GB2447872A - Fuel Cell Stack Assembly - Google Patents

Fuel Cell Stack Assembly Download PDF

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Publication number
GB2447872A
GB2447872A GB0706319A GB0706319A GB2447872A GB 2447872 A GB2447872 A GB 2447872A GB 0706319 A GB0706319 A GB 0706319A GB 0706319 A GB0706319 A GB 0706319A GB 2447872 A GB2447872 A GB 2447872A
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United Kingdom
Prior art keywords
fuel cell
cell stack
fuel
condenser
exhaust
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GB0706319A
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GB0706319D0 (en
Inventor
Robert Morgan
Robert Leah
Bruce Girvan
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Ceres Intellectual Property Co Ltd
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Ceres Intellectual Property Co Ltd
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Priority to GB0706319A priority Critical patent/GB2447872A/en
Publication of GB0706319D0 publication Critical patent/GB0706319D0/en
Publication of GB2447872A publication Critical patent/GB2447872A/en
Withdrawn legal-status Critical Current

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H01M8/04067—Heat exchange or temperature measuring elements, thermal insulation, e.g. heat pipes, heat pumps, fins
    • H01M8/04074—Heat exchange unit structures specially adapted for fuel cell
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
    • H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
    • H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
    • H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
    • H01M8/04164—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal by condensers, gas-liquid separators or filters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04225—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells during start-up
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/06—Combination of fuel cells with means for production of reactants or for treatment of residues
    • H01M8/0606—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants
    • H01M8/0612—Combination of fuel cells with means for production of reactants or for treatment of residues with means for production of gaseous reactants from carbon-containing material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
    • H01M8/241—Grouping of fuel cells, e.g. stacking of fuel cells with solid or matrix-supported electrolytes
    • H01M8/2425—High-temperature cells with solid electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/10—Fuel cells with solid electrolytes
    • H01M8/12—Fuel cells with solid electrolytes operating at high temperature, e.g. with stabilised ZrO2 electrolyte
    • H01M2008/1293—Fuel cells with solid oxide electrolytes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
    • H01M8/04014—Heat exchange using gaseous fluids; Heat exchange by combustion of reactants
    • H01M8/04022—Heating by combustion
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M8/00—Fuel cells; Manufacture thereof
    • H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
    • H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
    • H01M8/04268—Heating of fuel cells during the start-up of the fuel cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/30—Hydrogen technology
    • Y02E60/50—Fuel cells

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  • Life Sciences & Earth Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Sustainable Development (AREA)
  • Sustainable Energy (AREA)
  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Fuel Cell (AREA)

Abstract

The present invention is concerned with fuel cell stack assemblies, particularly with heat exchanger configurations within fuel cell stack assemblies, configured to enhance system efficiency and/or the speed and rate of warm-up of the fuel cell stack. It achieves this by employing a condenser in fluid communication with fuel cell exhaust gas to recover heat and switching means to switch between condensing water from the exhaust gas and not condensing water from the exhaust gas e.g. by bypassing the condenser or reducing cooling therein.

