KR20070025834A - Hybrid power supply system of fuel cell and battery with linear voltage profile and operating method - Google Patents

Abstract

A hybrid power supply system of a fuel cell and a battery having a linear voltage profile and a driving method thereof are provided to assemble the battery and the fuel cell simply and smoothly by using a section similar to an output voltage profile of the fuel cell in a charging/discharging voltage profile of the battery. A fuel cell(FC) generates electric energy by reacting a fuel and an oxidizing agent electrochemically. A battery is recharged, and supports the output of the fuel cell. A power conversion device receives at least one of a first output of the fuel cell and a second output of the battery, and supplies power to an internal or external electric load. A control device controls the charging/discharging operation of the battery, in order for a charging/discharging voltage of the battery to be located in a linear charging/discharging voltage section.

Description

Hybrid power supply system of fuel cell and battery with linear voltage profile and operating method}

1 is a graph showing a conventional fuel cell, a supercapacitor and a charge / discharge curve of a battery.

2 is a graph illustrating the operating principle of a hybrid power supply system according to a preferred embodiment of the present invention.

3A is a view for explaining a material that can be used as a cathode active material of a battery in a hybrid power supply system according to an embodiment of the present invention.

3B is a graph illustrating a charging profile of a battery coupled to a hybrid power supply system according to an embodiment of the present invention.

3C is a graph illustrating a discharge profile of another battery coupled to a hybrid power supply system according to an embodiment of the present invention.

4 is a diagram illustrating a hybrid power supply system according to a first embodiment of the present invention.

5 is a flowchart illustrating a method of driving a hybrid power supply system according to a second embodiment of the present invention.

Explanation of symbols on the main parts of the drawings

110: fuel cell 120: BOP (balance of plant)

130: battery 140: battery charge control device

150: power converter 160: controller

The present invention relates to a power supply system using a fuel cell, and more particularly, to a hybrid power supply system and a method for driving the same, which can be easily and smoothly combined with a fuel cell using a battery having a linear voltage profile. It is about.

Fuel cells are a type of power generation system that generates electrical energy by electrochemically reacting oxidants (oxygen) with fuels such as hydrogen, natural gas, methanol, coal, petroleum, biomass gas, and landfill gas. A fuel cell is a pollution-free power generator that has high energy efficiency and does not emit NOx, SOx, etc. Depending on the type of electrolyte used, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer It is classified into an electrolyte fuel cell, an alkaline fuel cell and the like. Each of these fuel cells operates basically on the same principle, but differs in the type of fuel used, operating temperature, catalyst, and electrolyte.

Polymer electrolyte membrane fuel cells (PEMFCs) have significantly higher output characteristics, lower operating temperatures and faster start-up and response characteristics than other fuel cells, and are portable such as portable electronic devices. A wide range of applications, such as power supplies for transportation such as power sources or automotive power sources, as well as distributed power supplies such as stationary power plants for houses and public buildings.

The fuel cell system described above continuously produces electrical energy while fuel is supplied. The energy efficiency of most fuel cell systems decreases significantly during low power periods. That is, for example, an auxiliary device such as a pump for supplying fuel and air to the fuel cell consumes a certain amount of electrical energy of the fuel cell, and thus the power of the auxiliary device relative to the total power generated in the fuel cell during a low output period. This is because the consumption rate is increased.

In addition, fuel cell systems generally use a combination of supercapacitors or batteries, such as an electric double layer capacitor (EDLC), due to their lack of output characteristics for applications requiring large power. In other words, when an electrical transient occurs such as a sudden load change, the fuel cell system may not respond quickly due to a limit condition such as fuel supply, and thus, its output characteristics may be remarkably changed. Therefore, the existing hybrid power supply system using a fuel cell is configured to compensate for the occurrence of an electrical transient on the system output side by combining an auxiliary power supply such as a battery or a supercapacitor.

Charge and discharge curves of the above-described fuel cell, supercapacitor and battery are shown in FIG. 1. As shown in FIG. 1, the supercapacitor E has a small energy capacity but has a large output, and the charge / discharge voltage profile is similar to the output voltage profile of the fuel cell FC, thereby for power backup of the fuel cell system. It is used a lot. For example, power supplies, such as supercapacitors, are being used to meet energy efficiency and improve energy efficiency during periods of reduced efficiency of fuel cell systems, ie, during low output periods. This is because supercapacitors have high power density and high charge and discharge efficiency.

