WO2009054658A1

WO2009054658A1 - Apparatus for measuring concentration of liquid fuel using strain gauge and fuel cell including apparatus

Info

Publication number: WO2009054658A1
Application number: PCT/KR2008/006209
Authority: WO
Inventors: Nam Hyuk Kim; Ju Ho Lee; Yun Mi Kim; Dong Il Kim
Original assignee: Dongjin Semichem Co., Ltd.
Priority date: 2007-10-23
Filing date: 2008-10-21
Publication date: 2009-04-30
Also published as: KR20090041172A; TW200933968A; KR100932229B1

Abstract

Provided is an apparatus for measuring the concentration of a liquid fuel and a fuel cell employing the apparatus. The apparatus includes a main body unit comprising a flow channel for the liquid fuel; a strain membrane deformed according to a change in the concentration of the liquid fuel and disposed in the main body unit so that the strain membrane is partly exposed to the flow channel; a strain gauge disposed on the strain membrane and detecting the strain of the strain membrane as a change in electrical resistance of the strain gauge; and a circuit unit electrically connected to the strain gauge and converting the change in electrical resistance of the strain gauge into an output signal.

Description

APPARATUS FOR MEASURING CONCENTRATION OF LIQUID FUEL USING STRAIN GAUGE AND FUEL CELL

INCLUDING APPARATUS

Technical Field

[1] An illustrative embodiment of the present invention relates to an apparatus for measuring the concentration of a liquid fuel and a fuel cell including the apparatus, and more particularly, to an apparatus for measuring the concentration of a liquid fuel, which uses a strain gauge, and a fuel cell including the apparatus. Background Art

[2] Direct liquid fuel cells, which are pwer generation systems that generate electricity by an electrochemical reaction of oxygen that is an oxidizer with an organic compound fuel, such as methanol or ethanol, have high energy and pwer density, since they directly use a liquid fuel such as methanol, need no peripheral device such as a reformer, and easily store and supply fuel.

[3] Among direct liquid fuel cells, direct methanol fuel cells (DMFCs) that use methanol and water as a fuel mixture produce electrode reactions including an oxidization reaction (an anode electrode reaction) in which fuel is oxidized and a reduction reaction (a cathode electrode reaction) in which hydrogen ions and oxygen are reduced. In the oxidization reaction, methanol and water react with each other, carbon dioxide, hydrogen ions, and electrons are generated, and the hydrogen ions are transferred to a cathode electrode via membrane. In the reduction reaction, hydrogen ions, electrons transferred via an external circuit, and oxygen react with each other, and water is generated. Therefore, the overall reaction of the DMFCs produces water and carbon dioxide by a reaction of methanol and oxygen. In this regard, the DMFCs must use methanol in the form of methanol solution rather than pure methanol, that is, a low concentration aqueous methanol solution which is a mixture of pure methanol and water that is generated in a DMFC system or has previously been stored therein. When the DMFCs use high concentration methanol, the methanol is cross-overed (the methanol passes through membrane and then), a cell voltage is reduced due to oxidization of the methanol.

[4] However, when the DMFCs use low concentration methanol, they need a very bulky fuel storage tank, which is not appropriate for the design of the DMFCs. To address this problem, concentrated methanol or pure methanol is used as fuel, and such a high concentration methanol is diluted according to the operating conditions of the DMFCs. Therefore, methanol sensors for precisely measuring the concentration of the methanol are essential to DMFC systems that use high concentration methanol as fuel.

[5] Methods of measuring the concentration of various liquid fuels such as methods of using a membrane electrode assembly (MEA), of using the viscosity of fuel, of using a differential pressure, and the like, and apparatuses for performing the methods have been conventionally used.

[6] US Patent No. 6,303,244 discloses a methanol sensor for detecting the concentration of methanol in a circulation tank but does not disclose the detailed structure of the methanol sensor.

