Patent Publication Number: US-2015064586-A1

Title: Solid oxide fuel cell system

Description:
TECHNICAL FIELD 
     The present invention pertains to a solid oxide fuel cell system, and more particularly to a solid oxide fuel cell for generating electricity by steam reforming fuel and reacting the resulting hydrogen with oxidant gas. 
     BACKGROUND ART 
     Solid oxide fuel cells (“SOFCs” below) are fuel cells which operate at a relatively high temperature in which, using an oxide ion conducting solid electrolyte as electrolyte, with electrodes attached to both sides thereof, fuel gas is supplied to one side thereof and oxidizer (air, oxygen, or the like) is supplied to the other side thereof. 
     Japanese Published Unexamined Patent Application 2012-3850 (Patent Document 1) discloses a solid oxide fuel cell. In this solid oxide fuel cell, when a fuel cell operating at a high temperature is turned off, air is supplied to the air electrode side of the fuel cell stack while continuing to supply a small amount of fuel and fuel-reforming water, and the temperature inside the fuel cell module is reduced by the cooling effect of this air. I.e., in this fuel cell, during the stopping step fuel continues to be supplied even after the extraction of power from the fuel cell module is stopped, while at the same time the fuel cell stack is cooled by delivering a large volume of cooling air. Next, when the cell stack temperature has been reduced to less than the fuel cell oxidation temperature, the supply of fuel is stopped, after which only the supply of cooling air is continued until the temperature drops sufficiently, and the fuel cell is safely turned off. 
     A fuel cell which performs a “shutdown stop” is also known, whereby in the stopping step, power is extracted and the supply of fuel, fuel reforming water, and generating air (air fed to the air electrode side) is completely stopped. 
     Japanese Published Unexamined Patent Application 2010-27579 (Patent Document 2) discloses a fuel cell system. In this fuel cell system, during an emergency stop the feed pump for supplying fuel to the reformer, the reform water pump for supplying water for steam reforming, and the air blower for feeding air to the air electrode side of the cell stack are stopped. Thereafter, when the feed pump and the reforming water pump are restarted under emergency stop operation control, fuel gas which had been adsorbed by the adsorber is fed to the reformer and steam reforming is carried out using water supplied from the reform water pump, even if the supply of fuel gas from the fuel supply source is cut off. By this means, reforming fuel is supplied to the cell stack electrode over a predetermined period even after the supply of fuel gas is cut off, and oxidation of fuel electrodes by reverse flow of air is prevented. 
     Furthermore, Japanese Published Unexamined Patent Application 2012-138186 (Patent Document 3) discloses a high temperature-triggered fuel cell system. In this high temperature-triggered fuel cell system, during an emergency stop the raw fuel pump for supplying fuel gas is stopped and the reforming pump for supplying water to the reformer is activated. When the reforming water pump is activated, the water supplied expands in volume due to vaporization inside the reformer. Therefore even if the supply of raw fuel gas from the fuel supply source is cut off, the fuel gas remaining in the fuel gas supply line downstream of the reformer is pushed toward the fuel cell (cell stack) side by the pressure of the volumetrically expanded steam. Oxidation of the fuel electrode by a reverse flow of air is thus prevented. 
     PRIOR ART REFERENCES 
     Patent Documents 
     Patent Document 1 
     JP 2012-3850 
     Patent Document 2 
     JP 2010-27579 
     Patent Document 3 
     JP 2012-138186 
     SUMMARY OF THE INVENTION 
     Problems the Invention Seeks to Resolve 
     In the fuel cell set forth in Japanese Published Unexamined Patent Application 2012-3850 (Patent Document 1), fuel is supplied until the fuel cell stack drops to a predetermined temperature, even during the stopping step, resulting in the problem of wasted fuel which does not contribute to electrical generation. If the supply of fuel is stopped and cooling air is supplied before the cell stack temperature is sufficiently reduced, cooling air supplied to the air electrode side of the individual fuel cells flows in reverse to the fuel electrode, and reverse flowing air oxidizes fuel electrodes in the individual fuel cells, damaging the cells. It is therefore necessary to continue supplying fuel until the temperature of the individual fuel cells drops to below the oxidation temperature, and to prevent the reverse flow of cooling air supplied to the air electrodes of individual fuel cells. Note that the time until the temperature of a fuel cell stack which had been operating drops to below the oxidation temperature depends on the insulating performance of the fuel cell module, etc., but in general runs from one hour to several hours, during which time fuel which does not contribute to electrical generation must be continually supplied. 
     On the other hand, in a shutdown stop the supply of fuel and fuel reforming water is completely stopped in a short time, so wastage of fuel can be constrained. Also, in a shutdown stop the supply of fuel is stopped with the fuel cell stack in a high temperature state, so the supply of cooling air fed to the air electrode of the fuel cell stack is also stopped together with the stopping of the fuel supply, thereby avoiding reverse flow of air to the fuel electrode side and oxidation of the fuel electrode. 
     However, the present inventor has discovered the new technical problem that when a shutdown stop is performed, oxidation of fuel electrodes in the individual fuel cells occurs and cells are degraded even if the air fed to the air electrode side of the fuel cell stack is stopped when the fuel supply is stopped, leading to cell damage. 
     At the same time, in the fuel cell system set forth in Japanese Published Unexamined Patent Application 2010-27579 (Patent Document 2), fuel gas which had been adsorbed onto an adsorber can be fed for a fixed period of time to, the cell stack by activating a feed pump even after the supply from the fuel supply source is cut off, thereby preventing oxidation of fuel electrodes. However, in this fuel cell system fuel is pre-stored, requiring the special provision of an adsorber. Because the fuel cell heat capacity is extremely large, a long time is required until the temperature declines to one at which oxidation of fuel electrodes can be avoided, and it is difficult to cause this large amount of fuel continually supplied over this long time period to be adsorbed by an adsorber. For this reason, fuel electrode oxidation cannot be sufficiently suppressed in the fuel cell system set forth in Japanese Published Unexamined Patent Application 2010-27579. Also, because an outfeed pump and reforming water pump are activated to feed fuel to the cell stack fuel in this fuel cell system, the problem arises that fuel electrode oxidation cannot be avoided in emergencies when the power supplies to operate each pump are lost, in addition to the supply of fuel from the fuel supply source. 
     Moreover, in the high temperature-activated fuel cell system set forth in Japanese Published Unexamined Patent Application 2012-138186 (Patent Document 3), a reforming water pump is activated immediately after the raw fuel pump is stopped, and residual fuel is pushed out to the cell stack side by the volumetric expansion of water vaporized in the reformer. An adsorber for adsorbing fuel is therefore unnecessary in this high temperature-activated fuel cell system. In this high temperature-activated fuel cell system, however, residual fuel is actively pushed out by the pressure from vaporized steam, and fuel flowing out of the cell stack is combusted. Therefore it is extremely difficult to continue to supply residual fuel until the temperature declines to a temperature at which oxidation of the cell stack will not occur, and oxidation of fuel electrodes cannot be prevented. Also, in this high temperature-activated fuel cell system, the problem arises that oxidation of fuel electrodes cannot be avoided in emergencies when the power supply needed to operate the water pump is lost. 
     Therefore the present invention has the object of providing a solid oxide fuel cell system capable of executing a shutdown stop while fully suppressing oxidation of individual fuel cells. 
     Means for Resolving Problems 
     In order to resolve the above-described problems, the present invention is a solid oxide fuel cell system for generating electricity by steam reforming fuel and reacting the resulting hydrogen with oxidant gas, having: a fuel cell module comprising a fuel cell stack, a fuel supply apparatus for supplying fuel to this fuel cell module, a water supply apparatus for supplying water for steam reforming to a fuel cell module, an oxidant gas supply apparatus for supplying oxidant gas to the oxidant gas electrode side of the fuel cell stack, a reformer disposed inside the fuel cell module for steam reforming fuel supplied from a fuel supply apparatus using water supplied from a water supply apparatus, a fuel/exhaust gas passageway for guiding fuel/exhaust gas from a fuel supply apparatus through a reformer and fuel electrodes on fuel cell units constituting a fuel cell stack, to outside a fuel cell module; and a controller for controlling the fuel supply apparatus, water supply apparatus, oxidant gas supply apparatus, and the extraction of power from the fuel cell module; whereby the controller comprises a shutdown stop circuit for executing a shutdown stop to stop the supply of fuel and water and the generation of electricity, and the fuel/exhaust gas passageway functions as a mechanical pressure retention means, maintaining a pressure oxidant gas electrode side within the fuel cell module at a pressure higher than atmospheric pressure, and maintaining the pressure on the fuel electrode side at a pressure higher than the pressure on the oxidant gas electrode side, after the shutdown stop circuit executes the shutdown stop, until the fuel electrode temperature drops to a predetermined oxidation suppression temperature at which the risk of fuel electrode oxidation is diminished. 
     In the invention thus constituted, fuel and water are respectively supplied by a fuel supply apparatus and a water supply apparatus to a reformer disposed inside a fuel cell module, and the reformer steam reforms the fuel. Reformed fuel is supplied to the fuel electrode side of the individual fuel cell units which make up the fuel cell stack. At the same time, oxidant gas is supplied by an oxidant gas supply apparatus to the oxidant gas electrode side of the fuel cell stack. A fuel/exhaust gas passageway guides fuel/exhaust gas from a fuel supply apparatus through a reformer and through fuel electrodes in the fuel cell units constituting a fuel cell stack, to outside the fuel cell module. The controller comprises a shutdown stop circuit, and controls the extraction of power from the fuel supply apparatus, water supply apparatus, oxidant gas supply apparatus, and fuel cell module. The fuel/exhaust gas passageway functions as a mechanical pressure retention means (mechanical means for retaining pressure), maintaining a pressure oxidant gas electrode side within the fuel cell module at a pressure higher than atmospheric pressure, and maintaining the pressure on the fuel electrode side at a pressure higher than the pressure on the oxidant gas electrode side until, after the supply of fuel and water and the generation of electricity has been stopped by a shutdown stop circuit, the fuel electrode temperature drops to a predetermined oxidation suppression temperature at which the risk of fuel electrode oxidation is diminished. 
     In conventional solid oxide fuel cell systems, the supply of fuel, supply of reforming water, extraction of power from the fuel cell module, and supply of oxidant gas are all simultaneously stopped when performing a shutdown stop. In the conventional art, the reason for stopping the supply of oxidant gas at the same time as the supply of fuel and the extraction of power are stopped is due to the risk of a reverse flow of oxidant gas to the fuel cell unit fuel electrode side and resulting damage to fuel electrodes when fuel is stopped and only oxidant gas is supplied when the fuel cell stack temperature is higher than the oxidation temperature immediately after extraction of power is stopped. 
     However, the inventor has discovered the new problem that there are cases in which oxidant gas oxidizes the fuel electrodes even when the supply of oxidant gas is thus simultaneously stopped. This problem is the cause of temperature differentials between the fuel electrode side and the oxidant gas electrode side of fuel cell units after the extraction of power is stopped. First, because the supply of oxidant gas for electrical generation is stopped on the oxidant gas electrode side of the fuel cell units, there is no cooling effect from oxidant gas, and the temperature tends to rise. At the same time, heat from electrical generation ceases in the fuel electrode side of the fuel cell units, since extraction of power is stopped. On the fuel electrode side of each of the fuel cell units, fuel remaining in the reformer, etc. flows in even after the supply of fuel by the fuel supply means is stopped. This fuel flowing into the fuel electrode side is produced by the endothermic steam reforming reaction in the reformer, and is generally at a lower temperature than that on the oxidant gas electrode side in the fuel cell units. Thus in comparison to the tendency for temperature to rise after the supply of fuel and extraction of fuel are stopped on the oxidant gas electrode side of the fuel cell units, the temperature tends to drop on the fuel electrode side due to dissipation of electrical generation heat and inflow of low temperature residual fuel. In parts where the temperature has dropped, surrounding gases contract and pressure drops; in parts where the temperature has risen, surrounding gases expand and pressure rises. As a result of these phenomena, the pressure on the oxidant gas electrode side of fuel cell units rises and pressure on the fuel electrode side falls, and this pressure differential can result in a reverse flow of oxidant gas from the oxidant gas electrode side to the fuel electrode side. 
     The present inventor solved this new technical problem by providing a mechanical pressure retaining means. I.e., a fuel/exhaust gas passageway which guides fuel/exhaust gas from a fuel supply apparatus through a reformer and through the fuel electrodes of each of the fuel cell units constituting a fuel cell stack, to outside the fuel cell module was constituted as a mechanical pressure retaining means. The fuel passageway is constituted to pass from a fuel supply apparatus through a reformer and the fuel electrodes in each fuel cell unit, to oxidant gas electrodes. An exhaust gas passageway is constituted to go from the oxidant gas electrode side in the fuel cell module to the atmosphere outside the fuel cell module. In the present invention, by appropriately distributing the balance of flow path resistance, etc. in each part of these fuel/exhaust gas passageways, the present embodiment succeeds, after a shutdown stop, at maintaining a pressure greater than atmospheric pressure on the oxidant gas electrode side and maintaining a pressure on the fuel electrode side greater than on the oxidant gas electrode side, until the temperature drops to the oxidation suppression temperature, at which the risk of fuel electrode oxidation diminishes. Reverse flow of oxidant gas from the oxidant gas electrode side to the fuel electrode side is by this means prevented. Using this mechanical pressure retention means, reverse flows of oxidant gas can be prevented until the temperature of fuel electrodes drops to the oxidation suppression temperature, thereby, suppressing the oxidation of fuel electrodes, even when the supply of fuel and commercial power to the solid oxide fuel cell system of the present invention is lost due to a natural disaster or the like. 
     In the present invention the mechanical pressure retention means is preferably constituted so that after a shutdown stop, fuel electrode side pressure is decreased while being maintained at a pressure higher than that on the oxidant gas electrode side, and is maintained at a higher pressure than atmospheric pressure even at the point in time when the fuel electrode temperature has dropped to the oxidation suppression temperature. 
     In the invention thus constituted, pressure on the fuel electrode side is maintained at a higher pressure than on the oxidant gas electrode side by a mechanical pressure retention means until falling to the oxidation suppression temperature. Here, in general solid oxide fuel cell system operates at a high temperature and has a large heat capacity, therefore the temperature drop behavior after a shutdown stop and the pressure drop behavior on the fuel electrode side and oxidant gas electrode side are not prone to be influenced by the outside air temperature, etc. The present inventor, taking note of this point, constructed a fuel/exhaust gas passageway such that the pressure drop on the fuel electrode side and oxidant gas electrode side after a shutdown stop occurs while maintaining a higher pressure on the fuel electrode side than on the oxidant gas electrode side. By so doing, reverse flows of oxidant gas can be prevented until the temperature drops to the oxidation suppression temperature using only a mechanical structure, without being substantially affected by outside air temperature or the like. 
     In the present invention the mechanical pressure retention means preferably has an outflow-side flow resistance section communicating with the fuel electrode side and oxidant gas electrode side of each fuel cell unit, and the flow path resistance of this outflow-side flow resistance section is set so that the pressure drop on the fuel electrode side after stopping the supply of fuel, water, and electrical generation is more gradual than the pressure drop on the oxidant gas electrode side. 
     The inventor discovered that the pressure change behavior on the fuel electrode side and oxidant gas electrode side after a shutdown stop changes with the balance of flow resistance in each part of the fuel/exhaust gas fuel passageway, and that by setting an appropriate balance, the pressure on the fuel electrode side can be maintained at a higher pressure than on the oxidant gas electrode side over a long time period. In the invention thus constituted, the fuel/exhaust gas passageway flow resistance balance can be adjusted, by setting the flow path resistance of the outflow-side flow resistance section, so the pressure drop on the fuel electrode side can with a simple structure be made more gradual than the pressure drop on the oxidant gas electrode side, and reverse flows of oxidant gas can be prevented. 
     In the present invention the mechanical pressure retention means preferably has an inflow-side flow resistance section for allowing the inflow of fuel to the fuel electrode side of the fuel cell units to flow. 
     The invention thus constituted, comprises an inflow-side flow resistance section for allowing the inflow of fuel to the fuel electrode side, therefore pressure fluctuations on the upstream side of the fuel cell units can be directly transferred to the fuel electrode side, and excessive pushing out of fuel remaining on the fuel electrode side to the oxidant gas electrode side can be prevented. This allows residual fuel to be accumulated for a long time period on the fuel electrode side using a mechanical pressure retention means. 
     In the present invention a cap formed as a separate body is preferably attached to the top end of the fuel cell units, and the outflow-side flow resistance section is constituted of elongated narrow tubes installed so as to extend upward from the caps; fine tubes function as a buffer section for preventing oxidation of fuel electrodes by oxidant gas penetrating from the top end thereof. 
     In the invention thus constituted, the outflow-side flow resistance section is constituted by elongated narrow fine tubes installed on caps formed as separate bodies, therefore flow path resistance can be accurately adjusted. By balancing the flow path resistance with each part, the outflow-side flow resistance section constitutes a mechanical pressure retention means, so the flow path resistance thereof must be precisely set. It is difficult, however, to set such precise flow path resistances using the materials which make up the fuel cell unit main body. Using the present invention, the outflow-side flow resistance section is installed on caps formed as a separate body, so the outflow-side flow resistance section can be formed of various materials amenable to precision machining, and the flow resistance section can be accurately set. 
     In the present invention the caps are preferably formed of metal so that heat on the oxidant gas electrode side can be easily conducted to the fuel electrode side. 
     In the invention thus constituted, the caps are constituted of metal, therefore thermal conductivity is high, and the heat on the fuel electrode side is easily conducted to the fuel electrode side. Therefore sudden drops in the temperature of fuel remaining on the fuel electrode side of fuel cell stack resulting in sudden shrinking of the volume of gas on the fuel electrode side and sucking in of oxidant gas on the oxidant gas electrode side to the fuel electrode side can be prevented. 
