Solar cell system

Abstract

To provide an on-vehicle solar cell system capable of simplifying a charging circuit that charges a battery with electric power from solar cells and of reducing a generation power loss and cost.SOLUTION: A solar cell system comprises: solar cells; a low-voltage battery; a high-voltage battery that has a voltage value higher than a voltage value of the low-voltage battery; a power conversion device; and a control device that controls an operation of the power conversion device. An open-end voltage of the solar cells is equal to or less than an allowable upper limit voltage of the low-voltage battery. The solar cells and the low-voltage battery are connected with each other directly, and the low-voltage battery and the high-voltage battery are connected with each other via the power conversion device. The power conversion device is a bi-directional DC / DC converter. The control device controls the power conversion device on the basis of the voltage value of the low-voltage battery. Thus, a charging circuit is simplified, and a power loss between the solar cells and the low-voltage battery is inhibited.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a solar cell system, and more particularly to an in-vehicle solar cell system used in a vehicle equipped with a high-voltage battery for driving and a low-voltage battery for electronic devices.

Background Art

[0002] The generated power of a solar cell is supplied to a high-voltage battery for driving an electric vehicle motor or a low-voltage battery (auxiliary battery) for in-vehicle electronic devices to charge these batteries.

[0003] Since the generated power of a solar cell varies according to the solar radiation amount and the optimum operating point voltage with good power generation efficiency changes, in order to change the charging destination to a battery with a voltage value with good power generation efficiency according to the optimum operating point voltage at that time, a control device for controlling the charge and discharge of the battery is required, and this control device usually consumes the power of the low-voltage battery.

[0004] Furthermore, the low-voltage battery also supplies power to other electronic devices, etc. If the generated power of the solar cell is directly charged to the high-voltage battery, the voltage of the low-voltage battery may drop too much.

[0005] Patent Document 1 discloses an in-vehicle charging control device that steps down the generated power of a solar cell and charges a low-voltage battery, and when the power amount of this low-voltage battery reaches or exceeds a specified amount, steps up the voltage of the low-voltage battery and charges the high-voltage battery. According to this in-vehicle charging control device, it is possible to prevent the power amount of the low-voltage battery from dropping too much.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in the in-vehicle charging control device described in Patent Document 1, converters are required between the solar cell and the low-voltage battery, and between the low-voltage battery and the high-voltage battery, making cost reduction difficult.

[0008] In addition, charging a high-voltage battery with electricity generated by solar panels involves two stages of power conversion losses.

[0009] This invention has been made in view of the problems of the prior art, and its objective is to provide an in-vehicle solar cell system that simplifies the charging circuit for charging a battery from a solar cell, thereby reducing power loss and lowering costs. [Means for solving the problem]

[0010] The inventors, after diligent research to achieve the above objective, discovered that the objective can be achieved by making the open-circuit voltage of the solar cell smaller than the allowable upper limit voltage of the low-voltage battery, directly connecting the solar cell and the low-voltage battery, and controlling the charging and discharging of these batteries using a bidirectional DC / DC converter installed between the low-voltage battery and the high-voltage battery based on the voltage value of the low-voltage battery. This led to the completion of the present invention.

[0011] In other words, the solar cell system of the present invention comprises a solar cell, a low-voltage battery, a high-voltage battery with a voltage value higher than that of the low-voltage battery, a power conversion device, and a control device for controlling the operation of the power conversion device. Furthermore, the open-circuit voltage of the solar cell is less than or equal to the allowable upper limit voltage of the low-voltage battery, the solar cell and the low-voltage battery are directly connected, the low-voltage battery and the high-voltage battery are connected via the power conversion device, the power conversion device is a bidirectional DC / DC converter, and the control device controls the power conversion device based on the voltage value of the low-voltage battery. [Effects of the Invention]

