Patent Publication Number: US-2020295386-A1

Title: Water exchanger for a fuel cell based power generator

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a water exchanger for a fuel cell based power generator, and a method of processing the same. 
     BACKGROUND 
     The run time of unmanned air systems (UAS), sometimes referred to as drones, is limited by their power sources. State of the art UAS use light-weight lithium ion/polymer batteries with specific energies that range from ˜200-300 Wh/kg, enabling flight times on the order of 20-60 minutes. For emerging applications including infrastructure inspection (e.g. roads, bridges, power lines, rail, pipelines, etc.) and package delivery, it may be desired to have greater flight times on battery charge, among other suitable applications. In some instances, greater than six-hour flight times are desired in order for such UAS to be commercially viable. 
     Existing obstacles include efficient energy storage and utilization. Proton exchange membrane (PEM) fuel cells for man-portable power and micro air vehicles require light-weight, small-size, and high-rate hydrogen sources. Commercially available hydrogen sources such as metal hydrides, compressed hydrogen in cylinders, or catalytic waterborohydride hydrogen generators are capable of high rate hydrogen generation, but are heavy and bulky. Further, state of the art gas-to-gas water exchangers (e.g., water exchangers used to exchange water between flowing gas streams without mixing) are too heavy, and have pressure drops that are too high, for use in airborne applications such as UAS. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a fuel cell based power generator according to an embodiment of the present disclosure. 
         FIGS. 2A and 2B  illustrate various perspective views of a water exchanger for a fuel cell based power generator according to an embodiment of the present disclosure. 
         FIG. 3  illustrates an angled cross-sectional view of a hollow tubular structure of a water exchanger according to example embodiments. 
         FIGS. 4A and 4B  illustrate process steps associated with forming a water exchanger for a fuel cell based power generator according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A water exchanger for a fuel cell based power generator is described herein. For example, an embodiment includes a plurality of hollow tubular structures, wherein each respective hollow tubular structure comprises a membrane that is selectively permeable to water vapor over other gases (e.g. air, hydrogen, oxygen, nitrogen). The selective membrane of the humidifier may be made from a broad variety of compositions, including (but not limited to) commercially available membranes based on: perfluorosulfonic acid (e.g. Nation®, Aquivion®, Gore-Select®, etc.); polybenzimidazole (e.g. Fumapem® AM); sulfonated polysulfone (e.g. Fumapem® SX); sulfonated polyarylene (e.g. Asahi CMV); poly(styrene-co-butadiene) (e.g. Asahi AMV); poly(styrene-co-divinylbenzene) (e.g. NEOSEPTA); silicone (e.g. PermSelect®). The membranes may also be made from a variety of proprietary or experimental chemistries under development for application as cation and anion exchange membranes. 
     A water exchanger in accordance with embodiments of the present disclosure is a light-weight, low pressure-drop water exchanger for a fuel cell based power system that is suitable for use in unmanned air systems (UAS) such as drones. For example, fuel cell based power systems incorporating a light-weight, low-pressure drop water exchanger in accordance with embodiments of the present disclosure can provide the increased UAS endurance (e.g., flight time) needed for emerging applications including infrastructure inspection (e.g. roads, bridges, power lines, rail, pipelines, etc.) and package delivery that is not available with state of the art UAS that use lithium ion/polymer batteries and provide flight times on the order of 20-60 minutes. 
     For example, a water exchanger in accordance with the present disclosure can have a reduced (e.g., lighter) weight as compared to state of the art gas-to-gas water exchangers, which can allow the fuel cell of the UAS to carry more fuel and thereby enhance the endurance of the UAS. Further, a water exchanger in accordance with the present disclosure can have a reduced (e.g., lower) pressure drop as compared to state of the art gas-to-gas water exchangers, which can allow for the use of light-weight blowers with reduced parasitic power consumption, which can further enhance the endurance of the UAS. 
     In the following description, reference is made to the accompanying drawings that form a part hereof. The drawings show by way of illustration specific embodiments which may be practiced. 
     These embodiments are described in sufficient detail to enable those of ordinary skill in the art to practice one or more embodiments of this disclosure. It is to be understood that other embodiments may be utilized, and that mechanical, electrical, and/or process changes may be made without departing from the scope of the present disclosure. 
     As will be appreciated, elements shown in the various embodiments herein can be added, exchanged, combined, and/or eliminated so as to provide a number of additional embodiments of the present disclosure. The proportion and the relative scale of the elements provided in the figures are intended to illustrate the embodiments of the present disclosure, and should not be taken in a limiting sense. 
     The figures herein follow a numbering convention in which the first digit or digits correspond to the drawing Fig. number and the remaining digits identify an element or component in the drawing 
     As used herein, “a” or “a number of” something can refer to one or more such things, while “a plurality of” something can refer to more than one such thing. For example, “a number of devices” can refer to one or more devices, while “a plurality of devices” can refer to more than one device. 
       FIG. 1  is a schematic diagram of a fuel cell based power generator  100  in accordance with an embodiment of the present disclosure. In the embodiment shown in  FIG. 1 , power generator  100  includes a fuel cell  110  and a hydrogen generator  115 . 
     As used herein, the term “fuel cell” can, for example, refer to an electrochemical cell that converts chemical energy from a fuel into electricity through an electrochemical reaction. For example, hydrogen can be provided to fuel cell  110  such that hydrogen is consumed in an electrochemical reaction to produce electricity, as is further described herein. An ambient air path  120  is configured to run ambient air past a cathode side of the fuel cell  110 , via ambient air path portion  122 . The ambient air path  120  is part of a cathode loop, which includes all the paths that ambient air circulates through, including interiors of components the ambient air passes through. 
     A reaction in the fuel cell  110  generates electrical power and adds water as a by-product to the ambient air path portion  122 . This water is then provided to the hydrogen generator  115 , which contains one or more fuels that release hydrogen responsive to exposure to water, which may be in vapor form. As used herein, the term “hydrogen generator” refers to a device which contains one or more fuels that release hydrogen responsive to exposure to water, which may be in the form of humidity. 
     The hydrogen generator  115  provides the released hydrogen to a recirculating hydrogen path  125 , which splits into two parts at junction  127 . The two parts include a primary path  126  and a secondary path  128 . The primary path  126  recirculates released hydrogen back to hydrogen generator  115 . The secondary path  128  runs past the anode side of the fuel cell  110  to provide the hydrogen to the fuel cell  110 . The secondary path  128  is part of an anode loop, which includes all the paths that hydrogen recirculates through, including interiors of components the ambient air passes through. 
     Hydrogen from the recirculating hydrogen path  125  reacts with oxygen from the ambient air path  120  in fuel cell  110 , producing electrical power, water vapor, and heat as reaction byproducts. The byproducts on the cathode side of the fuel cell  110  are removed from the fuel cell by the air flowing within ambient airflow path  120 . Leftover hydrogen and any inert gases that leak/permeate into the anode loop over time continue through the recirculating hydrogen path  125 . 
     In some embodiments, a cooling mechanism  132 , such as a fan or liquid cooling loop, can be used with the fuel cell portion system to assist in the removal of heat. In such an embodiment, most of the heat generated in the fuel cell is removed via this liquid cooling loop and rejected to ambient via a heat exchanger and/or fan, represented in block form as part of the cooling mechanism  132 . 