Description

Fuel Cell Stack Assembly The present invention is concerned with fuel
cell stack assemblies, particularly with heat exchanger configurations within fuel cell stack assemblies, configured to enhance system efficiency andlor the speed and rate of warm-up of the fuel cell stack.
Fuel cell stack assemblies (for example for domestic or industrial use) are available in a large number of configurations. In some configurations, for example, a fuel reformer is located upstream of a fuel cell stack, and the exhaust oxidised fuel gas stream from the fuel cell stack is passed through a condenser in order to condense out water vapour to recover the energy associated with the latent heat of evaporation. One major issue for fuel cell stack assembly operation, particularly for high-and intermediate-temperature fuel cell stack assemblies is achieving a rapid "start-up" of the fuel cell stack in order that it can go from cold (e.g. ambient temperature) to its optimal operational temperature as quickly as possible. As start-up of the fuel cell stack progresses, once its temperature exceeds a minimum point, its operational efficiency increases and it is able to oxidise more fuel and generate more heat, and hence become thermally more sustainable in operation. Thus, in order to effect a rapid start-up, it is important to introduce heat into the fuel cell stack. This is typically done by, for example, mixing fuel with an oxidant inlet stream and burning the fuel outside of the fuel cell stack, and passing the resultant hot gases through the oxidant side of the fuel cell stack, thus heating the whole fuel cell stack (see e.g. US 6042956). Alternatively, a burner can be provided thermally coupled to a reformer in order to supply heat energy to the reformer, and thus to the fuel gas flowing in the reformer, and help catalyse production of reformed fuel for oxidation, which then flows into and heats the fuel cell stack.
The present invention seeks to overcome the prior art disadvantages and provide an improved fuel cell stack and fuel cell stack assemblies. More particularly, it seeks to use the energy available in the fuel cell stack exhaust gas to provide an improved efficiency fuel cell stack warm-up by using a controllable off-gas condenser unit to help control an amount of heat energy that can be fed back to the fuel cell stack.
According to a first aspect of the present invention there is provided a fuel cell stack assembly comprising: (i) a fuel cell stack having fuel and oxidant inlets and fuel and oxidant exhaust outlets; (ii) a condenser in fluid communication with said fuel exhaust outlet; (iii) switching means for said condenser to switch between a first state in which in-use said condenser condenses water from exhaust gases passing from said fuel exhaust outlet and a second state in which in-use said condenser does not condense water from said exhaust gases passing from said fuel exhaust outlet; and (iv) control means to control said switching means.
Thus the fuel cell stack assemblies of the present invention are capable of switching between first and second states -in the first state the exhaust gases passing through the condenser has its temperature reduced so as to recover some or substantially all of the water by condensation and also recover the thermal energy associated with the associated latent heat of evaporation, which can be used to provide heat to another part of the system. In the second state the condenser does not actively condense water out from the exhaust gases (although of course depending upon the configuration of the system it may be that the condenser still causes slight, but not substantial, condensation due to its thermal mass and ability to absorb heat energy from passing exhaust gases and thus condense out water, but this is not an active condenser system and does not substantially cool the exhaust gases). In the second state, the outlet gas from the condenser unit is at a higher temperature than in the first state. Thus, the gas outlet temperature from the condenser unit can be controlled by operating between the two states.
In preferred embodiments, in-use the gas outlet temperature from the condenser unit is controlled by switching between the two states to achieve a set output temperature.
Thus, in such embodiments the fuel cell stack assembly additionally comprises condenser gas outlet temperature sensing means, the control means being adapted to control the switching means with reference to an in-use detected condenser gas outlet temperature.
In further preferred embodiments, this set output temperature is achieved by the switching means being configured such that it can be placed in an intermediate state between said first and second states in which in-use said condenser condenses water from a portion of exhaust gases passing from said fuel exhaust outlet.
