Abstract:
According to the present invention, access to a password area in a nonvolatile memory cannot be granted by simple supply of an address in a normal order. According to one preferable mode, for instance, a trap address is set in the password area so that reading information from the password area is permitted only when the password area is accessed without accessing the trap address, whereas when the password area is accessed the trap address, whereas when the password area is access through the trap address, information reading is inhibited, or meaningless data is output or the information in the password area is destroyed. This invention can make it harder to gain access to a password area which is used to protect against illegitimate copying and can provide a nonvolatile memory having a stronger copy protection capability.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a temperature controller (control system) for a fuel gas in a fuel cell system which generates an electric power by supplying a modified hydrocarbon gas as a fuel gas to a fuel cell. 
     BACKGROUND OF THE INVENTION 
     A fuel cell system is an electric power system mainly composed of a fuel cell. The fuel cell generates an electric power by supplying hydrogen as a fuel gas to a hydrogen pole of the fuel cell and supplying an oxidizing gas containing oxygen gas such as air to an oxygen pole of the fuel cell. The fuel cell system directly converts a chemical energy to an electric energy and has a high power efficiency. In addition, the fuel cell system is assumed to be very clean power generating system which discharges negligible amount of environmental contaminating substances and, thus, has been under the examination of applicability to a wide variety of fields. 
     In such a fuel cell system, from the view points of difficulty to handle hydrogen and diversification of fuel sources, a gas except for hydrogen is utilized as a fuel gas. For example, a hydrocarbon fuel such as methane (CH 4 ) or methanol (CH 3 OH) is reformed in a reformer to generate hydrogen, and the modified gas comprising hydrogen as a main ingredient is utilized in many cases. In the case of a vehicle like an automobile, on which fuel cell is carried, the use of hydrogen is inconvenient in the requirement of a long period of time for filling hydrogen and in the difficulty in carrying a large amount of hydrogen, resulting in a shortened mileage. For this reason, it has been considered that a liquid fuel like methanol is charged into an automobile, to be utilized as the fuel by modifying the liquid fuel in the reformer to generate a gas containing hydrogen as a main ingredient. Since methanol can be charged just like refueling and a mileage in this case is in no way to inferior to that in the case of the present automobile utilizing gasoline, making it possible to treating the automobile just like gasoline based car. What is more, in the case of utilizing methanol, since the methanol molecule only has one hydrogen atom, the amount of hydrogen generated is large and the proportion of carbon dioxide discharged is small in comparison with any other hydrocarbon fuel having a larger number of carbon atoms. 
     With reference to FIG. 5, the conventional fuel cell system  50  will be specifically described. The hydrocarbon fuel (methanol in this case) is introduced to a reformer  61  of a fuel gas generator  60  together with water and air at which the hydrocarbon fuel is modified to produce a fuel gas. While carbon monoxide (hereinafter sometimes referred to as “CO” ) is contained in the resulting fuel gas generated in the reformer only in a trace amount, CO, which poises the catalyst in a fuel cell  52 , is converted into carbon dioxide by a CO remover  62  to be removed. The chemical reaction within the CO remover  62 , of course, has an optimal temperature range. If the temperature is lower than this range, the proportion of converting (removing) CO becomes low, and conversely, if it is higher than this range, there is a possibility to bring about “converse shift” or “methanation” where the hydrogen generated unduly undergoes oxidation. 
     Since the fuel gas generated in the reformer  61  has a high temperature (e.g., 300° C.), the fuel gas is allowed to cool down to an appropriate temperature (e.g., 100° C.) by means of a heat exchanger  71   in  at an inlet side, and then introduced into the carbon monoxide remover  62 . The fuel gas from which CO has been removed by the CO remover  62  is introduced into the fuel cell  52 . Since the chemical reaction within the carbon monoxide remover  62  is exothermic, the temperature of the fuel gas is increased (e.g., 180° C.). On the other hand, the working temperature of the solid macromolecule fuel cell is from normal temperature to approximately 150° C., a heat exchanger  71   out  at an outlet side is placed between the CO remover  62  and the fuel cell  52  to cool the fuel gas (e.g., cooled to 80° C.). Subsequently, an electric powder is generated due to the reaction between the fuel gas supplied at the side of the hydrogen pole and air supplied at the side of the oxygen pole to supply electric power to a motor etc. 
