Abstract:
A fuel cell system includes a fuel cell apparatus and a heat storage device. The heat storage device is in fluid communication with and responsive to the fuel cell apparatus and is adapted to receive and store heat from and deliver heat to the fuel cell apparatus.

Description:
BACKGROUND OF THE INVENTION  
     The present disclosure relates generally to fuel cell systems, and particularly to thermal management of fuel cell systems. 
     Fuel cell systems may employ electrolysis modules in combination with fuel cell modules, thereby providing a regenerative fuel cell system. A typical fuel cell module receives hydrogen fuel, from either the electrolysis module or through an intermediate hydrogen storage device, and oxygen to generate electricity and product water, while a typical electrolysis module receives process water from a water storage device and electricity to produce hydrogen, oxygen, and byproduct water. Another byproduct of both the electrolysis and fuel cell modules is heat, which is typically distributed throughout the fuel cell system via the product and process water. With the presence of water, it is preferable to operate the fuel cell system at a temperature above the freezing temperature of water, typically zero degree Celsius but with some variation depending on pressure. However, in cold climates or at high altitudes, such cold temperatures are unavoidable. With high altitude airships (HAA), for example, the ambient temperature may reach as low as −55 degree-Celsius or below. In such environments, auxiliary heating systems, such as electric heaters, may be used to prevent water freezing. However, such auxiliary systems have high energy demands. Accordingly, it would be advantageous to have an intelligent fuel cell system that can utilize available thermal energy to maintain operating temperatures above the freezing temperature of water. 
     SUMMARY OF THE INVENTION  
     In one embodiment, a fuel cell system includes a fuel cell apparatus and a heat storage device. The heat storage device is in fluid communication with and responsive to the fuel cell apparatus and is adapted to receive and store heat from and deliver heat to the fuel cell apparatus. 
     In another embodiment, a method of storing and transferring heat within a fuel cell system includes transferring heat originating from an electrolysis module, a fuel cell module, or a hydrogen storage device, to a heat storage device and storing the heat thereat, and transferring heat from the heat storage device to the electrolysis module, the fuel cell module, the hydrogen storage device or a water storage device. In so doing, the temperatures of the electrolysis module, the fuel cell module, the hydrogen storage device and the water storage device are maintained above a predefined temperature. 
     In a further embodiment, a fuel cell system includes a fuel cell apparatus, means for storing a portable heat transfer medium, and means for communicating the portable heat transfer medium between a heat storage device and the fuel cell apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
       Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures: 
         FIG. 1  is an exemplary fuel cell system for implementing an embodiment of the invention; 
         FIG. 2  is an exemplary side view of an energy conversion module for use in the system of  FIG. 1 ; and 
         FIGS. 3–6  are exemplary fuel cell systems depicting alternative heat transfer arrangements to that depicted in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     An embodiment of the invention provides a fuel cell system having a thermal management arrangement for maintaining the temperature of the fuel cell system components above a predefined temperature, such as freezing for example. 
       FIG. 1  is an exemplary embodiment of a fuel cell system (FCS)  150  having a regenerative fuel cell apparatus (RFC)  200  and a heat storage device (HSD)  300  in fluid communication with and responsive to RFC  200 . As used, and described further, herein, HSD  300  is a device for storing heat and not merely for transferring heat from a heat source to a heat sink. As also used herein, fluid communication is intended to apply to a gas, a liquid or a solid as long as the heat transfer medium is portable and therefore fluidly communicable from one location to another. In an embodiment, RFC  200  includes an electrolyzer (electrolysis module)  210 , a hydrogen storage device  220 , a fuel cell (fuel cell module)  230 , a water storage device  240 , and interconnected fluid communication paths  212  (hydrogen),  222  (hydrogen),  232  (product water),  242  (process water). A distinction between FCS  150  and RFC  200  is that FCS  150  includes HSD  300 , whereas RFC  200  does not. RFC  200  is regenerative in that electrolyzer  210  uses water and generates hydrogen for use at fuel cell  230 , and fuel cell  230  uses hydrogen and generates water for use at electrolyzer  210 . RFC  200  and HSD  300  include a thermal conduit system (TCS)  400  (represented by dashed lines) for transferring heat from RFC  200  to HSD  300  and vice versa. Within TCS  400  is a thermal switch (alternatively a heat transfer unit)  410  that directs the flow of a heat transfer medium, such as water for example, according to the operating characteristics of FCS  150 , to be discussed below. A heat transfer unit  420  at water storage device  240  provides a means for transferring heat from HSD  300  to water storage device  240 . A controller  500 , in signal communication with RFC  200 , HSD  300 , the ambient atmosphere  95 , and the system environment  152 , includes a processor  510  that controls the heat transfer between RFC  200  and HSD  300  in response to signals from thermal sensors  90 ,  154 ,  214 ,  224 ,  234 ,  244 , and  302 . An alternative embodiment of controller  500  may be viewed as controlling the system defined by FCS  150 . Fluid