     In some embodiments, as shown in  FIG. 1 , the secondary path  128  can include a purge valve  129  that purges inert gases (e.g. nitrogen, water vapor) that build up over time in the anode loop into an ambient airflow path portion  123  of the ambient airflow path  120 . These gases are purged periodically by actuating the purge valve  129 , for example, based on predetermined timing or a sensed parameter like fuel cell voltage or hydrogen concentration. In some embodiments, the valve may be slightly open most of the time to continuously remove the inert gases, with most of the hydrogen flowing to and being consumed by the anode of the fuel cell. 
     In some embodiments, the fuel cells provide current to a controller  135  that charges a Li-ion battery or batteries  130 . The controller  135  also provides power to a load, such as the UAS. In some implementations, the batteries can provide the ability to supply higher and more dynamic levels of power than simply utilizing the fuel cells directly, which can be slower to respond and not normally be able to provide high levels of power that may be required for operation of the UAS in a desired manner, such as accelerating sufficiently while carrying a load. 
     Controller  135  may comprise a microprocessor, circuitry, and other electronics to receive data representative of sensed pressure, temperature, and other parameters and utilize control algorithms, such as proportional/integral/derivative (PID) or other type of algorithms to control mechanisms to modify the parameters to meet one or more different setpoints. Controller  135  may also be referred to as a power management module or controller  135 . In some embodiments, control may be based on proportional controller. 
     In some embodiments, the fuel cell based power generator  100  has a system configuration (implemented in a X590 form factor battery package in one embodiment) and its operating principle is schematically depicted in  FIG. 1 . Hydrogen generator  115 , in various embodiments, is a replaceable and disposable “fuel-cartridge” unit that generates H 2  for a H 2 /oxygen proton exchange membrane (PEM) fuel cell  110 , and a permanent unit that, in some embodiments, includes PEM fuel cell  110 , Li-ion recharge battery  130  as an output stage to interface with an external load, and the controller  135  that controls electronic and fluidic control circuits (e.g., controlling one or more fluid movement apparatuses) to dynamically sense and optimize the power generator  100  under varying load and environmental conditions. 
     Ambient air serves as the fuel cell power generator  100  oxygen source, carrier gas for water vapor, and coolant gas for the fuel cell stack and H 2  generator. A first fluid movement apparatus (e.g., a fan)  140  draws in fresh air from ambient via an inlet  142  and circulates it over the cathode side of the fuel cell stack at  121  via the ambient air path or passage  120 . 
     Since the fuel cell  110  reaction is exothermic, the temperature of the fuel cell  110  increases and may be measured by a first temperature sensor  143  associated with fuel cell  110 , which is positioned to measure the temperature of the fuel cell  110 . The temperature sensor is shown in block form and may be placed anywhere such that it is thermally coupled to the fuel cell  110  to provide a reliable measurement of the temperature of the fuel cell  110 . Sensor  143  may comprise multiple temperature sensors. In one embodiment, one of the temperature sensors is coupled to provide data representative of the temperature proximate the anode, and another coupled to provide data representative of the temperature proximate the cathode of the fuel cell  110 . The temperature data is provided to the controller  135  for use in controlling to one or more setpoints. A fuel cell set point temperature of the fuel cell  110  is indicated as 60° C., which has been found to be a temperature at which the fuel cell  110  functions most efficiently. 
     In further embodiments, the set point may vary between 40° C. and 80° C., and may vary further depending on the configuration and specific materials utilized in fuel cell  110  and system  100 . Different optimal set points for the fuel cell may be determined experimentally for different fuel cells and may be found to be outside the range specified above. 
     The fuel cell temperature is modified via cooling mechanism  132  (e.g., liquid cooling loop with liquid pump, heat exchanger, and fan) under control of controller  135  that receives temperature information from first temperature sensor  143 . The first temperature sensor  143  may include separate temperature sensors to sense temperatures of both the anode side and cathode side of the fuel cell  110 . 
     In some embodiments, the fuel cell temperature and hydrogen generator temperature can be controlled separately. Separately controllable fans and or fluid pumps may be used for such independent control. The power management module may control various pressures and temperatures via the various mechanisms using one or more of PID control, proportional control, or other type of algorithm. Temperatures may be controlled within desired temperature ranges defined by upper and lower temperature thresholds. 
     While the fuel cell  110  is producing electrical power as well as heat, ambient air flowing within path  120  delivers oxygen to the fuel cell  110  cathode and removes water vapor generated by the reaction in the fuel cell  110 . The hot, humid air continues down path  120  to a first water exchanger  155 . The water exchanger  155  extracts water from the hot, humid ambient air and passes the extracted water into the hydrogen flow path  124  (anode loop). The hot, somewhat drier air continues down path  122 ′ to a second water exchanger  157 , where heat and water is passed into the cathode loop. This heat and water raise the temperature and humidity of the incoming ambient air, which improves fuel cell performance. After exiting the second water exchanger, the warm dry air is exhausted to the ambient at  160 . 
     Water exchanger  155 , and the operation of water exchanger  155 , is further described herein. For instance, water exchanger  155  can be a light-weight, low pressure-drop water exchanger, as will be further described herein. 
     The extracted water from the ambient air path is then provided to the recirculating hydrogen path to create humid hydrogen (H 2 ) at  124 . This humid H 2  then flows to the hydrogen generator where water therein interacts with the fuel to generate additional hydrogen. 
     The hydrogen generator  115  also has a set point temperature at which it operates most efficiently. The temperature may be measured by sensing the temperature of the hydrogen as it exits the hydrogen generator  115  as represented by the position of a sensor  133 , which may be a temperature sensor and also may include a pressure sensor. The hydrogen generator experiences an exothermic reaction and has an optimal operating set point is shown as 80° C., but may vary from 60° C.-100° C. or outside the range depending on the composition of the hydrogen generator used. 
     The hydrogen generator temperature may be controlled by varying the speed of one or more cooling mechanisms  131  positioned to remove heat from the hydrogen generator. The cooling mechanism may be positioned on the outside of the hydrogen generator or positioned proximate the hydrogen generator to effect cooling of the hydrogen generator. For example, in some embodiments, the hydrogen generator temperature is modified by an external cooling mechanism (e.g., a fan, blower, etc.) positioned, for instance, on the surface of the generator. The cooling mechanism may be controlled via the controller  135  using RD or other control algorithms, such as proportional control. The hydrogen generator could also be cooled using a liquid cooling loop and associated liquid pump, heat exchanger, and fan. 
     The humid hydrogen  124  flows into the hydrogen generator  115 , where the water reacts with the fuel and generates hydrogen. The now dry hydrogen leaves the hydrogen generator and flows into blower  165 , which raises the pressure. 
     The higher pressure dry hydrogen then progresses down the path  125  to a split  127  where some of the dry H 2  enters a primary path  126  and some dry H 2  enters a secondary path  128 . 