     As described above, it is important to control the temperature of the fuel gas at the inlet and outlet of the carbon monoxide remover  61  in the fuel cell system  50 , and the temperature is controlled by a temperature control system  70 . 
     The temperature control system  70  has a circulating channel  76  for circulating a coolant medium (cooling water), having a radiator  72 , a thermostat  73  for controlling the temperature of the coolant medium, a circulating pump  75  and the like in addition to the heat exchanger  71   in  at an inlet side and the heat exchanger  71   out  at an outlet side. In this temperature control system  70 , the coolant medium circulating within the circulating channel  76  is controlled so as to keep its temperature at a constant level. 
     However, since the temperature control system  70  as described above has a configuration that the temperature of the coolant medium is controlled within a constant level by a thermostat  73 , and the temperature of the fuel gas is controlled by the coolant medium having the constant temperature, the temperature of the fuel gas is decided basically by the ability of the heat exchanger  71 , the temperature of the fuel gas at the inlet of the heat exchanger  71  and the flow amount. For this reason, the temperature of the fuel gas at the inlet of the CO remover  62  and that at the inlet of the fuel cell  52  cannot be controlled at a desirable level in a precision manner. Particularly, when the thermal load at the heat exchanger  71  is rapidly increased, as in the case where the temperature of the fuel gas is rapidly increased, the temperature control of the fuel gas utilizing the coolant medium having a constant temperature, has a restriction and, thus the temperature of the fuel gas at the outlet of the heat exchanger  71  is unduly increased. This problem cannot be solved if the flow amount of the coolant medium (cooling water) become variable. 
     Furthermore, in the case of carrying the fuel cell system on an automobile, since the fuel cell system is used in the state of a high variation in the thermal load of the fuel cell  52 , it is required to keep the temperature of fuel gas at a constant level, quickly corresponding to the variation in the load according to driving operation. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is, therefore, to provide a temperature control system for controlling a temperature of a fuel gas in a fuel cell system, which can quickly respond to the sharp variation in thermal load in an exchanger to control the temperature of the fuel gas to a desirable value in a precision manner. 
     We have studied to solve the problems associated with the prior art and to attain the object described above. As a result, we have found the fact that when a mechanism for controlling the temperature of the fuel gas is added to a thermostat for controlling the temperature by keeping the temperature of a coolant medium at a constant level, the temperature can be controlled at a desirable level in a precision manner due to the synergism between them, and completed the present invention. 
     The temperature control system for controlling a fuel gas of fuel cell of the present invention comprises a fuel reformer for reforming a hydrocarbon fuel into a reformed gas mainly comprising hydrogen, carbon monoxide remover or removing a carbon monoxide in the reforming gas and said reformed fuel gas is supplied to said fuel cell, said temperature control system comprising: 
     at least one heat exchanger which exchanges heat between the fuel gas and a coolant medium, said heat exchanger being placed at an inlet side and/or outlet side of said carbon monoxide remover, 
     a radiator which radiates the heat exchanged by said heat exchanger, 
     a thermostat which is connected to said radiator and a radiator bypass channel which bypasses said radiator, said thermostat being actuated by said coolant medium at a predetermined temperature, so as to decrease the flow of the coolant medium from the radiator when the temperature of the coolant medium is lower than said predetermined value, and increase the flow of the coolant medium from the radiator when the temperature of the coolant medium is higher than said predetermined value, 
     a thermostat bypass control valve connected to said radiator and said heat exchanger, 
     a control unit which detects the temperature of said fuel gas and/or said coolant medium, and controls said thermostat bypass control valve based on said detected temperature, so as to open said thermostat bypass control valve when said detected temperature is higher than a second predetermined temperature and, and to close said thermostat bypass control valve when said detected temperature is lower than said second predetermined temperature. 
     According to the temperature control system of the present invention, the thermostat bypass control valve is subjected to the coolant medium flowing within the thermostat to be bypassed to control the temperature of the fuel gas flowing toward the heat exchanger irrelevant to the temperature set by the thermostat. The temperature of the coolant medium flowing toward the heat exchanger is decided by the temperature of the coolant medium at the outlet of the thermostat, the temperature of the coolant medium at the outlet of the radiator, and the mixing ratio of both coolant media. The heat exchanger may be placed either in the inlet side or at the outlet side of the carbon monoxide remover, or at both sides of the carbon monoxide remover. If the heat exchanger is placed at the inlet side of the carbon monoxide remover, the temperature of the fuel gas at the inlet of the carbon monoxide remover is controlled. If the heat exchanger is placed at the outlet side of the carbon monoxide remover, the temperature of the fuel gas at the outlet of the carbon monoxide remover is controlled. 