On the other hand, the battery B has a very large energy capacity per weight compared to the supercapacitor E, but the charge / discharge voltage profile is different from the output voltage profile of the fuel cell FC. Therefore, in a hybrid system of a fuel cell and a battery, an expensive controller for smoothly operating the fuel cell FC and the battery B should be used. As such, when a battery is used as an auxiliary power supply in a hybrid power supply system using a fuel cell, a smooth combination of the battery and the fuel cell is used due to the charge / discharge characteristics of the battery having a flat section or a plateau. This is difficult. In other words, the conventional hybrid power supply system that requires a combination of a fuel cell and a battery has a disadvantage of using an expensive controller together.

The present invention has been derived in view of the above-described problems, and an object of the present invention is to simply and smoothly implement a combination of a battery and a fuel cell by using a section similar to the output voltage profile of the fuel cell in the charge / discharge voltage profile of the battery. To provide a hybrid power supply system that can be.

Another object of the present invention is to eliminate the need for an expensive controller in a hybrid power supply system of a fuel cell and a battery, and to maximize the efficiency of a fuel cell system by using a battery having a very large energy capacity compared to a supercapacitor. To provide a hybrid power supply system.

In order to achieve the above object, according to a first embodiment of the present invention, a fuel cell that generates electrical energy by electrochemically reacting a fuel and an oxidant, a rechargeable battery and a battery assisting the output of the fuel cell, A power converter that receives at least one of the first output and the second output of the battery and supplies power to an internal or external electrical load, and a controller that controls the battery so that the charge / discharge voltage of the battery is present in the linear charge / discharge voltage section There is provided a hybrid power supply system comprising a.

Preferably, the controller senses the voltage of the battery and controls the battery charging so that the battery is charged to the maximum voltage of the linear charge / discharge voltage section. If the detected battery voltage is equal to or greater than the maximum voltage, the controller stops charging the battery. The control device may further include a sensing unit for sensing a voltage of the battery, a comparator for comparing the sensed signal with a reference signal, and a control signal for enabling or disabling the charging of the battery according to the signal level output from the comparator. It includes a control signal generator for generating a. Here, the reference signal is a signal corresponding to the maximum voltage of the linear charge / discharge voltage section.

The battery also has a plurality of cells that are electrically connected in series.

In addition, the above-described hybrid power supply system is controlled by a control device, and further includes a battery charge control device for charging the battery using the output of the fuel cell or the external commercial power source.

In addition, the battery has at least two linear charge and discharge voltage intervals. The controller controls charging / discharging of the battery such that the charging / discharging voltage of the battery is located in at least two linear charging / discharging voltage sections by using at least two functions.

According to a second embodiment of the present invention, a method of driving a hybrid power supply system between a battery and a fuel cell, the method comprising: setting a linear charge / discharge section of a battery and a maximum voltage of the section, and detecting a battery voltage; And, if the sensed battery voltage is less than the maximum voltage, charging the battery, and if the sensed battery voltage is greater than or equal to the maximum voltage, stopping the charging of the battery.

Preferably, the above-described hybrid power supply system sets a minimum voltage of a linear charge / discharge section of a battery, detects an output voltage and an output current of a fuel cell, and detects a sudden decrease in output power of the fuel cell. Discharging the battery, and discharging the battery if the sensed battery voltage is below the minimum voltage.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following description, when a part is combined with another part, this includes not only a case where the part is directly coupled, but also a case where other elements are interposed therebetween. In addition, in the following description, fuel means water and oxygen as fuel of a wide range other than narrow-line fuel containing hydrogen, such as methanol and natural gas. However, the fuel described below is defined as the fuel of consultation. In the drawings, the thickness or size of each element is exaggerated for clarity and convenience of explanation. In the drawings, like reference numerals refer to similar or identical elements.

2 is a graph illustrating the operating principle of a hybrid power supply system according to a preferred embodiment of the present invention.

