An electric-generating capacity which a solar cell can generate is determined depending on an irradiance level of a light and, for example, the electric-generating capacity when there is no irradiation of the light is zero. On the other hand, an electric power corresponding to an operation state of an electronic apparatus is required for a power source for driving the electronic apparatus irrespective of the irradiance level of the light. Therefore, it is obvious that the power source for stably driving the electronic apparatus cannot be composed of only the solar cell.
A hybrid power source system in which a solar cell and a secondary cell are combined with each other, and the secondary cell is used as an electric power buffer is known as a system in which a power source for stably driving an electronic apparatus is configured by using a solar cell. In this system, when the electric-generating capacity of the solar cell exceeds the electric power with which the electronic apparatus is driven, the secondary cell is charged in such a way that the excessive electric power which the solar cell generates is stored in the secondary cell. On the other hand, when the electric-generating capacity of the solar cell falls below the electric power with which the electronic apparatus is driven, the secondary cell is discharged so that the electronic apparatus is driven by the solar cell and the secondary cell.
The configuring of the hybrid power source system results in that there is no necessity for the solar cell to respond to a maximum power consumption of the electronic apparatus, and thus it is only necessary for the solar cell to supply the power consumption of the electronic apparatus on the average. As a result, it is possible to miniaturize the size of the solar cell. The hybrid power source system of the solar cell and the secondary cell can realize both of the stable supply of the electric power, and miniaturization of the solar cell, and thus is a system which is very effective for an electronic apparatus for which the miniaturization and the portableness promotion are aimed.
On the other hand, in a portable electronic apparatus such as a personal computer or a mobile phone, the power consumption has a tendency to be increased along with the high performance promotion and multi-function promotion thereof. Thus, a fuel cell is expected as a next-generation power source, for the portable electronic apparatus, which can cope with this tendency. In the fuel cell, a fuel is supplied to a negative electrode (anode) side, so that the fuel is oxidized, and air or oxygen is supplied to a positive electrode (cathode) side, so that oxygen is reduced. Thus, an oxidation-reduction reaction between the fuel and oxygen is caused in terms of the entire fuel cell. At this time, a chemical energy which the fuel has is efficiently converted into an electrical energy which is in turn taken out. The fuel cell has a feature that the fuel cell can be continuously used as a power source by supplying thereto the fuel unless the fuel cell breaks down.
Although various kinds of fuel cells have been proposed, a polymer electrolyte fuel cell (PEFC) using a hydrogen ion-conducting polymer film as an electrolyte is suitable as a portable power source because the electrolyte is a solid and is in no danger of flying-apart, the polymer electrolyte fuel cell can be operated at a lower temperature than that in any other type fuel cell, for example, at temperatures of about 30° C. to about 130° C., a start time thereof is short, and so forth.
Various kinds of combustible materials such as hydrogen and methanol can be used as the fuel of the fuel cell. However, a gas fuel such as hydrogen is unsuitable for the miniaturization and lightweight because the gas fuel requires a high-pressure container or hydrogen storing alloy for storage. On the other hand, although a liquid fuel such as methanol has an advantage that the liquid fuel is easy to store, a fuel cell utilizing a system for taking out hydrogen from the liquid fuel by using a reformer is unsuitable for the miniaturization because a construction thereof becomes complicated. Contrary to those, a direct methanol fuel cell (DMFC) in which methanol is directly supplied to an anode to be caused to make a reaction without reforming methanol has a feature that the fuel is easy to store, a construction thereof is simple, and the miniaturization is easy. Conventionally, the DMFC has been combined with the PEFC in many cases to be studied as one kind of PEFC, and thus is most highly expected as a power source for portable electronic apparatuses.
However, since an output density of the DMFC is relatively small, when the electric power with which the portable electronic apparatus is driven is tried to be generated by the fuel cell by itself, it is feared that the size of the fuel cell becomes too large. Therefore, even in the fuel cell such as the DMFC, it is effective to compose a hybrid power source system together with the secondary cell having the large output density, such as a lithium-ion cell.
