Patent Publication Number: US-2005120731-A1

Title: Electrochemical cell cooling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority from U.S. Provisional Patent Application Ser. No. 60/516,285 filed Nov. 3, 2003. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to an electrochemical cell system and, more particularly, to a cooling system for an electrochemical cell system.  
     BACKGROUND OF THE INVENTION  
      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 where it reacts electrochemically in the presence of the electrolyte to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen or air is introduced to the second electrode where the oxidant reacts electrochemically in the presence of the electrolyte and a 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 first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the first and second electrodes respectively are:
 
H 2 →2H + +2e −   (1)
 
½O 2 +2H + +2e − →H 2 O   (2) 
 
      The external electrical circuit withdraws electrical current and thus receives electrical power from the fuel cell. The overall fuel cell reaction produces electrical energy as shown by the sum of the separate half-cell reactions shown in equations 1 and 2. Water and heat are typical by-products of the reaction.  
      In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, either stacked one on top of the other or placed side by side. The series of fuel cells, referred to as a fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds in the housing to the electrodes. The fuel cell is cooled by either the reactants or a cooling medium. The fuel cell stack also comprises current collectors, cell-to-cell seals and insulation while the required piping and instrumentation are provided external to the fuel cell stack.  
      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 cells 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 the coolant flows through the fuel cell stack. Outside the fuel cell stack, the coolant temperature is readjusted by a heat exchanger to within a predetermined temperature range. Depending on the system configuration and operation conditions, the coolant may be cooled or heated. Typical coolant includes deionized water, pure water, any non-conductive water, ethylene glycol, or a mixture thereof.  
      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 disposed downstream of the fuel cell stack and draws coolant therefrom. In this case, the fuel cell stack is referred to as operating under negative pressure of coolant. A fuel cell cooling system that can operate under both positive and negative pressure conditions was disclosed in U.S. patent application Ser. No. 10/184,104. However, the fuel cell cooling system uses two coolant circulation loops, two circulation pumps and various control valves. The system configuration is complex, which results in high parasitic load and low system efficiency.  
      There remains a need for a simplified cooling system that can provide an electrochemical cell with both positive and negative coolant pressure conditions in an efficient manner.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the invention to provide a simplified cooling system that can provide an electrochemical cell with both positive and negative coolant pressure conditions in an efficient manner.  
      The invention therefore provides a system for cooling an electrochemical cell, the system including a coolant circulation loop for supplying a coolant to an electrochemical cell for absorbing heat from the cell. The coolant circulation loop has a coolant container; a pump for circulating the coolant through the coolant circulation loop; and a heat exchanging portion for cooling the coolant. The system also includes a pressure-adjusting flow line having a pressure-adjustment device in fluid communication with the coolant container for selectively increasing or decreasing fluid pressure in the container thereby selectively enabling either positive-pressure cooling or negative-pressure cooling of the electrochemical cell.  
      In one embodiment, the flow line includes a venturi in fluid communication with the container and a shut-off valve downstream of the venturi for selectively pressurizing or depressurizing the container and loop. When the shut-off valve is open, the fluid flows through an inlet and outlet of the venturi and exhausts to atmosphere, causing the venturi to induce a negative pressure condition in the container and loop. When the shut-off valve is closed, the fluid is forced to flow through a bottom port of the venturi into the container, thus causing a positive pressure condition in the loop. As a refinement, a variable-flow valve can be provided upstream of the venturi to regulate the fluid pressure in the flow line, thus enabling fine adjustment of the pressurization or depressurization of the container and loop. As a further refinement, a bypass with a flow constriction can be provided to facilitate coolant temperature control.  
      The invention further provides a method of cooling an electrochemical cell. The method includes the step of circulating a coolant through a loop having the electrochemical cell and a coolant container. The method also includes the step of flowing a fluid through a pressure-adjusting flow path in fluid communication with the container for selectively increasing or decreasing pressure in the container and thus in the loop.  
      In one embodiment, the step of flowing the fluid through the pressure-adjusting flow path includes the step of flowing the fluid through a venturi.  
      In another embodiment, the step of flowing the fluid through the pressure-adjusting flow path includes the step of selectively operating a shut-off valve downstream of the venturi, whereby the valve is operated between an open position permitting fluid to exhaust to atmosphere, thus inducing a depressurization of the coolant container, and a closed position, wherein fluid is forced to vent into the container, thus causing a pressurization of the coolant container.  