     In the present invention the mechanical pressure retention means preferably maintains the pressure on the fuel electrode side at a pressure higher than that on the oxidant gas electrode side until the fuel electrode temperature drops to 400° C. 
     In the invention thus constituted, the pressure on the fuel electrode side is maintained at a pressure higher than the pressure on the oxidant gas electrode side until the fuel electrode temperature drops to 400° C., therefore fuel electrode oxidation can be reliably suppressed. 
     In the present invention the mechanical pressure retention means preferably maintains the pressure on the fuel electrode side at a pressure higher than that on the oxidant gas electrode side until the fuel electrode temperature drops to 350° C. 
     In the invention thus constituted, the pressure on the fuel electrode side is maintained at a pressure higher than the pressure on the oxidant gas electrode side until the fuel electrode temperature drops to 350° C., therefore fuel electrode oxidation can be even more reliably suppressed. 
     In the present invention, after executing a shutdown stop the shutdown stop circuit preferably stops the fuel supply apparatus, the oxidant gas supply apparatus, and the water supply apparatus until the fuel electrode temperature falls to the oxidation suppression temperature. 
     In the invention thus constituted, the fuel supply apparatus, oxidant gas supply apparatus, and water supply apparatus are stopped after a shutdown stop until the fuel electrode temperature drops to the oxidation suppression temperature, and a reverse flow of oxidant gas can be prevented until the fuel electrode temperature drops to the oxidation suppression temperature, even during emergencies when fuel and commercial power sources are simultaneously lost. 
     In the present invention, after executing a shutdown stop the shutdown stop circuit preferably operates the oxidant gas supply apparatus for a predetermined time, then stops the oxidant gas supply apparatus until the fuel electrode temperature drops to the oxidation suppression temperature. 
     In the invention thus constituted, the oxidant gas supply apparatus is operated for a predetermined time after a shutdown stop, so immediately after a shutdown stop the temperature on the oxidant gas electrode side is decreased, a balance in temperature is established between the fuel electrode side and the oxidant gas electrode side, and the temperature inside the fuel cell module is lowered. By this means, after an oxidant gas supply apparatus which has been operated for a predetermined time is stopped, an initial state is established when the prevention of reverse flow of oxidant gas is started by the mechanical pressure retention means, and reverse flows can be reliably prevented until the temperature drops to the oxidation suppression temperature. 
     In the present invention, immediately before execution of a shutdown stop, the shutdown stop circuit preferably decreases the amount of electricity generated to a fixed value and increases the amount of oxidant gas supplied by the oxidant gas supply apparatus. 
     In the invention thus constituted, immediately before execution of a shutdown stop, the amount of electricity generated decreases to a fixed value and the amount of oxidant gas supplied is increased, so the temperature and pressure on the fuel electrode side and oxidant gas electrode side at time of shutdown stop can be set to appropriate values. In this way, after a shutdown stop an initial state can be established when the prevention of reverse flow of oxidant gas is started by a mechanical pressure retention means, and reverse flows can be reliably prevented until the temperature drops to the oxidation suppression temperature. 
     In the present invention the shutdown stop confidential information preferably has a pressure retention operation circuit for raising the pressure on the fuel electrode side after the fuel electrode temperature has declined to the oxidation suppression temperature, so that a pressure decrease on the fuel electrode side occurring in with the decline in temperature on the fuel electrode side can be suppressed. 
     In the invention thus constituted, the provision of a mechanical pressure retention means and pressure retention operation enables the prevention of a reverse flow of air from the air electrode side to the fuel electrode side. The risk of fuel electrode oxidation is well reduced by the mechanical pressure retention means, but when unanticipated external disturbances occur, such as changes in atmospheric pressure outside design values, there is a risk that the pressure on the fuel electrode side cannot be maintained at a temperature at which there is no oxidation of fuel electrodes whatsoever (the oxidation lower limit temperature) using only this mechanical means. The pressure retention operation circuit therefore executes a pressure retention operation to raise the pressure on the fuel electrode side after the fuel electrode temperature has declined to the oxidation suppression temperature. Pressure retention control is executed after the fuel electrode temperature has dropped to the oxidation suppression temperature, so the pressure on both the fuel electrode side and the oxidant gas electrode side drops greatly, and only a slight pressure compensation on the fuel electrode side is sufficient. Also, because pressure retention operation is executed with the risk of oxidation sufficiently reduced, oxidation of the fuel electrodes can be reliably prevented without implementing precise controls. 
     In the present invention the shutdown stop circuit is preferably constituted to execute stoppage of fuel supply and electrical generation by an emergency stop mode and a normal stop mode, and the shutdown stop circuit does not execute control using a pressure retention operation circuit for stopping in the emergency stop mode. 
     In the invention thus constituted, no control by a pressure retention operation circuit is executed when stopping by the emergency stop mode. The simultaneous loss of fuel and power supplied to the solid oxide fuel cell system is anticipated as a circumstance in which the emergency stop mode is executed; in such circumstances the pressure retention operation is not executed, and the risk of fuel electrode oxidation can be decreased by a mechanical pressure retention means alone. By not executing pressure retention operation, secondary problems occurring when active control is executed can be avoided in the abnormal conditions when the emergency stop mode is executed. Moreover, the certainty of avoiding fuel electrode oxidation is diminished by not executing a pressure retention operation in the emergency stop mode, but since the frequency with which the emergency stop mode is executed is extremely low, decreasing the risk of fuel electrode oxidation using a mechanical pressure retention means enables the effect on the durable life span of the solid oxide fuel cell system to be reduced to a negligible level. 
     In the present invention the normal stop modes preferably include a program stop mode for executing a stop at a pre-planned opportune time, and the shutdown stop circuit executes control by a pressure retention operation circuit when stopping by the program stop mode. 
     In the invention thus constituted, pressure retention operation is executed in the program stop mode, so the pressure retention operation is executed at a pre-planned time. For this reason no problems are prevented even when a pressure retention operation is executed after a shutdown stop, and a long period of time is required until the pressure retention operation stops. Also, because the program stop mode is anticipated to be executed to respond to an intelligent meter, it is executed at a high frequency, and the durable lifespan of the solid oxide fuel cell system can be extended by executing pressure retention operation during stop modes which executed at a high frequency, thereby reliably avoiding the risk of fuel electrode oxidation. 
     In the present invention, when stopping in the normal stop mode the shutdown stop circuit preferably executes a temperature drop operation for dropping the temperature on the oxidant gas electrode side of the fuel cell stack immediately after the fuel supply and electrical generation are stopped, whereas when stopping in the emergency stop mode, no temperature drop operation is executed. 
     In the invention thus constituted, when stopping in the emergency stop mode, no temperature drop operation is executed after a shutdown stop, and the risk of fuel electrode oxidation is diminished by the mechanical pressure retention means alone. On the other hand, when stopping in the normal stop mode, the temperature and pressure on the fuel electrode side and oxidant gas electrode side can be dropped at the point when mechanical pressure retention is started by performing a temperature drop operation after a shutdown stop, and the risk of a reverse flow of oxidant gas can be further diminished during the time until the fuel electrode temperature drops to the oxidation suppression temperature. 
     Effect of the Invention 
     Using the solid oxide fuel cell system of the present invention, a shutdown stop can be executed while sufficiently suppressing fuel electrode oxidation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overview schematic diagram showing a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 2  is a front elevation cross section showing a fuel cell module in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 3  is a sectional diagram along line III-III in  FIG. 2 . 
         FIG. 4  is a partial cross section showing fuel cell units of a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 5  is a perspective view showing a fuel cell stack in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 6  is a block diagram showing a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 7  is a perspective view showing the reformer in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 8  is a perspective view showing the interior of a reformer with the top plate removed of the reformer removed, in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 9  is a plan view showing a cross section of the flow of fuel inside a reformer in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 10  is a perspective view showing a metal case and air heat exchanger housed within a housing in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 11  is a cross section showing the positional relationship between insulation used in a heat exchanger and a vaporizing section in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 12  is a timing chart showing an example of the supply amounts of fuel, etc. and temperatures of various parts, in the startup step of a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 13  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 14  is a timing chart schematically showing in time line form an example of the stopping behavior when a stop mode 1 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 15  is a diagram explaining in time line form the control, temperature and pressure inside the fuel cell module, and the state of the tip portion of fuel cell units when stop mode 1 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 16  is a timing chart schematically showing in time line form an example of the stopping behavior when a stop mode 2 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 17  is a diagram explaining in time line form the control, temperature and pressure inside the fuel cell module, and the state of the tip portion of fuel cell units when stop mode 2 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 18  is a timing chart schematically showing in time line form an example of the stopping behavior when a stop mode 3 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 19  is a timing chart showing an expanded view immediately after a shutdown stop in stop mode 3 of a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 20  is a diagram explaining in time line form the control, temperature and pressure inside the fuel cell module, and the state of the tip portion of fuel cell units when stop mode 3 is executed in a solid oxide fuel cell system according to a first embodiment of the present invention. 
         FIG. 21  is a flow chart of the supply of water in pre-shutdown operation. 
         FIG. 22  is a timing chart showing a variant example of stop mode 3. 
         FIG. 23  is a timing chart schematically showing stopping behavior on a timeline when stop mode 4 is executed in a solid oxide fuel cell system according to a second embodiment of the present invention. 
         FIG. 24  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system according to a second embodiment of the present invention. 
         FIG. 25  is a timing chart schematically showing in time line form an example of the stopping behavior when a stop mode 4 is executed in a solid oxide fuel cell system according to a second embodiment of the present invention. 
         FIG. 26  is a diagram explaining in time line form the control, temperature and pressure inside the fuel cell module, and the state of the tip portion of fuel cell units when stop mode 4 is executed in a solid oxide fuel cell system according to a second embodiment of the present invention. 
         FIG. 27  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system according to a variant example of the second embodiment of the present invention. 
         FIG. 28  is a timing chart schematically showing in a time line an example of stopping behavior in a conventional solid oxide fuel cell system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Next, referring to the attached drawings, we discuss a solid oxide fuel cell system (SOFC) according to an embodiment of the present invention. 
       FIG. 1  is an overview schematic of a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention. As shown in  FIG. 1 , the solid oxide fuel cell system (SOFC) of the first embodiment of the present invention is furnished with a fuel cell module  2  and an auxiliary unit  4 . 
     Fuel cell module  2  comprises a housing  6 ; a metal case  8  is built into the interior of housing  6 , mediated by insulation  7 . Fuel cell assembly  12 , which performs an electricity generating reaction using fuel gas and oxidant gas (air), is disposed on generating chamber  10 , under case  8 , which is a sealed space. This fuel cell assembly  12  comprises 10 fuel cell stacks  14  (see  FIG. 5 ); fuel cell stacks  14  comprise 16 fuel cell units  16  (see  FIG. 4 ). Thus fuel cell assembly  12  has 160 fuel cell units  16 , and all of these fuel cell units  16  are connected in series. 
     A combustion chamber  18 , being a combustion section, is formed at the top of the above-described generating chamber  10  in case  8  of fuel cell module  2 ; residual fuel and residual oxidant (air) not used in the electrical generation reaction is combusted in this combustion chamber  18  to produce an exhaust gas. Furthermore, case  8  is covered by insulation  7 , and diffusion to the outside atmosphere of heat inside fuel cell module  2  is suppressed. 
     Reformer  20  for reforming fuel is disposed at the top of combustion chamber  18 ; combustion heat from residual gas heats reformer  20  to a temperature at which the reforming reaction can occur. Disposed above this reformer  20  is an air-heat exchanger  22 , being a heat exchanger for heating generating air using combustion gas from residual gas to pre-heat the generating air. 
     Next auxiliary unit  4  comprises a pure water tank  26  which stores water obtained by condensing moisture contained in exhaust from fuel cell module  2  and purifies it using a filter, and a water flow volume regulator unit  28  (a “water pump” or the like driven by a motor) for adjusting the flow volume of water supplied from this water storage tank. Auxiliary unit  4  comprises a gas shutoff valve  32  for shutting off gas supplied from a municipal gas or other fuel supply source  30 , a desulfurizer  36  for removing sulfur from fuel gas, a fuel flow regulator unit  38  (a motor-driven “fuel pump” or the like) for regulating the flow volume of fuel gas, and a valve  39  for shutting off fuel gas flowing out from fuel flow regulator unit  38  during a loss of power. Furthermore, auxiliary unit  4  comprises: an electromagnetic valve  42  for shutting off air, which is the oxidant gas supplied from air supply source  40 , reforming air flow regulator unit  44  and generating air flow regulator unit  45  (a motor-driven “air blower” or the like), which regulate the flow volume air, first heater  46  for heating reforming air supplied to reformer  20 , and second heater  48  for heating air supplied to the electrical generating chamber. This first heater  46  and second heater  48  are provided in order to efficiently raise the temperature at startup, but may also be omitted. 
     Next, connected to fuel cell module  2  is a hot-water production device  50 , supplied with exhaust gas. Tap water is supplied from water supply source  24  to this hot water production device  50 ; this tap water becomes hot water using the heat of the exhaust gas, and is supplied to an external hot water holding tank, not shown. 
     A control box  52  for controlling the amount of fuel gas supplied, etc. is connected to fuel cell module  2 . 
     In addition, an inverter  54  serving as an electrical power extraction unit (electrical power conversion unit) for supplying electrical power generated by the fuel cell module to the outside is connected to fuel cell module  2 . 
     Next, referring to  FIGS. 2 and 3 , we explain the internal structure of the fuel cell module in a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention.  FIG. 2  is a side elevation cross section showing a fuel cell module in a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention;  FIG. 3  is a cross section along line III-III in  FIG. 2 . 
     As shown in  FIGS. 2 and 3 , fuel cell assembly  12 , reformer  20 , and air heat exchanger  22  are disposed in order starting from the bottom, as described above, in case  8  within housing  6  of fuel cell module  2 . 
     A reformer introducing pipe  62  for introducing pure water, reformed fuel gas, and reforming air is attached to the end portion side surface on the upstream side of reformer  20 . 
     Reformer introducing pipe  62  is a round pipe extending from the side wall surface at one end of reformer  20 ; it bends 90° and extends essentially in a plumb direction, penetrating the top end surface of case  8 . Note that reformer-introducing pipe  62  functions as a water-introducing pipe for introducing water into reformer  20 . T-pipe  62   a  is connected to the top end of reformer-introducing pipe  62 , and piping for supplying fuel gas and pure water is respectively connected to both end portions of a pipe extending in approximately the horizontal direction of this T-pipe  62   a . Water supply piping  63   a  extends diagonally upward from one end of T-pipe  62   a . Fuel gas supply piping  63   b  extends horizontally from the other end of T-pipe  62   a , then is bent in a U shape and extends approximately horizontally in the same direction as water supply piping  63   a.    
     At the same time, in order starting upstream, vaporizing section  20   a , blending section  20   b , and reforming section  20   c  are formed in the interior of reformer  20 , and reforming section  20   c  is filled with reforming catalyst. Fuel gas and air, blended with steam (pure water) introduced into reformer  20 , is reformed using the reforming catalyst with which reformer  20  is filled. Reforming catalysts in which nickel is applied to the surface of aluminum spheres, or ruthenium is applied to the surface of aluminum spheres, are used as appropriate. 
     A fuel gas supply line  64  is connected to the downstream end of reformer  20 ; this fuel gas supply line  64  extends downward, then further extends horizontally within a manifold formed under fuel cell assembly  12 . Multiple fuel supply holes  64   b  are formed on the bottom surface of the horizontal portion  64   a  of fuel gas supply line  64 ; reformed fuel gas is supplied into manifold  66  from these fuel supply holes  64   b . A pressure fluctuation-suppressing flow resistance section  64   c  in which the flow path is narrowed is provided in the middle of the vertical portion of fuel gas supply pipe  64 , and flow path resistance in the fuel gas supply flow path is thereby adjusted. Adjustment of flow path resistance is discussed below. 
     A lower support plate  68  provided with through holes supporting the above-described fuel cell stack  14  is attached at the top of manifold  66 , and fuel gas in manifold  66  is supplied into fuel cell units  16 . 
     At the same time, an air heat exchanger  22  is provided over reformer  20 . 
     Also, as shown in  FIG. 2 , an ignition apparatus  83  for starting the combustion of fuel gas and air is disposed on combustion chamber  18 . 
     Next, referring to  FIG. 4 , we discuss fuel cell units  16 .  FIG. 4  is a partial cross section showing fuel cell units of a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention. 
     As shown in  FIG. 4 , individual fuel cells  16  comprise individual fuel cells  84  and inside electrode terminals  86 , which are metal caps respectively connected to both side portions of individual fuel cells  84 . 
     Fuel cell  84  is a tubular structure extending vertically, equipped with a cylindrical internal electrode layer  90 , on the inside of which is formed a fuel gas flow path  88 , a cylindrical external electrode layer  92 , and an electrolyte layer  94  between internal electrode layer  90  and external electrode layer  92 . This internal electrode layer  90  is a fuel electrode through which fuel gas passes, and has a (−) polarity, while the external electrode layer  92  is an air-contacting electrode with a (+) polarity. 
     The internal electrode terminals  86  attached at the top and bottom ends of individual fuel cells  84  have the same structure, therefore here we specifically discuss internal electrode terminal  86  attached at the top end. The top portion  90   a  of inside electrode layer  90  comprises an outside perimeter surface  90   b  and top end surface  90   c , exposed to electrolyte layer  94  and outside electrode layer  92 . Inside electrode terminal  86  is connected to the outer perimeter surface of inside electrode layer  90  through conductive seal material  96 , and is electrically connected to inside electrode layer  19  by direct contact with the top end surface  90   c  of inside electrode layer  90 . Fuel gas flow path fine tubing  98  communicating with inside electrode layer  90  fuel gas flow path  88  is formed at the center portion of inside electrode terminal  86 . 