[0012] According to the present invention, the open-circuit voltage of the solar cell is made smaller than the allowable upper limit voltage of the low-voltage battery, and the solar cell and the low-voltage battery are directly connected. Based on the voltage value of the low-voltage battery, the charging and discharging of these batteries are controlled by a bidirectional DC / DC converter installed between the low-voltage battery and the high-voltage battery. As a result, the charging circuit is simplified, and power loss between the solar cell and the low-voltage battery is prevented, providing an in-vehicle solar cell system. [Brief explanation of the drawing]

[0013] [Figure 1] This is a system configuration diagram showing an example of the configuration of the solar cell system of the present invention. [Figure 2] This graph shows the relationship between the temperature and solar radiation of a solar cell and the optimal operating point voltage of the solar cell. [Figure 3] This diagram shows the relationship between the internal voltage, external voltage, internal resistance, and charging current of a low-voltage battery. [Figure 4] This graph shows an example of the changes in the internal and external voltages of a low-voltage battery when switching it from a charged state to a discharged state. [Figure 5] This graph shows an example of the changes in the internal and external voltages of a low-voltage battery when switching it from a discharged state to a charged state. [Figure 6] This flowchart illustrates an example of the operation of the control device for the solar cell system of the present invention. [Modes for carrying out the invention]

[0014] The solar cell system of the present invention will be described in detail. As shown in Figure 1, the solar cell system of the present invention comprises a solar cell, a low-voltage battery, a high-voltage battery, a power conversion device, and a control device.

[0015] The low-voltage battery is a battery that supplies power to in-vehicle electronic devices and the like. The high-voltage battery is a battery having a voltage value higher than the voltage value of the low-voltage battery, and supplies power to a driving motor and the like. The solar cell and the low-voltage battery are directly connected, and the low-voltage battery and the high-voltage battery are connected via the power conversion device.

[0016] In the solar cell system of the present invention, since the open-circuit voltage of the solar cell is not higher than the allowable upper limit voltage of the low-voltage battery, it is possible to directly charge the low-voltage battery without stepping down the generated power of the solar cell. Therefore, a converter between the solar cell and the low-voltage battery is unnecessary, the charging circuit can be simplified and the cost can be reduced, and power loss due to the converter can be prevented.

[0017] The lower limit value of the open-circuit voltage of the solar cell is not particularly limited as long as it has a voltage capable of charging the low-voltage battery, but a higher value is preferable from the viewpoint of charging efficiency.

[0018] [[ID=II]]In the present invention, the "allowable upper limit voltage" is the upper limit voltage set in advance to prevent battery failure due to overcharging, and refers to the voltage when the battery is 100% charged. The "allowable lower limit voltage" refers to the voltage set in advance to prevent battery deterioration due to over-discharge, and these are stored in the control device.

[0019] Preferably, the solar cell has its generated output voltage fluctuation suppressed by the solar radiation amount. When a part of the solar cell is shaded, in the case of a solar cell in which solar cells or solar cell modules are connected in series, the generated output voltage will decrease. By configuring the solar cell such that solar cells or solar cell modules are mainly connected in parallel, the fluctuation of the generated output voltage can be suppressed, and the generated voltage value of the solar cell can be controlled to the optimum operating voltage according to the voltage value of the low-voltage battery.

[0020] Furthermore, since the power converter installed between the low-voltage battery and the high-voltage battery is a bidirectional DC / DC converter, it is possible to charge the low-voltage battery and the high-voltage battery mutually. The control device controls the power converter based on the voltage value of the low-voltage battery, and mutually supplies power between the low-voltage battery and the high-voltage battery.

[0021] Therefore, in addition to supplying power from the low-voltage battery to the high-voltage battery to charge it, when the solar panel is not supplying power and the voltage of the low-voltage battery drops, power can be supplied from the high-voltage battery to the low-voltage battery to charge it. This prevents the low-voltage battery from being over-discharged and dropping too low in voltage, thereby suppressing the degradation of the low-voltage battery.

[0022] The control device sets a reference voltage value for controlling charging between the low-voltage battery and the high-voltage battery according to the optimal operating point voltage of the solar cell, compares the voltage value of the low-voltage battery with the set reference voltage value, controls the bidirectional DC / DC converter, and supplies power to the low-voltage battery and the high-voltage battery mutually.

