Abstract:
An external gas humidifier for fuel cell transfers recycled high-temperature waste heat produced by the fuel cell to a reactant gas, such as hydrogen or air, which is guided into the external gas humidifier. The heated reactant gas is treated in the external gas humidifier to effectively increase the moisture content of the reactant gas within a shortened time. When the humidified reactant gas enters into the fuel cell, it enables a polymeric membrane in the fuel cell to be well humidified to thereby enhance the power generation efficiency and service life of the fuel cell.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a gas humidifier for a proton exchange membrane fuel cell, and more particularly to an external gas humidifier for fuel cell that utilizes recycled high-temperature waste heat produced by the fuel cell to heat a reactant gas and humidifies the heated reactant gas for the same to have an increased moisture content within a shortened time. 
   BACKGROUND OF THE INVENTION 
   A proton exchange membrane fuel cell (PEMFC) is a device which directly uses hydrogen (H 2 ) and oxygen (O 2 ) in an electrochemical reaction to generate electricity. The PEMFC has low operating temperature, short turn-on time, high energy density, low pollution, and wide applications, and is therefore a technique widely researched and promoted all over the world. 
   A typical PEMFC mainly consists of a proton exchange membrane (PEM), a catalyst layer, a gas diffusion layer (GDL), and a bipolar plate. The proton exchange membrane (PEM) is a solid-state polymeric membrane, such as Nafion® membranes from DuPont®, Aciplex® membranes from Asashi Chemical, BAM® (Ballard Advanced Material) membranes from Ballard, and Gore-Select® membranes from Gore. The PEM is used in a PEMFC mainly to isolate the reactant gas at the cathode from that at the anode, and to isolate electrons. The PEM conducts only water molecules (H 2 O) and hydrogen ions (H + ). Therefore, this type of polymeric membrane is a gas impermeable membrane, which conducts hydrogen ions (H + ) but not electrons (e − ). When the hydrogen ions (H + ) are conducted via this type of polymeric membrane, they must be carried by water molecules. Therefore, the higher the moisture content of the polymeric membrane is, the better the hydrogen ions (H + ) conducting is. Thus, it is important to increase the moisture content of the polymeric membrane to obtain better hydrogen ions (H + ) conducting, and accordingly, to maintain the PEMFC in good performance. 
   The methods and designs for humidifying a reactant gas for the fuel cell may be generally divided into two types, namely, internal and external humidification, that have their respective advantages and drawbacks. Regarding the external humidification, an external humidifier has to be provided outside the fuel cell. The external humidifier disadvantageously occupies additional space and requires additional power supply to a heater provided therein for increasing the temperature and humidity of the reactant gas. However, the external humidifier also has many advantages, such as providing stable humidifying amount, capable of controlling and regulating humidifying amount, capable of handling a relatively large amount of gas humidifying, and easy to maintain and repair. Regarding the internal humidification, humidifying mechanisms are internally added to a fuel cell. The internal humidifying mechanisms have the advantages of having small volume without occupying too much space, omitting additional humidifier and heater to save the cost therefor, and directly utilizing recycled waste heat or water produced by the fuel cell itself. However, the internal humidifying mechanisms also have some drawbacks, such as involving complicate pipeline design and complicate connection to the fuel cell, uneasy to control and regulate the humidifying amount, uneasy to get saturated humidified gas when the load is high, and uneasy to maintain and repair. 
   In recent years, many reactant gas humidifying designs for fuel cell systems have been made or improved. U.S. Pat. Nos. 5,482,680 and 5,527,363 disclose a fuel cell stack having a humidifying section. This type of fuel cell stack has disadvantageously largely increased volume and weight, and the flow field design for the fuel, oxidizer, and water in the fuel cell stack is very complicate. U.S. Pat. No. 6,406,807 teaches the forming of water spray holes on ribs or lands between the gas passages on a carbon plate, so as to humidify the PEM directly. U.S. Pat. No. 6,403,249 discloses a battery with a membrane type humidifying section to directly humidify a reactant gas. U.S. Pat. No. 6,207,312 discloses a self-humidifying design, in which an interdigitated flow field and a membrane type humidifying section provided on a carbon plate are adopted. U.S. Pat. No. 6,066,408 teaches the addition of a wick in the gar flow field, and the use of water adding holes to supplement the water content of the wick. U.S. Pat. No. 5,998,054 discloses a humidifying design in which water is sprayed at a front end of every gas flow field on the carbon plate. U.S. Pat. No. 5,952,119 discloses a gas diffusion layer formed from a carbon fabric, on which hydrophilic fine threads are sewed at regular intervals, so that supplemented water is distributed over the PEM via the hydrophilic fine threads. And, U.S. Pat. No. 5,965,288 teaches an external water-permeable membrane humidifier to humidify a reactant gas. 
