Title of Invention

A COOLANT FLOW FIELD PLATE FOR POLYMER ELECTROLYTE MEMBRANE FUEL CELL

Abstract

A coolant flow field plate for polymer electrolyte membrane fuel cell said plate being made of an electrically conducting material and formed in a substantially planar surface, said plate comprising a coolant flow field formed on said planar surface;at least one supplier passage included in the flow field for surface; supplying coolant to said fuel cell and at least one collector passage included in the flow field for collecting hot coolant from said fuel cell during operation thereof; a plurali substantially symmetric flow sectors included in the flow field,each of the said sectors having a separate inlet communicating with the supplier passage and a separate outlet communicating with the collector passage; at least one set of substantially parellel flow channels being formed in the said planar surface,each of said flow sectors being partitioned to demarcraet the said flow channels in to a plurality of sets of coolant flow channels.

Full Text

Summary BACKGROUND OF THE INVENTION 1 .Field of the Invention The present invention relates generally to fuel cell and, more particularly, to a coolant flow field plate for use in a PEMFC. 2. Related Art Fuel cells are electrochemical energy conversion devices, defined as electrochemical cells that can continuously transform the chemical energy of a fuel and oxidant to electrical energy by an isothermal process invoMng an essentially invariant electrode- electrolyte system. The basic components of a typical fuel cell are anodes, cathodes and electrolyte. The electrodes act as reaction sites or catalysts where the electrochemical transformation of the fuel or oxidant occurs, producing d-c power and water. Classification of fuel cells is based upon type of elecfarolyte used. PEMFC uses solid polymer or more specifically, perfluorinated sulphonic acid polymer electrolyte membrane as electrolyte that is sandwiched between anode and cathode. Both anode and cathode contain platinum supported on carbon as electrocatalyst. In a hydrogen-fueled PEM fuel cell, hydrogen is consumed at the anode, producing protons and electrons as shown in equation [1]. The protons are conducted through tiie PEM membrane to the cathode, while the electrons travel firom fuel cell anode through an external circuit load to the cathode. At the cathode, oxygen reacts with proton and eletron at the electi-ocatelyst sites to yield water as shown in equation [2]. Thus, the overall fuel cell reaction becomes the combination of equations [1] and |2] as showm in equation [3]. Anode Reaction: 2H2 -►2hr + 4e" [1] Catttode Reaction: 02 + 4l-r+4e" -►2H2O [2] Overall Reaction: 2H2+O2 -► 2 H2O+Elech"ic Power [3J A fuel cell steck is the arrangement of single cells connected elecbi 2 anode and cathode respectivety and provide for removal of water formed during operation of the cell. The overall fuel cell reaction as described in equation (3) is exothermic in nature. The heat of dissociation in the electrochemical reaction, ohmic resistance, and various polarization losses define the amount of heat generation in fuel cell. The problem of thermal management of fuel cell stack is well known almost since its inception. Efficient heat removal and operating the stack at Isothermal condition is important from the viewpoint of thermal stability and cooling scheme. Numerous heart: transfar analysis and models have been devek>ped for various types of fuel ceils to determine the temperature profile, power density distribution etc. Heat transfer from a fuel cell stack primarily depends on flow geometry of the racant gases, type of coolant and its flow geometry, themiat conductivity of the stack materials etc. The amount olf heat generation in a fuel cell varies with current density and heat dissipation occurs in three directions: i) in a plate heat dissipates in x and y direction and ii) in the z direction across the stack. The coolant flow field and the amount of coolant describe the temperature profile in a plate whereas the number of coolant plate in a stack along with the plate temperature profile dicate the temperature gradient across the stack. Accordingly, at the time of the present invention, fuel cell designers continued to search for an improved coolant flow field design so as to minimise the temperature gradient across the coolant plate and the fuel cell stack vis - a - vis to optimise the perfbnnance of PEM fuel cells. Description of the Prior Art A typical prior art coolant flow plate includes in a major surface thereof a single continuous open- faced coolant flow channel having a coolant inlet at one