Abstract:
Proton-exchange membrane fuel-cell power generating equipment includes a heat exchanger coupled to a process burner and, therethrough, to a fan. When water needs to be heated, such as during startup, the water is circulated through the heat exchanger and the process burner is operated (ignited) to heat the water. When the water needs to be cooled, such as when a hot water reserving tank is full, the water is circulated through the heat exchanger and the fan is operated, but the process burner is not operated, to cool the water. Water is circulated through part or all of a water system to prevent freezing while the system is stopped. Optionally, the process burner is operated to heat the circulated water. The heat exchanger and other heat exchangers in the system are arranged to efficiently recover heat from burners, a fuel-cell cooling system and exothermic processes.

Description:
FIELD OF THE INVENTION 
   The present invention relates to proton-exchange membrane fuel cell power generating equipment and, more particularly, to exhaust heat recovery and freezing prevention devices in such equipment. 
   BACKGROUND ART 
   Conventional proton-exchange membrane fuel cell power generating equipment suitable as a small power supply typically comprises a reformer for reforming fuel gas, such as natural gas, town gas, methanol, liquefied petroleum gas (LPG), or butane, to hydrogen rich gas; a CO transformer for transforming carbon monoxide to CO 2 ; CO removing apparatus for removing the carbon monoxide; a process gas burner for burning hydrogen until each reactor becomes stable during startup; a fuel cell for chemically reacting the hydrogen with oxygen from the air to generate power; a water tank for storing water that is treated by water treating apparatus using an ion-exchange resin or the like to cool an electrode part of the fuel cell and moisten the reaction air; a heat exchanger for recovering heat from exhaust gas from the reformer, the fuel cell, the process gas burner, or the like to produce hot water; and a hot water reserving tank for reserving the hot water. 
   A solid polymer electrolyte membrane used in such proton-exchange membrane fuel cell power generating equipment functions as a proton conductive electrolyte by containing water. The proton-exchange membrane fuel cell is operated by saturating the reaction air or reaction gas, such as fuel gas, with steam and supplying it to the electrode part. 
   When fuel gas containing hydrogen is fed to a fuel electrode and air is fed to an air electrode, a fuel electrode reaction for decomposing hydrogen molecules to hydrogen ions and electrons is performed in the fuel electrode, and an electrochemical reaction for generating water from oxygen with hydrogen ions in the air electrode occurs. Thus, the electrons moving through an external circuit from the fuel electrode to the air electrode carry power to a load and generate water on the air electrode side.  FIG. 7  is a diagram of a conventional proton-exchange membrane fuel cell power generating equipment system (PEFC equipment GS). PEFC equipment GS typically includes an exhaust heat recovery device RD in addition to a fuel cell  6 . The exhaust heat recovery device RD is coupled to a hot water reserving tank  50 , heat exchanges  32 ,  46 ,  71 , and pumps  33 ,  47 ,  72  through a hot water circuit or the like. 
   The fuel cell  6  has fuel gas feeding apparatus comprising a desulfurizer  2 ; a reformer  3 ; a CO transformer  4 ; CO removing apparatus  5  and the like; reaction air feeding apparatus comprising an air pump  11 , a water tank  21  (distinct from the hot water reserving tank  50 ), and the like; electrodes, such as a fuel electrode  6   a  and an air electrode  6   k ; and cooling apparatus of the fuel cell  6 , comprising the water tank  21 , a pump  48 , a cooling section  6   c , and the like. 
   Power generated by the fuel cell  6  is increased in voltage by a direct current DC/DC converter (not shown) and is supplied to the commercial power supply via an electric distribution system cooperation inverter (not shown). The power is supplied from the power supply to houses or offices to be used for illumination or electric equipment, such as air conditioners. 
   The PEFC equipment GS uses the fuel cell  6  to generate power and uses heat generated at the same time to produce hot water from city water, accumulates the hot water in the hot water reserving tank  50 , and supplies the hot water, such as for use in a bath or kitchen. 
   In the fuel gas feeding apparatus of the PEFC equipment GS, raw fuel  1 , such as natural gas, town gas, methanol, LPG, or butane, is supplied to the desulfurizer  2 , and here sulfur components are removed from the raw fuel. The raw fuel, having passed through the desulfurizer  2 , is pressurized by a pressurizing pump  10  and supplied to the reformer  3 . The raw fuel, while being supplied, is mixed with steam produced by feeding hot water from the water tank  21  through a water pump  22  and heating the hot water in a heat exchanger  17 . The reformer  3  produces reformed gas containing hydrogen, carbon dioxide, and carbon monoxide. The reformed gas produced in the reformer  3  is supplied to the CO transformer  4 , and here the carbon monoxide contained in the reformed gas is transformed to carbon dioxide. The gas from the CO transformer  4  is supplied to the CO removing apparatus  5 . In the CO removing apparatus  5 , untransformed carbon monoxide in the gas supplied from the CO transformer  4  is reduced to 10 ppm or less, and water gas (reformed gas) having a high hydrogen concentration is supplied to the fuel electrode  6   a  of the fuel cell  6  through a pipe  64 . The amount of hot water supplied from the water tank  21  to the reformer  3  is adjusted to control moisture concentration in reformed gas. 
