Patent Publication Number: US-2018048001-A1

Title: Hydrogen storage container

Description:
TECHNICAL FIELD 
     The present invention relates to a hydrogen storage container having an inside resin layer, a barrier layer, and an outside resin layer arranged in this order from the inside. 
     BACKGROUND ART 
     As is well known, in electric power generation in a fuel cell, it is necessary to supply a fuel gas such as a hydrogen gas to an anode. Therefore, for example, a fuel-cell vehicle having the fuel cell is equipped with a hydrogen storage container filled with the hydrogen gas. The fuel-cell vehicle is driven by supplying oxygen in the atmosphere as an oxygen-containing gas to a cathode of the fuel cell, supplying the hydrogen gas from the hydrogen storage container, reacting the hydrogen gas with the oxygen to generate electricity, and using the electricity to actuate a driving source. 
     In general, the hydrogen storage container is made up of a main body of a liner and a shell enclosing the liner. The liner is composed of a resin material such as a polyethylene naphthalate or a high-density polyethylene (HDPE), and the shell is composed of a fiber-reinforced material such as an FRP. Thus, the hydrogen storage container can be formed by covering a resin liner with a carbon fiber such as FRP or the like. 
     For example, Japanese Laid-Open Patent Publication No. 2000-220794 proposes a high-pressure container for hydrogen storage, which has inside and outside resin layers composed of the polyethylene naphthalate, and further has an intermediate layer interposed between the resin layers. Thus, when the high-pressure container is filled with a high-pressure hydrogen gas, the inside resin layer comes into contact with the hydrogen gas. 
     The intermediate layer acts as a barrier layer for blocking permeation of the hydrogen gas, and is made from a material such as an ethylene-vinyl alcohol copolymer (EVOH), as disclosed in the publication. Adhesive resin layers may be formed between the inside resin layer and the intermediate layer and between the intermediate layer and the outside resin layer if necessary. 
     SUMMARY OF INVENTION 
     In the technology described in Japanese Laid-Open Patent Publication No. 2000-220794, the inside resin layer is arranged inside the intermediate layer (the barrier layer) for the purpose of achieving a sufficient pressure resistance in the hydrogen storage container. The polyethylene naphthalate in the inside resin layer is inferior in hydrogen barrier ability to metal materials. Therefore, by forming the intermediate layer as the barrier layer, the hydrogen gas is prevented from permeating through the container and being diffused into the atmosphere. In other words, lowering of the hydrogen gas pressure is prevented in the hydrogen storage container. 
     However, in this technology, the inside resin layer may be cracked and deteriorated at a relatively early stage in the use of the hydrogen storage container disadvantageously. 
     A principal object of the present invention is to provide a hydrogen storage container capable of preventing an inside resin layer from being cracked due to hydrogen molecules in a high-pressure hydrogen gas stored in the container. 
     According to an aspect of the present invention, there is provided a hydrogen storage container including: 
     an inside resin layer having at least an inner layer, which is brought into contact with a hydrogen gas when the hydrogen gas is introduced into the hydrogen storage container; 
     a barrier layer configured to block permeation of the hydrogen gas, and arranged outside the inside resin layer; and 
     an outside resin layer containing a resin, and arranged outside the barrier layer, 
     wherein 
     the inside resin layer contains a polyethylene-based resin, and 
     the thickness X of the inside resin layer and the thickness Y of the barrier layer satisfy the following inequality (1): 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         75 
                         Y 
                       
                       ) 
                     
                     × 
                     
                       10 
                       
                         - 
                         4 
                       
                     
                   