end and a coolant outtet at the other end. The channel extends across the major surface between a coolant feed inlet and an exhaust outiet. The channel is typically of rectangular shape in cross section and the dimenstons are being 1.0 mm deep and 4.0 mm across the opening. The coolant inlet and outiet are connected to the coolant feed and exhaust respectively. The coolant enters the coolant inital of the serpentine flow channel and exits through the coolant outiet after traversing a major central area of the plate tiirough the plurality of passes fomfied by channel. Such a single serpentine coolant flow field forces the coolant flow to traverse the entire flow fleld area of Vne plate. However, this type of coolant flow fields results in a relatively tow heat ptek-up from the plate. As the coolant entera the coolant flow field, it picks-up heat and thereby, the temperature of the coolant increases. Thus, the temperature gradient between the plate and the coolant decreases continuously as the coolant traverses along the serpentine path of the 3 / novel features of the present invention are not restricted to a particular geometric shape. It should further be understood that flow field 16 need not be limited to centrally located in surface 12 as shown in Fig. 1 to realise the advantages of the present invention. The outer periphery of the flow field 16 is fonned by borderiands 20. 22, 24 and 26 in conjunction with the active area of the electrode of fuel cell. The outer peripheries 20-26 may correspond generally in size to that of the gaskets 28. Flow field 16 is effective in transporting coolant during operation of fuel cell. It should be understood that within the present context the term coolant refers to a coolant such as water, air, glycol-water mixture or any kind of fluid that has cooling property. The flow field plate 10 includes a network of inlet flow passages 32 to supply coolant from a common inlet source 30 to flow field 16 during operation of fuel cell. Further, the plate 10 includes a network of outlet flow passages 42 to discharge the hot coolant from the flow field through a common discharge port 40 during operation of fuel cell. The illustrative embodiment. Fig. 1. includes two inlet flow passages 34A and 34B communicating with the common coolant inlet 30 and two outlet flow passages 44A and 44B communicating with the common discharge port 40.^-^"^ Flow fiekj 16 includes a plurality of substantially symmetric flow sectors 52, with the illustrative emt}odiment including two flow sectors 52, designated as 52A and 52B respectively. A substantially straight interior borderland 50 forms the boundaries of the flow sectors 52A and 528. Flow sectors 52A and 528 are substantially symmetric in size and shape and have substantially same average path length that the coolant traverses during operation of fuel cell. The additional features & functions of flow field 16 will be discussed and illustrated in Fig. 2 with particular reference to flow sector 52A. These features and functions are equally applicable to flow sector 528. Each of the flow sectors 52A and 528 includes a plurality of substantially symmetiic parallel flow channels 62 formed in the generally central portion 14 of the substantially planar surface 12 of flow field plate 10. Fig.2 shows the section view of channels 62 that are open-faced or open top. Accordingly, the plurality of substantially parallel lands 64 are formed with the substantially parallel flow channels 62. As shown in Fig.2, each channel 62 has a width 66 and a depth 68 ttiat are substantially constant tiiroughout tiie channel length. Each channel 62 has a substantially trapezoidal cross section. However, the channel cross section may have other shapes like square, rectangular eto. with rounded corner. Each land has a width 70 and a top surface 72, which supports ttie backside of tiie bipolar plate of fuel ceil and thus, maintains tiie electrical continuity of fuel cell. The widtiis 66 and 70 of tiie channel 62 and land 64 respectively, are so chosen to maximise the contact area and minimise the pressure drop while the coolant fiows through tiie channel 62. 