The fuel cell stack assemblies of the present invention are particularly applicable to solid oxide fuel cells. In such fuel cells and fuel cell stacks, a fuel inlet and an oxidant (e.g. air) inlet is provided, together with an exhaust fuel outlet and an exhaust oxidant outlet. In use, oxygen ions from the oxidant gas stream migrate across an electrolyte layer and combine on the opposite side with fuel gas stream elements, typically hydrogen andlor carbon monoxide, to generate water and carbon dioxide, and this now partially or fully reacted fuel gas is referred to below as the fuel exhaust gas stream. In intermediate-or high-temperature fuel cells, this fuel exhaust gas stream is hot (450 -1100 C) and typically contains non-oxidised fuel which can subsequently be oxidised to release further heat energy. The water generated in the reaction of oxidant with fuel can be condensed to its liquid phase to release additional energy, which can be used elsewhere in systems. The exhaust oxidant can simply be exhausted from the system, if desired.
In use, the fuel cell stack can be considered to produce first stage exhaust gases, which are then passed through or bypass the condenser to give second stage exhaust gases.
With the switching means in its first state, the second stage exhaust gases are condenser-cooled, whereas with the switching means in its second state, the second stage exhaust gases are not condenser-cooled.
In certain aspects, the switching means comprises a condenser bypass. In other aspects of the present invention, the switching means comprises switching for a coolant flow with the first state having an active coolant flow through the condenser which absorbs heat from the exhaust gases, and in the second state the coolant flow being switched off, meaning that there is no active cooling of exhaust gases as they pass through the condenser.
Thus, as the exhaust gases pass through or bypass the condenser with the switching means in the second state, the exhaust gases retain substantial thermal energy. This energy can be put to convenient use.
Generally speaking, the heat energy retained by the second stage exhaust gases when the switching means is in its second state can be retained within the fuel cell stack assembly and passed either upstream or downstream of the fuel cell stack.
In certain aspects, the system additionally comprises a reformer upstream of the fuel cell stack for reforming an inlet fuel with water in order to produce hydrogen and carbon oxides. In some cases, such as with steam reforming, the process of reforming is an endothermic one and therefore requires heat energy to be added to the catalytic area of the reformer from an external source. The exhaust gases exiting the condenser or bypassing the condenser can be passed through a heat exchanger thermally coupled with the reformer, and heat energy from the exhaust gases directly transferred to the reformer, allowing it to operate.
In further aspects of the present invention, such a system is provided with a burner, which is thermally coupled to the reformer, and the exhaust gases from or bypassing the condenser containing un-oxidised fuel are burnt in the burner and heat energy transferred to the reformer to which it is thermally coupled. This thennal coupling can be a direct one in the form of a heat exchanger part of the burner providing a direct thermal coupling between the burner and the reformer. Alternatively, the thermal coupling can be an indirect one in which exhaust gases exit the burner and are then passed to a heat exchanger which is thermally coupled to the reformer. For example, the heat exchanger can form a part of the reformer.
In this way, the heat energy generated by the fuel cell stack is returned to it via the reformer, helping warm the fuel cell stack and enhance its operational efficiency, thus generating additional heat and in turn further heating the fuel cell stack.
In further aspects of the present invention, the heat energy contained in the second stage exhaust gases when the switching means is in its second position are used to heat a downstream element of the fuel cell stack assembly. Examples of downstream fuel cell stack elements include a second fuel cell stack into which the second stage exhaust gases are fed as fuel with or without additional fuel being added to the second stage exhaust gases. Such second fuel cell stacks can be thermally coupled to the first fuel cell stack if desired.
Thus, the fuel cell stack assembly preferably comprises a second fuel cell stack having fuel and oxidant inlets and fuel and oxidant exhaust outlets, said second fuel cell stack fuel inlet being in fluid communication with said first fuel cell stack such that in-use exhaust fuel can pass from said first fuel cell stack to said second fuel cell stack fuel inlet. More preferably, the water vapour concentration in the fluid flow from said first fuel cell stack fuel exhaust outlet to said second fuel cell stack fuel inlet is controlled by said switching means.
