Abstract:
A hydrogen storage tank for containing solid-state hydrogen storage media, and method for determining the hydrogen fill level in the tank. The tank has at least one compartment for storing the storage media, passages for transporting hydrogen gas to and from the compartment, and a heat distribution system for establishing a substantially uniform temperature in the storage media. A look-up table is generated for the tank that relates hydrogen pressure changes in the tank versus hydrogen fill levels in the tank as a function of temperature. The tank is filled to full capacity by adsorbing hydrogen on the storage media. Thereafter, hydrogen is released from the storage media for a time period during which there is a substantially constant demand on the tank, the change in pressure of the released hydrogen gas is measured, and the look-up table is applied to determine the hydrogen fill level in the tank.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/814,435, filed Jun. 16, 2006, the contents of which are incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    This invention was made with United States Government support from Edison Materials and Technology Center (EMTEC), Contract No. EFC-H2-3-1C. The Government has certain rights in this invention. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to hydrogen storage tanks and methods. More particularly, this invention relates to hydrogen storage tank adapted to contain solid-state hydrogen storage media, such as nano-porous silicon (npSi), and to methods for determining fill levels in such tanks. 
         [0004]    Hydrogen-based fuel cell technologies are being considered for a wide variety of power applications, including but not limited to mobile applications such as vehicles as an attractive alternative to the use of petroleum-based products. Hydrogen-based fuel cells are also readily adaptable for use as energy sources in numerous and such diverse applications as cellular phones to space ships. They have the further desirable attribute of producing water vapor as their only byproduct and are thus environmentally benign. However, hydrogen storage remains a challenge because of its very low heat value per volume compared to fossil fuels. As such, efficient storage of hydrogen is vitally important for cost-effective system implementation. When compared to storage for conventional chemical fuels or electric energy sources, existing hydrogen storage technologies lack the convenience of gasoline for delivery and storage capacity (energy density per unit weight), and lack the flexibility of electrical energy stored in batteries and capacitors. Therefore, for fuel cells to reach their full commercial potential, improved hydrogen storage technologies are needed. 
         [0005]    Prior methods of storing hydrogen fall broadly into two categories. The first category involves storing hydrogen chemically within a convenient chemical molecule, usually an aliphatic organic compound such as methane, octane, etc., and then pre-processing the fuel as needed, such as by catalytic reforming, to release elemental hydrogen plus carbon oxides. This method suffers two important drawbacks: carbon dioxide byproduct is a “greenhouse gas” that some believe contributes to global warming and is therefore environmentally undesirable; and the additional weight of the chemical molecule and the reformer reduce the efficiency of the entire process, making it less attractive from a cost and performance standpoint. 
         [0006]    The second category involves mechanical or adsorptive storage of elemental hydrogen in one of three forms: compressed gas, cryogenically-refrigerated liquid, or chemisorbed onto active surfaces. Of these methods, compressed gas storage is the most straightforward and is a mature technology. However, compressed gas cylinders are quite heavy, needing sufficient strength to withstand pressures of many thousands of pounds per square inch. This weight is a considerable drawback for portable applications, and in any usage compressed gas cylinders must be treated with care because they represent a safety hazard. 
         [0007]    Cryogenic storage of hydrogen is also well known, being used in industrial plants and as a rocket fuel. Liquid hydrogen is remarkably dense from a specific energy point of view (kilowatts per kilogram), but requires a considerable amount of additional energy to maintain the nearly absolute zero temperatures needed to keep hydrogen in a liquid state. Liquid hydrogen also requires a heavy mass of insulation, and these factors conspire to make cryogenic storage impractical for portable and small-scale applications. 
         [0008]    Chemisorption as used herein means the adsorption of a given molecule onto an active surface, typically of a solid or a solid matrix. Chemisorption is typically reversible, although the energy of adsorption and the energy of desorption are usually different. Various catalysts and surface preparations are possible, providing a wide range of possible chemistries and surface properties for a given storage problem. Chemisorption of hydrogen has been studied extensively, and substances such as metal hydrides, palladium, and carbon nanotubes or activated carbon have been used to adsorb and desorb hydrogen. In particular, hydrogen storage in absorbed solids such as metal hydrides, metal oxides and other inorganic surfaces, carbon nanotubes/fibers, carbon fullerene, etc., has recently been considered as a promising method with the advantages of high volumetric hydrogen density and improved safety. However, prior hydrogen chemisorption techniques with these solid-state storage media have fallen short of the goals of efficiency, convenience, and low system cost for several reasons. 