   In the reaction air feeding apparatus, air is fed from the air pump  11  to the water tank  21 , and the reaction air is whipped in the hot water in the water tank  21  and is fed to a gas phase section  53 , thereby moistening the reaction air. The air is moistened to facilitate the reaction in the fuel cell  6 . The moistened reaction air is fed to the air electrode  6   k  of the fuel cell  6  from the water tank  21  through a pipe  25 . The fuel cell  6  generates power by an electrochemical reaction of the hydrogen of the reformed gas fed to the fuel electrode  6   a  with oxygen in the air supplied to the air electrode  6   k  through the air pump  11  and the gas phase section  53  in the water tank  21 . 
   The cooling apparatus of the fuel cell  6  is arranged along with the electrodes  6   a ,  6   k  of the fuel cell  6 , and prevents the fuel cell  6  from being overheated by heat of the electrochemical reaction. The cooling apparatus circulates water from the water tank  21  as a coolant to a cooling section  6   c  with a pump  48 , and the coolant maintains a proper temperature in the fuel cell  6  (for example, 70–80° C.) for the power generation. 
   The chemical reaction in the reformer  3  is an endothermic reaction, so that a burner  12  provides heat to the reformer  3  to maintain the chemical reaction. To the burner  12 , raw fuel is supplied through a pipe  13 , air is fed through a fan  14 , and unreacted hydrogen is supplied from the fuel electrode  6   a  through a pipe  15 . During startup of the PEFC equipment GS, the raw fuel is supplied through the pipe  13  to the burner  12 . When the temperature of the fuel cell  6  becomes stable, the supply of the raw fuel through the pipe  13  is stopped, and, instead, the unreacted hydrogen (off-gas) discharged from the fuel electrode  6   a  is supplied through the pipe  15  to continue the combustion. 
   The chemical reactions performed in the CO transformer  4  and the CO removing apparatus  5  are exothermic reactions. During their operation, the CO transformer  4  and the CO removing apparatus  5  are cooled to prevent the CO transformer and the CO removing apparatus from reaching a reaction temperature. Predetermined chemical reactions and power generation occur in the reformer  3 , the CO transformer  4 , the CO removing apparatus  5 , and the fuel cell  6 . 
   Heat exchangers  18  and  19  are installed between the reformer  3  and the CO transformer  4 , and between the CO transformer  4  and the CO removing apparatus  5 , respectively. The water supplied from the water tank  21  circulates in the respective heat exchangers  18 ,  19  via pumps  23 ,  24 , and cools respective gasses fed from the reformer  3  and the CO transformer  4 . Another heat exchanger (not shown) may be also installed between the CO removing apparatus  5  and the fuel cell  6  to cool gas fed from the CO removing apparatus  5 . 
   The heat exchanger  17  is connected to an exhaust system  31  of the reformer  3 . When water is supplied from the water tank  21  via a pump  22 , the heat exchanger  17  vaporizes the water to produce steam, and the steam mixes with the raw fuel and is fed to the reformer  3 . 
   The PEFC equipment GS has a process gas burner (PG burner)  34 . During-startup of the PEFC equipment GS, the composition of the reformed gas fed through the reformer  3 , the CO transformer  4 , and the CO removing apparatus  5  does not reach a defined stable value preferable for the operation of the fuel cell  6 . Therefore, the gas cannot be fed to the fuel cell  6  until the composition becomes stable. Until each reactor becomes stable, the gas is guided to the PG burner  34  and burned in it. A fan  37  feeds air for combustion to the PG burner  34 . 
   After each reactor becomes stable and the CO concentration in the gas reaches a defined value (for example, 10–20 ppm or lower), the gas is guided to the fuel cell  6  for power generation. Unreacted gas (off-gas) that cannot be used for power generation in the fuel cell  6  is initially guided to the PG burner  34  and burned, and, after the temperature of the fuel cell  6  becomes stable, the unreacted gas is guided to the burner  12  of the reformer  3  through a pipe  15 . 
   Until each reactor becomes stable in temperature, on-off valve  91  is closed, and the reformed gas is fed to the PG burner  34  through the duct  35  and on-off valve  36 . Even after the reactors become stable in temperature, until the temperature of the fuel cell  6  becomes stable in a range that is appropriate for producing electricity (for example, 70–80° C.), the on-off valve  91  is opened while the on-off valve  92  is closed, and the reformed gas is fed to the PG burner  34  through a duct  38  and an on-off valve  39 , and the gas is burned in the PG burner. When the temperature of the fuel cell  6  becomes stable and appropriate for continuous power generation, both the on-off valve  91  and the on-off valve  92  are opened, while the on-off valve  36  and the on-off valve  39  are closed, and the unreacted gas (off-gas) is fed from the fuel cell  6  to the burner  12  through a duct  15 . 
   City water is supplied to the hot water reserving tank  50  through an inlet  61 . The water in the hot water reserving tank  50  is heated by exhaust heat generated from the PEFC equipment GS, and the heated water is supplied through the hot water supply pipe  62  to, for example, a kitchen, lavatory, or bath. 
   PEFC equipment typically includes several water circuits for recovering heat from exhaust gases. For example, heat is recovered from these exhaust gases and stored in the hot water reserving tank  50 . Exhaust system  31  is connected to heat exchanger  32 , in addition to heat exchanger  17 , and the water in the hot water reserving tank  50  circulates in the heat exchanger  32  via pump  33  to recover heat from the exhaust gases passing through the exhaust system  31 . Heat exchanger  46  is connected to exhaust system  45  of the PG burner  34 , and the water in the hot water reserving tank  50  circulates in the heat exchanger  46  via pump  47  for exhaust heat recovery. 