                   &lt; 
                   X 
                   ≦ 
                   
                     70 
                      
                     
                       D 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein D stands for a diffusion coefficient of the polyethylene-based resin, measured at 50° C. by a differential-pressure method. 
     Since the polyethylene-based resin has a relatively lower hydrogen barrier ability as described above, hydrogen molecules can enter the polyethylene-based resin of the inside resin layer. As a result of research in view of this problem, the present inventors have found that the inside resin layer made from the polyethylene-based resin is deteriorated relatively readily for the following reason. That is, once the hydrogen molecules enter the inside resin layer, the inside resin layer maintains such an entering state even after the hydrogen gas is discharged from the container (i.e. the container is depressurized) to operate the fuel cell. 
     Furthermore, the inventors have found that the barrier layer can maintain a sufficient barrier ability when the thicknesses of the barrier layer and the inside resin layer satisfy a particular condition. 
     In the present invention, the thickness X of the inside resin layer is controlled within a range satisfying the above inequality (1) based on the above findings. The hydrogen molecules that have entered the inside resin layer having the controlled thickness can be diffused in the inside resin layer and removed from the inside resin layer when the container is depressurized. In other words, the hydrogen molecules that have entered the inside resin layer do not remain in the inside resin layer, and are removed from the inside resin layer and released to the internal space of the hydrogen storage container. Thus, the state in which the hydrogen molecules are introduced into the inside resin layer is eliminated. Consequently, the inside resin layer (for example, composed of the polyethylene-based resin) can be prevented from being deteriorated due to the hydrogen molecules. 
     In addition, a sufficient pressure resistance can be achieved by forming the inside and outside resin layers, and permeation of the hydrogen gas can be prevented by forming the barrier layer. In other words, lowering of the hydrogen gas pressure in the container can be prevented. It is to be understood that the hydrogen permeability of the barrier layer is lower than those of the inside and outside resin layers. 
     As described above, the hydrogen storage container having a good pressure resistance, a good hydrogen barrier ability, and an excellent durability can be obtained by using the above structure. 
     For example, the polyethylene-based resin of the inside resin layer is preferably a high-density polyethylene (HDPE). In the case of using the HDPE, the inside resin layer can be easily formed at low cost. 
     The HDPE has a diffusion coefficient D of 4.62×10 −10  m/second, measured at 50° C. by the differential-pressure method. Based on this value and the inequality (1), the thickness of the inside resin layer is preferably controlled to be 1.5 mm or less. In most of conventional hydrogen storage containers, the inside resin layers have thicknesses of 3 mm or more. In the present invention, the thickness of the hydrogen storage container can be reduced, and accordingly the weight thereof can be reduced. 
     The polyethylene-based resin of the inside resin layer may be a low-density polyethylene (LDPE). The LDPE has a diffusion coefficient D of 4.45×10 −10  m/second measured at 50° C. by the differential-pressure method. Therefore, in this case, based on this diffusion coefficient value and the inequality (1), the thickness of the inside resin layer is preferably controlled to be 1.47 mm or less. 
     The thickness of the inside resin layer may be 1.4 mm or less as long as the inequality (1) is satisfied. In this case, the thickness and weight of the hydrogen storage container can be further reduced. 
     The inside resin layer may have the inner layer and an adhesive layer. In this case, the inner layer is attached to the barrier layer with the adhesive layer interposed therebetween. Therefore, the inner layer and the barrier layer are firmly bonded with the adhesive layer, so that the hydrogen molecules or the hydrogen gas can be prevented from remaining between the inner layer and the barrier layer. 
     The material of the barrier layer is preferably a resin having a small hydrogen permeability coefficient. Specific examples of such resins include ethylene-vinyl alcohol copolymer resin. 
     An adhesive layer may be formed between the barrier layer and the outside resin layer to bond the barrier layer and the outside resin layer. In this case, the outside resin layer is attached to the barrier layer with the adhesive layer interposed therebetween. Thus, the barrier layer and the outside resin layer are firmly bonded via the adhesive layer. Therefore, even hypothetically assuming that the hydrogen gas permeates through the barrier layer, the hydrogen gas can be prevented from remaining between the barrier layer and the outside resin layer. Consequently, the outside resin layer can be prevented from being peeled off from the barrier layer. 
