A method for improving the efficiency of a fuel cell power supply system

Abstract

The present invention discloses a method for improving the efficiency of a fuel cell power supply system, comprising the following steps: step S1, applying a partial power conversion structure to a fuel cell system and establishing a mathematical model of the efficiency of the fuel cell system and the number of fuel cells connected in series, selecting the number of fuel cells connected in series that makes the system most efficient, and improving the system efficiency; step S2, on the basis of step S1, establishing a fuel flow rate and the efficiency model of the fuel cell system, obtaining an optimal efficiency curve of the fuel cell system, and further improving the system efficiency when the system operates on the optimal efficiency curve; step S3, formulating a power control strategy, and through the power control strategy, making the fuel cell power supply system operate in the optimal efficiency area of ​​the optimal efficiency curve of the fuel cell system in step S2, and further improving the system efficiency.

Description

Technical Field

[0001] The present invention relates to the field of fuel cell power supply systems, and in particular to a method for improving the efficiency of fuel cell power supply systems. Background Art

[0002] The pollution caused by fuel vehicles to the environment is becoming more and more serious. In order to reduce the pollution caused by vehicles to the environment, the world is vigorously developing efficient, clean and pollution-free electric vehicles. Fuel cells have high efficiency in the entire economic operation area, so for electric vehicles, fuel cells are very attractive power sources. The core factor affecting its commercialization potential is the cost of electricity required by the fuel cell system. Therefore, improving operating efficiency is a key issue in designing a cost-effective fuel cell battery system with high market competitiveness.

[0003] For improving efficiency, most of them reduce equivalent loss through control strategy and track the maximum efficiency point to improve the efficiency of fuel cells. The present invention applies partial power conversion structure to fuel cell system and optimizes fuel cell efficiency and system efficiency from the perspective of fuel flow rate, which has good effect. Most of them do not apply partial power conversion structure to fuel cell system. This is because the output voltage range of fuel cell is wide. When its voltage is low, the partial power conversion efficiency of the system may be lower than the full power conversion efficiency. This paper solves this problem through the selection scheme of the number of fuel cells in series and the optimal efficiency curve scheme. Other related literature only studies the influence of fuel flow rate on fuel cells under a single working condition, and none of them converts the influence of fuel flow rate on fuel cells into the optimal efficiency curve. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a method for improving the efficiency of a fuel cell power supply system. The method for improving the efficiency of a fuel cell system by selecting the number of fuel cells connected in series and a power control strategy based on the optimal efficiency curve of the fuel cell system is simpler and more efficient than other solutions.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for improving the efficiency of a fuel cell power supply system, comprising the following steps:

[0006] Step S1, applying a part of the power conversion structure to the fuel cell system and establishing a mathematical model of the fuel cell system efficiency and the number of fuel cells connected in series, selecting the number of fuel cells connected in series that makes the system most efficient, and improving the system efficiency;

[0007] Step S2: On the basis of step S1, a fuel flow rate and fuel cell system efficiency model is established to obtain an optimal efficiency curve of the fuel cell system. When the system operates on the optimal efficiency curve, the system efficiency is further improved.

[0008] Step S3, formulate a power control strategy, through which the fuel cell power supply system is operated in the optimal efficiency region of the optimal efficiency curve of the fuel cell system in step S2, thereby further improving the system efficiency.

[0009] A further improvement of the technical solution of the present invention is that the step of improving the system efficiency in step S1 is:

[0010] Step S11, convert the fuel cell voltage into a quantity related to the number of fuel cells connected in series: the fuel cell voltage is equal to the product of the voltage of one fuel cell unit and the number of fuel cells connected in series, then

[0011]

[0012] where v cell_min With v cell_max are the minimum and maximum values ​​of the fuel cell unit, respectively;

[0013] Step S12, finding the number of fuel cells connected in series to achieve the highest system efficiency, the specific steps are:

[0014] Step S121, first find the gain range and output voltage v of the converter o Relationship:

[0015]

[0016] Among them, v o is the converter output voltage, N is its transformer ratio, and M is the converter output voltage gain;

