WO2008034253A1

WO2008034253A1 - Control of relative humidity in fuel cell systems

Info

Publication number: WO2008034253A1
Application number: PCT/CA2007/001689
Authority: WO
Inventors: Hao Tang; Dingrong Bai; Jean-Guy Chouinard; David ELKAÏM
Original assignee: Hyteon Inc.
Priority date: 2006-09-19
Filing date: 2007-09-19
Publication date: 2008-03-27

Abstract

The present system and method relates to the operation of PEM fuel cell stacks, particularly to a method for operating a fuel cell system, the method comprising: measuring operating parameters of the fuel cell system; calculating an initial inlet relative humidity of a reactant at an inlet of a fuel cell stack in the fuel cell system using the operating parameters; determining a quantity of water to be transferred into the reactant that is required for the initial inlet relative humidity to correspond to a desired inlet relative humidity; and providing the quantity of water to the reactant, thereby obtaining the desired inlet relative humidity.

Description

CONTROL OF RELATIVE HUMIDITY IN FUEL CELL SYSTEMS .

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of US Provisional Patent Application filed on September 19, 2006 and bearing serial number 60/845,513.

TECHNICAL FIELD

The present invention relates to the field of fuel cells, and more particularly, to the active control of the relative humidity of reactants of fuel cell stacks in fuel cell systems or in fuel cell testing facilities.

BACKGROUND OF THE INVENTION

Proton Exchange Membrane (PEM) fuel cell stacks are preferably operated at 100% anode/cathode reactant relative humidity to attain their maximum performance and lifetime. This is especially important for stationary applications where a lifetime of at least five, and ideally ten, years is required. The stack lifetime can be greatly reduced if anode and/or cathode reactant relative humidities (RHs) are either higher (flooding) or lower than 100% (drying) . Therefore, RH control during fuel cell operation is a core aspect of fuel cell technologies.

In most PEM fuel cell systems or testing facilities, internal or external humidifiers can be used for anode and/or cathode reactant humidification. In principle, the relative humidity (RH) , or the dew point of the reactant, is determined by humidification capacity of the humidifier membrane, humidifier temperature, and reactant mass flow rate. With the aging of the membranes, however, the humidifier performance and capacity declines compared to those of its initial values. Therefore, it is highly desirable that the reactant humidity be real-time monitored and precisely controlled during the lifetime of fuel cell operation.

In some fuel cell systems or fuel cell testing facilities, RH or dew point sensors are used to provide feedback for reactant humidity monitoring and controlling. However, both the high cost and instability to operate at high humidity conditions (close to, or over 100% RH) limits their practical applications. There is therefore a need for a simple, low cost but yet precise method to control reactant RH in fuel cell systems or testing facilities.

SUMMARY OF THE INVENTION

In accordance with a first broad aspect of the present invention, there is provided a method for operating a fuel cell system, the method comprising: measuring operating parameters of the fuel cell system,- calculating an initial inlet relative humidity of a reactant at an inlet of a fuel cell stack in the fuel cell system using the operating parameters; determining a quantity of water to be transferred into the reactant that is required for the initial inlet relative humidity to correspond to a desired inlet ■ relative humidity; and providing the quantity of water to the reactant, thereby obtaining the desired inlet relative humidity.

In accordance with a second broad aspect of the present invention, there is provided a fuel cell system comprising: a fuel cell stack; a reactant humidifying module; devices for measuring operating parameters during operation of the fuel cell system,- and a control module adapted to: calculate an initial inlet relative humidity of a reactant at an inlet of the fuel cell stack in the fuel cell system using the operating parameters; determine a quantity of water to be transferred into the reactant that is required for the initial inlet relative humidity to correspond to a desired inlet relative humidity; and set parameters of the fuel cell system to provide the quantity of water to the reactant, thereby obtaining the desired inlet relative humidity.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

Fig. 1 is a block diagram of an embodiment of the fuel cell system including two humidifiers;

Fig.2 is a block diagram illustrating a water injection device in accordance with an embodiment;

Fig. 3 is a block diagram of an embodiment of the fuel cell system including outlet measuring devices,-

Fig. 4 is a block diagram of an embodiment of the fuel cell system including a single humidifier; and

Fig. 5 is a flow chart of an embodiment of the method for controlling the RH.

