BACKGROUND OF THE INVENTION [0002]'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: H2<2H++2e- (1) % °2 + 2H+ + 2e~ H20 (2) [0003] 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.

[0004] 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. The fuel cell stack, housing and associated hardware constitute a fuel cell module. For the purposes of the present specification, the term"fuel cell"refers to either a single fuel cell or a fuel cell stack having a plurality of fuel cells.

[0005] A typical way to test the performance of a fuel cell is to connect the fuel cell to a stand-alone fuel cell testing station. A fuel cell testing station usually tests the fuel cell by simulating harsh, real-world operating conditions so as to optimize fuel cell efficiency, durability, robustness, etc. A fuel cell testing station enables fuel cell engineers and technicians to diagnose problems with fuel cell designs and to study particular facets of the fuel cell's performance, such as start-up, shut-down, or the effects of a sudden massive power demand.

Of course, as persons of ordinary skill in the art will appreciate, a fuel cell testing station can be used for testing many other aspects of a fuel cell's performance.

[0006] While subjecting the fuel cell to various operating conditions, the testing station monitors various system parameters indicative of the performance of the fuel cell.

For example, a fuel cell testing station is usually capable of supplying reactants, e. g. hydrogen and air, and/or coolant, to the fuel cell with various temperature, pressure, flow rates and/or humidity. A fuel cell test station may also change the load of the fuel cell and hence change the voltage output and/or current of the fuel cell.

A fuel cell test station typically monitors individual cell voltages within a fuel cell stack, current flowing through the fuel cell, current density, temperature, pressure or humidity at various points within the fuel cell.

[0007] Some fuel cell test stations are known in the art, notably those developed and commercialized by the Applicant, Greenlight Power Technologies of Burnaby, B. C, Canada and its parent, Hydrogenics Corporation of Mississauga, Ontario, Canada. Using these prior-art testing stations, engineers and technicians have been able to test fuel cells for optimizing their performance. These prior-art testing stations supply coolant to the fuel cell and then recirculate the coolant back to the testing station. In order to regulate the coolant flow, the prior- art testing stations included a basic flow rate control system involving a simple feedback loop between a flow meter and a flow control valve. At the outset, the operator of the testing station would have to estimate the waste heat generated by the stack before testing in order to determine what flow rate to set. Once set, the simple flow-rate control loop would endeavor to maintain a constant flow rate. However, it was found that the testing stations implementing this simple control loop suffered from undesirably high error margins in the stack's temperature control due to variations in the actual performance of the stack.

[0008] Thus, there remains a need for a fuel cell testing station having improved coolant flow control.

SUMMARY OF THE INVENTION [0009] It is therefore an object of the invention to provide a fuel cell testing station with improved coolant flow control.

[0010] In accordance with one aspect of the present invention, a station for testing a fuel cell includes reactant supply lines connected between the station and the fuel cell for supplying reactants to the fuel cell for generating electricity, byproduct return lines connected between the fuel cell and the station for returning byproducts to the station, an electrical line for returning to the station electrical current generated by the fuel cell, and a coolant supply line connected between the station and the fuel cell for supplying coolant to the fuel cell. The coolant supply line includes a flow control valve, a flow meter and a first temperature sensor. A coolant return line is also connected between the fuel cell and the station for returning coolant to the station. The coolant return line includes a second temperature sensor.

The station further includes a coolant flow controller for controlling a coolant flow rate by actuating the flow control valve in response to feedback from the flow meter and from the first and second temperature sensors.

[0011] In accordance with another aspect of the present invention, a method of testing a fuel cell stack includes the steps of supplying reactants from the station to the fuel cell for generating electricity and returning byproducts and electrical current from the fuel. cell to the station. The method also includes the steps of supplying a coolant from the station to the fuel cell to absorb heat from the fuel cell, measuring a flow rate of the coolant, and measuring a coolant temperature increase across the fuel cell. The method further includes the step of controlling a flow-regulating valve in response to the measured flow rate and the coolant temperature increase across the fuel cell.