   Heat is also recovered from exothermic chemical processes and stored in water tank  21 . Water returned from the heat exchangers  18 ,  19  by pumps  23 ,  24 , and coolant circulating in the cooling section  6   c  of the fuel cell  6 , through water duct  73  and pump  48 , flow into the water tank  21 . 
   Water refilling apparatus  68  is connected to the water tank  21  to maintain a water level in the tank  21 . The water refilling apparatus  68  includes an electromagnetic valve  56 , a supply tank  67 , and a pump  74 . The supply tank  67  temporarily reserves water from city water refilling apparatus  69  and water recovered from the fuel cell  6  through a pipe  70 . The supply tank  67  supplies the water to the water tank  21  as needed to maintain the water level in the tank  21 . Water generated from the fuel cell  6  includes drain water obtained by a cooling system that includes the heat exchanger  71 , the hot water reserving tank  50 , circulating pump  72 , as well as the water (condensate) contained in the gas exhausted from the fuel electrode  6   a.    
   The city water refilling apparatus  69  is connected to the water source  78  and includes an inlet  52  and an electromagnetic valve  76 , which is opened by water level controller  77  when a water level gauge  79  detects an insufficient amount of water in the supply tank  67 . The supply tank  67  is refilled through the inlet  52  and water treating apparatus (ion-exchange resin)  51 , which uses the water pressure of the water source  78 . 
   The water tank  21  includes a water level controller LC for keeping a water level sufficient to form an air section (gas phase section)  53  in the upper part of the tank and a temperature adjusting apparatus TC for keeping the temperature of the water in the tank  21  in a predetermined range. The water level controller LC includes a water level gauge  54  and an electromagnetic valve  56  for monitoring the water level in the water tank  21  and adding water as needed. Air passing through the water tank  21  is moisturized before being supplied to the fuel cell to facilitate the fuel cell reaction. The water level controller LC controls the water level so as to form the gas phase section  53  in the upper part of the water tank  21 . When the water level decreases, the water level controller LC operates the pump  74 , adjusts the opening of the electromagnetic valve  56  to feed treated water from the supply tank  67 , through the pipe  84 , into the water tank  21 . The controller LC thus keeps the water level in the water tank  21  within the predetermined range. 
   A wave breaking plate  55  prevents the water level detection by the water level gauge  54  from being destabilized by foaming. The temperature control apparatus TC keeps the temperature of the water in the water tank  21  at a predetermined value or range, for example 60–80° C., so as to properly moisturize the reaction air supplied to the air electrode  6   k  of the fuel cell  6 . 
   The water in the water tank  21  is heated by a heating device  63  installed in the water tank  21  if necessary. 
   The heat exchange between the water in the hot water reserving tank  50  and the heat exchanger in the fuel cell power generating equipment GS, as shown in  FIG. 7 , uses: a first circuit R 1  between the tank and the first heat exchanger  32 , through which combustion exhaust gas from the burner  12  of the reformer  3  passes; a second circuit R 2  between the tank and the second heat exchanger  46  through which combustion exhaust gas from the PG burner  34  passes; and a third circuit R 3  between the tank and a third heat exchanger  71  through which non-reacted oxygen gas exhausted from the air electrode of the fuel cell  6  passes. In other words, the heat exchange is performed between water fed from the hot water reserving tank  50  and the combustion exhaust gases or unreacted oxygen gas from the first heat exchanger  32 , the second heat exchanger  46 , and the third heat exchanger  71 . 
   The PEFC equipment GS as discussed above is configured as a cogeneration system for power generation and heat utilization, so that the power generating efficiency of the fuel cell is relatively high and the water used in the system is effectively recycled. 
   However, when the hot water reserving tank  50  is full of hot water of a predetermined temperature, and the hot water cannot be discharged to the outside through the hot water supply pipe  62 , additional exhaust heat from the gases cannot be recovered. Therefore, in order to keep the temperature of the coolant in the fuel cell  6  in a predetermined range, another cooling apparatus, such as a radiator (not shown), must be installed or operation of the equipment must be stopped. The installation of the cooling apparatus increases cost, and it makes reducing the size of the PEFC equipment GS difficult. 
   A layer of water at room temperature lies at the bottom of the hot water reserving tank  50 , and a hot water layer with a lower density lies in the upper part of the tank. If no hot water is drawn from the hot water reserving tank  50  for an extended period of time while the system operates, the water in the bottom of the tank is heated by the heat exchanger and moves to the upper part, so that the hot water layer gradually increases and finally the tank is entirely filled with hot water. 
   When hot water is drawn from the hot water tank  50 , hot water in the upper part is taken out to reduce the hot water layer, and city water is added into the bottom in proportion to the amount of hot water drawn off, resulting in an increase of the cooler water layer. Therefore, the temperature of the water fed from the bottom of the hot water reserving tank  50  to the heat exchangers is not constant, and heat exchanging efficiencies of the heat exchangers fluctuate. Furthermore, combustion exhaust gases passing through the three heat exchangers  32 ,  46 ,  71  contain gases having different temperatures. Consequently, the heat exchanging efficiencies fluctuate according to the temperature fluctuations of the water fed from the hot water reserving tank  50 , resulting in low efficiency when the temperature difference is small. 