It will be noted that throughout the appended drawings, like features are identified by like reference numerals.

DETAILED DESCRIPTION

Figure 1 illustrates one embodiment for a system of the present invention. External anode and cathode humidifiers are present. Alternatively, a single, integrated external humidifier can also be provided for both anode and cathode.

The fuel cell system 98 includes a fuel cell stack 102, a humidity control module 100, a cathode humidifier 104 and an anode humidifier 106. A stream of cathode reactant 110 enters the cathode humidifier 104 and a stream of humidified cathode reactant 118 exits the cathode humidifier 104 and is directed to the fuel cell stack 102. A stream of water 114 goes through the cathode humidifier 104 to humidify the cathode reactant 110. A stream of anode reactant 108 enters the anode humidifier 106 and a stream of humidified anode reactant 116 exits the anode humidifier 106 and is directed to the fuel cell stack 102. A stream of water 112 goes through the anode humidifier 106 to humidify the anode reactant 108.

Several devices for measuring operating parameters are present in the fuel cell system 98. In this particular embodiment, a first mass flow meter 126 is used to measure the mass flow rate of the anode reactant flow 108. A second mass flow meter 124 measures the mass flow rate of the cathode reactant stream 110. A third mass flow meter 128a measures the mass flow rate of the water stream 112 supplying the anode humidifier 106. A fourth mass flow meter 128b measures the mass flow rate of the water stream 114 supplying the cathode humidifier 104. All four of these mass flow meters 128a, 128b, 124, and 126 are connected to the humidity control module 100 as illustrated by 130, 132, 140 and 142, respectively so that the humidity control module 100 receives the data recorded from the mass flow meters, based on which commands will be generated and communicated to fluid delivering devices (e.g. pumps). Furthermore, the fuel cell system 98 illustrated in figure 1 comprises a pressure sensor 144a to measure the pressure of the anode reactant stream 116 at the anode inlet of the fuel cell stack 102 and a pressure sensor 144b to know the pressure of the cathode reactant stream 118 at the cathode inlet of the fuel cell stack 102. Both pressure sensors 144a and 144b are connected to the humidity control module 100. Additionally, the fuel cell system 98 illustrated in figure 1 may comprise temperature measurement means (e.g. thermocouple) 119a, 119b to measure the temperatures of streams 116 and 118, as well as to measure coolant inlet and outlet (not shown) .

It should be noted that the mass flow meters can be replaced by any device which permits the mass flow rate of the reactant and/or water streams to be identified. For example, weighing devices or pre- calibrated pumps can be used.

It should be noted that the water streams 112 and 114 can come from external sources independent of the fuel cell stack 102 or can come from recycled sources of the fuel cell system 98.

It should also be noted that the humidifiers can be replaced by any humidification device known by a person skilled in the art such as a bubbler or an enthalpy wheel. Alternatively, the water can be directly injected in the reactant stream by a water injection device, as illustrated in figure 2.

Furthermore, the control module 100 is connected to the fuel cell stack 102 and the humidifiers 104, 106 as illustrated by 136, 134 and 138, respectively. Temperature sensors can be integrated into the humidity control module 100 to monitor the temperature of the humidifiers 104, 106 and the fuel cell stack 102. Alternatively, temperature recording devices may be provided externally to the humidity control module 100 as additional parameter recording devices.

The humidity control module 100 receives the data coming from the different devices for measuring the operating parameters and calculates the RH of the reactants. If the calculated RH of at least one of the reactant does not correspond to a predetermined value, the humidity control module calculates the quantity of water to be transferred to the reactant in its corresponding humidifier that is required for the RH to correspond to the desired relative humidity.

Then, the humidity control module 100 proceeds to the adjustment of at least one of the fuel cell system parameters. Some fuel cell system parameters are the anode/cathode mass flow rate in the humidifier, the water mass flow rate into the humidifier, the humidifier temperature and the pressure difference across the membrane of the humidifier in the case of a membrane-type humidifier. If the RH of the anode reactant does not correspond to its associated predetermined value, the humidity control module 100 adjusts at least one of the water mass flow rate, the anode reactant mass flow rate, the anode humidifier temperature and the pressure difference across the membrane in the anode humidifier. If the RH of the cathode reactant does not correspond to its associated target value, the humidity control module 100 adjusts at least one of the water mass flow rate, the cathode reactant mass flow rate, the cathode humidifier temperature and the pressure difference across the membrane in the cathode humidifier. In an embodiment, the humidity control module 100 can increase/decrease the anode/cathode humidifier temperature to increase/decrease the relative humidity of the reactant, respectively.