     This fuel gas flow path fine tubing  98  is elongated fine tubing disposed to extend in the axial direction of individual fuel cells  84  from the center of inside electrode terminals  86 . Therefore a predetermined pressure loss occurs in the flow of fuel gas flowing from manifold  66  ( FIG. 2 ) through the fuel gas flow path fine tubing  98  in the lower inside electrode terminals  86  into fuel gas flow path  88 . Fuel gas flow path fine tubing  98  on the lower inside electrode terminals  86  therefore acts as an inflow-side flow resistance section, and the flow path resistance thereof is set at a predetermined value. A predetermined pressure loss also occurs in the flow of fuel gas flowing out from fuel gas flow path  88  through fuel gas flow path fine tubing  98  in upper inside electrode terminals  86  into combustion chamber  18  ( FIG. 2 ). Therefore fuel gas flow path fine tubing  98  on the upper inside electrode terminals  86  acts as an outflow-side flow resistance section, and the flow path resistance thereof is set at a predetermined value. 
     Inside electrode layer  90  is formed, for example, from at least one of a mixture of Ni and zirconia doped with at least one type of rare earth element selected from among Ni, Ca, Y, Sc, or the like; or a mixture of Ni and ceria doped with at least one type of rare earth element; or any mixture of Ni with lanthanum gallate doped with at least one element selected from among Sr, Mg, Co, Fe, or Cu. 
     The electrolyte layer  94  is formed, for example, from at least one of the following: zirconia doped with at least one type of rare earth element selected from among Y, Sc, or the like; ceria doped with at least one type of selected rare earth element; or lanthanum gallate doped with at least one element selected from among Sr or Mg. 
     Outside electrode layer  92  is formed, for example, from at least one of the following: lanthanum manganite doped with at least one element selected from among: Sr or Ca; lanthanum ferrite doped with at least one element selected from among Sr, Co, Ni, or Cu; lanthanum cobaltite doped with at least one element selected from among Sr, Fe, Ni, or Cu; silver, or the like. 
     Next, referring to  FIG. 5 , we explain fuel cell stack  14 .  FIG. 5  is a perspective view showing a fuel cell stack in a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention. 
     As shown in  FIG. 5 , fuel cell stack  14  comprises 16 fuel cell units  16 ; these fuel cell units  16  are disposed in 2 rows of 8. The fuel cell units  16  is supported on the bottom side by a ceramic elongated lower support plate  68  ( FIG. 2 ); on the top side it is supported by 2 approximately square upper support plates  100 , on both sides of which are 4 of the fuel cell units  16  at both ends. Through holes through which inside electrode terminals  86  can penetrate are provided on this lower support plate  68  and outer support plates  100 . 
     In addition, a collector  102  and an external terminal  104  are attached to fuel cell units  16 . This collector  102  is integrally formed to connect a fuel electrode connecting portion  102   a , electrically connected to inside electrode terminal  86  attached to inside electrode layer  90  serving as fuel electrode, and an air electrode connecting portion  102   b , electrically connected to the external perimeter of outside electrode layer  92  serving as air electrode. A silver thin film is formed as an air electrode-side electrode over the entirety of the outside surface of outside electrode layer  92  (air electrode) on each of the individual fuel cells units  16 . The contact by air electrode connecting portion  102   b  with this thin film surface results in an electrical connection between current collector  102  and the entire air electrode. 
     Furthermore, positioned at the ends of fuel cell stack  14 , two external terminals  104  are respectively connected to the inside electrode terminals  86  on fuel cell units  16 . These external terminals  104  are connected to the inside electrode terminals  86  on fuel cell units  16  at the edges of adjacent fuel cell stacks  14 , and as described above, all 160 of the fuel cell units  16  are connected in series. 
     Next, referring to  FIG. 6 , we explain the types of sensors attached to a solid oxide fuel cell system (SOFC) according to the present embodiment.  FIG. 6  is a block diagram of a solid oxide fuel cell system (SOFC) according to a first embodiment of the present invention. 
     As shown in  FIG. 6 , a solid oxide fuel cell system  1  is furnished with a control section  110 ; connected to this control section  110  are an operating device  112  provided with operating buttons such as “ON” or “OFF” for user operation, a display device  114  for displaying various data such as a generator output value (Watts), and a notification device  116  for issuing warnings during abnormal states and the like. A microprocessor, memory, and program to operate these (the above are not shown) are built into control section  110 ; auxiliary unit  4 , inverter  54 , and the like are controlled by these, based on input signals from each of the sensors. Note that this notification device  116  may also be connected to a management center in a remote location to inform the management center of abnormal states. 
     Next, signals from the various sensors described below are input to control section  110 . 
     First, flammable gas detection sensor  120  has the purpose of detecting gas leaks, and is attached to fuel cell module  2  and auxiliary unit  4 . 
     The purpose of flammable gas detection sensor  120  is to detect whether CO in the exhaust gas, which is supposed to be discharged to the outside via exhaust gas conduit  80 , has leaked into the external housing (not shown) which covers fuel cell module  2  and auxiliary unit  4 . 
     A water reservoir state detection sensor  124  detects the temperature and amount of hot water in a water heater (not shown). 
     An electrical power state detection sensor  126  detects current, voltage, and the like in inverter  54  and in a distribution panel (not shown). 
     A generator air flow detection sensor  128  detects the flow volume of generating air supplied to generating chamber  10 . 
     A reforming air flow volume sensor  130  detects the flow volume of reforming air supplied to reformer  20 . 
     A fuel flow volume sensor  132  detects the flow volume of fuel gas supplied to reformer  20 . 
     A water flow volume sensor  134  detects the flow volume of pure water supplied to reformer  20 . 
     A water level sensor  136  detects the water level in pure water tank  26 . 
     Pressure sensor  138  detects pressure on the upstream side outside reformer  20 . 
     An exhaust temperature sensor  140  detects the temperature of exhaust gas flowing into hot water producing device  50 . 
     As shown in  FIG. 3 , generating chamber temperature sensor  142  is disposed on the front surface side and rear surface side around fuel cell assembly  12 , and has the purpose of detecting the temperature near fuel cell stack  14  and estimating the temperature of fuel cell stack  14  (i.e., of the fuel cells  84  themselves). 
     A combustion chamber temperature sensor  144  detects the temperature in combustion chamber  18 . 
     Exhaust gas chamber temperature sensor  146  detects the temperature of exhaust gases in exhaust gas chamber  78 . 
     Reformer temperature sensor  148  detects the temperature of reformer  20 ; it calculates the reformer  20  temperature from the intake and exit temperatures on reformer  20 . 
     An outside temperature sensor  150  detects the outside temperature when a solid oxide fuel cell system (SOFC) is placed out of doors. Sensors to detect outside atmospheric humidity and the like may also be provided. 
     Signals from these various sensors are sent to control section  110 ; control section  110  sends control signals to water flow regulator unit  28 , fuel flow regulator unit  38 , reforming air flow regulator unit  44 , and generating air flow regulator unit  45  based on data from the sensors, and controls the flow volumes in each of these units. 
     Next, referring to  FIGS. 7 through 9 , we explain the detailed constitution of reformer  20 . 
       FIG. 7  is a perspective view of reformer  20 ;  FIG. 8  is a perspective view showing the interior of reformer  20  with the top panel removed.  FIG. 9  is a plan view cross section showing the flow of fuel inside reformer  20 . 
     As shown in  FIG. 7 , reformer  20  is a cuboid metal box, filled internally with a reforming catalyst for reforming fuel. A reformer introducing pipe  62  is connected on the upstream side of reformer  20  for introducing water, fuel, and reforming air. In addition, a fuel gas supply pipe  64  is connected on the downstream side of reformer  20  for discharging fuel reformed in the interior of reformer  20 . 
     As shown in  FIG. 8 , a vaporizing section  20   a , being a vaporization chamber, is installed on the upstream side inside reformer  20 ; a blending section  20   b  is installed on the downstream side thereof, adjacent to vaporizing section  20   a . Furthermore, a reforming section  20   c  is installed on the downstream side, adjacent to blending section  20   b . A winding, serpentine passageway is formed within steam generating section  20   a  by the disposition of multiple partitioning plates. Water introduced into reformer  20  is vaporized at an elevated temperature inside vaporizing section  20   a  and becomes steam. Moreover, blending section  20   b  is constituted by a chamber having a predetermined volume; a winding, serpentine passageway is also formed on the interior thereof by the placement of multiple partitioning plates. Fuel gas and reforming air introduced into reformer  20  are mixed with steam produced in vaporizing section  20   a  as they pass through the winding passageway of blending section  20   b.    
     At the same time, a winding passageway is also formed inside reforming section  20   c  by the disposition of multiple partitioning plates, and this passageway is filled with catalyst. When a mixture of fuel gas, steam, and reforming air which has passed through vaporizing section  20   a  is introduced, a partial oxidation reforming reaction and a steam reforming reaction occur in reforming section  20   c . In addition, when a mixture of residual gas and steam is introduced, only the steam reforming reaction occurs in reforming section  20   c.    
     Note that in the present embodiment the vaporizing section, blending section, and reforming section are constituted as a single piece, but as a variant example a reformer comprising only a reforming section can be provided, with a blending section and vaporization chamber placed adjacent thereto on the upstream side. 
     As shown in  FIGS. 8 and 9 , fuel gas, water, and reforming air introduced into vaporizing section  20   a  of reformer  20  flows in a serpentine manner in a direction traversing reformer  20 ; the introduced water vaporizes and become steam. A steam/blending section partition  20   d  is provided between vaporizing section  20   a  and blending section  20   b ; a partition opening  20   e  is installed on this steam/blending section partition  20   d . This partition opening  20   e  is a rectangular opening placed in approximately half the upper region in approximately one half of one side of steam/blending section partition  20   d.    
     A blending/reforming section partition  20   f  is installed between blending section  20   b  and reforming section  20   c ; in this case a narrow flow path is formed by the placement of multiple communicating holes  20   g  in this blending/reforming section partition  20   f . Fuel gas and the like blended inside blending section  20   b  flows into reforming section  20   c  through these communicating holes  20   g.    
     After flowing in the longitudinal direction at the center of reforming section  20   c , fuel and the like which has flowed into reforming section  20   c  is split into two parts and reversed; the two passageways then reverse again and are merged, facing the downstream end, and flow into fuel gas supply pipe  64 . As it passes through the passageway, winding in the serpentine manner described, fuel is reformed by the catalyst with which the passageway is filled. Note that in some cases, boiling occurs inside vaporizing section  20   a , by which a certain quantity of water is suddenly vaporized in a short time period, causing internal pressure to rise. However, a chamber of predetermined volume is constituted in blending section  20   b , and narrow passageways are formed in blending/reforming section partition  20   f , making it difficult for sudden fluctuations inside vaporizing section  20   a  to affect reforming section  20   c . Therefore the narrow passageways between blending section  20   b  and blending/reforming section partition  20   f  function as a pressure fluctuation absorption means. 
     Next, referring newly to  FIGS. 10 and 11 , and again to  FIGS. 2 and 3 , we explain details of the structure of air heat exchanger  22 , which is a heat exchanger for oxidant gas used for electrical generation.  FIG. 10  is a perspective view showing metal case  8  and air heat exchanger  22  housed inside housing  6 .  FIG. 11  is a cross section showing the positional relationship between the vaporization chamber insulation and the vaporizing section. 
     As shown in  FIG. 10 , air heat exchanger  22  is a heat exchanger disposed at the top of case  8  in the fuel cell module  2 . As shown in  FIGS. 2 and 3 , a combustion chamber  18  is formed inside case  8 , and multiple fuel cell units  16  and reformer  20 , etc. are housed therein, therefore air heat exchanger  22  is positioned above these. Air heat exchanger  22  recovers heat from combustion gas combusted in combustion chamber  18  and discharged as exhaust and uses this heat to pre-heat air for electrical generation introduced into fuel cell module  2 . As shown in  FIG. 10 , vaporization chamber insulation  23 , being internal insulation, is sandwiched between the top surface of case  8  and the bottom surface of air heat exchanger  22 . I.e., vaporization chamber insulation  23  is disposed between reformer  20  and air heat exchanger  22 . In addition, insulation  7  ( FIG. 2 ), being outside insulation, covers the outside of the air heat exchanger  22  and case  8  shown in  FIG. 10 . 
     As shown in  FIGS. 2 and 3 , air heat exchanger  22  has multiple combustion gas pipes  70  and generating air flow paths  72 . As shown in  FIG. 2 , an exhaust gas collection chamber  78  is installed at one end portion on the multiple combustion gas pipes  70 ; this exhaust gas collection chamber  78  communicates with each of the combustion gas pipes  70 . An exhaust gas discharge pipe  82  is connected to exhaust gas collection chamber  78 . The other end portion of each of the combustion gas pipes  70  is left open; these open end portions communicate with the inside case  8  of combustion chamber  18  through communication openings  18   a.    
     Combustion gas pipes  70  are multiple metal round tubes directed in the horizontal direction; each round tube is respectively horizontally disposed. On the other hand generating air flow paths  72  are constituted by spaces outside each of the combustion gas pipes  70 . An oxidant gas introducing pipe  74  ( FIG. 10 ) is connected at the end portion on the exhaust gas discharge pipe  82  side of generating air flow paths  72 ; air outside fuel cell module  2  is introduced into generating air flow paths  72  through oxidant gas introducing pipe  74 . Note that, as shown in  FIG. 10 , oxidant gas introducing pipe  74  projects outward in the horizontal direction from air heat exchanger  22 , parallel to exhaust gas discharge pipe  82 . Furthermore, a pair of connecting flow paths  76  ( FIG. 3 ,  FIG. 10 ) are connected to both sides of the other end portion of generating air flow paths  72 , and generating air flow paths  72  and each of the connecting flow paths  76  are respectively joined through outlet ports  76   a.    
     As shown in  FIG. 3 , generating air supply paths  77  are respectively placed on both side surfaces of case  8 . Each of the connecting flow paths  76  erected on both sides of air heat exchanger  22  respectively communicates with the top portion of generating air supply paths  77  installed on both sides of case  8 . A large number of jet outlets  77   a  are arrayed in the horizontal direction at the bottom portion of each of the generating air supply paths  77 . Air for electrical generation which has been supplied through each of the generating air supply paths  77  is jetted from the many jet outlets  77   a  toward the bottom side of fuel cell stack  14  in fuel cell module  2 . 
     A flow straightening plate  21 , being a partition, is attached to the ceiling surface inside case  8 , and an opening portion  21   a  is provided in flow straightening plate  21 . 
     Flow straightening plate  21  is a plate material horizontally disposed between the ceiling surface of case  8  and reformer  20 . This flow straightening plate  21  is constituted to align the flow of gas flowing upward from combustion chamber  18 , guiding it to the entrance of air heat exchanger  22  (communication opening  8   a  in  FIG. 2 ). Generating air and fuel gas flowing upward from combustion chamber  18  flows into the top side of flow straightening plate  21  through opening portion  21   a  formed at the center of flow straightening plate  21 , then flows leftward in  FIG. 2  through exhaust passageway  21   b  between the top surface of flow straightening plate  21  and the ceiling surface of case  8 , and is guided to the entrance of air heat exchanger  22 . As shown in  FIG. 11 , opening portion  21   a  is placed above reforming section  20   c  on reformer  20 ; gas rising through opening portion  21   a  flows to exhaust passageway  21   b  on the left side of  FIG. 2  and  FIG. 11 , on the opposite side of vaporizing section  20   a . Therefore in the space above vaporizing section  20   a  (the right side in  FIGS. 2 and 11 ), the flow of exhaust is slower than in the space above reforming section  20   c , and acts as a gas retaining space  21   c  in which the flow of exhaust is detained. 
     A vertical wall  21   d  is installed at the edge of opening portion  21   a  in flow straightening plate  21  over the entire perimeter thereof; this vertical wall  21   d  causes the flow path to be narrowed from the space at the bottom side of flow straightening plate  21  flowing into exhaust passageway  21   b  on the top side of flow straightening plate  21 . Furthermore, a suspended wall  8   b  ( FIG. 2 ) is also installed over the entire perimeter of the edge of communication opening  8   a  to allow exhaust passageway  21   b  and air heat exchanger  22  to communicate; the flow path flowing into air heat exchanger  22  from exhaust passageway  21   b  is narrowed by this suspended wall  8   b . The flow path resistance in the passageway by which exhaust reaches the outside of fuel cell module  2  through air heat exchanger  22  from combustion chamber  18  is adjusted by the provision of this vertical wall  21   d  and suspended wall  8   b . Adjustment of flow path resistance is discussed below. 
     Vaporization chamber insulation  23  is attached to the bottom surface of air heat exchanger  22  so as to cover essentially the entirety thereof. Therefore vaporization chamber insulation  23  is disposed over the top of the entire vaporizing section  20   a . This vaporization chamber insulation  23  is disposed to constrain the direct heating of the bottom surface of air heat exchanger  22  by high temperature gases inside exhaust passageway  21   b  and gas holding space  21   c  formed between the top surface of flow straightening plate  21  and the ceiling surface of case  8 . Therefore during operation of fuel cell module  2 , the amount of heat directly transferred from exhaust accumulated in the exhaust passageway at the top of vaporizing section  20   a  is low, and the temperature around vaporizing section  20   a  can easily rise. After fuel cell module  2  has been stopped, the disposition of vaporization chamber insulation  23  means that heat dissipation from reformer  20  is constrained, making it difficult for heat around vaporizing section  20   a  to be robbed by air heat exchanger  22 , so the decline in the temperature of vaporizing section  20   a  is made gradual. 
     Note that in order to suppress dissipation of heat to the outside, vaporization chamber insulation  23  is outside insulation covering the entirety of case  8  and air heat exchanger  22  in fuel cell module  2 , disposed inside insulation  7 , separate from insulation  7 . Insulation  7  has higher thermal insulation characteristics than vaporization chamber insulation  23 . I.e., the heat resistance between inside and outside surfaces of insulation  7  is greater than the heat resistance between the inside and outside surface of vaporization chamber insulation  23 . In other words, if insulation  7  and vaporization chamber insulation  23  are constituted of the same material, insulation  7  will be thicker than vaporization chamber insulation  23 . 
