Abstract:
A fuel cell cooling system and a method of operating the fuel cell cooling system. The fuel cell cooling system has a first coolant circulation loop for selectably supplying coolant to a fuel cell and a second coolant circulation loop for selectably supplying coolant to the fuel cell. The method comprises: (a) selectably connecting one of the first coolant circulation loop and the second coolant circulation loop to a coolant inlet and a coolant outlet of the fuel cell for fluid communication therewith; (b) selectably disconnecting the other of the first coolant circulation loop and the second coolant circulation loop from the coolant inlet and the coolant outlet of the fuel cell to impede fluid communication therewith; (c) when the first circulation loop is connected with the coolant inlet and the coolant outlet of the fuel cell for fluid communication therewith, providing a positive pressure to coolant in the first coolant circulation loop upstream from the coolant inlet of the fuel cell; and (d) when the second circulation loop is connected with the coolant inlet and the coolant outlet of the fuel cell for fluid communication therewith, providing a negative pressure to coolant in the second coolant circulation loop downstream from the coolant outlet of the fuel cell.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a fuel cell cooling system. More particularly, the present invention relates to a fuel cell cooling system in which the fuel cell is capable to operate under either positive or negative pressure of coolant.  
         BACKGROUND OF THE INVENTION  
         [0002]    Fuel cells have been proposed as a clean, efficient and environmentally friendly source of power which can be utilized for various applications. A fuel cell is an electrochemical device that produces an electromotive force by bringing the fuel (typically hydrogen) and an oxidant (typically air) into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode, i.e. the anode, where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations. The electrons are conducted from the anode to a second electrode, i.e. the cathode, through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the cathode. Simultaneously, an oxidant, such as oxygen gas or air is introduced to the cathode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product. The anode may alternatively be referred to as a fuel or oxidizing electrode, and the cathode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:  
         H   2     -&gt;       2        H   +       +     2        e   -                       1   2          O   2       +     2        H   +       +     2        e   -         -&gt;       H   2        O                           
 
           [0003]    The external electrical circuit withdraws electrical current and thus receives electrical energy as shown by the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction. Accordingly, the use of fuel cells in power generation offers potential environmental benefits compared with power generation from combustion of fossil fuels or by nuclear activity. Some examples of applications are distributed residential power generation and automotive power systems to reduce emission levels.  
           [0004]    In practice, fuel cells are not operated as single units. Rather fuel cells are connected in series, stacked one on top of the other, or placed side-by-side, to form what is usually referred to as a fuel cell stack. The fuel, oxidant and coolant are supplied through respective delivery subsystems to the fuel cell stack. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally to the fuel cell stack.  
           [0005]    As fuel cell reactions are exothermic, heat generated within the fuel cell stack has to be dissipated to ensure that the fuel cells operate within an optimal temperature range. One of the commonly used methods of cooling a fuel cell stack is providing coolant flow passages within the fuel cell stack having a coolant inlet and a coolant outlet, and running liquid coolant through the fuel cell stack. A coolant circulation loop is typically provided, which includes a circulation pump and a heat exchanger. The circulation pump supplies the coolant to the coolant inlet of the fuel cell stack and draws the coolant from the coolant outlet. The coolant absorbs heat generated in the fuel cell stack, as it flows through the fuel cell stack. Outside the fuel cell stack, the coolant is cooled by a heat exchanger to within a predetermined temperature range. Typical coolant includes deionized water, pure water, any non-conductive water, ethylene glycol, the mixture thereof, etc.  
           [0006]    The heat exchanger in the coolant circulation loop can be a radiator. Alternatively, the heat exchanger can be an isolation heat exchanger in which two fluids exchange heat in a non-mixing manner. In this case, another coolant circulation loop is provided. Depending on the system configuration and fuel cell power capacity, a heater may be provided in the coolant circulation loop either downstream or upstream of the heat exchanger to heat the coolant, thereby maintaining the temperature of the coolant within a desired range.  