     The secondary path  128  is located adjacent the anode side of the fuel cell to provide hydrogen to the fuel cell, while the primary path can be located further away from the fuel cell. This configuration allows for a large amount of hydrogen to recirculate continuously through the system in a hydrogen loop (to efficiently extract the water from the cathode via the ambient air path water exchanger  155 ) while flowing a smaller amount of hydrogen to the fuel cell via secondary path  128 . 
     The secondary path  128  can be a dead end with a purge valve  129  therein that allows inert gasses (e.g., nitrogen, water vapor) to be purged from the anode stream by actuating the valve periodically (e.g., based timing or a sensed parameter such as fuel cell voltage or oxygen concentration). Because some water vapor is included in the inert gas, it is desirable to purge the inert gas into the cathode stream  122  upstream of the primary water exchanger  155 , so that the water vapor can be recovered via water exchangers  155  and  157 . 
     The anode loop pressure as measured by sensor  133  is controlled by varying the blower  165  fan speed, which controls the amount of water recovered from through the water exchanger  155 , and hydrogen generated in the hydrogen generator  115 . Higher blower fan speeds lead to higher anode loop pressures, for example, pressures slightly above ambient pressure by 1-10 psig. 
     Steady state operation of the fuel cell based power generator can be achieved by: 
     1) Controlling cathode blower speed based on power demand from load; 
     2) Controlling anode blower speed based on anode loop pressure (e.g., measured via pressure sensor  133 ); 
     3) Controlling fuel cell cooling based on fuel cell temperature (e.g., via cooling mechanism  132 ); and/or 
     4) Controlling pump/fan speed control for cooling mechanism  131  (e.g., fan, blower, cooling loop, etc.) associated with the hydrogen generator (e.g., mounted on the outside of the hydrogen generator) based on hydrogen generator temperature. 
     In some embodiments, as air passes by the fuel cell stack  110  from the ambient air path  120  and the secondary path  128  of the recirculating hydrogen path  125 , oxygen and hydrogen are consumed by the fuel cell  110 , and water vapor and waste heat are removed by the ambient air at fuel cell cathode  121 . 
     The power generated in the fuel cell stack may be fed to controller  135  which may include power management circuitry. The circuitry conditions the power and provides it as electricity to a load as indicated by contacts  180 . 
     One or more sensors may measure, in addition to the temperature sensor previously described, humidity, and/or pressure throughout the system  100 . Data provided by the sensors, as well as the electrical load and/or charge state of the charge storage device  130  are used by the control controller  135  to determine and set the various fluid movement apparatus speeds to control the temperature of the elements to corresponding set points. Power management circuitry  135  can include a controller, as is further described herein. 
     Fuel consumption may also be monitored via controller  135  or other power monitoring device, and the remaining capacity may be displayed via a display on the fuel cell power generator packaging as driven by controller  135  in various embodiments. In some embodiments, greater than  95 % fuel utilization may be achieved through an optimized LAH fuel formation (e.g., through one or more of porosity, particle size/distribution, rate enhancing additives, or other formulation characteristics). 
     In some embodiments, the LAH-water reaction generates heat (˜150 kJ/mol LAH, exothermic) leading to a rise in temperature in the fuel. The temperature may be monitored along with controlling the speed of the hydrogen generator cooling fan to maintain the temperature at a desired set point for optimal operation. 
     Electrochemical system power performance can substantially degrade at low temperatures (−40° C.) due to slower reaction kinetics and lower electrolyte conductivity. The hybrid fuel cell may avoid freezing problems by: 1) using water in vapor form, 2) adjusting airflow to prevent water vapor condensation, 3) using heat generated by the fuel cell stack and H 2  generator to regulate their temperatures, 4) Insulating certain system components, and 5) using electrically power heaters to control the temperature of certain system components. In some embodiments, noryl plastic packaging (e.g., consistent with the type used on the Saft BA5590) may be used. Many different types of plastics and/or other materials (e.g., that provide low weight yet sufficient tolerance to the operating parameters and environmental conditions of the generator) may be used. 
     Hydrogen generator  115  in some embodiments is a high-rate hydrogen generator suitable for man-portable power and micro air vehicle applications that provides four to five times the hydrogen of commercially available hydrogen sources of the same size and weight. Many different hydrogen producing fuels, such as LAH may be used. In further embodiments, the hydrogen producing fuel may, for example, include AlH 3 , LiAlH 4 , NaAlH 4 , KAlH 4 , MgAlH 4 , CaH 2 , LiBH 4 , NaBH 4 , LiH, MgH 2 , Li 3 Al 2 , CaAl 2 H 8 , Mg 2 Al 3 , alkali metals, alkaline earth metals, alkali metal silicides, or combinations of one or more thereof. 
     The term “about” as used herein can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range, and includes the exact stated value or range. 
     The term “substantially” as used herein refers to a majority of, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more, or 100%. 
     Hydrogen-Generating Composition for a Fuel Cell 
     In various embodiments, the present disclosure provides a hydrogen-generating composition for a fuel cell. 
     The hydrogen-generating composition reacts with water to generate hydrogen gas. The phase of the water contacted with the hydrogen-generating composition to generate the hydrogen gas can be any suitable phase, such as liquid water (e.g., in a pure state, diluted state, or such as having one or more compounds or solvents dissolved therein) or gaseous water (e.g., water vapor, at any suitable concentration). The generated hydrogen gas can be used as the fuel for a hydrogen-consuming fuel cell. 
     The hydrogen-generating composition can be in any suitable form. The hydrogen-generating composition can, for example, be in the form of a loose powder or a compressed powder. The hydrogen-generating composition can also be in the form of grains or pellets (e.g., a powder or grains compressed into pellets). The hydrogen-generating composition can have any suitable density, such as, for example, about 0.5 g/cm 3  to about 1.5 g/cm 3 , or about 0.5 g/cm 3  or less, or less than, equal to, or greater than about 0.6 g/cm 3 , 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4 g/cm 3 , or about 1.5 g/cm 3  or more. 
     In some embodiments, the hydrogen-generating composition is substantially free of elemental metals. In some embodiments, the hydrogen-generating composition can be substantially free of elemental aluminum. 
     Hydride 
     The hydrogen-generating composition may include one or more hydrides. The one or more hydrides can form any suitable proportion of the hydrogen-generating composition, such as about 50 wt % to about 99.999 wt %, about 70 wt % to about 99.9 wt %, about 70 wt % to about 90 wt %, or about 50 wt % or less, or less than, equal to, or greater than about 52 wt %, 54, 56, 58, 60, 62, 64, 66, 68, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 92, 94, 96, 98, 99, 99.9, 99.99, or about 99.999 wt % or more. 
     The hydride can be any suitable hydride, such that the hydrogen-generating composition can be used as described herein. The hydride can be a compound in which one or more hydrogen centers (e.g., one or more hydrogen atoms, or a group that includes one or more hydrogen atoms) having nucleophilic, reducing, or basic properties. 