     Next we explain the flow of fuel, generating air, and fuel gas during an electrical generation operation by solid oxide fuel cell system  1 . 
     First, fuel is introduced into reformer  20  vaporizing section  20   a  through fuel gas supply piping  63   b , T-pipe  62   a , and reformer introducing pipe  62 , and pure water is introduced into vaporizing section  20   a  through water supply piping  63   a , T-pipe  62   a , and reformer introducing pipe  62 . Therefore supplied fuel and water is merged in T-pipe  62   a , and is introduced into vaporizing section  20   a  through reformer introducing pipe  62 . During an electrical generating operation, vaporizing section  20   a  is heated to a high temperature, therefore pure water introduced into vaporizing section  20   a  is vaporized relatively quickly to become steam. Vaporized steam and fuel are blended inside blending section  20   b  and flow into the reforming section  20   c  in reformer  20 . Fuel introduced into reforming section  20   c  together with steam is here steam reformed into a fuel gas rich in hydrogen. Fuel reformed in reforming section  20   c  descends through fuel gas supply pipe  64  and flows into manifold  66 , which is a dispersion chamber. 
     Manifold  66  is a cuboid space of a relatively large volume disposed on the bottom side of fuel cell stack  14 ; a large number of holes disposed on the top surface thereof communicate with the inside of the fuel cell units  16  constituting fuel cell stack  14 . Fuel introduced into manifold  66  flows out through the large number of holes on the top surface thereof, through the fuel electrode side of fuel cell units  16 , i.e. the interior of fuel cell units  16 , from the top end thereof. When hydrogen gas, which is fuel, passes through the interior of fuel cell units  16 , it reacts with oxygen in the air passing outside fuel cell units  16 , which are air electrodes (oxidant gas electrodes), producing a charge. Residual fuel remaining unused for electrical generation flows out from the top ends of the fuel cell units  16  and is combusted inside the combustion chamber  18  disposed at the top of fuel cell stack  14 . 
     At the same time, oxidant gas, which is the air used for electrical generation, is fed through oxidant gas introducing pipe  74  by generating air flow regulator unit  45 , which is a generating oxidant gas supply apparatus, into fuel cell module  2 . Air fed into fuel cell module  2  is introduced into generating air flow paths  72  in air heat exchanger  22  through oxidant gas introducing pipe  74 , and is preheated. Preheated air flows out to each of the connecting flow paths  76  through each of the exit ports  76   a  ( FIG. 3 ). Generating air flowing into each of the connecting flow paths  76  flows through the generating air supply paths  77  formed on both sides of fuel cell module  2 , and is jetted from a large number of jet outlets  77   a  into generating chamber  10  toward fuel cell stack  14 . 
     Air jetted into generating chamber  10  contacts the outside surface of fuel cell units  16 , which is the air electrode side (oxidant gas electrode side) of fuel cell stack  14 , and a portion of the oxygen in the air is used for electrical generation. Air jetted into the bottom portion of generating chamber  10  through jet outlets  77   a  rises inside generating chamber  10  as it is used for electrical generation. Air which has risen inside generating chamber  10  causes fuel flowing out from the top end of fuel cell units  16  to combust. The combustion heat from this combustion heats the vaporizing section  20   a , blending section  20   b , and reforming section  20   c  of the reformer  20  disposed on top of fuel cell stack  14 . After heating the upper reformer  20 , combustion gas produced by the combustion of fuel passes through the upper opening portion  21   a  on reformer  20  and flows into the top side of flow straightening plate  21 . Combustion gas flowing into the top side of flow straightening plate  21  is directed through the exhaust passageway  21   b  constituted by flow straightening plate  21  into communication opening  8   a , which is the entrance to air heat exchanger  22 . Combustion gas flowing into air heat exchanger  22  from communication opening  8   a  flows into the end portion of each of the opened combustion gas pipes  70 , is subjected to heat exchange with generating air flowing in generating air flow paths  72  on the outside of each of the combustion gas pipes  70 , and is collected in exhaust gas collection chamber  78 . Exhaust gas collected in exhaust gas collection chamber  78  is discharged through exhaust gas discharge pipe  82  to outside fuel cell module  2 . Vaporization of water in vaporizing section  20   a  and the endothermic steam reforming reaction in reforming section  20   c  are promoted, and generating air inside air heat exchanger  22  is preheated. 
     Next, referring newly to  FIG. 12 , we explain control in the startup step of solid oxide fuel cell system  1 . 
       FIG. 12  is a timing chart showing an example of the various supply amounts and temperatures of different parts in the startup step. Note that the scale markings on the vertical axis in  FIG. 12  indicate temperature, and the supply amounts of fuel indicate the increases and decreases thereof in a summary manner. 
     In the startup step shown in  FIG. 12 , the temperature of the fuel cell stack  14  at room temperature is raised to a temperature at which electricity can be generated. 
     First, at time t0 in  FIG. 12 , the supply of generating air and reforming air is started. Specifically, control section  110 , which is a controller, sends a signal to generating air flow regulator unit  45 , which is an apparatus for supplying oxidant gas for generation, activating same. As described above, generating air is introduced into fuel cell module  2  via generating air introducing pipe  74 , and flows into generating chamber  10  through air heat exchanger  22  and generating air supply paths  77 . Control section  110  sends a signal to reforming air flow regulator unit  44 , which is an apparatus for supplying oxidant gas for reforming, activating same. Reforming air introduced into fuel cell module  2  passes through reformer  20  and manifold  66  into fuel cell units  16 , and flows out of the top end thereof. Note that at time t0, because fuel is still not being supplied, no reforming reaction takes place inside reformer  20 . In the present embodiment, the supply amount of generating air started at time t0 in  FIG. 12  is approximately 100 L/min, and the supply amount of reforming air is approximately 10.0 L/min. 
     Next, supply of fuel is begun at time t1, a predetermined time after time t0 in  FIG. 12 . Specifically, control section  110  sends a signal to fuel flow regulator unit  38 , which is a fuel supply apparatus, activating same. In the present embodiment, the supply amount of fuel started at time t1 is approximately 5.0 L/min. Fuel introduced into fuel cell module  2  passes through reformer  20  and manifold  66  into fuel cell units  16  and flows out of the top end thereof. Note that at time t1, because the reformer temperature is still low, no reforming reaction takes place inside reformer  20 . 
     Next, a step for igniting supplied fuel is started at time t2 after the elapse of a predetermined time from time t1. Specifically, the ignition step control section  110  sends a signal to ignition apparatus  83  ( FIG. 2 ), which is an ignition means, igniting the fuel flowing out of the top end of the fuel cell units  16 . Ignition apparatus  83  generates repeated sparks in the vicinity of the top end of fuel cell stack  14 , igniting fuel flowing out from the top end of the fuel cell units  16 . 
     When ignition is completed at time t3 in  FIG. 12 , the supply of reforming water is started. Specifically, control section  110  sends a signal to water flow volume regulator unit  28  ( FIG. 6 ), which is a water supply apparatus, activating same. In the present embodiment, the amount of water supplied starting at time t3 is 2.0 cc/min. At time t3, the fuel supply apparatus is maintained at the previous level of approximately 5.0 L/min. The amount of generating air and reforming air supplied is also maintained at the previous values. Note that at time t3, the ratio O 2  of oxygen O 2  in reforming air to carbon C in fuel is approximately 0.32. 
     After ignition has occurred at time t3 in  FIG. 12 , supplied fuel flows out from the top end of each individual fuel cell unit  16  as off-gas, and is here combusted. This combustion heat heats reformer  20  disposed above the fuel cell stack  14 . Here vaporization chamber insulation  23  is disposed above reformer  20  (at the top of case  8 ), and by this means the temperature of the reformer  20  rises suddenly from room temperature immediately following the start of fuel combustion. Because outside air is introduced into air heat exchanger  22  disposed over vaporization chamber insulation  23 , the temperature of air heat exchanger  22  is low, particularly immediately after start of combustion, and so can easily become a cooling source. In the present embodiment, because vaporization chamber insulation  23  is disposed between the top surface of case  8  and the bottom surface of air heat exchanger  22 , movement of heat from reformer  20  disposed at the top inside case  8  to air heat exchanger  22  is constrained, and heat tends to retreat to the vicinity of reformer  20  inside case  8 . In addition, the space on the top side of flow straightening plate  21  at the top of reformer  20  is constituted as a gas holding space  21   c  ( FIG. 2 ) in which fuel gas flow is slowed, therefore a double insulation around vaporizing section  20   a  is achieved, and the temperature rises even more rapidly. 
     Thus by the rapid rise in the temperature of vaporizing section  20   a  it is possible to produce steam in a short time after off-gas begins to combust. Also, because reforming water is supplied to vaporizing section  20   a  in small amounts at a time, water can be heated to boiling with a very small heat compared to when a large amount of water is stored in vaporizing section  20   a , and the supply of steam can be rapidly started. Furthermore, since water flows in from water flow regulator unit  28 , excessive temperature rises in the vaporizing section  20   a  caused by delays in the supply of water, as well as delays in the supply of steam, can be avoided. 
     Note that when a certain amount of time has elapsed after the start of off-gas combustion, the temperature of air heat exchanger  22  rises due to exhaust gas flowing into air heat exchanger  22  from combustion chamber  18 . Vaporization chamber insulation  23 , which insulates between reformer  20  and air heat exchanger  22 , is placed on the inside of heat insulation  7 . Therefore vaporization chamber insulation  23  does not suppress the dissipation of heat from fuel cell module  2 ; rather it is disposed in order to cause the temperature of reformer  20 , and particularly vaporizing section  20   a  thereof, to rise rapidly immediately following combustion of off-gas. 
     Thus at time t4, when the temperature of reformer  20  has risen rapidly, the fuel and reforming air flowing into reformer section  20   b  via vaporizing section  20   a  causes the partial oxidation reforming reaction shown in Expression (1). 
       C m H n   +x O 2   →a CO 2   +b CO+ c H 2   (1)
 
     Because this partial oxidation reforming reaction is an exothermic reaction, there are local sudden rises in the surrounding temperature when the partial oxidation reforming reaction takes place inside reformer section  20   b.    
     On the other hand, in the present embodiment the supply of reforming water starts from time t3 immediately following confirmation of ignition, and the temperature of vaporizing section  20   a  rises suddenly, therefore at time t4, steam is produced in vaporizing section  20   a  and supplied to reformer section  20   b . I.e., after the off-gas has been ignited, water is supplied starting at a predetermined duration prior to when reformer section  20   b  reaches the temperature at which the partial oxidation reforming reaction occurs, and at the point when the partial oxidation reforming reaction temperature is reached, a predetermined amount of water is held in vaporizing section  20   a , and steam is produced. Therefore when the temperature rises suddenly due to the occurrence of the partial oxidation reforming reaction, a steam reforming reaction takes place in which reforming steam and fuel being supplied to reformer section  20   b  react. This steam reforming reaction is the endothermic reaction shown in Expression (2); it occurs at a higher temperature than the partial oxidation reforming reaction. 
       C m H n   +x H 2 O→ a CO 2   +b CO+ c H 2   (2)
 
     Thus when time t4 in  FIG. 12  is reached, a partial oxidation reforming reaction takes place inside reformer section  20   b , and the temperature rise caused by the occurrence of the partial oxidation reforming reaction causes the steam reforming reaction to simultaneously occur. Therefore the reforming reaction which takes place in reformer section  20   b  starting at time t4 is an auto-thermal reforming reaction (ATR) indicated by Expression (3), in which the partial oxidation reforming reaction and the steam reforming reaction are both present. I.e., the ATR step is started at time t4. 
       C m H n   +x O 2   +y H 2 O→ a CO 2   +b CO+ c H 2   (3)
 
     Thus in solid oxide fuel cell system  1  according to a first embodiment of the present invention, water is supplied during the entire period of the startup step, and no partial oxidation reforming reactions (POX) occurs independently. Note that in the timing chart shown in  FIG. 12 , the reformer temperature at time t4 is approximately 200° C. This reformer temperature is lower than the temperature at which the partial oxidation reforming reaction occurs, but the temperature detected by reformer temperature sensor  148  ( FIG. 6 ) is the average temperature in reformer section  20   b . In actuality, even at time t4 reformer section  20   b  has partially reached the temperature at which partial oxidation reforming reactions occur, and a steam reforming reaction is also induced by the reaction heat of the partial oxidation reforming reaction that does arise. Thus in the present embodiment, after ignition the supply of water begins before the time when reformer section  20   b  reaches the partial oxidation reforming reaction temperature, and no partial oxidation reforming reaction occurs independently. 
     Next, when the temperature detected by reformer temperature sensor  148  reaches approximately 500° C. or greater, the system transitions from the ATR1 step to the ATR2 step at time t5 in  FIG. 12 . At time t5, the water supply amount is changed from 2.0 cc/min to 3.0 cc/min. The previous values are maintained for the fuel supply amount, reforming air supply amount, and generating air supply amount. The ratio S/C for steam and carbon in the ATR2 step is thereby increased to 0.64, whereas the ratio between reforming air and carbon O 2 /C is maintained at 0.32. Thus by increasing the steam to carbon ratio S/C while holding fixed the reforming air to carbon ratio O 2 /C, the amount of steam-reformable carbon is increased without reducing the amount of partial oxidation reformable carbon. By so doing, the temperature of reformer section  20   b  can be raised and the amount of steam-reformed carbon increased, while reliably avoiding the risk of carbon deposition in reformer section  20   b.    
     Furthermore, the system transitions from the ATR2 step to the ATR3 step when the temperature detected by generating chamber temperature sensors  142  reaches approximately 400° C. or greater at time t6 in  FIG. 12 . The reforming air supply amount is thus changed from 5.0 L/min to 4.0 L/min, and the reforming air supply amount is changed from 9.0 L/min to 6.5 L/min. The previous values are maintained for the water supply amount and generating air supply amount. The ratio S/C for steam and carbon in the ATR3 step is thereby increased to 0.80, whereas the ratio between reforming air and carbon O 2 /C is reduced to 0.29. 
     Furthermore at time t7 in  FIG. 12 , when the temperature detected by generating chamber temperature sensors  142  reaches approximately 550° C. or greater, the system transitions to the SR1 step. In conjunction with this, the fuel supply amount is changed from 4.0 (L/min) to 3.0 L/min, and the water supply amount is changed from 3.0 (cc/min). Supply of reforming air is stopped, and the generating air supply amount is maintained at the previous value. Thus in the SR1 step, the steam reforming reaction is already occurring within reformer section  20   b , and the steam to carbon ratio S/C is set to 2.49, appropriate for steam reforming the entire amount of supplied fuel. At time t7 in  FIG. 12 , the temperatures of both reformer  20  and fuel cell stack  14  have risen sufficiently, therefore the steam reforming reaction can be stably brought about even if no partial oxidation reforming reaction is occurring in reformer section  20   b.    
     Next, at time t8 in  FIG. 12 , when the temperature detected by generating chamber temperature sensors  142  reaches approximately 600° C. or greater, the system transitions to the SR2 step. In conjunction with this, the fuel supply amount is changed from 3.0 (L/min) to 2.5 L/min, and the water supply amount is changed from 7.0 (cc/min) to 6.0 cc/min. The generating air supply amount is maintained at the previous value. In the SR2 step the water to carbon ratio S/C is thus set to 2.56. 
     Moreover, after the SR2 has been executed for a predetermined time, the system transitions to the electrical generation step. In the electrical generating step, power is extracted from fuel cell stack  14  to inverter  54  ( FIG. 6 ), and electrical generation is begun. Note that in the electrical generation step, fuel is already reformed by steam reforming in reformer section  20   b . Also, in the electrical generation step, the fuel supply amount, generating air supply amount, and water supply amount are changed in response to the output power demanded of fuel cell module  2 . 
     Next, referring to  FIGS. 13 through 22  and  FIG. 28 , we explain the stopping of a solid oxide fuel cell system  1  according to a first embodiment of the present invention. 
     First, referring to  FIG. 28 , we explain the behavior at the time of shutdown stop in a conventional solid oxide fuel cell system.  FIG. 28  is a timing chart schematically showing on a time line an example of stopping behavior in a conventional solid oxide fuel cell system. 
     First, at time t501 in  FIG. 28 , a shutdown stop operation is performed on a fuel cell which had been generating electricity. Thus the fuel supply amount, reforming water supply amount, and generating air supply amount are brought to zero without waiting for the temperature inside the fuel cell module to decline, and the current (generating current) extracted from the fuel cell module is also brought to zero. I.e., the fuel, water, and generating air supply to the fuel cell module are stopped in a short time period, and the extraction of power from the fuel cell module is stopped. Even when there is a loss in supply of fuel and electricity to the solid oxide fuel cell system due to natural disaster, etc., the stopping behavior is the same as in  FIG. 28 . Note that each of the supply amount, current, and voltage graphs in  FIG. 28  merely illustrates a change trend, and does not indicate specific values. 
     As a result of the stopping of power extraction at time t501, the voltage value produced in the fuel cell stack rises (however, current is zero). Because the supply of generating air is brought to zero at time t501, the fuel cell stack is naturally cooled over a long period after time t501 without forcibly feeding air into the fuel cell module. 
     If, hypothetically, air continues to be supplied into the fuel cell module after time t501, the pressure inside the fuel cell module will rise due to the fed-in air. On the other hand, the supply of fuel has already been stopped, so pressure inside the fuel cell units starts to drop. Air fed into the generating chamber of the fuel cell module may therefore conceivably flow in reverse on the fuel electrode side within the fuel cell units. Since the fuel cell stack is in a high temperature state at time t501, a reverse flow of air on the fuel electrode side leads to oxidation of the fuel electrode and damage to the fuel cell units. To avoid this, in a convention fuel cell, as shown in  FIG. 28 , generating air was also quickly stopped immediately after the supply of fuel was stopped by a shutdown, even if the supply of power was lost. 