[0017] Step S122: Establish the maximum value of the fuel cell output voltage v cell_max and the minimum value v cell_min , bus voltage v bus The maximum output voltage of the converter is v omax and the minimum value v omin Relationship:

[0018]

[0019] Step S123: According to the relationship between steps S121 and S122, the maximum number of series connections n is calculated. max The optimal number of fuel cells in series is:

[0020]

[0021] A further improvement of the technical solution of the present invention is that the step of improving the efficiency of the fuel cell system in step S2 is:

[0022] S21. Establish the efficiency model of fuel flow rate and fuel cell system: the efficiency η of the fuel cell system system Transformed into fuel cell efficiency η FC and partial power conversion efficiency η global Relationship:

[0023] η system =η global ·η PPC ,

[0024] Among them, the fuel cell efficiency η FC :

[0025]

[0026] Among them, I FC is the fuel stack output current, v FC is the fuel cell output voltage, E nern is the Nernst voltage, P aux is the auxiliary system loss, v FC and P aux :

[0027] v FC =EV act -V con -V ohm ,

[0028]

[0029] Where E is the Nernst voltage, V act is the activation polarization voltage, V con is the concentration polarization voltage, V ohm is the ohmic polarization voltage, k is the proportionality coefficient, Q is the flow rate in L / s, η is the fan efficiency, η N is the torque, P fix is the fixed loss;

[0030] S22. Based on the fuel flow rate and efficiency model of the fuel cell system established in step S21, draw a three-dimensional efficiency surface of the fuel cell system, find and extract the points in the three-dimensional efficiency surface with the output current as a constant, the fuel flow rate as an independent variable, and the maximum efficiency of the fuel cell system, mark each optimal flow rate point, and obtain the optimal efficiency curve of the fuel cell system. Adjust the fuel flow rate through the controller to make the system run on the efficiency curve, thereby improving the system efficiency.

[0031] A further improvement of the technical solution of the present invention is that the three-dimensional efficiency surface of the fuel cell system in step S22 is divided into a light load area, an economic operation area, and a maximum power area in turn. The area where the efficiency of the fuel cell system is greater than or equal to 0.75 is the economic operation area, the area on the left side of the economic operation area where the efficiency of the fuel cell system is lower than 0.75 is the light load area, and the area on the right side of the economic operation area where the efficiency of the fuel cell system is lower than 0.75 is the maximum power area.

[0032] A further improvement of the technical solution of the present invention is that in step S3, based on the optimal efficiency curve of the fuel cell system obtained in step S2, a battery is used to adjust the output power of the fuel cell system so that it operates in the optimal efficiency area on the optimal efficiency curve of the fuel cell system, thereby further improving the system efficiency.

[0033] The further improvement of the technical solution of the present invention is that the economic operation area in the optimal efficiency curve of the fuel cell system in step S3 is divided into four control areas, the first control area is the light load area and the partial economic operation area, the second control area is the system optimal efficiency area, the third control area is a region slightly larger than the optimal efficiency area, the fourth control area is the maximum power area, P SOFC_x The fuel cell power at the boundary of each area, the output power control principle of the battery-regulated fuel cell system is:

[0034] 1) When the battery is not fully charged and the load power is less than P SOFC_2 When the fuel cell supplies power to the battery and the load at the same time, the fuel cell power does not exceed P SOFC_2 ;

[0035] 2) When the battery SOC is lower than the minimum value and the load power is lower than P SOFC_3 , the fuel cell supplies power to the battery and the load at the same time, and the fuel cell power does not exceed P SOFC_3 ;

[0036] 3) When the battery still has residual discharge power and the load power is greater than P SOFC_2 , the battery and fuel cell supply power to the load at the same time, and the fuel cell power cannot be lower than P SOFC_2 .

