Abstract:
A power generator has a hydrogen flow path through which moisture is induced to flow to a hydrogen-containing fuel that reacts with the moisture to produce hydrogen. The moisture passes to the hydrogen flow path through a water exchange membrane from a water vapor flow path. A fuel cell between the hydrogen flow path and the water vapor flow path reacts with the hydrogen in the hydrogen flow path to produce electricity, and to also principally produce the moisture in the water vapor flow path.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to electrical power generators. 
     BACKGROUND OF THE INVENTION 
     There is an ongoing but urgent need for lighter weight sources of electricity for many applications, such as battlefield applications, unmanned aircraft, and unattended wireless sensors. Presently, the power source that is primarily used is the lithium battery. However, the energy capability of lithium batteries, for a given weight of the battery, is limited. 
     It is desirable to increase the power of electrical sources by many times, thus making it possible to materially increase the operating life of electronic equipment without adding weight. 
     The assignee of the present invention has previously developed a high performance micropower source  10  shown in  FIG. 1 . The micropower source  10  includes a water reservoir  12 , a water permeable membrane  13 , a solid fuel reservoir  14 , and a pneumatic valve  16 . The water reservoir  12  may be arranged to hold, for example, a small amount of water such as a few cc. The solid fuel reservoir  14  contains, for example, a solid fuel such as a few grams of a metal hydride such as LiAlH4. 
     The water reservoir  12  is connected to the solid fuel reservoir  14  by way of the pneumatic valve  16 . As the water in the water reservoir  12  evaporates and permeates through the water permeable membrane  13 , the resulting water vapor diffuses through the pneumatic valve  16  and reacts with the fuel in the solid fuel reservoir  14  to produce hydrogen gas. The pneumatic valve  16  senses the internal hydrogen gas pressure, and regulates the diffusion of the water vapor to maintain a fixed internal hydrogen gas pressure, usually slightly above atmospheric pressure. 
     One or more fuel cells  18  of the micropower source  10  convert the hydrogen gas to an output voltage. For example, the fuel cells  18  may be miniature proton exchange membrane (PEM) fuel cells each having a 1 mm diameter Nafion membrane, with one side in contact with air, that converts hydrogen to a dc output voltage. 
     One the advantages of using water vapor over liquid water is that water vapor produces essentially a 100% complete reaction with the solid fuel without the caking and clogging typically observed with the use of liquid water. 
     The micropower source  10  can generate up to 1.25 Watt-hours per gram of its weight, compared to 0.25 Watt-hours per gram from typical primary lithium batteries. The stored energy (3.1 Watt-hours) of the micropower source  10  requires only about 2.5 grams of fuel weight (e.g., metal hydride and water), and the additional non-fuel components can add as little as 0.8 grams weight to the micropower source  10 , so that the overall specific energy of the micropower source  10  is 0.95 Watt-Hours per gram, about four times better than a typical primary lithium battery. 
       FIG. 2  shows an exemplary construction of the micropower source  10  configured as a C-cell sized power generator. The micropower source  10  is housed in a cylindrical housing  20  which houses the water reservoir  12  and the solid fuel reservoir  14  such that the solid fuel reservoir  14  forms a core that is surrounded by the water reservoir  12 . The fuel in the solid fuel reservoir  14  may be in the form of solid metal hydride pellets  15 . The pneumatic valve  16  includes a valve disk  22  that cooperates with a valve seat  24  to regulate the amount of water vapor diffusing from the water reservoir  12  to the solid fuel reservoir  14 . The valve disk  22  is coupled to a valve diaphragm  26  by way of a valve stem  28  so that, as the internal hydrogen gas pressure changes, the diaphragm deflects to move the valve disk  22  in relation to the valve seat  24  to regulate the amount of water vapor diffusing from the water reservoir  12  to the solid fuel reservoir  14 . 
     The fuels cells  18  are in contact with air and hydrogen. Porous barriers  21  hold the water in the water reservoir  12  and the solid metal hydride pellets  15  in their proper positions. 