     Moreover, after the elapse of 6 to 7 hours following a shutdown stop, when the temperature inside the fuel cell module has dropped to under a lower limit temperature for fuel electrode oxidation, air is again supplied into the fuel cell module (not shown). Such supplying of air is executed with the object of discharging remaining fuel gas, but when the fuel cell stack temperature has dropped to under a lower limit temperature for fuel electrode oxidation, there will be no fuel electrode oxidation even if a reverse flow of oxygen to the fuel electrode has occurred. 
     However the present inventor realized that even if a shutdown stop is performed in this type of conventional fuel cell, there still a risk of a reverse flow of air to the fuel electrode side, causing oxidation of the fuel electrode. 
     A reverse flow of air from the air electrode side to the fuel electrode side occurs based on a pressure differential between the inside (fuel electrode side) and the outside (air electrode side) of the fuel cell unit. In the state prior to a shutdown stop, when fuel gas and generating air are being supplied, reformed fuel is being fed under pressure to the fuel electrode side of the fuel cell unit. On the other hand, generating air is also being fed into the air electrode side of the fuel cell unit. In this state, the pressure on the fuel electrode side of the fuel cell unit is higher than the pressure on the air electrode side, and fuel is jetted from the fuel electrode side to the air electrode side of the fuel cell unit. 
     Next, when the supply of fuel gas and generating air is stopped by a shutdown stop, fuel is jetted from the fuel electrode side, which was in a high pressure state, to the low pressure air electrode side. Since the pressure on the air electrode side inside the fuel cell module is also higher than the outside air pressure (atmospheric pressure), after a shutdown stop air on the air electrode side inside the fuel cell module (and fuel gas jetted from the fuel electrode side) is exhausted through the exhaust pathway to outside the fuel cell module. Therefore after a shutdown stop, the pressures at both the fuel electrode side and the air electrode side of the fuel cell unit decline, ultimately converging to atmospheric pressure. Therefore the behavior of the declining pressure on the fuel electrode side and air electrode side are affected by the flow path resistance between the fuel electrode side and air electrode side of the fuel cell unit, the flow path resistance between the air electrode side inside the fuel cell module and the outside air, and so forth. Note that in a state whereby pressures on the fuel electrode and air electrode side are equal, air on the air electrode penetrates the fuel electrode side through diffusion. 
     In actuality, however, because the interior of the fuel cell module is at a high temperature, the pressure behavior after a shutdown stop is also affected by temperature changes on the fuel electrode side and the air electrode side. For example, if the temperature on the fuel electrode side of the fuel cell unit drops more suddenly than on the air electrode side, the volume of fuel gas inside the fuel cell unit shrinks, causing a drop in pressure on the fuel electrode side and a reverse flow of air. Thus the pressure on the fuel electrode side and air electrode side after a shutdown stop is affected by the flow path resistance in each part of the fuel cell module, the temperature distribution and stored heat amounts within the fuel cell module, and so forth, and changes in an extremely complex manner. 
     The gas component present on the fuel electrode side and air electrode side of the fuel cell unit can be estimated based on the fuel cell stack output pressure when no current is being extracted (output current=0). As shown by the heavy dotted line in  FIG. 28 , immediately after the shutdown at time t501, the output voltage from the cell stack suddenly rises (part A in  FIG. 28 ). This is because immediately after a shutdown stop, a large amount of hydrogen is present on the fuel electrode side, and air is present on the air electrode side, while the current extracted from the cross section is set at 0. Next, the output pressure of the cell stack drops suddenly (part B in  FIG. 28 ), but this is assumed to be due to the fact that the hydrogen present on the fuel electrode side of each of the fuel cell units flows out so that the concentration of hydrogen on the fuel electrode side declines, while the outflowing hydrogen causes the concentration of air on the air electrode side to decline. 
     Next, the output voltage from the cell stack drops with the passage of time, and when the temperature inside the fuel cell module has dropped to below the oxidation lower limit temperature (part C in  FIG. 28 ), the output voltage has dropped greatly. In this state it is estimated that there is almost no hydrogen remaining on the fuel electrode side of each of the fuel cell units, and in a conventional fuel cell the fuel electrode would be exposed to the risk of oxidation. In reality it is believed that in most cases in a conventional fuel cell a phenomenon arises whereby the pressure on the fuel electrode side falls more than the pressure on the air electrode side before the temperature inside the fuel cell module drops to below the oxidation lower limit temperature, producing an adverse effect on the fuel cell units. 
     Also, depending on the fuel cell module structure and the operating conditions prior to the shutdown stop, a phenomenon (not shown) arises whereby the temperature at the top of the fuel cell module rises after a shutdown stop, notwithstanding that the supply of fuel has been stopped. I.e., for about an hour after a shutdown stop, the temperature inside the fuel cell module in some cases rises more than during the electrical generation operation. This type of temperature rise is believed to be caused by the fact that the endothermic steam reforming reaction which had been occurring inside the reformer during an electrical generation operation ceases to occur when the supply of fuel stops, while at the same time the fuel remaining inside the fuel cell units and in the manifold which distributes fuel to those units continues to be combusted in the combustion chamber even after the supply of fuel stops. 
     Thus while on the one hand the temperature near the reformer inside the fuel cell module is rising, the heat of electrical generation ceases to be produced in the fuel cell stack due to the cessation of current extraction from the fuel cell stack. Therefore in contrast to the rise in pressure accompanying the rise in temperature at the top of the fuel cell stack, the pressure inside the fuel cell units drops due to the fall in temperature. As a result of this temperature slope inside the fuel cell module, the pressure on the fuel electrode side in each of the fuel cell units in some cases becomes lower than the pressure on the air electrode side. In such cases, there is a high potential that air on the air electrode side outside the fuel cell unit will reverse flow to the interior fuel electrode side, damaging the fuel cell unit. 
     In the solid oxide fuel cell system  1  of the first embodiment of the present invention, an appropriate value is set for the flow path resistance in each part within the fuel cell module, and the risk of oxidation of the fuel electrode is greatly suppressed by the control provided by the shutdown stop circuit  110   a  ( FIG. 6 ) built into control section  110 . 
     Next, referring to  FIGS. 13 through 22 , we explain the stopping of a solid oxide fuel cell system  1  according to a first embodiment of the present invention. 
       FIG. 13  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system  1  according to a first embodiment of the present invention. The purpose of the  FIG. 13  flow chart is to determine which of the stop modes to select based on predetermined conditions; during operation of the solid oxide fuel cell system  1 , this flow chart sequence is repeated at a predetermined time interval. 
     In step S 1  of  FIG. 13 , a determination is made as to whether the supply of fuel gas from fuel supply source  30  ( FIG. 1 ) and the supply of power from a commercial power source have been stopped. If the supplies of both fuel gas and power have been stopped, the system advances to step S 2 ; at step S 2 , stop mode 1, which is the emergency stop mode, is selected, and one iteration of the processing in the  FIG. 13  flow chart is completed. When stop mode 1 is selected, it is assumed that the supply of fuel gas and power has been stopped by a natural disaster or the like; the frequency with which this type of stoppage occurs is expected to be extremely rare. 
     On the other hand, when at least either fuel gas or power is being supplied, the system advances to step S 3 , and in step S 3  a decision is made as to whether the supply of fuel gas has been stopped and power is being supplied. When the supply of fuel gas has been stopped and power is being supplied, the system advances to step S 4 ; in cases other than this the system advances to step S 5 . In step S 4 , stop mode 2, which is one of the normal stop modes, is selected, and one iteration of the  FIG. 13  flow chart processing is completed. When stop mode 2 is selected, a temporary stoppage of the supply of fuel gas due to construction or the like is assumed to have occurred in the fuel gas supply path; the frequency with which this type of stopping occurs is expected to be low. 
     Furthermore, in step S 5  a determination is made as to whether a stop switch has been operated by a user. If a user has operated a stop switch, the system advances to step S 6 ; if the switch has not been operated, the system advances to step S 7 . In step S 6 , stop mode 3 is selected as one of the switch stop modes among the normal stop modes, is selected, and one iteration of the  FIG. 13  flow chart processing is completed. The presumed situation for a selection of stop mode 3 is that a user of the solid oxide fuel cell system  1  has been absent for a long period, such that operation of solid oxide fuel cell system  1  was intentionally stopped over a relatively long time period; the frequency of this type of stoppage is not believed to be very great. 
     In step S 7 , on the other hand, a determination is made of whether the stop is a regular stop, executed at a pre-planned regular timing. If the stop is a regular stop, the system advances to step S 8 ; if it is not a regular stop, one iteration of the  FIG. 13  flow chart is completed. In step S 6 , stop mode 3 is selected in the same way as a switch stop mode, as one of the program stop mode among the normal stop modes and one iteration of the  FIG. 13  flow chart processing is completed. Responding to an intelligent meter installed on fuel supply source  30  is assumed as a circumstance for executing a program stop mode. I.e., in general if an intelligent meter (not shown) is installed on fuel supply source  30 , and there is no period longer than approximately 1 hour during which the supply of gas is completely turned off over approximately a 1 month interval, the intelligent meter judges that a gas leak is occurring and cuts off the supply of fuel gas. In general, therefore, solid oxide fuel system  1  should be stopped for roughly 1 hour or more approximately once per month. As a result, it is anticipated that a stoppage by stop mode 4 will be performed at a frequency of approximately once per month, and that this will be the most frequently performed stoppage. 
     Note that if the supply of power is stopped and the supply of fuel gas is continued, neither of the stop modes will be selected according to the  FIG. 13  flow chart. In such cases, in a solid oxide fuel cell system  1  according to the embodiment, electrical generation can be continued by activating auxiliary unit  4  using power produced by fuel cell stack  14 . Note that the invention can also be constituted to stop electrical generation if the supply of power stays stopped continuously over a predetermined long time period. 
     Next, referring to  FIGS. 14 through 22 , we explain the stop processing in each stop mode. 
       FIG. 14  is a timing chart schematically showing on a time line an example of the stopping behavior when stop mode 1 (step S 2  in  FIG. 13 ) is executed in a solid oxide fuel cell system  1  according to a first embodiment of the present invention.  FIG. 15  is a diagram explaining on a time line the control, the temperature and pressure inside the fuel cell module, and the state of the tip portion of fuel cell units when stop mode 1 is executed in a fuel cell apparatus according to a first embodiment of the present invention. 
     First, at time t101 in  FIG. 14 , when a shutdown stop is performed the supply of fuel by fuel flow regulator unit  38 , the supply of water by water flow volume regulator unit  28 , and the supply of generating air by generating air flow regulator unit  45  are stopped in a short time period. The extraction of power from fuel cell module  2  by inverter  54  is also stopped (output current=0). When stop mode 1 is executed, fuel cell module  2  is left alone in this state after a shutdown stop. For this reason, fuel which had been present on the fuel electrode side of the fuel cell units  16  is jetted to the air electrode side through gas flow path fine tubing  98  ( FIG. 4 ) based on the pressure differential relative to the external air electrode side. Also, air present on the air electrode side of the fuel cell units  16  (and fuel jetted from the fuel electrode side) is discharged through air-heat exchanger  22 , etc. to the outside of fuel cell module  2  based on the pressure differential between pressure on the air electrode side (the pressure inside generating chamber  10  ( FIG. 1 )) and atmospheric pressure. Therefore after a shutdown stop, pressure on the fuel electrode and the air electrode side of each of the fuel cell units  16  drops naturally. 
     However, gas flow path fine tubing  98 , which is an outflow-side flow resistance section, is installed on the top end portion of the fuel cell units  16 , and a vertical wall  21   d  and suspended wall  8   b  ( FIG. 2 ) are erected in exhaust pathway  21   b . The flow path resistance of this gas flow path fine tubing  98  is set so that after the fuel supply and power are stopped, the pressure drop on the fuel electrode side is more gradual than the pressure drop on the air electrode side. In solid oxide fuel cell system  1 , by tuning the flow path resistance appropriately in each part of these fuel and exhaust pathways, fuel is made to remain over long time periods even after a shutdown stop on the fuel electrode side of fuel cell units  16 . For example, if the flow path resistance in the exhaust path leading from generating chamber  10  to the outside air is too small relative to the flow path resistance in gas flow path fine tubing  98 , the pressure on the air electrode side after a shutdown stop will drop suddenly, causing the pressure differential between the fuel electrode side and the air electrode side to increase so that the outflow of fuel from the fuel electrode side is actually increased. Conversely, if the exhaust path flow path resistance is too great relative to the flow path resistance of gas flow path fine tubing  98 , the pressure drop on the air electrode side will be gradual compared to the pressure drop on the fuel electrode side, and the pressures on the fuel electrode side and the air electrode side will approach one another, increasing the risk of a reverse air flow to the fuel electrode side. 
     Thus in the present embodiment the fuel and/or exhaust gas pathways guiding fuel and/or gas to outside fuel cell module  2  from fuel flow regulator unit  38  through reformer  20 , and the fuel electrodes in each of fuel cell units  16 , are tuned as described above. Therefore even when left alone after a shutdown stop, pressure on the fuel electrode side drops while maintaining a higher pressure than the air electrode side, and maintains a higher pressure than atmospheric pressure until the fuel electrode temperature drops to the oxidation suppression temperature, so the risk of fuel electrode oxidation can be well suppressed. As shown in  FIG. 14 , in the solid oxide fuel system  1  of the embodiment, after a shutdown stop is performed at time t101 the output voltage from fuel cell stack  14  shown by the heavy dotted line temporarily rises significantly and then drops, but that drop is less than in a conventional solid oxide fuel cell system ( FIG. 28 ), and a relatively high voltage continues for a long time period. In the example shown in  FIG. 14 , a relatively high voltage is maintained after a shutdown stop until the fuel electrode side and air electrode side temperatures drop to the oxidation suppression temperature at time t102. This indicates that fuel remains on the fuel electrode side of the fuel cell units  16  until time t102, when the temperature falls to the oxidation suppression temperature. 
     Note that in this Specification, “oxidation suppression temperature” refers to the temperature at which the risk of oxidation of the fuel electrodes is sufficiently reduced. The risk of fuel electrode oxidation declines gradually as the temperature falls, ultimately reaching zero. Therefore the risk of fuel electrode oxidation can be sufficiently reduced even with an oxidation suppression temperature slightly higher than the oxidation lower limit temperature, which is the minimum temperature at which oxidation of the fuel electrode can occur. In a standard fuel cell unit, this oxidation suppression temperature is thought to be about 350° C. to 400° C., and the oxidation lower limit temperature about 250° C. to 300° C. 
     I.e., in the solid oxide fuel cell system  1  of the embodiment, the fuel/exhaust gas pathway is constituted so that after a shutdown stop and until the fuel electrode temperature drops to the oxidation suppression temperature, the pressure on the air electrode side within fuel cell module  2  is maintained at higher than atmospheric pressure, and the pressure on the fuel electrode side is maintained at a pressure higher than the pressure on the air electrode side. Therefore the fuel/exhaust gas pathway functions as a mechanical pressure retention means for extending the time until the pressure on the fuel electrode side approaches the pressure on the air electrode side. 
       FIG. 15  is a diagram explaining the operation of stop mode 1; the top portion shows a graph schematically depicting pressure changes on the fuel electrode side and air electrode side; the middle portion shows the control operations by control section  110  and the temperature inside fuel cell module  2  on a time line, and the bottom portion shows the state at the top end portion of the fuel cell units  16  at each point in time. 
     First, a normal electrical generation operation is being performed prior to the shut down in the middle portion of  FIG. 15 . In this state, the temperature inside fuel cell module  2  is approximately 700° C. As shown in the bottom portion (1) of  FIG. 15 , fuel gas remaining without being used for electrical generation is jetted out from gas flow path fine tubing  98  at the top end of fuel cell units  16 , and this jetted out fuel gas is combusted at the top end of gas flow path fine tubing  98 . Next, when the supply of fuel gas, reforming water, and generating air is stopped by the shutdown stop, the flow volume of jetted out fuel gas declines and, as shown in the bottom portion (2) of  FIG. 15 , the flame is extinguished at the tip of gas flow path fine tubing  98 . Because gas flow path fine tubing  98  is formed to be long and narrow, flame is quickly extinguished when the flame is pulled into gas flow path fine tubing  98  as the result of a decrease in the flow volume of fuel gas. Quick extinction of the flame means the consumption of fuel gas remaining inside the fuel cell units  16 , etc. is suppressed, and the time during which remaining fuel can be maintained on the fuel electrode side is extended. 
     As shown in the lower portion (3) of  FIG. 15 , even after the flame is extinguished following a shutdown stop, the pressure inside fuel cell units  16  (on the fuel electrode side) is higher than outside (the air electrode side), therefore jetting of fuel gas from gas flow path fine tubing  98  is continued. Also, as shown in the upper portion of  FIG. 15 , immediately after a shutdown stop the pressure on the fuel electrode side is higher than the pressure on the air electrode side, and each pressure declines with this relationship maintained intact. The pressure differential between the fuel electrode side and the air electrode side declines together with the decline in jetting of fuel gas after a shutdown stop. 
     The amount of fuel gas jetted from gas flow path fine tubing  98  declines as the pressure differential between the fuel electrode side and the air electrode side diminishes (bottom portions (4), (5) of  FIG. 15 ). At the time of a shutdown stop, on the other hand, reformed fuel gas, unreformed fuel gas, steam, and water also remain inside reformer  20 , and fuel gas not reformed by residual heat is reformed by steam even after a shutdown stop. Remaining water is also vaporized by residual heat into steam, since the reformer  20  integrally comprises a vaporizing section  20   a . Because there is volumetric expansion due to the reforming of fuel gas and vaporization of water in reformer  20 , fuel gas which had been remaining inside the reformer  20 , fuel gas supply pipe  64 , and manifold  66  ( FIG. 2 ) is pushed out in sequence into the fuel cell units  16  (on the fuel electrode side). A pressure drop on the fuel electrode side accompanying the jetting of fuel gas from gas flow path fine tubing  98  is in this way suppressed. 