[0023] Specifically, the control device obtains the optimal operating point voltage of the solar cell and sets a reference voltage value for controlling the voltage of the low-voltage battery that simultaneously satisfies the relationships between equations (1), (2), and (3) below, within the range of voltage values ​​in which the low-voltage battery operates normally, which has been stored in advance. (Upper limit of first charging voltage - Optimal operating point voltage) × 2 < (Optimal operating point voltage - Lower limit of first discharge voltage) ... Equation (1) Allowable upper voltage value ≥ First charging voltage upper limit > First discharge voltage lower limit > Second discharge voltage lower limit ≥ Allowable lower voltage value ... Equation (2) Allowable upper voltage limit ≥ First charging voltage upper limit > Second charging voltage upper limit > Second discharge voltage lower limit ≥ Allowable lower voltage limit ...Equation (3) However, in the above formulas (1) to (3), The permissible upper voltage limit is the maximum voltage value at which a low-voltage battery will not be overcharged, and the permissible lower voltage limit is the minimum voltage value at which a low-voltage battery will not be over-discharged. Furthermore, the first charging voltage upper limit is the upper voltage value when charging the low-voltage battery from the solar cell, and the first discharge voltage lower limit is the lower voltage value when discharging the low-voltage battery from the solar cell. The second charging voltage upper limit is the upper voltage value when charging the low-voltage battery when no power is being supplied from the solar panel to the low-voltage battery, and the second discharge voltage lower limit is the lower voltage value when discharging the low-voltage battery when no power is being supplied from the solar panel to the low-voltage battery.

[0024] In the solar cell system of the present invention, the solar cell and the low-voltage battery are directly connected, and the voltages of the solar cell and the low-voltage battery are the same. Therefore, it is necessary to consider not only the power generation efficiency of the solar cell but also the charging efficiency of the low-voltage battery.

[0025] In such a solar cell system, as the voltage between the solar cell and the low-voltage battery deviates from the optimal operating point voltage, the efficiency decreases more rapidly on the higher voltage side than on the lower voltage side. Therefore, the operating range during charging from the solar cell to the low-voltage battery is set to be wider on the lower voltage side.

[0026] By setting a first charging voltage upper limit and a first discharge voltage lower limit that satisfy the above formula (1), and charging the low-voltage battery from the solar cell within these limits, it is possible to achieve both the power generation efficiency of the solar cell and the charging efficiency from the solar cell to the low-voltage battery, enabling highly efficient charging.

[0027] Then, when the voltage value of the low-voltage battery exceeds the first upper limit of the charging voltage, power is supplied from the low-voltage battery to the high-voltage battery, and when it falls below the first lower limit of the discharge voltage, the power supply from the low-voltage battery to the high-voltage battery is stopped. This prevents the low-voltage battery from being overcharged by the power generated by the solar panels.

[0028] Furthermore, when the second discharge voltage falls below the lower limit, power is supplied from the high-voltage battery to the low-voltage battery, and when the second charge voltage exceeds the upper limit, power supply from the high-voltage battery to the low-voltage battery is stopped. This prevents the voltage of the low-voltage battery from dropping too low, thus suppressing its degradation.

[0029] As shown in Figure 2, the optimal operating point voltage changes depending on the temperature and solar irradiance of the solar cell. The optimal operating point voltage can be obtained from the relationship between the temperature and solar irradiance of the solar cell (as shown in Figure 2) and the optimal operating point voltage, which is stored in the control device beforehand, and from the temperature and solar irradiance obtained from temperature sensors and solar irradiance sensors installed on the solar cell.

[0030] If the temperature of the solar cell cannot be obtained, the optimal operating voltage may be set as the optimal operating voltage, which is determined from the relationship between the solar radiation at the highest solar cell temperature and the optimal operating voltage among the pre-set relationships between solar cell temperature and solar radiation.