communication paths  212 ,  222 ,  232 ,  242 , and TCS  400  include pumps and valves (not shown) that are in signal communication with and controlled by controller  500  for providing the desired water flow. While embodiments of the invention depict water as a heat transfer medium, which is readily mixable with product and process water (system water)  236 ,  246 , alternative portable heat transfer mediums may also be employed where the heat transfer medium and the product/process water are kept isolated from one another. Such alternative portable heat transfer mediums include but are not limited to salt media, saline solution, and phase change material, for example. In an embodiment using a portable heat transfer medium other than water, heat transfer unit  410  serves to limit the interaction between the heat transfer medium and the system water. If a non-water heat transfer medium is employed, then the predefined temperature referred to herein is that temperature at which the heat transfer medium is not effective for transferring heat. For example, the predefined temperature for water may be the freezing temperature of 0-degrees-Celcius (degC.) (but may be other than 0-deg C. depending on the purity level and ambient conditions, such as pressure), the predefined temperature of a saline solution may be substantially lower than 0-deg C., and the predefined temperature of a phase change material may be substantially higher than 0-deg C. In general, the predefined temperature is that temperature at which the heat transfer medium becomes immobile and ineffective in transferring heat. 
     Electrolyzer  210  and fuel cell  230  may, depending on system demand, include any number of energy conversion modules  100 , best seen by referring to  FIG. 2 , which either generate hydrogen (electrolyzer  210 ) or electricity (fuel cell  230 ) depending on the manner employed. Referring now to  FIG. 2 , energy conversion module  100  includes an electrolyte  118  disposed between and in ionic communication with electrodes  114 ,  116 . 
     When energy conversion module  100  is employed as an electrolyzer  210 , electrode  116  is in fluid communication with a water source  240 , while electrode  114  is in fluid communication with fuel cell  230 , preferably via a phase separation device (not shown) and hydrogen storage device  220 . In response to electrodes  114 ,  116  being energized via a power-in device  156 , such as a battery (not shown) or energized fuel cell  230  for example, electrolyzer  210  effectuates the separation of water from water storage  240  to produce hydrogen that is stored at hydrogen storage device  220  for subsequent use at fuel cell  230 . Unprocessed water, a byproduct of electrolyzer  210 , is returned to water storage device  240 . 
     When energy conversion module  100  is employed as a fuel cell  230 , electrode  114  is in fluid communication with a hydrogen supply, such as hydrogen storage device  220  or energized electrolyzer  210  for example, while electrode  116  is in fluid communication with an oxygen supply  151 , via ambient atmosphere  95  and a blower (not shown) for example. In response to the reaction of hydrogen ions and oxygen at electrolyte  118  and between electrodes  114 ,  116 , fuel cell  230  effectuates the recombination of hydrogen and oxygen to produce electricity, designated as power-out  158 , for external consumption, and product water  236 , which is stored at water storage device  240 . 
     The operation of RFC  200  also results in the production of heat at electrolyzer  210  when operating in electrolysis mode ( FIG. 3 ), and at fuel cell  230  when operating in fuel cell mode ( FIG. 4 ). Instead of discharging this byproduct heat to ambient  95 , an embodiment of FCS  150  employs HSD  300  and TCS  400  in conjunction with controller  500  to maintain the temperature of various system components above a predefined temperature, such as the freezing temperature of water for example, thereby enabling cold ambient operation of FCS  150 , such as at high altitudes for example. 
     Referring now to  FIG. 3 , which depicts FCS  150  operating in electrolysis mode, electrolyzer  210  receives power from power-in device  156  and process water  246  from water storage device  240 , thereby producing hydrogen, in line  212 , and exhaust water, in line  213 , as discussed above. Heat generated at electrolyzer  210  is discharged in the exhaust water as represented by q Elect, Out    600 , which is directed through thermal switch  410  to HSD  300  as represented by q HSD, In    605 . HSD  300  retains the thermal energy represented by q HSD, In    605  by insulative means, and discharges heat represented by q HSD, Out    610  as demanded by controller  500 , discussed in more detail below. The heat represented by q HSD, Out    610  may be recirculated back into HSD  300  or directed to hydrogen storage device  220  via thermal switch  410 . In an alternative embodiment that includes a metal hydride as a hydrogen storage medium, a quantity of heat represented by q Hydride, Out    615  is released as hydrogen storage device  220  is filled with hydrogen gas. This quantity of heat, represented by q Hydride, Out    615 , is directed to water storage device  240  via fuel cell  230 . That portion of q Hydride, Out    615  entering fuel cell  230  is represented by q FC, In    620 , and that portion entering water storage device  240  is represented by q Water Tank, In    625 . In an embodiment where a metal hydride is not employed as a hydrogen storage medium, then q Hydride, Out    615  is zero. The heat loss occurring at fuel cell  230  is represented by q FC, Loss    630 , which represents the amount of energy to avoid sub-freezing temperatures at fuel cell  230 . In determining whether heat needs to be transferred from HSD  300  to other system components, processor  510  at controller  500  evaluates the following equation:
 