     Furthermore, since reforming section  20   c  inside reformer  20  is filled with catalyst, its flow path resistance is relatively large. Hence when remaining water is vaporized in vaporizing section  20   a , steam flows into reforming section  20   c , and also reverse flows toward reformer-introducing pipe  62  ( FIG. 2 ). After this reformer-introducing pipe  62  extends approximately horizontally from the side surface of vaporizing section  20   a , it is bent to extend approximately vertically upward. Therefore reverse-flowing steam rises vertically upward inside reformer-introducing pipe  62  and reaches the T-pipe  62   a  connected to the top end of reformer-introducing pipe  62 . Here reformer-introducing pipe  62  extending from vaporizing section  20   a  is disposed on the interior of the covering case  8  on thermal insulation  7 , so the temperature is high. Also, the top end portions of reformer-introducing pipe  62 , and T-pipe  62   a , are positioned outside thermal insulation  7 , so their temperature is low. Hence steam which has risen up in reformer-introducing pipe  62  contacts the top end portion of reformer-introducing pipe  62  and T-pipe  62   a , which are at a low temperature, and is cooled and condenses, producing water. 
     Water produced by condensation falls from T-pipe  62   a  and the top end portion of reformer-introducing pipe  62  onto the inside wall surface at the bottom of reformer-introducing pipe  62  where it is again heated so that it rises and flows once more into vaporizing section  20   a . Since reformer-introducing pipe  62  is bent, water droplets dropping down after condensing do not directly flow into vaporizing section  20   a , but rather fall onto the inner wall surface at the bottom portion of reformer-introducing pipe  62 . Therefore the part of reformer-introducing pipe  62  disposed inside thermal insulation  7  functions as a pre-heating portion for preheating supplied or condensed water, and the top end portions of reformer-introducing pipe  62  and T-pipe  62   a , which are lower in temperature than this preheating section, function as condensing sections. 
     There are cases in which steam that has risen up inside reformer-introducing pipe  62  reverse flows from T-pipe  62   a  to water supply pipes  63   a . However, water supply pipe  63   a  is disposed at an incline to face upward from T-pipe  62   a , therefore even when steam condenses inside water supply pipes  63   a , the condensate flows from water supply pipes  63   a  toward T-pipe  62   a  and falls into reformer-introducing pipe  62 . Also, as shown in  FIG. 2 , the bottom portion of reformer-introducing pipe  62  is placed in proximity at the inside of thermal insulation  7  to intersect exhaust gas discharge pipe  82 . Therefore a heat exchange is performed between reformer-introducing pipe  62 , being the preheating section, and exhaust gas discharge pipe  82 ; heating is also accomplished by exhaust heat. 
     Thus a portion of the steam vaporized in vaporizing section  20   a  reverse flows to reformer-introducing pipe  62 ; this produces a condensate, which is again vaporized in vaporizing section  20   a . Therefore even after the supply of water is stopped during a shutdown stop, remaining water is vaporized a little at a time inside vaporizing section  20   a , and water is vaporized over a relatively long period after a shutdown stop. Moreover, after extending from the side surface of vaporizing section  20   a , reformer-introducing pipe  62  is bent to extend approximately vertically upward, penetrating thermal insulation  7 . Therefore the location where reformer-introducing pipe  62  penetrates thermal insulation  7  is separated from the region vertically above reformer  20 , making it difficult for the heat of reformer  20  to escape through the site of penetration of thermal insulation  7  by reformer-introducing pipe  62 , so there is no extraordinary loss of thermal insulation characteristics caused by reformer-introducing pipe  62 . 
     On the other hand, the water vaporization occurring inside reformer  20  can occur suddenly depending on the distribution of temperatures inside vaporizing section  20   a , etc. In such cases, the pressure inside vaporizing section  20   a  rises suddenly, so a high pressure is transferred to the downstream side, and there is a risk that fuel gas inside the fuel cell units  16  will suddenly be jetted to the air electrode side. However, because a pressure fluctuation-suppressing flow resistance section  64   c  is provided on fuel gas supply pipe  64 , sudden eruptions of fuel gas within the fuel cell units  16  based on sudden rises in pressure inside reformer  20  are suppressed. Also, because gas flow path fine tubing  98  ( FIG. 4 ) is also installed at the bottom end of the fuel cell units  16 , sudden pressure rises inside the fuel cell units  16  are suppressed by the flow path resistance of gas flow path fine tubing  98 . Therefore the gas flow path fine tubing  98  and pressure fluctuation-suppressing flow resistance section  64   c  at the bottom end of the fuel cell units  16  function as a mechanical pressure retaining means for maintaining a high pressure on the fuel electrode side. 
     By thus using a mechanical pressure retaining means, the drop in pressure on the fuel electrode side of the fuel cell units  16  is suppressed over a long time period following a shutdown stop. When 5 to 6 hours have elapsed following a shutdown stop and the temperature inside fuel cell module  2  has dropped to 400° C., both the fuel electrode side and the air electrode side of the fuel cell units  16  fall to essentially atmospheric pressure, and air on the air electrode begins to diffuse to the fuel electrode side (bottom portions (6), (7) in  FIG. 16 ). However, gas flow path fine tubing  98  and the top end portion of fuel cell  84  where no outside electrode layer  92  is formed (the A part of bottom portion (6) in  FIG. 15 ) are not oxidized even if air penetrates, so this part functions as a buffer portion. In particular, because gas flow path fine tubing  98  is formed to be long and narrow, the buffer portion is elongated, and oxidation of the fuel electrode is unlikely to occur even if air penetrates from the top end of the fuel cell units  16 . Close to the oxidation suppression temperature, the oxidation which occurs even when the fuel electrode temperature is low and air is contacting the fuel electrode is minute, and since the frequency with which stop mode 1 is executed is extremely low, the adverse effects resulting from oxidation can in substance be ignored. Moreover, as shown in the lower portion (8) of  FIG. 15 , after the fuel electrode temperature declines to below the oxidation lower limit temperature, there is no change to fuel electrodes even if the fuel electrode side of the fuel cell units  16  is filled with air. 
     Next, referring to  FIGS. 16 and 17 , we explain stop mode 2. 
       FIG. 16  is a timing chart schematically showing on a time line an example of the stopping behavior when stop mode 2 (step S 4  in  FIG. 13 ) is executed in a solid oxide fuel cell system  1  according to a first embodiment of the present invention.  FIG. 17  is a diagram explaining on a time line the control, the temperature inside the fuel cell module, the pressure, and the state of the front end portion of the fuel cell units when stop mode 2 is executed. 
     First, stop mode 2 is a stop mode executed when only the supply of fuel gas has been stopped. At time t201 in  FIG. 16 , when a shutdown stop is performed, the supply of fuel by fuel flow regulator unit  38  and the supply of water by water flow volume regulator unit  28  stops in a short time period. The extraction of power from fuel cell module  2  by inverter  54  is also stopped (output current=0). When stop mode 2 has been executed, the shutdown stop circuit  110   a  built into control section  110  executes a temperature drop operation immediately after a shutdown stop at time t201, causing the generating air flow regulator unit  45  to operate at maximum output over a predetermined heat discharge time. Note that in the embodiment, the predetermined heat discharge time is approximately 2 minutes, during which water flow volume regulator unit  28  is stopped. Furthermore, at time t202 in  FIG. 16 , after generating air flow regulator unit  45  has been stopped, the system is left alone, as in stop mode 1. 
     In a stoppage by stop mode 2, air is fed after a shutdown stop to the air electrode side of fuel cell units  16  under temperature drop operation. Thus in part A of  FIG. 16 , the temperature on the air electrode side is more suddenly reduced than in the stop mode 1 case ( FIG. 14 ). As described above, after the supply of fuel is completely stopped up until the temperature of fuel cell stack  14  drops to the oxidation suppression temperature, there is a danger that fuel electrodes will be oxidized and damaged, therefore the supply of air was always stopped. However, the inventor discovered that generating air can be supplied safely over a predetermined time period even immediately after the supply of fuel is stopped. 
     I.e., immediately after a shutdown stop, sufficient fuel gas remains on the fuel electrode side of the fuel cell units  16 , and since this is being jetted out from the top end of the fuel cell units  16 , there is no reverse flow of air to the fuel electrode side caused by feeding air to the air electrode side. In other words, feeding in air in this state under temperature drop operation does cause the pressure on the air electrode side to rise, but the pressure on the fuel electrode side is still higher than the pressure on the air electrode side. The gas flow path fine tubing  98  installed at the top end of the fuel cell units  16  is a constricting flow path with a narrowed flow path cross sectional area, by which the flow velocity of fuel gas flowing out of the fuel cell units  16  is increased. Therefore gas flow path fine tubing  98 , which is installed at the top end, functions as an accelerating portion for increasing the flow velocity of fuel gas. Moreover, after the supply of air has been stopped at time t202, the system is left alone as in stop mode 1, and the pressure on the fuel electrode side is maintained by a mechanical pressure retention means for a predetermined time at a higher level than the pressure on the air electrode side. In stop mode 2, however, because the high temperature air and fuel gas accumulating inside fuel cell module  2  is discharged under temperature drop operation, the natural leaving alone of the system is started from a temperature lower than in stop mode 1. The risk of a reverse flow of air before the fuel electrode temperature drops to the oxidation suppression temperature is therefore decreased. Thus after a shutdown stop, the pressure reduction on the fuel electrode side becomes more gradual than the drop in pressure on the air electrode. Since the temperature inside the fuel cell module  2  is averaged under temperature drop operation, the risk is diminished that fuel gas on the inside of fuel cell units  16  will suddenly shrink and air will be pulled in on the fuel electrode side. 
     Furthermore, after a shutdown stop air is fed to the air electrode side under temperature drop operation, therefore the flame at the top end of gas flow path fine tubing  98  is more quickly extinguished, and consumption of remaining fuel is suppressed. Immediately after a shutdown stop, a large amount of fuel gas jetted from fuel cell units  16  flows out on the air electrode side of fuel cell units  16  without being combusted. In stop mode 2, after a shutdown stop air is fed into the air electrode side and jetted fuel gas is discharged together with air, so the risk that fuel gas which has flowed away from the fuel electrodes will contact the air electrodes and cause a partial reduction of the air electrodes is avoided. 
       FIG. 17  is a diagram explaining the operation of stop mode 1; the top portion shows a graph schematically depicting pressure changes on the fuel electrode side and air electrode side; the middle portion shows the control operations by control section  110  and the temperature inside fuel cell module  2  on a time line, and the bottom portion shows the state at the top end portion of the fuel cell units  16  at each point in time. 
     First, in the middle portion of the  FIG. 17 , an electrical generation operation is being performed before a shutdown stop, and temperature drop operation is executed after the shutdown stop. After a temperature drop operation of approximately 2 minutes, generating air flow regulator unit  45  is stopped, following which the system is left alone as in stop mode 1. However in stop mode 2, the temperature inside fuel cell module  2  at the starting point of leaving the system alone (time t202 in  FIG. 16 ), and the pressure on the fuel electrode side and on the air electrode side, are reduced more than in stop mode 1. For this reason the risk of air penetrating to the fuel electrode side before the fuel electrode temperature drops to the oxidation suppression temperature is still further diminished. 
     Next, referring to  FIGS. 18 through 22 , we explain stop mode 3. 
       FIG. 18  is a timing chart schematically showing in time line form an example of the stopping behavior when stop mode 3 (step S 6  in  FIG. 13 ) is executed in a solid oxide fuel cell system  1  according to a first embodiment of the present invention.  FIG. 19  is a timing chart showing an expanded view immediately after a shutdown stop.  FIG. 20  is a diagram explaining in time line form the control, the temperature inside the fuel cell module, the pressure, and the state of the tip portion of the fuel cell units when stop mode 3 is executed.  FIG. 22  is a timing chart showing a variant of stop mode 3. 
     First, stop mode 3 is a stop mode which is executed when a stop switch has been operated by a user, or with a program stop. As shown in  FIGS. 18 and 19 , temperature drop operation is also executed during stop mode 3, but the temperature drop operation in stop mode 3 comprises a first temperature drop step prior to the complete stopping of power extraction from fuel cell stack  14 , and a temperature drop step after power extraction is stopped. The second temperature drop step is the same as the temperature drop operation in stop mode 2, and the first temperature drop step is executed as pre-shutdown operation before power extraction is stopped. 
     In the example shown in  FIG. 19 , a stop switch is operated by a user at time t301, and pre-shutdown operation, which is the first temperature drop step, is started. In pre-shutdown operation, the output to inverter  54  by fuel cell module  2  is first stopped. This results in a sudden drop in the current and power extracted from fuel cell module  2 , as shown by the light dot and dash line in  FIG. 19 . Note that in pre-shutdown operation, the current output to inverter  54  from fuel cell module  2  is stopped, but extraction of a degree of weak current (approximately 1 A) for operating the solid oxide fuel cell system  1  auxiliary unit  4  is continued for a predetermined time. Therefore even after the generated current has greatly decreased at time t301, a weak current is extracted from fuel cell module  2  during pre-shutdown operation. As shown by the dotted line in  FIG. 19 , the output voltage on fuel cell module  2  rises as extracted current drops. Thus in pre-shutdown operation, the amount of power extracted is restricted, and electrical generation at a predetermined power is continued while a weak current is extracted, therefore since a part of the supplied fuel is used for electrical generation, an extraordinary increase in surplus fuel not used for electrical generation is avoided, and the temperature inside fuel cell module  2  is decreased. 
     Moreover, in pre-shutdown operation, after time t301 the fuel supply amounts shown by the dotted line and the reforming water supply amounts shown by the light solid line in  FIG. 19  are linearly decreased. On the other hand, the amount of generating air supplied, as shown by the thick dot and dash line, is linearly increased. Therefore during pre-shutdown operation, more air is supplied than the amount corresponding to the power extracted from fuel cell module  2 . By increasing the air supply amount in this way, robbing of heat from reformer  20  and a rise in temperature within fuel cell module  2  are suppressed. Continuing, in the example shown in  FIG. 19 , at time t302 approximately 20 seconds after time t301, the fuel supply amount and water supply amount are reduced to the supply amounts which correspond to the weak current extracted from fuel cell module  2 ; thereafter a reduced supply amount is maintained. By reducing the fuel supply amount and water supply amount in this way as pre-shutdown operation, air current turbulence within fuel cell module  2  caused by the sudden stopping of a large flow volume of fuel when the fuel supply is fully stopped is prevented, and large quantities of fuel are kept from accumulating in reformer  20  and manifold  66  after the supply of fuel is completely stopped. Note that after time t301, the temperature of the air on the air electrode side inside fuel cell module  2 , shown by the heavy solid line in  FIG. 19 , is reduced by lowering the fuel supply amount and increasing the air supply amount. However, a large heat quantity is still accumulated in thermal insulation  7 , etc. surrounding fuel cell module  2 . Also, while the current output to inverter  54  is stopped under pre-shutdown operation, the supply of fuel and water are continued, therefore even if the supply of generating air is continued, no reverse flow of air to the fuel electrode side occurs within the fuel cell units  16 . Therefore the supply of air can be safely continued. 
     In the example shown in  FIG. 19 , from time t301 at which pre-shutdown operation is started until time t303 approximately 2 minutes later, the fuel supply amount and reforming water supply amount are brought to zero, and current extracted from fuel cell module  2  is also brought to zero and a shutdown stop effected. Note that in the example shown in  FIG. 19 , at time t303 the water supply amount is increased slightly immediately before the current extracted from fuel cell module  2  is brought to zero. This increase in the water supply amount adjusts the water amount at the time of a shutdown stop so that an appropriate amount of water remains in vaporizing section  20   a . This control of the water supply amount is discussed below. 
     In the example shown in  FIG. 19 , even after the shutdown stop at time t303, the supply of generating air is continued as a second temperature drop step in the temperature drop operation (although electrical generation is completely stopped). By so doing, the air in fuel cell module  2  (on the air electrode side of fuel cell stack  14 ), the remaining fuel combustion gas, and the fuel from the fuel electrode side of fuel cell stack  14  after a shutdown stop are discharged, so the second temperature drop step functions as an exhaust step. At time t303 in the embodiment, after the supply of fuel has been completely stopped, the supply of a large volume of generating air is continued for a predetermined time until time t304. The amount of generating air supplied is increased up to the maximum air supply amount during pre-shutdown operation, after which it is maintained at the maximum value. 
     As shown in  FIG. 18 , at time  304  the system is left alone, as in stop mode 1, after the supply of generating air is stopped. However, in stop mode 3 a first temperature drop step is executed before a shutdown stop, and a second temperature drop step is executed after a shutdown stop, therefore the temperature decrease in part A of  FIG. 18  is greater than in stop modes 1 and 2, and the leaving alone of the system is started from a lower temperature and a lower pressure state. 
       FIG. 20  is a diagram explaining the operation of stop mode 3; the top portion shows a graph schematically depicting pressure changes on the fuel electrode side and air electrode side; the middle portion shows the control operations by control section  110  and the temperature inside fuel cell module  2  on a time line, and the bottom portion shows the state at the top end portion of the fuel cell units  16  at each point in time. 
     First, before the stop switch is operated in the middle portion of  FIG. 20 , a generating operation is being performed; after the stop switch is operated a pre-shutdown operation step is executed, being the first temperature drop step. In the pre-shutdown operation step, the amount of fuel gas supplied is decreased, as shown in the bottom portion (1) of  FIG. 20 , causing the flame at the top end of the fuel cell units  16  to decline in size, as shown in the lower portion (2) of the figure. Since the amounts of fuel gas supplied and electricity generated are decreased in this way, the temperature inside fuel cell module  2  is decreased more than during the electrical generation operation. After an approximately 2 minute pre-shutdown operation step, a shutdown stop is performed. After the shutdown stop, generating air is supplied for 2 minutes by generating air flow regulator unit  45  as a second temperature drop step. After the second temperature drop step, generating air flow regulator unit  45  is stopped, following which the system is left alone, as in stop mode 1. 