In one embodiment of the device, the humidity control module 100 can increase/decrease the anode/cathode reactant mass flow rates to decrease/increase the RH, respectively.

In one embodiment of the device, the humidity control module 100 can increase/decrease the anode/cathode humidifier water mass flow rate to increase/decrease the RH, respectively.

In another embodiment of the device, the humidity control module 100 can increase/decrease the pressure difference across the membrane in the anode/cathode humidifier to increase/decrease the relative humidity, respectively. In the case of a membrane-type humidifier, the pressure difference induces a water partial pressure difference across the membrane and affects the water transfer rate across the membrane as demonstrated by the following equation:

where Q_H2o is water transfer rate from the water side to the reactant side, D_H2o is the water diffusion coefficient in the membrane, S is water solubility in the membrane, L is membrane thickness, and Pi and P₂ are H₂O partial pressures from the water and reactant sides, respectively. One way to increase/reduce the water transfer from the water side to the reactant side is to increase/reduce the partial pressure difference (Pi-P₂) . It should be understood that any technique to vary the water transfer rate through the humidifier known by a person skilled in the art can be used.

In addition, any combination of the above-described parameter-adjusting techniques may be used to modify the RH.

It should be understood that other measures known to a person skilled in the art may be taken to adjust the humidifier parameters and thereby adjust the RH.

In an embodiment of the fuel cell system, the mass flow meters can be located before the humidifiers. In this case, the mass flow meters communicate the water mass flow rate, the anode reactant mass flow rate and the cathode reactant mass flow rate to the humidity control module.

In an embodiment of the fuel cell system, a bubbler-type humidification system can be used to humidify the reactant. In this case, the humidity control module monitors the parameters of the bubbler-type humidifier to determine the RH of the reactants and varies those parameters to adjust the RH of the reactant. For example, a water heating means such as a heat exchanger or an electric heater can be included in the bubbler-type humidifier to adjust the humidifier temperature which varies the transfer rate of water to the reactant.

In another embodiment of the fuel cell system, the humidifier is replaced by a water injection device which directly injects a determined quantity of water into the reactant stream as illustrated in Fig. 2. The fuel cell system 300 includes a conduit 304 connected to the fuel cell stack 302. A stream of reactant molecules 310 flows into the conduit 304 following direction 312. Before the stream of reactant molecules 310 reaches the inlet of the fuel cell stack, water 308 is injected in the stream of reactant by the water injection module 306. The water injection module can also be carried out inside the stack, or by any appropriate means known to those skilled in the art. The humidity control module can calculate the RH of the reactant by monitoring the quantity of water that the water injection module injects into the reactant stream. The humidity control module can adjust the RH of the reactant by varying the quantity of water injected into the reactant stream.

Fig. 3 illustrates a fuel cell system 160 which includes all of the components of the fuel cell system 98 illustrated in figure 1 in addition to two sensing devices 162 and 164. The sensing devices 162 and 164 are connected to the humidity control module 100 as illustrated by 168 and 170, respectively.

In an embodiment of the system, the sensing devices 162 and 164 include a device or a combination of devices that measure the weight of the water contained in the reactant at the outlet of the fuel cell stack. For example, the sensing devices 162 and 164 can include heat exchangers which condense the water from the fuel cell stack exhausting streams and devices to weigh the condensed water. The sensing devices 162 and 164 further include pressure and temperature sensors to measure the temperature and the pressure at the outlet of the fuel cell stack. The weight of condensed water, the temperature and the pressure at the outlet of the fuel cell stack are sent to the humidity control module 100 which calculates the water mass flow rates of the reactants at the outlet of the fuel cell stack. Furthermore, the reactant mass flow rate at the output of the fuel cell stack can be calculated starting from the fuel cell stack performances such as the total current of the fuel cell stack and from the reactant's mass flow rate at the inlet of the fuel cell stack. The humidity control module calculates the RH of the reactants at the outlet of the fuel cell stack using the calculated mass flow rates of water and reactants and the measured pressure and temperature at the outlet of the fuel cell stack. Any method known by a person skilled in the art to determine the reactant mass flow rate and to measure the weight of water at the outlet of the fuel cell stack can be used.