[7] US Patent No. 6,488,837 discloses a methanol sensor comprising an MEA, an anode current collector, a cathode current collector, and a current sensor. The current sensor measures a current of a short circuit across the anode current collector and the cathode current collector so as to provide output signal functionally associated with the concentration of methanol of a methanol aqueous solution. That is, a change in electrical conductivity of the methanol aqueous solution according to the concentration of methanol is measured as an output current.

[8] US Publication Patent No. 2006/0272943 discloses an electrochemical sensor for measuring the concentration of an aqueous solution fuel comprising an MEA, two current collectors, an anode endplate, and a cathode endplate. The electrochemical sensor provides an output signal according to the concentration of the aqueous solution fuel. In more detail, a change in electrical conductivity of liquid fuel according to the concentration of organic fuels such as methanol, ethanol, or a formic acid and inorganic fuels such as sodium or potassium borohydrides is measured as output current.

[9] US Patent No. 6,536,262 discloses a method of measuring the concentration of alcohol by installing a bypass line in a liquid fuel supply line and using a differential pressure in an inlet and outlet of the bypass line.

[10] Korean Publication Patent No. 2006-0064978 discloses an apparatus for measuring the concentration of methanol, the apparatus comprising a sensor that generates an electrical signal by a deformation of a supprt beam caused by a change in the viscosity of a liquid fuel.

[11] In addition to the concentration measuring methods, methods of using a refractive index ( www.ti.com ), an ultrasound wave ( www.murata.com ), mass density of a liquid fuel ( www.mems-issys.com ), and the like have been used. Disclosure of Invention Technical Problem

[12] An illustrative embodiment of the present invention provides an apparatus for measuring the concentration of a liquid fuel, which uses a strain gauge .

[13] An illustrative embodiment of the present invention provides an apparatus for easily measuring the concentration of liquid fuel by which manufacturing costs are reduced because of a simple structure thereof.

[14] An illustrative embodiment of the present invention provides a fuel cell including the apparatus for measuring the concentration of a liquid fuel. Technical Solution

[15] According to an aspect of the present invention, there is provided an apparatus for measuring the concentration of a liquid fuel, the apparatus comprising: a main body unit comprising a flow channel for the liquid fuel; a strain membrane deformed according to a change in the concentration of the liquid fuel and disposed in the main body unit so that the strain membrane is partly exposed to the flow channel; a strain gauge disposed on the strain membrane and detecting the strain of the strain membrane as a change in electrical resistance of the strain gauge; and a circuit unit electrically connected to the strain gauge and converting the change in electrical resistance of the strain gauge into an output signal.

[16] According to another aspect of the present invention, there is provided a direct methanol fuel cell employing the above apparatus for measuring the concentration of a liquid fuel.

Description of Drawings

[17] The above and other features and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which:

[18] FIG. 1 is a cross-sectional view illustrating an apparatus for measuring the concentration of a liquid fuel, according to an embodiment of the present invention;

[19] FIG. 2 is a cross-sectional view illustrating a strain membrane and a strain gauge stacked thereon, as shown in FIG. 1, according to an embodiment of the present invention;

[20] FIG. 3 is a cross-sectional view illustrating a strain membrane and the strain gauge stacked thereon, according to another embodiment of the present invention;

[21] FIG. 4 is an expanded perspective view illustrating the strain gauge shown in FIG. 1 according to an embodiment of the present invention; and [22] FIGS. 5 and 6 are views for explaining the operation of the apparatus for measuring the concentration of a liquid fuel, shown in FIG. 1, according to an embodiment of the present invention. Mode for Invention

[23] Hereinafter, an apparatus for measuring the concentration of a liquid fuel according to embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[24] FIG. 1 is a cross-sectional view illustrating an apparatus 10 for measuring the concentration of a liquid fuel according to an embodiment of the present invention. FIG. 2 is a cross-sectional view illustrating a strain membrane 12 and a strain gauge 13 stacked thereon, as shown in FIG. 1, according to an embodiment of the present invention.