     As described above, at the time of a shutdown stop the pressure on the fuel electrode side of the fuel cell units  16  is higher than the pressure on the air electrode side, therefore fuel gas on the fuel electrode side jets out from the top end of the fuel cell units  16  even after the fuel supply is stopped. In addition, flame resulting from the combustion of fuel gas is extinguished at the time of a shutdown stop. After a shutdown stop, the quantity of fuel gas jetted from the top end of each individual fuel cell unit  16  is greatest immediately after a shutdown stop, then declines gradually. This large amount of fuel gas jetted immediately after a shutdown stop is discharged to the outside of fuel cell module  2  by generating air supplied in the second temperature drop step (exhaust step). Even after the exhaust step ends, fuel gas is jetted from the top ends of the fuel cell units  16 , but the quantity of that fuel gas is relatively small. 
     For this reason hydrogen, which is the fuel gas jetted after completion of the exhaust step, accumulates at the top portion inside fuel cell module  2  (above the fuel cell stack  14 ), but jetted fuel gas makes no substantial contact with the air electrodes in the fuel cell units  16 . Therefore fuel gas is reduced subjected to a reduction reaction by contact with high temperature air electrodes, and there is no degradation of the air electrodes. In pre-shutdown operation prior to a shutdown stop, water is supplied so that an appropriate amount of water within a predetermined range of quantities is accumulated within vaporizing section  20   a . Therefore in the exhaust step following a shutdown stop, the pressure on the fuel electrode side of the fuel cell units  16  is increased by the vaporization of water in vaporizing section  20   a , and an appropriate amount of fuel gas is jetted from the top end of the fuel cell units  16 . Fuel gas jetted during the exhaust step is quickly exhausted from fuel cell module  2 . Since an appropriate amount of fuel gas is jetted out in the exhaust step, it does not occur after the exhaust step that an excessive amount of fuel gas is jetted out from the fuel cell units  16 , degrading the air electrodes. 
     Here, in stop mode 3, after completion of the exhaust step, the temperature inside fuel cell module  2  at the starting point of leaving the system alone (time t304 in  FIG. 18 ), and the pressure on the fuel electrode side and on the air electrode side, are reduced more than in stop mode 1. Also, in stop mode 3 the fuel gas supply amount and water supply amount prior to shutdown stop are fixed at a predetermined value by the pre-shutdown operation step. This leads to a decline in the degree of variability of pressure and temperature distribution, etc. when the system is initially left alone, which is dependent on the operating state during electrical generation, and the system is always left alone from an appropriate starting state. Therefore the risk of air invading the fuel electrode side before the fuel electrode temperature drops to the oxidation suppression temperature is extremely low. 
     Next, referring to  FIG. 21 , we explain the supply of water in pre-shutdown operation. 
       FIG. 21  is a flow chart of the supply of water in pre-shutdown operation; during the operation of solid oxide fuel cell system  1 , this is repeatedly executed at a predetermined time interval by shutdown stop circuit  110   a . First, at step S 11  in  FIG. 21 , a determination is made as to whether pre-shutdown operation has started. If pre-shutdown operation has started, the system advances to step S 12 ; if it has not started, one iteration of the  FIG. 21  flow chart is completed. 
     Next, in step S 12 , as a water supply securing step, a hot water radiator (not shown) built into hot water production device  50  ( FIG. 1 ) is activated for 2 minutes. This hot water radiator heats water by performing a heat exchange with high temperature exhaust gas from fuel cell module  2 , recovering discharged heat in the exhaust gas. At the same time, the exhaust gas contains steam, and a heat exchange is carried out between this steam and the hot water radiator, whereby the steam turns into water due to cooling and condenses. By activating the hot water radiator, the amount of cooling of exhaust gas increases, and the amount of condensed water increases. The increased condensed water is recovered and stored in pure water tank  26  ( FIG. 1 ). Water recovered in this pure water tank  26  is utilized as water for steam reforming after passing through filter processing, etc. (not shown). Water produced by this processing in step S 12  is utilized to supply water during pre-shutdown operation. Note that while the amount of water used during pre-shutdown operation and pressure retention operation is slight, high temperature exhaust gas containing large amounts of steam is suddenly cooled by the hot water radiator (not shown), therefore the needed water can be sufficiently acquired in 2 minutes during pre-shutdown operation. 
     Next, in step S 13 , time line data W0 for the quantity of electrical generation during the 10 minutes immediately prior to time t301 in  FIG. 19 , when pre-shutdown operation is begun, is read into control section  110 . Also, an average value W1 for 10 minutes of read-in electrical generation amount time line data W0 is calculated in step S 14 . Next, at step S 15 , a difference W2 is calculated between the solid oxide fuel cell system  1  maximum rated generating amount and the average value W1. In addition, at step S 16  an insufficient water quantity Q1 is also calculated based on the difference W2. Finally, in step S 17 , the quantity Q1 of water calculated to be lacking is supplied before the end of pre-shutdown operation (immediately before time t303 in  FIG. 19 ), and one iteration of the flow chart in  FIG. 21  is completed. 
     As a result of supplying this insufficient water quantity Q1, approximately the same quantity of reforming water is accumulated in vaporizing section  20   a  as when a shutdown stop is performed following continuous operation at the maximum rated generating amount. Vaporization of this water in the exhaust step following a shutdown stop (time t303-t304 in  FIG. 19 ) causes the pressure on the fuel electrode side of the fuel cell units  16  to increase, and an appropriate amount of fuel gas is jetted from the top end of the fuel cell units  16 . 
     Next, referring to  FIG. 22 , we explain a variant example of stop mode 3. 
     In the variant example shown in  FIG. 22 , the way in which generating air is supplied in the second temperature drop step differs from  FIG. 19 . As shown in  FIG. 22 , in this variant example, after a shutdown stop is performed at time t303, generating air is supplied in the maximum amount until time t304. At time t304, the amount of generating air supplied is decreased in stages, and supply is continued at the reduced supply amount until time t305. The interval between time t303 and t304 is preferably set at approximately 2 to 5 minutes, and the interval between time t304 and t305 is set at approximately 2 to 20 minutes. 
     In this variant example, high temperature air on the air electrode side is quickly discharged by supplying a large quantity of generating air under high pressure on the fuel electrode side immediately after a shutdown stop. At the same time, if a certain amount of time has elapsed since a shutdown stop and pressure on the fuel electrode has dropped, reducing the amount of generating air supplied causes high temperature air to be discharged while avoiding the risk of reverse flow. 
     Using a solid oxide fuel cell system  1  of a first embodiment of the present invention, by providing a mechanical pressure retention means, reverse flows of air can be prevented until the temperature of fuel electrodes drops to the oxidation suppression temperature, thereby suppressing the oxidation of fuel electrodes. I.e., a mechanical pressure retention means is formed by the fuel/exhaust gas passageway ( FIG. 2 ) which guides fuel/exhaust gas from fuel flow regulator unit  38  through reformer  20 , and the fuel electrodes in the fuel cell units  16  which constitute fuel cell units  16 , to outside fuel cell module  2 . The fuel passageway is constituted to run from fuel flow regulator unit  38  through reformer  20  and the fuel electrodes of fuel cell units  16  to the air electrode side. The exhaust gas passage is arranged to run from the air electrode side within fuel cell module  2  to the atmosphere outside fuel cell module  2 . In the present embodiment, by appropriately distributing the balance of flow path resistance, etc. in each part of these fuel/exhaust gas passageways, the present embodiment succeeds, after a shutdown stop, at maintaining a pressure greater than atmospheric pressure on the air electrode side and maintaining a pressure on the fuel electrode side greater than on the air electrode side, until the temperature drops to the oxidation suppression temperature, at which the risk of fuel electrode oxidation diminishes. In this way, reverse flows of air from the air electrode side to the fuel electrode side are prevented. Using this mechanical pressure retention means, reverse flows of air can be prevented until the temperature of fuel electrodes drops to the oxidation suppression temperature, thereby suppressing the oxidation of fuel electrodes, even when the supply of fuel and commercial power to the solid oxide fuel cell system of the present embodiment is lost due to a natural disaster or the like. 
     Also, using the solid oxide fuel cell system of the present embodiment, pressure on the fuel electrode side can, by the mechanical pressure retention means, be decreased while maintaining a higher pressure than on the air electrode until the temperature drops to the oxidation suppression temperature. Here, in general solid oxide fuel cell system  1  operates at a high temperature and has a large heat capacity, therefore the temperature drop behavior after a shutdown stop (time t101 in  FIG. 14 ) and the pressure drop behavior on the fuel electrode side and air electrode side are not prone to be influenced by the outside air temperature, etc. The present inventor, taking note of this point, constructed a fuel/exhaust gas passageway such that the pressure drop on the fuel electrode side and air electrode side after a shutdown stop occurs while maintaining a higher pressure on the fuel electrode side than on the air electrode side (upper portion of  FIG. 15 ). By so doing, reverse flows of air can be prevented until the temperature drops to the oxidation suppression temperature using only a mechanical structure, without being substantially affected by outside air temperature or the like. 
     Furthermore, the inventor discovered that the pressure change behavior on the fuel electrode side and air electrode side after a shutdown stop ( FIG. 14 , time t101) changes with the balance of flow resistance in each part of the fuel/exhaust gas fuel passageway, and that by setting an appropriate balance, the pressure on the fuel electrode side can be maintained at a higher pressure than on the air electrode side over a long time period. Using the solid oxide fuel cell system  1  of the present embodiment, the fuel/exhaust gas passageway flow resistance balance can be adjusted by setting the flow path resistance of fuel gas flow path fine tubing  98  ( FIG. 4 ), so the pressure drop on the fuel electrode side can with a simple structure be made more gradual than the pressure drop on the air electrode side (top portion of  FIG. 15 ), and reverse flows of air can be prevented. 
     The solid oxide fuel cell system  1  of the present embodiment comprises a fuel gas flow path fine tubing  98  ( FIG. 4 ) for causing fuel to flow into the fuel electrode side, therefore pressure fluctuations on the upstream side of fuel cell units  16  can be prevented from transferring directly to the fuel electrode side, thereby causing fuel remaining on the fuel electrode side to be pushed out excessively to the air electrode side. This allows residual fuel to be accumulated for a long time period on the fuel electrode side using a mechanical pressure retention means. 
     In addition, using the solid oxide fuel cell system  1  of the present embodiment, fuel gas flow path fine tubing  98  is constituted by elongated narrow tubes installed on inside electrode terminals  86  ( FIG. 4 ) formed as separate bodies, so that flow path resistance can be correctly set. By balancing the flow path resistance with each part, fuel gas flow path fine tubing  98  constitutes a mechanical pressure retention means, so the flow path resistance thereof must be precisely set. It is difficult, however, to set such precise flow path resistances using the materials which make up the individual fuel cell unit  16  main body. In the present embodiment, fuel gas flow path fine tubing  98  is installed on inside electrode terminals  86 , formed as separate bodies, therefore fuel gas flow path fine tubing  98  can be constituted by various materials amenable to precision working, so flow path resistance can be accurately set. 
     Using solid oxide fuel cell system  1  of the present embodiment, inside electrode terminals  86  ( FIG. 4 ) is constituted of metal, and therefore has a high thermal conductivity so that heat on the air electrode side is easily conducted to the fuel electrode side. Therefore sudden drops in the temperature of fuel remaining on the fuel electrode side of fuel cell stack  14  resulting in sudden shrinking of the volume of gas on the fuel electrode side and sucking in of air on the air electrode side to the fuel electrode side can be prevented. 
     Moreover, using the solid oxide fuel cell system  1  of the present embodiment, the pressure on the fuel electrode side is maintained at a higher level than the pressure on the air electrode side until the temperature of the fuel electrode drops to 350° C.-400° C. or below (top portion of  FIG. 15 ), so oxidation of the fuel electrodes can be reliably suppressed. 
     Also, using the solid oxide fuel cell system  1  of the present embodiment, fuel flow regulator unit  38 , generating air flow regulator unit  45 , and water flow volume regulator unit  28  are stopped ( FIG. 14 , times t101-t102) until the fuel electrode temperature drops to the oxidation suppression temperature, so reverse flows of air can be prevented until the fuel electrode temperature drops to the oxidation suppression temperature, even in emergencies when fuel and commercial power are simultaneously lost. 
     Moreover, using the solid oxide fuel cell system  1  of the present embodiment, generating air flow regulator unit  45  is operated ( FIG. 16 , time t201-t202) for a predetermined time after a shutdown stop ( FIG. 16 , time t201), therefore immediately after a shutdown stop the temperature on the air electrode side is decreased and the temperature balanced on the fuel electrode side and the air electrode side, and the temperature inside fuel cell module  2  is decreased. By this means, after a generating air flow regulator unit  45  which has been operated for a predetermined time is stopped, an initial state is established when the prevention of reverse flow of air is started by the mechanical pressure retention means, and reverse flows can be reliably prevented until the temperature drops to the oxidation suppression temperature. 
     Also, using the solid oxide fuel cell system  1  of the present embodiment, immediately prior to a shutdown ( FIG. 19 , time t303), the amount of electricity generated is reduced to a fixed value and the amount of air supplied is increased ( FIG. 19 , time t301-t303), so the temperature and pressure on the fuel electrode side and air electrode side at time of shutdown stop can be set to an appropriate value. In this way, after a shutdown stop an initial state can be established when the prevention of reverse flow of air is started by a mechanical pressure retention means, and reverse flows can be reliably prevented until the temperature drops to the oxidation suppression temperature. 
     Next, referring to the  FIGS. 23 through 26 , we discuss the control of a solid oxide fuel cell system according to a second embodiment of the present invention. 
     In the solid oxide fuel cell system of the present embodiment, processing at time of stopping differs from the above-described first embodiment. Therefore here we will explain only the parts which differ between the first embodiment and second embodiment of the present invention; the description in the first embodiment shall be modified and used for those parts which are the same, and an explanation thereof omitted. 
       FIG. 24  is a block diagram showing a solid oxide fuel cell system according to a second embodiment of the present invention. As shown in  FIG. 23 , the constitution of solid oxide fuel cell system  200  according to the present embodiment is the same as the first embodiment except that pressure retention operation circuit  110   b  is built into control section  110 . The same reference numerals are used for the same constituent parts, and an explanation thereof is omitted. Below, the same reference numerals are assigned as in the first embodiment, even for constituent parts not shown in  FIG. 23 . 
       FIG. 24  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system  200  according to a second embodiment of the present invention. In addition to the stop modes 1-3 in the first embodiment, this embodiment comprises a stop mode 4, which is executed for a program stop. The purpose of the  FIG. 24  flow chart is to determine which of the stop modes to select based on predetermined conditions; during operation of the solid oxide fuel cell system  1 , this flow chart sequence is repeated at a predetermined time interval. 
     First, at step S 2  in  FIG. 24 , a judgment is made of whether the supply of fuel gas from fuel supply source  30  ( FIG. 1 ) and the supply of power from a commercial power source has been stopped. If the supplies of both fuel gas and power have been stopped, the system advances to step S 22 ; at step S 22 , stop mode 1, which is the emergency stop mode, is selected, and one iteration of the processing in the  FIG. 24  flow chart is completed. The conditions under which stop mode 1 is selected and the processing executed in stop mode 1 are the same in the first embodiment, so an explanation thereof is here omitted. 
     On the other hand, when at least either fuel gas or power is being supplied, the system advances to step S 3 , and in step S 23  a decision is made as to whether the supply of fuel gas has been stopped and power is being supplied. When the supply of fuel gas has been stopped and power is being supplied, the system advances to step S 24 ; in cases other than this the system advances to step S 25 . In step S 24 , stop mode 2, which is one of the normal stop modes, is selected, and one iteration of the  FIG. 24  flow chart processing is completed. The conditions under which stop mode 2 is selected and the control executed in stop mode 2 are the same in the first embodiment, so an explanation thereof is here omitted. 
     Furthermore, in step S 25  a determination is made as to whether a stop switch has been operated by a user. If a user has operated a stop switch, the system advances to step S 26 ; if the switch has not been operated, the system advances to step S 27 . In step S 26 , stop mode 3, which is one of the switch stop modes among the normal stop modes, is selected, and one iteration of processing the  FIG. 24  flow chart is completed. In this embodiment, stop mode 3 is selected only when the stop switch has been operated by a user. The control executed in stop mode 3 is the same as in the first embodiment, therefore an explanation thereof is here omitted. 
     In step S 27 , on the other hand, a determination is made of whether the stoppage is a regular stoppage, executed at a pre-planned regular timing. If the stop is a regular stop, the system advances to step S 28 ; if it is not a regular stop, one iteration of the  FIG. 24  flow chart is completed. In step S 28 , stop mode 4, which is one of the program stop modes among the normal stop modes, is selected, and one iteration of processing the  FIG. 24  flow chart is completed. In the present embodiment, stop mode 4 is selected only when a regular stop is being implemented. Responding to an intelligent meter installed on fuel supply source  30  is assumed as a circumstance for the selection of stop mode 4. Therefore in the present embodiment it is anticipated that a stoppage by stop mode 4 will be performed at a frequency of approximately once per month, which is the most frequently performed stoppage. 
     Note that if the supply of power is stopped and the supply of fuel gas is continued, neither of the stop modes will be selected according to the  FIG. 24  flow chart. In such cases, in a solid oxide fuel cell system  1  according to the embodiment, electrical generation can be continued by activating auxiliary unit  4  using power produced by fuel cell stack  14 . Note that the invention can also be constituted to stop electrical generation if the supply of power stays stopped continuously over a predetermined long time period. 
     Next, referring to  FIGS. 25 and 26 , we explain stop mode 4. 
       FIG. 25  is a timing chart schematically showing on a time line an example of the stopping behavior when stop mode 4 (step S 28  in  FIG. 24 ) is executed in a solid oxide fuel cell system according to a first embodiment of the present invention.  FIG. 26  is a diagram explaining on a time line the control, the temperature inside the fuel cell module, the pressure, and the state of the front end portion of the fuel cell units when stop mode 4 is executed. 