         [0009]    In the case of metal hydrides, metal oxides, and other inorganic surfaces, storage efficiencies typically are relatively low and the adsorption/desorption process is highly dependent upon exacting chemistry. These factors combine to make such approaches less than sufficiently robust for many commercial applications. Even so, metal hydrides are considered leading candidates for hydrogen storage. However, metal hydride materials expand during hydriding cycles to the extent that large stresses are generated on the material (typically particles) and the hydriding container, such that stringent requirements are imposed in container design. Furthermore, these stresses tend to fragment the metal hydride particles into finer and finer powders in an uncontrollable manner, resulting in material movement and segregation in the storage container, an increased tendency for entrainment of hydride fines in the hydrogen gas streams released from the particles, and the risk of plugged gas filters and high internal pressures within the hydriding container. Secondly, hydrogen charging (adsorption) and release (desorption) require heating of the metal hydride particles at a level of about 14.6 MJ/kg, which poses a significant thermal management challenge and leads to slow refueling. 
         [0010]    In other materials, such as carbon nanotubes, the efficiency of hydrogen adsorbed per unit weight of matrix is higher than metal hydrides, metal oxides, and other inorganic surfaces, but desorption requires high temperatures that raise the risk of combustion. Additionally, the present cost of carbon nanostructures is relatively high, and control over material properties can be quite difficult in high-volume manufacturing. 
         [0011]    Hydrogenated surfaces in silicon have also been employed, as disclosed in US. Pat. Nos. 5,604,162, 5,605,171, and 5,765,680, the disclosures of which are incorporated herein by reference. In each of these references, the adsorbed molecule is the radioactive hydrogen isotope tritium ( 3 H), and the objective is the storage of this isotope to enable its safe transport, typically to a waste handling or storage facility, or to serve as a means for providing radioactive energy to power a light source. These prior methods of chemisorption do not, however, provide for desorption of hydrogen from a silicon storage medium. In fact, conventional methods of chemisorption are generally designed to prevent desorption. Further, these conventional methods of chemisorption fail to teach methods by which the storage capacity of a silicon matrix can be increased. 
         [0012]    As a solution to the forgoing, a system for storage and retrieval of elemental hydrogen on a porous silicon media is described in U.S. Published Patent Application No. 2004/0241507 to Schubert et al., the disclosure of which is incorporated herein by reference. As a solid-state hydrogen storage media, nano-porous silicon (npSi) has a theoretical hydrogen capacity of about 6.6 weight percent, and has unique characteristics capable of enhancing hydrogen charging and recharging processes. As such, there is a need for hydrogen storage tanks adapted for containing npSi, as well as methods for their use and operation. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    The present invention is directed to hydrogen storage tanks adapted to contain solid-state hydrogen storage media, such as nano-porous silicon (npSi), and methods for determining hydrogen fill levels in such tanks. 
         [0014]    According to one aspect of the invention, a hydrogen storage tank is provided having a plurality of separate compartments for storing the storage media. Passages are provided for transporting hydrogen gas to the tank and into each compartment and for transporting hydrogen gas out of each compartment. Heat is distributed within and among the compartments so as to enable establishing a substantially uniform temperature in the storage media within the compartments. Means is provided for desorbing hydrogen from the storage media, and a sensor senses the pressure of hydrogen gas released from the storage media within the compartments. 
         [0015]    According to another aspect of the invention, a method for determining the hydrogen fill level in a hydrogen storage tank entails configuring the tank to have at least one compartment for storing the storage media, passages for transporting hydrogen gas to the tank and into the compartment and for transporting hydrogen gas out of the compartment, and means for distributing heat within the compartment so as to establish a substantially uniform temperature in the storage media within the compartment. A look-up table is generated for the tank that is derived from a series of plots relating changes in pressure of the hydrogen gas in the tank versus fill levels of hydrogen in the tank as a function of temperature. The tank is filled to full capacity by flowing hydrogen gas through the passages and into the compartment so as to adsorb hydrogen on the storage media. Thereafter, hydrogen is released from the storage media for a time period during which there is a substantially constant demand on the tank, the change in pressure of the hydrogen gas released from the storage media is measured during the time period, and the look-up table is applied to determine the hydrogen fill level in the tank. 