     The “thermostat bypass valve” used herein is intended to include those which cannot control the opening of the valve (ON/OFF valve) as well as those which can control the opening of the valve in a voluntary manner. In the case of the thermostat bypass valve which can control the opening degree of the valve in a voluntary manner, the term “opening the thermostat bypass control valve” used herein includes the operation of valve in such a manner that the flow of the coolant medium is increased at a voluntary proportion. Also, in such a thermostat bypass valve, the term “closing the thermostat bypass valve” used herein includes the operation of valve in such a manner that the flow of the coolant medium is decreased at a voluntary proportion. 
     The term “detected temperature being within a prescribed level” used herein means that the detected temperature (preferably, the temperature of the fuel gas at the outlet of the heat exchanger) is within the temperature range tolerable for operating the CO remover or the fuel cell under the optimal temperature condition. The detected temperature may be the temperature of the fuel gas flowing towards the radiator or the temperature of the fuel gas flowing toward the heat exchanger or the temperature calculated from the combination of these temperatures by the control unit. 
     The temperature control system according to the present invention may be configured so that said temperature control system has the heat exchangers at the inlet and the outlet sides of the CO remover and, said circulating channel connects the heat exchangers in series to circulate the fuel gas from the heat exchanger placed at the outlet of the CO remover toward the heat exchanger placed at the inlet of the CO remover. 
     By such a configuration, the heat exchange between the coolant medium and the fuel gas is carried out in a counter flow. 
     The temperature control system according to the present invention may also be configured so that said temperature control system has the heat exchangers at the inlet and the outlet sides of the CO remover and, said circulating channel connects the heat exchangers in parallel. 
     According to this configuration, the temperatures of the fuel gas at the inlet and the outlet of the CO remover are controlled through own channels different from other channels of the heat exchangers. The differences in temperature Δt between the coolant medium and the fuel gas in all heat exchangers may be large. 
     In this configuration, the temperature control system may further be configured so that each heat exchanger has the thermostat, the thermostat bypass valve, and the circulating pump. 
     According to such a configuration, due to each heat exchanger having the thermostat, the thermostat bypass valve, and the circulating pump, the heat exchangers can control the temperature without any affect of the other heat exchanger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a fuel cell system according to the first embodiment of the present invention; 
     FIG. 2 is a block diagram of a fuel cell system according to the second embodiment of the present invention; 
     FIG. 3 is a block diagram of a fuel cell system according to the third embodiment of the present invention; 
     FIG. 4 is a drawing showing the control flow of the temperature control system; and 
     FIG. 5 is a block diagram of the conventional fuel cell system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described by referring to the following embodiments. However, the present invention is not restricted thereto. 
     First Embodiment 
     By referring to FIG. 1, a first embodiment of the present invention will be described. FIG. 1 is a block diagram of a fuel cell system according to the first embodiment of the present invention. The fuel cell system  1  shown in this figure is carried on a fuel cell electric vehicle (FCEV). 
     [Configuration] 
     The fuel cell system  1  according to the first embodiment is mainly composed of a fuel cell  2 , a fuel gas generator  10 , and a temperature control system  20 . The fuel cell  2  of the fuel cell system  1  is a solid macromolecule type fuel cell to which a fuel gas and an oxidizing gas are supplied to electrochemically generate an electric power (see FIG.  1 ). 
     As the fuel gas for supplying a hydrogen pole of the fuel cell  2 , a methanol-modified fuel gas is utilized in which methanol as a hydrocarbon fuel is modified in a generator  10  in order to be accorded with variation in the fuel source and in order to be operated just like gasoline carried vehicle. Air is used as the oxidizing gas to be supplied to the oxygen pole of the fuel cell. 
     The fuel gas generator  10  is mainly composed of a reformer  11  and a CO remover  12 . The reformer  11  in this embodiment modifies methanol in the presence of a catalyst according to the following reactions to generate a fuel gas comprising hydrogen as a main ingredient.                           