[0037] Due to the adoption of the above technical solution, the technical progress achieved by the present invention is:

[0038] The present invention provides a method for improving the efficiency of a fuel cell power supply system. First, the number of fuel cells connected in series and the fuel flow rate are selected to obtain an optimal efficiency curve, and power control is performed based on this curve. The fuel cell system operating area is selected through battery regulation to improve the efficiency of the fuel cell system. Compared with other control strategies, the present invention enables the fuel cell to have a wider output power range, which is beneficial to the development of fuel cell power supply systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Power supply system for fuel cell;

[0040] Figure 2 is the relationship between fuel voltage and converter voltage;

[0041] Figure 3 is the range of the number n of fuel cells connected in series;

[0042] Figure 4 is the relationship between the fuel cell voltage, current and flow rate;

[0043] Figure 5 VI characteristic curves at different flow rates;

[0044] Figure 6 To optimize the fuel cell voltage and converter voltage changes;

[0045] Figure 7 is the fuel cell power transfer diagram;

[0046] Figure 8 The effect of flow rate change on fuel cell efficiency;

[0047] Fig. 9 is the three-dimensional efficiency diagram of the fuel cell;

[0048] Fig.10 is the optimal efficiency curve of the fuel cell;

[0049] Fig.11 is the fuel cell voltage at different positions;

[0050] Fig.12 is the relationship between the LLC converter output voltage and frequency;

[0051] Fig.13 It is the system's optimal efficiency zone;

[0052] Fig.14 Parameter design flow chart;

[0053] Fig.15 Fuel cell control area;

[0054] Fig.16 Fuel cell power control flow chart. DETAILED DESCRIPTION

[0055] A method for improving the efficiency of a fuel cell power supply system comprises the following steps:

[0056] Step S1, applying a part of the power conversion structure to the fuel cell system and establishing a mathematical model of the fuel cell system efficiency and the number of fuel cells connected in series, selecting the number of fuel cells connected in series that makes the system most efficient, and improving the system efficiency.

[0057] The fuel cell power supply system adopts a partial power conversion structure. However, since the fuel cell has a wide output voltage range, there are two problems when the system adopts a partial power conversion structure:

[0058] 1) When the fuel cell power increases, the fuel cell voltage decreases. At this time, the voltage at the output end of the converter connected in series with the fuel cell continues to increase, causing the power flowing through the converter to increase, which is not conducive to improving the system efficiency;

[0059] according to Figure 1 The efficiency expression of the fuel cell system based on the partial power conversion structure can be expressed as:

[0060]

[0061] according to Figure 1 , Equation 1 can be converted into the relationship between fuel cell voltage and efficiency:

[0062]

[0063] From formula 2, we can see that the partial power conversion efficiency η of the fuel cell system is global Can be converted into the fuel cell output voltage v FC , bus voltage v bus , converter efficiency η converter The relationship between bus With η converter If both remain unchanged, then η global Only with v FC About, when v FC The larger the system efficiency η global The higher the v FC The larger the system efficiency η global The lower.

[0064] 2) In some power conversion systems, the voltage range of the fuel cell is consistent with the output voltage range of the converter, which makes the voltage gain of the converter larger and increases the difficulty of converter parameter design.

[0065] The fuel cell voltage has a wide voltage range, such as Figure 2As shown in the figure, as the current starts to increase, the fuel cell voltage changes from v smin Rise to v smax , and since the bus voltage remains unchanged, the converter output voltage is v omin Rise to v omax . Where △v=v smax -v smin =v omax -v omin , it can be explained that the wide voltage range of the fuel cell leads to a larger output voltage gain of the converter, which increases the difficulty of converter design.

[0066] Therefore, the voltage range design of the fuel cell system first needs to determine the bus voltage v according to the power level of the system design. bus , and then determine the number of fuel cells in series n, and the determination of n is related to the required voltage of the fuel cell, and needs to satisfy the partial power conversion constraint relationship and the converter gain constraint relationship at the same time.

[0067] Step S11, first convert the fuel cell voltage into a quantity related to the number of fuel cells connected in series: the fuel cell voltage is equal to the product of the voltage of one fuel cell unit and the number of fuel cells connected in series, then

[0068]

[0069] where v cell_min With v cell_max are the minimum and maximum values ​​of the fuel cell unit respectively. The fuel cell voltage can be expressed in the coordinate system as the slopes of the two zero crossing points are v cell_min With v cell_max The straight line, such as Figure 3 shown.