 q   FC, Loss   &lt;q   Elect, Out   +q   Hydride, Out   +q   HSD, Net   −q   Losses   Equa. 1.
 
where
 
 q   HSD, Net   =q   HSD, In   −q   HSD, Out  
 
and
 
q Losses =heat lost to ambient atmosphere.
 
     In general, it is desirable for the amount of energy loss in FCS  150  during electrolysis mode to be less than the overall energy available from the surrounding systems in order to maintain above-freezing temperatures; that is, T FCS  (temperature of FCS  150 )&gt;0 deg-C. (degree Celsius). If Equation 1 is true, then in an embodiment T FCS &gt;0 deg-C., and if Equation 1 is false, then T FCS &lt;0 deg-C. By evaluating Equation 1, controller  500  can determine whether the rate of heat transfer from HSD  300  to other system components should be modified. Additionally, by evaluating Equation 1 in combination with information from thermal sensors  90 ,  154 ,  214 ,  224 ,  234 ,  244 , and  302 , controller  500  can modify the flow of water, or more generally the flow of portable heat transfer medium, using pumps and valves (not shown but discussed above), for efficient use of the available thermal energy, or controller  500  can determine whether FCS  150  should be changed from electrolysis mode to another mode, thereby changing the rate of heat production. 
     Referring now to  FIG. 4 , which depicts FCS  150  operating in fuel cell mode, fuel cell  230  receives hydrogen from hydrogen storage device  220  and oxygen from oxygen supply  151 , thereby producing electricity, depicted as power out  158 , and product water  236  that is stored at water storage device  240 , as discussed above. Heat generated at fuel cell  230  is discharged in fuel cell cooling water  238  as represented by q FC, Out    635 , which is directed to water storage device  240  for subsequent use. In an alternative embodiment, q FC, Out    635  also represents the heat generated at fuel cell  230  and delivered to water storage device  240  via fuel cell product water  236 . In a further alternative embodiment, q FC, Out    635  represents the summation of the heat generated at fuel cell  230  and discharged in fuel cell cooling water  238  plus the heat generated at fuel cell  230  and delivered to water storage device  240  via fuel cell product water  236 . HSD  300 , under the control of controller  500  as discussed in more detail below, discharges the thermal energy represented by q HSD, Out    610  to thermal switch  410 , which directs a portion of the heat represented by q Hydride, In    640  to hydrogen storage device  220 , and a portion of the heat represented by q HSD, In    605  back to HSD  300 . The heat represented by q Hydride, In    640  is that amount of heat, in an alternative embodiment employing a metal hydride as a hydrogen storage medium, that is extracted to cool the metal hydride before storing the hydrogen gas in hydrogen storage device  220 . In an embodiment where a metal hydride is not employed as a hydrogen storage medium, then q Hydride, In    640  is zero. Process water  246  from water storage device  240  provides heat represented by q Elect, In    645  to electrolyzer  210 , which retains some heat to keep electrolyzer  210  from freezing and discharges some lost heat represented by q Elect, Loss    650  to system environment  152 , the term q Elect, Loss    650  represents the total amount of energy to avoid sub-freezing temperatures at electrolyzer  210 . In determining whether heat needs to be transferred from HSD  300  to other system components, processor  510  at controller  500  evaluates the following equation:
 
 q   Elect, Loss   &lt;q   FC, Out   +q   Hydride, Out   +q   HSD, Net   −q   Losses   Equa. 2.
 