     Methanol and water are supplied into the reformer  11  and, thus, the fuel gas generated has a composition comprising hydrogen, carbon dioxide, nitrogen, and a trace amount of carbon monoxide. Methanol and water are converted into gaseous states in an evaporator (not shown), and then are supplied into the reformer  11 . 
     The fuel cell  2  in this embodiment is a solid macromolecule type and, thus, it is not desirable to supply the gas containing CO into the fuel cell  2  as is, in terms of the fact that the platinum catalyst is poisoned. Consequently, the fuel gas from which CO is removed in the CO remover  12  is supplied into the fuel cell  2 . Air is supplied into the CO remover  12  at which a selective oxidization of carbon monoxide is brought about in the presence of a catalyst to convert CO into CO 2 , whereby CO is removed from the fuel gas. The oxidation in the CO remover  12  is exothermic and, thus, the fuel gas passing through the CO remover  12  is heated up (for example 100-180° C.). 
     The CO remover  12  and the fuel cell  2  have optimal operation temperatures. For example, if the temperature of the fuel gas within the CO remover  12  is unduly low, the conversion (removal rate) of CO is low, and conversely if it is too high, hydrogen undergoes oxidation to bring about “converse shift” or “methanation”. Consequently, the fuel gas just generated in the reformer  11  which has a high temperature (e.g., 300° C.) is cooled to a prescribed temperature (e.g., 100° C.) before entering in the CO remover  12 . 
     Since the operation temperature of the fuel cell  2  is from normal temperature to 150° C., the supply of the fuel gas having the temperature higher than the operation temperature is problematic in terms of heat resistance of a macromolecular electrolyte membrane and drying of the membrane. Consequently, the fuel gas exiting the CO remover  12  is cooled down to a prescribed temperature (e.g., 80° C.) before entering the fuel cell  2 . 
     The cooling (temperature control) of the fuel gas is carried out by the temperature control system (see FIG.  1 ). The temperature control system  20  is mainly composed of a heat exchanger  21 , a radiator  22 , a thermostat  23 , a thermostat bypass control valve  24 , a circulating pump  25 , and a circulating channel  26 . The temperature control system  20  controls the temperature of the fuel gas due to the heat exchange between the coolant medium and the fuel gas by the circulation of the coolant medium. 
     The temperature control system  20  in this embodiment cools the temperature of the fuel gas. 
     In this embodiment, as the heat exchanger  21 , heat exchangers  21   in  and  21   out  are placed at inlet and outlet side of the CO remover  12 . As the heat exchanger  21 , a shell and tube type heat exchanger may be utilized. Within the heat exchanger  21 , the heat exchange between the fuel gas which is a high temperature fluid and the coolant medium which has a low temperature is carried out. Water or such is utilized as the coolant medium. 
     The heat exchanger  21   in  cools the fuel gas flowing toward the CO remover  12  and the heat exchanger  21   out  cools the fuel gas flowing toward the fuel cell  2 . In the heat exchanger  21 , for example when two CO removers are provided, i.e., the CO remover is divided into two removers, an intermediate heat exchanger may be placed between these two CO removers. This ensures the temperature control of the fuel gas in much more secure manner. 
     The radiator  22  is provided in order to discharge the heat received by the coolant medium out of the temperature control system  20 , such as in atmosphere. As the radiator  22  to be used is one which can cool the temperature of the coolant medium to not more than the temperature set by the thermostat  23 , which will be described later on. 
     The thermostat  23  actuates depending upon the temperature of the fuel gas passing therethrough. If the temperature of the fuel gas is lower than the temperature set by the thermostat  23 , (e.g., when the flow amount of the fuel gas is decreased), the thermostat  23  decreases the flow amount of the coolant medium. Conversely, if the temperature of the fuel gas is higher than the temperature set by the thermostat  23 , (e.g., when the flow amount of the fuel gas is increased), the thermostat  23  increases the flow amount the coolant medium. This makes the temperature of the coolant medium to maintain at the temperature set by the thermostat  23 . 
     A thermostat bypass control valve  24  is fit to a thermostat bypass channel  23   a  which bypasses the thermostat  23 . The thermostat bypass control valve  24  detects the temperature(s) of the fuel gas and/or the coolant medium, and actuates on the basis of the detected temperature(s) to open or close the thermostat bypass channel  23   a . The thermostat bypass control valve  24  may be one which can freely control the opening of the valve. By freely controlling the opening of the valve, the temperature of the coolant medium can be carefully controlled. 