   In addition, if the water in the tank  21  decreases in temperature and freezes the water tank  21 , the fuel cell  6 , as well as piping lines and valves may be damaged, resulting in a malfunction of the equipment. 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the invention to provide proton-exchange membrane fuel cell power generating equipment that addresses the problems discussed above. According to one aspect of the presently disclosed system, PEFC equipment maintains the temperature of the coolant of the fuel cell  6  within a predetermined range, without additional cooling apparatus and without stopping operation of the power generating equipment, even if the hot water reserving tank  50  becomes full of hot water and the hot water can not be discharged from the system. 
   According to another aspect of the presently disclosed system, the PEFC equipment prevents freezing of the water system while operation of the power generating equipment is stopped. This prevents damage to, for example, the water tank  21 , the fuel cell  6 , the hot water reserving tank  50 , the heat exchangers  32 ,  46 ,  71 , the piping system, the valves, the pumps, and the pipes in the water system. 
   According to yet another aspect of the presently disclosed system, the PEFC equipment provides improved exhaust heat recovery from the plural heat exchangers. 
   According to another aspect of the presently disclosed system, the PEFC equipment recovers heat from a fuel cell cooling subsystem. 
   According to yet another aspect of the presently disclosed system, the PEFC equipment heats a fuel cell using a combination of hot gas and hot liquid. 
   According to one embodiment of the present invention, proton-exchange membrane fuel cell power generating equipment includes a reformer for reforming fuel gas, such as natural gas, town gas, methanol, LPG, or butane, to hydrogen rich gas; a CO transformer for transforming carbon monoxide; CO removing apparatus for removing the carbon monoxide; a process gas burner for burning hydrogen until each reactor becomes stable; a fuel cell for generating power using hydrogen; a water tank for storing water for cooling the fuel cell; a heat exchanger for recovering heat from combined exhaust gas of the reformer, the fuel cell and/or the process gas burner and heating the water to produce hot water; and a hot water reserving tank for reserving the hot water. 
   The power generating equipment has a line for circulating water between the hot water reserving tank and the heat exchanger connected to the process gas burner. The temperature of the water is maintained by exchanging heat in the heat exchanger to either heat or cool the water in the water tank, as necessary. If the water in the water tank becomes too hot, i.e., the water reaches a predetermined high temperature, the hot water is circulated through the line to cool the water by radiation from the line. Optionally, a fan that is otherwise used for feeding air to the process gas burner for combustion is operated to cool the hot water as the hot water flows through the heat exchanger. During this time, the process burner is not operated, i.e., gas is not burned in the process burner. During this time, the heat exchanger acts as a cooler, and the cooled water is fed back to the water tank. If the water in the water tank becomes too cool, i.e., the water reaches a predetermined low temperature, the water can be circulated through the heat exchanger connected to the process burner and the process burner can be operated, i.e. gas is burned in the process burner, to heat the water. Thus, the temperature of the hot water in the water tank is kept within a predetermined range, without ceasing operation of the power generating equipment or installing separate cooling apparatus, such as a radiator. 
   A fuel cell cooling subsystem circulates water from a water tank (distinct from the hot water reserving tank) through the fuel cell and then back to the water tank. At startup time, if the water in the water tank is below a predetermined temperature, the process gas burner is operated and water is circulated through the heat exchanger connected to the process burner. This heated water is circulated through a heat exchanger in the water tank to heat the water in the water tank. Since the water in the water tank circulates through the fuel cell, the fuel cell is heated to an operating temperature more quickly. Once the water in the water tank reaches the predetermined temperature, the process burner and the water circulation through the heat exchanger in the water tank are stopped. 
   The fuel cell can also be heated, such as during startup, by circulating warm water through the cooling section of the fuel cell and/or by blowing hot air through the air electrode of the fuel cell. The air can be heated in the fuel cell cooling subsystem water tank. 
   During operation of the fuel cell, if the water in the water tank of the fuel cell cooling subsystem increases and reaches a predetermined temperature, water is circulated through the heat exchanger in the water tank to recover heat from the water in the water tank. The recovered heat is transferred to the hot water reserving tank. Maintaining the water in the fuel cell cooling subsystem at a relatively constant value or in a predetermined range increases the efficiency of chemical reactions. As noted above, if the water in the hot water reserving tank becomes too hot, water is circulated through the heat exchanger connected to the process burner to “dump” the excess heat. 
   The power generating equipment has a control system which prevents freezing of water in the equipment while the equipment is stopped. If the control system detects a risk of freezing, hot water from the hot water reserving tank is circulated through part or all of the water system. The process gas burner can, but need not, be operated to heat the water. 
   If the control system detects a risk of freezing in the water tank of the fuel cell cooling subsystem, hot water from the how water reserving tank is circulated through the heat exchanger in the water tank of the fuel cell cooling system, thereby preventing freezing. If the control system detects a risk of freezing in the fuel cell, warm water from the fuel cell cooling system water tank is circulated through the cooling section of the fuel cell, thereby preventing freezing. 