Thus, in a fuel cell stack assembly comprising first and second fuel cell stacks thermally coupled together, the second fuel cell stack can be relatively small and low-powered compared to the first fuel cell stack and can be provided with the hot second stage exhaust gases as an inlet fuel, thus heating the second fuel cell stack and helping it oxidise a greater proportion of its inlet fuel. The lower thermal mass of the second fuel cell stack (when compared to the first fuel cell stack) means that it heats up relatively quickly and is rapidly able to attain a high-efficiency operating temperature and generate additional heat. When thermally coupled to the first fuel cell stack, the second fuel cell stack can thus provide a source of heat to the first fuel cell stack and thus help increase its operational efficiency whilst at the same time generating output electrical energy.
Preferably, the thermal mass of the second fuel cell stack is less than 25, 50, 66, 75, 80, 85, 90 or 95% of the thermal mass of the first fuel cell stack.
In further aspects of the present invention, the first and second fuel cell stacks need not be thermally coupled to one another.
A wide range of configurations of condenser can be used. As mentioned above, the switching means can switch between a first state in which exhaust gases (i.e. exhaust fluid) is passed through the condenser and condensation of water occurs, and a second state in which a condenser bypass is used, exhaust gases thus not passing through the condenser and water not being condensed.
Preferably, the switching means is configured to switch between the first and second states with reference to an in-use detected temperature. More preferably, it is configured such that when the detected temperature is below a pre-defined temperature, the switching means is in the first state, and when the detected temperature is at or above the pre-defined temperature, the switching means is in the second state.
Preferably the detected temperature is a temperature of the fuel cell stack. Other temperatures which can be detected include the fuel cell stack exhaust temperature of either the fuel side or air side, and the stack surround temperature.
Preferably, the pre-determined temperature is set by the temperature close to that at which the fuel cell stack starts to become thermally self-sufficient or at which the reformer catalytic temperature becomes thermally active. For an intermediate temperature solid oxide fuel cell system where the fuel cell electrolyte operates between 450 -650 C, such a stack temperature is preferably between 450 C -650 C. For a high temperature solid oxide fuel cell where the fuel cell electrolyte operates between 700-1000 C, the pre-determined temperature is preferably between 700 C -1000 C.
Preferably, the switching means is triggered by the amount of condensate required.
Thus, if more condensate is required then the switching means switches to the first state, and if less condensate is required then it switches to the second state.
The condenser itself typically comprises a fluid flow passage for the exhaust fluid, and a fluid flow passage for a coolant fluid, the exhaust fluid and coolant fluid flow passages being thermally coupled to one another. In its simplest form, the thermal coupling can be by way of a heat exchanger. Examples of heat exchangers include counter-current heat exchangers and co-flow heat exchangers, in either plate heat exchanger design or shell and tube heat exchanger design.
Preferably, the condenser comprises an exhaust gas inlet and an exhaust gas outlet.
in certain aspects of the present invention, the coolant fluid is water. Other coolant fluids appropriate for specific fuel cell stack assemblies will be readily apparent to a person of ordinary skill in the art.
The switching means can comprise a temperature sensor upstream of the condenser, a thermostat, and valve means configured such that when the sensed temperature is below a pre-determined value the valve means is set by the thermostat such that the condenser is in the second state, and when the sensed temperature is at or above the pre-determined value the valve means is set by the thermostat such that the condenser is in the first state.
The pre-determined value need not be the optimal operating temperature of the fuel cell stack, and instead other temperatures can be set as desired -it may be decided that the pre-determined temperature should be set such that once the fuel cell stack has been given a "kick-start' by the condenser being in the second state, the condenser can be switched to the first state, even if the fuel cell stack itself is not at its optimal operating temperature.
The temperature sensor itself can be placed so as to determine the temperature of the exhaust fuel flow from the fuel cell stack. Alternatively, the temperature sensor can be placed so as to determine the temperature of the fuel cell stack itself or a part of it. The fundamental role of the temperature sensor is to determine the temperature of the electrolyte in the fuel cell stack so as to determine whether the temperature of the fuel cell stack needs increasing in order to increase the conversion rate of the fuel and thus fuel cell stack efficiency.