     First, as described above, stop mode 4 is a stop which is executed on the order of once per month in order to respond to an intelligent meter (not shown), and is the most frequently executed stop mode. Therefore when executing stop mode 4, oxidation of fuel electrodes must be more reliably prevented, since even a slight negative effect from the oxidation of the fuel electrodes on fuel cell units  16 , etc. imparts a large effect on the durability of fuel cell stack  14 . Stopping by stop mode 4 is executed periodically based on a program built into shutdown stop circuit  110   a.    
     First, at time t401 in  FIG. 25 , at a predetermined prior to the shutdown stop time planned in the shutdown stop circuit  110   a  program, shutdown stop circuit  110   a  executes a temperature drop operation. As in stop mode 3, in stop mode 4 the temperature drop operation is executed by a first temperature drop step and a second temperature drop step. I.e., in pre-processing for stopping, which is the first temperature reduction step, the output of generated power to inverter  54  by fuel cell module  2  is first stopped, and only the extraction of a weak current (appropriate 1 A) for the operating solid oxide fuel cell system auxiliary unit  4  is continued. In pre-shutdown operation, as noted above, the water supply flow for pre-shutdown operation shown in  FIG. 21  is also executed. 
     Moreover, in pre-shutdown operation, after time t401 the fuel supply amounts shown by the dotted line and the reforming water supply amounts shown by the heavy dotted line in  FIG. 25  are linearly decreased. On the other hand, the amount of generating air supplied, as shown by the heavy dot and dash line, is increased. In stop mode 4, the first temperature drop step is continued for 10 minutes after time t401, a longer period than stop mode 3. 
     At time t402, when 10 minutes have elapsed following time t401, shutdown stop circuit  110   a  executes a shutdown stop. When a shutdown stop is performed, the supply of fuel by fuel flow regulator unit  38  and the supply of water by water flow volume regulator unit  28  are stopped in a short period of time. The extraction of power from fuel cell module  2  by inverter  54  is also stopped (output current=0). 
     Shutdown stop circuit  110   a  executes the second temperature drop step out of the temperature drop operations after a shutdown stop at time t402, and generating air flow regulator unit  45  is operated at maximum output for appropriate 2 minutes. In addition, at time t403 in  FIG. 25 , as in stop mode 1, after generating air flow regulator unit  45  is stopped, the system is left alone. 
     Furthermore, in stop mode 4, at time t404 when approximately 5 hours have elapsed after a shutdown stop, and the temperature inside fuel cell module  2  has fallen to a predetermined temperature, shutdown stop circuit  110   a  activates pressure retention operation circuit  110   b  ( FIG. 23 ). In this embodiment, when the temperature inside fuel cell module  2  drops to a predetermined temperature of 400° C., the pressure on the fuel electrode side of fuel cell units  16  also falls, and approaches the pressure on the air electrode side. Pressure retention control circuit  110   b  sends a signal to water flow volume regulator unit  28 , thereby activating it. The activation of water flow volume regulator unit  28  results in water being supplied to vaporizing section  20   a  in reformer  20 . The interior of fuel cell module  2  is still at a temperature of approximately 400° C. even at time t404 when approximately 5 hours have elapsed after a shutdown stop, so water supplied to vaporizing section  20   a  is vaporized there. Note that in this embodiment, water is supplied intermittently, and the water supply amount is set at approximately 1 mL per minute; this water supply amount value is below the minimum water supply amount in the electrical generation operation. 
     Vaporization and expansion of water in vaporizing section  20   a  raises the pressure inside the fuel gas passageway from reformer  20  through fuel gas supply pipe  64  and manifold  66  ( FIG. 2 ) up to the fuel cell units  16 . Thus pressure drops on the fuel electrode side of fuel cell units  16  are suppressed, and a reverse flow of air to the fuel electrode side is more reliably prevented. Note that the flow paths for vaporizing section  20   a , reforming section  20   b , and reforming section  20   c  in reformer  20  are all formed in a serpentine shape, making it difficult for the effects of a pressure rise to be transferred downstream even if water suddenly vaporizes inside vaporizing section  20   a . Sudden rises in pressure on the inside of fuel cell units  16  (the fuel electrode side) caused by sudden vaporization, such that fuel gas accumulated therein is jetted out in large quantities over a short time period, can thus be prevented. 
     Pressure fluctuation-suppressing flow resistance section  64   c  ( FIG. 2 ), which is installed midway on fuel gas supply pipe  64 , and gas flow path fine tubing  98 , which is an inflow-side flow resistance section installed at the bottom end of the fuel cell units  16 , also suppress sudden pressure rises on the fuel electrode side, and cause fuel gas to remain for long time periods on the fuel electrode side. 
     Pressure retention control circuit  110   b  stops water flow volume regulator unit  28  at time t405 in  FIG. 25  when the temperature inside fuel cell module  2  has dropped to the oxidation temperature; thereafter fuel cell module  2  is left alone. 
     Furthermore, at time t406 when the temperature inside fuel cell module  2  has further dropped, shutdown stop circuit  110   a  sends a signal to reforming air flow regulator unit  44  and generating air flow regulator unit  45 , activating those units. By this means the fuel gas pathways such as reformer  20 , fuel gas supply pipe  64 , and manifold  66 , and the internal fuel electrodes in the fuel cell units  16 , are purged by air. The inside of exhaust gas passageways such as the air electrode side inside generating chamber  10 , the exhaust pathway  21   b , and in air-heat exchanger  22  are also purged by air. By purging fuel gas passageways and fuel electrodes, steam which had been held within these locations is condensed, and oxidation by condensate water on the fuel gas passageways and fuel electrodes is prevented. By purging the inside of exhaust gas passageways, condensation within exhaust gas passageways of steam discharged from the fuel electrodes is prevented. Also, by purging the air electrode side within generating chamber  10 , a reduction reaction by discharged fuel gas from the fuel electrode side is prevented. 
       FIG. 26  is a diagram explaining the operation of stop mode 4; the top portion shows a graph schematically depicting pressure changes on the fuel electrode side and air electrode side; the middle portion shows the control operations by control section  110  and the temperature inside fuel cell module  2  on a time line, and the bottom portion shows the state at the top end portion of the fuel cell units  16  at each point in time. 
     First, before shutdown stop in the middle portion of  FIG. 26 , electrical generation is occurring, and 10 minutes before the shutdown stop time planned in the program, a first temperature drop step under temperature drop operation is executed. In stop mode 4, because the first temperature drop step is executed for approximately 10 minutes, the temperature inside fuel cell module  2  at the time of shutdown stop, as well as the pressure on the fuel electrode side and the air electrode side, are decreased by a greater amount than in stop mode 3. After a shutdown stop, generating air is supplied for approximately 2 minutes as the second temperature drop step of the temperature drop operation, and generating air flow regulator unit  45  is stopped. After generating air flow regulator unit  45  is stopped, the system is left alone, as in stop mode 3. Here, in stop mode 4, when the system is initially left alone (time t403 in  FIG. 25 ), the temperature inside fuel cell module  2  and the pressures on the fuel electrode side and the air electrode side are decreased even more than in stop mode 3. For this reason the risk of air penetrating to the fuel electrode side before the fuel electrode temperature drops to the oxidation suppression temperature is still further diminished. 
     In addition, in stop mode 4 at the point in time when the pressure on the fuel electrode side of the fuel cell units  16  has approached the pressure on the air electrode side by being left alone, pressure retention operation circuit  110   b  is activated, and the pressure on the fuel electrodes in the fuel cell units  16  is increased. Under pressure retention operation, reformed fuel gas which had been accumulating in manifold  66  and in fuel gas supply pipe  64  ( FIG. 2 ), etc. is first fed a little at a time to the fuel electrodes in the fuel cell units  16 , then the unreformed fuel gas which had remained inside reformer  20  is fed a little at a time to the fuel electrodes. In addition, after all the unreformed fuel gas is fed in, the steam vaporized in vaporizing section  20   a  is fed in a little at a time to fuel electrodes in fuel cell units  16 . At the point when pressure retention operation circuit  110   b  is activated, the temperature at the fuel electrodes in the fuel cell units  16  has fallen to close to the oxidation suppression temperature, therefore even if a reverse flow of air to the fuel electrode side occurs, the effect thereof is minute. However, the program stop which executes stop mode 4 is the most often executed stop mode, therefore the risk of fuel electrode oxidation is even further reduced, and the effects of oxidation of each fuel cell units  16  are reduced to a minimum. 
     As indicated on the left side of the upper portion of  FIG. 26 , in stop modes 1 through 3 the pressure on the fuel electrode side of fuel cell units  16  drops after a shutdown stop and approaches the pressure on the air electrode side around the time the fuel electrode temperature drops down to the region of the oxidation suppression temperature. In response, in stop mode 4, as shown on the right side of the upper portion of  FIG. 26 , pressure retention operation by pressure retention operation circuit  110   b  is executed in the region where pressure on the fuel electrode side approaches the pressure on the air electrode side, and a drop in pressure on the fuel electrode side below that on the air electrode side is more reliably prevented. 
     As shown in the bottom portion of  FIG. 26 , when the system is left alone after completion of the second temperature drop step (“left alone 1” in the middle portion of  FIG. 24 ), the fuel gas which had been accumulating at the fuel electrodes in the fuel cell units  16  flows out a little at a time, and at the end of that time the air on the air electrode side can in some cases begin to diffuse to the fuel electrode side (bottom portion (1) in  FIG. 26 ). However, because pressure retention operation is started, the pressure of the steam produced inside vaporizing section  20   a  causes fuel gas accumulated inside the fuel gas passageways on the downstream side of reformer  20  to again move into the fuel cell units  16 , so that the concentration of fuel gas inside the fuel electrodes again rises (lower portion (2) of  FIG. 26 ). Later, as well, steam is produced within vaporizing section  20   a  under pressure retention operation, so the outflowing portion of fuel gas from the fuel electrodes in the fuel cell units  16  is compensated by the fuel gas which had been accumulating in the fuel gas passageway, and a reverse flow of air to the fuel electrode is prevented. Furthermore, in the pressure retention operation terminating phase, as shown in the bottom portion (3) of  FIG. 26 , even if accumulated fuel gas has almost completely flowed out, steam produced by the pressure retention operation fills in the fuel electrodes in fuel cell units  16 , so a reverse flow of air to the fuel electrodes is reliably prevented. 
     In addition, after completion of the pressure retention operation, the system is left alone (“left alone 2” in the middle portion of  FIG. 26 ), following which reforming air and generating air are supplied (reforming and electrical generation are not performed), and a purge is executed. Thus the fuel gas and steam remaining on the fuel electrode side of fuel cell units  16  are discharged, and the fuel gas remaining on the air electrode side in generating chamber  10  is also discharged from fuel cell module  2 . By this means, oxidation of the fuel electrodes in the fuel cell units  16  is reliably avoided in the most frequently executed stop mode 4. 
     In the solid oxide fuel cell system  200  of the second embodiment of the present invention, the provision of a mechanical pressure retention means and pressure retention operation enables the prevention of a reverse flow of air from the air electrode side to the fuel electrode side. 
     As explained with respect to the first embodiment of the invention, the risk of fuel electrode oxidation is well reduced by the mechanical pressure retention means, but when unanticipated external disturbances occur, such as changes in atmospheric pressure outside design values, there is a risk that the pressure on the fuel electrode side cannot be maintained at a temperature at which there is no oxidation of fuel electrodes whatsoever (the oxidation lower limit temperature) using only this mechanical means. Pressure retention control circuit  110   b  therefore executes a pressure retention operation ( FIG. 25 , time t404-t405) for increasing the pressure on the fuel electrode side after the fuel electrode temperature has declined to the oxidation suppression temperature. Pressure retention control is executed after the fuel electrode temperature has dropped to the oxidation suppression temperature, therefore the pressure on both the fuel electrode side and the air electrode side has dropped greatly, and it is sufficient just to slightly compensate the pressure on the fuel electrode side. Also, because pressure retention operation is executed with the risk of oxidation sufficiently reduced, oxidation of the fuel electrodes can be reliably prevented without implementing precise controls. 
     Using the solid oxide fuel cell system  200  of a second embodiment of the present invention, no control by pressure retaining control circuit  110   b  is executed when stopping under the emergency stop mode (stop mode 1) ( FIG. 24 , step S 22 ). The simultaneous loss of fuel and power supplied to solid oxide fuel cell system  200  is anticipated as a circumstance in which the emergency stop mode is executed; in such circumstances the pressure retention operation ( FIG. 25 , time t404-t405) is not executed, and the risk of fuel electrode oxidation can be decreased by a mechanical pressure retention means alone. By not executing pressure retention operation, secondary problems occurring when active control is executed can be avoided in the abnormal conditions when the emergency stop mode is executed. Moreover, the certainty of avoiding fuel electrode oxidation is diminished by not executing a pressure retention operation in the emergency stop mode, but since the frequency with which the emergency stop mode is executed is extremely low, decreasing the risk of fuel electrode oxidation using a mechanical pressure retention means enables the effect on the durable life span of solid oxide fuel cell system  200  to be reduced to a negligible level. 
     Also, using the solid oxide fuel cell system  200  of the present embodiment a pressure retention operation ( FIG. 25 , time t404-t405) is executed in the program stop mode ( FIG. 24 , step S 28 ; stop mode 4), so the pressure retention operation is executed at a pre-planned time. For this reason no problems are prevented even when a pressure retention operation is executed after a shutdown stop, and a long period of time is required until the pressure retention operation stops. Also, because the program stop mode is anticipated to be executed to respond to an intelligent meter, it is executed at a high frequency, and the durable lifespan of solid oxide fuel cell system  200  can be extended by executing pressure retention operation during stop modes which executed at a high frequency, thereby reliably avoiding the risk of fuel electrode oxidation. 
     Also, using the solid oxide fuel cell system  200  of the present embodiment, no temperature drop operation is executed when stopping in the emergency stop mode (stop mode 1), and the fuel electrode oxidation risk can be decreased by the mechanical pressure retention means alone. On the other hand when stopping by a normal stop mode (stop modes 3, 4), the temperature and pressures on the fuel electrode side and air electrode side can be dropped at the time when mechanical pressure retention is started ( FIG. 25 , time t403) by performing a temperature drop operation ( FIG. 25 , time t402-t403) after a shutdown stop, and the risk of a reverse flow of air during the time when the temperature of the fuel electrodes is being decreases to the oxidation suppression temperature can be further diminished. 
     Also, in the second embodiment of the present invention described above, stop mode 3 was executed when a stop switch was operated by a user ( FIG. 24 , step S 25 ), but as a variant example, stop mode 2 could also be executed, as shown in  FIG. 27 .  FIG. 27  is a flow chart of the stopping decision which selects a stop mode in a solid oxide fuel cell system according to a variant example of the second embodiment of the present invention. I.e., in this variant example, stop mode 2 is executed when fuel gas is stopped and only electricity is being supplied ( FIG. 27 , step S 33 →S 34 ), and when a stop switch has been operated by a user ( FIG. 27 , step S 35 →S 34 ). According to this variant example, if a stop switch is operated, a shutdown stop is executed without executing pre-shutdown operation (the first temperature drop step), therefore controls for a shutdown stop can be quickly completed after a user operates the stop switch. 
     We have described above a preferred embodiment of the present invention, but various changes may be added to the above-described embodiments. 
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               1 : solid oxide fuel cell system 
               2 : fuel cell module 
               4 : auxiliary unit 
               7 : insulation (heat storage material) 
               8 : case 
               8   a : communication opening 
               8   b : suspended wall 
               10 : generating chamber 
               12 : fuel cell assembly 
               14 : fuel cell stack 
               16 : fuel cell units (individual solid oxide fuel cells) 
               18 : combustion chamber (combustion section) 
               20 : reformer 
               20   a : vaporizing section (vaporization chamber) 
               20   b : blending section (pressure fluctuation absorption means) 
               20   c : reforming section 
               20   d : steam/blending section partition 
               20   e : partition opening 
               20   f : blending/reforming section partition 
               20   g : communicating holes (narrow flow paths) 
               21 : flow straightening plate (partition wall) 
               21   a : opening portion 
               21   b : exhaust pathway 
               21   c : gas holding space 
               21   d : vertical wall 
               22 : air heat exchanger (heat exchanger) 
               23 : vaporization chamber insulation (internal insulation) 
               24 : water supply source 
               26 : pure water tank 
               28 : water flow regulator unit (water supply apparatus) 
               30 : fuel supply source 
               38 : fuel flow regulator unit (fuel supply apparatus) 
               39 : valve 
               40 : air supply source 
               44 : reforming air flow regulator unit (reforming oxidant gas supply apparatus) 
               45 : generating air flow regulator unit (generating oxidant gas supply apparatus) 
               46 : first heater 
               48 : second heater 
               50 : hot water production device (heat exchanger for waste heat recovery) 
               52 : control box 
               54 : inverter 
               62 : reformer introducing pipe  62 : reformer introducing pipe (water introducing pipe, preheating section, condensing section) 
               62   a : T-pipe  62   a  (condensing section) 
               63   a : water supply piping 
               63   b : fuel gas supply piping 
               64 : fuel gas supply pipe 
               64   c : flow resistance section for pressure fluctuation suppression 
               66 : manifold (dispersion chamber) 
               76 : air introducing pipe 
               76   a : jet outlets 
               82 : exhaust gas discharge pipe 
               83 : ignition apparatus 
               84 : individual fuel cells 
               85 : exhaust valve 
               86 : inside electrode terminals (caps) 
               98 : fuel gas flow path fine tubing (inflow-side flow resistance section, outflow-side flow resistance section; constricted flow path, acceleration section) 
               110 : control section (controller) 
               110   a : shutdown stop circuit 
               112 : operating device 
               114 : display device 
               116 : warning device 
               126 : electrical power state detecting sensor (demand power detection means) 
               132 : fuel flow volume sensor (fuel supply amount detection sensor) 
               138 : pressure sensor (reformer pressure sensor) 
               142 : generating chamber temperature sensor (temperature detection means) 
               148 : reformer temperature sensor 
               150 : outside air temperature sensor