This invention relates to circulating an antifreeze solution from a reservoir through water channels of porous, hydrophilic water transport plates and back to the reservoir; the mixture enters the fine pores of the water transport plates which are warmed by the heat of the fuel cell process, thereby evaporating water which may include product water (but not antifreeze) from the plates into the process oxidant flow channels, cooling the fuel cells. Water is condensed out of the process air oxidant exhaust and returned to re-mix with the concentrated antifreeze.
It is known that water produced at the cathodes of fuel cells has to be removed from the cathodes in order to prevent the water from blocking the flow of oxidant gas, such as air, from reaching the electrodes. It is also known that fuel cells, when operating, must be cooled to keep the fuel cells at a proper operating temperature. Some fuel cells are cooled only by conduction of heat into cooler plates which are interspersed between some or all of the fuel cells.
One known type of fuel cells employ reactant gas flow field plates which are porous and hydrophilic, having fine pores to allow water to pass from the cathode into the oxidant reactant gas flow channels, and to allow water to pass from the fuel reactant gas flow channels toward the membrane. These are typically called water transport plates. Cooling is typically accomplished by sensible heat transfer to water in the water flow channels formed in or adjacent to the water transport plates.
It has been known to cool fuel cells by evaporation, typically by providing atomized water to the reactant gas streams, which water evaporates, thereby cooling the stack.
In fuel cells which have employed separate cooler plates, the use of an antifreeze mixture as coolant in place of water is known. The use of separate cooler plates requires a fuel cell stack to occupy a larger volume than it would without cooler plates. Similarly, atomizing water into reactant gas streams for evaporative cooling requires additional equipment, which increases cost and volume and presents difficulty, especially at shut down, for fuel cell power plants operating in freezing environments.
In any of the cases referred to, even when antifreeze is used in cooler plates, the requirement to eliminate all water from the stack and auxiliary plumbing before freezing, or to otherwise accommodate the likelihood of freezing temperatures during fuel cell power plant shut down poses additional difficulties, requiring apparatus that adds cost and volume, which are most undesirable when a fuel cell is used as a power source for an electric vehicle.
Objects of the invention include: reducing the volume of a fuel cell power plant; eliminating or reducing freezable water in a fuel cell power plant system; improving fuel cell power plant for use where freezing temperatures may be encountered when the fuel cell is not operating; avoiding having freezable liquid in contact with moving parts in a fuel cell power plant; shortening fuel cell power plant startup time by reducing cell stack thermal mass; and improved fuel cell power plant.
According to the invention, fuel cells in a fuel cell power plant are evaporatively cooled by evaporation of at least some of the water in an antifreeze mixture with a freeze depressing substance in the porous, hydrophilic reactant gas flow field plates, which typically have reactant gas flow channels extending from a surface of reactant flow field plates opposite from coolant passageways. The antifreeze coolant mixture circulates through the coolant passageways in or adjacent the reactant gas flow field plates. A more concentrated mixture returns to a coolant reservoir. The evaporation of water from the antifreeze mixture and product water into the reactant streams (primarily the cathode) cools the fuel cell stack. At least some water vapor is condensed out of at least the oxidant reactant gas stream exiting from the stack, the condensed water being returned to the mixture in the accumulator. To avoid diluting the antifreeze mixture, less than all of the water vapor in the air exhaust may be condensed. The rate of condensing may be controlled using a condensate controller to ensure proper water balance, such as a variable flow cooling fan for the condenser, or by cooling the air in the condenser with a controlled circulation of antifreeze.
A pump is used to pump the antifreeze mixture in a conventional fashion similar to the manner of circulating coolant water in conventional fuel cells. Since only the antifreeze is present in the pump, freezing during shutdown is not a problem.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.
