WO2007064338A1

WO2007064338A1 - Evaporative cooling of fuel cells employing antifreeze solution

Info

Publication number: WO2007064338A1
Application number: PCT/US2005/043942
Authority: WO
Inventors: Jeremy P. Meyers; Ryan J. Balliet
Original assignee: Utc Fuel Cells, Llc
Priority date: 2005-12-01
Filing date: 2005-12-01
Publication date: 2007-06-07

Abstract

A fuel cell power plant (19) has a stack of fuel cells (20) cooled by a mixture of water with a non-volatile, miscible fluid that sufficiently depresses the freezing point, such as polyethylene glycol (PEG). The water and fluid are mixed in a reservoir (21), a small pump (22, 60) flows the mixture through coolant channels (28) in or adjacent water transport plates (29); heat of the catalytic reaction warms the water transport plates causing water to evaporate therefrom thereby cooling the stack. The PEG is non-volatile at stack operating temperature and does not evaporate; concentrated PEG is returned (33) to the reservoir (21). Water in the process air flow channels (41), including evaporated process water, is recovered in a condensation-rate-controlled (53, 54)) condenser (46) in communication (48) with the reservoir (21) for remixture with the concentrated PEG solution. Hydrophobic gas diffusion layers (72) shield the proton exchange membrane (70) from the PEG.

Description

Evaporative Cooling of Fuel Cells Employing Antifreeze Solution

Technical Field

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.

Background Art

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.

Disclosure of Invention

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.

Brief Description of the Drawings Fig. 1 is a block illustration of the invention.

Fig. 2 is a partial perspective view of an embodiment of the invention.

Fig. 3 is a fragmentary view of a variation of Fig. 1.

Fig. 4 is a fragmentary view of an alternative to the embodiment of Fig. 2.

Fig. 5 is a sectioned, side elevation view, with sectioning lines omitted for clarity, of portions of fuel cells useful with the invention.

Mode(s) for Carrying Out the Invention Referring to Fig. 1, a fuel cell power plant 19 has a stack 20 of fuel cells.

The concept of the present invention is illustrated by the density of stippling to provide a rough indication of the fraction of fluid in the fuel cell stack coolant that is antifreeze 23, such as polyethylene glycol (PEG) or other non-volatile, miscible fluid that sufficiently suppresses the freezing point of a mixture with water. In Fig. 1, PEG and water are mixed in a reservoir 21, assisted by a pump 22, so as to achieve the desired freeze point of the mixture. Because the pump is in an antifreeze solution, the pump will not be rendered inoperable due to freezing conditions.

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 Fig. 4, hereinafter) in the stack 20, in which the coolant channels are formed within or in fluid communication with, porous, hydrophilic water transport plates 29, which have fine pores that contain a water-antifreeze mixture.

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 ot 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. Fig. 2 illustrates a portion of the fuel cell power plant 19 including the fuel cell stack 20, which employs evaporative cooling. Air is provided to the air inlet manifold 42 and proceeds through the oxidant flow field channels 41 (Fig. 1) to the air exit manifold 45 and thence into the condenser 46. The air outflow from the condenser 46 is above the water line 51 of the water reservoir 21. The cool dried air is expelled to exhaust 47. The coolant for the condenser 46 may comprise ambient air as illustrated by arrows 52, the volume of which is controlled by a condenser controller 53 that varies the speed of a flow fan 54 in order to adjust the condensation rate as needed. The condenser 46 may serve as a manifold, and the air exit manifold 45 may then be omitted. 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 Fig. 1 hereinbefore) to the top of the fuel cell stack 20, and through the coolant exit manifold 32. Coolant flowing out of the coolant exit manifold 32 is recirculated over the conduit 33 to the reservoir 21. The water in the coolant mixture entering through coolant inlet manifold 26 replaces that which is evaporated into the process air channels 41, as described with respect to Fig. 1 hereinbefore. To ensure that adequate water will be present in the fine pores for evaporation, the pump 22 (Fig. 1) may be disposed at the inlet 37 to draw the coolant into the reservoir 21 from the line 33 as shown in Fig. 3. Or, a pump 60 (Fig. 4) may be used in the line 33 or in any other suitable location to ensure adequate coolant circulation. Usually, a pump will be required in order to assure that the flow of antifreeze mixture is sufficient to provide enough water so that evaporation will occur throughout all portions of all of the fuel cells, and to prevent the antifreeze component from partially or completely filling the pores of the water transport plates.