     The detected temperature may be, for example, the temperature obtained by detecting the temperature of the fuel gas at the outlet of the heat exchanger  21  (the terms “inlet” and “outlet” are based on the fuel gas). A control unit CU as shown in FIG. 1 controls the conditions of the thermostat bypass control valve  24 . 
     The lower limit of the temperature of the fuel gas, which can be realized in the temperature control system  20  having the configuration shown in FIG. 1 is the temperature of the coolant medium which is cooled by passing through the radiator. On the other hand, the limit thereof is the temperature set by the thermostat  23 . To be specific, the temperature of the coolant medium flowing toward the heat exchanger  21   in  at the outlet side can be freely changed within the range from the temperature at the outlet of the radiator  22  to the temperature set by the thermostat  23 . 
     The circulating pump  25  is a device for circulating the coolant medium within the coolant medium  20 . The circulating channel  26  is a closed channel for circulating the coolant medium, having piping connected to the heat exchanger  21 , the radiator  22  etc. 
     In some cases, in order to warm up the heat exchanger  21 , the CO remover  12  etc. according to warming up the fuel cell system  1 , a heating means for heating the fuel gas may be placed before the heat exchanger  21   in  at the inlet side or a heating means for heating the coolant medium may be placed within the circulating channel  26 . By placing a heating means downstream of the thermostat  23 , the temperature of the heat-control medium can be set at a level higher than the temperature set by the thermostat  23 . 
     [Actuation] 
     Subsequently, an embodiment of actuation of the temperature control system  20  in the fuel cell system  1  described above will be described. In this embodiment, the thermostat bypass control valve  24  used herein is one which can freely control the opening of the valve. 
     1) Variation with Small Thermal Load; 
     As for variation with small thermal load, due to the function of controlling the temperature possessed by the thermostat  23 , the temperature of the fuel gas is controlled corresponding to the variation in thermal load in the heat exchanger  21 . In this course, although the opening of the thermostat bypass control valve  24  is not regarded, a constant opening of the thermostat bypass control valve  24  is preferred in view of controlling the thermostat  23  only due to the thermostat bypass control valve  24  (Of course, the opening may be gradually changed.) 
     (1) Specifically, in such a case that the accelerator of the automobile is slightly pushed the flow amount of the fuel gas supplied to the fuel cell  2  is increased according to the degree of pushing the accelerator. At the same time, the amount of fuel gas passing through the heat exchanger  21  is increased. This makes the thermal load at the heat exchanger  21  increase. When the thermal load is increased, the heat transfer amount of the fuel gas to the coolant medium is increased, resulting in an increased thermal load at the heat exchanger  21 . The increase in thermal load makes the heat transfer amount of the fuel gas to the coolant medium large, increasing the temperature of coolant medium. 
     When the temperature of the coolant medium becomes higher than the temperature set by the thermostat, the thermostat  23  increases the flow amount of the coolant medium whose temperature has been cooled down to be lower than the set temperature, and decreases the flow amount of the coolant medium bypassing the heat exchanger  21  through the bypass channel  23   a  having a temperature higher than the set temperature. This increases the flow amount of the coolant medium passing through the heat exchanger  22  to keep the coolant medium flowing forward the heat exchanger  21   in  at a constant temperature. 
     Specifically, by the function of the thermostat  23  to keep the coolant medium at the set temperature, the flow amount of the coolant medium passing through the heat exchanger  21  is increased to accelerate the radiation in the radiator  22 . As a result, the flow amount of the even when the thermal load is increased by increasing the flow amount of the fuel gas, the fuel gas at the outlet of the heat exchanger is kept at the prescribed temperature (e.g., 100° C., 800° C. shown in FIG.  1 ). 
     (2) Conversely, in such a case that the accelerator of the automobile which has been slightly pushed is returned to the original state to decrease the flow amount of the fuel gas resulting in decrease in the thermal load at the heat exchanger  21 , the actuation reverse to the case of (1) is carried out. To be specific, in order to deal with the decrease in the thermal load, the amount of the coolant medium passing through the heat radiator  22  is decreased by the thermostat  23  to decrease the radiation. This keeps the temperature of the fuel gas at the outlet of the heat exchanger  21  at a prescribed value. 
