Classifications

• • 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
• • 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
  • H01M8/04029—Heat exchange using liquids
• • 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
  • H01M8/04126—Humidifying
• • 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

Definitions

• Fuel cells are known as devices for the direct conversion of the chemical energy of a fuel (such as pure hydrogen or hydrogen mixtures obtained by hydrocarbon steam reforming) to electric energy without involving a combustion step as instead occurs in the conventional generators based on the combustion-compressed vapour production-turbine expansion cycle or on the cycle comprising gas turbines.
• a fuel such as pure hydrogen or hydrogen mixtures obtained by hydrocarbon steam reforming
• the absence of such combustion step sets the fuel cells free from the constraints of the known Camot's principle and imparts them an intrinsically higher energy efficiency than the conventional generators.
• Fuel cells can be divided into families depending on the operating temperature, which is 60-90°G for the proton-exchange membrane types (commonly identified as PEIV ⁇ FC), 180-250°C for the types making use of phosphoric acid as the electrolyte (PAFC), 0Q-70Q for the types in which the electrolyte consists of s mixture of molten carbonates (MCFC) and 800-1000°C for those in which the electrolyte is a solid state ion-conducing oxide (SOFC).
• PEIV ⁇ FC proton-exchange membrane types
• PAFC electrolyte
• 0Q-70Q for the types in which the electrolyte consists of s mixture of molten carbonates
• SOFC solid state ion-conducing oxide
• Whe eas each y e of fuel cells has its merits in terms of energy efficiency, promptness in following the power demand transients, start-up rapidity, maintenance in conditions of zero power generation, it's a matter of fact that the presence of liquid aggressive electrolytes (phosphoric acid, molten carbonates) or the high temperatures (600-1000 "C) impose the use of sophisticated construction materials and particular measures in the design and operation, particularly as concerns the remarkable thermomechanical solicitations.
• PAFC, MCFC and SPFC-type fuel cells are normally considered to be practically destined to the construction of big units with power exceeding 1 megawatt, such as for example distributed electric power plants, wherein the operation is subjected to transients of moderate entity and wherein some form of supervision by specialised personnel is available.
• membrane fuel cells are substantially solid-state devices, hence free of the problems associated to corrosivity, as is the case with phosphoric acid and molten carbonates, to safety, since in the case of liquid electrolytes the separation of the fuel from the air reactant is ensured just by the capillary forces holding the electrolyte inside the pores of an inert matrix, and finally to the temperature as seen for the conductive solid oxides.
• membrane fuel cells being devices characterised by an extremely reduced electrode and membrane mass, have a particular capability of following steep power demands and of reaching the nominal power generation in very low times even starting from a zero generation condition. This set of characteristics makes fuel cells very attractive for their use in the automotive and also in the stationary field for small power applications, as is the case, very interesting from a commercial standpoint, of systems directed to be installed in private houses, hotels, hospitals.
• membrane fuel cells present however also some inconveniences: among these, particularly relevant is the need of having to maintain the proton-ejcehange membranes, whose conductivity is a function of the water content, in a fully hydrated state.
• Membranes are usually 20 to 100 micron thick films consisting of a polymer on whose backbone groups with an acidic function, usually sulphonic groups, are inserted.
• the polymer must be resistant to the highly aggressive action of per ⁇ Midic and radicalic compounds generated as intermediate products of reaction of air.
• the polymers currently used for the production of membranes presently available on the market as commercial or experimental products are invariably perfluorinated polymers characterised by high chemical inertia, even though a remarkable research activity is directed to the development of non-fluorinated polymers, for instance having an aromatic structure, whose long term chemical inertia and whose mass production scale economics have still to be demonstrated.
• the acidic functions, and in particular the sulphonic groups, must be dissociated: the resulting free electric charge determines in fact a particular space orientation of the polymer chains with formation of reticular channels along which proton migration takes place.
• the dissociation which is thus a required passage for channel formation, occurs only in the presence of certain water amounts, resulting in the need of keeping the membrane hydrated.
• the hydration condition is the outcome of a delicate equilibrium between water of reaction and water extracted from the gases flowing through the fuel cells during operation, in particular from the air and fuel respectively fed on the cathode side and on the anode side.
• the water extraction may become dangerously high for the membrane hydration when the fuel cell is operated at near-ambient pressure, hence with particularly elevated gas volumetric flow-rates.
• the framework doesn't change significantly even when the fuel cell anode side is directly supplied with gas coming from the steam reforming unit which may contain, depending on the operative conditions, 50 to 75% hydrogen and which has a volumetric flow-rate indicatively equivalent up to 40-50% of the volumetric flow-rate of air.
• the capability of withdrawing water is further reduced due to the fact that the gas coming from the steam reforming unit is normally saturated with water vapour.
• the water withdrawal capacity is high due both to the stoichiometric excess employed, resulting in a remarkable volumetric flow- rate particularly for the case of near-ambient pressure operation, and to the reduced moisture content present in the ambient air intake of fans or compressors used for the cell feeding.