5 The inventors have determined that the width 66 and depth 68 of each flow channel 62 preferably ranges from 1mm to 4mm and 1mm to 3mm respectively. In the illustrative embodiment, the width 66 and depth 68 of channel 62 are kept at 4 mm and 1 mm respectively for the flow field 16 size of about 400 cm2. It should be understood that these two parameters might vary as a function of the overall size of flow field 16. Similarly, the inventors have decided the widthi 70 of each land 64 may range from about 1mm to 4mm and preferably is 4 mm for the illustrative embodiment. Each of the flow sectors 52A and 52B is partitioned to sutxJivide the plurality of substantially parallel flow channel 62 into a plurality of sets 74 of the flow channels 62, with each set 74 includes a plurality of the flow channels 62. According to the present embodiment illustrated in Fig. 1. each flow sedors 52A and 52B includes three sets 74 of flow channels 62 which are numt>ered 74A, 74B and 74C and 74D, 74E and 74F respectively. Each of the sets 74 includes five of the flow channels 62 as illustrated in Fig. 1. However, each set 74 of flow channels 62 may have different numbers of the flow channels 62. The specific choice of number of flow sectors 52, the number of sets of flow channels 74 per flow sector 52 and number of flow channels 62 per set 74 depend on the shape and size of the flow field 16 and several other design parameters such as maximum allowable pressure drop, coolant flow, rate, etc. that vary fi"om application to application. For optimum design, the inventors have determined two sectors 52A and 528, three sets 74 per sector 52 and flve flow channels 62 per set 74 for a rectangular shaped flow field 16 having 400-cm2 area. The three sets 74A-C are disposed in serial flow relationship with one another while the five flow channels 62 of each set 74 are In parallel flow relationship with one another. The coolant is supplied from a common inlet of a fuel cell stack and gets distributed to reach at the Individual coolant plate inlet 30. From the common inlet 30 the coolant is bifurcated through inlet channel passages 34A and 348 to enter the respective flow sectors 52A and 528 as shown by arrowheads 76A and 766. After the coolant enters the first set 74A of flow sector 52A, it flows through each of the substentially parallel channels 62 to enter the second set 748 and so on. Thus, before the coolant reaches tiie outlet channel passage 44A, it undergoes three passes to traverse the entire area of the corresponding sector 52A. The coolant follows the same flow pattern for the other sector 528 of the flow field 16 and considered to be within the scope of the present invention. The average path length of the coolant throuh each flow sectors 52A and 528 is substantially equal, thereby exposing each portion of the flow field plate 10 to the same flow conditions. Hence, the present flow field 16 provides an even distribution of coolant across the entire area and maintains equal pressure drop with minimum temperature gradient across the flow field plate 10. SUMMARY OF THE PRESENT INVENTION The present invention is related to a coolant flow field design for use In a PEM fuel cell using a gaseous fuel, more specifically hydrogen, and a gaseous oxidant, more speclficaily oxygen or air. The coolant flow field, according to a preferred embodiment of the present invention, comprises a substantially planar surface having a generally central portion and a coolant flow field in the generally central portion. The flow field is used to transport coolant during operation of the fuel cell. The plate further includes at least one inlet flow passage for supplying the coolant to the flow field and at least one outlet flow passage to collect the discharges from the flow field during the operation of the fuel cell. In other preferred embodiment, the coolant flow field plate may include the following additional structural features and functions. The flow field includes two substantially symmetric separate flow sectors having separate inlet and outiet flow passage communicating with common coolant inlet and outiet respectively. Each flow sector comprises a plurality of sut)stantiany parallel flow channels formed in tiie substantially planar plate surface. Each sector is partitioned to subdivide the plurality of coolant flow channels into three sets of flow channels disposed in serial flow relationship with one another. Each set of flow channels includes a plurality of flow channels, which are disposed in parallel flow relationship with one another. Consequentiy, the coolant traverses each sector in a plurality of passes and tiie coolant flows through a plurality of channels during each pass. Each of the plurality of substantially parallel flow channels comprises an open faced flow channel, and the plurality of flow channels defines a plurality of lands, witii the flow channels and tiie lands being interdigitated. Each of the flow channels has a sutstantially square cross section and includes a length and a depth, with the depth being substantially constant throughout the length. Each of the lands include a width, and a ratio of the width of the channels