Tensioning band with compensation structure has a corrugated profile

Abstract

The invention relates to tensioning band (10) for strapping fuel cell stack (20), wherein the fuel cell stack (20) is configured to be subjected to predetermined cyclically varying dimensions and wherein the tensioning band includes two end portions (12a, 12b) and an elastic compensation structure (14a, 14b) capable of compensating for the predetermined length variations of the tensioning band (10) corresponding to the cyclically varying dimensions of the object, wherein, in a tightened configuration of the tensioning band (10), at least a centre portion the compensation structure (14a,15 14b) has a periodic, corrugated profile with an amplitude A and period L. It is proposed that, in a first region around a maximum and a minimum turning point of the corrugated profile, a curvature of the profile is at least 5% smaller than the curvature of a sine profile with the same amplitude A and period L at its maximum or minimum turning points.

Description

[0001] Tensioning band with compensation structure has a corrugated profile

[0002] The invention relates to a tensioning band for fuel cell stacks according to claim 1 , to a fuel cell stack including a tensioning band and to a method for assembling such a fuel cell stack .

[0003] Fuel cells are used to provide electrical energy through an electrochemical reaction . To increase the usable power, several fuel cells can be connected in series to form a fuel cell stack . Each of the fuel cells comprises a membrane electrode assembly with an anode , a cathode and a proton-conducting membrane separating the anode from the cathode , which is coated with a catalyst to promote the electrochemical reaction . Furthermore , in a fuel cell stack, at least the central fuel cells on both sides of the membrane are provided with bipolar plates for supplying the reactants and, if necessary, a coolant . In addition, gas diffusion layers are used to distribute the reactants supplied in the bipolar plates as evenly as possible over the entire surface of the catalyst-coated membrane . Since the open-circuit voltage of a fuel cell is typically about 1 volt (V) and the maximum permissible voltage in the high-voltage system of the fuel cell stack of the fuel cell device is 330 V, for example , 330 membrane electrode assemblies , 228 bipolar plates and two end plates can form the fuel cell stack .

[0004] This plurality of fuel cells combined in a fuel cell stack is generally pressed with the help of tension elements with a force in the range of several tons in order to achieve sufficient contact pressure on the catalyst-coated membrane to reduce ohmic losses and to avoid leaks in the inserted seals due to the high compression . The internal resistance of the compressed fuel cells depends on the compression force , which has an optimum value at which the internal resistance is at its lowest .

[0005] However, it should be noted that forces occur during the operation of the fuel cell stack that can lead to an increase or reduction of the pressing force . The increase in pressing force is caused by a thermal expansion of the components used, by the pressure used for the supply and distribution of the reactants and by a swelling of the membrane used when it is hydrated .

[0006] A reduction in the compression force can occur due to negative thermal expansion at low or decreasing temperatures or due to the settling behavior of the gas diffusion layers , which occurs after the first operation of the fuel cell stack and increases with increasing usage time and thus age of the fuel cell stack .

[0007] The document EP 1 870 952 A2 discloses a fuel cell stack according to the preamble of claim 1 with a tensioning band having an elastic compensation structure with a wavy or zig-zag profile , wherein the band ends are attached to a bottom end plate of the fuel cell stack or connected on a side of the fuel cell stack . The document DE 103 43 766 Al discloses a further example of a wavy or corrugated compensation structure .

[0008] Moreover , the assembly of fuel stacks may be challenging because the tensioning elements are to be pre-tensioned in an appropriate and even manner, preferably in a cost-saving manner and avoiding complex mechanical elements . The even and controlled pre-tensioning of the tensioning elements ultimately leads to efficient , high-quality, and long-lifetime fuel cell stacks .

[0009] The invention is based on the problem providing a tensioning band with a structure increasing the elastic range and cycle lifetime , an improved fuel cell stack, and a method for assembling a fuel cell stack .

[0010] According to a first aspect of the invention, the problem is solved by a device having the features of claim 1 . Advantageous embodiments of the invention are defined in the dependent claims .