• the prior art discloses various devices directed to saturate the air feed with water vapour at temperatures close to the fuel cell working temperature. The basic idea is in fact to eliminate the water-extracting capacity of air along the whole fuel cell crossing, during which the air temperature rises anyway to values close to those of the cell.
• the device moreover requires level control instrumentation, pumps for feeding water, purge flow-rate control to prevent the build up of impurities inevitably present, albeit in traces, in the water to be evaporated, coming from the condensation of vapour contained in the exhaust gases of the system before their release to the atmosphere, all of this involving non negligible additional costs.
• a similar procedure is claimed in US 6,350,535, wherein the air feed is added with atomised liquid water and the mixture so obtained is made progress across a heat exchanger whereto the required thermal energy for evaporating the water is provided. This device presents the same inconveniences discussed for the previous case.
• a humidification method comprising humidification cells intercalated to the fuel cells of a stack: the humidification cells thereby work practically as cooling cell wherein the cooling is ensured by the evaporation of the water required to saturate the incoming air-flow.
• the humidification temperature results higher than that relative to the above discussed external saturators, although somehow lower than the fuel cell temperature since a certain thermal difference is in any case required to maintain an adequate heat exchange rate.
• This type of device it is no more possible to employ additional heat sources to increase the humidification temperature until making it practically coincide with the cell temperature.
• the efficiency of the device is associated to the thermal level established in the stack, decreasing as the temperature decreases. The device is thus practically inactive during the start-up of the stack and during the periods of low power generation when the temperature is significantly lower than that of nominal operation.
• enthalpic unit In US 2001/0015501 the use of a unit generally defined as enthalpic unit is disclosed.
• This unit consists of a vessel subdivided into two compartments by a selective water-permeable membrane: the two compartments are supplied respectively with air at ambient temperature to be delivered to the stack and with water vapour-saturated hot exhaust air from the stack outlet.
• This type of air-feed conditioning is acceptable in the case of pressurised operation, where the air volumetric flow-rate is substantially reduced and may have a temperature above ambient under the effect of compression, while being arguable and relatively less reliable for near-ambient pressure operation.
• a device functioning in a similar way is disclosed in DE 199 18 849 wherein the water and heat transfer does not take place through a selective membrane, but rather employing a rotating drum subdivided into sectors whose internal walls are provided with a film of hygroscopic material, for instance a lithium salt.
• US 6,406,807 discloses a direct water injection inside the fuel cells: the evaporation effectively withdraws the heat generated during operation and at the same time generates the vapour partial pressure required to maintain a correct membrane hydration.
• This method still presents a remarkable critical point as that the water amounts have to be careful calibrated with respect to the power generation to prevent the two opposite risks of hydration loss (injection of insufficient amounts of water) and of flooding of the porous electrodes (injection of excessive amounts of water): all this requires injection pumps, water distributors inside the fuel cells and relevant controls, which besides implying additional costs introduce problems of functioning reliability as well.
• the stable operation of membrane fuel cells, single or arranged in stacks is obtained by cooling the non-humidified air feed to an adequate temperature.
• the cooling of the non-humidified air feed is obtained by means of a suitable unit consisting of a fan or compressor followed by a heat exchanger wherein heat is extracted making use of ambient air as the coolant, followed in its turn by an expander wherein the air pressure is reduced down to the level required for achieving the desired thermal level.
• the expander is connected to the fan or compressor so as to achieve a partial recovery of energy.
• the region of membrane most exposed to the risk of dehydration is the one immediately adjacent to the inlet of gases, and in particular to the air inlet to which future reference will be made hereafter.
• This region is subject to a quick evaporation of the product water generated by the reaction between the air itself, the proton migrating across the membrane and the electrons flowing through the external circuit. If the rate of evaporation is higher than that of water formation the membrane undergoes a progressive dehydration hindering the proton migration with a consequent conductivity and performance loss.
• the polymer structure is subject to a slow process of structural reorganisation making the conductivity loss irreversible.
• the mechanical characteristics of the membrane and in particular the plastic reserve thereof steeply decay, with an intolerable increase of the damage frequency in form of porosities and micro-fractures especially localised in the regions of higher mechanical strain, such as for instance the edges and the optional irregularities of the electrode surface.
• the prior art is substantially directed either to ensure the air feed humidification, which however, being not complete for the above mentioned reasons, softens the risks of dehydration without eliminating the same, or to employ electrodes provided with reduced porosity in the gas inlet region, capable of hindering the diffusion of the product water thus better preserving the membrane hydration, but at the cost of a performance loss and of non negligible complications in the production phase.
• the inventors on the basis of a wide testing, have surprisingly found that it is possible to make a fuel cell operate in a stable fashion in a wide range of current density as required by the practical applications, with voltages representative of good energy conversion efficiencies, with stoichiometric factors equalling those of common use in the prior art (indicatively comprised between 1.2 and 3) also at near-ambient pressure and with no pre-humidification of the air feed, if the air feed is pre-cooled below 35°C and preferably below 30°C, with an optimum value around 25 ⁇ 2°C.