Then, a hybrid power source system is proposed in Patent Document 1 which will be described later in which a fuel cell and a secondary cell are connected in parallel with a load, and at least one of the fuel cell and the secondary cell supplies an electric power to the load. FIG. 4 is a graph for explaining an operation of the power source system described above based on current-voltage characteristics of a fuel cell and a secondary cell. It is noted that voltages of the fuel cell and the secondary cell shown in FIG. 4 are not voltages of single cells, but are voltages of cell stacks in each of which plural cells are connected in series. In addition, since in a current Ir of the secondary cell, a discharge direction is taken as being positive (Ir>0), when the charging is carried out, the current Ir is negative (Ir<0).
As shown in FIG. 4, the current-voltage curve of the fuel cell has a sigmoid-like shape, and thus a generated voltage is relatively, largely reduced as a generated current is increased. The reason for this is because in the fuel cell, activation polarization, resistance polarization, and diffusion polarization remarkably appear in order with an increasing generated current. On the other hand, although the current-voltage curve of the secondary cell such as the lithium-ion cell is high in linearity, and thus a discharge voltage is gradually reduced due to the resistance polarization or the like when a discharge current is increased, a gradient of the current-voltage curve is relatively small, and thus an internal resistance is small. Similarly, during charging, although a charging voltage gradually rises when a charging current is increased, a gradient thereof is small. An open voltage Vr0 of the secondary cell is changed depending on a charging state representing how much the secondary cell is charged.
In this power source system, when there is no external load, all of the electric power generated by the fuel cell is used for charging of the secondary cell. When let Vc be the voltage at this time, let Ifc be the magnitude of the generated current of the fuel cell at this time, and let −Irc be the magnitude of the charging current of the secondary cell at this time, since the following relationship is fulfilled,Ifc=−Irc the voltage Vc is determined as a voltage (>Vr0) fulfilling this relationship in FIG. 4. When the external load is not zero, but is small, a part of the electric power generated by the fuel cell is used for driving of the load, and a surplus electric power is used for charging of the secondary cell. The voltage at this time is smaller than Vc, and is larger than Vr0.
When the load is larger and the voltage is smaller than Vr0, the discharging of the secondary cell is caused, and thus the load is driven by the fuel cell and the secondary cell. In this case, in order that each of the cells may effectively function, as shown in FIG. 4, the current-voltage curves of the two cells need to cross with each other in a suitable area. If this situation is met, when the load is relatively small, and a voltage V1 with which the load is driven is larger than a voltage Vx at an intersection point X, the currents supplied from the fuel cell and the secondary cell, as shown in FIG. 4, are determined as If1 and Ir1, respectively. Since If1>Ir1, the electric power is mainly supplied from the fuel cell. On the other hand, when the load is large and a drive voltage V2 is smaller than Vx, the currents supplied from the fuel cell and the secondary cell, as shown in FIG. 4, are determined as If2 and Ir2, respectively. Since If2<Ir2, in this case, the electric power supplied from the secondary cell exceeds the electric power supplied from the fuel cell.
It is understood that while the load is increased, so that the drive voltage is decreased from V1 to V2 through Vx, the generated current from the fuel cell is merely increased from If1 to If2, whereas the discharge current from the secondary cell is largely increased from Ir1 to Ir2, a most part of the power consumption increased for this period of time is supplied from the secondary cell. In addition, when an increase in the power consumption is borne by only the fuel cell, as can be seen from FIG. 4, the generated voltage of the fuel cell is reduced so as to be smaller than a minimum voltage with which the load can be driven. As described above, when the power source is composed of only the fuel cell, the fuel cell needs to grow in size in such a way that the generated voltage equal to or larger than the minimum drive voltage can be maintained. In addition, when the secondary cell which is excellent in the output density is connected in parallel with the fuel cell, thereby configuring the hybrid power source, the fuel cell can be miniaturized and the entire power source system can be miniaturized in turn.