      In another embodiment, the step of flowing the fluid through the pressure-adjusting flow path further includes the step of variably adjusting a variable-flow valve disposed upstream of the venturi, thereby finely adjusting the pressurization or depressurization of the coolant container.  
      In yet another embodiment, the method also includes the step of partially bypassing the electrochemical cell whereby a portion of the coolant flows through a bypass while a remainder of the coolant flows through the electrochemical cell.  
      The present invention further provides a system for cooling an electrochemical cell that includes a coolant circulation loop for supplying a coolant to an electrochemical cell for absorbing heat from the cell. The coolant circulation loop has a coolant container; a pump for circulating the coolant through the coolant circulation loop; and a heat exchanger for cooling the coolant. The system further includes a pressure-adjusting flow line that has a venturi in fluid communication with the coolant container for selectively increasing or decreasing fluid pressure in the container; and a valve downstream of the venturi, the valve adapted to be opened to depressurize the coolant circulation loop or to be closed to pressurize the coolant circulation loop.  
      In one embodiment, the system further includes a pressure controller for controlling a variable valve located upstream of the venturi in response to a pressure feedback signal from a pressure transducer in the coolant circulation loop and a temperature controller for controlling a cooling rate of the heat exchanger in response to a temperature feedback signal from a temperature sensor in the coolant circulation loop.  
      The present invention thereby provides a system and method for simple and efficient cooling of an electrochemical cell under either positive or negative coolant pressure conditions. This invention therefore enables efficient and cost-effective cooling of electrochemical cells, such as those utilized in fuel cell stacks. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, in which:  
       FIG. 1  shows a simplified schematic of an electrochemical cell cooling system in accordance with a first embodiment of the present invention;  
       FIG. 2  shows a simplified schematic of an electrochemical cell cooling system in accordance with a second embodiment of the present invention; and  
       FIG. 3  is a schematic of an electrochemical cell cooling system with pressure and temperature feedback control loops in accordance with a third embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  shows, in schematic form, an electrochemical cell cooling system, generally designated by reference numeral  10 , in accordance with a first embodiment of the present invention. Although the present invention can be used to efficiently cool fuel cell systems, it is not limited to application in fuel cell systems. Rather, it is applicable to any electrochemical cell system where liquid coolant is needed to control the operating temperature of the electrochemical cell.  
      As shown in  FIG. 1 , the cooling system  10  includes a coolant circulation loop  100  for circulating a coolant through or past an electrochemical cell  50  for absorbing heat from the exothermic reaction of a working electrochemical cell  50 . The coolant circulation loop  100  includes a coolant container  140 , which is also known as an overflow tank or storage tank. The coolant container  140  stores an excess volume of coolant at a nominal operating pressure, which can be adjusted in a manner to be described below.  
      As illustrated in  FIG. 1 , the coolant circulation loop  100  includes a pump  120 , e.g. a variable speed pump, which drives the coolant through the loop  100 . The coolant circulation loop  100  also includes a heat exchanging portion, such as a heat exchanger  70 , e.g. a radiator, but which is preferably a high-efficiency plate heat exchanger. Where ambient conditions suffice to cool the coolant, the heat exchanging portion can simply be an exposed section of the coolant circulation loop itself.  
      Optionally, as shown in  FIG. 1 , the coolant circulation loop  100  further includes a filter  60 . As is known in the art, as coolant flows along conduits and pipes, it may pick up impurities and ions. To keep the coolant non-conductive so that the coolant does not short the electrochemical cell  50  when flowing therethrough, the filter  60  may be provided to filter out the impurities and ions. For example, a de-ionizing filter is provided when de-ionized water is used as the coolant. Different types of filters can be provided, as is known in the art, depending on the type of coolant being used and the piping used to construct the loop. The filter  60  may be omitted altogether where filtration is considered unnecessary or where the effects of impurities on system performance are negligible.  