• stoichiometric factor as known to the experts of the field, it is intended the ratio between moles of fed reactant and moles of reactant required by the reaction stoichiometry.
• Figure 2 is a comparison of the performances of the stack in figure 1 with those of a stack also working at 1.5 bar absolute but with the known parameters of the current technology, in particular with air feed pre-humidified at 80°C and stoichiometric factor of 2.5: the voltage and power curves are respectively indicated with (101) and (201). Surprisingly, the two performances show only marginal differences.
• the air feed temperature is decreased to the specified values, the reduced water vapour tensions in the air inlet region substantially reduce the local evaporation rate and the amount of water that can be evaporated.
• the air moving across the fuel cells, progressively increases its temperature with a simultaneous increase of the water vapour tensions and with a progressive evaporation: however such evaporation results distributed along the cell active area.
• the merit of the present invention might be distributing the water evaporation avoiding the hazardous concentration thereof in limited areas as ⁇ Q
• Figure 3 shows the result of a series of tests carried out adjusting the cooling water temperature and/or flow-rate so as to vary the outlet exhaust air temperature up to the limit where a performance loss began to be noticed, likely associated to the onset of membrane dehydration. The tests were carried out at 25°C with a stoichiometric factor of 2.
• the maximum temperature of the outlet exhaust air (ordinate value) compatible with a stable operation in time using the indicated stoichiometric factor is a function of the working pressure (abscissa value): if the limiting factor is effectively the onset of membrane dehydration the result of figure 3 is then unsurprising since lower air volumetric flow-rates correspond to higher pressures and hence, all other conditions being equal, the capacity of withdrawing vapour decreases, which permits increasing the outlet temperature.
• the temperature and/or flow-rate of the cooling water or in the more general case of the coolant fluid
• the ambient or near-ambient pressure operation is particularly interesting under the system reliability standpoint, considering the fact that the employed machines, particularly the air feed fan, and the seals, particularly the stack peri etrical gaskets which have a very high overall linear development, result not too critical.
• cooling water it is necessary to maintain the flow-rate thereof within reasonable limits in order to contain the energy consumption for the circulation within acceptable values: this condition practically results in a temperature variation between fuel cell inlet and outlet of about 10-15°C (with inlet temperatures of about 45-60°C), corresponding to flow-rates around 25-40 litre/hour/m 2 . In any case the cooling water outlet temperature results substantially coincident with that of exhaust air.
• Figure 5 represents a scheme of realisation of a conditioning device capable of taking the air feed to the above temperature levels: (1) indicates the air feed aspiration tubing, (2) the fan whose purpose is to ensure the required flow-rate with a modest overpressure, indicatively from 1.1 to 1.5 bar absolute (the described scheme is applicable, although with lesser advantages, to systems operating under pressures of 2-5 bar absolute, in which case (2) is a suitable compressor), (3) the delivery tubing of air exiting the fan (or compressor), (4) a heat exchanger directed to cool down the air warming up in the fan (or compressor) (2) using ambient air (5), (6) is the air feed delivery tubing to the expander (7), for instance a rotating expander, in which the air pressure is partially reduced with production of mechanical work, that is transmitted to the fan (2), and with temperature decrease, (8) the delivery tubing to the fuel cell stack (9), and (10) the exhaust air outlet tubing.
• (1) indicates the air feed aspiration tubing
• the fan whose purpose is to ensure the required flow-rate with a modest overpressure, indicative
• the temperature level that can be reached in the expander is a function of the pressure in (8) to pressure in (6) ratio: for a given pressure in (8) (stack operating pressure) the temperature level attainable by expansion is only a function of pressure in (6) (practically the fan or compressor (2) outlet pressure) and of the temperature in (6), in its turn a function of the ambient air temperature, which in the most unfavourable conditions (design conditions) is assumed to be 4Q-45 . In principle, very low temperatures can be achieved, even below 0°C, but this would require excessive pressures in (6) with unacceptable energy consumption. Moreover, as previously discussed, interesting results are already obtained with temperatures of about 25°C. Using this value as the optimal cooling level, the following data are obtained in the case of a 9 kW stack consisting of 100 fuel cells, each of 225 cm 2 active area, and 100 cooling cells intercalated thereto fed with demineralised water:
• the heat exchanger (4) represents no critical component for the system design, as it simply has to cool down the air-feed flow from a temperature at the fan outlet which in the worst of cases is lower than 80X, to a temperature of delivery to the expander around 50X (typically 50+3X): this may be easily accomplished making use of a second ambient air flow as the cooling fluid, considering also the modest thermal content of the non-humidified air feed.
• Figure 6 indicates how, wishing to maintain the temperature of the air-feed to the stack at 25X, the compression work is characterised by a multiplying factor (indicated as ⁇ on the ordinate axis) which is a function of ambient temperature (on the abscissa) and of the stack working pressure: curve (401) is relative to an absolute working pressure of 1.05 bar, curve (402) to 1.1 bar, curve (403) to 1.5 bar, curve (404) to 2.0 bar.
• ⁇ on the ordinate axis

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