HK1197774A - Method of managing the operation of a hybrid system - Google Patents

Description

Method for controlling operation of a hybrid system

Technical Field

The invention relates to a method of controlling the operation of a hybrid direct current source comprising a fuel cell stack, a battery and a DC/DC converter comprising an input and an output, the converter input being connected to the output of the fuel cell stack and the output being connected to a variable load in parallel with the battery, the fuel cell stack being formed by a plurality of electrochemical cells adapted to generate electricity from fuel and oxidizing gas.

Background

Electrochemical cell assemblies (often referred to as stacks) connected in series are well known. The electrochemical unit thus assembled can be formed, for example, by a battery element or a fuel cell. Fuel cells are electrochemical devices for the direct conversion of chemical energy into electrical energy. For example, one type of fuel cell includes an anode and a cathode with a proton exchange membrane disposed therebetween, which is commonly referred to as a polymer electrolyte membrane. Such membranes only allow protons to pass between the anode and cathode of the fuel cell. At the anode, diatomic hydrogen reacts to produce H that can pass through the polyelectrolyte membrane⁺Ions. Electrons generated by this reaction reach the cathode through a circuit external to the fuel cell, thereby generating an electric current. Since individual fuel cells typically produce only low voltages (about 1 volt), fuel cells are often connected in series to form a fuel cell stack capable of producing higher voltages, which is the sum of the voltages of each cell.

When used in the automotive industry, these fuel cell stacks are associated with a battery to form a hybrid system 100. The system connects the fuel cell stack 102 in parallel with the battery so that the fuel cell stack or battery 106 simultaneously or individually powers the vehicle 108 via a common portion known as a bus. This mixing also allows the fuel cell stack to recharge the battery that will provide power to the vehicle. As shown in fig. 1, when the hybrid system uses a DC/DC converter 104 connected to the output 102 of the fuel cell stack, the hybrid system is referred to as "active". The DC/DC converter 104 is used to regulate the voltage levels of the fuel cell stack 102 and the battery 106 and to regulate the power provided by the fuel cell stack 102.

Adjusting the power requirements implements a control strategy to distribute power between the fuel cell stack 102 and the battery 106 according to the power requirements of the automotive electric engine and system constraints. The system constraints that the control strategy must take into account are the maximum voltage and current of the fuel cell stack and the battery, the temperature range that must not be exceeded, the battery state of charge, i.e. when the battery is already 100% charged, for example, it cannot be charged any more.

One of the control strategies of this hybrid system consists in regulating the battery state of charge around a nominal value without being able to reach a maximum or minimum charge of said battery. In this way, the battery never needs to be charged from the outside, since it is recharged by the fuel cell stack, and when the vehicle is in a braking phase, it can be recharged by recovering the kinetic energy of the vehicle. This means that the fuel cell stack provides the average power consumed by the vehicle's electric engine, while the battery acts as an energy buffer to charge or discharge energy. This strategy is implemented by using a DC/DC converter to regulate the bus voltage at a constant value.

One drawback of this known strategy is that no measures are implemented to prevent the fuel cell stack from operating at an open circuit voltage ("OCV"). "open circuit voltage" refers to the operating region where the voltage per cell is above 0.85-0.9V/cell. This voltage is known to severely shorten the life of the fuel cell stack. Therefore, it is not desirable for the fuel cell stack to operate in this mode. At constant pressure, the fuel cell stack operates in open circuit voltage mode when the load current is small.

An open circuit voltage mode of operation may occur when a minimum current is applied to the fuel cell stack at a constant pressure. In fact, this solution avoids the so-called open circuit mode that occurs when the voltage is higher than 0.85-0.9V/cell. As the current decreases, the voltage increases at a constant pressure. The current value determines the power value and does not always consume the power supplied, especially when the state of charge of the battery is close to 100%, and the battery can no longer be charged.

Another condition that may cause the fuel cell stack to operate in an open circuit mode is when the pressure is reduced. Reducing the pressure at low power reduces the battery voltage, thereby avoiding open circuit mode. However, it must be taken into account that the pressure variation dynamics are much slower than the current variation dynamics, and that the pressure reduction only occurs when the current is consumed. The current value directly affects the pressure reduction speed. Thus, if the power of the fuel cell stack momentarily goes from several kilowatts to zero watts, it is impossible to avoid the open circuit mode because there is no longer any current to reduce the pressure. Likewise, if the power of the fuel cell stack must quickly change from a few watts of power at low pressure to a few kilowatts of power at high pressure, the pressure must increase before the current increases. This approach necessarily results in the fuel cell stack entering open circuit mode for a short period of time, damaging the fuel cell stack.