           [0007]    The coolant in the coolant circulation loop is usually pumped into the fuel cell stack. Hence, the fuel cell stack is usually referred to as operating under positive pressure of coolant. Alternatively, the circulation pump may be placed downstream of the fuel cell stack and draws coolant from the fuel cell stack. In this case, the fuel cell stack is referred to as operating under negative pressure of coolant. Prior fuel cell cooling systems can only provide either positive or negative pressure to the fuel cell stack. However in some cases, such as in fuel cell testing systems, in order to test the ability of a fuel cell stack to operate under different cooling conditions, it may be desirable to provide a fuel cell cooling system that is capable of operating a fuel cell stack under both positive pressure and negative pressure and switching between the two operating conditions. Although reversing the direction of the circulation pump may provide the desired pressure conditions, this cannot always satisfy the operational requirements for particular system configurations. For example, some components in the fuel cell system, such as pressure or flow regulators or even the fuel cell stack itself, may not work with the reversed flow direction of coolant. As a result, significant changes to the fuel cell system must be made to test the system under both positive and negative pressure conditions.  
           [0008]    There remains a need for a fuel cell cooling system that can provide the fuel cell with both negative and positive pressure conditions without changing the system configuration.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of an aspect of the present invention is to provide an improved fuel cell cooling system.  
           [0010]    In accordance with an aspect of the present invention, there is provided a fuel cell cooling system comprising: (a) a first coolant circulation loop for supplying a coolant to a fuel cell, (b) a second coolant circulation loop for supplying the coolant to the fuel cell, and (c) coolant directing means for selectively directing the coolant from one of the first and second coolant circulation loops into the fuel cell and for impeding coolant flow from the other of the first and second coolant circulation loops into the fuel cell. The first coolant circulation loop has a first circulation means for effecting a positive pressure in the coolant upstream of the fuel cell to circulate the coolant through the fuel cell. The second coolant circulation loop has a second circulation means for effecting a negative pressure in the coolant downstream of the fuel cell to circulate the coolant through the fuel cell  
           [0011]    An object of a second aspect of the present invention is to provide an improved method of operating a fuel cell cooling system.  
           [0012]    In accordance with a second aspect of the present invention, there is provided a method of operating a fuel cell cooling system. The fuel cell cooling system has a first coolant circulation loop for selectably supplying coolant to a fuel cell and a second coolant circulation loop for selectably supplying coolant to the fuel cell. The method comprises: (a) selectably connecting one of the first coolant circulation loop and the second coolant circulation loop to a coolant inlet and a coolant outlet of the fuel cell for fluid communication therewith; (b) selectably disconnecting the other of the first coolant circulation loop and the second coolant circulation loop from the coolant inlet and the coolant outlet of the fuel cell to impede fluid communication therewith; (c) when the first circulation loop is connected with the coolant inlet and the coolant outlet of the fuel cell for fluid communication therewith, providing a positive pressure to coolant in the first coolant circulation loop upstream from the coolant inlet of the fuel cell; and (d) when the second circulation loop is connected with the coolant inlet and the coolant outlet of the fuel cell for fluid communication therewith, providing a negative pressure to coolant in the second coolant circulation loop downstream from the coolant outlet of the fuel cell.  
           [0013]    The present invention provides a fuel cell cooling system that is capable of cooling a fuel cell under both positive and negative pressures. The components in the cooling system of the present invention do not need to be reconfigured to work in different pressure conditions. This is particularly desirable in fuel cell testing systems. The present invention has many advantages over the prior art when employed in fuel cell cooling systems having low flow rates. Increasing the turbulence of the coolant by mixing coolant in the first and second coolant circulation loops increases heat exchange efficiency in the coolant circulation loop. This in turn renders better control of the temperature of the coolant flowing through the fuel cell. Therefore, fuel cell is ensured to operate under optimum temperature and hence it is operating more efficiently.  