[0070] When the horizontal axis n increases, v smin With v smax As it increases, the converter voltage v omin With v omax It decreases accordingly, while △v increases continuously.

[0071] n min Determination of:

[0072] The present invention determines n according to the partial power conversion constraint min The partial power conversion constraint is that the system adopting partial power conversion must be better than the full power conversion structure, that is, it only needs to ensure that v smin Greater than or equal to v omax , then Figure 3 As shown, when n increases from 0, v smin Increase, v omax Decrease, when n is greater than or equal to the critical value n minWhen smin Equal to v omax , then n min The lower limit of .

[0073] The minimum number of series connections n min As shown in Formula 4:

[0074]

[0075] Step S12, n max Determination of:

[0076] like Figure 3 As shown in Figure 1, as n increases, the output voltage range △v of the converter continues to increase. However, the output voltage gain of the converter is limited, so n must have a maximum value n. max This causes the converter output voltage gain to reach its maximum limit.

[0077] Step S121: Taking the LLC converter as an example, first find the gain range and output voltage v of the LLC converter. o Relationship:

[0078]

[0079] Among them, v o is the converter output voltage, N is its transformer ratio, and M is the converter output voltage gain;

[0080] Step S122: Establish the maximum value of the fuel cell output voltage v cell_max and the minimum value v cell_min , bus voltage v bus The maximum output voltage of the converter is v omax and the minimum value v omin Relationship:

[0081] Formula 6 is the relationship between the maximum and minimum values ​​of the fuel cell voltage, the bus voltage and the converter output voltage:

[0082]

[0083] Step S123: According to the relationship between steps S121 and S122, the maximum number of series connections n is calculated. max The optimal number of fuel cells in series: Substituting equation 5 into equation 6 yields the maximum number of fuel cells in series that meets the converter gain requirement n. max , as shown in Formula 7:

[0084]

[0085] In the present invention, when n is n min and n maxWhen n is between n and n, the basic requirements of some power conversion systems and converters can be met. And from formula 2, it can be seen that when n is n min To n max When moving, v smin Increase, v omax Decrease, maximum power at η global Continuously increases, so the present invention selects n=n max As the number of fuel cells in series, it can ensure that η global The maximum voltage can be guaranteed to reach the converter within this voltage range. bus 、Number of series connections n max , fuel cell voltage v cell_min n max -v cell_max n max and the converter output voltage (v bus -v cell_max n max )~(v bus -v cell_min n max ).

[0086] Step S2: On the basis of step S1, a fuel flow rate and fuel cell system efficiency model is established to obtain an optimal efficiency curve of the fuel cell system. When the system operates on the optimal efficiency curve, the system efficiency is further improved. The specific process is as follows:

[0087] In order to further improve the partial power conversion efficiency η of the fuel cell system global The present invention starts with the power supply voltage to optimize the problem of low system efficiency in some voltage ranges and large converter output voltage range caused by the wide range power supply for partial power conversion. To this end, it is necessary to first analyze the factors affecting the fuel cell voltage, and its fuel cell model is shown below.

[0088] The fuel cell voltage can be expressed as:

[0089] V FC =EV act -V con -V ohm (8)

[0090] Where V FC is the fuel cell voltage, V act ,V con ,V ohm , are respectively the activation polarization partial pressure, concentration polarization partial pressure, and ohmic polarization partial pressure of the fuel cell. The Nernst voltage E is related to the gas partial pressure and can be expressed as:

[0091]

[0092] Where E0 is the standard voltage, R is the universal gas constant, T is the operating temperature of the fuel cell (unit: Kelvins), F is the Faraday constant, and P is H2 is the hydrogen partial pressure, P O2 is the oxygen partial pressure, P H2O P is the water partial pressure. H2 , P O2 , P H2O The partial pressure expression is as follows:

[0093]

[0094] Among them, τ H2 , τ O2 , τ H2O Represent the response time constants of hydrogen, oxygen and water vapor respectively. N H2 in 、N O2 in 、N H2O in It is positively correlated with the flow rate by the input molar mass of hydrogen, oxygen and water respectively.