[0012] In accordance with yet another aspect of the present invention, a fuel cell testing system includes a fuel cell and a fuel cell testing station. The station has reactant supply lines connected between the station and the fuel cell for supplying reactants to the fuel cell for generating electricity; byproduct return lines connected between the fuel cell and the station for returning byproducts to the station; an electrical line for returning to the station electrical current generated by the fuel cell; a coolant supply line connected between the station and the fuel cell for supplying coolant to the fuel cell, the coolant supply line including a flow control valve, a flow meter and a first temperature sensor ; a coolant return line connected between the fuel cell and the station for returning coolant to the station, the coolant return line including a second temperature sensor; and a coolant flow controller for controlling a coolant flow rate by actuating the flow control valve in response to feedback from the flow meter and from the first and second temperature sensors.

[0013] The testing station, method and system in accordance with the present invention thereby enable a fuel cell to be tested with improved flow control. The testing station, method and system. more optimally control the rate of flow of coolant to the fuel cell so that a desired temperature differential across the fuel cell can be maintained.

BRIEF DESCRIPTION OF THE DRAWINGS [0014] Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, in which: [0015] FIG. 1 is a schematic view of a fuel cell testing system in accordance with an embodiment of the present invention; and [0016] FIG. 2 is a schematic view of a fuel cell testing system in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0017] FIG. 1 shows, in schematic form, a fuel cell testing system, generally designated by reference numeral 2, in accordance with one embodiment of the present invention.

[0018] As shown in FIG. 1, the fuel cell testing system 2 includes a fuel cell 10 connected to a fuel cell testing station 20 via a plurality of fluid lines, which may be tubes, pipes or other types of conduits. The fuel cell testing station 20 supplies fuel and oxidant to the fuel cell 10 via a fuel supply line 12 and an oxidant supply line 14, respectively. For the purposes of the specification, the fuel supply line 12 and the oxidant supply line 14 together constitute"reactant supply lines".

[0019] The fuel cell testing station 20 also supplies coolant to the fuel cell 10 via a coolant supply line 16 and has a coolant return line 18 running from the fuel cell back to the test station for recirculation of coolant to the test station. The fuel cell 10 may operate in a dead- end mode, in which fuel and oxidant are supplied to the fuel cell and react therein without leaving the fuel cell.

Alternatively, the fuel and/or oxidant may flow through the fuel cell and be recirculated to the station. In this latter case; a fuel return line 22 and/or an oxidant return line 24 are provided, for recirculation of non-reacted fuel and/or oxidant to the test station 20. One or more other lines may be provided to return reaction byproducts to the station. For the purposes of this specification, "byproduct return lines"shall include the fuel return line 22 and the oxidant return line 24, both of which may also conduct water (a reaction byproduct) back to the station.

[0020] As is known in the art, the fuel cell 10 generates electrical power in DC form. This DC current is collected using current-collectors disposed in the fuel cell stack.

Electrical current is then carried back to the test station via an electrical line 32. The test station 20 may, of course, include any number of meters, instruments, sensors or transducers, e. g. for measuring amperage and voltage of the electrical power generated by the fuel cell. The test station 20 can be designed to not only monitor and display the power, amperage and voltage generated by the fuel cell, but also (optionally) to include hardware or software for processing and/or correlating the electrical parameters to the other system parameters and operating conditions to provide useful diagnostic tools for operators or users.

[0021] A coolant flow controller 30 of the test station 20 servos the flow of the coolant, e. g. de-ionized (DI) fluid, through the fuel cell 10 (or stack) to control the temperature increase across the cell or stack to a set value specified by an operator or user of the test station 20. The station 20 includes a variable frequency drive (VFD) 34 and a flow control valve 36, both of which operate as flow-regulating actuators that are actuated by signals sent by the flow controller 30.

[0022] The station 20 further includes an inline flow meter 38 for measuring the flow rate in the coolant supply line 16. The coolant supply line 16 also includes a first temperature sensor 40, e. g. a thermocouple. The first thermocouple 40 is disposed in the-coolant supply line 16 at or near the inlet to the fuel cell 10 to measure a first (inlet) temperature T1. The coolant return line 18 includes a second temperature sensor 42, e. g. a thermocouple. The second thermocouple 42 is disposed in the coolant return line 18 at or near the outlet to the fuel cell 10 to measure a second (outlet) temperature T2.