[0011] The inventors propose a tensioning band for strapping fuel cell stack, wherein the fuel cell stack is configured to be subj ected to predetermined cyclically varying dimensions and wherein the tensioning band includes two end portions and an elastic compensation structure capable of compensating for the predetermined length variations of the tensioning band corresponding to the cyclically varying dimensions of the obj ect , wherein , in a tightened configuration of the tensioning band, at least a centre portion the compensation structure has a periodic , corrugated profile with an amplitude A and period L .

[0012] The inventors propose that , in a first region around a maximum and a minimum turning point of the corrugated profile , a curvature of the profile is at least 5% , alternatively at least 10% or at least 20% , smaller than the curvature of a sine profile with the same amplitude A and period length L at its maximum or minimum turning points .

[0013] The region with the higher curvature should preferably amount to a substantial fraction of the period length L, in particular at least 5% of the period length L, preferably at least 10% of the

[0014] The tensioning band according to the invention is preferably a metal band with a pre-shaped corrugated profile and other structures such as gripping or engaging structures or end loops .

[0015] By using such a profile , the local curvature of the band near the maxima and minima varies less than in known bands with sinusoidal or zig-zag profiles . Considering that the material strengthening of the spring steel material depends on the bending radius or bending curvature , this shape ensures that the bending strength is more uniform, whereas profiles with non- uniform curvature would result in non-uniform bending strength, wherein portions with reduced strength would exhibit inelastic behaviour and / or fatigue at lower threshold tension forces and / or at lower cycle numbers .

[0016] In the context of the invention the expression "curvature" refers to the inverse curvature radius . In the context of the invention, the expression "cyclically" may refer to periodic temperature changes resulting in cyclic thermal expansion / shrinking cycles or other cycles , such as cyclic filling and emptying of a high-pressure tank resulting in cyclic expansion of the diameter of the tank .

[0017] Further, in the context of the invention, a profile is considered to be "essentially" composed of regions having certain curvature values if the latter is true besides of small sections interconnecting, wherein the total length of such sections is preferably less than 10% of the total length of the corrugated portion, more preferably less than 20% of the total length of the corrugated portion when measured along the shape of the band .

[0018] The inventors further propose that , in at least one second region between adj acent maximum and minimum turning points of the corrugated profile , the curvature is larger than the curvature at the maximum and minimum turning points of the corrugated profile .

[0019] In a preferred embodiment of the invention, the corrugated profile can be represented as wherein L is the period length of the periodic , corrugated profile , H ( x ) is the height above the average height of the profile and 0 . 05 1 . The profile of the tensioning band according to the invention is not perfectly sinusoidal but includes non-negligible higher harmonics , in particular the first harmonic ai , which contributes to the total elasticity of the band specifically in the range of tensioning forces occurring in practical use in fuel cells .

[0020] It is further proposed that end portions of the compensation structure are formed so as to include an outermost portion shaped as a circular arc extending over an angle of approximately 30 ° , followed by an adj acent portion with inversed curvature extending over an angle of approximately 60 ° . In this context , "approximately" is to be understood as indicating a tolerance margin of ±10 ° .

[0021] It is further proposed that the tensioning band includes two compensation structures arranged in positions and having configurations mirror-symmetrical to each other .

[0022] According to a further aspect of the invention, the inventors propose a fuel cell stack including a top end plate and a bottom end plate , a stack with a plurality of bipolar plates sandwiched between the end plates , and at least one tensioning band, wherein the tensioning band includes two end portions and at least elastic compensation structure , wherein the tensioning band extends around the entire fuel cell stack including the top end plate and the bottom end plate .

[0023] It is proposed that the elastic compensation structures are provided in lateral parts of the tensioning band extending in the stacking direction and that the end portions are arranged and connected at or above the top end plate and / or below the bottom of the fuel cell stack . By closing the tensioning band on the top of the fuel cell stack, the force will be evenly distributed over the sides due to the symmetrical tensioning and friction effects . The same applies for closure below the bottom end plate .

[0024] According to a further aspect of the invention, the invention proposes a fuel cell stack including a top end plate and a bottom end plate , a stack with a plurality of bipolar plates sandwiched between the end plates , and at least one tensioning band, wherein the tensioning band includes two end portions and at least elastic compensation structure .

[0025] It is proposed that the elastic compensation structures extend in the stacking direction and that the end portions connected to both the top end plate and the bottom end plate . Preferably, at least one the end portions includes a gripping structure formed as a hole configured for engagement of a hook provided the top end plate and / or the bottom end plate . Like in the case of closing the tensioning band on the top of below the bottom of the fuel cell stack, the provision of multiple tensioning bands and multiple , points of attachment in a symmetrical distribution facilitates an even, symmetrical force distribution .

[0026] In a preferred embodiment of the fuel cell stack or of the tensioning band according to the invention, the tensioning band includes engagement or gripping structures for a tensioning mechanism in top portions of the lateral parts of the tensioning band .

[0027] A cost-saving and reliable assembly can be achieved when the end portions are connected by welding or by pressing .

[0028] Alternatively, the end portions are connected by a tensioning mechanism.

[0029] A further aspect of the invention relates to a method for assembling a fuel cell stack as described above . The method comprising the steps of sandwiching the stack with a plurality of bipolar plates sandwiched between the end plates , pulling the two end portions while exerting pressure onto the top end plate of the fuel cell stack and connecting the end portions of the tensioning band to each other or to the end plates .

[0030] Further features and advantages will be apparent from the following description of the embodiments and figures . The entire description, claims and figures disclose features of the invention in specific embodiments and combinations . The person skilled in the art will also consider the features individually and combine them into further combinations or sub-combinations to adapt the invention, as defined in the claims , to his needs or to specific fields of application .

[0031] The figures illustrate the following : Fig . 1 tensioning band according to a first embodiment of the invention;

[0032] Fig . 2 a detailed profile view of compensation structure in a tensioning band according to the invention;

[0033] Fig . 3 a system including a tensioning band and a fuel cell stack;

[0034] Fig . 4 an illustrative fuel cell stack with leaf springs according to the prior art and two different embodiments of tensioning bands according the invention;

[0035] Fig . 5 a fuel cell stack according to the invention an illustration of the assembly method according to the invention .

[0036] Fig . 1 shows a tensioning band 10 according to a first embodiment of the invention . The tensioning band 10 is designed for strapping a fuel cell stack 20 ( Fig . 3 ) as an obj ect with a force equal to or lower than a yield force Fwy, wherein the obj ect is configured to be subj ected to predetermined cyclically varying dimensions and the tensioning band 10 is made of a spring steel band with a Young' s modulus E , a yield strength Re and a cross-sectional area A . The tensioning band 10 is preshaped in a roughly rectangular shape corresponding to the shape of the fuel cell stack 20 ( cf . Fig . 3 ) and includes two end portions 12a, 12b formed as loops for attaching a closing- and tightening structure and two elastic compensation structures 14a, 14b configured to be arranged on the vertical side faces of the fuel cell stack 20 and being capable of compensating for the predetermined length variations AL of the tensioning band 10 corresponding to the cyclically varying dimensions of the obj ect . The two compensation structures 14a, 14b are arranged in positions and having configurations mirror-symmetrical to each other with regard to a middle point P of the band . The lower part 14c of the tensioning band 10 is flat and configured to pass below the bottom side of the fuel cell stack 20 .

[0037] In both a tightened and a non-tightened configuration of the tensioning band 10 , at least a centre portion the compensation structure 14a, 14b has a periodic, corrugated profile with an amplitude A and period L , wherein the amplitude A and period L obviously depend on the tensioning state . .

[0038] Moreover , in second regions between adj acent maximum and minimum turning points , specifically between each maximum or minimum turning point and the adj acent point of inflection where the curvature becomes zero and then changes its sign, the curvature is larger than the curvature of a sine profile with the same amplitude A and period L at the corresponding points , possibly even larger than the curvature at the maximum and minimum turning points of the corrugated profile .

[0039] In a preferred embodiment of the invention, the corrugated profile can be represented as wherein L is the period length of the periodic , corrugated profile , H ( x ) is the height above the average height of the profile

[0040] Fig . 2 illustrates the profile compensation structure 14a, 14b in a tightened configuration of the tensioning band 10 for the case ai / ao = 0 . 1 and an= 0 for n b 2 . In this configuration, the compensation structure 14a , 14b has a corrugated profile with a period L and amplitude A which is not sinusoidal but rather essentially composed of portions formed as flattened arcs with alternating curvature directions . The local curvature of the profiles , which is represented as the second derivative of a graph describing the profile , is represented likewise along with the local curvature of the sinusoidal profile with the same amplitude and period length .

[0041] According to the invention, in a first region R1 around a maximum and a minimum turning point of the corrugated profile , a curvature of the profile is at least 5 % smaller than the curvature of the sine profile with the same amplitude A and period L at its maximum or minimum turning points . Moreover , in second regions R2 between adj acent maximum and minimum turning points of the corrugated profile , the curvature is larger than the curvature at the maximum and minimum turning points of the corrugated profile .

[0042] Note that , in the context of this description, when comparing curvatures , reference is made to the absolute values of the curvatures , in particular to absolute values of a second derivative of a graph describing the profile , specifically a centre line of the tensioning band in its thickness direction when viewed from the side .

[0043] This shape ensures that the bending strength is uniform, whereas profiles with non-uniform curvature would result in non-uniform bending strength, wherein portions with reduced strength would exhibit inelastic behaviour and / or fatigue at lower threshold tension forces and / or at lower cycle numbers .

[0044] Fig . 3 illustrates a fuel cell stack 20 according to the invention . The fuel cell stack 20 has specifications according to which the height and width varies in a cyclical manner depending on the temperature , wherein the tensioning band 10 is adapted to its purpose by setting the maximum stroke AL of the two compensation structures 14a, 14b , such that the compensation structures 14a, 14b combined accommodate the total variations of the circumference of the fuel cell stack 20 .

[0045] The fuel cell stack 20 includes a top end plate 16a and a bottom end plate 16b, a stack with a plurality of bipolar plates 18 sandwiched between the end plates 16a, 16b , and, in the case of Fig . 3 , three tensioning bands 10 , wherein each of the tensioning bands 10 includes two end portions 12a , 12b and two elastic compensation structures 14a, 14b . The tensioning band 10 extends around the entire fuel cell stack including the top end plate 16a and the bottom end plate 16b .

[0046] The elastic compensation structures 14a, 14b are provided in lateral parts of the tensioning band 10 extending in the stack- ing direction and that the end portions 12a, 12b are arranged and connected at or above the top end plate 16a of the fuel cell stack 20 such that the tensioning band 10 can be closed at the top of the fuel cell stack 20 . In an alternative embodiment of the invention, the tensioning band 10 could be closed below the bottom of the fuel cell stack 20 .

[0047] Fig . 4 illustrates another example of a fuel cell stack 20 according to the invention . For illustrative purposes , Fig . 4 not only illustrates two different examples of tensioning bands 10a , 10b according to the invention but further a tensioning system 24 according to the prior art including leaf springs 24a or disc springs arranged between a top plate 24b and the stack of bipolar plates 18 as compensation structures .

[0048] In top portions of each of the lateral parts of the tensioning bands 10a, 10b , engagement or gripping structures 22a formed as a punched-out hole for a tensioning mechanism are provided .

[0049] The tensioning bands 10a , 10b are further provided with embossments 26 ensuring a lateral offset of the lateral , vertical parts of the tensioning bands 10a, 10b to make sure that a sufficient distance between the valleys of the compensation structures 14a and the bipolar plates 18 is preserved to avoid a short-circuit . The tensioning mechanism is illustrated in Fig . 5 .

[0050] Depending on the circumstances , the end portions 12a , 12b are connected by welding or by pressing and punching pressing .

[0051] Alternatively, the end portions 12a, 12b are connected by a tensioning mechanism 28 as illustrated for one of the tensioning bands in Fig . 3 .