       FIG. 3  summarizes the chemical processes in the micropower source  10 . 
     The present invention relates to a power source which provides higher power levels, specific energies, and/or energy densities. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a power generator comprises a hydrogen flow path, a humid air flow path, a fuel cell, and a water exchange membrane. The hydrogen flow path contains a hydrogen-containing fuel and a flow inducer, the hydrogen-containing fuel reacts with moisture in the hydrogen flow path to produce hydrogen, and the flow inducer induces a flow in the hydrogen flow path. The fuel cell is between the hydrogen flow path and the humid air flow path such that the fuel cell reacts with the hydrogen in the hydrogen flow path to produce electricity and such that moisture in the humid air flow path is produced by the reaction of the fuel cell. The water exchange membrane is between the hydrogen flow path and the humid air flow path and permits the moisture in the water vapor flow path to be supplied to the hydrogen flow path. 
     According to another aspect of the present invention, a method of generating electricity comprises the following: supplying moisture to a hydrogen flow path; inducing a flow in a hydrogen flow path, wherein the flow contains the moisture, and wherein the hydrogen flow path contains a hydrogen-containing fuel that reacts with the moisture to produce hydrogen; and, directing the hydrogen produced by the hydrogen-containing fuel to a fuel cell that reacts with the hydrogen to produce electricity. 
     According to still another aspect of the present invention, a method of generating electricity comprises the following: reacting moisture with a metal hydride in a hydrogen flow path so as to produce hydrogen; directing the hydrogen through the hydrogen flow path to a fuel cell that reacts with the hydrogen to produce electricity and moisture; and, supplying the moisture produced by the fuel cell to the hydrogen flow path as the principal moisture that reacts with the metal hydride. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawing in which: 
         FIG. 1  is a schematic diagram of a micropower source previously developed by the assignee of the present invention; 
         FIG. 2  shows an exemplary construction of the micropower source shown in  FIG. 1 ; 
         FIG. 3  shows the chemical processes for the micropower source shown in  FIG. 1 ; 
         FIG. 4  is a schematic diagram of a closed-cycle micropower source in accordance with one embodiment of the present invention; and, 
         FIG. 5  shows an exemplary construction of the closed-cycle micropower source shown in  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     The micropower source  10  discussed above is extremely simple and efficient for mW level power generation. However, the micropower source  10  employs a slow natural evaporation and diffusion rate of moisture from its water reservoir  12 , that significantly limits power generation. Moreover, the water that is produce by the fuel cells  18  is exhausted as waste, which limits the smallest size ands weight of the fuel required by the micropower source  10 . As can be seen from the chemical reaction sequence of  FIG. 3 , in principle it is not necessary to re-supply water to the micropower source  10  because the natural action of the fuel cells  18  produces water at exactly the rate required by the chemical reaction between the water vapor and the fuel. 
       FIG. 4  shows a closed-cycle micropower source  30  in accordance with one embodiment of the present invention. The closed-cycle micropower source  30  includes a hydrogen flow path  32  and a humid air flow path  34 . A flow inducer  36  such as a fan, pump, impeller, etc. induces a flow through the hydrogen flow path  32 . Similarly, an inducer such as a fan, pump, impeller, etc. (not shown in  FIG. 4 ) induces a flow through the humid air flow path  34 . Atomized water droplets may be initially injected (not shown) into either the hydrogen flow path  32  or the humid air flow path  34  in order to produce the initial humidification of hydrogen in the hydrogen flow path  32 . 
     The flow of humidified hydrogen in the hydrogen flow path  32  passes through a fuel reservoir  38  which contains, for example, a solid fuel such as a metal hydride. Although many metal hydrides can be used in the fuel reservoir  38 , LiBH4 and LiAlH4 contain a good amount of hydrogen energy per unit weight. For example, LiBH4 contains about 1.7 times as much energy as LiAlH4, and thus could potentially give 36% hydrogen by initial weight. At depletion, the fuel will have gained a factor of 3.9 times in weight due to accumulated oxygen. However, the chemical reaction between LiBH4 and water vapor produces an alkaline “slurry” which reduces the reaction rate resulting in a chemical reaction that typically only proceeds to 30% of completeness. LiAlH4 does not show this problem, and gives close to 100% completeness. If methods (e.g., added acids or catalysts) can eliminate this effect in LiBH4, then LiBH4 may be a better fuel than LiAlH4. 