[25] Referring to FIG. 1, the apparatus 10 for measuring the concentration of liquid fuel comprises a main body unit 11, the strain membrane 12, the strain gauge 13, and a circuit unit 15.

[26] A main flow channel 11a and a bypass flow channel 1 Ib of the liquid fuel are formed in the main body unit 11. Although both the main flow channel 11a and the bypass flow channel 1 Ib are formed in the present embodiment, the bypass flow channel 1 Ib may not be formed. However, when a pressure deviation occurs along the main flow channel 11a, the bypass flow channel 1 Ib removes the pressure deviation, so that the pressure deviation can exclude an effect on the strain of the strain membrane 12 that will be described later. Alcohol such as methanol or ethanol may be used as the liquid fuel. However, the present invention is not limited thereto and various other organic compounds can be used as the liquid fuel. The main body unit 11 includes a groove l ie containing the strain membrane 12, the strain gauge 13, and the circuit unit 15.

[27] The main body unit 11 is designed to be insensitive to changes in pressure and temperature since such changes influence the strain of the strain membrane 12, which results in an increase of an error in measuring the strain. Therefore, it is necessary to correct a change in the strain of the strain membrane 12 according to the changes in pressure and temperature. The main body unit 11 may further comprise a variety of devices for correcting pressure and/or a temperature (not shown).

[28] The strain membrane 12 is disposed in the groove 1 Ic of the main body 11 in such a manner that at least a part of the strain membrane 12 can be exposed to the main flow channel 1 Ia or the bypass flow channel 1 Ib. In the present embodiment, the strain membrane 12 is partly exposed to the bypass flow channel 1 Ib. The strain membrane 12 is deformed according to a change in the concentration of the liquid fuel. In more detail, the higher the concentration of the liquid fuel, the greater the strain of the strain membrane 12. Therefore, the strain membrane 12 may be formed of various materials according to the type of the liquid fuel. Also, the strain membrane 12 has an enough deformation and restoring force so as to increase or reduce the strain thereof according to the change in the concentration of the liquid fuel.

[29] In the meantime, the strain membrane 12 may have a thickness of between about 10 μm and about 500 μm . If the thickness of the strain membrane 12 is less than 10 μm , it is difficult to restore the strain membrane 12 to its original state when the concentration of the liquid fuel decreases again after having increased. If the thickness of the strain membrane 12 is greater than 500 μm , the strain of the strain membrane 12 is extremely reduced according to the change in the concentration of the liquid fuel.

[30] The strain membrane 12 has a double membrane structure including a first membrane 12a and a second membrane 12b with reference to FIGS. 1 and 2. In more detail, the second membrane 12b is disposed on a surface of the first membrane 12a, i.e., a surface of the first membranel2a oppsite to the main flow channel 1 Ia and the bypass flow channel 1 Ib. The first membrane 12a contains at least one material that is deformed according to the change in the concentration of the liquid fuel. The second membrane 12b contains at least one material that is not deformed according to the change in the concentration of the liquid fuel. In more detail, the first membrane 12a may be formed of a material through which at least a part of the liquid fuel can pass. If the first membrane 12a is deformed according to the change in the concentration of the liquid fuel, the second membrane 12b stacked on the first membrane 12a is deformed. Thereafter, the strain gauge 13 stacked on the second membrane 12b is deformed, which changes an electrical resistance of the strain gauge 13, thereby detecting the change in the concentration of the liquid fuel. This will be described in more detail later.

[31] The second membrane 12b may be waterproof so that the liquid fuel is not transmitted. If the second membrane 12b is water permeable, the liquid fuel sequentially transmits the first membrane 12a and the second membrane 12b and wets the strain gauge 13, which causes an electrical short of the strain gauge 13. To prevent the electrical short from occurring, it is necessary to additionally install a separate waterproof membrane or change the structure of the apparatus 10 for measuring the concentration of liquid fuel, which complicates a manufacturing process and structure of the apparatus 10 for measuring the concentration of liquid fuel. Meanwhile, the second membrane 12b can be water permeable so that the liquid fuel is transmitted. In this case, the strain gauge 13 must be waterproof.