[0095] Concentration polarization voltage V con It is caused by the mass transfer resistance of reactants and products when they pass through the porous electrode, and its voltage can be calculated as:

[0096]

[0097] Where IL is the limiting current density, as shown below:

[0098]

[0099] Among them, C O2 is the oxygen flow concentration, D O2eff is the effective diffusion coefficient of oxygen, Lc is the cathode thickness, A cell is the battery area.

[0100] According to the fuel cell output voltage model, it can be seen that the flow rate can change the Nernst voltage and the concentration polarization partial pressure. The present invention increases the input molar mass by increasing the fuel flow rate and the oxygen flow rate, which can increase the hydrogen and oxygen partial pressure, reduce the concentration polarization partial pressure, and thus increase the fuel cell output voltage. The increase in the fuel cell Nernst voltage and the reduction in the polarization partial pressure can improve the efficiency of the fuel cell stack, while v SOFC Increase, also make v o Reduce and improve the partial power conversion efficiency η of the fuel cell system global .

[0101] The present invention uses a unit surface area of ​​6.1413cm2 composed of n max Based on this fuel cell model, the present invention uses hydrogen flow rate and output current as independent variables and fuel cell voltage as dependent variable to draw a three-dimensional voltage surface diagram of the fuel cell. Figure 4 It can be seen that at any current, as the flow rate increases, the fuel cell voltage continues to increase:

[0102] Figure 4 The fuel cell VI characteristic curves at four different flow rates are extracted, such as Figure 5 As shown, as the flow rate increases, v cell_min n rises to v' cell_min If the fuel cell flow rate is reduced at light load, the maximum fuel cell voltage v can be reduced at the same time. smax , thereby increasing v omin , reducing the converter output voltage range, thereby reducing the difficulty of converter parameter design.

[0103] At this time, compared with before optimization, the minimum output voltage of the fuel cell converter is v cell_min n increases v' cell_min n, maximum output voltage of fuel cell v smax By v cell_max n is reduced to v' cell_max n, then the efficiency at maximum power is η global increases, and at the same time, the required converter voltage gain decreases, as shown in Figure 6 shown.

[0104] The flow rate cannot be increased indefinitely. Fuel cells have auxiliary systems. As the flow rate increases but the molar mass of the fuel cell output decreases, the internal pressure of the fuel cell will inevitably increase, thereby increasing the loss of the fuel cell auxiliary system.

[0105] like Figure 7 As shown in the figure, Stack is a fuel cell stack. The fuel cell stack needs to provide energy Paux to the auxiliary system, and then part of the energy Pcon flows through the converter, and most of the energy Pbus is directly sent to the bus.

[0106] S21. Based on step S1, the present invention establishes a fuel flow rate and fuel cell system efficiency model, and specifically analyzes the impact of the flow rate on the system efficiency.

[0107] First, the fuel cell stack efficiency can be expressed as:

[0108]

[0109] The fuel cell auxiliary system includes air supply subsystem, hydrogen supply subsystem, thermal management subsystem, exhaust gas treatment subsystem and other systems. According to the impact of changes in hydrogen and oxygen flow rates on the losses of these systems, the system is divided into hydrogen and oxygen compressor system (energy consumption increases with increasing air volume), fuel recovery system (fixed energy consumption), and other systems (fixed energy consumption).

[0110] The compressor is mainly used to provide air for the fuel cell, and the hydrogen flow rate is controlled by the solenoid valve on the hydrogen storage tank. Since the hydrogen fuel and control are supplied at a ratio of 1.145 / 0.21, the hydrogen flow rate can be used as an independent variable to analyze the auxiliary system loss and the entire system loss. The increase in hydrogen and oxygen flow rate will increase the compressor system loss, and the relationship is as follows:

[0111]

[0112] Among them, P motor is the motor power, P blower is the compressor power, Q is the flow rate in L / s, P is the pressure in Kpa, η is the fan efficiency, η N is the torque.

[0113] For a fuel cell system, increasing the flow rate will inevitably increase its pressure, which conforms to the following relationship:

[0114]

[0115] but

[0116]

[0117] Where k is the proportionality coefficient between rated power and rated speed, Q is the flow rate in L / s, η is the fan efficiency, η N is the torque.

[0118] The auxiliary system power loss can be expressed by the following formula:

[0119]

[0120] Fuel cell efficiency η FC :

[0121]

[0122] Among them, I FC is the fuel stack output current, v FC is the fuel cell output voltage, E nern is the Nernst voltage, P aux is the auxiliary system loss, v FC The formula is as follows:

[0123] v FC =EV act -V con -V ohm , (19)

[0124] Where E is the Nernst voltage, V act is the activation polarization voltage, V con is the concentration polarization voltage, V ohm is the ohmic polarization voltage.

[0125] Fuel cell system efficiency η system Transformed into fuel cell efficiency η FC and the partial power conversion efficiency η of the fuel cell system global Relationship:

[0126] η system =η global ·η PPC (20).

[0127] S22. The present invention builds a fuel cell simulation according to the efficiency model in the previous section, such as Figure 8 As shown in the figure, taking the maximum power condition as an example, the fuel cell current at this time is fixed, the flow rate is set as the independent variable, and the efficiency is the dependent variable for verification. From the waveform diagram, it can be seen that there is indeed a maximum efficiency point. When the flow rate continues to increase, although the fuel cell voltage and power are increasing, the auxiliary system loss is also increasing, resulting in a decrease in fuel cell efficiency.

[0128] To this end, the present invention uses matlab to establish an efficiency model. Finally, the following three-dimensional efficiency surface diagram is obtained. Fig. 9 It can be seen from the figure that each output current must correspond to a unique flow rate, so that the fuel cell efficiency is the highest at the current. Fig.10 The optimal efficiency point under each current is extracted to obtain the optimal flow rate efficiency diagram. Among them, the three-dimensional efficiency surface of the fuel cell system is divided into a light load area, an economic operation area, and a maximum power area. The area where the fuel cell system efficiency is greater than or equal to 0.75 is the economic operation area, the area on the left side of the economic operation area where the fuel cell system efficiency is less than 0.75 is the light load area, and the area on the right side of the economic operation area where the fuel cell system efficiency is less than 0.75 is the maximum power area. Since it is necessary to limit the output voltage of the fuel cell at low power, thereby increasing the minimum output voltage of the converter, the flow rate in the light load area is limited, and other areas are selected according to the optimal flow rate.

[0129] Based on the optimal flow rate diagram, it can be seen that at the optimal flow rate, the output voltage of the fuel cell when the fuel cell outputs the maximum current is v' cell_min n, the corresponding converter output voltage is v bus -v' cell_minn. This voltage range shows good partial power conversion performance in the full range of the fuel cell on the basis of meeting the optimal flow rate, and the maximum k value is (v bus -v' cell_min n) / v' cell_min n, which is much smaller than 1, means that the power flowing through the converter is only a small part of the power generated by the fuel cell. And the converter output voltage range △v is from v cell_max nv cell_min n is reduced to v' cell_max n-v' cell_min n, which is conducive to reducing the difficulty of converter parameter design. In summary, the fuel cell voltage is different under different flow rates, and the flow rate selection is different at different fuel cell operating points. The present invention divides the fuel cell operation area into a light load area, an economic operation area, and a maximum power area according to efficiency and power. The present invention effectively reduces the maximum voltage of the fuel cell through the flow rate, increases the minimum voltage of the fuel cell, improves the partial power conversion effect, and reduces the difficulty of converter parameter design.

[0130] Step S3, formulate a power control strategy, through which the fuel cell power supply system is operated in the optimal efficiency zone of the optimal efficiency curve of the fuel cell system in step S2, and a battery is used to adjust the output power of the fuel cell system so that it operates in the optimal efficiency zone on the optimal efficiency curve of the fuel cell system, thereby further improving the system efficiency.