   In one embodiment, a means for detecting a possibility of freezing is a means for detecting the temperature of the water tank. When the temperature of the water tank is below a predetermined value, hot water is circulated through part or all of the water system to prevent freezing. 
   In another embodiment, the means for detecting a possibility of freezing is a means for detecting the temperature of the fuel cell body. In yet another embodiment, the means for detecting a possibility of freezing is a means for detecting the temperature of the atmosphere in the power generating equipment. 
   The proton-exchange membrane fuel cell power generating equipment of the present invention has the following control system. When the temperature of the water, for example, in the water tank  21 , falls to about 2° C. while the power generating equipment is not operating, or the temperature of the fuel cell  6  body or the temperature of the atmosphere in the power generating equipment falls to a point where a risk of freezing exists, the control system operates the process gas burner  34 , heats hot water in the hot water reserving tank  50 , circulates hot water in a part or the whole of the water system, including the water tank  21 , and operates the pump  48  to circulate hot water in the cooling section  6   c  of the fuel cell  6  to increase the temperature of the fuel cell  6  body, thereby preventing freezing. Optionally, the control system circulates the hot water in the hot water reserving tank  50 , without operating the process burner  34 . The control system prevents damage due to freezing of the fuel cell  6  body, the water tank  21 , the fuel cell  6 , the hot water reserving tank  50 , and the water system, including heat exchangers  32 ,  46 ,  71 , the piping system, valves, pumps, and pipes, saving maintenance manpower in a cold region or in the winter season and improving reliability. The power generating equipment can, therefore, be used as, for example, a small power supply for a household in a cold region. 
   The hot water reserving tank and the plural heat exchangers installed in the fuel cell power generating equipment are interconnected by piping to form a loop-like (serial) duct. Water in the hot water reserving tank passes sequentially through the respective heat exchangers via the duct to heat the water. The water passes through the heat exchangers in an order, i.e. through heat exchangers operating at lower temperatures, then through heat exchangers operating at progressively higher temperatures and finally to the hot water reserving tank. The first heat exchanger is heated by combustion exhaust gas from the reformer burner of the reforming apparatus. The second heat exchanger is heated by combustion exhaust gas from the PG (process gas) burner. The third heat exchanger is thermally coupled to the fuel cell. The fourth heat exchanger is connected to a duct, through which the combined combustion exhaust gases from the reformer burner and the PG burner and the non-reacted oxygen gas from the fuel cell flow. 
   In the above-mentioned fuel cell power generating equipment, the loop-like duct extends from the hot water reserving tank, through the fourth heat exchanger, the third heat exchanger, and the first heat exchanger (in that order), then back to the hot water reserving tank. A first selector valve is installed in the duct between the heat exchanger and the hot water reserving tank. A branch duct branches from an intermediate point between the first selector valve and the second heat exchanger. The branch duct connects to the water tank. On the upstream side of the water tank in the branch duct, a second selector valve is installed. When the water temperature of the water tank is at or above a predetermined temperature, during power generation by the fuel cell, the first selector valve is closed and the second selector valve is opened to pass water through the branch duct to recover heat from the water tank. When the water temperature of the water tank is below the predetermined temperature, the first selector valve is opened and the second selector valve is closed, and no water flows through the branch duct. 
   Unlike the prior art, in which the hot water reserving tank is connected to each of several heat exchangers by a separate circuit, according to the present disclosure, a series of ducts form a loop-like duct that connects plural heat exchangers in series, and the water flows sequentially through the heat exchangers in a particular order, i.e., through heat exchangers operating at progressively higher temperatures. Therefore, even though the temperature of the water from the hot water reserving tank fluctuates, the heat exchanging efficiencies of the heat exchangers is increased. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings, in which: 
       FIG. 1  is a system diagram of proton-exchange membrane fuel cell power generating equipment in accordance with an exemplary embodiment of the present invention; 
       FIG. 2  is a diagram of an exemplary hot water flow in the heat recovery device in the proton-exchange membrane fuel cell power generating equipment of  FIG. 1 ; 
       FIG. 3  is a diagram of another hot water flow in the heat recovery device in the proton-exchange membrane fuel cell power generating equipment of  FIG. 1 ; 
       FIG. 4  is a diagram of an exemplary hot water flow for preventing freezing in the proton-exchange membrane fuel cell power generating equipment of  FIG. 1 ; 
       FIG. 5  is a diagram of another exemplary the hot water flow for preventing freezing in the proton-exchange membrane fuel cell power generating equipment of  FIG. 1 ; 
       FIG. 6  is a block diagram of a part of the heat recovery route of an embodiment of the power generating equipment of  FIG. 1 ; and 
       FIG. 7  is a system diagram of conventional proton-exchange membrane fuel cell power generating equipment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  to  FIG. 6  illustrate the proton-exchange membrane fuel cell power generating equipment in accordance with an exemplary embodiment of the present invention. Elements in  FIGS. 1–6  that are similar to those in  FIG. 7  have the same reference numbers as in  FIG. 7 , and duplicate descriptions for those elements are omitted. 