     The hydrogen atom in the hydride can be bonded to a more electropositive element or group. For example, the hydrogen can be chosen from an ionic hydride (e.g., a hydrogen atom bound to an electropositive metal, such as an alkali metal or alkaline earth metal), a covalent hydride (e.g., compounds including covalently bonded hydrogen and that react as hydride, such that the hydrogen atom or hydrogen center has nucleophilic properties, reducing properties, basic properties, or a combination thereof), a metallic hydride (e.g., interstitial hydrides that exist within metals or alloys), a transition metal hydride complex (e.g., including compounds that can be classified as covalent hydrides or interstitial hydrides, such as including a single bond between the hydrogen atom and a transition metal), or a combination thereof. 
     The hydride can be chosen from magnesium hydride (MgH 2 ), lithium hydride (LiH), aluminum hydride (AlH 3 ), calcium hydride (CaH 2 ), sodium aluminum hydride (NaAlH 4 ), sodium borohydride (NaBH 4 ), lithium aluminum hydride (LiAlH 4 ), ammonia borane (H 3 NBH 3 ), diborane (B 2 H 6 ), palladium hydride, LaNi 5 H 6 , TiFeH 2 , and a combination thereof. The hydride can be chosen from lithium aluminum hydride (LiAlH 4 ), calcium hydride (CaH 2 ), sodium aluminum hydride (NaAlH 4 ), aluminum hydride (AlH 3 ), and a combination thereof. The hydride can be lithium aluminum hydride (LiAlH 4 ). 
     In some embodiments, the hydrogen-generating composition only includes a single hydride and is substantially free of other hydrides. In some embodiments, the hydrogen-generating composition only includes one or more hydrides chosen from lithium aluminum hydride (LiAlH 4 ), calcium hydride (CaH 2 ), sodium aluminum hydride (NaAlH 4 ), and aluminum hydride (AlH 3 ), and is substantially free of other hydrides. 
     In various embodiments, the hydrogen-generating composition only includes the hydride lithium aluminum hydride (LiAlH 4 ), and is substantially free of other hydrides. In some embodiments, the hydrogen-generating composition can be substantially free of simple hydrides that are a metal atom directly bound to a hydrogen atom. In some embodiments, the hydrogen-generating composition can be substantially free of lithium hydride and beryllium hydride. 
     In various embodiments, the hydrogen-generating composition can be substantially free of hydrides of aluminum (Al), arsenic (As), boron (B), barium (Ba), beryllium (Be), calcium (Ca), cadmium (Cd), cerium (Ce), cesium (Cs), copper (Cu), europium (Eu), iron (Fe), gallium (Ga), gadolinium (Gd), germanium (Ge), hafnium (Hf), mercury (Hg), indium (In), potassium (K), lanthanum (La), lithium (Li), magnesium (Mg), manganese (Mn), sodium (Na), neodymium (Nd), nickel (Ni), lead (Pb), praseodymium (Pr), rubidium (Rb), antimony (Sb), scandium (Sc), selenium (Se), silicon (Si), samarium (Sm), tin (Sn), strontium (Sr), thorium (Th), titanium (Ti), thallium (TI), vanadium (V), tungsten (W), yttrium (Y), ytterbium (Yb), zinc (Zn), zirconium (Zr), hydrides of organic cations including (CH 3 ) methyl groups, or a combination thereof. In some embodiments, the hydrogen-generating composition can be substantially free of one or more of lithium hydride (LiH), sodium hydride (NaH), potassium hydride (KH), magnesium hydride (MgH 2 ), calcium hydride (CaH 2 ), lithium aluminum hydride (LiAlH 4 ), sodium borohydride (NaBH 4 ), lithium borohydride (LiBH 4 ), magnesium borohydride Mg(BH 4 ) 2 , sodium aluminum hydride (NaAlH 4 ), or mixtures thereof. 
     In various embodiments, the hydrogen-generating composition includes a metal hydride (e.g., an interstitial intermetallic hydride). Metal hydrides can reversibly absorb hydrogen into their metal lattice. The metal hydride can be any suitable metal hydride. 
     The metal hydride can, for example, be LaNi 5 , LaNi 4.6 Mn 0.4 , MnNi 3.5 Co 0.7 Al 0.8 , MnNi 4.2 Co 0.2 Mn 0.3 Al 0.3 , TiFe 0.8 Ni 0.2 , CaNi 5 , (V 0.9 Ti 0.1 ) 0.95 Fe 0.05 , (V 0.9 Ti 0.1 ) 0.95 Fe 0.05 , LaNi 4.7 Al 0.3 , LaNi 5−x Al x  wherein x is about 0 to about 1, or any combination thereof. The metal hydride can be LaNi 5−x Al x  wherein x is about 0 to about 1 (e.g., from LaNi 5  to LaNi 4 Al). The metal hydride can form any suitable proportion of the hydrogen-generating composition, such as about 10 wt % to about 99.999 wt %, or about 20 wt % to about 99.5 wt %, or about 10 wt % or less, or less than, equal to, or greater than about 15 wt %, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 99.5, 99.9, 99.99, or about 99.999 wt % or more. Any metal hydride that is described in U.S. Pat. No. 8,172,928, incorporated by reference herein in its entirety, can be included in the present hydrogen-generating composition. 
     The hydrogen-generating composition can include both a metal hydride (e.g., an interstitial intermetallic hydride, such as LaNi 5−x Al x  wherein x is about 0 to about 1), and a chemical hydride (e.g., an ionic hydride or a covalent hydride, such as magnesium hydride (MgH 2 ), lithium hydride (LiH), aluminum hydride (AlH 3 ), calcium hydride (CaH 2 ), sodium aluminum hydride (NaAlH 4 ), sodium borohydride (NaBH 4 ), lithium aluminum hydride (LiAlH 4 ), ammonia borane (H 3 NBH 3 ), diborane (B 2 H 6 ), palladium hydride, LaNi 5 H 6 , TiFeH 2 , and a combination thereof). 
     Metal Oxide 
     In various embodiments, the hydrogen-generating composition can include one or more metal oxides. In some embodiments, the hydrogen-generating composition can be free of metal oxides. The one or more metal oxides can form any suitable proportion of the hydrogen-generating composition, such as about 0.001 wt % to about 20 wt % of the hydrogen-generating composition, about 1 wt % to about 10 wt %, or about 0.001 wt % or less, or less than, equal to, or greater than about 0.01, 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, or about 20 wt % or more. 
     The metal oxide can be any suitable metal oxide, such that the hydrogen-generating composition can be used as described herein. The metal oxide can be zirconium (IV) oxide, hafnium (IV) oxide, titanium (IV) oxide, or a combination thereof. The metal oxide can be titanium (IV) oxide. 
     The hydrogen-consuming fuel cell can include an anode, a cathode, and an electrically-insulating ion-conducting electrolyte (e.g., a membrane, such as a proton exchange membrane, or PEM) separating the anode and cathode, wherein at least one of the anode or cathode undergoes a chemical reaction that consumes hydrogen and generates an electrical potential across the electrodes. In some embodiments, the cathode of the fuel cell consumes hydrogen gas and generates electrons and hydrogen ions. 
     The hydrogen ions can travel across the electrolyte to the cathode, while the electrons can travel to the cathode via an electrical circuit connecting the anode to the cathode. At the cathode, the hydrogen ions can react with oxygen gas and the electrons produced by the anode to form water. 