where
 
 q   HSD, Net   =q   HSD, In   −q   HSD, Out  
 
and
 
q Losses =heat lost to ambient atmosphere.
 
     In general, it is desirable for the amount of energy loss in FCS  150  during fuel cell mode to be less than the overall energy available from the surrounding systems in order to maintain above-freezing temperatures; that is, T FCS &gt;0 deg-C. If Equation 2 is true, then in an embodiment T FCS &gt;0 deg-C., and if Equation 2 is false, then T FCS &lt;0 deg-C. By evaluating Equation 2, controller  500  can determine whether the rate of heat transfer from HSD  300  to other system components should be modified. Additionally, by evaluating Equation 2 in combination with information from thermal sensors  90 ,  154 ,  214 ,  224 ,  234 ,  244 , and  302 , controller  500  can modify the flow of water for efficient use of the available thermal energy as discussed above, or controller  500  can determine whether FCS  150  should be changed from fuel cell mode to another mode, thereby changing the rate of heat production. 
     In response to FCS  150  operating in idle mode, that is, when electrolyzer  210  is not operating in electrolysis mode and fuel cell  230  is not operating in fuel cell mode, processor  510  at controller  500  evaluates the following equation to determine whether heat needs to be transferred from HSD  300  to other system components:
 
 q   Elect, Loss   +q   FC, Loss   &lt;q   HSD, Net   −q   Losses   Equa. 3
 
     In general, it is desirable for the amount of energy loss in FCS  150  during idle mode to be less than the overall energy available from the surrounding systems in order to maintain above-freezing temperatures; that is, T FCS &gt;0 deg-C. If Equation 3 is true, then in an embodiment T FCS &gt;0 deg-C., and if Equation 3 is false, then T FCS &lt;0 deg-C. By evaluating Equation 3, controller  500  can determine whether the rate of heat transfer from HSD  300  to other system components should be modified. Additionally, by evaluating Equation 3 in combination with information from thermal sensors  90 ,  154 ,  214 ,  224 ,  234 ,  244 , and  302 , controller  500  can modify the flow of water for efficient use of the available thermal energy as discussed above, or controller  500  can determine whether FCS  150  should be changed from an idle mode to a non-idle mode, thereby providing a source of additional heat. 
       FIGS. 5 and 6  depict alternative embodiments whereby controller  500  controls the heat transfer among and between components of FCS  150 . As depicted in  FIG. 5 , HSD  300  is in fluid communication with hydrogen storage device  220  and heat transfer unit  410  via TCS  400  (dashed lines), and electrolyzer  210  is in fluid communication with heat transfer unit  410 , fuel cell  230 , and water storage device  240  via fluid paths  216 ,  218 , and  233 . As depicted in  FIG. 6 , HSD  300  is in fluid communication with heat transfer unit  410 , hydrogen storage device  220 , and fuel cell unit  230  via TCS  400 , and electrolyzer  210  is in fluid communication with heat transfer unit  410  and water storage device  240  via fluid paths  216  and  219 . Other embodiments for controlling the heat transfer among and between components of FCS  150 , while not depicted, may also be employed. As discussed above, processor  510  at controller  500  utilizes thermal sensors  90 ,  154 ,  214 ,  224 ,  234 ,  244 , and  302 , Equations 1–3, and pumps and valves (not shown), to manage the heat transfer among and between the various components of FCS  150  to maintain above-freezing temperatures. 
     Some embodiments of the invention have the advantage of: low weight and low cost as a result of the absence of an auxiliary heating system; high energy efficiency as a result of the fuel cell system using byproduct heat; an absence of coolants that can pollute proton exchange membranes (PEM); and, reduced complexity as a result of controlled information processing. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.