However, in the power source system described above, the current-voltage curves of the fuel cell and the secondary cell need to cross with each other in a suitable area. In addition thereto, the system characteristics are perfectly determined by these current-voltage characteristics. In a simple system in which either the solar cell or the fuel cell, and the secondary cell are merely connected in parallel with each other like this example, since the characteristics of the solar cell or the fuel cell, and the characteristics of the secondary cell are mutually restricted, and the characteristics when the secondary cell is charged, and the characteristics when the secondary cell is discharged are mutually restricted, there is a limit to an improvement in the energy efficiency, the stability, and the convenience. For example, the energy efficiency is dominated by the characteristics of each of the cells and the charging state of the secondary cell, and the stability is largely influenced by the secular change or the like of the characteristics of each of the cells. In addition, it is impossible to use the various kinds of charging systems such as the high-efficiency charging and the fast charging appropriately.
In addition, in the hybrid power source system, the prevention of the overcharging of the secondary cell is a very important problem. For example, when the lithium-ion cell is used as the secondary cell, the overcharging causes dangerousness such as smoke generation and firing, and explosion as the case may be. Since the solar cell generates the electric power when the light is irradiated to the solar cell, there is required some sort of mechanism for usually detecting the charging state of the secondary cell, and stopping the charging when the full charging is reached. In addition, since the electric power generation is carried out even in a passive type fuel cell not having means for controlling the supply of the fuel on a steady basis, the same mechanism is required.
Then, a portable power source apparatus with a battery charger including a DC/DC converter, a current controlling circuit, and an overcurrent preventing circuit is proposed in Patent Document 2 which will be described later. FIG. 5(a) is a schematic diagram showing a configuration of a power source apparatus 100.
As shown in FIG. 5(a), in the power source apparatus 100, a solar cell 101 for generating an electric power by receiving a solar light is connected to an electric double layer capacitor 103 through a backflow preventing diode 102, and the electric power which the solar cell 101 generates is temporarily stored in the capacitor 103. After the electric power stored in the capacitor 103 is converted into a suitable voltage by a DC/DC converter 105, the resulting voltage is supplied to each of a load 104 and a secondary cell 106.
In addition, in the power source apparatus 100, as a feature thereof, a current controlling circuit 107 is provided between the DC/DC converter 105, and the secondary cell 106 and the load 104, and an overcurrent preventing circuit 108 is provided between the current controlling circuit 107 and the secondary cell 106.
FIG. 5(b) is a schematic diagram of the current controlling circuit 107 exemplified in Patent Document 2. The current controlling circuit 107 is configured similarly to a control circuit of a general series regulator. That is to say, division resistors 111 and 112 divide an output voltage to give a reference voltage Vref1. A constant voltage diode 113 generates a standard voltage. A control transistor 114 and a load resistor 115 thereof amplify a difference between the standard voltage (strictly, a sum of the standard voltage, and a base-to-emitter voltage of the control transistor 114 and the reference voltage Vref1, and controls an operation of a power transistor 116 depending on small and large of the difference. The power transistor 116 is inserted in series in the load 104, and a conductive property thereof is controlled by the control transistor 114 in such a way that an output voltage thereof becomes constant. A concrete configuration of the overcurrent preventing circuit 108 is not shown in Patent Document 2.
In the power source apparatus 100, when the electric power generated by the solar cell 101 exceeds the electric power with which the load 104 is driven, the secondary cell 106 is charged with the electric power for a surplus. When the secondary cell 106 reaches a full charging state, the charging for the secondary cell 106 is stopped by the overcurrent preventing circuit 108. Therefore, the secondary cell 106 is prevented from being overcharged. At this time, since the surplus electric power is accumulated in the electric double layer capacitor 103, a voltage of the capacitor 103 rises. As a result, since the voltage applied to the load 104 is controlled to a predetermined voltage by the current controlling circuit 107 even when the output voltage from the DC/DC converter 105 rises, an excessive voltage is prevented from being applied to the load 104.