[0131] The voltages in different regions of the fuel cell's optimal efficiency curve are as follows: Fig.11 As shown. When the parameters are designed to match the most efficient operating area of ​​the LLC converter with the economic operating area of ​​the fuel cell, the efficiency of the system in the economic operating area can be optimized. The optimal efficiency point of the LLC converter is the resonance point. As the frequency decreases further, the efficiency decreases, but compared with the working area above the resonance point, the efficiency is higher. Therefore, the working area on the left side of the LLC resonance point is matched with the economic operating area and the maximum power area as much as possible to further improve the efficiency of the economic operating area and reduce the loss of the converter in the maximum power area.

[0132] For LLC converter, when the bus voltage is determined, the transformation ratio directly determines the voltage range on the left side of the resonance point and the voltage range on the right side of the resonance point. Fig.12 As shown:

[0133] Among them, v o_fs is the converter output voltage corresponding to the resonance point. To match the converter resonance point with the fuel cell's highest efficiency point, the converter output voltage v when operating at the resonance point is o_fs Equal to the voltage v when the fuel cell efficiency is the highest s_eff The key is the transformer ratio n T choice.

[0134] Its expression is as follows:

[0135]

[0136] Transformation ratio n T , so that v o_fs The voltage of the LLC converter is the same as the maximum efficiency point of the fuel cell, which means that the LLC converter resonance point, that is, the maximum efficiency point of the converter, coincides with the maximum efficiency point of the fuel cell, and ensures that when the fuel cell operates at 0.85n-0.93n, the LLC converter is also in the highest efficiency range. Fig.13 As shown:

[0137] After completing the efficiency zone matching, according to Fig.14 The flowchart shown can complete the resonant parameter design of the corresponding converter.

[0138] According to the economic operation area and the output capacity of the battery, the economic operation area is divided into four control areas, among which P SOFC_x is the fuel cell power at the boundary of each region, as Fig.15 shown.

[0139] In the figure, the second control area is the system's optimal efficiency area, the first control area is the light load area and partial economic operation area, the third control area is an area slightly larger than the optimal efficiency area, and the fourth control area is the maximum power area.

[0140] Basic principles of control such as Fig.16 As shown:

[0141] 1) When the battery is not fully charged and the load power is less than P SOFC_2 When the fuel cell supplies power to the battery and the load at the same time, the fuel cell power does not exceed P SOFC_2 ;

[0142] 2) When the battery SOC is lower than the minimum value and the load power is lower than P SOFC_3 , the fuel cell supplies power to the battery and the load at the same time, and the fuel cell power does not exceed P SOFC_3 ;

[0143] 3) When the battery still has residual discharge power and the load power is greater than P SOFC_2 , the battery and fuel cell supply power to the load at the same time, and the fuel cell power cannot be lower than P SOFC_2 .

Claims

1. A method for improving the efficiency of a fuel cell power supply system, characterized in that: The steps include: Step S1, applying a part of the power conversion structure to the fuel cell system and establishing a mathematical model of the fuel cell system efficiency and the number of fuel cells connected in series, selecting the number of fuel cells connected in series that makes the system most efficient, and improving the system efficiency; Steps to improve system efficiency are: Step S11, convert the fuel cell voltage into a quantity related to the number of fuel cells connected in series: the fuel cell voltage is equal to the product of the voltage of one fuel cell unit and the number of fuel cells connected in series, then where v cell_min With v cell_max are the minimum and maximum values ​​of the fuel cell unit, respectively; Step S12, finding the number of fuel cells connected in series to achieve the highest system efficiency, the specific steps are: Step S121, first find the gain range and output voltage v of the converter o Relationship: Among them, v o is the converter output voltage, N is its transformer ratio, and M is the converter output voltage gain; Step S122: Establish the maximum value of the fuel cell output voltage v cell_max and the minimum value v cell_min , bus voltage v bus The maximum output voltage of the converter is v omax and the minimum value v omin Relationship: Step S123: According to the relationship between steps S121 and S122, the maximum number of series connections n is calculated. max The optimal number of fuel cells in series is: Step S2: On the basis of step S1, a fuel flow rate and fuel cell system efficiency model is established to obtain an optimal efficiency curve of the fuel cell system. When the system operates on the optimal efficiency curve, the system efficiency is further improved. The steps for improving the efficiency of the fuel cell system include: S21. Establish the efficiency model of fuel flow rate and fuel cell system: the efficiency η of the fuel cell system system Transformed into fuel cell efficiency η FC and partial power conversion efficiency η global Relationship: or system =the global ·or FC Among them, the fuel cell efficiency η FC : Among them, I FC is the fuel stack output current, v FC is the fuel cell output voltage, E nern is the Nernst voltage, P aux is the auxiliary system loss, v FC and P aux : v FC =EV act -V con -V ohm , Where E is the Nernst voltage, V act is the activation polarization voltage, V con is the concentration polarization voltage, V ohm is the ohmic polarization voltage, k is the proportionality coefficient, Q is the flow rate in L / s, η is the fan efficiency, η N is the torque, P fix is the fixed loss; Step S3, formulate a power control strategy, through which the fuel cell power supply system is operated in the optimal efficiency region of the optimal efficiency curve of the fuel cell system in step S2, thereby further improving the system efficiency.

2. A method for improving the efficiency of a fuel cell power supply system according to claim 1, characterized in that: The step of improving the efficiency of the fuel cell system in step S2 also includes: S22. Based on the fuel flow rate and efficiency model of the fuel cell system established in step S21, draw a three-dimensional efficiency surface of the fuel cell system, find and extract the points in the three-dimensional efficiency surface with the output current as a constant, the fuel flow rate as an independent variable, and the maximum efficiency of the fuel cell system, mark each optimal flow rate point, and obtain the optimal efficiency curve of the fuel cell system. Adjust the fuel flow rate through the controller to make the system run on the efficiency curve, thereby improving the system efficiency.

3. A method for improving the efficiency of a fuel cell power supply system according to claim 2, characterized in that: In step S22, the three-dimensional efficiency surface of the fuel cell system is divided into a light load area, an economic operation area, and a maximum power area in sequence. The area where the fuel cell system efficiency is greater than or equal to 0.75 is the economic operation area, the area on the left side of the economic operation area where the fuel cell system efficiency is lower than 0.75 is the light load area, and the area on the right side of the economic operation area where the fuel cell system efficiency is lower than 0.75 is the maximum power area.

4. A method for improving the efficiency of a fuel cell power supply system according to claim 3, characterized in that: In step S3, based on the optimal efficiency curve of the fuel cell system obtained in step S2, a battery is used to adjust the output power of the fuel cell system so that it operates in the optimal efficiency area on the optimal efficiency curve of the fuel cell system, thereby further improving the system efficiency.

5. A method for improving the efficiency of a fuel cell power supply system according to claim 4, characterized in that: The economic operation area in the optimal efficiency curve of the fuel cell system in step S3 is divided into four control areas. The first control area is the light load area and the partial economic operation area. The second control area is the system optimal efficiency area. The third control area is a region slightly larger than the optimal efficiency area. The fourth control area is the maximum power area. SOFC_x The fuel cell power at the boundary of each area, the output power control principle of the battery-regulated fuel cell system is: 1) When the battery is not fully charged and the load power is less than P SOFC_2 When the fuel cell supplies power to the battery and the load at the same time, the fuel cell power does not exceed P SOFC_2 ; 2) When the battery SOC is lower than the minimum value and the load power is lower than P SOFC_3 , the fuel cell supplies power to the battery and the load at the same time, and the fuel cell power does not exceed P SOFC_3 ; 3) When the battery still has residual discharge power and the load power is greater than P SOFC_2 , the battery and fuel cell supply power to the load at the same time, and the fuel cell power cannot be lower than P SOFC_2 .

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