In an embodiment, the sensing devices 162 and 164 include temperature and pressure sensors and the water mass flow rate at the outlet of the fuel cell stack can be calculated by the humidity control module 100 without the weight of the water contained in the reactant exiting the fuel cell stack. In this case, the humidity control module 100 calculates the water mass flow rates at the outlet of the fuel cell stack by starting from the water mass flow rate at the inlet of the fuel cell stack and the fuel cell stack performances and taking into account the water transfer between the anode side and the cathode side in the fuel cell stack. It should be noted that any method known by a person skilled in the art to calculate the water transfer between the anode side and the cathode side in the fuel cell stack can be used and falls within the scope of the present device.

One method to calculate the water mass flow rate at the outlet of the fuel cell stack takes into account the water mass flow rate at the inlet of the fuel cell stack, the quantity of water produced during the fuel cell stack operation and the water transfer rate between the anode side and the cathode side in the fuel cell stack.

During fuel cell stack operation, some of the anode water will be transferred to the cathode side along with proton mitigation (water drag) while some of cathode water will be transferred to the anode side due to water back diffusion.

Monitoring the real time water transfer rate between anode and cathode sides during fuel cell operation is very important to manage the water balance in the fuel cell stack and hence to improve the fuel cell stack performance and lifetime.

The water transfer rate from the cathode side to the anode side can be calculated using the following equation:

W_c-_a= Wi - W_m (1)

where W_c__a is the cathode side to anode side water transfer rate, W_ra is the measured or calculated rate of actual cathode water flowing outside the fuel cell stack, and Wi is the calculated ideal rate of cathode water flowing outside the fuel cell stack assuming that the water transfer rate between anode and cathode is nil, and equals the sum of the water flowrate brought into the stack by cathode oxidants and the water produced at the cathode side of the stack.

If W_c__a is positive, the water is transferred from the cathode side to the anode side into the fuel cell stack, while a negative value W_c__a corresponds to a transfer of water from the anode side to the cathode side.

Alternatively, the transfer rate from the anode side to the cathode side can be used. The humidity control module 100 receives the data coming from the different devices for measuring the operating parameters and calculates the RH of the reactants at the inlet and outlet of the fuel cell stack 102. The humidity control module 100 compares the calculated inlet and outlet RH values to desired inlet and outlet RH values. To adjust the RH of the reactants at the inlet of the fuel cell stack 102, the humidity control module 100 calculates the quantity of water to be transferred to the reactants in their corresponding humidifier, so that the RH of the reactants corresponds to their respective desired RH value. The humidity control module 100 varies at least one of the water mass flow rate in the cathode humidifier, the cathode reactant mass flow rate, the cathode humidifier temperature and the pressure difference across the membrane in the cathode humidifier in order to transfer the calculated quantity of water to the reactant and to achieve the desired value of reactant RH at the inlet of the fuel cell stack 102. In order to adjust the RH of the reactants at the outlet of the fuel cell stack 102, the humidity control module 100 varies the temperature of the fuel cell stack 102.

In an embodiment of the system, only the RH of the cathode reactant is adjusted at the inlet and outlet of the fuel cell stack.

In another embodiment of the system, only the RH of the anode reactant is adjusted at the inlet and outlet of the fuel cell stack.

Alternatively, the RHs of both the anode and cathode reactants are controlled. Fig. 4 illustrates a fuel cell system 200 which includes a fuel cell stack 202, a humidity control module 204 and a single integrated humidifier 206 to humidify a stream of cathode reactant 210 and a stream of anode reactant 208. Flow meters 218, 216 and 214 are used to measure the mass flow rates of the water stream 212, the stream of cathode reactant 210 and the stream of anode reactant 208, respectively. The water stream 212 goes through the humidifier 206 to humidify the reactants. At the outlet of the humidifier 206, the stream of humidified anode reactant 222 and the stream of cathode reactant 220 enter the fuel cell stack 202. Pressure sensors 224a and 224b measure the pressure of the cathode reactant and the anode reactant, respectively, at the inlet of the fuel cell stack 202. The humidity control module 204 is connected to the water flow meter 218, the cathode flow meter 216, the humidifier 206, the anode flow meter 214, the anode pressure sensor 224b, the cathode pressure sensor 224a and the fuel cell stack 202 operating temperatures as illustrated by 228, 230, 232, 242, 244, 246 and 248. In this embodiment, the parameters of this single humidifier 206 can be adjusted in order to vary the RH of the cathode reactant and/or the anode reactant .