           [0014]    Additionally, while the invention is described and claimed as providing a “cooling system”, more generally the system can provide both cooling and heating of the fuel cell  10 . The coolant is thus more generally a heat transfer fluid. References to “cooling” and related terms should be construed accordingly. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show a preferred embodiment of the present invention and in which:  
         [0016]    [0016]FIG. 1 illustrates a schematic flow diagram of a first embodiment of a fuel cell cooling system according to the present invention; and  
         [0017]    [0017]FIG. 2 illustrates a schematic flow diagram of a second embodiment of the fuel cell cooling system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1, this shows a schematic flow diagram of a first embodiment of a fuel cell cooling system  1  according to the present invention. The fuel cell cooling system  1  generally comprises a fuel cell  10 , a coolant storage tank  20 , a first coolant circulation loop  100  and a second coolant circulation loop  200 . In known manner, the fuel cell  10  has a coolant inlet  12  and a coolant outlet  14  for coolant to flow through the fuel cell  10  and absorb heat generated in the fuel cell reaction. For clarity, lines unique of the first coolant circulation loop  100  are indicated with dash lines. It is to be understood that in the present invention, “fuel cell” is used to indicate a fuel cell stack comprising a plurality of fuel cells or just a single fuel cell. In addition, the present invention is applicable to any type of fuel cell.  
         [0019]    As shown in FIG. 1, the first coolant circulation loop  100  comprises a first supply line  150 , a first return line  160 , a coolant inlet line  300  and a coolant outlet line  400 . The first supply line  150  of the first coolant circulation loop  100  is in fluid communication with the coolant storage tank  20 . A first coolant circulation pump  130  draws coolant from the coolant storage tank  20  and supplies it along the first supply line  150  to a first three-way valve  70  which, in one position, fluidly connects the first supply line  150  with the coolant inlet line  300 . The coolant inlet line  300  is in turn in fluid communication with the coolant inlet  12  of the fuel cell  10 . Then, the coolant continues to flow along the coolant inlet line  300  into the fuel cell  10 . In this case, the fuel cell  10  is operating under positive pressure of coolant. Then in known manner, the coolant flows through the fuel cell  10 , absorbs heat within the fuel cell  10  and leaves the fuel cell  10  via the coolant outlet  14 . From the coolant outlet  14 , the coolant flows along the coolant outlet line  400  which is in fluid communication with the coolant outlet, to a second three-way valve  80 . In one position, the second three-way valve  80  fluidly connects the coolant outlet line  400  with first return line  160 . Hence, the coolant flows from the second three-way valve  80  along the first return line  160  back to the coolant storage tank  20 .  
         [0020]    A first heat exchanger  140  is disposed in the first coolant circulation loop  100  to regulate the temperature of the coolant supplied to the fuel cell  10  so that a desired amount of heat generated within the fuel cell  10  is absorbed and hence the fuel cell  10  can operate under optimum temperature. In FIG. 1, the first heat exchanger  140  is disposed in the first supply line  150 . However, it is to be understood that the first heat exchanger  140  may also be disposed in the first return line  160 . It may be a radiator, or an isolation liquid-liquid heat exchanger. In the latter case, an additional cooling loop is required as is known in the art.  
         [0021]    When the fuel cell cooling system  1  is operating under low coolant flow rate, for example, less than 1 liter per minute, heat loss in the conduits or pipes is relatively great. In order to prevent coolant temperature becoming too low when the coolant is circulated back to the fuel cell  10 , a heater (not shown) may be desired. In addition, during initial start-up of the fuel cell  10 , coolant is at a relatively low temperature. The heater helps to heat up the coolant during start-up to bring the coolant to desired temperature more rapidly. Such a heater may be disposed in the first supply line  150  or the first return line  160 , either upstream or downstream of the first heat exchanger  140 . Alternatively, the heater, for example an electric heater, may form an integral part of the coolant storage tank  20 .  