   The proton-exchange membrane fuel cell power generating equipment GS 1  of the present invention shown in  FIG. 1  includes a heat exchanger HEX at the back of the heat exchanger  71  for extracting heat from a combination of gases exhausted from several sources, including the heat exchanger  32  of the exhaust system  31 , the heat exchanger  46  of the exhaust from the process gas burner  34 , and the air electrode  6   k  of the fuel cell  6 . The equipment also includes a line L 1  for circulating hot water through a heat exchanger located in the water tank  21 . The water in the hot water reserving tank  50  recovers exhaust heat through it&#39;s the water&#39;s circulation among the heat exchanger HEX and the heat exchangers  71 ,  32 ,  46 . Pump P circulates the water among these heat exchangers and the hot water reserving tank  50 . The power generating equipment has a line L 2  for feeding the hot water to the hot water reserving tank  50  when the hot water does not need to be fed to the water tank  21  through the line L 1 . The line L 1  has an on-off valve  82 , and the line L 2  has an on-off valve  81 . A water pipe  73  has a thermometer T 1  indicating the temperature of coolant flowing through the pipe. A thermometer T 2  is disposed in the water tank  21  for detecting the temperature of the water in the tank  21 . The proton-exchange membrane fuel cell power generating equipment GS 1  of the present invention is similar to the equipment shown in  FIG. 7 , however, among other things, the power generating equipment GS 1  includes a heat recovery device RD  1 . 
   1. (During Startup of the Proton-exchange Membrane Fuel Cell Power Generating Equipment GS 1  of the Present Invention) 
   During startup of the fuel cell  6 , the fan  37  and the PG burner  34  are activated. If the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is lower than a predetermined value (for example, less than 80° C.), the on-off valve  81  is closed and the on-off valve  82  is opened to circulate water (that has been heated by recovering exhaust heat) into the line L 1  to heat the water in the water tank  21 . (See  FIG. 2 .) The table in  FIG. 2  indicates open/close states of the on-off valves  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 . 
   If the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is reaches or exceedsthe predetermined value (for example, 80° C. or higher), the on-off valve  81  is opened and the on-off valve  82  is closed to feed water (that has been heated by recovering exhaust heat) to the hot water reserving tank  50  through the line L 2 . (See  FIG. 3 .) The table in  FIG. 3  indicates open/close states of the on-off valves  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 ). 
   2. (During Power Generation by the Proton-Exchange Membrane Fuel Cell Power Generating Equipment GS 1  of the Present Invention) 
   During power generation by the fuel cell  6 , the fan  37  and the PG burner  34  are typically stopped. If the hot water reserving tank  50  is not filled with hot water and the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is lower than the predetermined value (for example, lower than 80° C.), the on-off valve  81  is opened and the on-off valve  82  is closed so as not to feed hot water to the line L 1 , but to feed the water (that has been heated by recovering exhaust heat) to the water reserving tank  50  through the line L 2 . (See  FIG. 3 .) The table in  FIG. 3  indicates open/close states of the on-off valve  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 . 
   If the hot water reserving tank  50  is not filled with hot water, but the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is equal to or greater than the predetermined value (for example, 80° C. or higher), the on-off valve  81  is closed and the on-off valve  82  is opened to circulate water through the line L 1  to cool the water in the water tank  21 . (See  FIG. 2 .) Heat is recovered from the tank  21  and stored in the hot water reserving tank  50 . The table in  FIG. 2  indicates open/close states of the on-off valve  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 . 
   3. (During Power Generation by the Proton-Exchange Membrane Fuel Cell Power Generating Equipment GS 1  of the Present Invention, if the Hot Water Reserving Tank  50  is Filled with Hot Water) 
   If the hot water reserving tank  50  is filled with hot water of a predetermined temperature during power generation by the fuel cell  6 , and the hot water is not supplied to the outside through the hot water supply pipe  62 , additional exhaust heat of the PEFC equipment GS 1  cannot be recovered. Therefore, if the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is equal to or greater than the predetermined value (for example, 80° C. or higher), the fan  37  for feeding air to the PG burner  34  is operated (without operating the PG burner) to decrease the temperature of the hot water using the heat exchanger  46  as a cooler. The cooled water is circulated through the line L 1  by closing the on-off valve  81  and opening the on-off valve  82  to cool the water in the water tank  21 . (See  FIG. 2 .) The table in the  FIG. 2  indicates open/close states of the on-off valve  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 . 
   If the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is lower than the predetermined value (for example, lower than 80° C.), the on-off valve  81  is opened and the on-off valve  82  is closed, so as not to feed hot water to the line L 1 , but to feed the water (that has been heated by recovering exhaust heat) to the hot water reserving tank  50  through the line L 2 . (See  FIG. 3 .) The table in the  FIG. 3  indicates open/close states of the on-off valves  81 ,  82  and operating or stopping of the fan  37  and the PG burner  34 . 
   The on-off valves  81 ,  82  can be opened or closed manually, and the fan  37  and the PG burner  34  can be operated or stopped manually, too. However, preferably, these operations are performed automatically with a controller (not shown). 
   4. (If Water in the Water Tank  21  is Likely to Freeze While Operation of the Proton-Exchange Membrane Fuel Cell Power Generating Equipment GS 1  of the Present Invention is Stopped) 
   As shown in  FIG. 4 , if the temperature of the water tank  21  becomes less than or equal to a predetermined value (for example, 2° C. or lower), such that there is a possibility that water in the system will freeze, a controller (not shown) sends a signal to the PG burner  34 , the fan  37 , the on-off valves  81 ,  82 , and the pump P to activate the fan  37  and to ignite the PG burner  34 . The controller also closes the on-off valve  81  of the line L 2 , opens the on-off valve  82  of the line L 1 , and operates the pump P to circulate hot water from the hot water reserving tank  50 , including water whose temperature is increased by recovering heat at the heat exchanger  46  connected to the PG burner  34 , resulting in heating the water of the water tank  21 . The table in  FIG. 4  indicates open/close states of the on-off valves  81 ,  82  and operating states of the fan  37  and the PG burner  34 . 