     In the present invention, a diffusion distance is calculated based on the diffusion coefficient of the polyethylene-based resin measured at 50° C. by the differential-pressure method, and the thickness of the inside resin layer of the polyethylene-based resin is controlled to be equal to or less than the diffusion distance. Therefore, when the hydrogen storage container is depressurized, the hydrogen molecules that have entered the inside resin layer can be diffused in the inside resin layer and released from the inside resin layer to the internal space of the container. Consequently, the state, in which the hydrogen molecules are introduced into the inside resin layer, is eliminated, so that the inside resin layer can be prevented from being cracked (i.e. deteriorated) due to the hydrogen molecules. Thus, the hydrogen storage container can have an improved durability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall, schematic, longitudinal sectional view of a hydrogen storage container according to an embodiment of the present invention; 
         FIG. 2  is an enlarged sectional view in the thickness direction, of main parts of the hydrogen storage container of  FIG. 1 ; 
         FIG. 3  is an enlarged sectional view of main parts of an inner layer in the hydrogen storage container of  FIG. 1 , from which hydrogen molecules are removed; and 
         FIG. 4  is a schematic sectional view in the thickness direction, of a test specimen composed of an HDPE, observed after the test specimen is exposed to a pressurized hydrogen atmosphere. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Several preferred embodiments of a hydrogen storage container of the present invention will be described in detail below with reference to the accompanying drawings. 
       FIG. 1  is an overall, schematic, longitudinal sectional view of a hydrogen storage container  10  according to an embodiment. The hydrogen storage container  10  is a high-pressure container to be filled with a high-pressure hydrogen gas. For example, the hydrogen storage container is installed in a vehicle body to form a fuel-cell vehicle. 
     An opening  12  is formed at one end of the hydrogen storage container  10 , a pipe joint is attached to the opening  12 , and a pipe for supplying a hydrogen gas from the hydrogen storage container  10  to an anode of a fuel cell or a pipe for feeding a hydrogen gas from a hydrogen supply source into the hydrogen storage container is connected to the pipe joint. The fuel cell, the hydrogen supply source, the pipe, and the pipe joint are not shown in the drawing. 
     The hydrogen storage container  10  is made up of an inside resin layer  14 , a barrier layer  16 , and an outside resin layer  18  as main components. As shown in the enlarged view of  FIG. 2 , the inside resin layer  14  has two layers of an inner layer  20  and a first adhesive layer  22 . A second adhesive layer  24  is interposed between the barrier layer  16  and the outside resin layer  18 . In this embodiment, the inner layer  20  and the outside resin layer  18  comprises a high-density polyethylene (HDPE) resin, and the barrier layer  16  comprises an ethylene-vinyl alcohol copolymer (EVOH) resin. The first adhesive layer  22  and the second adhesive layer  24  preferably comprise a polyethylene-based resin, particularly preferably comprise a low-density polyethylene (LDPE) resin. 
     In this case, the inner layer  20  and the outside resin layer  18  can be easily prepared at low cost because the HDPE resin is inexpensive and easily workable. A sufficient pressure resistance can be achieved by forming the inner layer  20  and the outside resin layer  18 . 
     The inner layer  20  and the barrier layer  16  can be bonded sufficiently firmly with the first adhesive layer  22 , and the barrier layer  16  and the outside resin layer  18  can be bonded sufficiently firmly with the second adhesive layer  24 . This is because the polyethylene-based resin in the first adhesive layer  22  and the second adhesive layer is a modified resin, which can adhere to both of the HDPE and EVOH resins. Therefore, a region between the inner layer  20  and the barrier layer  16  and a region between the barrier layer  16  and the outside resin layer  18  can be sufficiently sealed to prevent entry of hydrogen molecules  26  into the regions. 
     Furthermore, the barrier layer  16  acts to block permeation of the hydrogen gas. Thus, even hypothetically assuming that the hydrogen molecules  26  enter the inner layer  20  as shown in  FIG. 2 , further diffusion of the hydrogen molecules  26  is prevented by the barrier layer  16 . Also the first adhesive layer  22  and the second adhesive layer  24 , as well as the inside resin layer  14  and the outside resin layer  18 , can act to block the permeation (diffusion) of the hydrogen gas. Therefore, diffusion of the hydrogen gas into the atmosphere is prevented. 
     In the above structure, the total of the thickness x 1  of the inner layer  20  and the thickness x 2  of the first adhesive layer  22 , i.e. the thickness X of the inside resin layer  14  containing the inner layer  20  and the first adhesive layer  22 , is more than 0 and equal to or less than a predetermined value. A method for obtaining the predetermined value will be described below. 
     In this method, in a case where the hydrogen storage container  10  is filled with the hydrogen gas and is then depressurized until a crack is generated in the inner layer  20 , t c  represents a time from the depressurization start to the crack generation, and L c  represents a movement distance of the hydrogen molecule  26  in the inner layer  20  within the time t c . L c  and t c  satisfy the following formula (2): 
       L c =k√{square root over (Dt c )}  (2)
 