         [0016]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  schematically represents a cross-sectional view of a hydrogen storage tank adapted to contain porous silicon according to a first embodiment of the invention. 
           [0018]      FIGS. 2 and 3  schematically represent cross-sectional views of the hydrogen storage tank taken along section lines  2 - 2  and  3 - 3  of  FIG. 1 . 
           [0019]      FIG. 4  schematically represents an end view of the hydrogen storage tank as viewed from line  4 - 4  of  FIG. 1 . 
           [0020]      FIG. 5  schematically represents a cross-sectional view of a hydrogen storage tank adapted to contain porous silicon according to a second embodiment of the invention. 
           [0021]      FIG. 6  schematically represents a cross-sectional view of the hydrogen storage tank taken along section line  6 - 6  of  FIG. 5 . 
           [0022]      FIG. 7  is a graph depicting a relationship between fill level and pressure change as a function of time within the hydrogen storage tanks of  FIGS. 1 through 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The present invention provides hydrogen storage systems with particular application for automotive fuel cells. The storage systems include integrated storage tanks adapted to contain solid-state storage media, including but not limited to nano-porous silicon (npSi) powders. The tanks are equipped with heat distribution systems, light sources, and electric field generators to facilitate hydrogen charge and discharge processes and fill level checks carried out with the tanks. An algorithm is preferably employed to determine the fill level of hydrogen in the tanks. 
         [0024]      FIGS. 1 through 4  depict a first configuration of such a hydrogen storage tank  10 . The tank  10  is divided into several compartments  12  by dividers  14  shown as extending the full height of the tank  10  so that the compartments  12  are substantially parallel to each other. Each compartment  12  is adapted to contain a solid-state storage media such as a npSi powder. The tank  10  is equipped with hydrogen inlet/outlet ports  16  for transporting hydrogen gas to each compartment  12  to allow for adsorption of hydrogen on the storage media, and for transporting hydrogen gas from the compartments  12  following desorption of hydrogen from the storage media. Each port  16  is outfitted with a porous filter  18  that extends into its corresponding compartment  12  to trap contaminants that might otherwise be transported to and from the compartments  12  with the hydrogen gas. As shown, the porous filters  18  can extend nearly the full length of each compartment  12  to facilitate the flow of hydrogen gas. A hydrogen manifold  20  interconnects the ports  16 , and a flow meter or pressure sensor  22  is connected to the hydrogen manifold  20  to enable monitoring of the hydrogen gas entering and leaving the tank  10 . The manifold  20  is preferably provided with a check valve  24  with an integrated flow sensor  26  to help prevent backwash of contaminants into the tank  10 . 
         [0025]    The tank  10  is also preferably equipped with light sources, such as an infrared (IR) emitter  28 , mounted on both sides of each divider  14  for the purpose of generating photons within each compartment  12 . Because silicon is at least partially transparent to light in the infrared region (wavelengths of about 700-5000 nm) and hydrogen has a prime absorption peak at a wavelength of about 660 nm and therefore adsorbs light in this region of the infrared spectrum, photon energy can be effectively used to promote the release of hydrogen from npSi contained within the compartments  12 . The IR emitters  28 , which preferably emit light in wavelengths of about 100 to about 850 nm, can have a variety of configurations, for example, a diffuse light source with a homogeneous illumination capability, a directed light source that is lensed and tailored to the geometry of the tank  10 , and/or a series of light sources that are individually controlled to give greater spatial control of hydrogen release from npSi. The distribution density of the IR emitters  28  within the tank  10  may be determined by its penetration capability though npSi. 
         [0026]    In  FIGS. 2 and 4 , the tank  10  can be seen to contain U-shaped heat exchanger tubes  30  located within each compartment  12 . The tubes  30  form part of a heat distribution system for the tank  10  intended to maintain a substantially uniform temperature throughout each compartment  12  and among the several compartments  12 . In particular, the tubes  30  can be used to remove heat from the compartments  12  during the hydrogen adsorption process, and to import waste heat from the fuel cell for use during the desorption process. The ends of each U-shaped tube  30  are connected to either an inlet or outlet coolant manifold  32 , as seen in  FIGS. 2 and 4  (the tubes  30  and their manifolds  32  are omitted from  FIG. 1  for clarity.) The type of coolant, the number of heat exchanger tubes  30 , the diameters of the tubes  30  and manifolds  32 , the coolant flow rate, inlet temperatures, etc., can be selected by those skilled in the art to achieve minimum refueling time and maximum gravimetric efficiency for the tank  10 . 