to the width of the lands ranges about 1.0 to 1 .S.The flow field plate is made of a non-porous electrically conductive graphite material. The main advantage of the coolant flow field plate of the present invention is the ability to pick up heat from the coolant plate uniformly and thus, eliminate formation of any hot spot and maintain minimum temperature gradient across the plate with low pressure drop. COMPARATIVE ANALYSIS For comparative analysis, the surface of the coolant flow field 16,as illustrated in Rg.1 is divided into sixty-four nodes. Fig.3, the prior art coolant flow field plate, illustrates a rectangular shaped coolant flow field 82 having a single serperttine coolant flow channel 84 in a plurality of passes as 7 coolant flow field plate. The heat pick-up becomes less and less as the coolant flows from inlet to outlet along the coolant flow field. The non-unifbmi heat pick-up from the major surface of the plate results in non-uniform temperature distribution of the said plate and may lead to the fonnation of hot spot zone. The ionic conductivity of the polymer membrane depends upon water ccHitent of the polymer membrane and formation of such hc4 sp(H zone may attenuate the ionic conductivity of the polymer membrane and hence, the performance of fuel cell. Additionally, such a single serpentine channel results in a relativeiy high coolant flow path length which creates a substantial pressure drop and thus, requires additional parasitic power to pump the coolant into the said coolant flow fiekl. DRAWING DESCRIPTION BRIEF DESCRIPTION OF THE DRAWINOS The advantages of the present invention of the coolant flow field can be elaborated from the subsequent detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings, wherein: Fig. 1 is the plan view of a coolant flow field plate according to the present inventkHi; Fig. 2 is an and section of Fig. 1 showing the groove on an enlarged scale; Fig. 3 is the plan view of a coolant flow field plate according to the prior art; Fig. 4 is the plan view of a coolant flow field plate according to the prior art; Detailed Description Referring to the drawings (Figs. 1-3), all the elements of each figure are numbered and same numerals have been used for the similar elements throughout. In Fig. 1, the specific construction and function of the coolant flow field plate 10 of the present invention are discussed in greater detail. The flow fleld plate is 10 is made of an electrically conductive material and is preferably made from non-porous graphite blocks. However, other conventional electrically conductive materials like electrically-conductive polymers, metals, etc. may also be used to fat>ricate coolant flow fiekl plate 10. Plate 10 includes a substantially planar surface 12, having a generally central portion 14. and a flow fleld 16 formed in the generally central portion 14 of surface 12. In the illustrated embodiment, tiie flow fleld platelO and the flow field 16 are shown to have a generally square and rectangular shapes \ as viewed In plan respectively. However, It should be understood that the 4 discussed earlier. The surface of the coolant flow field 82 has also been divided into sixty-four nodes for comparative analysis. These nodes are assumed to be connected by rods of thermal resistance. Heat transfer from coolant flow field of fuel cell stack primarily depends on flow geometry of the reactant gases, type of coolant and Its flow geometry, thermal conductivity of the coolant plate material etc. The amount of heat generation varies with current density and heat dissipates in x and y direction of the coolant flow field plate. For a given coolant flow field design, a two - dimensional heat transfer analysis gives the temperature profile of the coolant flow field plate. For this purpose, the inventors applied a general equation [4] as given below. Qi+S(T,-T,)/R„ [A] Wherein: • Qi " heat generated in the I th node; • T1 = Temperature of the I th node; • T1 = Temperature of the J th node; • R1 = Resistance due to I and J th nodes; Assuming that at steady state the net heat in at any node is zero, the temperature of every node has been computed. Due to symmetry the coolant plate 10 can be subdivided into equal four parts. As the node temperatures of the opposite parts are mirror image to each other, it is sufficient if the temperatures of any one part i.e. sixteen nodes (64 x 1/4) are calculated. For the present embodiment, the temperatures of one part of the coolant flow field plate 10 as shown In Flg.1 are given In Tablol. The same for the prior art coolant flow field plate Is shown in Table 2. As can be seen from Flg.1, the central portion has the