     In the formula (2), k is a proportionality constant, and D is a diffusion coefficient of the material measured at 50° C. by a differential-pressure method. The differential-pressure method is well known, and therefore detailed explanation thereof is omitted. 
     In a case where the thickness X is larger than the movement distance L c , even after the hydrogen is supplied from the hydrogen storage container  10  to the anode in order to operate the fuel cell (even after the depressurization of the hydrogen storage container  10  is started), a state in which the hydrogen molecule  26  is introduced into the inner layer  20 , is maintained. In contrast, in a case where the movement distance L c  is equal to or less than the thickness X, after the hydrogen storage container  10  is depressurized, the hydrogen molecule  26  can be removed from the inner layer  20  as shown in  FIG. 3 . The hydrogen molecule  26  can be moved by a distance equal to or larger than the thickness X in this case. Therefore, the thickness X is controlled to a value more than 0 and equal to or less than L c . Thus, X and L c  satisfy the following inequality (3): 
       0&lt;X≦L c   (3)
 
     In the formula (2), the proportionality constant k is a constant value, and t c  is not changed or is changed only negligibly. Thus, both of k and t c  in the formula (2) can be considered as constant values. Then, a constant K is defined as a product of k and t c   1/2  as shown in the following formula (4): 
       K=k√{square root over (t c )}  (4)
 
     The following formula (5) is derived from the formulae (2) and (4): 
       L c =K√{square root over (D)}  (5)
 
     In the inside resin layer  14 , the thickness x 2  of the first adhesive layer  22  is negligibly smaller than the thickness x 1  of the inner layer  20 . Thus, x 1  and x 2  satisfy the condition of x 1 &gt;&gt;x 2 . Therefore, the thickness x 1  of the inner layer  20  may be regarded as the thickness X of the inside resin layer  14  as described hereinafter. 
     Next, for example, L c  is determined using a test specimen  30  shown in  FIG. 4 . The test specimen  30  is composed of the HDPE resin, and has a thickness X′ of 7 mm. 
     The test specimen  30  is left at 50° C. in a pressurized hydrogen atmosphere for a predetermined time. The exposed surfaces (end surfaces) of the test specimen  30  are pressed by the pressurized hydrogen gas. Then, the pressure of the atmosphere is reduced to a predetermined pressure. After this pressurized hydrogen treatment, the test specimen  30  is cut in the thickness direction. The cut surface is shown in  FIG. 4 . 
     In  FIG. 4 , cracks  32  are generated in a region enclosed by virtual lines M 1  and M 2 . As shown in  FIG. 4 , the cracks  32  are generated in the internal region of the test specimen  30 , and are not generated in the vicinity of the end surfaces. The distances m 1  and m 2  between the end surfaces and the virtual lines M 1  and M 2  are both 1.5 mm. Thus, each of the virtual lines M 1  and M 2  (the region with the cracks  32  generated) is separated a distance of 1.5 mm away from the end surface. 
     Consequently, the distance m 1 , m 2  from the end surface to the virtual line M 1 , M 2 , i.e. the thickness of a region with no cracks  32  generated, corresponds to the movement distance L c  of the hydrogen molecule  26 . Thus, the movement distance L c  is determined to be 1.5 mm. 
     The diffusion coefficient D of the HDPE, measured at 50° C. by the differential-pressure method, is 4.62×10 −10  m/second. In this case, the constant K is calculated to be 70 by plugging in 4.62×10 −10  m/second for D and 1.5 mm for L c  in the formula (5). The thickness X of the inside resin layer  14  is set to be equal to or less than L c  as described above, and thus may be 70×D 1/2  or less. Therefore, based on the above (3) and (5), the thickness X and the diffusion coefficient D of the inside resin layer  14  satisfy the following inequality (6): 
       0&lt;X≦70√{square root over (D)}  (6)
 
     Next, the relation between the thickness X of the inside resin layer  14  and the thickness Y of the barrier layer  16  will be studied below. In this embodiment, the barrier layer  16  contains the EVOH as described above. In this case, when the barrier layer  16  has a water absorption of 2% by weight or more, it is difficult to ensure the barrier ability. The EVOH has a density of about 1.0 g/cm 3 . Therefore, when the barrier layer  16  with the thickness Y [mm] has a water absorption of 2% by weight, the water vapor permeation amount is 0.002Y [g/cm 2 ]. 
     In a test specimen that contains the inner layer and the first adhesive layer  22  and has the total thickness of 0.1 cm, a water vapor permeation rate was measured at 85°. The measured water vapor permeation rate was 1.5×10 −5  [g/cm 2 ·24 h]. Thus, when water vapor permeates through the inside resin layer  14  having the thickness of X mm in a 24-hour period, the water vapor permeation amount is 1.5×10 −5 /X [g/cm 2 ]. 
     In order to ensure the barrier ability of the barrier layer, the amount of the water vapor permeating through the inside resin layer  14  needs to be less than a water vapor permeation amount at which the water absorption of the barrier layer  16  is 2% by weight. Thus, it is necessary to satisfy the condition of the following inequality (7): 
       1.5×10 −5   /X&lt; 0.002 Y   (7)
 
     This formula can be simplified in terms of X to obtain the following inequality (8): 
         X &gt;(75/ Y )×10 −4   (8)
 
     Based on the (6) and (8), the thickness X of the inside resin layer  14  is controlled in view of satisfying the following inequality (1): 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         75 
                         Y 
                       