         [0027]    According to an optional aspect of the invention, electric fields can be applied to modify the surface energy of the silicon within the tank  10 . Depending on whether an adsorption or desorption cycle is being performed, the electric fields can be employed to either loosen or strengthen the bonds between hydrogen and the storage media, and thereby facilitate the charging and discharging of hydrogen from the tank  10 . The electric fields can be generated field plates having a variety of configurations, including large field plates  34  on opposite sides of the tank  10  as shown in  FIG. 1 , field plates integrated within the compartments  12 , or a series of field plates that, in a preferred embodiment, enable finer control over hydrogen release. 
         [0028]    An npSi material in the form of powders within the compartments  12  may be suspended in a heat-conductive foam (not shown) that forms part of the heat distribution system for the tank  10  to maintain a uniform temperature throughout each compartment  12  and among the several compartments  12 . However, by accommodating a heat-conductive foam within the compartments  12 , the powder packing density within each compartment  12  may be limited, hence lowering the volumetric percentage of hydrogen capacity of the tank  10 . 
         [0029]      FIGS. 5 and 6  schematically illustrates a second configuration of a hydrogen storage tank  50  that differs from that of  FIGS. 1 through 4  with respect to the techniques employed for packing of the storage media and heat distribution within the storage media. In  FIG. 5 , npSi powders (not shown) are packed within individual packing cells or boxes  52 , which in turn are individually stacked in columns within the hydrogen storage tank  50 . Inlet/outlet ports  56  connected to a hydrogen manifold  60  (shown only in  FIG. 5  for clarity) are provided for distributing hydrogen to each box  52  for adsorption by the npSi powders within the boxes  52 , and subsequently removing the hydrogen desorbed by the npSi powders. A layer of filter material  58  is located between the ports  56  and the boxes  52  to trap contaminants that might be entrained in the hydrogen flowing to and from the boxes  52 . In the embodiment shown in  FIGS. 5 and 6 , the boxes  52  are required to be made of a porous material, such as a sintered material that is sufficiently porous to provide hydrogen flow channels therethrough. Alternatively, the tank  50  is required to include porous filter tubes connected to the ports  56  to enable hydrogen flow to and from the boxes  52 , similar to the tubes  18  shown in  FIG. 1 . 
         [0030]    Each box  52  includes holes for the insertion of two sets of U-shaped cooling tubes  70 . The inlet and outlet ends of each cooling tube  70  are coupled to either an inlet or outlet manifold  72  (shown only in  FIG. 6  for clarity). The tubes  70  are tightly fitted to the holes in the boxes  52  during assembly to form an effective tube-fin system with good thermal contact and heat transfer characteristics. In addition to being porous, preferred materials for the packing boxes  52  are thermally conductive to enable the walls  54  of the boxes  52  to function as thermal conducting fins in combination with the cooling tubes  70 . A light source, such as IR emitters  68 , and an electric field plate  74  are preferably installed at each box  52  for effective spatial control of the charging and releasing processes. Optimal sizes for the boxes  52  are determined in part by the heat conductivity, light penetration capability, and electrical field distribution made possible by the materials from which the boxes  52  are made, the type and strength of the IR emitters  68 , and strength of the electric field generated by the field plates  74 . 
         [0031]    The tank  50  illustrated in  FIGS. 5 and 6  has two notable advantages. First, the assembly process for the tank  50  is uncomplicated and the powder packing density within each box  52  can be easily controlled. Secondly, because direct handling of very fine npSi powders might pose possible health problems, the powders can be packed in the boxes  52  as part of the manufacturing process, reducing the potential for human exposure to the powders and facilitating transport of the powders to the assembly site of the hydrogen storage tank  50 . The tank  50  can also be modified to include, for example, an integrated tube-fin system that contacts the boxes  52  and promotes heat transfer throughout the interior of the tank  50 , and optionally a light source and/or electric field plate installed on each of these fins. 