Referring to FIG. 2, in the hybrid power supply system, the charge / discharge voltage of the battery B is similar to the output curve of the fuel cell FC among the charge / discharge voltage-current curves of the battery B, for example, the A and B sections. By controlling the combination of the fuel cell and the battery to achieve a simple and smooth. Rechargeable batteries have upper and lower limits of battery voltage for smooth reversible charging and discharging. The upper and lower limits of the battery voltage are approximately 80% and 20% of the full capacity of the battery. Suitable batteries for this embodiment include a lithium ion secondary battery.

Lithium ion secondary battery is a kind of lithium secondary battery that uses lithium ion (Li +) as intercalation and deintercalation without using lithium metal as a negative electrode active material, LiCoO ₂ / Carbon structure Examples thereof include lithium ion secondary batteries and lithium ion secondary batteries using LiMn ₂ O ₄ as a positive electrode. Each cell of such a lithium ion secondary battery has a very high potential of 4.2V, which can be used very efficiently by connecting several in series and / or in parallel. For example, if a sufficient number of cells are connected, a voltage of up to 400V can be obtained.

In addition, the output voltage of a multi-cell rechargeable battery of a lithium ion secondary battery depends on the number of cells connected in series and the specific chemical selected for the cell. In addition, since the lithium ion secondary battery does not have a memory effect phenomenon that occurs in nickel-containing batteries (Ni-Cd and Ni-Mh), even if it is frequently charged at any time, almost does not affect the life.

In addition, a battery that can be employed in the present embodiment includes all lithium ion secondary batteries having a linear discharge voltage profile in a specific section. Therefore, in the present invention, a battery may be manufactured and used to have a linear profile in a desired discharge voltage section. For example, when a variety of active materials having a plateau in a particular section is properly mixed, the plate section of the produced battery is changed according to the amount of use. In the present invention, a desired voltage section is used by mixing various active materials using this principle. A battery having a linear discharge voltage profile can be manufactured and used.

As such, in the present invention, a simple and smooth combination with the fuel cell is achieved by using the linear charge / discharge section of the battery.

3A is a view for explaining a material that can be used as a cathode active material of a battery in a hybrid power supply system according to a preferred embodiment of the present invention.

As shown in FIG. 3A, a battery employed in the hybrid power supply system of the present invention may be manufactured using various anode oxides that may form electrodes of the battery. For example, LiF, LiMiVO ₄ , Li _{1 - x} NiO ₂ , Li _{1 - x} CoO ₂ , Li _{1 - x} Mn ₂ O ₄ , Li _x MnO ₄ , Li _x MoS ₂ , Li _x TiS ₂ , Li _x MoO ₂ , By properly combining materials such as Li _x WO ₃ , Li _x Coke, and Li _x Graphite, a battery exhibiting a linear voltage profile in a desired charge / discharge period may be implemented. Of course, a material such as Li _x Ni _y Co _z Al _(1-yz) O ₂ may be used in addition to the above material, but is not limited thereto.

In the above-described case, graphite starting from soft carbon or hard carbon may be used for the negative electrode. Here, soft carbon refers to a carbonaceous material which can be converted into graphite by heat treatment, and is also referred to as digraphitizable carbon. Hard carbon refers to a carbonaceous material which, for example, continues to be in a state in which crystallographic orientation is scattered even after heat treatment at 3000 ° C., and does not change into graphite, also called non-graphitizable carbon. In the battery using hard carbon as a negative electrode, since the voltage drops slowly at the same time as the discharge, a relatively linear discharge voltage curve section can be easily obtained.

For example, _{Li x Ni 1/3 Co 1} /3 Al 1/3 O 2 and LiMn a ₂ O ₄ 5: The mixture was combined with 5 as the positive electrode active material of the battery, as shown in Figure 3b, the high A battery with a wide charge range and a linear range can be obtained. In the present embodiment, 90% of the active material, 5% of the conductive material, and 5% of the binder were used for the positive electrode active material. 95% natural graphite and 5% binder were used as the negative electrode active material. Electrolyte was used EC: DEC 1: 1, 1.3M LiPF6. The capacity is 1000㎃h class and is charged at a rate of about 2C ~ 10C. The discharge profile is substantially the same as the charge profile. As can be seen in Figure 3b, the manufactured battery can exhibit a nearly linear discharge profile in the interval of about 4.2V to 3.5V at a discharge rate of 2C.