Disclosure of Invention

It is an object of the present invention to provide a method for operation of a hybrid system comprising a fuel cell stack and a battery, which optimizes the performance of the hybrid system and increases the lifetime of the fuel cell stack.

The invention therefore relates to a method of controlling the operation of a hybrid continuous current power supply comprising a fuel cell stack, a battery and a DC/DC converter comprising an input and an output, the input of the converter being connected to the output of the fuel cell stack and the output being connected to a variable load in parallel with the battery, the fuel cell stack being formed by a plurality of electrochemical cells adapted to generate electricity from fuel and oxidizing gas, the method being characterized in that it comprises the steps of:

a) providing a fuel stream and an oxidizing gas stream to each of the electrochemical cells;

b) defining a set point representative of a power demand;

c) monitoring a fuel pressure and an oxidizing gas pressure in the fuel cell stack;

d) monitoring the output voltage of the fuel cell stack, the output current of the fuel cell stack and the cell voltage;

e) changing an output current of the fuel cell stack and a pressure of the fuel cell stack via the DC/DC converter, thereby changing an output power of the fuel cell stack;

f) controlling operation of the hybrid continuous current power supply by applying the following test:

-1) measuring the cell voltage and comparing it with a first predetermined critical value, if the cell voltage exceeds said first predetermined critical value, reducing the output current of the fuel cell stack and restarting step f), otherwise moving to a second test;

-2) measuring the output current of the fuel cell stack and comparing said output current with a second predetermined critical value, if the output current falls below said second predetermined critical value, reducing the pressure and restarting step f), otherwise moving to a third test;

-3) measuring the output voltage of the fuel cell stack and comparing said output voltage with a third predetermined critical value,

-if the output voltage of the fuel cell stack is below said third predetermined critical value, moving to a fourth test,

-if the output voltage of the fuel cell stack exceeds said third predetermined critical value, reducing the pressure, measuring the cell voltage and comparing it with a first predetermined critical threshold value, if the cell value falls below said first predetermined critical threshold value, keeping the output current of the fuel cell stack constant and resuming step f), otherwise moving to a fourth test; and is

-4) measuring the battery charge and comparing it with a fourth predetermined critical value, modifying the output current of the fuel cell stack and restarting said step f) if the battery charge is different from said fourth predetermined critical value, otherwise moving to the first test.

One advantage of the present invention is to provide longer stack life. In fact, with the strategy of the invention, the cell voltage values and the voltage values of the fuel cell stack are not too high. Therefore, the cells do not easily reach values that could cause them to explode, and the cells of the fuel cell stack do not face the risk of damage. By arranging the hybrid system of the present invention in this manner, the strategy of the present invention allows for longer use.

Advantageous embodiments of the inventive method form the subject matter of the dependent claims.

In a first advantageous embodiment, step f) comprises a fifth test performed in the event of success of the fourth test, which comprises measuring the output current of the fuel cell stack and comparing said output current with a second predetermined threshold, increasing the output current of the fuel cell stack to reach said second predetermined threshold and restarting said step f) if the output current of said fuel cell stack is lower than said second predetermined threshold, and otherwise increasing the pressure until a maximum pressure value is reached.

In a second advantageous embodiment, in test 4), the output current of the fuel cell stack is increased and step f) is restarted if the state of charge of the cell is lower than said fourth predetermined critical value, otherwise the output current of the fuel cell stack is decreased and said step f) is restarted.

In another advantageous embodiment, the maximum pressure value is 2.5 bar.

In another advantageous embodiment, the hybrid continuous current power supply further comprises a regulating circuit for recovering data related to the operation of the hybrid system and for sending control signals to the fuel cell stack and the DC/DC converter.

Drawings

The objects, advantages and features of the mixing system according to the invention will become more apparent from the following detailed description of at least one embodiment thereof, given by way of non-limiting example only and illustrated in the accompanying drawings, wherein:

fig. 1 is a schematic diagram of a known mixing system.