The temperature differential across the cell or stack is thus AT = T2-Ti, which is generally a positive value since the fuel cell reaction is exothermic and thus the temperature of the coolant increases as it flows from the first thermocouple to the second thermocouple. The amount of heat absorbed by the coolant is calculated as the product of the temperature differential AT, the mass flow rate of the coolant and the heat capacity of the coolant.

Of course, not all heat generated by the fuel cell is absorbed by the coolant passing through the cell so that the calculated heat absorbed by the coolant (based on the measured AT) is less than the actual heat generated by the fuel cell or stack.

[0023] The flow meter 38 and the thermocouples 40,42 operate respectively as flow-rate and temperature sensors, providing feedback signals to the coolant flow controller 30. The coolant flow controller 30 implements a control algorithm which adjusts the flow rate of the coolant to a flow-rate setpoint to control the temperature increase across the fuel cell 10 (or stack). The coolant flow controller 30 continually or intermittently actuates or "servos"the valve 36 and the (optional) VFD 34 in response to feedback signals from the thermocouples and the flow meter. The dotted lines in FIG. 1 represent communication lines for signals/data to be exchanged between the flow controller 30 and the VFD, valve and flow meter. (Feedback links from the thermocouples are not shown.) [0024] The flow controller 30 can work harmoniously with a PID-type flow control loop operating between the flow meter and the valve. The flow controller 30 can provide and then continually (or intermittently) update the flow-rate setpoint for the PID loop based on feedback signals from the thermocouples and flow meter. In other words, the PID loop can be nested (as a"slave"loop) within the control algorithm of the flow controller 30. The PID loop simply adjusts the flow rate based on the setpoint, which is in turn determined (and hence indirectly controlled) by the flow controller 30, i. e. the"master"controller.

Alternatively, the flow controller 30 can directly control both the VFD 34 and the valve 36 in response to feedback signals from the thermocouples and flow meter, thus dispensing with the need for a"nested"PID loop.

[0025] In the embodiment of FIG. 1, flow rate is regulated by both the valve 36 and the VFD 38. As is known in the art, a variable-frequency drive (VFD) is an electronic controller that adjusts the speed of an electric motor by modulating the power being delivered. Variable-frequency drives provide continuous control, matching motor speed to the specific demands of the work being performed. The VFD 34, as will be explained below, is an optional component which can be used to extend the range of flow rates.

[0026] The flow controller's control algorithm implements a simple energy balance equation for the stack operating parameters to determine the initial flow rate to supply the stack. After a user-configurable time, the algorithm adjusts the flow rate to the stack to correct any offset that exists in the temperature increase based on a simple heat transfer equation. The correction stage will repeat indefinitely on a timed cycle until the temperature increase across the stack is within an acceptable"dead- band"or error margin, in which case no correction should be applied. Both equations have proportional gain constants that can be displayed on a graphical user interface (GUI) and modified using any suitable user input device. In one embodiment, the parameters of the control algorithm for the fuel cell testing station 10 can be varied using suitable software operating on a personal computer (or by changing settings on a rack-mounted module or bench-top unit).

Preferably, the proportional gain constants can be adjusted while the algorithm is operating, without"resetting"or interrupting the algorithm.

[0027] As would be appreciated by those of ordinary skill in the art, the test station 20 can include a computer with data acquisition cards connected to various sensors, such as to a cell voltage monitoring board, which is a component known in the art. The test station 20 could also include fuel cell testing software to automate aspects of the fuel cell testing.

[0028] Furthermore, the test station 20 could include any number of safety alarms, automatic shut-offs, and auxiliary data acquisition cards for receiving signals from additional transducers that may be needed for conducting less common or customized tests.

[0029] As noted above, the flow controller 30 implements a control algorithm designed to regulate coolant flow rate to provide the desired temperature differential across the cell or stack. The following variables are used by the control algorithm to determine the flow rate and flow correction: M = coolant flow rate in liters per minute (LPM) c, = coolant heat capacity kJ/kg*Kelvin DT = desired temperature increase in degrees Celsius V = average CVM value in volts I = current measured at the shunt in amps N = number of cells in stack D = fluid density in kg per liter KE = efficiency constant Kp = proportional reaction constant.

[0030] As is known by those of ordinary skill in the art of fuel cell technology, the CVM voltage (Cell Voltage Monitoring voltage) is the electrical potential across a fuel cell.