[32] In the present embodiment, if the liquid fuel is alcohol, the first membrane 12a may be a polymer membrane comprising at least one of a sulfonic acid group (SO ₃H), a phosphoric acid group ( H ₂PO₄ ), a hydroxyl group (OH), a carboxyl group (COOH), and the like, and the second membrane 12b may be a polymer membrane comprising at least one of poly vinylidene fluoride (PVDF), polyetheretherketone (PEEK), polycarbonate (PC), and the like but the present invention is not limited thereto.

[33] Hereinafter, 1 ike reference numerals in FIGS. 1 and 2 denote like elements or portions of like elements.

[34] Referring to FIG. 3, the strain membrane 12 may be a single membrane containing at least one material that is deformed according to the change in the concentration of the liquid fuel. In this case, the strain membrane 12 may be formed of the same material as that of the first membrane 12a and have a thickness of between about 10 μm and about 500 μm . However, to prevent an electrical short from occurring, the strain gauge 13 may comprise a waterproof insulation substrate (not shown), a metal resistance line (not shown) disposed inside the insulation substrate, and an insulated lead line (not shown).

[35] The strain gauge 13 is a device that is disposed on the strain membrane 12 and detects the strain of the strain membrane 12 according to the change in the concentration of the liquid fuel as a change in electrical resistance.

[36] FIG. 4 is an expanded perspective view illustrating the strain gauge 13 shown in FIG.

1 according to an embodiment of the present invention. Strain is a deformation that occurs in an object, and defined as a ratio of the length of stretch or compression with regard to the original length of the object when being subjected to tensile or compressive force. That is, strain is a term indicating the measure of deformation of an object induced by an internal or external force.

[37] Referring to FIG. 4, the strain gauge 13 may be one of two types of strain gauges; an electrical strain gauge and a mechanical strain gauge. The electrical strain gauge measures the strain of an object by measuring a change in electrical resistance of the strain gauge attached to the object in which deformation has occurred. The mechanical strain gauge mechanically measures the strain of an object by mechanically measuring a minute distance change between two points of the object before and after its deformation. In the present embodiment, although the electrical strain gauge is used as shown in FIG. 4, the present invention is not limited thereto but the mechanical strain gauge can be used. Although a single axis gauge is illustrated in FIG. 4, the present invention is not limited thereto and an electrical strain gauge such as a two-axis rosette gauge or a three-axis rosette gauge also can be used.

[38] The strain gauge 13 includes a very thin metal resistance line 13b on or in an insulation substrate 13a such as paper or plastic. The metal resistance line 13b is in the form of a coil having both ends connected to two lead lines 14. The strain gauge 13 is a sensor for detecting a small mechanical strain as an electric signal. In more detail, if the strain gauge 13 is attached to the surface of the strain membrane 12, the strain gauge 13 can measure a minute dimension change, i.e., strain, which occurs on the surface of the strain membrane 12 as a change in electrical resistance, and find out the concentration of the liquid fuel from the size of the change. In more detail, the strain gauge 13 is an application of electrical resistance in which a strain (deformation) of the length of a resistance device is proportional to a rate of resistance change in the resistance device. Also, a ratio of the strain of length and the rate of resistance change is referred to as a gauge factor, which is provided by a strain gauge manufacturer. Therefore, the gauge factor makes it pssible to calculate a strain of length from a change in the amount of electrical resistance. The strain of length is proportional to the concentration of the liquid fuel, thereby obtaining the concentration of the liquid fuel from the change in the amount of electrical resistance. The measured strain is represented as an output signal such as an output voltage or output current of a bridge circuit that will be described later.

[39] When a tensile force F is applied to the strain gauge 13, the correlation between a gauge factor K, a rate of length change ΔL/L , a strain ε , and a rate of electrical resistance change ΔR/R is expressed as equation 1 below.