[0031] The control device described above preferably controls the power passed through the bidirectional DC / DC converter when supplying power from a low-voltage battery to a high-voltage battery to 10% or more of the maximum power that the bidirectional DC / DC converter can use in continuous operation (hereinafter referred to as "rated power").

[0032] In a bidirectional DC / DC converter, the conversion efficiency decreases as the transmitted power decreases. Therefore, it is preferable to set the transmitted power when supplying power from a low-voltage battery to a high-voltage battery to its rated power. By controlling the transmitted power to 10% or more of the rated power, losses associated with power conversion can be reduced. The above rated power is stored in the control device in advance as the upper limit of the power that passes through the bidirectional DC / DC converter.

[0033] Furthermore, the power passed through the bidirectional DC / DC converter when supplying power from the high-voltage battery to the low-voltage battery may be set to the rated power of the converter, or it may be controlled so that the internal voltage of the low-voltage battery, as described later, becomes the upper limit of the second charging voltage.

[0034] The control device described above may use the external voltage (voltage between terminals) of the low-voltage battery as the voltage value of the low-voltage battery, but it is preferable to use the internal voltage as the voltage value of the low-voltage battery.

[0035] Figure 3 shows the relationship between the internal voltage, external voltage, internal resistance, and charging current of a low-voltage battery. As shown in Figure 3, because there is a resistance component inside the battery, the value of the external voltage fluctuates due to the influence of the current when charging or discharging a low-voltage battery.

[0036] In other words, during charging, the external voltage is higher than the internal voltage, and during discharge, the external voltage is lower than the internal voltage. Therefore, as shown in Figures 4 and 5, switching the power supply direction between the low-voltage and high-voltage batteries causes the external voltage to fluctuate, and this fluctuation results in the external voltage showing a voltage value that supplies power in the opposite direction to the power supply direction that was just switched.

[0037] By having the control device control the power converter based on the internal voltage of the low-voltage battery, the influence of fluctuations in external voltage due to charging and discharging currents is eliminated, and frequent switching of the power converter can be prevented.

[0038] The internal voltage of a low-voltage battery can be calculated using the following formula (4) from the measured external voltage of the low-voltage battery, the charging current of the low-voltage battery, and the preset internal resistance of the low-voltage battery. Internal voltage = External voltage - Internal resistance × Charging current ... Equation (4)

[0039] Furthermore, unnecessary switching control of the power converter can be prevented by setting a first discharge voltage lower limit and a second charge voltage upper limit, taking into account the voltage values ​​that fluctuate due to charging and discharging.

[0040] When switching a low-voltage battery from a charging state to a discharging state, the voltage drops as shown in Figure 4. Therefore, the first discharge voltage lower limit should be set lower than the voltage drop caused by the effects of charging and discharging.

[0041] Specifically, by setting the first discharge voltage lower limit to satisfy the following equation (5), unnecessary switching control of the power converter can be prevented. Lower limit of first discharge voltage < [Upper limit of first charge voltage + (Upper limit of first charge voltage) 2 (-4 × 1st target power + rated output of solar cell × internal resistance of low-voltage battery) 0.5 ]×0.5...Equation (5)

[0042] Furthermore, when switching a low-voltage battery from a discharged state to a charging state, the voltage increases as shown in Figure 5. Therefore, the upper limit of the second charging voltage should be set higher than the voltage increase caused by the effects of charging and discharging.

[0043] Specifically, by setting the upper limit of the second charging voltage to satisfy the following equation (6), unnecessary switching control of the power converter can be prevented. Second charging voltage upper limit > Second discharge voltage lower limit + (Second target power + Rated power of solar cell) / Second discharge voltage lower limit × Internal resistance of low-voltage battery ... Equation (6)

[0044] However, in equations (5) and (6), the first target power is the power that passes through the power converter when the solar cell is charging the low-voltage battery, and the second target power is the power that passes through the power converter when the solar cell is charging the low-voltage battery. Furthermore, the rated power of the solar cell is the power under the standard test conditions specified in JIS C8918, and is pre-stored in the control device.