   SUMMARY OF THE INVENTION 
   A primary object of the present invention is to provide an external gas humidifier for fuel cell capable of increasing the moisture content of a reactant gas within a shortened time, and thereby enables a polymeric membrane in the fuel cell to be well humidified to enhance the power generation efficiency and service life of a PEMFC. 
   To achieve the above and other objects, the external gas humidifier for fuel cell according to a preferred embodiment of the present invention includes a barrel, a preheating serpentine, and a gas disperser. The gas disperser isolates a humidifying liquid in the barrel from a gas chamber formed in the barrel below the gas disperser. The preheating serpentine has a coiled section immersed in the humidifying liquid in the barrel, and an outlet located in the gas chamber. When a reactant gas is guided into the preheating serpentine, the reactant gas and the humidifying liquid exchange heat at the coiled section of the preheating serpentine. The heated reactant gas enters into the gas chamber via the outlet of the preheating serpentine, and leaves the gas chamber via the gas disperser in the form of tiny bubbles, which enter into the humidifying liquid above the gas disperser to increase the temperature and humidity of the reactant gas. Meanwhile, high-temperature waste heat produced by the fuel cell in the electrochemical reaction is recycled, and the waste heat is transferred via the humidifying liquid in the barrel to the reactant gas to reduce power consumption needed to heat the humidifying liquid. The large amount of tiny bubbles of the reactant gas entering into the humidifying liquid largely increase a contact area between the reactant gas and the humidifying liquid, and the retention time of the reactant gas in the humidifying liquid, and accordingly enhance the effect of humidifying the reactant gas. 
   On the other hand, at least one gas baffle plate is provided in the barrel in front of a gas outlet to reduce a dew point of the reactant gas, so that super-saturated water and gas molecules are collected and condensed to prevent flooding in the fuel cell caused by such super-saturated moisture and gas. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings, wherein 
       FIG. 1  is a schematic view of an external gas humidifier for fuel cell according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Please refer to  FIG. 1  that is a schematic view of an external gas humidifier for fuel cell  1  according to a preferred embodiment of the present invention. For the purpose of simplicity, the external gas humidifier for fuel cell  1  is also briefly referred to as the gas humidifier  1  throughout the specification, the abstract, and the appended claims. The gas humidifier  1  of the present invention is mainly used to heat and humidify a reactant gas for a fuel cell (not shown). The reactant gas may be hydrogen (H 2 ) or air. A fuel cell mainly consists of a proton exchange membrane (PEM), a catalyst layer, a gas diffusion layer (GDL), and a bipolar plate. When a proton exchange membrane fuel cell (PEMFC) operates, an oxidation reaction of hydrogen (H 2 ) occurs at the anode while a reduction reaction of oxygen (O 2 ) occurs at the cathode. First, the reactant gas is catalyzed by a catalyst, so that hydrogen is decomposed into hydrogen ions (H + ) and electrons (e − ), as indicated by the following chemical formula: H 2 →2H + +2e − . Electrons (e − ) flow from the anode to a circuit outside the cell to work against a load before flow to the cathode. Meanwhile, hydrogen ions (H + ) pass through the proton exchange membrane to move from the anode to the cathode, and combine with oxygen molecules (O 2 ) and electrons (e − ) to produce water (H 2 O) and heat, as indicated by the following chemical formula: ½O 2 +2H + +2e − →H 2 O +heat. When the hydrogen ions (H + ) are produced at the anode, a potential drop exists in the cell to cause the hydrogen ions to continuously move toward the cathode due to ion conducting. When the hydrogen ions (H + ) move due to ion conducting, they must be accompanied by several water molecules. That is, the hydrogen ions move in the form of hydrated ion, as indicated by the following chemical formula: H + (H 2 O) n . Therefore, water molecules would continuously move toward the cathode when the fuel cell operates. At this point, water must be properly supplemented to avoid an overly dried PEM, which would reduce the ability of hydrogen ions conducting and result in largely reduced power generating performance of the PEMFC. 