     The temperature control system  20  in this embodiment is mainly designed to effectively absorb (radiate out) the increased thermal load when the thermal load is increased due to the increase in the flow amount of the fuel gas based on the condition of small flow amount of the fuel gas (e.g., at the idling state of the automobile i.e., the condition of small thermal load at the heat exchanger  21 ). From the same viewpoint, the opening of the thermostat bypass control valve  24  is preferably configured so as to block the bypass channel in the state where the thermal load at the heat exchanger  21  is the smallest. 
     2) Variation in Big Thermal Load: 
     As for the big thermal load, since the thermostat  23  cannot control the temperature in a quick manner, the temperature is controlled by the thermostat bypass control valve  24 . 
     (1) Specifically, in such a case that the accelerator of the automobile is rapidly pushed, the flow amount of the fuel gas supplied to the fuel cell  2  is rapidly increased according to the degree of pushing the accelerator. In this course, the thermal load at the heat exchanger  21  is sharply increased, and the heat transfer amount of the fuel gas to the coolant medium is also quickly increased. However, since there is a limitation to control the temperature of the coolant medium having a prescribed temperature by the thermostat  23  and, thus, the temperature of the fuel gas at the outlet of the heat exchanger  21  is unduly increased. 
     In this embodiment, the temperature of the fuel gas at the heat exchanger  21  is detected, and based on the detected temperature, the opening of the thermostat bypass control valve  24  is increased when the detected temperature is higher than the prescribed temperature. At this time, the coolant medium whose temperature has been cooled down by the radiator  22  to be lower than the temperature set by the thermostat  23  bypasses the thermostat  23  and flows downstream of the thermostat  23 . This can decrease the temperature of the coolant medium irrelevant to the temperature set by the thermostat  23 , making it possible to introduce the coolant medium having a temperature lower than the set temperature to the heat exchanger  21  (i.e., making it possible to have a large difference Δt in both temperatures). 
     What is more, since the opening of the thermostat bypass control valve  24  can be quickly controlled, the temperature of the coolant medium can be decreased with good responsibility to the variation in the thermal load. 
     Consequently, even when the thermal load at the heat exchanger  21  is increased, the temperature of the coolant medium can be quickly decreased to absorb the increase in the thermal load in a quick manner, keeping the fuel gas at a constant temperature. 
     (2) Conversely, in such a case that the accelerator of the automobile which has been rapidly pushed is returned rapidly (or gradually) to the original state to decrease the flow amount of the fuel gas resulting in decrease in the thermal load at the heat exchanger  21  (returning to the original state), the actuation reverse to the case of (1) is carried out. To be specific, by decreasing the opening of the thermostat bypass control valve  24  to decrease the flow amount of the coolant medium bypassing the thermostat  23 , the temperature is controlled by the thermostat  23  utilizing the coolant medium having a constant temperature. 
     The thermostat bypass control valve  24  is controlled by the control unit CU by detecting the temperatures of the outlets of the heat exchangers  21   in  and  21   out , etc. and judging how to control by the control unit CU on the basis of the detected values. 
     As described above, by the addition of the temperature control due to the thermostat bypass control valve  24  to the temperature control of the fuel gas due to the thermostat  23 , the use of the thermostat bypass control valve  24  makes it possible to quickly deal with the quick increase in the thermal load at the heat exchanger  21 , which cannot be dealt (or followed up) by the thermostat  23  and, thus, the temperature of the fuel gas can be controlled in a precision manner in all cases. 
     For instance, in the automobile on which the fuel cell system is carried, the temperature control system in this embodiment is applicable to rapid variation in the thermal load due to acute acceleration or acute reduction, and slight variation in the thermal load at the heat exchanger such as slight acceleration or slight reduction from a running at a constant speed of high, middle or slow speed. 
     Second Embodiment 
     By referring to FIG. 2, a second embodiment of the present invention will be described. FIG. 2 is a block diagram of a fuel cell system according to the second embodiment of the present invention. The fuel cell system  1  shown in this figure is carried on a fuel cell electric vehicle (FCEV). 
     [Configuration] 
     The fuel cell system  1  according to the second embodiment has the same configuration as that of the first embodiment, unless otherwise mentioned. Consequently, the parts and the elements overlapped with the first embodiment have the same symbols and the repetition thereof will be omitted or described briefly. 