   If a temperature detecting means (thermometer) (not shown) detects that the temperature of the fuel cell  6  body is less than or equal to a predetermined value (for example, 2° C. or lower), such that there is a possibility that the fuel cell  6  body will freeze, the PG burner  34  is activated and the controller (not shown) sends a signal to the pump  48  to operate the pump  48  to circulate the hot water in a cooling section  6   c  of the fuel cell  6  to increase the temperature of the fuel cell  6  body to prevent freezing. 
   If a temperature detecting means (thermometer) (not shown) detects that the temperature of the atmosphere in the fuel cell power generating equipment GS 1  is less than or equal to a predetermined value (for example, 2° C. or lower), such that there is a possibility that the water system will freeze, the PG burner  34  is activated and the controller (not shown) sends a signal to the pump  48  to operate the pump  48  to circulate hot water in a cooling section  6   c  of the fuel cell  6  to increase the temperature of the fuel cell  6  body to prevent freezing. 
   The operation of the process gas burner  34  has been described above. However, the hot water of the hot water reserving tank  50  could be circulated in a part or the whole of the water system without operating the process gas burner  34  to prevent freezing. 
   If the temperature (measured by the thermometer T 2 ) of the water in the tank  21  is equal to or greater than 10° C., for example, a controller (not shown) sends a signal to the PG burner  34 , the fan  37 , and the pump P (exhaust heat recovery pump) to stop operation of them. Thus, the pump P, the PG burner  34 , and the fan  37  are intermittently operated to prevent freezing. 
   5. (If Water in the Water System, Including the Hot Water Reserving Tank  50 , has a Possibility of Freezing While Operation of the Proton-Exchange Membrane Fuel Cell Power Generating Equipment GS 1  of the Present Invention is Stopped) 
   If the temperature of the water in the tank  21  is equal to or greater than the predetermined value (for example, 2° C. or higher) but a temperature monitored and measured by another thermometer (not shown) installed in the water system, including the hot water reserving tank  50 , is less than or equal to the predetermined value (for example, 2° C. or lower), such that there is a possibility of freezing, a controller (not shown) sends a signal to the PG burner  34 , the fan  37 , the on-off valves  81 ,  82 , and the pump P to activate the fan  37  and operate (ignite) the PG burner  34 , and opens the on-off valve  81  of the line L 2 , closes the on-off valve  82  of the line L 1 , and operates the pump P to circulate water through the heat exchanger  46  connected to the PG burner  34 , thereby heating the water of the water system, including the hot water reserving tank  50 . (See  FIG. 5 .) The heavy line shows flow of the hot water, and the table indicates open/close states of the on-off valves  81 ,  82  and operating states of the fan  37  and the PG burner  34 . 
   If the water temperature of the water system, including the hot water reserving tank  50 , is 10° C. or warmer, for example, the controller (not shown) sends a signal to the PG burner  34  and the fan  37  to stop operation of them. Thus, the pump P, the PG burner  34 , and the fan  37  are intermittently operated to prevent freezing. 
   6. (Exhaust Gas Heat Recovering Method 1) 
   The fuel cell  6  normally operates at about 80° C., but heat generation due to the electrochemical reaction sometimes increases this temperature. To prevent such a temperature increase, water is supplied from the water tank  21  to the cooling section  6   c  of the fuel cell  6  by the pump  48  to cool the fuel cell  6 . After cooling the fuel cell  6 , the water is returned to the water tank  21 , but the amount of water in the tank  21  gradually decreases. Therefore, water is added as needed. Pure water is produced by a purifying procedure that uses an ion-exchange resin  51  and a reserve supply tank  67 . Moisture (including moisture from unreacted oxygen gas) collected from the third heat exchanger  71  is also fed to the supply tank  67 . 
   During operation of the fuel cell power generating equipment, water in the bottom of the hot water reserving tank  50  (at approximately 20° C. or room temperature, for example) is taken out by the pump P and fed to the fourth heat exchanger HEX through the first duct S 1 , as shown in  FIG. 1 . The exhaust gas passing through the fourth heat exchanger HEX is a mixture of gases. This mixture includes the combustion exhaust gas from the reformer burner  12 , the combustion exhaust gas from the PG burner  34 , and the non-reacted oxygen gas from the fuel cell  6 . The combustion exhaust gas from the reformer burner  12  passes through the heat exchanger  17  and the first heat exchanger  32 , so that its temperature is decreased before reaching the heat exchanger HEX. The combustion exhaust gas from the PG burner  34  passes through the second heat exchanger  46 , so that its temperature is also decreased before reaching the heat exchanger HEX. The unreacted oxygen gas from the fuel cell  6  passes through the third heat exchanger  71 , so that its temperature is also decreased on its way to the heat exchanger HEX. Therefore, the temperature level of the mixed gas passing through the fourth heat exchanger HEX is low, about 50–60° C. 