[40] K = (ΔR/R)/(ΔL/L) = (ΔR/R)/ε, i.e., ΔR/R = K*ε (1)

[41] The circuit unit 15 is electrically connected to the strain gauge 13 via the lead lines

14 and converts a change in the electrical resistance of the strain gauge 13 into an output signal, output voltage, or output current. In the present embodiment, the circuit unit 15 comprises a part of whiston bridge and selectively an amplifier. The constitution and effect of the whiston bridge and the amplifier will be described in detail later.

[42] FIGS. 5 and 6 are views for explaining the operation of the apparatus 10 for measuring the concentration of a liquid fuel, shown in FIG. 1, according to an embodiment of the present invention.

[43] FIG. 5 is a cross-sectional view illustrating the main body unit 11 with omitted the bypass flow channel 1 Ib, the strain membrane 12 and the strain gauge 13 partly extracted from the same elements as those included in the apparatus 10 for measuring the concentration of liquid fuel shown in FIG. 1. FIG. 6 is a view for explaining a method of measuring a change in the electrical resistance detected by the strain gauge 13 as a change in an output voltage.

[44] Referring to FIGS. 5 and 4, when the liquid fuel flows along the main flow channel

1 Ia in a direction of an arrow, the liquid fuel partly flows into the first membrane 12a through the surface of the first membrane 12a. Such a flow of the liquid fuel deforms the first membrane 12a so that the first membrane 12a is bent in a particular direction and thus the second membrane 12b attached to the first membrane 12a is bent in the same direction in which the first membrane 12a is bent. Such bending of the second membrane 12b provides bending of the strain gauge 13 attached to the second membrane 12b in the same direction in which the second membrane 12b is bent. Such a deformation of the strain gauge 13 results in a change in electrical resistance of the metal resistance line 13b disposed in the strain gauge 13.

[45] Referring to FIGS. 6 and 1, the strain gauge 13 is electrically connected to the circuit unit 15 and constitutes a resistor R _! of four resistors R _! through R₄ which constitute the whiston bridge.

[46] With regard to the structure of the whiston bridge, the four resistors R j through R₄ are symmetrically connected between points a through d as shown in FIG. 6, an galvanometer G is connected between points c and d, and a voltage is applied to the whiston bridge, so that current flows through the circuit unit 15 and thus a voltage drop occurs in each of the four resistors R j through R₄. If each voltage at the point c and point d, between which the galvanometer G is connected, become equal, the potential difference between the points c and d becomes '0' and thus no current flows. Thus, a needle of the galvanometer G indicates '0'. The voltage drop at each of the four resistors R₁ through R₄ is proportional to the amount of resistance. Therefore, the output voltage of the whiston bridge is '0' under the condition that the four resistors R j through R₄ satisfy equation 2 below.

[47] R₁*R₃ = R₂*R₄ (2)

[48] wherein, R₁ denotes a strain gauge resistance, and R ₂ through R₄ denote dummy resistances.

[49] Although the resistance values of the dummy resistances R ₂ through R₄ do not change, the strain gauge resistance R j varies according to the deformation of the strain gauge 13, so that the resistance variation results in a change in the output voltage Vo. [50] The relationship among the output voltage Vo, the gauge factor K, the strain ε , and the input voltage Vi is expressed as equation 3 below. That is, the output voltage Vo is proportional to the strain ε.

[51] ΔVo = l/4*K*ε*Vi (3)

[52] The whiston bridge shown in FIG. 6 is just an example that is to be applied to the apparatus for measuring the liquid fuel of the present invention and other modifications of the whiston bridge can be employed in the apparatus for measuring the liquid fuel.

[53] Although only one strain gauge 13 is used to measure the strain of the strain membrane 12 in the present embodiment, the present invention is not limited thereto and a two-axis or three-axis strain gauge of cross type can be used to correct a temperature change influencing the strain of the strain membrane 12, remove noise, and/or measure the strain that changes in vertical and horizontal directions.