[0045] Next, the operation of the control device will be explained. A flowchart illustrating the operation of the control device is shown in Figure 6.

[0046] The control device first obtains the temperature of the solar cell from the temperature sensor and the amount of solar radiation from the solar radiation sensor (step S1).

[0047] Step S2 obtains the optimal operating point voltage of the solar cell from the solar cell temperature and solar irradiance obtained in step S1. The optimal operating point voltage is set based on a pre-set relationship between the solar cell temperature and solar irradiance. If solar radiation cannot be obtained from the solar radiation sensor, the solar radiation may be estimated from the date and time. Furthermore, if the temperature of the solar cell cannot be obtained, the optimal operating voltage will be determined from the relationship between the solar cell temperature and solar irradiance that is highest among the pre-set relationships between solar cell temperature and solar irradiance.

[0048] Step S3 is used to set the first upper limit of the charging voltage, the first lower limit of the discharging voltage, the second upper limit of the charging voltage, and the second lower limit of the discharging voltage, which serve as the control reference, using the optimal operating point voltage of the solar cell obtained in step S2 and equations (1) to (3) and equations (5) and (6) above.

[0049] The external voltage (terminal voltage) of the low-voltage battery is obtained from the voltage sensor, and the low-voltage battery charging current is obtained from the current sensor (step S4).

[0050] In step S4, the internal voltage of the low-voltage battery is obtained from the external voltage and charging current acquired in step S4 and the internal resistance of the low-voltage battery which is stored in the control device beforehand, using the above formula (4) (step S5).

[0051] Determine whether the internal voltage of the low-voltage battery is greater than the first upper limit of the charging voltage. If the answer is Yes, proceed to step S7. If the answer is No, proceed to step S10 (step S6).

[0052] If the determination in step S6 above is Yes, the power supply direction of the power converter is set from the low-voltage battery to the high-voltage battery (step S7).

[0053] Again, the external voltage (terminal voltage) of the low-voltage battery is obtained from the voltage sensor, the charging current of the low-voltage battery is obtained from the current sensor, and the internal voltage of the low-voltage battery is obtained using the above formula (4) (step S8).

[0054] In step S8, determine whether the low-voltage battery internal voltage obtained is below the first discharge voltage lower limit. If it is No, return to step S6 and repeat until it becomes Yes. If it becomes Yes, return to step S1 (step S9).

[0055] If the determination in step S6 above is No, then it is determined whether the internal voltage of the low-voltage battery is below the second discharge voltage lower limit. If Yes, proceed to step S11. If No, return to step S1 (step S10).

[0056] If the determination in step S10 is Yes, the power supply direction of the power converter is set from the high-voltage battery to the low-voltage battery (step S11).

[0057] Again, the external voltage (terminal voltage) of the low-voltage battery is obtained from the voltage sensor, the charging current of the low-voltage battery is obtained from the current sensor, and the internal voltage of the low-voltage battery is obtained using the above formula (4) (step S12).

[0058] In step S12, it is determined whether the low-voltage battery internal voltage obtained is greater than the second charging voltage upper limit. If the answer is No, the process returns to step S12 and repeats until the answer is Yes. Once the answer is Yes, the process returns to step S1 (step S13).

[0059] In this way, by supplying power between the low-voltage battery and the high-voltage battery based on the voltage value of the low-voltage battery, the output voltage of the solar cell can be made to follow the optimal operating point voltage with a simple circuit configuration, and losses associated with power conversion between the solar cell and the low-voltage battery can be prevented.

Claims

1. A solar cell system comprising a solar cell, a low-voltage battery, a high-voltage battery with a voltage value higher than that of the low-voltage battery, a power converter, and a control device for controlling the operation of the power converter, The open-circuit voltage of the above solar cell is less than or equal to the allowable upper limit voltage of the above low-voltage battery. The solar cell and the low-voltage battery are directly connected, and the low-voltage battery and the high-voltage battery are connected via the power conversion device. The above power conversion device is a bidirectional DC / DC converter, A solar cell system characterized in that the control device controls the power converter based on the voltage value of the low-voltage battery.