Figure 2 is a schematic diagram of an electrochemical system operating according to a first mode of operation of the present invention.

Figure 3 shows a diagram of the operation of the fuel cell stack during operation of the method according to the invention.

Figure 4 shows the operating points of the fuel cell stack during operation according to the method of the invention.

Detailed Description

In the following description, all components of the fuel cell stack known to those skilled in the art will be described only in a simplified manner.

Fig. 2 shows a schematic view of a mixing system 1 according to the invention. The hybrid system 1 comprises a fuel cell stack 2, i.e. a plurality of electrochemical cells mounted in series. The fuel cell stack 2 is powered by the subtraction of a fuel such as hydrogen and an oxidant such as oxygen. The reactions between the fuel and the oxidant are attenuated to produce a fuel cell voltage. The water resulting from the abatement of the reaction between the fuel and the oxidant may be discharged via a recirculation loop equipped with a circulation pump. The hybrid system 1 further comprises means for storing energy 6, such as one or more batteries or super capacitors. In the remainder of the description, the means for storing electric energy will be assumed to be a battery 6, but the use of a plurality of batteries is not excluded. The battery 6 supplies a battery voltage and is connected in parallel with the fuel cell stack 2 so that both the fuel cell stack 2 and the battery 6 are connected to a variable load 8. The variable load 8 may be, for example, an automobile engine.

The hybrid system 1 further comprises a DC/DC converter 4 having an input unit and an output unit. The output of the fuel cell stack 2 is connected to the input unit of the DC/DC converter 4, and therefore this means that the voltage provided by the fuel cell stack 2 enters the DC/DC converter 4. The connection point of the variable load 8 and the battery 6 is connected to the output unit of the DC/DC converter 4. It is obvious that the input unit comprises a plurality of inputs and the output unit comprises a plurality of outputs.

The DC/DC converter 4 is also arranged to control the hybrid system 1, since the DC/DC converter 4 is able to adjust the voltage level by controlling the current of the fuel cell stack 2. Also, the DC/DC converter may regulate the power provided by the fuel cell stack 2.

In practice, the role of the DC/DC converter 4 is to control the hybrid system 1 so that the battery 6 and the fuel cell stack 2 together operate on the power load 8. The function of the DC/DC converter is also to distribute the power supplied by the fuel cell stack between the load (the engine in automotive applications) and the battery. Of course, the control of the hybrid system 1 may be subject to constraints such as voltage and current limits of the fuel cell stack 2, voltage and current limits of the battery 6, state of charge limits of the battery 6, temperature limits that cannot be exceeded, and the like.

The hybrid system 1 further comprises a regulating circuit 10, shown in fig. 2 and used for recovering data related to the operation of the hybrid system 1, and for sending control signals to the fuel cell stack 2 and the DC/DC converter 4. The data relating to the operation of the hybrid system 1 include the current set-point of the fuel cell stack and the battery 6, as well as the power set-point and all measured data, i.e. the voltage and current of the battery 6 and the fuel cell stack 2, the pressure P of the fuel cell stack and the power generated by the hybrid system. The signal sent by the regulating circuit 10 to the fuel cell stack 2 is used to vary the pressure P. The signal sent by the regulating circuit 10 to the DC/DC converter 4 is used to vary the voltage by controlling the current of the fuel cell stack 2.

The fuel cell stack 2 is characterized by its cell voltage curve according to current and pressure. FIG. 4 illustrates that the voltage of the fuel cell stack varies as a function of current, and more specifically that the cell voltage increases with current (i.e., when the power P is_TRACIncreasing) and decreases. It is noted that at very low currents, the cell voltage is high and may damage the cell. The cell voltage-current curve is also characterized in that it depends on the pressure P. In fact, for each pressureForce values, there is a cell voltage-current curve. Note that for a given current, the cell voltage increases with pressure P. Thus, an increase in pressure P provides higher power. Thus, the operating point can be selected by changing the current Ifc and the pressure P.