     The fuel reservoir  38  may contain the required amount, such as a few grams, of the metal hydride that reacts with water vapor in the hydrogen flow to generate dry hydrogen. The dry hydrogen passes through a heat exchanger  40  in the hydrogen flow path  32  to maintain ambient temperature. That is, the exothermic heat generated by the reaction of the water vapor and the metal hydride in the fuel reservoir  38  (about 15 Watts) can be passively passed by the heat exchanger  40  to ambient air. 
     The dry hydrogen in the hydrogen flow path  32  passes by a water exchange membrane  42 , such as a polymer permeation-exchange barrier. The large relative humidity difference between the moist flow in the humid air flow path  34  and the dry hydrogen in the hydrogen flow path  32  induces water vapor permeation across the water exchange membrane  42  into the dry hydrogen gas without a latent heat change. Breathable polymers (e.g., Goretex, PEBA, PVOH, Nafion, etc.) have very high permeability to water vapor and can be used for the water exchange membrane  42 . Thus, water vapor from the humid air flow path  34  passes to the hydrogen flow path  32  in order to humidify the hydrogen in the hydrogen flow path  32 . 
     The humidified hydrogen then flows past the membrane electrode assembly (MEA) of one or more fuel cells  44 , such as Nafion fuel cells, where part of the hydrogen (about 2 cc/sec) is consumed at the anode of the membrane electrode assembly of the fuel cells  44  so as to generate electrical power and evolve water at the cathode of the membrane electrode assembly. Accordingly, replacement hydrogen is generated by the fuel in the fuel reservoir  38 , matching the hydrogen consumption rate of the fuel cells  44 . 
     The flow of air or other carrier in the humid air flow path  34  counter-flows past the cathode of the fuel cells  44  and is humidified by the water evolved by the fuel cells  44 . This water vapor is extracted downstream back to the hydrogen by the water exchange membrane  42 . If the water cycle is completely efficient, the system naturally stays in balance as shown in  FIG. 3 , with water and hydrogen generated exactly as needed, according to the electrical power withdrawn. If the water cycle is not completely efficient, a small reservoir can be provided in the humid air flow path  34  or the hydrogen flow path  32  so as to inject water that is otherwise lost in the closed cycle micropower source  30 . Thus, in practice, a small water injection system (not shown) may be necessary for start-up and replacement of lost water. 
       FIG. 5  shows an exemplary construction of the closed-cycle micropower source  30  of  FIG. 4 . Hydrogen circulates continuously in the hydrogen flow path  32  into a lower manifold  46  that distributes the hydrogen flow upwards through an array of vertical tubes  48  in a central manifold  49 . The vertical tubes  48  are surrounded by downwardly flowing humid air in the humid air flow path  34 . The hydrogen in the hydrogen flow path  32  flows upwards through the vertical tubes  48  and is collected from the vertical tubes  48  by an upper manifold  50  under inducement from the flow inducer  36 . The upper portions of the vertical tubes  48  are coated with fuel cell electrode material and the lower portions of the vertical tubes  48  are coated with the water exchange membrane  42 . The vertical tubes  48  in the central manifold  49  contain the fuel such as a solid fuel in the form of metal hydride fuel rods. The vertical tubes  48 , for example, may be Nafion polymer tubes. 