         [0032]    The external geometries of both tanks  10  and  50  can be optimized for performance and for the space available in the intended application, such as a vehicle. As shown in  FIGS. 1 through 6 , the hydrogen storage tanks  10  and  50  can have rectangular external shapes with rounded edges to fit within a rectangular vehicle space and minimize stress concentrations, though it should be recognized that other tank geometries and non-uniform shapes are also within the scope of the invention. IR emitters  28  and  68  and electrical field plates  34  and  74  may also be distributed non-uniformly within the tanks  10  and  50  to accommodate various geometrical requirements, so as to substantially achieve the same IR radiation density and electrical strength at all points within the tanks  10  and  50 . 
         [0033]    A desirable feature for hydrogen storage tanks is the ability to determine the fill level of hydrogen in the tank, both during fueling and to monitor the degradation of the hydrogen storage capacity of the solid-state storage media over time. The tanks  10  and  50  described above are well suited for making such a capability practical. To achieve this functionality, the tank  10 / 50  is assumed to have an approximately uniform (though not necessarily steady-state) temperature throughout its compartments  12 / 52  due to the heat distribution capabilities noted above for the tanks  10  and  50 . Also consistent with the foregoing, the tank  10 / 50  preferably has IR emitters  28 / 68  (or another suitable light source) positioned and having sufficient power so that substantially all regions within the tank  10 / 50  are exposed to the IR radiation from the emitters  28 / 68 . It is further assumed that, as in the case when npSi is used as the hydrogen storage media, elevated temperatures, IR radiation, and electric fields cause or promote the release of hydrogen from the storage media within the tank  10 / 50 . 
         [0034]    The relative fraction of hydrogen released during a given exposure to IR radiation from the emitters  28 / 68  changes with both the fill level of the tank  10 / 50  and its temperature, which can be characterized by the change in pressure as measured by the sensor  22 / 62  at the hydrogen manifold  20 / 60  of the tank  10 / 50  when the IR emitters  28 / 68  are on. Because the effect of IR radiation will change as the fill level changes over time due to hydrogen being drawn from the tank  10 / 50 , a normalized value can be calculated by measuring the change in hydrogen pressure (ΔP RUN ) while the IR emitters  28 / 68  are on for a set period of time, and normalizing this measurement to a change in hydrogen pressure (ΔP FULL ) that was measured when the hydrogen fill level within the tank  10 / 50  was at full capacity, as well as at various known but lesser fill levels. A family of curves can be empirically derived in this manner showing normalized pressure change (ΔP RUN /ΔP FULL ) against fill levels for a variety of temperatures. The data from such curves can be stored in a look-up table, providing the basis for an algorithm by which the approximate fill level within the tank  10 / 50  can be determined during subsequent operation of a fuel cell (or other device) that draws hydrogen from the tank  10 / 50 . 
         [0035]      FIG. 7  represents an example of plots (characterization curves) that constitute such a look-up table, and depicts temperature-dependent relationships between normalized pressure change (ΔP RUN /ΔP FULL ) and fill level within a hydrogen storage tank. Such a look-up table is preferably established for each tank geometry and each type of solid-state media to be used. 
         [0036]    Because the capacity of solid-state hydrogen storage media can degrade over time and the capacity of the tank  10 / 50  can diminish due to poisoning by hydrocarbons, oxygen, water, and nitrous oxides, the algorithm used to determine the fill level is preferably recalibrated each time the tank  10 / 50  is refilled to capacity (hydrogen is adsorbed on the npSi powder). Recalibration entails measuring the tank hydrogen pressure (P FULL ) with the sensor  22 / 62  immediately following each refill of the tank  10 / 50 , and then remeasuring the change in hydrogen pressure (ΔP FULL ) with the sensor  22 / 62  while the IR emitters  28 / 68  are on for a set period of time. 
         [0037]    During subsequent operation of a fuel cell (or other device) that draws hydrogen from the tank  10 / 50 , the fill level within the tank  10 / 50  can be determined by intermittent flashing of the IR emitters  28 / 68  at times when the demand for hydrogen from the tank  10 / 50  is constant, preferably zero. The update rate can be variable, but should not be faster than some minimum time T min , which may be in a range of, for example, about 10 to about 100 seconds. As a nonlimiting example, the update rate could be about 180 seconds, during which the emitters  28 / 68  might be on for about 15 seconds. While the interior of the tank  10 / 50  is subjected to IR radiation from the emitters  28 / 68  for a specified period of time, the change in hydrogen pressure (ΔP RUN ) is measured with the sensor  22 / 62 , and the measured pressure change is normalized to the pressure change (ΔP FULL ) measured when the tank  10 / 50  was last refilled to capacity. Thereafter a fill level can be determined by comparing the normalized value to the characterized values in the look-up table. The result can be reported as a fill level to the storage system and the hydrogen tank user. 
         [0038]    To aid in the understanding of the concept, a pseudocode for preparing a software or hardware implementation of this algorithm is shown below in Table I. 
         [0000]    
       