2. The above control device is Obtain the optimal operating point voltage of the solar cell, Set a reference voltage value for controlling the voltage value of a low-voltage battery that simultaneously satisfies the relationships of equations (1), (2), and (3) below. The voltage value of the above low-voltage battery is When the first charge limit is exceeded, power is supplied from the low-voltage battery to the high-voltage battery, and when the discharge voltage falls below the first discharge limit, power supply from the low-voltage battery to the high-voltage battery is stopped. The solar cell system according to claim 1, characterized in that power is supplied from the high-voltage battery to the low-voltage battery when the second discharge voltage falls below the lower limit, and power supply from the high-voltage battery to the low-voltage battery is stopped when the second charge voltage exceeds the upper limit. (First charging voltage upper limit - Optimal operating point voltage) × 2 < (Optimal operating point voltage - First discharge voltage lower limit) ... Equation (1) Allowable upper voltage value ≥ First charging voltage upper limit > First discharge voltage lower limit > Second discharge voltage lower limit ≥ Allowable lower voltage value ... Equation (2) Allowable upper voltage limit ≥ First charging voltage upper limit > Second charging voltage upper limit > Second discharge voltage lower limit ≥ Allowable lower voltage limit ... Equation (3) However, in the above formulas (1) to (3), The permissible upper voltage limit is the maximum voltage value at which a low-voltage battery will not be overcharged, and the permissible lower voltage limit is the minimum voltage value at which a low-voltage battery will not be over-discharged. The first upper charge limit is the upper voltage limit when the power converter is stopped, and is the threshold for the power converter to start discharging the low-voltage battery. The first lower discharge limit is the threshold for the power converter to stop discharging the low-voltage battery. The second discharge voltage lower limit is the voltage lower limit when the power converter is stopped, and is the threshold for the power converter to start charging the low-voltage battery. The first discharge voltage lower limit is the threshold for the power converter to stop charging the low-voltage battery.

3. The solar cell system according to claim 2, characterized in that the above-mentioned optimal operating point voltage is set from a predetermined relationship between the temperature of the solar cell and the amount of solar radiation.

4. When the temperature of the solar cell cannot be obtained, The solar cell system according to claim 2, characterized in that the optimal operating voltage is the optimal operating voltage determined from the relationship between the temperature and solar radiation of the solar cell with the highest temperature and solar radiation among the relationships between the temperature and solar radiation of the solar cell that are set in advance.

5. The above control device is When supplying power from a low-voltage battery to a high-voltage battery, The solar cell system according to claim 2, characterized in that the power passing through the power converter is controlled to 10% or more of the maximum power that can be used in a preset continuous operating state.

6. The solar cell system according to claim 5, characterized in that the control device sets the internal voltage of the low-voltage battery to the voltage value of the low-voltage battery and controls the power conversion device.

7. The above control device is When supplying power from a low-voltage battery to a high-voltage battery, The solar cell system according to claim 6, characterized in that the above-mentioned first discharge lower limit is set to a value that satisfies the following formula (5). First discharge lower limit < [First charging voltage upper limit + (First charging voltage upper limit) ２ (-4 × 1st target power + rated output of solar cell × internal resistance of low-voltage battery) ０．５ ]×0.5...Formula (5) However, in equation (5), the first target power is the power that passes through the power converter when the low-voltage battery is discharged by the power converter.

8. The above control device is When supplying power from a high-voltage battery to a low-voltage battery, The solar cell system according to claim 6, characterized in that the second upper limit of charging is set to a value that satisfies the following formula (6). Second charge limit > Second discharge voltage lower limit + (Second target power + Rated power of solar cell) / Second discharge voltage lower limit × Internal resistance of low-voltage battery ... Equation (6) However, in equation (6), the second target power is the power that passes through the power converter when charging the low-voltage battery with the power converter.

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