For example, as illustrated in FIG. 3C, a battery was manufactured using a mixture of LiNiCoAlO ₂ and LiMn ₂ O ₄ at 4: 6 as a cathode active material of the battery. In this embodiment, the discharge rate of the manufactured battery was 10C and the cutoff voltage was 2.5V. In addition, the manufactured battery exhibited a nearly linear discharge profile section in a voltage section of about 3.6V to 3.0V and a voltage section of about 3.0V to 2.5V. This indicates that the present invention can be implemented in combination with a fuel cell using a battery having at least two linear sections. In other words, if the control device applies different ratios or functions in the two sections, respectively, in the predetermined voltage range from the specific voltage to the specific voltage range of the battery, and in the predetermined voltage range below the specific voltage, the fuel cell and the battery, respectively. The combination with is easy to implement. In this case, since the battery uses a wide range of charge and discharge profiles, the battery can be used more efficiently.

As described above, in the present invention, since the charge / discharge linear voltage section of the battery is used, output control can be easily performed by combining the battery with the fuel cell. In this case, the charging and discharging of the battery is controlled by checking the charging voltage of the battery such that the battery is waiting to be discharged in a linear section. Therefore, according to the present invention, the control circuit can be simplified, a low-cost controller can be used, and a hybrid power supply device having a simple structure combining a fuel cell and a battery can be easily implemented.

4 is a diagram illustrating a hybrid power supply system according to a first embodiment of the present invention.

Referring to FIG. 4, the hybrid power supply system can maximize system efficiency by simply and smoothly coupling a battery having a very large energy capacity to a fuel cell. To this end, the hybrid power supply system includes a fuel cell 110, a fuel cell support unit (balance of plant, BOP, 120), a battery 130, a battery charge controller 140, a power converter 150, and a controller ( 160).

Specifically, the fuel cell 110 is any type of fuel cell that generates electrical energy by electrochemically reacting an oxidant (oxygen) with fuel such as hydrogen, natural gas, methanol, coal, petroleum, biomass gas, landfill gas, and the like. It includes. In particular, in this embodiment, any one of a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid oxide fuel cell, a polymer electrolyte fuel cell, an alkaline fuel cell, and a direct methanol fuel cell can be used.

The fuel cell support unit 120 is a pump, a valve, a fan, or the like for supplying fuel, air, water, etc. to the fuel cell 110, and a reaction temperature and operation of the reformer (not shown) and the fuel cell 110. A device such as a heat exchanger for adjusting the temperature and the like. In addition, the fuel cell support unit 120 is a circulation tank (not shown) for circulating the fuel and water to reuse the fuel and water from the device for improving the operating efficiency of the fuel cell 110, for example, the fuel cell 110. And the like.

The battery 130 has a linear discharge voltage profile in at least a predetermined period and includes a secondary battery that can be recharged by the fuel cell 110 or an external commercial power source. As the battery 130 described above, a lithium ion secondary battery is suitable.

Further, the battery 130 has a structure in which a plurality of unit cells or cells B1, B2, B3, B4, ..., Bn are connected in series. In this structure, when the battery 130 is charged or recharged, the current source is connected through all cells connected in series. Since the cells may react differently to the charging current when the battery is charged, the battery 130 is coupled to the battery charge control device 140 that can independently control the charging and discharging of each cell.

The battery charge controller 140 controls each cell B1, B2, B3, B4,..., Bn in order to prevent the overcharged specific cell from reacting with another cell and damaging the cell during the battery 130 charging cycle. It includes a plurality of bypass switching means (S1, S2, S3, S4, ..., Sn) coupled in parallel to bypass the overcurrent applied to a specific cell.

The power converter 150 supplies a constant voltage to the battery 130 regardless of whether the on-off states of the plurality of bypass switching means S1, S2, S3, S4,..., Sn are changed. As the power converter 150, a conventional constant voltage regulator may be used.

In addition, the power converter 150 supplies power of the fuel cell 110 and / or the battery 130 to an external electrical load 200, the fuel cell support unit 120, the controller 160, and the like. . In this case, the power converter 150 may include a means for converting the output of the fuel cell 110 and / or the battery 130 according to the device using the power. Such means include, for example, existing DC-DC converters, DC-AC converters, and the like.