In another embodiment of the fuel cell system illustrated in figure 3, only the RH of the anode reactant is controlled .

In another embodiment of the fuel cell system illustrated in figure 3, only the RH of the cathode reactant is controlled.

In an embodiment of the fuel cell system, the humidity control module further controls the RH of the anode and/or cathode reactant at the outlet of the fuel cell stack. The RH of a reactant can be determined using the following equation:

where P_w and P_ws are the water partial pressure and the water saturation partial pressure of the reactant, respectively, at the measured stack inlet temperature T and pressure .

The water partial pressure of the reactant P_w can be determined starting from the measured pressure P of the reactant at the inlet of the fuel cell stack using the following equation:

Pw=VwP ⁽3⁾

where y_w is the molar fraction of water.

The molar fraction of water y_w can be determined using equation (3) :

where M_w and M_R are the molecular weight of water and the molecular weight of the reactant, respectively, and W_w is the humidity ratio.

The humidity ratio W_w is defined by equation (4) :

where m_w is the measured/calculated mass flow rate of water and m_R is the measured reactant mass flow rate. In the case of a humidifier (such as a bubbler-type or a membrane-type humidifier) , it is assumed that all water entering the humidifier is transferred to the reactant .

The water saturation partial pressure P_ws at the measured stack inlet temperature T can be determined using any method known by a person skilled in the art

One method to calculate the water saturation partial pressure P_ws (in psia) at the measured reactant inlet temperature T is to use the following equations:

In(P₁J = CJT_R +C₂+C₃T_K+CJ_R ² +C_ST_R ³+C₆ ln(T_K) (6)

where T_R is the absolute temperature, which relates to the temperature T (in ⁰C) by:

T_R=-T + 491.67 (7)

and

C₁ =-1.0440397x10"

C₂ =-1.1294650x10' C, =-2.7022355 xlO^"2

-5 :s)

C₄ =+1.2890360x10 C₅ =-2.4780681 xlO^"9 C₆ = +6.5459673

The water saturation partial pressure P_Ws (in atm) at the measured reactant inlet temperature T can also be determined using the following empirical equation (9) :

log₁₀(P_viJ = -2.1794 + 0.02593r-9.1837xl0^"5r² +1.4454XlO-⁴T³

(9) It should be understood that any method known by a person skilled in the art to calculate the water saturation partial pressure P_ws can be used without departing from the scope of the present invention.

The dew point T_d is the temperature at which the water partial pressure of the reactant P_w is equal to the water saturation partial pressure of the reactant. At the dew point T_d, the RH is equal to 100%.

If the dew point T_d is between 0 and 93°C, the dew point can be calculated using the following equations:

T_d =C₁+C,a + C₉a² +C_]Oa^} +C_πP/¹⁹⁸⁴ (10)

where Td is expressed in ⁰F and P_w in psia and

C₇ =100.45 C₈ =33.193

C₉ =2.319

(11) C₁₀ =0.17074

C_n =1.2063

If one wants to operate the fuel cell stack with a reactant having a 100% RH, the fuel cell stack has to be operated at a temperature equal to the dew point temperature Td, which is possible by adjusting the water partial pressure P_w of the reactant .

Fig. 5 illustrates an embodiment of the method to control the RH of the reactant in a fuel cell system. The humidity control module obtains the temperature and the pressure of the reactant at the inlet of the fuel cell stack from the stack temperature sensor and the pressure sensor, respectively. The water mass flow rate from the mass flow meter associated with the water source and the mass flow rate of the reactant from the mass flow meter associated with the reactant are also obtained.

The RH of the reactant is calculated following the method previously described using equations (2) -(5) and equations (6)- (8) or (9) .