                       ) 
                     
                     × 
                     
                       10 
                       
                         - 
                         4 
                       
                     
                   
                   &lt; 
                   X 
                   ≦ 
                   
                     70 
                      
                     
                       D 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     In the case of controlling the thickness X of the inside resin layer  14  (the thickness x 1  of the inner layer  20 ) within this range, when the hydrogen storage container is depressurized, the hydrogen molecules  26  that have entered the inner layer  20  can be diffused in the inner layer  20  and discharged to the internal space of the hydrogen storage container  10 . Thus, the hydrogen molecules  26  can be returned into the internal space of the hydrogen storage container  10 . Consequently, the state, in which the hydrogen molecules  26  are introduced into the inner layer  20 , can be eliminated, whereby the inner layer can be prevented from being deteriorated due to the hydrogen molecules  26 . 
     The inner layer  20  may contain an LDPE resin. The LDPE has a diffusion coefficient D of 4.45×10 −10  m/second, measured at 50° C. by the differential-pressure method. In this case, the movement distance L c  of the hydrogen molecule  26  is calculated to be 1.47 mm by plugging in 4.45×10 −10  m/second for D and the above obtained value 70 for K in the formula (5). Thus, in the case where the inner layer  20  contains the LDPE resin, the thickness X of the inside resin layer  14  (the thickness x 1  of the inner layer  20 ) may be controlled to be 1.47 mm or less. Consequently, in the same manner as above, the state, in which the hydrogen molecules  26  are introduced into the inner layer  20 , can be eliminated when the hydrogen storage container  10  is depressurized. Thus, also in this case, the inner layer  20  can be prevented from being deteriorated due to the hydrogen molecules  26 . 
     The thickness X of the inside resin layer  14  can be controlled to 1.4 mm or less. In this case, the thickness of the hydrogen storage container  10  can be further reduced. 
     In any case, since the thicknesses X and Y are controlled to satisfy the condition of the inequality (1), the water vapor (moisture) can be prevented from permeating through the inner layer  20  and reaching the barrier layer  16 . Therefore, lowering of the barrier ability of the barrier layer  16  is avoided, whereby leakage of the hydrogen gas from the hydrogen storage container  10  can be prevented. 
     The present invention is not particularly limited to the above-described embodiments, and various changes and modifications may be made therein without departing from the scope of the invention. 
     For example, the outside resin layer  18  may be covered with a carbon fiber or the like to form a shell structure. 
     One or both of the first adhesive layer  22  and the second adhesive layer  24  may be omitted. In the case of not using the first adhesive layer  22 , the inner layer  20  may be used as the inside resin layer, and its thickness x 1  may be controlled to a value of more than 0 and not more than 70×D 1/2 . 
     EXAMPLES 
     Multilayer test specimens were each produced by stacking a first layer of HDPE resin, a first adhesive layer of LDPE resin, a barrier layer of EVOH resin, a second adhesive layer of LDPE resin, and a second layer of HDPE resin in this order. The multilayer test specimens were different from each other in total thickness of the first layer and the first adhesive layer. The total thicknesses of the first layer and the first adhesive layer were set to be 0.3 mm, 1 mm, 3 mm, 4 mm, and 5 mm respectively. 
     Each of the multilayer test specimens was left in a pressurized hydrogen atmosphere at 50° C. for a predetermined time. In this treatment, the exposed surfaces of the first and second layers were pressed by the pressurized hydrogen gas. Then, the hydrogen gas pressure was reduced to a predetermined pressure, and each specimen was cut in the thickness direction. 
     Thus-obtained exposed cut surface of the first layer was evaluated with respect to whether a crack was generated or not. The results are shown in Table 1 in relation to the total thicknesses of the first layer and the first adhesive layer. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Total thickness of first layer and 
                   
               
               
                 first adhesive layer [mm] 
                 Crack 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 0.3 
                 Not generated 
               
               
                 1 
                 Not generated 
               
               
                 3 
                 Generated 
               
               
                 4 
                 Generated 
               
               
                 5 
                 Generated 
               
               
                   
               
            
           
         
       
     
     As shown in Table 1, the crack was not generated when the total thickness of the first layer and the first adhesive layer was 1 mm or less, whereas the crack was generated when the total thickness was 3 mm or more. As is clear from the test results of the test specimens and a sample consisting of the HDPE resin, the cracking in the inside resin layer of the hydrogen storage container can be prevented by controlling the thickness X of the inside resin layer, which corresponds to the total thickness of the first layer and the first adhesive layer, to be 1.5 mm or less.