[0031] For measuring the current, I, the shunt referred to above is merely a resistor installed in the load circuit which is thus capable of taking accurate measurements of the current produced by the cell or stack. The shunt can be located in the line between the cell or stack and the load box but is preferably built into the load box. The load box is the device within the station or associated with the station that controls and dissipates the electrical power generated by the cell or stack.

[0032] The efficiency constant, KE, is the ratio of heat absorbed by the coolant to the total heat generated by the cell or stack. In other words, KE represents the percentage of waste heat generated by the cell or stack that transfers to the coolant. As will be understood by those of ordinary skill in the art, it is desirable that the efficiency constant approach 1 so that almost all of the heat generated by the cell or stack is removed by the coolant.

[0033] Similarly, those of ordinary skill in the art of process control will recognize the proportional reaction constant, Kp, as the term that controls the magnitude of the response to a feedback change in a control algorithm.

Typical values for the proportional reaction constant in this algorithm would range from 0.8 to 1.2.

[0034] Regarding the desired temperature increase, the variable DT can be expressed in any temperature scale provided that the heat capacity Cv is expressed consistently in the same system of units.

[0035] The equation to determine the initial flow rate (M) is: M = KE* (I*V*N* ( (1. 25/V)-1) *60000)/ (cv*DT*D) [0036] The equation to determine the flow rate correction (M') is: MT = KP*M* (achieved DT)/(required DT) [0037] The control algorithm is expected to become unstable at low flow rates, as the desired temperature increase may not be generated by the stack. There is thus a user-configurable minimum flow rate. If the flow-rate setpoint drops below this minimum threshold, the minimum flow rate is set, and the setpoint is not permitted to grow exponentially downwards. If the flow rate does drop below the minimum flow rate or the temperature differential AT reaches negative values, a warning message is displayed on screen to alert the user that the lower limit has been reached. The operator should then switch to constant flow mode, and the flow will remain at minimum flow until an input is received to increase the flow rate (e. g. increased fuel cell stack load). Optionally, the software running the control algorithm could include code that prevents a user from entering a flow rate that is too low.

[0038] The control algorithm will be upset by changes in current, since the efficiency, environmental thermal losses, and coolant heat dissipation will all change suddenly. By the same token, if the algorithm is working well, and a small current change is effected, the algorithm will keep track of the flow setpoint, since these factors have already been compensated by the correction equation.

If the change in current is greater than the user- configurable percentage, the algorithm will re-estimate the flow rate based on the first equation, and use the servo equation (i. e. the flow rate correction equation) to approach the final flow rate on a timed cycle. If the change in current is less than the user-configurable percentage, the initial estimate of the new flow rate can be determined as: M'= M* (I')/(I) where I'is the correction current value.

[0039] Thus, when the fuel cell stack is under no load or very low power, the coolant supply will enter a constant flow mode.

[0040] FIG. 2 is a schematic view of a fuel cell testing system in accordance with another embodiment of the present invention. In this second embodiment, the testing station 20 regulates coolant flow rate without the variable- frequency drive that was present in the previous embodiment. The flow controller 30 receives feedback signals from the first and second temperature sensors (thermocouples) 40,42 and from the flow meter 38. Based on these feedback signals, the flow controller 30 then actuates the valve 36 to modulate the flow of the coolant through the coolant supply line 16. While this second embodiment does not offer the range of flow rates available in the first embodiment (due to the absence of the VFD), the second embodiment is nonetheless simpler and less expensive to implement.

[0041] In summary, therefore, the coolant flow controller is operatively connected to the flow-control valve and optionally to the variable frequency drive (VFD), which together operate as flow-regulating actuators for regulating the flow rate of the collant. It bears emphasizing that the variable frequency drive is optional.

The VFD is simply used to extend the range of flow rates.

[0042] The testing station 20 described herein can be used, with appropriate modifications where necessary, to test any type of fuel cell, such as Proton Exchange Membrane (PEM) cells, Direct Methanol Fuel Cells (DMFC), Solid Oxide Fuel Cells (SOFC) and Molten Carbonate Fuel Cells (MCFC).

[0043] 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 invention is therefore intended to be limited solely by the scope of the appended claims.