[54] The strain gauge 13 and the circuit unit 15 are connected by using either two wire connection or three wire connection method. Three wire connection method makes it possible to minimize an error in the strain according to the resistance of the connection between the strain gauge 13 and the circuit unit 15. Two wire connection method is used to connect the strain gauge 13 and the circuit unit 15(that is, a part of whiston bridge) with reference to FIG. 6.

[55] Since an output voltage Vo of the whiston bridge generates a very weak output voltage signal, an amplifier (not shown) may be used to amplify the output voltage Vo. The amplifier may be, for example, a differential amplifier.

[56] The apparatus for measuring the concentration of a liquid fuel having the above configuration can be applied to the biotechnology industry, the automobile industry, fuel cells, and the like.

[57] The apparatus for measuring the concentration of a liquid fuel may be used as a d irect liquid fuel cell, which generally uses alcohol such as methanol or ethanol as a liquid fuel. A fuel cell using methanol is referred to as a direct methanol fuel cell that uses a methanol aqueous solution having the concentration of between 1 and 2 M.

[58] The performance of a fuel cell varies according to a change in the concentration of methanol, and methanol having a constant concentration must be supplied so as to operate a direct methanol fuel cell system for a long period of time. Therefore, the direct methanol fuel cell system needs an apparatus for measuring the concentration of methanol, which is capable of precisely controlling the concentration of methanol.

[59] If the apparatus for measuring the concentration of the liquid fuel of the present invention is applied to the direct methanol fuel cell, a strain membrane and a stain gauge that have a simple structure and incur low manufacturing costs make it possible to easily and precisely measure a change in the concentration of methanol. [60] While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

[1] 1. An apparatus for measuring the concentration of a liquid fuel, the apparatus comprising: a main body unit comprising a flow channel for the liquid fuel; a strain membrane deformed according to a change in the concentration of the liquid fuel and disposed in the main body unit so that the strain membrane is at least partly exposed to the flow channel; a strain gauge dispsed on the strain membrane and detecting the strain of the strain membrane as a change in electrical resistance of the strain gauge; and a circuit unit electrically connected to the strain gauge and converting the change in electrical resistance of the strain gauge into an output signal.

[2] 2. The apparatus of claim 1, wherein the strain membrane is a single membrane containing at least one material that is deformed according to the change in the concentration of the liquid fuel.

[3] 3. The apparatus of claim 2, wherein the strain gauge is waterproof.

[4] 4. The apparatus of claim 1, wherein the strain membrane is a double membrane or a multiple membrane comprising a first membrane containing at least one material that is deformed according to the change in the concentration of the liquid fuel, and a second membrane stacked on the first membrane and containing at least one material that is not deformed according to the change in the concentration of the liquid fuel.

[5] 5. The apparatus of claim 4, wherein the second membrane is disposed on a surface oppsite to the flow channel of the first membrane, and is waterproof.

[6] 6. The apparatus of claim 5, wherein the liquid fuel is alcohol, and the second membrane is a plymer membrane containing at least one selected from a group consisting of poly vinyidene fluoride (PVDF), plyetheretherketone (PEEK), and plycarbonate (PC).

[7] 7. The apparatus of claim 2 or claim 4, wherein the liquid fuel is alcohol, and the single membrane or the first membrane is a plymer membrane containing at least one selected from a group consisting of a sulfonic acid group (SO ₃ H ), a phosphoric acid group ( H ₂PO₄ ), a hydroxyl group (OH), and a carboxyl group (COOH).

[8] 8. The apparatus of claim 1, wherein the strain membrane has a thickness of between 10 μm and 500 μm .

[9] 9. The apparatus of claim 1, wherein the circuit unit comprises at least a part of a whiston bridge.

[10] 10. The apparatus of claim 9, wherein the circuit unit further comprises an amplifier.

[11] 11. A direct methanol fuel cell employing an apparatus for measuring the concentration of a liquid fuel in any one of claims 1 through 6 and claims 8 through 10.