     As shown in FIG. 2, the fuel cell system  1  of the second embodiment is different from that of the first embodiment in that the circulating channel  26  connects the heat exchanger  21   in  at the inlet side and the heat exchanger  21   out  at the outlet side in parallel. Other parts such as the fuel cell  2  and the fuel gas generator  10  are the same as those of the first embodiment. To be specific, the coolant medium exiting the circulating pump passes through the separate channels and flows toward two directions, i.e., towards the heat exchanger  21   in  at the inlet side and towards the heat exchanger  21   out  at the outlet side, respectively, and they join after exiting the heat exchanger  21   in  and the heat exchanger  21   out , after which the joined flow goes towards the radiator  22  and a radiator bypass channel  22   a . The second embodiment is different from the first embodiment in this point. 
     The proportion of the flow amount of the coolant medium flowing forward the heat exchanger  21   in  at the inlet side to that flowing forward the heat exchanger  21   out  at the outlet side may be previously decided depending upon the specification of the heat exchangers  21   in  and  21   out , etc. 
     [Actuation] 
     Subsequently, an embodiment of actuation of the temperature control system  20  in the fuel cell system  1  described above will be described. In this embodiment, the thermostat bypass control valve  24  used herein is one which can freely control the opening of the valve similar to the first embodiment. 
     1) Variation with Small Thermal Load: 
     As for variation with small thermal load, due to the function of controlling the temperature possessed by the thermostat  23  as in the first embodiment, the temperature of the fuel gas is controlled corresponding to the variation in thermal load in the heat exchanger  21 . In the case where the thermal load is increased or vice versa, the thermostat  23  acts the same function as in the first embodiment. 
     2) Variation in Big Thermal Load: 
     As for the big thermal load, since the thermostat  23  cannot control the temperature in a quick manner as in the first embodiment, the temperature is controlled by the thermostat bypass control valve  24 . In the case where the thermal load is increased or vice versa, the thermostat bypass control valve  24  acts the same function as in the first embodiment. 
     To be specific, when the thermal load is rapidly increased, the opening of the thermostat bypass control valve  24  is increased to bypass the thermostat  23  and to direct the coolant medium having a low temperature flowing from the radiator  22  towards downstream of the thermostat  23 . This makes it possible to the temperature of the coolant medium flowing towards the heat exchangers ( 21   in  and  22   out ) lower than the set temperature. Accordingly, the temperature control system  20  of the second embodiment is applicable to a rapid increase in the thermal load at the heat exchanger  21 , and can control the fuel gas at the outlet of the heat exchanger  21  in a precision manner. 
     As described above, by connecting the circulating channel  26  in parallel, the use of the thermostat bypass control valve  24  makes it possible to quickly deal with the quick increase in the thermal load at the heat exchanger  21 , which cannot be dealt (or followed up) by the thermostat  23  and, thus, the temperature of the fuel gas can be controlled in a precision manner in all cases as in the first embodiment. 
     Particularly, by connecting the circulating channel  26  in parallel, the temperature difference Δt between the coolant medium and the fuel gas at the outlet side of the heat exchanger  21   in  at the inlet side may be large, minimizing the heat exchanger  21   in  at the inlet side. 
     Third Embodiment 
     By referring to FIG. 3, a second embodiment of the present invention will be described. FIG. 3 is a block diagram of a fuel cell system according to the third embodiment of the present invention. The fuel cell system  1  shown in this figure is carried on a fuel cell electric vehicle (FCEV). 
     [Configuration] 
     The fuel cell system  1  according to the third embodiment has the same configuration as that of the first embodiment and that of the second embodiment, unless otherwise mentioned. Consequently, the parts and the elements overlapped with the first and the second embodiments have the same symbols and the repetition thereof will be omitted or described briefly. 
     As shown in FIG. 3, the fuel cell system  1  of the third embodiment is composed of the heat exchanger  21   in  at the inlet side and the heat exchanger  21   out  at the outlet side connected in parallel, each having the thermostat bypass control valve  24  and the thermostat bypass channel  23   a , and the circulating pump  25 . 
     The proportion of the flow amount of the coolant medium flowing forward the heat exchanger  21   in  at the inlet side to that flowing forward the heat exchanger  21   out  at the outlet side may be freely controlled by adjusting the blow amounts of the circulating pumps  25  and  25 . 