         [0022]    Still referring to FIG. 1, the second coolant circulation loop  200  comprises a second supply line  250 , a second return line  260 , a bypass line  270 , the coolant inlet line  300  and the coolant outlet line  400 . The second supply line  250  of the second coolant circulation loop  200  is in fluid communication with the coolant storage tank  20  and supplies coolant along the second supply line  150  to the first three-way valve  70 . As mentioned above, in one position, the three-way valve  70  fluidly connects the first supply line  150  of the first coolant circulation loop  100  with the coolant inlet line  300 . In the other position, the first three-way valve  70  fluidly connects the second line  250  of the second coolant circulation loop  200  with the coolant inlet line  300 , and hence cuts off the fluid communication between the first supply line  150  and the coolant inlet line  300 . Then, the coolant from the second supply line  250  flows along the coolant inlet line  300  into the fuel cell  10 . In known manner, the coolant flows through the fuel cell  10 , absorbs heat within the fuel cell  10  and leaves the fuel cell  10  via the coolant outlet  14 . From the coolant outlet  14 , the coolant flows along the coolant outlet line  400  which is in fluid communication with the coolant outlet  14 , to the second three-way valve  80 . As mentioned above, in one position, the second three-way valve  80  fluidly connects the coolant outlet line  400  with the first return line  160 . In the other position, the second three-way valve  80  fluidly connects the coolant outlet line  400  with second return line  260  and hence cuts off the fluid communication between the coolant outlet line  400  and the first return line  160 . Then, the coolant flows from the second three-way valve  80  along the second return line  260  back to the coolant storage tank  20 . A second coolant circulation pump  230  is disposed in the second return line  260  of the second coolant circulation loop  200 . It draws coolant from the fuel cell  10  and returns the coolant to the coolant storage tank  20 . As the fuel cell  10  is located adjacent the inhalant side of the second coolant circulation pump  230 , in this case the fuel cell  10  is operating under negative pressure of coolant.  
         [0023]    As shown in FIG. 1, a first pressure regulating valve  90  is disposed in the coolant inlet line  300  upstream of and adjacent the coolant inlet of the fuel cell  10 . The first pressure regulating valve  90  regulates the flow of coolant supplied to the fuel cell  10  in either positive or negative pressure operation. Particularly, in negative pressure operation, the pressure regulating valve  90  regulates the amount of coolant flow through the fuel cell  10 . Hence, when the second coolant circulation pump  230  continuously draws coolant from the fuel cell  10 , the first pressure regulating valve  90  regulates the negative pressure under which the fuel cell  10  operates, without changing the speed of the second coolant circulation pump  230 .  
         [0024]    A bypass line  270  is connected between the coolant storage tank  20  and a position in the second return line  260  upstream of the second coolant circulation pump  230 , i.e. the inhalant side of the second coolant circulation pump  230 . A second pressure regulating valve  60  is disposed in the bypass line  270  to regulate the amount of coolant supplied directly from the coolant storage tank  20  to the inhalant side of the second coolant circulation pump  230 . The second pressure regulating valve  60  is normally closed. The second pressure regulating valve  60 , by opening to different extents and hence supplying a portion of the coolant to the inhalant side of the second coolant circulation pump  230 , reduces the negative pressure under which the fuel cell  10  operates to different extents. In known manner, the valve  60  can be a conventional pressure regulating valve, that effectively regulates the pressure drop across the fuel cell  10 . This provides an additional mechanism of controlling negative pressure. It is to be understood that the bypass line  270  does not necessarily start from the coolant storage tank  20 . It may start from any location upstream of the fuel cell  10 , either in the first coolant circulation loop  100  or the second coolant circulation loop  200 . Likewise, the bypass line  270  does not necessarily end at a position in the second return line  260  upstream of second coolant circulation pump  230 . It may end at a position in the coolant outlet line  400 .  
         [0025]    Similar to the first heat exchanger  140  described above in the first coolant circulation loop  100 , a second heat exchanger  240  is disposed in the second coolant circulation loop  200  to regulate the temperature of the coolant. In FIG. 1, the second heat exchanger  240  is located in the second return line  260  of the second coolant circulation loop  200 . However, it may also be located in the second supply line  250 . Again, the second heat exchanger may be a radiator or an isolation liquid-liquid heat exchanger. It is to be understood that the first or second heat exchanger  140 ,  240  may be disposed in the coolant inlet line  300  or coolant outlet line  400 . In this case, only one heat exchanger is needed. Additional heat exchangers may be provided as desired. As mentioned above, a heater may be provided. Such a heater may be disposed in the second supply line  250  or the second return line  160 , either upstream or downstream of the second heat exchanger  240 . Alternatively, the heater, for example an electric heater, may form an integral part of the coolant storage tank  20 . In this case, only one heater is needed.  