Also provided according to the present invention is a method of operation of a fuel cell stack assembly, said fuel cell stack assembly comprising: (i) a fuel cell stack having a fuel and oxidant inlet and an exhaust outlet; (ii) a condenser in fluid communication with said exhaust outlet; (iii) switching means for said condenser to switch between a first state in which said condenser condenses water from exhaust gases passing from said fuel cell stack exhaust outlet and a second state in which said condenser does not condense water from said exhaust gases passing from said fuel cell stack exhaust outlet; and (iv) control means to control said switching means, said method comprising the steps of providing said fuel cell stack with fuel and oxidant streams and operating said fuel cell stack to produce an oxidant exhaust stream and a fuel exhaust stream, determining the temperature of said fuel cell stack or said fuel exhaust stream, and switching said condenser between said first state when said temperature is greater than or equal to a pre-determined temperature and said second state when said temperature is below said pre-determined temperature.
The invention will be further apparent from the following description with reference to the figures of the accompanying drawings which show by way of example only forms of fuel cell stack assembly. Of the Figures: Figure 1 shows a first fuel cell stack assembly configuration; Figure 2 shows a second fuel cell stack assembly configuration; Figure 3 shows a third fuel cell stack assembly configuration; Figure 4 shows a fourth fuel cell stack assembly configuration; Figure 5 shows a fifth fuel cell stack assembly configuration; Figure 6 shows a first condenser configuration; Figure 7 shows a second condenser configuration; Figure 8 shows a third condenser configuration; and Figure 9 shows a fourth condenser configuration.
As can be seen from the assembly of Figure 1, fuel F and water are passed into reformer R where the fuel F is reformed to produce hydrogen and carbon oxides which pass into the fuel inlet of fuel cell stack FCS. Oxygen (as a component part of air) enters the fuel cell stack FCS through an oxidant inlet, and the fuel is oxidised to produce oxidation products (including water and carbon dioxide), heat and electricity. Within the fuel cell stack FCS, the fuel and oxidant are not in gaseous communication with one another, and instead oxidation of the fuel occurs by the passage of oxygen ions from the oxidant stream on the oxidant side of the fuel cell across an ion-permeable electrolyte layer to the fuel side of the fuel cell where the oxygen ions combine with the fuel. Electron flow along an external electrical circuit upon which a load can be placed occurs between the fuel and oxidant sides of the electrolyte layer to complete the reaction.
The oxidised fuel stream (including the oxidation products such as water and carbon dioxide, as well as non-oxidised fuel) is then exhausted from the fuel cell stack FCS to condenser C and passes temperature sensor S which detects its temperature, and which is in communication with thermostat T which compares the detected temperature to a pre-determined temperature Ti. If the detected temperature is greater than or equal to Ti then thermostat T actuates valve V to switch condenser C into a first state in which coolant water flows into condenser C and passes through a heat exchanger (not shown) which is thermally coupled to the exhaust fuel stream so as to transfer heat from the exhaust fuel stream to the coolant fluid, thus resulting in condensation of water from the exhaust fuel stream and the recovery of the heat of evaporation. The exhaust fuel then passes from condenser C to burner B where it is mixed with the oxygen-depleted exhaust oxidant stream from the fuel cell stack FCS and is ignited so as to burn off any remaining fuel in the exhaust fuel stream. Burner B is thermally coupled to reformer R and the heat energy released is transferred to the reformer R and its endotherinic reformation process uses the energy.
When the detected temperature is below TI then thermostat T actuates valve V to switch condenser C into a second state in which coolant water does not flow into condenser C and does not pass through the heat exchanger and therefore heat is not transferred from the exhaust fuel stream to the coolant fluid. The un-cooled exhaust fuel then passes from condenser C to burner B where it is mixed with the oxygen-depleted exhaust oxidant stream from the fuel cell stack FCS and is ignited so as to burn off any remaining fuel in the exhaust fuel stream. Burner B is thermally coupled to reformer R and the heat energy released together with the heat energy which would otherwise have been removed in condenser C is transferred to reformer R and its endothermic reformation process uses the energy.
Thus in the first state when the temperature of the exhaust fuel stream is greater than or equal to TI, the condenser operates to remove heat and water from the exhaust fuel stream, and the heat can be used elsewhere as desired.
In the second state when the temperature of the exhaust fuel stream is below TI, the condenser does not operate to remove heat and water from the exhaust fuel stream, and instead the heat passes directly to burner B and is then conducted across to reformer R. This is an efficient and effective way of effecting heating of the reformer and thus of the fuel cell stack inlet fuel stream and of the fuel cell stack itself more than prior art systems. It avoids the inevitable inefficiencies introduced by the condenser coolant system and allows for the direct recycling of heat into or upstream of the fuel cell stack during system start-up. This means that system start-up can occur more quickly than would otherwise be possible, and the overall efficiency and effectiveness of the fuel cell stack assembly is increased.