     [Actuation] 
     Subsequently, an embodiment of actuation of the temperature control system  20  in the fuel cell system  1  described above will be described. In this embodiment, the thermostat bypass control valve  24  used herein is one which can freely control the opening of the valve similar to the first and the second embodiments. 
     1) Variation with Small Thermal Load; 
     As for variation with small thermal load, due to the function of controlling the temperature possessed by the thermostat  23  as in the first embodiment, the temperature of the fuel gas is controlled corresponding to the variation in thermal load in the heat exchanger  21 . In the case where the thermal load is increased or vice versa, the thermostat  23  acts the same function as in the first embodiment. 
     As different from the second embodiment, since due to the thermostats  23  and  23  corresponding to the heat exchangers  21   in  and  21   out , the temperatures set by both thermostats  23  and  23  may be different from each other in this embodiment and, thus, the coolant media each having much more optimized temperature can be supplied to the heat exchangers  21   in  and  21   out . 
     2) Variation in Big Thermal Load: 
     As for the big thermal load, since the thermostat  23  cannot control the temperature in a quick manner as in the first embodiment, the temperature is controlled by the thermostat bypass control valve  24 . In the case where the thermal load is increased or vice versa, the thermostat bypass control valve  24  acts the same function as in the first embodiment. 
     To be specific, when the thermal load is rapidly increased, the opening of the thermostat bypass control valve  24  is increased to bypass the thermostat  23  and to direct the coolant medium having a low temperature flowing from the radiator  22  towards downstream of the thermostat  23 . This makes it possible to the temperature of the coolant medium flowing towards the heat exchangers ( 21   in  and  22   out ) lower than the set temperature. Accordingly, the temperature control system  20  of the second embodiment is applicable to a rapid increase in the thermal load at the heat exchanger  21 , and can control the fuel gas at the outlet of the heat exchanger  21  in a precision manner. 
     On the other hand, even when the thermal load at the heat exchanger  21  which has been increased will be returned to the original state, the temperature of the coolant medium can be returned to the original state by decreasing the opening of the thermostat bypass control valve  24 , whereby the variation in the thermal load can be applied to control the temperature of the fuel gas. 
     As described above, as in the first embodiment, the temperature control system according to the third embodiment of the present invention can quickly deal with the increase in the thermal load at the heat exchanger  21 , which cannot be dealt (or followed up) by the thermostat  23  and, thus, the temperature of the fuel gas can be controlled in a precision manner in all cases. 
     Also, in addition to the parallel connection of the circulating channels  26 , by placing the thermostat  23 , the thermostat bypass control valve  24 , and the like in each channel, the coolant media having different temperatures can be supplied to the heat exchangers  21   in  and  21   out , respectively, which further increases the precision of controlling the temperature of the fuel gas. 
     Control Flow of Control Unit 
     Next, one embodiment of controlling the thermostat bypass control valve  24  by the control unit CU will be described by referring to the flow chart shown in FIG.  4 . The fuel cell system  1  is carried on the automobile. 
     When a driver pushes the accelerator, the opening of the accelerator is increased (S 1 ), and the output of the motor is calculated on the basis of the opening of the accelerator (S 2 ). Subsequently, the output of the fuel cell is calculated (S 3 ), the amount of the fuel gas generated by the reformer  2  is calculated (S 4 ), the blowing amount of the water/methanol mixture is calculated (S 5 ), and then the target temperature of the coolant medium is calculated (S 6 ). Subsequently, the temperature of the coolant medium at the inlet of the heat exchanger  21  (S 7 ), the temperature of the coolant medium at the outlet of the heat exchanger  21  is detected (S 8 ), and the flow amount of the coolant medium at the outlet of the heat exchanger  21  is detected (S 9 ). Furthermore, on the basis of these calculated values and detected values, the opening of the thermostat bypass control valve  24  is calculated by the control unit CU (S 10 ), and the opening is adjusted. The control unit CU detects the temperature of the fuel gas at the inlet of the heat exchanger (S 11 ). While the detected temperature is fed back to the control unit CU, the opening of the thermostat bypass control valve  24  is calculated (S 10 ) to control the opening of the thermostat bypass control valve  24 . 
     While the invention has been described in detail with reference to the specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope thereof.