      In order to adjust the pressure in the coolant container  140  and hence in the loop  100 , a pressure-adjusting flow line  170  is provided. The pressure-adjusting flow line  170  includes a fluid supply source  180 , a variable-flow valve  200 , and a pressure-adjustment device  160 , e.g. a venturi. As shown in  FIG. 1 , the venturi is located downstream of the fluid supply source  180  and the variable-flow valve  200 . The venturi  160  has an inlet  161 , an outlet  162 , and a bottom port  163 . The bottom port  163  is in fluid communication with the coolant container  140  via a pressure line  190 . The pressure-adjusting flow line also includes a shut-off valve  300  located downstream of the venturi  160 .  
      The pressure-adjusting flow line utilizes, preferably, an inert gas, such as nitrogen which can be exhausted to atmosphere without raising concerns about pollution. Thus, in one embodiment, the fluid supply source  180  is a nitrogen storage tank.  
      In operation, the fluid (e.g., the nitrogen gas) flows along a pressure-adjusting flow line  170  from the fluid supply source  180  (e.g., the nitrogen storage tank) through the variable-flow valve  200  to an inlet  161  of the venturi  160 . Then the fluid (e.g., nitrogen) flows through the venturi  160  to the shut-off valve  300 . In one embodiment, the shut-off valve is a solenoid valve. Thereafter, the fluid (nitrogen gas) is vented into the environment, i.e., it is exhausted to atmosphere. As is known in the art, as the nitrogen flows through the venturi  160 , a negative pressure or vacuum is generated therein at the bottom port  163 . Since the bottom port  163  is in fluid communication with the coolant container  140  via the pressure line  190 , a negative pressure condition (i.e., a depressurization) is created in the coolant container  140 . This in turn results in a negative pressure in the coolant circulation loop  100 . Accordingly, the cooling system  10  provides the electrochemical cell  50  with a negative coolant pressure. During testing, Applicant has been able to obtain a pressure range of −3 psig to 60 psig. This range could be increased or decreased by modifying the fluid dynamic properties of the venturi.  
      The shut-off valve, e.g., the solenoid valve  300 , must be in an open position to allow the nitrogen to flow through the venturi  160 . In other words, the shut-off valve  300  must be open for the venturi  160  to induce depressurization of the container  140  and of the loop  100 . When the solenoid valve  300  is closed, the nitrogen enters the venturi  160  via the inlet port  161  and is forced to flow out via the bottom port  163 . The nitrogen is thus directed downwardly into the coolant container  140  via the pressure line  190 . This results in increased pressure in the coolant container  140  and hence increased pressure in the coolant circulation loop  100 . In this case, the cooling system  10  provides the electrochemical cell  50  with a positive pressure condition.  
      The pressure in the loop can thus be adjusted upwardly (positive pressure condition) or downwardly (negative pressure condition) by simply opening and closing the shut-off valve  300 . By simply opening and closing the shut-off valve  300 , the electrochemical cell  50  can operate under either positive or negative pressure condition.  
      The pressurization or depressurization of the container  140  and loop  100  can be finely adjusted by operating the variable-flow valve  200  located upstream of the venturi  160 . This is useful where a precise positive or negative pressure condition is sought, e.g., for optimal system performance. Alternatively, the variable-flow valve  200  can be used to control the rate of pressurization or depressurization. In one embodiment, a microprocessor control system can receive signals from sensors or transducers and adjust the opening or closing of the variable-flow valve  200  according to a performance-optimizing algorithm. In other words, the variable-flow valve  200  can be used to variably regulate, or throttle, the fluid flow through the pressure-adjusting flow line  170  so that a desired flow pressure (or flow rate) is achieved in the flow line  170  which, in turn, finely adjusts the pressurization or depressurization of the loop.  
      For fine tuning of the positive pressure condition, the solenoid valve  300  is closed, thus diverting the flow of fluid (nitrogen) through the bottom port  163 , through the pressure line  190  and into the container  140 , thus pressurizing the container  140 . The variable-flow valve  200  can be partially closed to diminish the fluid pressure in the flow line  170  and thus diminish the pressurization, or rate of pressurization, as the case may be, of the loop  100 . Conversely, for fine tuning of the negative pressure condition, the solenoid valve  300  is opened, thus inducing a negative pressure condition in the container and loop due to the venturi effect. The degree or rate of depressurization can then be finely tuned by adjusting the variable-flow valve  200 .  
