(a) Technical Field
The present invention relates to a membrane humidifier for a fuel cell. More particularly, it relates to a membrane humidifier for a fuel cell, which uniformly humidifies entire hollow fiber membranes from outer sides of the membrane to a central portion of the interior of the membrane humidifier to improve the distribution of wet air and dry air, thereby improving the humidification performance.
(b) Background Art
Fuel cell stack operation requires humidifying a polymer electrolyte membrane within the fuel cell stack. Accordingly, the fuel cell employs a membrane humidifier operated by moisture exchange between moisture from exhaust gas, which is a wet air discharged from the fuel cell stack, and dry air supplied from outside (e.g., environmentally supplied cool air.)
There are several types of humidifiers, such as a bubbler-type humidifier, an injection-type humidifier, and a humidifier using an adsorbent. However, since there is only a limited space on the package surface of a fuel cell vehicle, a small-sized membrane humidifier requiring no special power is used in the fuel cell. In particular, a membrane humidifier using hollow fiber membranes has been suitably used for a membrane humidifier for a fuel cell.
As illustrated in FIG. 5, a membrane humidifier 100 is included in an air supply system for supplying air (oxygen) to a fuel cell stack 200 of a fuel cell system, exterior dry air is supplied to the membrane humidifier 100 by an inhalation of an air blower 202 and exhaust gas discharged from the fuel cell stack 200 passes through an interior of the membrane humidifier 100 at the same time. At this time, the dry air is humidified while moisture contained in the exhaust gas passes through hollow fiber membranes within the membrane humidifier 100.
A conventional membrane humidifier including the hollow fiber membranes and its operation will be described with reference to FIG. 6 in more detail. As illustrated in FIG. 6, the conventional membrane humidifier 100 includes a housing 101. The housing 101 has a first inlet 102 for introducing dry air and a first outlet 103 for discharging humidified dry air. A hollow fiber membrane bundle 107 including a plurality of dense hollow fiber membranes 106 is received within the housing 101. Further, the housing 101 includes a second inlet 104 for inducing wet air discharged from the fuel cell stack and a second outlet 105 for discharging wet air at an opposite side thereof.
In operation, when discharged gas, i.e., wet air, reacted completely and discharged from the fuel cell stack is supplied to a side of the hollow fiber membrane bundle 107 from the second inlet 104 of the housing 101, moisture contained in the wet air is separated by a capillary action of each of the hollow fiber membranes 106 and the separated moisture is condensed while being permeated into the capillaries of the hollow fiber membranes 106 to move to the interior of the hollow fiber membranes 106. Subsequently, the wet air from which moisture is separated directly flows along the outside the hollow fiber membranes 106 and is discharged through the second outlet 105 of the housing 101.
In the meantime, exterior air (dry air) is supplied to the housing 101 through the first inlet 102 by force of the air blower and the exterior air supplied through the first inlet 102 flows through the interiors of the hollow fiber membranes 106. At this time, since the moisture separated from the wet air has already flowed to the interiors of the hollow fiber membranes 106, the dry air is humidified by the moisture and the humidified dry air is supplied to the side of the fuel cell stack through the first outlet 103.
However, since the hollow fiber membrane bundle 107 is very compact and has a plurality of dense hollow fiber membranes 106 therein, it is difficult for the wet air introduced through the second inlet 104 to permeate into the hollow fiber membrane bundle 107. Further, the diffusion rate of the wet air through the hollow fiber membranes is very slow, and thus it is very difficult for the wet air to permeate into the interiors of the hollow fiber membranes.
In particular, the wet air being passed through the outside of the hollow fiber membrane bundle 107 within the housing 101 often fails to permeate into a central portion of the hollow fiber membrane bundle 107 within the housing 101 as indicated with a dotted line in FIGS. 8 and 9 but mainly flows through an edge portion as indicated with an arrow in FIGS. 8 and 9. Thus, the diffusion rate of the wet air to the central portion of the hollow fiber membrane bundle 107 is very slow, thereby causing deterioration of humidification efficiency for dry air.
Further, since the large quantity of dry air introduced through the first inlet 102 of the housing 101 mainly flows through the central portion of the hollow fiber membrane bundle 107 (a part indicated with hidden lines in FIGS. 6 and 7), the hollow fiber membranes within the humidifier are under utilized and the general humidification efficiency of the humidifier is further deteriorated.
Thus, due to the above problems, the hollow fiber membranes 106 located at the central portion of the hollow fiber membrane bundle 107 fail to receive a sufficient amount of moisture, thus deteriorating general efficiency of the humidifier.
Such a problem can be identified through a simulation experimental result in FIG. 8. It can be clearly seen from FIG. 8 that most of the dry air flows through only the central portion of the hollow fiber membrane bundle 107. That is, the dry air introduced through the first inlet 102 of the housing 101 mainly flows through the central portion (a part indicated with hidden lines in FIGS. 6 and 7) of the hollow fiber membrane bundle 107 and the wet air introduced through the second inlet 104 flows through the edge portion of the hollow fiber membrane bundle 107. Accordingly, the humidification efficiency of the membrane humidifier is deteriorated, which further affects when the flow of the dry air is increased, i.e., a high power is output from the fuel cell stack.
As described above, the wet air supplied to the membrane humidifier is discharged after a reaction in the fuel cell, and water generated in the reaction, as well as vapor, is also supplied to the membrane humidifier together with the wet air. Therefore, in cold weather, water introduced into the membrane humidifier freezes and prevents the hollow fiber membranes from suitably performing its humidification activity. In addition, in cold weather the membrane humidifier can be used only after the frozen moisture of the hollow fiber membranes is melted. Further, since surfaces of the hollow fiber membranes of the conventional membrane humidifier are repeatedly frozen and melted, the hollow fiber membranes on the outer sides, i.e., the edge portions, of the hollow fiber membrane bundle 107 through which the wet air mainly flows are damaged or become disconnected (see FIG. 9).
Referring to FIG. 9, the hollow fiber membrane bundle 107 including the dense hollow fiber membranes 106 is mounted within the housing 101 of the membrane humidifier. In this case, opposite ends of the hollow fiber membrane bundle 107 are fixed to ends of an interior of the housing 101 by a potting material 108, so that the hollow fiber membrane bundle 107 is fixed. Therefore, disconnection of the hollow fiber membranes at the outer sides may occur due to a damage or breakage of the potting material 108 located on an outside end of the hollow fiber membrane bundle 107 through which the wet air mainly flows.
Even further, if the surfaces of the hollow fiber membranes are repeatedly frozen and melted to the extent that they become damaged, the damaged hollow fiber membranes eventually have a drastic effect on a performance of the fuel cell stack, and thus it is necessary to change the entire membrane humidifier.
Additionally, in manufacturing of the membrane humidifier, a large portion of the membrane humidifier is made of expensive hollow fiber membranes formed of a polymer material. In order to improve the humidification performance, more hollow fiber membrane bundles are used than necessary, thus increasing the manufacturing cost. In addition, due to the use of many hollow fiber membrane bundles, the size of the membrane humidifier is disproportionate compared to the performance of the membrane humidifier.
Furthermore, since the conventional membrane humidifier includes a single hollow fiber membrane module within which a plurality of hollow fiber membranes is received in the form of a bundle, the hollow fiber membranes are not uniformly distributed within the housing and are weighted to one side of the housing during manufacturing the membrane humidifier (see FIG. 10).