The assembly of Figure 2 essentially operates in the same manner as that of Figure 1, except that it is configured so that burner B is not thermally coupled to reformer R and instead exhaust gas from burner B passes through heat exchanger H of reformer R and heat is thus transferred to the inlet fuel F and/or inlet water streams.
The assembly of Figure 3 is configured as per Figure 1 except that burner B is not thermally coupled to reformer R and instead the hot exhaust gases from burner B pass to air pre-heater APH.
Figure 4 shows an assembly which comprises first and second fuel cell stacks FCS1 and FCS2 respectively. The first fuel cell stack FCS 1 is a relatively large high mass device, and oxidant exiting it is exhausted to the atmosphere. Instead of using a burner B, the exhaust fuel exiting condenser C is passed to the fuel inlet of a small, low mass fuel cell stack FCS2. Oxidant (oxygen as a component part of air) is also passed to the fuel cell stack FCS2, and with the condenser in its second state the second fuel cell stack FCS2 is thus provided with a hot inlet fuel stream, which effects a heating of the fuel cell stack and its rapid attainment of optimal operational conditions.
In the assembly of Figure 5, second fuel cell stack FCS2 is thermally coupled to first fuel cell stack FCS1, meaning that not only is second fuel cell stack FCS2 able to achieve its optimal operational conditions quickly, but also that heat energy from it is conducted across to first fuel cell stack FCSI, helping it increase its temperature and achieve its optimal operating conditions. In other embodiments (not shown), the oxidant outlet from first fuel cell stack FCS I feeds to the oxidant inlet to second fuel cell stack FCS2.
With the first fuel cell stack FCSI of relatively high mass when compared to second fuel cell stack FCS2. this means that second fuel cell stack FCS2 is able to rapidly increase its temperature and, with an increased temperature, increase its conversion efficiency for the remaining fuel in its inlet fuel stream and generate more heat. This positive feedback loop allows the efficient and rapid generation of additional heat which, due to the thermal coupling of the first and second fuel cell stacks FCS1 and FCS2, is conducted to the first fuel cell stack FCSI which helps it reach its optimal operational conditions.
Figure 6 shows a basic configuration of a condenser C and sensor S/thermostat T/valve V arrangement so as to switch coolant fluid flow according to temperatures detected by sensor S. Figure 7 is a more detailed view of Figure 6, showing a conventional heat exchanger arrangement H. Figure 8 shows an alternative arrangement for a heat exchanger, the one shown being a counter-current heat exchanger comprising first and second heat exchangers 1-Il and H2 where the incoming fluid heat is used to heat the outgoing condensed gas. This has double benefits in that the incoming gas is cooled prior to the condensing part of the condensing heat exchange unit and the outgoing gas is heated prior to leaving the heat exchange unit. Thus, in condensing mode, the condensor heat exchange part can be smaller as the inlet temperature of the gas is lower and therefore has less far to cool to reach dew point temperature. In addition, any downstream heating assembly can be smaller as the gas leaving the condensing heat exchange unit is hotter.
Figure 9 shows a condenser bypass circuit arrangement, with thermostat T actuating first and second valves Vi and V2 so as to switch between a first condenser state in which an exhaust fuel stream from a fuel cell stack flows through condenser C and does not pass along bypass circuit B, and a second state in which exhaust fuel stream from a fuel cell stack passes along bypass circuit B and does not enter condenser C. A one-way valve arrangement (not shown) is also provided to ensure that downstream of condenser C there is no back-flow of fluid along an undesired pathway (e.g. with the condenser in its first state, backflow into bypass circuit B; with the condenser in its second state, backflow into condenser C). Valves VI and V2 can be controlled in such a way as to control the amount of gas that bypasses the condenser unit and the amount of gas that flows through the condenser unit.
In further embodiments (not shown), the switching means is a condenser bypass.
In additional embodiments (not shown), a condensate drain set-up is included where the condensate can be drained from the condensing heat exchanger under gravity means or by pressurised means.
It will be appreciated that it is not intended to limit the present invention to the above examples only, many variants being readily apparent to a skilled person without departing from the scope of the appended claims.