maximum temperature and decreases towards the periphery of the plate. For a current density of 0.3A / cm 2 the calculated temperature gradient for the present invention Is found to be around 4° C whereas the same for the prior art coolant flow field plate is 12 C. The results of the comparative analysis shown In Table 1 and 2 Indicate that th© novel design of coolant flow field 10 of the present invention exhibits a substantially lower temperature gradient as comoared to the single serpentine cooiant flow field 82. The coolant refers to a cooling medium such as water, air, glycol-water mixture or any other fluid which has cooling properties. In the coolant flow field plate for polymer electrolyte membrane fuel cell, according to this invention, said plate is made of an electrically conducting material and formed in a substantially planar surface, said plate comprising a coolant flow field formed on said planar surface; at least one supplier passage included in the flow field for supplying coolant to said fuel cell and at least one collector passage included m the flow field for collecting hot coolant from said fuel cell during operation thereof; a plurality of substantially symmetric flow sectors included in the flow field, each of the said flow sectors having a separate inlet communicating with the supplier passage and a separate outlet communicating with the collector passage; at least one set of substantially parallel flow channels comprised in each flow sector, said flow channels being formed in the said planar surface, each of said flow sectors bemg partitioned to demarcate the said flow channels into a plurality of sets of coolant flow channels. We Claim 1. A coolant flow field plate for polymer electrolyte membrane fuel cell, said plate being . . . made of an electrically conducting material and formed in a substantially planar surface, said plate comprising a coolant flow field formed on said planar surface; at least one supplier passage included in the flow field for supplying coolant to said fuel cell and at least one collector passage included in the flow field for collecting hot coolant from said fuel cell during operation thereof; a plurality of substantially symmetric flow sectors included in the flow field, each of said flow sectors having a separate inlet communicating with the supplier passage and and a separate outlet communicating with the collector passage; at least one set of substantially parallel flow channels comprised in each flow sector, said flow channels being formed in the said planar surface, each of said flow sectors being partitioned to demarcate the said flow channels into a plurality of sets of coolant flow channels. I" 2. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in Claim 1 wherein the flow channels of each set are disposed in series flow relationship with one another. 3. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in Claim 1 or Claim 2 wherein the flow channels of each set are disposed in parallel flow relationship with one another. 4. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in. any one of the preceding Claims wherein the sets in each of the said sectors are equal in number. 5. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein five flow channels are provided in each set of a flow sector. 6. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein each flow channel is open at the top, the depth and width of each channel being substantially constant throughout the length thereof. 7. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein said channels form a plurality of substantially parallel lands. 8. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein the depth of each land is constant throughout the length thereof. 9. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein the ratio of the width of a channel to that of a land ranges from 1.0 to 1.55. 10. A coolant flow field plate for polymer electrolyte membrane fuel cell as claimed in any one of the preceding Claims wherein the said plate is made of a non-porous material. 11. A coolant flow field plate for polymer electrolyte membrane fuel cell, substantially as herein described with reference to, and as illustrated in, the drawings. Dated this the / day of /fL 1999

Documents:

0545-mas-1999 abstract.jpg

0545-mas-1999 abstract.pdf

0545-mas-1999 claims-duplicate.pdf

0545-mas-1999 claims.pdf

0545-mas-1999 correspondence-others.pdf

0545-mas-1999 correspondence-po.pdf

0545-mas-1999 description (complete)-duplicate.pdf

0545-mas-1999 description (complete).pdf

0545-mas-1999 drawings.pdf

0545-mas-1999 form-1.pdf

0545-mas-1999 form-19.pdf

0545-mas-1999 form-26.pdf