In this case, since the surplus electric power which the solar cell 101 generates has nowhere to go, there is feared a rise in the voltage of the capacitor 103 due to the accumulation of the surplus electric power. It is described in Patent Document 2 that the rise in the voltage of the capacitor 103 is suppressed because the surplus electric power is consumed by the control transistor 114 and the load resistor 115 thereof.
On the other hand, seven kinds of apparatuses are shown in Patent Document 3 which will be described later, as a power source apparatus with which under any weather, an electric power obtained from a solar cell can be utilized at a maximum. FIG. 6 is schematic diagrams showing configurations of the two kinds of power source apparatuses, each having a relation with the present invention, of them.
In a power source apparatus 200 shown in FIG. 6(a), a solar cell module 201 is connected to each of a load 203 and a secondary cell 204 through a backflow preventing diode 202. In this case, as a feature of the power source apparatus 200, the power source apparatus 200 is configured in such a way that a sum of a voltage for a forward voltage drop of the backflow preventing diode 202, and a voltage of the secondary cell 204 becomes approximately equal to an optimal operating voltage of the solar cell module 201. In the power source apparatus 200, when the electric power generated by the solar cell module 201 exceeds an electric power with which the load 203 is driven, the secondary cell 204 is charged with an electric power for a surplus. However, since the power source apparatus 200 is configured in the manner as described above, the electric power obtained from the solar cell module 201 can be utilized at a maximum.
In addition, as another feature of the power source apparatus 200, a shunt regulator 207 is connected in parallel with each of the load 203 and the secondary cell 204. The shunt regulator 207 is controlled in accordance with a reference voltage Vref1 obtained by dividing an output voltage by division resistors 205 and 206. Also, the shunt regulator 207 is set in such a way that a maximum value of the output voltage is suppressed to a predetermined voltage smaller than an overcharging voltage which causes overcharging of the secondary cell 204.
In the power source apparatus 200, since, while the secondary cell 204 does not reach a full charging state, and thus the secondary cell 204 is normally charged with the surplus electric power, the output voltage is held at a voltage smaller than the overcharging voltage by the charging, a voltage limitation by the shunt regulator 207 is not carried out. On the other hand, since the surplus electric power is not consumed by the charging, but is accumulated when the secondary cell 204 reaches the full charging state, the output voltage begins to be increased at once and is attempting to exceed the overcharging voltage of the secondary cell 204. At this time, a voltage limiting operation by the shunt regulator 207 is immediately exhibited, and the surplus electric power is shunted through the shunt regulator 207 and is then transformed into heat by a resistance component within the shunt regulator 207 to be abandoned. As a result, since the output voltage is held at the predetermined voltage smaller than the overcharging voltage of the secondary cell 204, the secondary cell 204 is prevented from being overcharged.
In a power source apparatus 300 shown in FIG. 6(b), after an electric power which a solar cell module 201 generates is converted into a suitable voltage by a DC/DC converter 301, the resulting voltage is supplied to each of a load 203 and a secondary cell 204. As a feature of the power source apparatus 300, the power source apparatus 300 is configured in such a way that division resistors 302 and 303 for giving a reference voltage Vref2 by dividing an output voltage from the solar cell module 201 are provided on an input side of the DC/DC converter 301, a difference between the reference voltage Vref2 and a built-in standard voltage is amplified by an error amplifier 304, and an operation of the DC/DC converter is controlled based on a magnitude of the amplified difference, whereby the output voltage from the solar cell module 201 is held at an optimal operating voltage thereof.
In addition, division resistors 205 and 206 for giving a reference voltage Vref1 by dividing an output voltage, and a comparator 305 for detecting the overcharging of the secondary cell 204 by comparing the reference voltage Vref1 and the standard voltage are provided on an output side of the DC/DC converter 301. Also, an AND circuit for stopping an operation of the converter when the overcharging is detected by the comparator 305 is provided within the DC/DC converter. Therefore, the output voltage from the DC/DC converter 301 is held at the predetermined voltage smaller than the voltage which causes the overcharging of the secondary cell 204, and thus the secondary cell 204 is prevented from being overcharged.