   Water heated by the fourth heat exchanger HEX is fed to the third heat exchanger  71  through the second duct S 2 . Heat is extracted from the non-reacted oxygen gas exhausted from the air electrode of the fuel cell  6 . The temperature level in the third heat exchanger  71  is about 70–80° C. 
   For the next step, the water is fed to the first heat exchanger  32  through the third duct S 3 . Heat is extracted from the combustion exhaust gas passing through the first heat exchanger  32 . This combustion exhaust gas is the combustion exhaust gas from the reformer burner  12 , but the gas passes through the heat exchanger  17  before entering the first heat exchanger  32 . As a result, the temperature level of the first heat exchanger  32  is about 100–120° C. 
   The hot water is further fed from the first heat exchanger  32  to the second heat exchanger  46  through the fourth duct S 4 , and heat is extracted from the combustion exhaust gas from the PG burner  34 . The temperature level of the second heat exchanger  46  is about 150–180° C. Hot water flows from the second heat exchanger  46  to the upper part of the hot water reserving tank  50  through the fifth duct S 5 . At this time, the first on-off valve  82  is opened and the second on-off valve  81  is closed. 
   The PG burner  34  is operated during startup, while the reformer  3  is unstable. (As noted above, the PG burner  34  is also sometimes operated when the power generation equipment is not operating.) Generally, after the reformed gas condition becomes stable, the PG burner  34  does not operate, and the second heat exchanger  46  does not extract heat during power generation. On the other hand, the burner  12  is operated to maintain the temperature of catalysts in the reformer  3  at a predetermined level during power generation. The required fuel is supplied by feeding the unreacted oxygen gas exhausted from the fuel electrode of the fuel cell  6  to the burner  12 , as discussed above. 
   The water in the bottom of the hot water reserving tank  50  passes through the heat exchangers sequentially in the order described above, i.e., from the lowest temperature heat exchanger through progressively higher temperature heat exchangers to become hot (about 60–70° C.). The water is returned to the upper part of the hot water reserving tank  50 . The heat exchange efficiency of each heat exchanger is high, because progressively warmer water passes through progressively hotter heat exchangers. Thus, the temperature differential between the water and each progressive heat exchanger is high. 
   7. (Exhaust Gas Heat Recovering Method 2) 
     FIG. 6  shows another exemplary embodiment of the exhaust heat recovering method of the present invention.  FIG. 6  is a block diagram of only a part of the structure of the fuel cell power generating equipment of  FIG. 1 . 
   A loop-like duct is formed that extends from the hot water reserving tank  50  through the fourth heat exchanger HEX, the third heat exchanger  71 , and the first heat exchanger  32 , in that order, then back to the hot water reserving tank  50 . A first selector valve V 1  is installed between the first heat exchanger  32  in the duct and the hot water reserving tank  50 . A branch duct is formed from an intermediate point between the first selector valve V 1  and the first heat exchanger  32 . The branch duct diverts hot water through a heat exchanger in the water tank  21  before returning to the hot water reserving tank  50 . Water in the water tank  21  is usually used to cool the fuel cell  6 . This diversion allows heat to be exchanged with the water in the water tank  21 . A second selector valve V 2  is installed on the upstream side of the water tank  21  in the branch duct. 
   If the temperature of the water in the tank  21  is at or above a predetermined value (for example, 80° C.) during power generation by the fuel cell  6 , the first selector valve V 1  is closed and the second selector valve V 2  is opened. The water taken out of the bottom of the hot water reserving tank  50  by the pump P is fed through the fourth heat exchanger HEX, the third heat exchanger  71 , the first heat exchanger  32  (in this order), and then (via the branch duct) through the heat exchanger in the water tank  21 . The water then returns back to the hot water reserving tank  50 . In this way, the water can recover heat from the water in the water tank  21  and transfer that heat to water in the hot water reserving tank  50 . 
   On the other hand, if the temperature of the water in the tank  21  is less than a predetermined value (for example, 76° C.), the first selector valve V 1  is opened and the second selector valve V 2  is closed. The water taken out of the bottom of the hot water reserving tank  50  is fed by the pump P to the fourth heat exchanger HEX, the third heat exchanger  71 , and the first heat exchanger  32  (in this order), then returned to the hot water reserving tank  50 . In this case, the water is not fed through the water tank  21  through the branch duct. In other words, heat is not recovered from the water tank  21 . 
   When the fuel cell power generating equipment is not operating, the fuel cell  6  is cool, and the water temperature in the water tank  21  can decrease. During cold weather, hot water is fed through the heat exchanger in the water tank  21 . For supplying hot water to the heat exchanger in the water tank  21 , the first selector valve V 1  is closed and the second selector valve V 2  is opened to allow hot water to flow through the branch duct to feed the hot water through the water tank  21 . Then, the water is returned to the hot water reserving tank  50 . 
   If the water in the water tank  21  is hot, air fed through the water tank  21  and then to the air electrode of the fuel cell  6  heats the fuel cell  6  in a short time, thereby shortening starting-up time of the system. 
   While the invention is described through the above-described exemplary embodiments, it will be understood by those of ordinary skill in the art that modifications to, and variations of, the illustrated embodiments may be made without departing from the inventive concepts disclosed herein. Accordingly, the invention should not be viewed as limited, except by the scope and spirit of the appended claims.