Referring to
The desired mixture, such as at the top 25 of the reservoir 21, is fed through a conventional coolant inlet manifold 26 into coolant channels 28 of the fuel cells (described with respect to
The fuel cell coolant channels 28 are connected to a coolant exit manifold 32 which is interconnected by means of a conduit 33 to the reservoir 21. At the inlet 37 where the coolant returns to the reservoir, the coolant may be substantially antifreeze 23 (e.g., PEG); that is to say, a very concentrated solution of antifreeze 23. However, this is remixed in the reservoir 21, such as by means of the pump 22, if desired; if the pump 22 is not necessary in any given embodiment of the invention, it may be omitted. Furthermore, other ways of assuring an adequate mixing of the returned antifreeze 23 with the rest of the fluid in the reservoir 21 may be used within the purview of the present invention.
The water transport plates 29 absorb heat generated in the catalytic reaction of oxygen and hydrogen. Although the antifreeze 23 is non-volatile at the operating temperature of the fuel cell stack, on the order of 60° C.-70° C. (140° F.-158° F.), water evaporates into the oxidant reactant gas stream flow channels 41 that receive oxidant, such as air from an air inlet manifold 42, cooling the fuel cells by the heat of vaporization. The saturated (or nearly saturated) air exits the fuel cells through an air exit manifold 45 and enters a condenser 46 where at least some water vapor is condensed out of the process air, the dried air flows to exhaust 47, and the condensate, which is essentially pure water, flows to the reservoir 21 directly or through a conduit 48. In the area 49 where the condensate enters the reservoir 21, the mixture is dilute. However, it is remixed with concentrated antifreeze 23 within the reservoir before reentering the fuel cells through the coolant inlet manifold 26.
Fuel provided to a fuel inlet manifold 55 flows to the left, then through a fuel turn manifold 56, after which fuel flows to the right and out through a fuel exit manifold 57; the exhausted fuel may be recycled or consumed in a related process.
Coolant from the reservoir 21 flows through a coolant conduit 60 to the coolant inlet manifold 26. The coolant passes into the coolant channels (as described with respect to
To ensure that adequate water will be present in the fine pores for evaporation, the pump 22 (
Because the PEG, or other antifreeze, has a viscosity many times higher than that of pure liquid water, the pressure drop across the coolant channels will be high, or, the coolant channels will have to be larger (deeper) to accommodate the coolant flow rates required to cool the stack. Deeper channels decrease the number of cells per unit of stack length compared to fuel cell stacks employing water transport plates and using evaporative cooling. The channel depth will nonetheless be shallower than in systems employing coolant water or similar systems employing an antifreeze mixture to cool the stack using the fluid sensible heat exchange. Thus, the invention will provide power density which is greater than traditional water or antifreeze cooling systems.
Detailed descriptions of fuel cells having water transport plates may be found in patent publication US2004/0106034.
Referring to
To prevent loss of fuel cell stack performance, the MEAs 70 must be shielded from the non-water component of the antifreeze mixture in the coolant channels 28. Therefore, the invention preferably employs fuel cells 63 with GDLs 72 which are treated, such as with polytetrafluoroethylene (PTFE) to be wet-proofed, or include an additional wet-proof layer.
If desired in any utilization of the invention, either the anode WTP 29a or the cathode WTP 29b may be solid. A solid WTP will block coolant from reaching the MEA on the side it is located. If the cathode WTP 29b is solid, water will reach the air (oxidant) flow field channels 66 by migration through the membranes of the MEAs 70 and GDLs 72. Alternatively, the surfaces of one of the WTPs 29, adjacent to the GDL 72, including the reactant gas flow field channels 65, 66, may be wet-proofed by treating with a wet-proofing material, such as PTFE, to shield the membrane from the PEG or other antifreeze on that side.
The coolant channels 28 may be formed by having grooves 75 on the opposite surface of the anode water transport plates 29a from the fuel reactant gas flow field channels 65 which match up with grooves 76 on the opposite surface of cathode water transport plates 29b from oxidant reactant gas flow field channels 66. Or, the grooves may be in only one plate 29a, 29b, the matching surface of the other plate 29b, 29a being flat.
Number | Date | Country | Kind |
---|---|---|---|
2006-137862 | May 2006 | JP | national |
2006-324195 | Nov 2006 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US2005/043942 | 12/1/2005 | WO | 00 | 6/12/2008 |