If the RH of the reactant does not correspond to a predetermined value of RH, the control module calculates the quantity of water that has to be transferred to the reactant stream in order to adjust the RH accordingly.

Once this quantity of water is known, various parameters of the fuel cell system may be adjusted to change the RH. For example, at least one of the water mass flow rate, the reactant mass flow rate, the humidifier temperature and the difference of pressure across the membrane of the humidifier is adjusted to transfer the calculated quantity of water to the reactant .

In an embodiment of the method, the reactant humidifier temperature can be increased/decreased to increase/decrease the RH of the reactant.

In an embodiment of the method, the reactant mass flow rate can be increased/decreased to decrease/increase the RH, respectively.

In an embodiment of the method, the humidifier water mass flow rate can be increased/decreased to increase/decrease the RH, respectively.

In an embodiment of the method, the pressure difference across the membrane in the humidifier can be increased/decreased to increase/decrease the relative humidity, respectively. The pressure difference induces a water partial pressure difference across the membrane and affects the water transfer rate across the membrane.

In another embodiment, a combination of these adjustments is also possible.

It should be understood that any means can be used to humidify the reactant and that the RH of the humidified reactant can be adjusted by varying at least one parameter of a reactant humidifying module. For example, a water injection device can be used and the quantity of injected water can be varied to adjust the RH of the reactant.

In an embodiment of the method, the RH of the reactants at the outlet of the fuel cell stack can also be controlled as illustrated in Fig. 4. Starting from the operating parameters of the fuel cell system, the RH of the reactants at the outlet of the fuel cell stack is calculated by the humidity control module following the methods described above and these calculated values of RH are compared to the desired RH values. If the calculated values of RH at the outlet of the fuel cell stack do not correspond to the corresponding desired values, the control module varies the temperature of the fuel cell stack to adjust the RH of the reactants .

It should be understood that the humidity control module can control the RH of either the anode reactant, the cathode reactant or both the anode and cathode reactants at the inlet and/or outlet of the fuel cell stack.

In an embodiment of the method, the mass flow rate of the reactant can be constant. In this case, the humidity control module receives the measured mass flow rate of the reactant from the reactant mass flow meter. The humidity control module also receives the reactant pressure, the water mass flow rate and the reactant temperature at the inlet of the fuel cell stack from the pressure sensor, the mass flow meter associated with the humidifier water and the fuel cell stack, respectively.

The RH of the reactant at the inlet of the stack is calculated by the humidity control module using the operating parameters previously received and equations (2)- (5) and equations (6) -(8) or (9). The calculated value of RH is compared to a predetermined value of RH. If the calculated value of RH is equal to the predetermined value of RH (or if the calculated value of RH meets some threshold requirements) , the operation of the fuel cell system continues without the change of any fuel cell system parameters.

If the calculated value of RH is above or below a threshold value, the quantity of water to be transferred to the reactant is calculated by the humidity control module. This quantity is adjusted by varying the temperature of the humidifier or varying the mass flow rate of the water stream.

If the calculated value of RH is above the threshold value, the humidifier temperature is decreased or the mass flow rate of the humidifier water is decreased, so that the quantity of water transferred to the reactant corresponds to the calculated quantity and, therefore the RH of the reactant corresponds to the predetermined value of RH.

If the calculated value of RH is below the threshold value, the humidifier temperature is increased or the mass flow rate of the humidifier water is increased, so that the quantity of water transferred to the reactant corresponds to the calculated quantity and, therefore the RH of the reactant corresponds to the predetermined value of RH.

Once the fuel cell system parameter (s) are adjusted, the humidity control module measures the reactant pressure, the water mass flow rate and the temperature of the reactant at the inlet of the fuel cell stack in order to calculate the new value of RH. This value is compared to the predetermined value of RH and if they are not equal, the humidity control module carries out another adjustment of the humidity parameter.

It should be understood that the humidity control module can control the RH of either the anode reactant, the cathode reactant or both the anode and cathode reactants .

The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims

I/WE CLAIM :

1. A method for operating a fuel cell system, the method comprising : measuring operating parameters of the fuel cell system; calculating an initial inlet relative humidity of a reactant at an inlet of a fuel cell stack in said fuel cell system using said operating parameters; determining a quantity of water to be transferred into said reactant that is required for said initial inlet relative humidity to correspond to a desired inlet relative humidity; and providing said quantity of water to said reactant, thereby obtaining said desired inlet relative humidity.