In addition, the power converter 150 may include another means for converting power supplied to the battery 130 when the battery 130 is charged, for example, another existing DC-DC converter. Here, the power supplied to the battery 130 may be an output of the fuel cell 110 or an external commercial power source (not shown).

The controller 160 includes circuitry for monitoring each cell B1, B2, B3, B4,..., Bn in the battery 130 to protect it from being overcharged. For example, the controller 160 includes a voltage detector 162, an A / D converter 164, a comparator 166, and a control signal generator 168.

The voltage detector 162 includes a plurality of pairs of input terminals a1, a2; b1, b2; c1, c2; d1 connected to both ends of the cells B1, B2, B3, B4, ..., Bn in the battery 130. , d2; ...; n1, n2). When these input terminals and the bypass switching means S1, S2, S3, S4, ..., Sn of the battery charge control device 140 are properly combined, not only the voltage between the cells of each cell but also the voltage between the batteries 130 as a whole. Etc. can be detected.

The A / D converter 164 converts the voltage detected by the voltage detector 162 into a digital signal. That is, the A / D converter 164 converts the detected analog voltage into a digital form so that the controller 160 can detect the detected analog voltage. The A / D converter 164 together with the voltage detector 162 constitutes a detector for sensing a voltage of the battery 130.

The comparator 166 compares the digital signal input from the sensor with the reference signal and transmits a larger or smaller value among the compared values to the control signal generator 168 according to the comparison result. Here, the reference signal is a signal having at least one of a maximum reference value corresponding to the maximum voltage of the linear discharge voltage section of the battery 130 and a minimum reference value corresponding to the lowest voltage of the linear discharge voltage section. The comparator 166 may be simply implemented using, for example, at least one operational amplifier.

The control signal generator 168 generates a control signal for charging or stopping charging of the battery 130 according to the signal level output from the comparator 166.

By the above-described configuration, the control device 160 senses the output voltage and the output current of the fuel cell 110 through the power converter 150, and in the transient state, that is, the fuel cell 110 is more than can afford When power is required and the output of the fuel cell 110 rapidly decreases, power conversion is performed so that power can be normally supplied to the load 200 by combining the output of the fuel cell 110 and the output of the battery 130 together. Control device 150.

On the other hand, the control device 160 may receive a predetermined detection signal from the sensor coupled to the fuel cell 110 and the fuel cell support unit 120, and may control the fuel cell 110 and the fuel cell support unit 120. . In addition, the controller 160 may communicate with the external electrical load 200 in a wired or wireless manner to control the system. In addition, the controller 160 may be configured as a microcomputer in the form of a chip, and may be coupled to a memory (not shown) in which a program for system control and linear charge / discharge voltage control of a battery is stored. In this case, the memory may be embedded in the controller 160 or coupled through a separate port.

Although the hybrid power supply system according to the present embodiment described above has a very large energy capacity, it is possible to simply and smoothly combine the battery with the fuel cell, which is difficult to combine because the charge / discharge profile is different from the output profile of the fuel cell. Therefore, it is possible to maximize the efficiency of the fuel cell system having a battery, that is, a hybrid power supply system.

5 is a flowchart illustrating a method of driving a hybrid power supply system according to a second embodiment of the present invention.

Referring to FIG. 5, a method of driving a hybrid power supply system includes the following processes. First, a linear charge / discharge section of a battery and a maximum voltage Vmax and a minimum voltage Vmin of the battery are set in the controller (302).

Next, the controller detects the charging voltage Vb of the battery (304). In operation 306, it is determined whether the detected charging voltage is smaller than the maximum voltage. As a result of the determination, if the charging voltage is smaller than the maximum voltage, the battery is charged (308). As a result of the determination, if the charging voltage is not smaller than the maximum voltage, the battery proceeds to the next step without charging the battery.

Next, the control device determines whether the detected charging voltage is greater than or equal to the maximum voltage (310). As a result of the determination, if the charging voltage is greater than or equal to the maximum voltage, the battery charging is stopped (312). As a result of the determination, if the charging voltage is not greater than or equal to the maximum voltage, the charging voltage of the battery is continuously monitored.