         [0026]    It is to be understood that the coolant storage tank  20  may receive coolant from an external coolant source. It is also to be understood that the first and second coolant circulation pumps  130  and  230  used in the present invention may be constant speed pumps or variable speed pumps.  
         [0027]    As can be appreciated from the description above, the fuel cell cooling system  1  of the present invention is capable of switching between two operation modes, a positive pressure mode and a negative pressure mode. In the positive pressure mode, coolant flows along the first coolant circulation loop  100 , while in the negative pressure mode, coolant flows along the second coolant circulation loop  200 . In the positive pressure mode, the first coolant circulation pump  130  operates and the second coolant circulation pump  230  is idle. In the negative pressure mode, the second coolant circulation pump  230  operates and the first coolant circulation pump  130  is idle. In other words, only one pump is working in either operation mode.  
         [0028]    Now referring to FIG. 2, this shows a schematic flow diagram of a second embodiment of a fuel cell cooling system according to the present invention. The second embodiment is particularly suitable for use in low flow rate fuel cell cooling systems. For simplicity, the elements in this embodiment that are identical or similar to those in the first embodiment are indicated with same reference numbers and for brevity, the description of these elements is not repeated.  
         [0029]    In this embodiment, a third coolant circulation loop  500  is provided. The first coolant circulation pump  130  draws coolant from the coolant storage tank  20  and supplies the coolant to the first supply line  150  and the third coolant circulation loop  500 . A third heat exchanger  520  and a filter  510  are disposed in the third coolant circulation loop  500 . The heat exchanger  520  regulates the temperature of the coolant in this loop  500  and the filter helps to purify the coolant. As in known in the art, as coolant flows along conduits and pipes, it picks up impurities particles and ions. To keep the coolant non-conductive so that the coolant does not short the fuel cell  10  when flowing therethrough, the filter  510  may be provided to filter out the impurities and ions. This is particularly useful when deionized water is used as the coolant. Depending on the type of coolant, the filter may be of different type or simply omitted.  
         [0030]    As shown in FIG. 2, a first flow regulating valve  30  is provided in the first supply line  150 , operating between open and closed positions. A second flow regulating valve  40  is connected between the first supply line  150  and the first return line  160 . The second flow regulating valve  40  operates between open and closed positions and connects to a position upstream of the first flow regulating valve  30  in the first supply line  150 . A third flow regulating valve  50  is provided in the first return line  160 , operating between open and closed positions. The third flow regulating valve  50  is disposed upstream of the position at which the second flow regulating valve  40  connects to the first return line  160 .  
         [0031]    When the fuel cell cooling system  2  operates in positive pressure mode, the first and third flow regulating valves  30  and  50  are in open position and hence permit coolant to flow along the first coolant circulation loop  100 . Meanwhile, the second flow regulating valve  40  is in closed position. The second coolant circulation pump  230  does not operate, as in the first embodiment. However, when the fuel cell cooling system  2  of the present invention operates under low flow rate of coolant (the flow rate in the first coolant circulation loop  100 ), e.g. less than 1 liter per minute, it may be desirable to operate the second coolant circulation pump  230 . When the second coolant circulation pump  230  operates, the first and second three-way valves  70  and  80  are still in such a position that permits coolant to flow in the first coolant circulation loop  100 . That is to say, the fluid communication between the second supply line  250  and the coolant inlet line  300 , and the fluid communication between the coolant outlet line  400  and the second return line  260  are respectively cut off. Therefore, the second coolant circulation pump  230  draws coolant from the coolant storage tank  20  via the bypass line  270  and returns the coolant to the tank  20  via the second return line  260 . This forms a complete circulation loop and coolant in this loop mixes with coolant in the first coolant circulation loop  100  in the coolant storage tank  20 . The coolant storage tank  20  in this embodiment preferably has an integral heating means, as in low flow rate, the heating means is usually used to prevent the coolant temperature from deviating too far from the optimum range, i.e. being too cold. The mixing of the coolant in the tank  20  creates turbulence in the coolant, thereby increasing heat transfer efficiency. Preferably, the second coolant circulation pump  230  operates at a higher flow rate than that of the first coolant circulation pump  130  to give even higher heat transfer efficiency. Similar techniques for obtaining higher heat exchange efficiency in low flow rate cooling systems is disclosed in the assignee&#39;s co-pending U.S. patent application Ser. No. ______.  