Claims (9)

1. A fuel cell stack assembly comprising: (i) a fuel cell stack having fuel and oxidant inlets and fuel and oxidant exhaust outlets; (ii) a condenser in fluid communication with said fuel exhaust outlet; (iii) switching means for said condenser to switch between a first state in which in-use said condenser condenses water from exhaust gases passing from said fuel exhaust outlet and a second state in which in-use said condenser does not condense water from said exhaust gases passing from said fuel exhaust outlet; and (iv) control means to control said switching means.
2. A fuel cell stack according to claim 1, said fuel cell stack being a solid oxide fuel cell stack.
3. A fuel cell stack according to either of the preceding claims, said switching means being selected from the group consisting of: coolant flow valve means, and a condenser bypass.
4. A fuel cell stack assembly according to any of the preceding claims, additionally comprising a reformer upstream of said fuel cell stack.
5. A fuel cell stack assembly according to claim 4, additionally comprising a burner thermally coupled with said reformer.
6. A fuel cell stack assembly according to any of the preceding claims, in-use the gas outlet temperature from said condenser unit being controlled by switching between said first and second states to achieve a set output temperature.
7. A fuel cell stack assembly according to claim 6, additionally comprising condenser gas outlet temperature sensing means, said control means being adapted to control said switching means with reference to an in-use detected condenser gas outlet temperature.
8. A fuel cell stack assembly according to claim 6 or 7, said switching means being configured such that it can be placed in an intermediate state between said first and second states in which in-use said condenser condenses water from a portion of exhaust gases passing from said fuel exhaust outlet.
9. A method of operation of a fuel cell stack assembly, said fuel cell stack assembly comprising: (i) a fuel cell stack having a fuel and oxidant inlet and an exhaust outlet; (ii) a condenser in fluid communication with said exhaust outlet; (iii) switching means for said condenser to switch between a first state in which said condenser condenses water from exhaust gases passing from said fuel cell stack exhaust outlet and a second state in which said condenser does not condense water from said exhaust gases passing from said fuel cell stack exhaust outlet; and (iv) control means to control said switching means, said method comprising the steps of providing said fuel cell stack with fuel and oxidant streams and operating said fuel cell stack to produce an oxidant exhaust stream and a fuel exhaust stream, determining the temperature of said fuel cell stack or said fuel exhaust stream, and switching said condenser between said first state when said temperature is greater than or equal to a pre-determined temperature and said second state when said temperature is below said pre-determined temperature.
GB0706319A 2007-03-30 2007-03-30 Fuel Cell Stack Assembly Withdrawn GB2447872A (en)

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2002117877A (en) * 2000-10-12 2002-04-19 Matsushita Electric Ind Co Ltd Solid polymer fuel cell system
US20030162065A1 (en) * 2001-05-23 2003-08-28 Shinji Miyauchi Fuel cell power generating device

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2002117877A (en) * 2000-10-12 2002-04-19 Matsushita Electric Ind Co Ltd Solid polymer fuel cell system
US20030162065A1 (en) * 2001-05-23 2003-08-28 Shinji Miyauchi Fuel cell power generating device
EP1396897A1 (en) * 2001-05-23 2004-03-10 Matsushita Electric Industrial Co., Ltd. Fuel cell power generating device
US20060246325A1 (en) * 2001-05-23 2006-11-02 Shinji Miyauchi Fuel-cell power-generation system and method

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