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 Fig. 5, fuel cells 63 which may be used to implement the present invention include anode water transport plates (WTPs) 29a having fuel reactant gas flow field channels 65 and cathode water transport plates 29b having oxidant reactant gas (air) flow field channels 66. A membrane electrode assembly (MEA) 70 includes a proton exchange membrane with catalyst on both surfaces. Gas diffusion layers

(GDLs) 72 are provided adjacent each surface of the MEAs 70. In the prior art (such as in the aforementioned patent publication US 2004/0106034) the GDLs are typically constructed from carbon fiber sheet material, and are usually wettable. In some known GDLs, there is an additional wet-proof layer deposited on or joined to the GDL to form a bi-layer. The carbon fiber layer may or may not be wet-proofed whether a bi-layer is used or not used.

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.

Claims

1. A fuel cell power plant (19) comprising: a stack (20) of fuel cells (63), each fuel cell including water transport plates (29) with coolant channels (28) formed therein or adjacent thereto and with fuel reactant gas flow field channels (65) and oxidant reactant gas flow field channels (66) having inlets and outlets, at least one of said plates being porous and hydrophilic; a source (42) of oxidant reactant gas in fluid communication with inlets of said oxidant reactant gas flow field channels; a source (55) of fuel reactant gas in fluid communication with inlets of said fuel reactant gas flow field channels; a coolant reservoir (21), each of said fuel cell coolant channels being in fluid communication with said coolant reservoir; a pump (22, 60) for circulating coolant from said reservoir, through said fuel cell coolant channels and back to said reservoir; characterized by: said coolant reservoir containing a coolant mixture 23 of water with a miscible, freeze depressing substance; and a condenser (46), connected to the outlet of at least one of said oxidant reactant gas flow field channels of said fuel cells, condensate of said condenser in fluid communication (48) with said reservoir, said coolant mixture migrating from said coolant channels into said at least one hydrophilic, porous water transport plate of each fuel cell and at least some water within said coolant mixture along with some process water evaporating into at least said reactant gas flow field channels of said at least one porous and hydrophilic plates of each fuel cell to cool said fuel cells, at least some of the water vapor in at least one of said reactant gas flow field channels being condensed in said condenser and returned to said reservoir where it mixes with coolant in said reservoir.

2. A power plant (19) according to claim 1 wherein: the reactant gas flow field is a fuel channel.

3. A power plant (19) according to claim 1 further characterized by: each fuel cell (63) including membrane electrode assembly (MEA) (70) having a membrane with catalyst on both surfaces thereof, said MEA configured to provide a wet-proofed barrier between at least one surface of said MEA and said coolant channels.

4. A power plant (19) according to claim 3 wherein said wet-proofed barrier comprises: at least one wet-proofed gas diffusion layer (72) adjacent said MEA (70) in each of said fuel cells.

5. A power plant (19) according to claim 3 wherein said wet-proofed barrier comprises: at least one bilayer (70) adjacent said MEA in each of said fuel cells.

6. A power plant (19) according to claim 3 wherein said wet-proofed barrier comprises: a solid water transport plate (29) on at least one side of said MEA (70).

7. A power plant (19) according to claim 1 further characterized by: said pump (22) being disposed at an inlet of said reservoir (21) receiving (33) circulating coolant from said coolant channels (28).

8. A power plant (19) according to claim 1 further characterized by: said pump (60) being disposed in a conduit (33) interconnecting said coolant channels (28) with an inlet (37) of said reservoir (21).

9. A power plant (19) according to claim 1 further comprising: a condenser controller (53, 54) for controlling the rate of condensation of water vapor in said condenser (46).

10. A power plant (19) according to claim 9 wherein: said condenser (46) is cooled by a stream of air (52) and said condenser controller is a controller (53) that varies the speed of an air fan (54).

11. A power plant (19) according to claim 9 wherein: said condenser (46) is cooled by a controlled flow of freeze-proof coolant through flow passages in said condenser.