When hybrid system 1 consumes low power, the ideal strategy involves obtaining Ufc the battery voltage adjusted to a determined value. This operation is necessary because the battery voltage Ufc-current Ifc curve as a function of pressure P shows that battery voltage Ufc can be maximized at low power (i.e., at low current). It is therefore necessary to regulate battery voltage Ufc by acting on current Ifc and pressure P. At high power, battery voltage Ufc does not require regulation because battery voltage Ufc decreases according to current Ifc. This control method is derived from the idea of reducing the operating pressure of the fuel cell stack 2 at low power to avoid the OCV mode. However, it must be taken into account that the voltage variation dynamics are much slower than the current variation dynamics (variation in seconds for pressure P; variation in milliseconds for current). It must also be considered that the pressure P reduction of the fuel cell stack 2 can occur only when the current is consumed, and the current value directly affects the pressure reduction speed.

To operate the hybrid system 1, the present invention proposes to implement the strategy forming the subject of the invention in order to operate the hybrid system in a safe manner while trying to obtain the longest possible lifetime.

To achieve this goal, the strategy of the invention shown in fig. 3 proposes to satisfy a certain number of criteria by performing a series of tests.

The first criterion specifies that the battery voltage Ubat must be below a first predetermined critical value. The first predetermined threshold value is a value selected according to the type of the battery 6. This value is a critical value beyond which the battery 6 is exposed to the risk of overload. Overloading the battery 6 damages the battery but more importantly leads to a risk of explosion. This risk cannot be envisaged because the type of battery 6 used contains toxic or corrosive products and, more importantly, the battery 6 mounted on the vehicle is prone to cause further damage. This explains why this first criterion (security criterion) is the most important and the criterion that must be fulfilled first.

The second criterion specifies that the fuel cell stack 2 preferentially covers the power PTRAC required by the load. This means that the battery must be used as little as possible and only the power difference between the power PTRAC required by the load and the maximum power supplied by the battery to the fuel cell stack 2 can be compensated. This criterion allows to give the battery 6 an optimal size so that it provides only the minimum necessary power. This means that the battery 6 is at least able to provide the power difference between the maximum power provided by the fuel cell stack 2 and the maximum power required by the load.

The third criterion specifies that the fuel cell stack output voltage Ufc be adjusted so that it does not exceed a third predetermined threshold. In practice, the voltage of each cell in the fuel cell stack 2 must not exceed a certain value, otherwise the cell may be damaged. Cell damage can lead to a shortened life of the fuel cell stack and a deterioration in its performance. The voltage Ufc may be the voltage of each cell or the total output voltage of the fuel cell stack.

The fourth criterion relates to keeping the charge level SOC of the battery at a desired constant level.

In order to meet these four criteria, the strategy according to the invention provides five tests via the regulating circuit to perform these tests to meet the defined criteria. These five tests involve performing parallel measurements and then performing successive comparisons. This means that all measurements needed for various tests are made at a given time. These tests are then performed. These tests are defined as follows: if the test condition is satisfied, an operation is performed to correct and repeat the test or the entire process, and if the test condition is not satisfied, the next test is performed. Since these tests are performed according to importance, it can be ensured that the most important criteria are always met.

The first test involves testing the battery voltage Ubat to see if it exceeds a first predetermined threshold Ubat _ max. The predetermined critical value Ubat _ max is selected according to the battery. For the present example, the limit is set to 288V, since the battery used has a maximum voltage of 350V, and the first predetermined critical value Ubat _ max is chosen to allow for leeway. If the first predetermined threshold is exceeded, it means that the battery receives too much current from the load. It is therefore apparent that the power provided by the fuel cell stack is too great. If the voltage condition is confirmed, the regulating circuit then sends a signal to the DC/DC converter 4 and the converter reduces the output current value Ifc of the fuel cell stack. This reduction in the current Ifc causes a reduction in the power of the fuel cell stack 2. Then, a new measurement is performed and the first test is repeated. In fact, since this is the test that represents the most important criterion, this criterion to be verified is the most important for enabling other criteria to be met. If the voltage condition is not confirmed, i.e. the battery voltage does not exceed the first predetermined critical value Ubat _ max, a second test is performed.

The second test comprises checking whether the output current Ifc of the fuel cell stack falls below a second predetermined threshold I _ pressure in order to then regulate the output voltage Ufc of the fuel cell stack. In fact, at the maximum voltage P, there is a current value Ifc of the fuel cell stack, called the second predetermined critical value I _ depress, below which the output voltage Ufc of the fuel cell stack becomes higher than a threshold value (here 0.85V), which damages the cells of the fuel cell stack and shortens their life.

Therefore, in the case where the output current Ifc of the fuel cell stack 2 falls below the second predetermined threshold value I _ pressure, the pressure P of the fuel cell stack 2 decreases, thereby decreasing the output voltage Ufc of the fuel cell stack. The test may also be performed by comparing the output voltage Ufc of the fuel cell stack to an equivalent voltage of a second predetermined threshold. This equivalent voltage is the voltage Ufc of the fuel cell stack 2 for the maximum pressure Pmax and for the current Ifc equal to the second predetermined threshold value. If the second test is negative, i.e. the output current Ifc of the fuel cell stack 2 is higher than the second critical threshold, a third test is performed. Otherwise, the previous test is repeated until the test condition is satisfied. The third test involves checking whether the output voltage Ufc of the fuel cell stack exceeds a third predetermined threshold Ufc _ max and then comparing the cell voltage to a fourth predetermined threshold. In fact, if the output voltage Ufc of the fuel cell stack exceeds the third predetermined threshold Ufc — max, there are two ways to reduce the voltage: either the stack output current Ifc must be reduced or the pressure P of the fuel cell stack 2 must be reduced. The battery voltage Ubat therefore does not exceed the first predetermined critical value Ubat _ max of the first test if the battery voltage Ubat is lower than the first predetermined critical threshold value. The output current Ifc of the fuel cell stack 2 therefore does not need to be reduced. The pressure P should then be reduced to reduce the output voltage Ufc of the fuel cell stack. This then protects the cells of the fuel cell stack 2 from overvoltage without reducing the charge of the battery 6, since the output voltage Ifc of the fuel cell stack is not changed.

Thus, the third test involves comparing the cell output voltage Ufc of the fuel cell stack 2 to a third predetermined threshold. If the cell output voltage of the fuel cell stack is above a third predetermined threshold, the pressure is reduced and the cell voltage is compared to a fourth threshold. If the cell voltage is below the fourth critical threshold, the current is held constant and the previous test is repeated. If the battery voltage is above the fourth critical threshold, the current is disregarded and the previous test is repeated.

The fourth test involves checking the state of charge SOC of the battery and modifying the value of the output current Ifc of the fuel cell stack accordingly. In fact, the battery state of charge SOC depends on the battery output current Ifc, so that if the current Ifc increases, the battery state of charge SOC increases. It is important that the state of charge SOC not be too high so that the battery can return high current. Thus, the state of charge SOC is compared to a fourth predetermined threshold SOC _ cons, which represents the desired state of charge SOC. If the state of charge SOC is different from the fourth predetermined threshold SOC _ cons, the regulation circuit determines whether the state of charge SOC is below or above the first predetermined threshold SOC _ cons. The regulating circuit commands an increase of the output current of the fuel cell stack if the state of charge SOC is lower than said fourth predetermined critical value SOC _ cons. Conversely, if the state of charge SOC is higher than said fourth predetermined threshold value SOC _ cons, the regulation circuit commands a reduction in the output current Ifc of the fuel cell stack 2. The result of this test is not important, i.e. whether the state of charge is equal to said fourth predetermined threshold value SOC _ cons is not important, and these tests will all be repeated. The state of charge SOC will be set between 0.6 and 0.8, i.e. the battery 6 is between 60% and 80% charged, and the state of charge never exceeds 0.9, i.e. 90%.

In a variant, a fifth test is performed. This fifth test involves monitoring the output current of the fuel cell stack so that the current never falls below the second predetermined threshold Ifc _ min. In fact, the current signature according to pressure P and fuel cell stack voltage Ufc shows: in order to reduce the output voltage Ufc of the fuel cell stack 2, the pressure P must be reduced before the current Ifc is reduced. The pressure P first decreases before the current Ifc decreases because at a constant current Ifc, the decrease in pressure P decreases the output voltage Ufc of the fuel cell stack, and the decrease in current Ifc causes the value of the output voltage Ufc of the fuel cell stack 2 to increase. The minimum current Ifc is the current: at this current, the pressure P is at a minimum, and an output voltage Ufc of the fuel cell stack equal to a third predetermined threshold is obtained. Below this second threshold Ifc _ min, the voltage Ufc therefore increases exponentially, leading to a risk of damaging the fuel cell stack.