     The water vapor reacts with the chemical hydride fuel in the hydrogen generator, and generates hydrogen in an exothermic reaction. The hydrogen is carried to a PEM fuel cell as illustrated in  FIG. 1  to generate electrical power. 
     The hydrogen generator  115  may be contained in a replaceable and disposable (recyclable) cartridge such as a container. The hydrogen generator  115  may be cylindrical in geometry in some embodiments. 
     During the electrochemical reaction in fuel cell  110  that produces energy, water vapor, and heat as reaction byproducts, the ambient air within the path  120  is heated and water is added resulting in hot, wet air traveling through the path at  122 . 
     The water exchanger  155  extracts water from the hot, wet air within ambient air path at  123 , and exhausts hot, dry air outside the power generator  100  at exhaust  160 . The set point temperature, which in some embodiments is 60° C., may, for example, vary from 40° C. to 80° C. in some embodiments, or outside that range depending on the type of water exchanger utilized as first water exchanger  155 . The extracted water from the ambient air path  120  is provided to the anode loop  125  to release additional hydrogen at  124  from hydrogen generator  115 . Temperature sensors in the anode and cathode loops may be used to determine and control the water exchanger  155  temperature. One or more sensors may be positioned proximate outlets of the water exchanger to provide a temperature data to the controller  135 . 
     As shown in the embodiment of  FIG. 1 , the power generator  100  can also include one or more other water exchangers, such as second water exchanger  157 . Second water exchanger  157  transfers heat and water vapor to the incoming air at inlet  142 , which improves fuel cell performance. In some embodiments a single water exchanger which combines the functions of the first and second water exchangers (e.g. has separate flow paths for the anode and cathode loops) is used to save weight. 
     Once the released hydrogen travels from hydrogen generator  115  through anode loop  125 , it progresses to junction  127  where some of the hydrogen enters a primary path  126  to be recirculated and some hydrogen enters a secondary path  128  to be provided for the electrochemical reaction in fuel cell  110 . 
     As described above, the electrochemical reaction in fuel cell  110  can produce energy. In some embodiments, the fuel cell  110  charges a charge storage device  130 . The charge storage device can be a rechargeable battery such as a lithium-ion battery, a capacitor, or any other suitable charge storage device. In other words, charge storage device  130  is coupled to power generator  100  such that charge storage device  130  receives electricity generated by fuel cell  110 . 
     In some implementations, the charge storage device  130  can provide the ability to supply higher and more dynamic levels of power than simply utilizing the fuel cell  110  directly, which can be slower to respond and not normally be able to provide high levels of power that may be required for operation of a UAS in a desired manner, such as accelerating sufficiently while carrying a load. In the embodiment of  FIG. 1 , power generated by the fuel cell  110  can be provided for storage in one or more charge storage devices  130 , and/or provided directly to the load from the controller  135 . 
     As illustrated in  FIG. 1 , power generator  100  can include controller  135 . Controller  135  can provide inputs to power generator  100  such that power generator  100  can run optimally, producing power to be stored in charge storage device  130  for use by a UAS, for example. For example, controller/power management electronics can manage flow of power from the fuel cell to the load, and/or control other aspects of power generation (e.g., regulation of temperatures, pressures, flow rates, etc.) 
     Controller  135  can provide inputs to power generator  100  in various ways such that power generator  100  can optimally generate power, as are further described herein. For example, in some embodiments, controller  135  can provide inputs to power generator  100  based on a pressure in anode loop  125 . In some embodiments, controller  135  can provide inputs to power generator  100  based on a current draw by the load (e.g., a UAS) from charge storage device  130 . However, embodiments of the present disclosure are not limited to control schemes for power generator  100 . For example, controller  135  can provide inputs for other system controls. For instance, controller  135  can control the temperature of the fuel cell/hydrogen generator, pressure in the anode loop, flow in anode and cathode loops, state of charge of charge storage device, anticipated changes in load from the device the power source is powering, etc. 
     As described above, in some examples controller  135  can provide inputs to power generator  100  based on a pressure in anode loop  125 . Controller  135  can receive a pressure reading in anode loop  125 , where the pressure in anode loop  125  is based on the blower fan speed of blower  165 . 
     The pressure reading received by controller  135  can be the pressure in anode loop  126 . The pressure in anode loop  126  can be the absolute pressure or the gauge pressure relative to the local ambient pressure. For example, a sensor included in anode loop  125  can determine the pressure in anode loop  125  and transmit the pressure to controller  135 . The pressure in anode loop  126  can allow controller  135  to determine a speed of blower  165  in order to allow hydrogen generator  115 , fuel cell  110 , first water exchanger  155  and/or second water exchanger  157  to operate optimally, as is further described herein. That is, the speed of blower  165  can affect operating parameters of the hydrogen generator  115 , fuel cell  110 , first water exchanger  155  and/or second water exchanger  157  according to the operational scheme of power generator  100  as described above. 
     Controller  135  can determine whether the pressure in anode loop  125  exceeds a threshold pressure. As an example, the sensor in anode loop  125  can determine the pressure in anode loop  125  is  8  pounds per square inch (PSI). Controller  135  can compare the received pressure to a threshold pressure to determine whether the received pressure exceeds the threshold pressure. The threshold pressure can be a predetermined pressure stored locally in memory included in controller  135 . 
     The threshold pressure can be a pressure range. For example, the pressure range can include an upper threshold pressure and a lower threshold pressure. For instance, operation of power generator  100  may occur optimally at a particular pressure of the anode loop  125 , and the particular pressure of the anode loop  125  can fall within the threshold pressure range. That is, the particular pressure of the anode loop  125  can be within the lower threshold pressure and the upper threshold pressure. 
     In some examples, controller  135  can determine the pressure in anode loop  125  is less than the lower threshold pressure. For example, the lower threshold pressure can be 5 PSI, and the controller  135  can determine the received pressure in the anode loop  125  is 4 PSI. Accordingly, controller  135  can determine the pressure in anode loop  125  is less than the lower threshold pressure. 
     A drop in pressure in anode loop  125  can, in some examples, correspond to a higher power requirement by the load from charge storage device  130 . For example, in response to more power being drawn by the load (e.g., by a UAS), more energy from fuel cell  110  may be needed to meet the demand. As the rate of hydrogen being used by fuel cell  110  increases to generate more energy, the pressure in anode loop  125  can drop, causing the pressure to fall below the lower threshold pressure. 
     In order to compensate for the rate of hydrogen being utilized by fuel cell  110  from hydrogen generator  115  increasing as a result of the increased load from the UAS, controller  135  can modify the speed of blower  165  to increase the hydrogen generation rate in the hydrogen generator. Controller  135  can modify the speed of blower  165  by increasing the blower speed such that blower  165  can provide more water vapor to the hydrogen generator  115 , which increases the hydrogen generation rate. Increasing the speed of blower  165  thus increases the pressure in anode loop  125  (e.g., to within the threshold pressure range as described above). As a result, operational parameters for various components of power generator  100  can be kept to within ideal operational limits. 
     In some examples, controller  135  can determine the pressure in anode loop  125  exceeds the upper threshold pressure. For example, the upper threshold pressure can be 12 PSI, and the controller  135  can determine the received pressure in the anode loop  125  is 14 PSI. Accordingly, controller  135  can determine the pressure in anode loop  125  has exceeded the upper threshold pressure. 