       FIG. 2  shows an electrochemical cell cooling system  20  in accordance with a second embodiment of the present invention. Same reference numbers are used herein to indicate same or similar components. In this second embodiment, the pressure-adjusting flow line  170  includes the same components as in the previous embodiment, namely a fluid supply source  180 , a variable-flow valve  200 , a venturi  160 , and a shut-off valve  300 . The flow line  170  is fluidly connected to the container as already described to provide either pressurization or depressurization of the container depending on whether the shut-off valve is open or closed.  
      As shown in  FIG. 2 , the coolant circulation loop  100  further includes a bypass line  220  that bypasses the electrochemical cell  50  from a bypass point  155  in the coolant circulation loop  100  which is upstream of the electrochemical cell  50 . This forms an inner bypass loop  230  starting from the container (or storage tank)  140  along the bypass line  220  and back to the container  140 . Coolant flowing through an outer loop (i.e., the coolant that does not traverse the bypass) flows through an optional filter  60 , the electrochemical cell  50  and a heat exchanging portion, e.g., the heat exchanger  70 . In one embodiment, a second variable-flow valve  150 , such as a solenoid valve, is disposed in the outer loop between the bypass point  155  and the electrochemical cell  50 .  
      As shown in  FIG. 2 , the bypass line  220  is preferably provided with a flow constriction  210 . The solenoid valve  150  also functions as a flow constriction. Hence, the coolant in the portion of the coolant circulation loop upstream of the electrochemical cell  50  and in the bypass line  220  is pressurized. This provides a relative fast flow rate in the bypass loop  230 . This is beneficial in terms of coolant temperature control. The coolant temperature can be maintained at a stable level in the bypass loop  230 . This in turn helps to control the temperature of the coolant supplied to the electrochemical cell  50  from the bypass point  155  via the solenoid valve  150 , as the coolant temperature at the bypass point  155  is relatively stable. In comparison with the first embodiment, the second embodiment provides better control of coolant temperature. However, it is more complex and more expensive due to the bypass and the additional valve.  
      It can be understood by those skilled in the art that heat exchangers, heaters, flow meters, temperature sensors, pressure gauges, etc can be disposed at various locations in the coolant circulation loop  100 . However, these are not essential to the present invention, and hence are not described herein in detail.  
      It is to be appreciated that the fluid used in the pressure-adjusting flow line  170  is not limited to nitrogen or inert gas. It can be any fluid that is compatible with the electrochemical cell environment. The pressure adjustment device is not limited to a venturi and can be any device that provides a similar function. The flow constriction means  210  in the bypass line  220  can be any suitable device, such as a valve. Alternatively, the bypass line  220  can simply have a smaller cross-sectional area than the line in the coolant circulation loop  100 . This also achieves flow constriction in the bypass line  220 . Other components in the present invention may also be replaced with other devices having similar function.  
       FIG. 3  shows an electrochemical cell cooling system  30  in accordance with a third embodiment of the present invention. Again, same reference numbers are used herein to indicate same or similar components. In this third embodiment, the pressure-adjusting flow line  170  includes the same components as in the previous embodiment, namely a fluid supply source  180 , a variable-flow valve  200 , a venturi  160 , and a shut-off valve  300 . The flow line  170  is fluidly connected to the container  140  as already described to provide either pressurization or depressurization of the container  140  depending on whether the shut-off valve  300  is open or closed.  
      As shown in  FIG. 3 , the coolant circulation loop  100  further includes a de-ionizing filter  700  and a particulate filter  900 . Although the de-ionizing filter is shown to be located in a second bypass line that is parallel to and upstream of the main bypass  220 , the de-ionizing filter may be located elsewhere as would be known by those of ordinary skill in the art. Similarly, while the particulate filter  900  is shown between the container  140  and the pump  120 , the particulate filter  900  can also be located elsewhere in the coolant circulation loop  100 . Optionally, as shown in  FIG. 3 , a heater  800  can be provided to heat the coolant to ensure that the coolant remains in the optimal temperature range.  
      As shown in  FIG. 3 , the electrochemical cell cooling system  30  further includes a pressure transducer  450  and a temperature sensor, e.g. a thermocouple  650 . While the pressure transducer  450  and the thermocouple  650  are shown between the second variable-flow valve  150  and the electrochemical cell  50 , these sensors may be located elsewhere in the coolant circulation loop  100  as would be known by those of ordinary skill in the art.  