         [0032]    When the fuel cell cooling system  2  operates in negative pressure mode and low flow rate of coolant (the flow rate in the second coolant circulation loop  200 ), e.g. less than 1 liter per minute, the first and third flow regulating valves  30  and  50  are in closed position. The second coolant circulation pump  230  operates to draw coolant from the fuel cell  10  and the first and second three-way valves  70  and  80  are in such a position that permits coolant to flow in the second coolant circulation loop  200 . That is to say, the fluid communication between the first supply line  150  and the coolant inlet line  300 , and the fluid communication between the coolant outlet line  400  and the first return line  160  are respectively cut off. Meanwhile, the second flow regulating valve  40  is in open position, and the first coolant circulation pump  130  operates to draw coolant from the coolant storage tank  20  and supplies the coolant to flow through the second flow regulating valve  40  into the first return line  160 . Then the coolant returns to the coolant storage tank  20  via the first return line  160 . This forms a complete circulation loop and coolant in this loop mixes with coolant in the second coolant circulation loop  200  in the coolant storage tank  20 . The mixing of the coolant in the tank  20  creates turbulence in the coolant and thereby increasing heat transfer efficiency. Preferably, in the negative pressure mode, the first coolant circulation pump  130  operates at a higher flow rate than that second coolant circulation pump  230  to give even higher heat transfer efficiency.  
         [0033]    Optionally, since the first and second three-way valves  70  and  80  selectively cut off the coolant flow in the first coolant circulation loop  100 , the first and third valves can be omitted. However, these two valves serve to minimize the amount of stagnant coolant in the first supply line  150  and part of the first return line  160 . Hence, the first and third valves  30  and  50  are preferably disposed adjacent to the second valve  40 . It is to be understood that in the second embodiment, the first heat exchanger  140  is disposed in the first return line  160 . However, it may also be disposed in the first supply line  150 . In addition, the first and second circulation pumps  130 ,  230  can be any type of pump commonly used. Preferably, at least the speed of one circulation pump is variable.  
         [0034]    It is also to be understood that, in known manner, various sensors and/or transmitters can be provided for measuring parameters of the coolant, such as temperature, pressure, flow rate, etc. The measured parameters can be sent to a processor (not shown) which in turn controls the operation of the heating means, the first and second pumps  130 ,  230 , and the heat exchangers  140 ,  240 . For example, sensors or transmitters can be provided adjacent the coolant inlet and outlet of the fuel cell  10  to monitor the temperature of the coolant, and hence the amount of heat removed from the fuel cell  10 . Similarly, sensors may also be provided adjacent the inlets and outlets of the coolant storage tank to monitor the temperature of the coolant, and hence the heating efficiency. The measured data is then sent to the processor for analysis. Then the process will control the operation of the components, such as increasing or decreasing the speed of the first or second pump, increasing or decreasing fan speed of radiators, if radiators are used as heat exchangers, increasing or decreasing heating, etc.  
         [0035]    It should also be appreciated that the present invention is not limited to the embodiments disclosed herein. It can be anticipated that those having ordinary skills in the art can make various modifications to the embodiments disclosed herein without departing from the fair meaning or the proper scope of the accompanying claims. For example, the number and arrangement of components in the system might be different, different elements might be used to achieve the same specific function. However, these modifications should be considered to fall within the scope of the invention as defined in the following claims.