IGBT or another semiconductor switch.

The signal source may be a processor configured to generate a broadband signal having one or a plurality of waveforms at different frequency points across a frequency range, or a regulator circuit configured to inject a broadband signal into the fuel-cell. The signal may be provided or injected as a current signal, or a voltage signal, or a combination of these.

The signal sent to the fuel cell may have a substantially smaller current than the current which the fuel cell provides towards the electric motor or optional battery, i.e. it has a small-signal value. In particular, the signal should not be of a strength or intensity such that it might damage the electric motor or battery or other

components, or that it interfere with normal operation of the complete system.

The response is analyzed by the signal monitor, which extracts the Electrical Impedance profile information from the response signal. With the response information, it is then possible to do Electrical Impedance Spectroscopy.

The signal monitor may comprise a measurement circuit for measuring one or both of a voltage and/or current response from the fuel-cell. The signal monitor may be capable of measuring a distribution of the frequency waveform, or of performing a signal analysis of the response.

Turning now to Figure 3, a topology is shown for Current Interrupt monitoring. In this topology, current stimuli 451 are sent from the converter 420 as a current signal. This is done at the converter, for example using the electrical switches in the converter. The current may be partially interrupted to generate the signal 451 , or the current may be completely interrupted. A response 452 is received and monitored at the signal monitor of the converter, with feedback from the fuel cell or fuel-cell stack. This feedback may be represented as a voltage response from the fuel cell or fuel-cell stack. The sending of the signal 451 may be started by a measurement request 453 to the converter, or it may be done at regular intervals, or according to a different plan or signal. An evaluated signal from the converter may be provided as 454.

Interruption of the current from the fuel cell imposes a square-wave signal on the current flow between fuel cell and converter. The response is analyzed by a signal monitor, which extracts the Current Interrupt profile information from the response signal. With the response information, it is then possible to do a Current Interrupt analysis or test.

Cell Voltage Monitoring may also be used for monitoring the state of fuel cell;

however, such a method may require a more complex wiring system and extra hardware. The information gathered using the instant methods can provide substantial insight into cell operation and changes within the fuel cell itself, in addition to the cell voltage.

The information from Current Interrupt monitoring may then be used for an analysis of the status of the fuel cell or fuel-cell stack, or for an analysis of the lifetime status of the fuel cell. This information may in turn be used to control the operation of the fuel cell, such as the operating temperature, or the amount of fuel provided to the cell, or the amount of air provided to the cell.

All measurements can be performed during normal operation of the fuel cell, i.e. in real time. The only constraint is that the superimposed signal or interruption of current does not hinder the operation of the electric motor, or cause the motor to be operated in an unsafe or undesirable operating range.

One aspect of fuel-cell operation which is important to monitor, is the humidity. The humidity of a cell or stack is determinative for the operating efficiency of the cell and the amount of electricity which is produced. The level of humidity is often related to the impedance of the cells, which can in turn be measured with the methods disclosed here. The impedance may be measured at differing frequencies, or at low or high frequencies. The impedance may also be measured with a step function, such as current interrupt.

The signal is supplied to the fuel cell or fuel cell stack, and the response is measured as a voltage or current response. From the response, the impedance of the cell or stack can be determined.

With the inventive methods described in the instant application, it is more easily possible to monitor the operation of a fuel-cell electrical energy source, and with the information gained, to improve efficiency and longevity. Reference numerals

210, 310, 410 Fuel-cell System 220, 320, 420 DC-DC Converter 230, 330, 430 DC-AC Inverter

351 , 451 Current Signal to FC

352, 452 Current Signal from FC

353, 453 Measurement request

354, 454 Measurement results

Claims

1. An electrical system for operating an electric motor (235) with a DC

electrical energy source (210), comprising a converter (220, 230) and a signal-monitoring device,

wherein the electric motor is electrically coupled to the converter, and the converter is electrically coupled to the energy source,

characterized in that

the converter is adapted to provide a signal (351 , 451 ) to the energy source, which signal is monitored (352, 452) by the signal-monitoring device.

2. The system of claim 1 wherein the energy source is a fuel cell and the signal provided to the energy source is a signal provided to a fuel cell.

3. The system of claim 2 wherein the fuel cell is a polymer electrolyte fuel cell.

4. The system of a previous claim wherein the signal provided to the energy source is a current stimulus.

5. The system of a previous claim wherein the signal is a current interrupt.

6. The system of a previous claim wherein the signal permits a current interrupt test.

7. The system of a previous claim wherein the converter is adapted to be coupled to additional DC electrical energy storage and/or additional DC electrical energy sources.

8. A method of operating a fuel cell (210) electrically coupled to a converter (220, 230), wherein the converter provides a signal (351 , 451 ) to the fuel cell, which signal is monitored (352, 452) by a signal-monitoring device.

9. The method of claim 8 wherein the signal is a small-signal current

stimulus such as a partial or a complete current interrupt.

10. An electrically-driven motor vehicle comprising an electric drive or traction motor (235), a fuel cell (210), and a system according to any of claims 1 -7, wherein the system is electrically coupled to the electric motor and the fuel cell.