     An increase in pressure in anode loop  125  can, in some examples, correspond to a lower power requirement by the load or the charge storage device  130 . For example, in response to lower power being used by the load (e.g., by a UAS) or by charge storage device  130 , power may be required from fuel cell  110 . As the rate of hydrogen being used by fuel cell  110  decreases to generate less power for charge storage device  130 , the pressure in anode loop  125  can increase, causing the pressure to increase above the higher threshold pressure. 
     In order to compensate for the rate of hydrogen being utilized by fuel cell  110  from hydrogen generator  115  decreasing as a result of the decreased load from the UAS or charge storage device  130 , controller  135  can modify the speed of blower  165 . Controller  135  can modify the speed of blower  165  by decreasing the blower speed such that blower  165  can provide less water vapor to hydrogen generator  115 , which decreases the hydrogen generation rate. Decreasing the speed of blower  165  can correspondingly decrease the pressure in anode loop  125  (e.g., to within the threshold pressure range as described above). As a result, operational parameters for various components of power generator  100  can be kept to within ideal operational limits. 
     Although the lower threshold pressure is described above as being 5 PSI and the upper threshold pressure is described above as being 14 PSI, embodiments of the present disclosure are not so limited. For example, the upper and lower threshold pressures can be any other pressure value. In some examples, the pressure values of the upper and lower threshold pressures can vary based on the load (e.g., the UAS), the type of hydrogen fuel utilized by hydrogen generator  115 , among other parameters. 
     As described above, modifying the speed of blower  165  can affect operating parameters of various components of power generator  100 . For example, modifying the speed of blower  165  to modify the pressure in anode loop  125  can maintain an inlet and outlet relative humidity of fuel cell  110  within a predetermined range, maintain an inlet and outlet relative humidity of hydrogen generator  115  within a predetermined range, maintain an inlet and outlet relative humidity of first water exchanger  155  and/or second water exchanger  155  within a predetermined range, and/or a temperature of first water exchanger  155  and/or second water exchanger  155  within a predetermined range, among other operating parameters and/or other operating parameters of other components of power generator  100 . 
     Various sensors can be utilized to monitor components of power generator  100 . For example, the various components of power generator  100  can include temperature sensors that can transmit temperatures of hydrogen generator  115 , fuel cell  110 , and/or first water exchanger  155  and/or second water exchanger  157  to controller  135 . In some examples, controller  135  can maintain operating temperatures of the hydrogen generator  115 , fuel cell  110 , and/or first water exchanger  155  and/or second water exchanger  157  utilizing a fan and/or fans (e.g., operation of the fan/fans can lower the operating temperatures). In some examples, controller  135  can maintain operating temperatures of the hydrogen generator  115 , fuel cell  110 , and/or first water exchanger  155  and/or second water exchanger  157  utilizing a pump circulating cooling fluid to the components of power generator  100  (e.g., operation of the pump circulating the cooling fluid can lower the operating temperatures). 
     As described above, in some examples controller  135  can provide inputs to power generator  100  based on a current draw by the load (e.g., a UAS) from charge storage device  130 . Controller  135  can receive an amount of current draw from charge storage device  130  coupled to fuel cell  110 . As described above, the charge storage device  130  receives electricity generated by fuel cell  110  in response to hydrogen being provided to an anode of fuel cell  110 . Hydrogen can be supplied to the anode via blower  165  by way of anode loop  125  and secondary path  128 . 
     The current draw from charge storage device  130  can be by the load in addition to that drawn by controller  135  and the mechanisms controlled by controller  135 . For example, a UAS may be drawing current from charge storage device  130  in order to operate. The current draw from charge storage device  130  can be measured by a sensor. For example, a current sensor can measure the current from charge storage device  130  to the load and transmit the current to controller  135 . 
     Controller  135  can determine whether the current draw from charge storage device  130  exceeds a threshold current draw. As an example, the current sensor can determine the current draw from charge storage device  130  to be 50 A. Controller  135  can compare the received current draw to a threshold current draw to determine whether the received current draw exceeds the threshold current draw. The threshold current draw can be a predetermined current draw stored locally in memory included in controller  135 . 
     The threshold current draw can be a current draw range. For example, the current draw range can include an upper threshold current draw and a lower threshold current draw. 
     In some examples, controller  135  can determine the current draw from charge storage device  130  exceeds the upper threshold current draw. For example, the upper threshold current draw can be 55 A, and the controller  135  can determine the current draw from charge storage device  130  is 60 A. Accordingly, controller  135  can determine the current draw from charge storage device  130  exceeds the upper threshold current draw. An increase in current draw from charge storage device  130  can indicate the load (e.g., the UAS) is utilizing more power. 
     In order to compensate for the increase in current draw by the load from charge storage device  130 , controller  135  can modify the speed of blowers  165  and  140 . Controller  135  can modify the speed of blowers  165  and  140  to provide more hydrogen from hydrogen generator  115  and air from ambient to fuel cell  110  by increasing a blower fan speed of blowers  165  and  140 . As a result of more hydrogen and air being provided to fuel cell  110 , fuel cell  110  can generate more electricity to provide to charge storage device  130  to compensate for the increase in power by the UAS so the UAS can continue to operate. 
     In some examples, controller  135  can determine the current draw from charge storage device  130  is less than the lower threshold current draw. For example, the lower threshold current draw can be 35 A. In an example, the controller  135  may determine the current draw from charge storage device  130  is 30 A. Accordingly, controller  135  can determine the current draw from charge storage device  130  (e.g., 30 A) is less than the threshold current draw (e.g., 35 A). The decrease in current draw from charge storage device  130  can indicate the load (e.g., the UAS) is utilizing less power. 
     In order to compensate for the decrease in current draw by the load from charge storage device  130 , controller  135  can modify the speed of blowers  165  and  140 . Controller  135  can modify the speed of blowers  165  and  140  to provide less hydrogen from hydrogen generator  115  and air from ambient to fuel cell  110  by reducing a blower fan speed of blowers  165  and  140 . As a result of less hydrogen and air being provided to fuel cell  110 , fuel cell  110  can generate less electricity to provide to charge storage device  130 . 
     Although the upper threshold current draw is described above as being 55 A and the lower threshold current draw is described above as being 35 A, embodiments of the present disclosure are not so limited. For example, the upper and lower threshold current draws can be any other current values. 
     The controller  135  may use PID type control algorithms to control the speed of blowers  165  and  140  to maintain the operating parameters of the various components of power generator  100 . Other control algorithms may be used in further embodiments, such as modeling and any other type of algorithm sufficient to control the operating parameters of the various components of power generator  100  by controlling the speed of blowers  165  and  140 . 
       FIGS. 2A and 2B  illustrate various views of a water exchanger  255  for a fuel cell based power generator according to an embodiment of the present disclosure. For example,  FIG. 2A  illustrates a first angled cross-sectional view of water exchanger  255 , and  FIG. 2B  illustrates a second angled cross-sectional view of water exchanger  255 . Water exchanger  255  can be, for example, water exchanger  155  and  157  previously described in connection with  FIG. 1 . 