[0052] Fig . 5 illustrates a fuel cell stack 20 according to the invention as well as an assembly method according to the invention . In the embodiment of Fig . 5 , the fuel cell stack 20 includes a top end plate 16a and a bottom end plate 16b , a stack with a plurality of bipolar plates 18 sandwiched between the end plates 16a, 16b , and a plurality of tensioning bands 10a - 10c . Each of the tensioning bands 10a - 10c includes two end portions 12a , 12b and one elastic compensation structure 14a extending in the stacking direction .

[0053] In the embodiment of Fig . 5 , each of the end portions 12a, 12b is connected one of the top end plate 16a and the bottom end plate 16b by a form-fitting engagement . Specifically, in the embodiment of Fig . 5 , the end portions 12a , 12b each include a gripping structure 22c formed as a hole configured for engagement of a hook 26 provided on the top end plate 16a and the bottom end plate 16b, respectively .

[0054] The tensioning bands 10 further include engagement or gripping structures 22a for a tensioning mechanism 28 provided in the upper end portions 12a of each tensioning band 10 .

[0055] The tensioning mechanism 28 is formed as a lifting column with a hook 28a configured to engage with the gripping structures 22a of the tensioning bands 10 . For assembly, the tensioning bands 10 are connected to the bottom end plate 16b with a hook and hole-connection . Then the hook 28a of the tensioning mechanism 28 is engaged with the gripping structure 22a and the tensioning mechanism 28 is operated to pull the upper end portion 12a of the band upward with a pre-tensioning force Fl . Meanwhile , a supporting force F2 oriented in a direction opposite to the pretensioning force Fl is exerted onto the top end plate 16a and the hook 26 of the top end plate is engaged with the gripping structure 22 c (hole ) of the tensioning band 10 .

[0056] While Fig . 5 illustrates only one tensioning mechanism 28 , the skilled person will understand that multiple tensioning mechanisms may be operated in a simultaneous manner for pretensioning multiple tensioning bands 10 , in particular on opposite sides of the fuel cell stack . It is noted that, while the corrugated portions being the elastic compensation structure 14a in tensioning bands 10 according to the invention are illustrated in Figs . 1 and 3 - 5 such that the band 10 is not twisted, i.e. the width direction of the band 10 does not change in the longitudinal direction of the band 10 and amplitudes of the waves are oriented perpendicular to the width direction, the band 10 may be twisted in other, nonillustrated embodiments of the invention, e.g. such that the waves in the elastic compensation structure 14a are rotated by 90° around the longitudinal direction of the tensioning band 10, i.e. that the band 10 is twisted by 90° above and below the compensation structure 14a.

[0057] List of reference numbers

[0058] 10 tensioning band 10a outermost portion

[0059] 10b adj acent portion

[0060] 12a, 12b end portions

[0061] 14a, 14b compensation structures

[0062] 14 c lower part of tensioning band 16a top end plate

[0063] 16b bottom end plate

[0064] 18 bipolar plates

[0065] 20 fuel cell stack

[0066] 22a, 22b engagement or gripping structures 26 hook

[0067] 28 tensioning mechanism

[0068] 28a hook

[0069] P middle point

Claims

Claims1. Tensioning band (10) for strapping fuel cell stack (20) , wherein the fuel cell stack (20) is configured to be subjected to predetermined cyclically varying dimensions and wherein the tensioning band includes two end portions (12a, 12b) and an elastic compensation structure (14a, 14b) capable of compensating for the predetermined length variations of the tensioning band (10) corresponding to the cyclically varying dimensions of the object, wherein, in a tightened configuration of the tensioning band (10) , at least a centre portion the compensation structure (14a, 14b) has a periodic, corrugated profile with an amplitude A and period L, characterized in that, in a first region around a maximum and a minimum turning point of the corrugated profile, a curvature of the profile is at least 5% smaller than the curvature of a sine profile with the same amplitude A and period L at its maximum or minimum turning points .

