In recent years, a fuel cell using a solid polymer membrane, such as perfluorocarbonsulfonic acid membrane, for the electrolyte membrane is attracting attention as a power source of electric cars or small stationary power-generating devices. Such solid polymer functions, in the hydrous state, as a proton conductive electrolyte but in the dry state, the proton conductivity decreases and at the same time, contact failure occurs between the solid polymer electrolyte membrane and the electrode to cause an abrupt-decrease in the output. Therefore, in the solid polymer-type fuel cell system, the supply gas is supplied after humidifying it so that the solid polymer electrolyte membrane is kept at a constant humidity. For this purpose, various studies are being made on the humidifying apparatus for humidifying a supply gas.
Japanese Unexamined Patent Publication (Kokai) No. 6-132038 discloses a technique of humidifying a supply gas, where the exhaust gas discharged from a fuel cell is supplied as a humidifying gas to a fuel cell by using a water vapor-permeable membrane.
Kokai No. 8-273687 discloses a technique of humidifying a supply gas of a fuel cell in a humidifying apparatus using hollow fiber membranes.
The humidifying apparatus for fuel cells is required to have capabilities of stably performing the humidification even when exposed for a long time to a temperature of about 80° C., which is an operating temperature of the fuel cell, or in an atmosphere where water vapor, oxygen, hydrogen and the like are present, efficiently performing the humidification even when a low-pressure gas is used, lowering the pressure loss of gas, and reducing the permeation of components other than water vapor.
For example, Kokai No. 2001-351660 proposes to provide auxiliary humidifying means comprising a condenser and a water injection valve, in addition to a hollow fiber water-permeable humidifying apparatus. This is proposed to solve the problem that scale-up made to increase the humidification amount leads to increase of the pressure loss even in a low-pressure operation and therefore, use of a hollow fiber water-permeable membrane-type humidifying apparatus is limited.
This indicates that, the humidifying apparatus for fuel cells using a hollow fiber membrane is still in need of improvements.
U.S. Pat. No. 6,464,755, by several inventors including the present inventors, discloses asymmetric hollow fiber gas separation membranes having a high gas permeation rate and a practical level of mechanical strength and being excellent in both water resistance and hot water resistance, and refers to the possibility that the asymmetric hollow fiber gas separation membranes can be used also for humidifying a supply gas of a fuel cell. However, in U.S. Pat. No. 6,464,755, the parameters useful in applying the hollow fiber gas separation membranes to a humidifier for fuel cells are not specifically studied and disclosed.
Kokai No. 2002-219339 discloses a humidifier module using hollow fiber membranes, where when the ratio L/A of the shortest length L drawn in the region filled with a hollow fiber membrane bundle to the length A of a diagonal line drawn in that region or the ratio D/L of the height D of a hollow fiber membrane bundle in a region filled with a hollow fiber membrane bundle to the shortest length L drawn in that region is in a specific range and a dry gas can be uniformly spread over an outer surface of each hollow fiber membrane in a cylindrical housing, so that the humidification ability of the humidifier can be enhanced. However, unlike the present invention, Kokai No. 2002-219339 is not an invention disclosing a method for solving the problem regarding the pressure loss of the hollow fiber membranes in the humidifier module.
An object of the present invention is to provide a humidifying apparatus which can stably perform the humidification even when exposed for a long time to a temperature of about 80° C., which is an operating temperature of a fuel cell, or in an atmosphere where water vapor, oxygen, hydrogen and the like are present, can elevate the humidification efficiency while lowering the pressure loss of gas even when a low-pressure gas is used, can reduce the permeation of components other than water vapor, is economical and can be suitably used for a fuel cell.