     As shown in  FIGS. 2A and 2B , water exchanger  255  can include a plurality of hollow tubular structures  202 . For instance, the plurality of hollow tubular structures  202  can comprise an array of hollow tubular structures  202 . 
     Each respective hollow tubular structure  202  can comprise a membrane that is selectively permeable to water vapor over atmospheric gases such as oxygen, nitrogen and also over hydrogen. The selective membranes may be made from a broad variety of compositions, including (but not limited to) commercially available membranes based on: perfluorosulfonic acid (e.g. Nation®, Aquivion®, Gore-Select®, etc.); polybenzimidazole (e.g. Fumapem® AM); sulfonated polysulfone (e.g. Fumapem® SX); sulfonated polyarylene (e.g. Asahi CMV); poly(styrene-co-butadiene) (e.g. Asahi AMV); poly(styrene-co-divinylbenzene) (e.g. NEOSEPTA); silicone (e.g. PermSelect®). The membranes may also be made from a variety of proprietary or experimental chemistries under development for application as cation and anion exchange membranes. The materials may be formed into reinforced membranes, such as for example W.L. Gore&#39;s GORE-SELECT which uses a reinforcing ePTFE (expanded polytetrafluoroethylene) layer to create a thin, strong membrane with higher water permeability and selectivity to other gases. 
     In one embodiment, the plurality of hollow tubular structures  202  can be formed, for example, by forming a plurality of flat sheet structures, each including a membrane material or reinforced membrane material, around a plurality of solid tubular (e.g., cylindrical) structures, as will be further described herein (e.g., in connection with  FIGS. 4A and 4B ). 
     The membrane material can be selectively permeable to water. If reinforcement is used, such as a PE material, the reinforcement may be a porous material. As such, the hollow tubular structures  202  can be used to exchange water (e.g., water vapor) between two flowing gas streams (e.g., air and hydrogen) without allowing the two gas streams to mix, as will be further described herein. 
     The membrane material can be a thin material as compared to the reinforcement material (e.g., the reinforcement material may be much thicker than the membrane material). For instance, the membrane material can have a thickness of 5 micrometers or less, and the reinforcement material, such as PE, can have a thickness of 100 micrometers. Further, the lumen (e.g., inner surface) of each respective hollow tubular structure  202  can be pressurized relative to (e.g. have a higher pressure than) the outer surface of each respective hollow tubular structure  202 , thereby causing the hollow tubular structures to inflate when a gas stream (e.g., hydrogen) flows therethrough. 
     The use of such a tensile membrane can enable an inherently light-weight design for the hollow tubular structures  202 , and therefore for water exchanger  255 . For example, each respective hollow tubular structure  202  may have a weight per unit area of 2.0-2.5 mg/cm 2 . Further, the design of the array of hollow tubular structures  202  can be used to reduce the pressure drop across the tubular structures while water is being exchanged between the two flowing gas streams. For example, an array having hollow tubular structures  202  with a larger diameter can have a reduced pressure drop on lumen (e.g., inner) side of the tubular structures, and an array having a larger spacing between each respective hollow tubular structure  202  can have a reduced pressure drop on the outside of the tubular structures (e.g., a larger diameter for, and a larger spacing between, the hollow tubular structures results in a smaller pressure drop). An example of a hollow tubular structure  202 , and an example method for processing an array of hollow tubular structures  202 , will be further described herein (e.g., in connection with  FIGS. 3 and 4A-4B ). 
     In one embodiment, the membrane, including any optional reinforcing material may have an inner tube (lumen) diameter of 0.5 mm to 2.0 mm. The membrane thickness may be 5 um to 50 um, and the tube length may be 10 cm to 40 cm. These measurements are for example only, and reflect a tradeoff between diameter, length, and thickness to achieve desired efficiency in water removal versus pressure required to operate efficiently, with total weight also taken into account. The use of hollow tube structures allows a significant reduction in pressure across the water exchanger enabling the use of lighter, less powerful fans. Other embodiments may deviate from these measurements for different applications. 
     As shown in  FIGS. 2A and 2B , water exchanger  255  can include a manifold  204  to which the plurality (e.g., array) of hollow tubular structures  202  can be coupled (e.g., attached). For instance, the plurality of hollow tubular structures  202  can be sealed to manifold  204  using a potting (e.g., a potting adhesive) material having a thickness of 0.5-2.0 mm. Further, manifold  204  can comprise a light-weight, stiff material, such as, for instance, a carbon fiber material, which can further enable the light-weight design for water exchanger  255 . 
     As shown in  FIGS. 2A and 2B , water exchanger  255  can include a packaging material  206  that encloses (e.g., surrounds) the plurality of hollow tubular structures  202 . Packaging material  206  can comprise, for instance, a carbon fiber material, or a biaxially-oriented polyethylene terephthalate (BoPET) material such as a Mylar film. As such, packaging material  206  can provide a thin, light-weight shell for the plurality of hollow tubular structures  202  that further enables the light-weight design for water exchanger  255 . 
     As shown in  FIG. 2A , during operation of water exchanger  255  (e.g., as a part of fuel cell based power generator  100  previously described in connection with  FIG. 1 ), water exchanger  255  can receive a stream of hot, dry hydrogen from a hydrogen generator (e.g., hydrogen generator  115  previously described in connection with  FIG. 1 ) via intake opening  214 , and water exchanger  255  can receive a stream of hot, wet (e.g. humid) air from a fuel cell (e.g., from the cathode exhaust of fuel cell  110  previously described in connection with  FIG. 1 ) via intake opening  212 . For example, water exchanger  255  can receive the hot, dry hydrogen through the lumen of each respective hollow tubular structure  202 , and water exchanger  255  can receive the hot, wet air across the outer surface of each respective hollow tubular structure  202 , such that the hydrogen flows inside the plurality of hollow tubular structures  202  and the air flows outside the plurality of hollow tubular structures  202 , as illustrated in  FIG. 2A . 
     As the hot, dry hydrogen flows through the lumen of each respective hollow tubular structure  202  and the hot, wet air flows across the outer surface of each respective hollow tubular structure  202 , water (e.g., water vapor) can be transferred from the wet air to the dry hydrogen through the membrane material of each respective hollow tubular structure  202 . For example, the water may be extracted from the wet air and provided to the dry hydrogen through the membrane material of each respective hollow tubular structure  202 . Further, the membrane material of each respective hollow tubular structure  202  can keep the hot, wet air and the hot, dry hydrogen separate during the transfer of the water from the air to the hydrogen. As such, the air and hydrogen do not mix while the water is being transferred. 
     After the water has been transferred from the wet air to the dry hydrogen, hot wet hydrogen is output from the other side of the lumen of each respective hollow tubular structure  202 . This hot, wet hydrogen can then be output from water exchanger  255  to the hydrogen generator via output opening  216  of the manifold  204 , as illustrated in  FIG. 2A . Further, after the water has been transferred from the wet air to the dry hydrogen, hot dry air is output (e.g., exhausted) from water exchanger  255  to ambient via output openings  218 - 1  and  218 - 2  of manifold  204 , as illustrated in  FIG. 2A . 