   As shown in  FIG. 1 , the external gas humidifier  1  includes a barrel  10 , a preheating serpentine  20 , a gas disperser  30 , a heat exchanger  40 , and a set of gas baffle plates  50 . The barrel  10  is a vertical cylindrical water tank defining an inner receiving space  11  for storing a humidifying liquid  12  therein. The receiving space  11  maybe externally provided with a heat-insulating layer to prevent leakage of heat of the humidifying liquid  12  from the barrel  10 . A liquid level controller  70  and thermocouple thermometers  61 ,  62  may be connected to the barrel  10  at predetermined positions. The liquid level controller  70  is mainly used to control and regulate a volume of the humidifying liquid  12  in the barrel  10 , so that a level of the humidifying liquid  12  in the barrel  10  is always maintained at a predetermined height. Basically, the liquid level is preferably higher enough to immerse a coiled section  23  of the preheating serpentine  20 . The thermocouple thermometers  61 ,  62  are mainly used to control and indicate a temperature of the humidifying liquid  12 . The barrel  10  is provided at a top with a liquid water inlet  111  and a gas outlet  112 . The humidifying liquid  12  is supplemented via the liquid water inlet  111 . 
   The preheating serpentine  20  includes an inlet  21 , an outlet  22 , and a middle coiled section  23  between the inlet  21  and the outlet  22 . With the continuously wound coiled section  23  immersed in the humidifying liquid  12 , the preheating serpentine  20  has an increased contact surface with the humidifying liquid  12  to obtain an enhanced heat exchange effect. 
   The gas disperser  30  is located in the barrel  10  near a bottom thereof, so that a gas chamber  31  is formed in the barrel  10  below the gas disperser  30 , and the humidifying liquid  12  in the receiving space  11  is isolated from the gas chamber  31 . The outlet  22  of the preheating serpentine  20  is located in the gas chamber  31 . The gas disperser  30  is of a porous plate structure having a very small pore size less than about 0.1 mm. The porous plate structure may be a foamed metal, a porous metal oxide, a porous carbon material, etc. 
   The set of gas baffle plates  50  includes at least one baffle plate, which has a plurality of fins  51  provided thereon and is downward inclined at a predetermined angle. 
   When the reactant gas, which is hydrogen (H 2 ) or air, is guided into the external gas humidifier  1  via the inlet  21  of the preheating serpentine  20  to flow through the coiled section  23 , which is completely immersed in the humidifying liquid  12  filled in the inner receiving space  11  of the barrel  10 , heat exchange occurs at the coiled section  23  between the reactant gas and the humidifying liquid  12 , so that the reactant gas has a raised temperature. High-temperature waste heat produced in the reaction of the fuel cell may be recycled for use as a heat source for heating the humidifying liquid  12 . The waste heat is guided into the heat exchanger  40  via a waste heat inlet  41 , and then flows through a heat exchange tube  43  of the heat exchanger  40  to exchange heat with the humidifying liquid  12  and thereby increase the temperature of the humidifying liquid  12 . The waste heat finally exits the heat exchanger  40  via a waste heat outlet  42  thereof. The heat exchange tube  43  may be a flat tube, a serpentine tube, or any other suitable configurations. 
   Thereafter, the reactant gas flows through the coiled section  23  and enters the gas chamber  31  via the outlet  22  of the preheating serpentine  20 . Since the gas disperser  30  is made of a porous material, when the reactant gas passes through the gas disperser  30 , a large amount of tiny bubbles are produced to enter into the humidifying liquid  12 . These tiny bubbles largely increase the contact surface of the reactant gas with the humidifying liquid  12 , and the retention time of the reactant gas in the humidifying liquid  12 . In this manner, the reactant gas is fully and effectively humidified, making it easy for the reactant gas to become fully saturated. Before the reactant gas fully humidified with water leaves the barrel  10  via the gas outlet  112 , it would first be intercepted by the set of gas baffle plates  50 . The fins  51  provided on the surfaces of the gas baffle plates  50  function to reduce a dew point of the reactant gas, so that water molecules in the reactant gas are collected and condensed, preventing any super-saturated reactant gas from entering into the fuel cell. It is known that a super-saturated gas tends to condense into liquid water and would cause flooding in the fuel cell to largely reduce the power generating efficiency of the fuel cell. With the above arrangements, the reactant gas may be effectively humidified and heated within a shortened time before entering into the fuel cell. 
   The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications in the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.