         
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Fill the tank 
               
               
                   
                 Set Level = 100% 
               
               
                   
                 Apply heat to the tank, preferably to steady state 
               
               
                   
                 Measure pressure at constant demand = P FULL   
               
               
                   
                 Turn on IR emitters 
               
               
                   
                 Measure pressure change ΔP FULL   
               
               
                   
                 Turn off IR emitters 
               
               
                   
                 Perform diagnostics on the tank condition: check if ΔP FULL  is low 
               
               
                   
                 During fuel cell operation: 
               
               
                   
                 When T &gt; T min  AND demand is relatively constant 
               
               
                   
                 Turn on IR emitters 
               
               
                   
                 Measure ΔP RUN   
               
               
                   
                 Turn off IR emitters 
               
               
                   
                 Compute ΔP RUN /ΔP FULL  and look up value on characterization 
               
               
                   
                 curves 
               
               
                   
                 Determine fill level and output to the system/users 
               
               
                   
                 Continue 
               
               
                   
                   
               
             
          
         
       
     
         [0039]    It should be noted that the capability for determining fill level with the present invention is not strictly dependent on the use of a light source (such as the IR emitters  28  and  68 ) to cause changes in hydrogen pressure at refill (ΔP FULL ) and subsequent to refill (ΔP RUN ), in that curves of the type shown in  FIG. 7  can be generated by relying solely on heat, electric fields, or other energy source to cause hydrogen to be released from the storage media. As such, the process steps outlined in Table I could be modified to omit the operation of the IR emitters and rely instead on measured changes in pressure (both ΔP RUN  and ΔP FULL ) during hydrogen desorption that occur at various elevated temperatures (e.g., T 1 , T 2  and T 3  in  FIG. 7 ) or when the storage media is subjected to electric field of various levels. Thereafter, hydrogen fill levels in the tank  10 / 50  could be assessed during periods of substantially constant demand by intermittently applying heat, electric fields, or other energy to the storage media, measuring the resulting pressure changes (ΔP RUN ), and calculating a normalized value (ΔP RUN /ΔP FULL ). 
         [0040]    While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.