      The pressure transducer  450  generates a pressure feedback signal which is sent to a PID (Proportional-Integral-Derivative) pressure controller  400 . A Proportional-Integral-Derivative controller is a standard component in industrial control applications which reads a sensor, subtracts the measurement from a desired “setpoint” to determine an “error”, and then drives the output toward the setpoint. As PID controllers are very well known in the art, their operation will not be described herein.  
      In addition to receiving the pressure feedback signal from the pressure transducer  450 , the PID pressure controller  400  also receives a pressure setpoint from a user or overall system control unit. The PID pressure controller  400  compares the pressure setpoint to the feedback signal from the pressure transducer  450  and then adjusts the first variable-flow valve  200  accordingly.  
      In addition to implementing control logic based on a generic PID transfer function, the PID pressure controller  400  can also include additional control logic designed to regulate the opening and closing of the shut-off valve  300 . However, preferably, the shut-off valve  300  is controlled by an overall system controller (not shown) which switches the shut-off valve  300  between the open and closed positions, i.e. between negative and positive pressure modes. If the pressure setpoint is less than the pressure measured by the pressure transducer  450 , then the overall system controller (or the PID pressure controller  400 ) generates a signal to open the shut-off valve  300 . As explained above, opening the shut-off valve  300  depressurizes the coolant container  140 , thus driving the pressure of the coolant in the first coolant circulation loop downwardly toward the pressure setpoint. If, on the other hand, the pressure setpoint is greater than the pressure measured by the pressure transducer  450 , then the overall system controller (or the PID pressure controller  400 ) generates a signal to close the shut-off valve  300 . Closing the shut-off valve  300  pressurizes the coolant container  140 , thus driving the pressure of the coolant in the first coolant circulation loop upwardly toward the pressure setpoint.  
      Similarly, the thermocouple  650  generates a temperature feedback signal which is sent to a PID temperature controller  610 . The PID temperature controller  610  also receives a temperature setpoint, which is a temperature value or setting input by a user or an overall system controller. The PID temperature controller  610  compares the temperature feedback signal from the thermocouple  650  to the temperature setpoint and then accordingly adjusts a third variable-flow valve  600 . The third variable-flow valve  600  regulates the flow of coolant in a second coolant circulation loop  620 . The second coolant circulation loop  620  circulates coolant through the heat exchanger  70  to absorb heat from the coolant circulating through the first coolant circulation loop  100 . Heat transferred at the heat exchanger  70  from the first coolant circulation loop  100  to the second coolant circulation loop  620  is then dissipated to the environment using, for example, an evaporative condenser  630  with a fan  640 . As will be appreciated by those of ordinary skill in the art, the coolant in the second coolant circulation loop  620  can be cooled by a variety of means that are known in the art of heat transfer. By regulating a flow rate of coolant in the second coolant circulation loop  620 , the PID temperature controller  610  therefore controls a cooling rate of the heat exchanger  70  and thus adjusts the temperature of the coolant in the first coolant circulation loop  100 .  
      Although PID controllers represent the best mode known to the Applicant of implementing control of the electrochemical cell cooling system, it should be understood that other controllers or other control logic can be substituted.  
      As alluded to in the foregoing paragraphs, the PID pressure controller  400  and the PID temperature controller  610  can be themselves controlled by a master controller and/or an overall system controller. The master controller and/or the overall system controller can thus provide pressure and temperature setpoints based on a power generation requirement for the electrochemical cell  50  or based on a variety of other factors, e.g. reactant consumption rates, ambient conditions, etc.  
      As would be appreciated by those of ordinary skill in the art, more than a single pump and/or heat exchanger can be provided in the embodiments described herein in order to optimize thermodynamic performance. The location of the pump(s) and heat exchanger(s) can also be varied without departing from the spirit and scope of the invention. Furthermore, various sensors and/or transducers can be provided for measuring parameters of the coolant, such as temperature and pressure (as shown in  FIG. 3 ) as well as other process parameters such as flow rate, humidity, etc. The measured parameters can be sent to one or more processors (like the PID controllers shown in  FIG. 3 ) which in turn control the operation of the pump(s), heat exchanger(s) and/or valves. For example, temperature sensors can be provided at the inlet and outlet of the electrochemical cell to monitor the temperature rise of the coolant and hence the amount of heat transferred from the electrochemical cell to the coolant.  
      Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.