Through the above-described process, the control device controls the charging and discharging of the battery so that the battery can wait in the linear charging and discharging section. As such, according to the present invention, by configuring the battery to be combined with the fuel cell by using the linear charge / discharge section of the battery, it is possible to simply and smoothly combine the battery having a larger energy capacity than the supercapacitor with the fuel cell. The following steps show useful applications of the present system.

Next, the controller detects the output voltage and the output current of the fuel cell while the battery is controlled to be charged to the maximum voltage of the linear charge / discharge section (314).

Next, the control device determines whether a transient state indicating a rapid decrease in the output power has occurred in the change of the output voltage and the output current which are continuously sensed (316). As a result of the determination, in the transient state, the control device discharges the battery to compensate for the power required in the transient state (318). If the result of the determination is not a transient state, the process ends.

On the other hand, after the step 318 of discharging the battery, the control device detects the charging voltage Vb of the battery again, and determines whether the detected charging voltage is less than or equal to the minimum voltage Vmin of the linear charging / discharging period of the battery. (320). As a result of the determination, when the charging voltage is lower than or equal to the minimum voltage, battery discharge is stopped (322). As a result of the determination, if the charging voltage is not below the minimum voltage, the process is terminated. According to the above-described configuration, the hybrid power supply system can respond quickly to the demands of the load by an appropriate combination of outputs of the fuel cell and the battery when a transient condition occurs.

As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various deformation | transformation by a person of ordinary skill in the art within the scope of the technical idea of this invention is carried out. This is possible.

According to the above-described configuration, by combining the battery and the fuel cell using the linear charge / discharge section of the battery, a hybrid power supply system of the battery and the fuel cell can be configured simply and smoothly. In addition, since the battery and the fuel cell having a very large energy density per weight compared to the supercapacitor can be easily combined, the efficiency of the hybrid power supply system can be maximized. In addition, by using the linear charge / discharge section of the battery, it is not necessary to use an expensive controller in a smooth combination with the fuel cell, thereby contributing to the miniaturization of the system.

Claims (11)

A fuel cell that generates electrical energy by electrochemically reacting a fuel and an oxidant; A battery that is rechargeable and assists in outputting the fuel cell; A power converter receiving at least one of the first output of the fuel cell and the second output of the battery and supplying power to an internal or external electrical load; And And a controller for controlling charging and discharging of the battery such that the charging and discharging voltage of the battery is located in a linear charging and discharging voltage section. The method of claim 1, The control device detects a voltage of the battery, and controls the battery charging so that the battery is charged to the maximum voltage of the linear charge-discharge voltage section. The method of claim 2, And the control device stops charging of the battery when the sensed battery voltage is greater than or equal to the maximum voltage. The method of claim 1, The battery has a hybrid power supply system having at least two linear charge and discharge voltage intervals. The method of claim 4, wherein And the control device controls charging / discharging of the battery such that the charging / discharging voltage of the battery is located in the at least two linear charging / discharging voltage sections using at least two functions. The method of claim 1, The controller may enable or disable the charging of the battery according to a sensing unit sensing a voltage of the battery, a comparator comparing the sensed signal with a reference signal, and a signal level output from the comparator. Hybrid power supply system comprising a control signal generator for generating a control signal for. The method of claim 6, And the reference signal is a signal corresponding to a maximum voltage of the linear charge / discharge voltage section. The method of claim 6, And the battery has a plurality of cells electrically connected in series. The method of claim 8, And a battery charge control device which is controlled by the controller and charges the battery using the fuel cell or an external commercial power source. In a method of driving a hybrid power supply system between a battery and a fuel cell, Setting a linear charge / discharge section of the battery and a maximum voltage of the section; Sensing the battery voltage; If the sensed battery voltage is less than the maximum voltage, charging the battery; And Stopping the battery charging if the sensed battery voltage is greater than or equal to the maximum voltage. The method of claim 10, Setting a minimum voltage of the linear charge / discharge section of the battery; Sensing an output voltage and an output current of the fuel cell; Detecting a sudden decrease in output power of the fuel cell; Discharging the battery; And And stopping the battery discharge if the detected battery voltage is less than or equal to the minimum voltage.

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