2. Tensioning band (10) according to claim 1, characterized in that, in at least one second region between adjacent maximum and minimum turning points of the corrugated profile, the curvature is larger than the curvature at the maximum and minimum turning points of the corrugated profile.

3. Tensioning band (10) according to claim 1 or 2, characterized in that the corrugated profile can be represented aswherein L is the period length of the periodic, corrugated profile, H(x) is the height above the average height of the profile and 0.051.

4. Tensioning band (10) according to one of the preceding claims ,characterized in that end portions (12a, 12b) of the compensation structure (14a, 14b) are formed as an outermost portion shaped as a circular arc extending over an angle of approximately 30°, followed by an adjacent portion with inversed curvature extending over an angle of approximately 60° .

5. Tensioning band (10) according to one of the preceding claims , characterized in that the curvature of the profile is at least 10% smaller than the curvature of a sine profile with the same amplitude A and period L at its maximum or minimum turning points .

6. Tensioning band (10) according to one of the preceding claims , characterized in that the curvature of the profile is at least 20% smaller than the curvature of a sine profile with the same amplitude A and period L at its maximum or minimum turning points .

7. Tensioning band (10) according to one of the preceding claims , characterized in that the tensioning band (10) includes two compensation structures (14a, 14b) arranged in positions and having configurations mirror-symmetrical to each other.

8. Fuel stack including a tensioning band (10) according to one of the preceding claims .

9. Fuel cell stack including a top end plate (16a) and a bottom end plate (16b) , a stack with a plurality of bipolar plates (18) sandwiched between the end plates (16a, 16b) , and at least one tensioning band (10) , wherein the tensioning band (10) includes two end portions (12a, 12b) and at least elastic compensation structure (14a, 14b) , wherein the tensioning band (10) extends around the entire fuelcell stack including the top end plate (16a) and the bottom end plate (16b) , characterized in that the elastic compensation structures (14a, 14b) are provided in lateral parts of the tensioning band (10) extending in the stacking direction and that the end portions (12a, 12b) are arranged and connected at or above the top end plate (16a) of the fuel cell stack (20) or below the bottom end plate (16b) .

10. Fuel cell stack including a top end plate (16a) and a bottom end plate (16b) , a stack with a plurality of bipolar plates (18) sandwiched between the end plates (16a, 16b) , and at least one tensioning band (10) , wherein the tensioning band (10) includes two end portions (12a, 12b) and at least elastic compensation structure (14a, 14b) , characterized in that the elastic compensation structures (14a, 14b) extend in the stacking direction and that the end portions (12a, 12b) connected to both the top end plate (16a) and the bottom end plate (16b) .

11. Fuel cell stack according to claim 10, characterized in that at least one the end portions (12a, 12b) includes a gripping structure (22c) formed as a hole configured for engagement of a hook provided the top end plate (16a) and / or the bottom end plate (16b) .

12. Fuel cell stack according to one of claims 9 to 11, characterized in that engagement or gripping structures (22a, 22b) for a tensioning mechanism are provided in top portions of the lateral parts .

13. Fuel cell stack according to claim 9, characterized in that the end portions (12a, 12b) are connected by welding.

14. Fuel cell stack according to claims 9, characterized in that the end portions (12a, 12b) are con-nected by pressing.

15. Fuel cell stack according claims 9, characterized in that the end portions (12a, 12b) are connected by a tensioning mechanism.

16. Fuel cell stack according to one of claims 9 to 15, characterized in that the tensioning band is a tensioning band according to one of claims 1 - 7.

17. Method for assembling a fuel cell stack according to one of claims 9 to 14, the method comprising the steps of sandwiching the stack with a plurality of bipolar plates (18) sandwiched between the end plates (16a, 16b) , pulling the two end portions (12a, 12b) while exerting pressure onto the top end plate of (16a) the fuel cell stack and connecting the end portions (12a, 12b) of the tensioning band (10) to each other or to the end plates (16a, 16b) .

18. Method according to claim 17, characterized by including the step of strapping the tensioning band (10) around the entire fuel cell stack including the top end plate (16a) and the bottom end plate (16b) .

Images