Humidifier for use with a fuel cell

Details Humidifier for use with a fuel cell Humidifier for use with a fuel cell

2001-01-19

2002-10-29

Bushey, C. Scott (Department: 1724)

Gas and liquid contact apparatus

Fluid distribution

Valved

C261S053000, C261S104000

Reexamination Certificate

active

06471195

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a humidifier for use with a fuel cell, and more particularly to a humidifier utilizing water permeable membranes.
BACKGROUND OF THE INVENTION
Fuel cell systems, especially solid polymer fuel cell systems are widely known as a power source for electric vehicles. In such fuel cell systems, a humidifier is used to moisture-exchange off gas, viz. moist gas discharged from a fuel cell, between its moisture and air as a dry gas, and to generate humidified air or humidified gas. Preferably, a humidifier used with such fuel cell systems is of a lower power consumption type, and is required compactness with less attachment space. For this reason, among many other humidifiers, such as a supersonic humidifier, a steam humidifier, a vaporizing humidifier and a nozzle injection type humidifier, a humidifier utilizing water permeable membranes, especially hollow fiber membranes is commonly used with a fuel cell.
A conventional humidifier utilizing hollow fiber membranes is disclosed in Japanese Laid-open Patent Publication No. HEI-7-71795. As shown in
FIG. 12
, a humidifier
100
comprises a housing
101
, on which is provided a first inlet
102
for introducing dry air and a first outlet
103
for discharging the dry air (humidified dry air). A bundle of hollow fiber membranes
104
consisting of a number of for example 5000 hollow fiber membranes is accommodated within the housing
101
.
At both ends of the housing
101
, fastening members
105
,
105
′ are provided for fixing the ends of the bundle
104
while leaving them open. Outside of the fastening member
105
is provided a second inlet
106
for introducing moist air or moist gas, and a second outlet
107
is provided outside of the fastening member
105
′ for discharging the moist air, moisture of which is separated and removed at the bundle of hollow fiber membranes
104
. The fastening members
105
,
105
′ are covered with a first head cover
108
and a second head cover
109
, respectively. And the second inlet
106
is formed on the first head cover
108
, while the second outlet
107
is formed on the second head cover
109
.
In the aforementioned humidifier
100
utilizing hollow fiber membranes, the moist air introduced from the second inlet
106
passes through the hollow fiber membranes forming the bundle of hollow fiber membranes
104
, and the moisture within the moist air is separated by capillary action of the hollow fiber membranes. The separated moisture moves outward of the hollow fiber membrane through a capillary tube of the membrane. The moisture-removed air is discharged from the second outlet
107
.
Meanwhile, dry air is supplied from the first inlet
102
. The dry air from the first inlet
102
flows outside of the hollow fiber membranes forming the bundle of hollow fiber membranes. Because the moisture separated from the moist air has moved outside of the hollow fiber membranes, the moisture humidifies the dry air. The humidified dry air is then discharged from the first outlet
103
.
However, as shown in
FIG. 13
, the conventional humidifier
100
has humidifying characteristics, on the basis of output of the fuel cell, in which the dew point of the humidified air decreases both in a low power output range where small amount of humidified air is required and a high power output range where large amount of humidified air is required, and the dew point of the humidified air increases in a middle power output range where middle amount of humidified air is required. Because dried electrolyte membranes affect fuel cells, the humidifying process is generally carried out on the basis of the low power output range or the high power output range. Therefore, the fuel cell tends to be excessively humidified in the middle power output range. As a result of the excessive humidification, if diffusion layers or electrodes of the fuel cell are immersed in water, free flow of the gas is interrupted in the fuel cell, which leads to a drawback in that expected performance is not achieved.
In order to improve the humidifying capability of the humidifier, a large number of hollow fiber membranes are required to provide extensive surface area of the membranes. However, with increasing the number of hollow fiber membranes, dry air flowing outside of the membranes tends to arise a drift, and the dry air does not equally flow throughout the housing. Thereby, the extensive surface area of the outer surface of the hollow fiber membranes cannot be used effectively, which leads to decreased humidifying efficiency and hence to a drawback that it is impossible to increase the dew point. This can also be said to other water permeable membranes.
In view of the above, the present invention seeks to provide a humidifier for use with a fuel cell, which can supply a fuel cell with humidified gas with stable dew points at the entire range from a low power output range to a high power output range, and which can utilize the extensive outer surface area of the hollow fiber membranes.
SUMMARY OF THE INVENTION
As the result of intensive researches, the inventors eventually complete the invention by focusing the reason why the dew point of the humidified air through the humidifier has the characteristics such as shown in FIG.
13
. Namely, in a low power output range of the fuel cell, the dew point of the humidified air cannot rise sufficiently, because the required amount of the humidified air is small and the operating pressure of the fuel cell is low. Meanwhile, in a high power output range of the fuel cell, the dew point of the humidified air cannot rise sufficiently, because the required amount of the humidified air is large.
According to the present invention, the objects of the invention can be accomplished by providing a humidifier for use with a fuel cell, comprising:
a plurality of combined water permeable membranes or water permeable devices, each of the water permeable membrane or water permeable device generating humidified gas by flowing therein different gases with different moisture contents and by moisture exchanging between the different gases so that one dry gas with smaller moisture content is humidified with the other moist gas with larger moisture content; and
flowing passage switching means for optionally switching flowing passages of said dry gas;
wherein said flowing passage switching means switches the flowing passage in accordance with a required amount of said humidified gas so as to selectively use particular water permeable membranes or a particular water permeable device from among the plurality of water permeable membranes or water permeable devices.
In such a construction, even if the required amount of the humidified gas is increased (output of the fuel cell is increased), control can be made with the flowing passage switching means so that the dew point of the humidified gas is converged within a certain extent. As a result, a humidifier suitable for use with a fuel cell can be provided. In this instance, in view of leveling the dew point, regardless of the required amount of the humidified gas, it is preferable to control the water permeable membranes or water permeable devices so as to be operable within a center region of the graph (FIG.
13
), where the dew point is stable. The “off gas” appeared in the preferred embodiment corresponds to the “moist gas”, and the “dry air” and “humidified air” correspond to the “dry gas” and “humidified gas”, respectively. Further, the “hollow fiber membranes” or “bundle of hollow fiber membranes” in the preferred embodiment corresponds to the “water permeable membranes”, and the “hollow fiber membrane module” corresponds to the “water permeable device”. The water permeable membranes may be in the form of a repeatedly folded film-like water permeable membrane or in the form of a winded roll.
According to a preferred embodiment of the present invention, each of the plurality of combined water permeable membranes or water permeable devices has a different humidifying capability.
In such a construction,

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