If this current condition is not observed, i.e. the stack current Ifc is higher than the second threshold Ifc _ min, this means that all defined criteria are fulfilled and the system operates in an optimal way. Thus, the fuel cell stack 2 can be operated at full speed, and to achieve this, the pressure is increased until the maximum pressure is achieved. This means that the fuel cell stack 2 can potentially reach maximum power.

Next, the process starts over, i.e. a new measurement is performed and the test is performed again.

It will be apparent that various modifications and/or improvements and/or combinations apparent to those skilled in the art may be made to the embodiments of the invention without departing from the scope of the invention as defined in the appended claims.

Claims (5)

1. A method for controlling the operation of a hybrid continuous current power supply (1) comprising a fuel cell stack (2), a battery (6) and a DC/DC converter (4) comprising an input unit and an output unit, the input of the converter (4) being connected to the output of the fuel cell stack and the output being connected to a variable load (8) in parallel with the battery, the fuel cell stack being formed by a plurality of electrochemical cells adapted to generate electricity from fuel and oxidizing gas, the method being characterized by comprising the steps of:

a) providing a fuel stream and an oxidizing gas stream to each of the electrochemical cells;

b) defining a set point representative of a power demand;

c) monitoring a fuel pressure and an oxidizing gas pressure in the fuel cell stack;

d) monitoring the output voltage of the fuel cell stack, the output current of the fuel cell stack and the cell voltage;

e) changing an output current (Ifc) of the fuel cell stack and a pressure (P) of the fuel cell stack via the DC/DC converter, thereby changing an output power of the fuel cell stack;

f) controlling operation of the hybrid continuous current power supply by applying the following test:

-1) measuring the battery voltage (Ubat) and comparing it with a first predetermined critical value (Ubat _ max), if the battery voltage exceeds said first predetermined critical value, reducing the output current (Ifc) of the fuel cell stack and restarting step f), otherwise moving to a second test;

-2) measuring the output current (Ifc) of the fuel cell stack and comparing said current with a second predetermined threshold value (I _ pressure), if the output current falls below said second predetermined threshold value (I _ pressure), reducing the pressure (P) and restarting step f), otherwise moving to a third test;

-3) measuring the output voltage (Ufc) of the fuel cell stack and comparing said voltage with a third predetermined critical value (Ufc _ max),

-if the output voltage (Ufc) of the fuel cell stack is below the third predetermined threshold value (Ufc _ max), moving to a fourth test,

-if the output voltage of the fuel cell stack exceeds said third predetermined critical value (Ufc _ max), reducing the pressure (P), measuring the battery voltage (Ubat) and comparing said voltage with a first predetermined critical threshold value (Ubat _ top), if the battery value falls below said first predetermined critical threshold value (Ubat _ top), keeping the output current (Ifc) of the fuel cell stack constant and restarting said step f), otherwise moving to a fourth test; and is

-4) measuring the battery state of charge (SOC) and comparing it with a fourth predetermined critical value (SOC _ cons), modifying the output current (Ifc) of the fuel cell stack and restarting said step f) if the battery state of charge is different from said fourth predetermined critical value (SOC _ cons), otherwise moving to the first test.

2. Control method according to claim 1, characterized in that step f) comprises a fifth test performed in the event of success of the fourth test, which fifth test comprises measuring the output current (Ifc) of the fuel cell stack and comparing said current with a second predetermined critical threshold value (Ifc _ min), increasing the output current (Ifc) of the fuel cell stack to said second predetermined critical threshold value (Ifc _ min) and restarting said step f) if the output current of said fuel cell stack is lower than said second predetermined critical threshold value (Ifc _ min), and otherwise increasing said pressure (P) until a maximum pressure value (Pmax) is reached.

3. The control method according to claim 1 or 2, characterized in that: in test 4), the output current (Ifc) of the fuel cell stack is increased and step f is restarted if the state of charge (SOC) of the battery is lower than said fourth predetermined critical value (SOC _ cons), otherwise the output current of the fuel cell stack is decreased and said step f is restarted).

4. Control method according to any one of claims 2 or 3, characterized in that the maximum pressure value (Pmax) is 2.5 bar.

5. Control method according to any of the preceding claims, characterized in that the hybrid continuous current power supply further comprises a regulating circuit (10) for recovering data related to the operation of the hybrid system and for sending control signals to the fuel cell stack and the DC/DC converter.