2. A method as claimed in claim 1, wherein said providing said quantity of water comprises adjusting an input of water into a membrane-type humidifier such that said quantity of water is transferred to said reactant.

3. A method as claimed in claim 1, wherein said providing said quantity of water comprises adjusting a temperature of a membrane-type humidifier to modulate said quantity of water transferred to said reactant.

4. A method as claimed in claim 1, wherein said providing said quantity of water comprises modifying a pressure difference across a membrane in a membrane-type humidifier.

5. A method as claimed in claim 1, wherein said providing said quantity of water comprises injecting said quantity of water directly into said reactant.

6. A method as claimed in claim 1, wherein said providing said quantity of water comprises adjusting a mass flow rate of said reactant as said quantity of water is being provided.

7. A method as claimed in claim 1, further comprising: calculating an initial outlet relative humidity of said reactant at an outlet of a fuel cell stack in said fuel cell system using said operating parameters; and adjusting a temperature of said fuel cell stack such that said initial outlet relative humidity be modified to correspond to a desired outlet relative humidity.

8. A method as claimed in claim 1, wherein said measuring operating parameters comprises measuring reactant inlet mass flow rate, stack inlet temperature, humidifying water mass flow rate, and stack inlet pressure.

9. A method as claimed in claim 1, wherein said calculating an initial inlet relative humidity comprises calculating an initial inlet anode relative humidity.

10. A method as claimed in claim 1, wherein said calculating an initial inlet relative humidity comprises calculating an initial inlet cathode relative humidity. ,

11. A fuel cell system comprising: a fuel cell stack,- a reactant humidifying module; devices for measuring operating parameters during operation of said fuel cell system,- and a control module adapted to calculate an initial inlet relative humidity of a reactant at an inlet of said fuel cell stack in said fuel cell system using said operating parameters ; determine a quantity of water to be transferred into said reactant that is required for said initial inlet relative humidity to correspond to a desired inlet relative humidity; and set parameters of said fuel cell system to provide said quantity of water to said reactant, thereby obtaining said desired inlet relative humidity.

12. A system as claimed in claim 11, wherein said reactant humidifying module is a membrane-type humidifier.

13. A system as claimed in claim 11, wherein said reactant humidifying module is a water injection device.

14. A system as claimed in claim 11, wherein said devices comprise at least one temperature sensor for measuring stack inlet temperature and reactant humidifying module temperature .

15. A system as claimed in claim 11, wherein said devices comprise a pressure sensor for measuring stack inlet pressure .

16. A system as claimed in claim 11, wherein said devices comprise at least one mass flow meter for measuring a mass flow rate of said reactant and a mass flow rate of humidifying water.

17. A system as claimed in claim 11, wherein said control module is adapted to set parameters of a reactant pump to modify a flow rate of said reactant during humidification.

18. A system as claimed in claim 12, wherein said control module is adapted to set parameters of said membrane- type humidifier by modifying a flow rate of water flowing therethrough.

19. A system as claimed in claim 12, wherein said control module is adapted to set parameters of said membrane-type humidifier by modifying a temperature thereof.

20. A system as claimed in claim 12, wherein said control module adapted to set parameters of said membrane- type humidifier by modifying a pressure difference across a membrane therein.

21. A system as claimed in claim 13, wherein said control module is adapted to set parameters of said water injection device by modulating a water injection rate thereof.

22. A system as claimed in claim 11, wherein said control module is further adapted to: calculate an initial outlet relative humidity of said reactant at an outlet of said fuel cell stack in said fuel cell system using said operating parameters; and adjust a temperature of said fuel cell stack such that said initial outlet relative humidity be modified to correspond to a desired outlet relative humidity.

23. A system as claimed in claim 22, wherein said devices comprise a temperature sensor for measuring stack outlet temperature and a pressure sensor for measuring stack outlet pressure.

24. A system as claimed in claim 11, wherein said control module calculates an initial anode relative humidity.

25. A system as claimed in claim 11, wherein said control module calculates an initial cathode relative humidity.