     As discussed above, hydrogen is created in hydrogen flow path  32  by the reaction of water vapor, from the humid air flow path  34  through the water exchange membrane  42  coating the lower portions of the vertical tubes  48 , with the metal hydride fuel, thus drying the hydrogen gas. Water vapor released at the fuel cell cathodes of the vertical tubes  48  is carried in a counter flow opposite to the flow of hydrogen by the humid air flow path  34  and permeates through the water exchange membrane  42  to the dry hydrogen gas, completing the water cycle. The heat exchanger  40  is not shown in  FIG. 5  but could be included in the hydrogen flow path  32  to maintain ambient temperature as discussed above. The counter flow in the humid air flow path  34  may be created by a flow inducer  52  such as a fan, pump, impeller, etc. Alternatively, counter flow in the humid air flow path  34  may be created by using aerodynamic pressure in moving vehicle applications. 
     The vertical tubes  48  and the lower and upper manifolds  46  and  50  are contained in a cylindrical housing  54 . The cylindrical housing  54  may be constructed of gas impermeable, metallized thin-walled plastic tubing for light weight. 
     Accordingly, all or the majority of the required water is provided by a closed cycle, so that very little water is required to be loaded, with a major savings in weight of water initially required and resulting higher specific energy. Instead of relying on the natural evaporation rate of water (about 1 e-7 cc/sec), an inducer, such as a micro water pump, may be used to initially inject water at the rate required for high electrical power (1.5 e-3 cc/sec of water is required to generate sufficient hydrogen for 10 Watts system output power). Suitable pumps, such as the MDP1304 micropump are commercially available, for example, from thinXXS Microtechnology. An ultrasonic atomizer can be used to initially mechanically inject micro-droplets of liquid water into the forced flow hydrogen gas stream, which initially produces sufficient humidified hydrogen for system startup. Such ultrasonic devices are used in household humidifiers. The flow rate of hydrogen gas required to transport 1.5 e-3 cc of water per second in vapor form at 100% RH at 5 C is about 200 cc/sec. Such gas flows can be produced by miniature low-power commercial fans (such as Sunon Inc #5F852, 0.25 W, 1.8 grams). 
     Since the water cycle will probably not be completely efficient, some additional water may be injected into the hydrogen stream from a small water reservoir. This injection rate may be used to control the hydrogen generation rate. 
     An appropriate flow rate of hydrogen gas is about 200 cc/sec. This flow rate ensures that the required amount of water can be transported as humid hydrogen gas. The 3.7 Watts of latent heat required for water evaporation is free, in the sense that it is supplied by heat conduction from the system&#39;s surroundings, and places no load on the electrical system. 
     The humid air flow path  34  also provides air to the cathode side of the fuel cells  44 . The air flow rate required is usually about twice the stochiometric rate (about 6 cc/sec) to avoid oxygen depletion at the cathode surface. 
     At 10 W output electrical power and with a typical cell voltage of 0.65 V, the fuel cells  44  are expected to be 50% efficient, and dissipate an additional 10 W as heat. In the event that Nafion is used for the fuel cells  44 , this dissipated heat should be helpful in keeping the Nafion warm, and producing water vapor at the cathode. 
     A small amount of a barrier material such as oil may be added to the metal hydride in order to slow its otherwise potentially explosive reaction with water, while still allowing an adequate reaction rate for controlled hydrogen generation. Liquid water can be kept from freezing by adding a small amount of alcohol. Alcohol reacts with LiAlH4 to also produce hydrogen. 
     Certain modifications of the present invention have been discussed above. Other modifications will occur to those practicing in the art of the present invention. For example, as described above, the heat exchanger  40  is used in the hydrogen flow path  32  to maintain ambient temperature. However, the heat exchanger  40  need not be used. 
     Also, as described above, the water exchange membrane  42  is used to pass water vapor from the moist air flowing through the humid air flow path  34  to the dry hydrogen flowing through the hydrogen flow path  32 . Alternatively, water vapor carried off in the humid air exiting the fuel cells  44  may be condensed by ducting this moist air past the cooler hydrogen gas, in effect using the latent heat of the cooling power. For example, condensed liquid water may be reintroduced into the hydrogen stream, such as by using a micropump and atomizer as described above for initial water injection. 
     Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.