     In one embodiment, the hot dry air can be captured from output openings  218 - 1  and  218 - 2  and routed to a second water exchanger  157  as shown in  FIG. 1 . The order of the water exchangers  155  and  157  in  FIG. 1  can be reversed (fuel cell cathode exchanger  157  first, and anode loop exchanger  155  second.) In still further embodiments, water exchangers  155  and  157  can be combined into a single water exchanger by segregating the cathode loop and anode loop flows on each of the intakes and outlets utilizing separate sets of tubes. 
       FIG. 3  illustrates an angled cross-sectional view of a hollow tubular structure  362  of a water exchanger for a fuel cell based power generator according to an embodiment of the present disclosure. Hollow tubular structure  362  can be an example of a hollow tubular structure  202  of water exchanger  255  previously described in connection with  FIGS. 2A-2B . Water exchanger  255  may include individual tubes as shown in  FIG. 3  supported between intake and output manifolds of the water exchanger, or may comprises sheets of sealed/laminated together tubes as shown in later figures. 
     As shown in  FIG. 3 , hollow tubular structure  362  can include a membrane material (e.g., film)  364 , and a reinforcement material  366  on the outer surface of (e.g., around) membrane material  364  that provides reinforcement for membrane material  364 . As shown in  FIG. 3 , and as previously described in connection with  FIGS. 2A-2B , membrane material  364  can be a thin material as compared to reinforcement material  366 . Further, hollow tubular structure  362  can have a light-weight design, and the lumen  368  of hollow tubular structure  362  can be pressurized relative to the outer surface  369  of hollow tubular structure  362 , as previously described in connection with  FIGS. 2A-2B . 
     In some embodiments, the tubular structure does not have a reinforcement material, or may have an interior reinforcement material, where material  364  comprises the reinforcement material. In one example, a three layer laminate of PFSA/porous reinforcement/PFSA may be used. Each layer may be approximately 5 um thick. 
     Membrane material  364  can be selectively permeable to water, and the reinforcement material  366  can be a porous material. As such, hollow tubular structure  362  can be used to exchange water (e.g., water vapor) between a stream of wet air flowing across the outer surface  369  of hollow tubular structure  362  and a stream of dry hydrogen flowing through the lumen  368  of hollow tubular structure  362  without allowing the two streams to mix, as previously described in connection with  FIGS. 2A-2B . 
       FIGS. 4A and 4B  illustrate process steps associated with forming a water exchanger for a fuel cell based power generator according to an embodiment of the present invention. For example,  FIGS. 4A and 4B  illustrate angled cross-sectional views of process steps associated with forming a plurality (e.g., array) of hollow tubular structures (e.g.,  462 - 1 ,  462 - 2 ,  462 - 3 , and  462 - 4 ) of a water exchanger such as, for instance, the plurality of hollow tubular structures  202  of water exchanger  255  previously described in connection with  FIGS. 2A and 2B . 
     The plurality of hollow tubular structures can be formed by forming a plurality of flat sheet structures around a plurality (e.g., array) of solid rods (e.g., cylindrical) structures. After forming the seals between the tubes (e.g. using a thermal or adhesive), the rods are removed and we are left with an array of hollow membrane tubes. For instance, in the example illustrated in  FIG. 2A , first sheet structure  472 - 1  and a second sheet structure  472 - 2  were formed around solid rod structures  474 - 1 ,  474 - 2 ,  474 - 3 , and  474 - 4 . However, embodiments are not limited to a particular number of sheet structures or solid rod structures. For instance, the array of solid rod structures can include any number of rows having any number of solid tubular structures, with flat sheet structures on each respective side (e.g., top and bottom) of each respective row being formed around the solid rod structures of that row in a manner analogous to that illustrated in  FIG. 2A . 
     Each respective flat sheet structure (e.g.,  472 - 1  and  472 - 2 ) can include a membrane material and a reinforcement material on the PFSA membrane material. The membrane material of each respective flat sheet structure can be a thin material as compared to the reinforcement material, as previously described in connection with  FIGS. 2A-2B . 
     The plurality of flat sheet structures (e.g., sheet structures  472 - 1  and  472 - 2 ) can be formed around the plurality of solid rod structures (e.g., rod structures  474 - 1 ,  474 - 2 ,  474 - 3 , and  474 - 4 ) by, for example, forming different respective portions of a first one of the flat sheet structures around a first portion of each respective solid rod structure, and forming different respective portions of a second one of the flat sheet structures around a second portion of each respective solid rod structure. For instance, as illustrated in  FIGS. 2A-2B , a different respective portion of flat sheet structure  472 - 1  is formed around the top half of each respective solid rod structure  474 - 1 ,  474 - 2 ,  474 - 3 , and  474 - 4  (e.g., a first portion of flat sheet structure  472 - 1  is formed around the top half of rod structure  474 - 1 , a second portion of flat sheet structure  472 - 1  is formed around the top half of rod structure  474 - 2 , etc.), and a different respective portion of flat sheet structure  472 - 2  is formed around the bottom half of each respective solid rod structure  474 - 1 ,  474 - 2 ,  474 - 3 , and  474 - 4  (e.g., a first portion of flat sheet structure  472 - 2  is formed around the bottom half of rod structure  474 - 1 , a second portion of flat sheet structure  472 - 2  is formed around the bottom half of rod structure  474 - 2 , etc.). That is, portions of two different sheet structures are formed around one respective solid rod structure to form one respective hollow tubular structure. 
     The first and second flat sheet structures can then be sealed together using an adhesive material or thermal bond such that the sheets conform to the array of rods between them. The sheets may be pressed together, vacuum sealed, laminated, heat sealed, adhered via adhesive material, or otherwise bonded together as indicated at  476 . Adhesive adds weight, whereas other means of bonding or sealing the sheets together avoid adding weight. 
     After the flat sheet structures have been sealed together, the solid rod structures can be removed, leaving a plurality of hollow tubular structures remaining. For instance, as shown in  FIGS. 4A-4B , solid rods  474 - 1 ,  474 - 2 ,  474 - 3 , and  474 - 4  can be removed to form hollow tubular structures  462 - 1 ,  462 - 2 ,  462 - 3 , and  462 - 4  having lumens  468 - 1 ,  468 - 2 ,  468 - 3 , and  468 - 4 , respectively. 
     In a further embodiment, the tube arrays may be formed in a flat configuration and inflated to form the tubes, obviating the need for rods. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art will appreciate that any arrangement calculated to achieve the same techniques can be substituted for the specific embodiments shown. This disclosure is intended to cover any and all adaptations or variations of various embodiments of the disclosure. 
     It is to be understood that the above description has been made in an illustrative fashion, and not a restrictive one. Combination of the above embodiments, and other embodiments not specifically described herein will be apparent to those of skill in the art upon reviewing the above description. 
     The scope of the various embodiments of the disclosure includes any other applications in which the above structures and methods are used. Therefore, the scope of various embodiments of the disclosure should be determined with reference to the appended claims, along with the full range of equivalents to which such claims are entitled. 
     In the foregoing Detailed Description, various features are grouped together in example embodiments illustrated in the figures for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the embodiments of the disclosure require more features than are expressly recited in each claim. 
     Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment.