WO2005008819A2

WO2005008819A2 - Evaporative coolants having low dielectric constant for use in fuel cells & other electrochemical reactor stacks

Info

Publication number: WO2005008819A2
Application number: PCT/CA2004/001086
Authority: WO
Inventors: Neil Fagan; K. Michael Flaherty; Kenneth W. Kratschmar; Eddy J. Peters; Adrian Corless; David Leboe; Rasvan Mihai; Peter J. Andrin; Eric W. Teather; Dennie T. Mah; Mike A. Hooper
Original assignee: Dupont Canada Inc.; Cellex Power Products, Inc.
Priority date: 2003-07-23
Filing date: 2004-07-22
Publication date: 2005-01-27
Also published as: WO2005008819A3; TW200511638A

Abstract

There is provided in the present invention a cooling system for an electrochemical reactor (10) that generates electrical energy and heat energy, wherein the reactor operates at a reactor temperature and a reactor pressure. The cooling system includes a coolant circulating in direct (11) or indirect (18) thermal contact with the reactor. The coolant has the following properties: a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10; a resistivity while circulated in the thermal contact of greater than 103 Ohm-cm; and a boiling point below the reactor temperature at the coolant pressure. The dielectric nature of the coolant prevents it from dissolving and attracting ions from the parent system, thereby allowing it to maintain its resistivity over extended periods of use. The coolant is in its liquid phase as it first comes into thermal contact with the reactor and a portion of the coolant is vaporized to a vapor by absorbing and removing excess heat energy from the reactor via sensible heat transfer and latent heat transfer to the coolant. Preferably, all of the coolant is vaporized to the vapor, and the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

Description

Evaporative Coolants Having Low Dielectric Constant For Use In Fuel Cells & Other Electrochemical Reactor Stacks

Field of the Invention:

[0001] The present invention relates to coolants and cooling systems for electrochemical reactor stacks, such as fuel cells, electrolysers and chemical reactors. In particular, the present invention relates to the use of coolants that absorb heat from the reactor stacks via latent heat transfer and that have low dielectric constants and high resistivity.

Background of the Invention:

[0002] Within an electrochemical reactor stack, which includes but is not limited to fuel cells (devices that are chemical reactors that generate electrical power from chemical feeds) or electrolysers (devices that generate chemical products from electrical power) or other electrical systems wherein heat is generated as a result of operation, there is a need to remove excess heat energy generated within the reactor to maintain a stable operating temperature. With respect to fuel cells, the fact that the reactor stack generates heat requires the presence of a cooling system that typically includes a circulating pump, plumbing, and heat exchanger or radiator. In a typical fuel cell, the cooling system components need to be made of expensive corrosion- resistant materials. In addition, a deionizer is typically included in the cooling loop to decrease the conductivity of the cooling fluid in order to prevent stack shunt currents

[0003] Typically, a liquid coolant, in particular deionised water with a high dielectric constant (80.1 at STP), is utilized to remove the excess heat energy by passing the coolant through the stack. The excess heat energy increases the liquid coolant temperature as it passes through the stack, resulting in thermal gradients within the stack.

[0004] With respect to fuel cells, multiple electrochemical cells may be contained within the fuel cell stack in series. This arrangement gives rise to significant electrical potential differences within the stack that can result in an electric current being driven through a conductive coolant. This transfer of electric current from the stack components to the coolant liquid can result in a corrosive attack of the stack components or in the generation of hazardous gases. Under certain conditions of fuel cell stack size, as typically seen in transportation and stationary power plants, there may exist sufficient conducted electrical voltage and current as to present a hazardous and a possibly lethal condition.

[0005] If water is used as the coolant, it can easily dissolve substances into ions due to its polarity and high dielectric constant, thus becoming a conductive coolant; therefore, it is necessary for the water to be periodically or continuously deionised to avoid these stray currents. Deionisation resins are used to deionise the water, however operation of commercial deionisation resins are limited to an upper temperature limit of about 50 to 60°C; therefore, the water coolant temperature entering the deionizer should not exceed the maximum operating temperature for the deionizing material and must be sub-cooled well below this temperature. The water coolant temperature exiting the stack doesn't necessarily need to be below 60 °C because the entire stream may be cooled to below 60 °C after leaving the stack and before entering the deionization filter. This involves a large amount of heat exchange capacity simply to manage the low temperature needs of the deionisation resin. Also, the temperature difference between the cooling fluid exit temperature from the radiator and the ambient environment is quite small, thereby necessitating a larger radiator to remove the excess heat energy. The large radiator is costly and takes up space.

[0006] The fuel cell stack can also be exposed to very cold environments, often in the neighbourhood of -40 °C; therefore, the coolant liquid should also be able to withstand freezing at these cold temperatures.

[0007] U.S. patent no. 4,824,740 issued to International Fuel Cell Corporation for an invention called "Fuel Cell Stack Cooling System". Disclosed as the coolant is a two- phase stack coolant water where water vapour & liquid are separated in a heat exchanger operating at a temperature lower than the fuel cell stack temperature. [0008] U.S. patent no. 4,769,297 issued to International Fuel Cell Corporation on Sept. 6, 1988 for an invention entitled "Solid Polymer Electrolyte Fuel Cell Stack Water Management System". The fuel cell is cooled from the anode side. Excess water is fed in the hydrogen reactant stream sufficient to hydrate the membrane, as well as provide water to the porous anode flow field plate for evaporative cooling. Thus, water is used as the coolant and it is in direct contact within the fuel cell.

[0009] U.S. patent no. 4,824,741 also issued to International Fuel Cell Corporation. The invention entitled "Solid Polymer Electrolyte Fuel Cell System With Porous Plate Evaporative Cooling" relates to the cooling of a fuel cell from the anode side. Water is pumped into the flat side of porous anode flow field plate (side not adjacent to the membrane electrode assembly). The water migrates through the plate to the lands to be absorbed by the membrane and to provide a moist surface from which water will evaporate to cool the fuel cell. This patent, therefore, teaches the use of water as the coolant, which is in direct contact within the fuel cell.

[0010] U.S. patent no. 5,206,094 relates to an invention called "Fuel Cell Evaporative Cooler". The fuel cell stack is cooled by evaporation of water into either the fuel or oxidant stream in a cooler adjacent to the stack. This system avoids two-phase flow separation by:

[0011] 1 - separately supplying the coolant and carrier gas so they are mixed within the cooler. The water is fed from a manifold to a spray bar at the inlet of every cooler; and

[0012] 2 - introducing the water and carrier gas on opposite sides of a porous plate and are mixed as they pass through the plate.

[0013] U.S. patent no. 6,117,577 issued to the Regents of the University of California for an invention called "Ambient Pressure Fuel Cell System". Excess air is fed to the cathode side of the fuel cell to provide evaporative cooling of the product water. An air blower is used that includes a controller responsive to the temperature of the stack. Thus, this system uses cathode air as the means to cool the stack. [0014] U.S. Patent no. 5,262,249 issued to International Fuel Cells Corporation for an invention called "Internally Cooled Proton Exchange Membrane Fuel Cell Device".

Disclosed is a means of cooling a fuel cell unit using a hermetically sealed casing featuring heat transfer substances of the type that evaporate at temperatures below those reached by the central zone of the casing and that condense at temperatures above those prevailing at a peripheral zone of the casing. This patent does not identify potential coolant fluids other than water. The disclosed invention keeps the heat transfer medium sealed in a closed casing while coolant fluid (i.e., water) is circulated in contact with the casing to remove the latent heat of vaporization absorbed by the heat transfer fluid. Furthermore, the patent makes no mention of shunt currents generated by a conductive coolant such as water and even states that it is unlikely that the cooling medium will be contaminated by circulation through the fuel cell environment.

[0015] U.S. patent application no. 20030047708 was published March 13, 2003 for an invention entitled "Novel Chemical Base For Fuel Cell Engine Heat Exchange Coolant/Antifreeze". Disclosed is the use of propanediol as a cooling fluid in fuel cells. Also disclosed are the requirements for a good cooling fluid, namely, the coolant which flows around the aluminium components of the fuel cell must be nonconductive to protect the cell from shorting out and to prevent electrical hazards. Also, the liquid should have an electrical resistivity greater than 250 kOhm-cm, a boiling point greater than 90 °C, a freezing point less than - -0 °C, a thermal conductivity greater than 0.4 W/m-k, a viscosity less than 1 cPs at 80 °C and less than 6 cPs at 0 °C, a heat capacity greater than 3 kJ/kg-K, and a durability greater than 5,000 hours of operation/3 years total time. The liquid should also be compatible with current cooling system materials. The combustible nature of 1,3 propanediol may be of concern for a closed-loop operating system such as within a fuel cell cooling loop. Propanediol features good electrical compatibility characteristics but boils above typical fuel cell operating temperatures (60-90 °C), making it useful only for sensible heat transfer; thermal gradients within the stack will thus result from its use. [0016] U.S. patent no. 6,374,907 issued to 3M Innovative Properties Company for an invention called "Hydrofluoroether As A Heat Transfer Fluid". Disclosed is a device and means for heat transfer that uses as the heat transfer fluid 3-ethoxy-perfluoro(2- methylhexane) having at least 95% purity. This patent discloses one type of heat transfer fluid to be employed over a broad temperature range in the liquid phase only.

[0017] US Patent no. 6416683B1, issued to E.I. DuPont de Nemours and Company, for an invention called "Compositions of a Hydrofluoroether and a Hydrofluorocarbon". Disclosed are compositions of at least one fluoroether and at least one fluorocarbon that may be used as, among other applications, refrigerants, heat transfer media and gaseous dielectrics. This patent discloses compositions of such hydrofluoroethers and hydrofluorocarbons that reduce the halocarbon global warming potential of the hydrofluorocarbon. Also disclosed in this patent are compositions of a fluoroether and a fluorocarbon that are azeotropic or azeotrope-like. This invention discloses compositions featuring boiling points well below typical fuel cell operating temperatures of 55-90 °C and generally below ambient temperature.

[0018] The present invention solves the problems of prior art systems by employing a coolant that vaporizes while cooling the electrochemical reactor by absorbing and removing heat via latent heat transfer. The coolant preferably conducts very little electrical current and has a very low capacity for the dissolution of ions (i.e., very low tendency to act as a solvent for potentially conductive ions) so that it may therefore remain non-conductive for extended periods. Also preferred is a coolant that withstands freezing in cold environments while minimizing balance of plant and extending stack component durability.

[0019] In particular, the present invention enables the elimination of a deioniser from the cooling system, and the elimination of customized, expensive corrosion-resistant heat transfer equipment for use in cooling the reactor. In addition, the present invention simplifies the temperature control system required by the reactor through the self-regulation of the reactor temperature provided by the coolant that evaporates at the desired stack operating temperature. Thermal gradients within the reactor are thus avoided via the use of the coolant latent heat of vaporization as opposed to more traditional sensible heat transfer whereby coolant fluid temperature changes as heat energy is transferred.

[0020] The disclosures of all patents/applications referenced herein are incorporated herein by reference.

Summary of the Invention:

[0021] According to one aspect of the invention there is provided a cooling system for an electrochemical reactor that generates electrical energy and heat energy, wherein the reactor operates at a reactor temperature, the cooling system comprising a coolant circulating in direct or indirect thermal contact with the reactor at a coolant pressure, wherein the coolant has:

[0022] (a) a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10;

[0023] (b) a resistivity while circulated in said thermal contact with the reactor of greater than 10³ Ohm-cm; and

[0024] (c) a boiling point below the reactor temperature at the coolant pressure,

[0025] wherein the coolant is in its liquid phase as it first comes into said thermal contact with the reactor and a portion of the coolant is vaporized to a vapor by absorbing and removing excess heat energy from the reactor via sensible heat transfer and latent heat transfer to the coolant.

[0026] Preferably, the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons. [0027] In a further embodiment of the present invention, there is provided a method of cooling an electrochemical reactor that generates electrical energy and heat energy, the method comprising the steps of:

[0028] (a) operating the reactor at a reactor temperature;

[0029] (b) circulating in direct or indirect thermal contact with the reactor a coolant at a coolant pressure;

[0030] (c) heating the coolant by absorbing and removing excess heat energy from the reactor via sensible heat transfer to the coolant; and

[0031] (d) vaporizing a portion of the coolant to a vapor by absorbing and removing excess heat energy from the reactor via latent heat transfer to the coolant,

[0032] wherein the coolant has a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10; a resistivity while circulated in said thermal contact with the reactor of greater than 10³ Ohm-cm; and a boiling point below the reactor temperature at the coolant pressure.

[0033] Preferably, the electrochemical reactor is an electrochemical fuel cell such as a proton exchange membrane fuel cell.

[0034] In yet a further aspect of the present invention, a method is provided of operating a fuel cell stack at a desired operating temperature, wherein the fuel cell stack generates electrical energy and excess heat energy, the method comprising the steps of:

[0035] (a) circulating in direct or indirect thermal contact with the fuel cell stack a coolant at a varying pressure;

[0036] (b) heating the coolant by absorbing and removing the excess heat energy from the fuel cell stack via latent heat transfer to the coolant so that a portion or all of the coolant is vaporized to a vapor; and [0037] (b) adjusting the pressure of the coolant so that the desired operating temperature is obtained,

[0038] wherein the coolant has a dielectric constant, at the operating temperature and pressure, of less than about 10; a resistivity while circulated in said thermal contact with the reactor of greater than 10³ Ohm-cm; and a boiling point below the reactor temperature at the reactor pressure.

[0039] Numerous other objectives, advantages and features of the present invention will also become apparent to the person skilled in the art upon reading the detailed description of the preferred embodiments, the examples and the claims.

Brief Description of the Drawings:

[0040] The preferred embodiments of the present invention will be described with reference to the accompanying drawing:

[0041] Fig. 1 is schematic representation of one embodiment of the cooling system of the present invention.

Detailed Description of the Preferred Embodiments:

[0042] The preferred embodiments of the present invention will now be described with reference to the accompanying figures.

[0043] In a preferred embodiment, the present invention consists of a cooling system for cooling an electrochemical reactor that generates electrical and heat energy. As shown schematically in Fig. 1, the electrochemical reactor 10 may be a fuel cell, an electrolyser or a chemical reactor. Reactor 10 generates excess heat that must be removed from the recator 10. Within reactor 10 are cooling channels 11 through which a coolant is circulated in direct or indirect thermal contact with reactor 10. The coolant enters cooling channels 11 as a liquid via inlet pipe 14. The liquid coolant is caused to circulate through the cooling channels 11 by use of a pump 12. The coolant liquid, while in cooling channels 11, absorbs and removes excess heat energy generated by the reactor 10 via sensible heat transfer and latent heat transfer. Thus, a portion, or in some cases all, of the coolant liquid entering cooling channels 11 exits the cooling channels 11 as a vapor because a portion or all of the coolant is vaporized. The partially or wholly vaporized coolant exits recator 10 via outlet 16 and is directed to heat exchanger/condenser 18 where the vaporized coolant is condensed back into a liquid and the coolant may optionally be further cooled. The cooled coolant exits heat exchanger/condenser 18 through conduit 20 and is directed to pump 12 where it is caused to circulate again through the cooling channels 11 in reactor 10. An accumulator 22 may be present in line 20 to allow for coolant volume expansion.

[0044] In a preferred embodiment, the present invention consists of the use of a coolant in an electrochemical reactor stack, such as a fuel cell, to remove excess heat energy generated by the reactor stack via latent heat transfer to the coolant or via a combination of latent and sensible heat transfer to the coolant depending on the physical state of the coolant at any given point in the coolant channels.

[0045] The coolant used in the present invention preferably has the following properties:

[0046] (a) A dielectric constant, at the reactor operating temperature at at the coolant pressure, of less than about 10, with a corresponding low capacity for the dissolution of ions (i.e., a low dielectric constant is generally indicative of a low tendency to act as a solvent for potentially conductive ions) such that the coolant liquid will maintain a resistivity while in use of greater than 10 Ohm-cm for extended periods;

[0047] (b) An initial electrical resistivity of greater than about 10⁷ Ohm- cm, preferably greater than about 10 Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm;

[0048] (c) A fluid or fluid composition boiling point that is below the electrochemical reactor operating temperature, at the operating coolant pressure of the reactor coolant loop (from about a light vacuum to as high as 60 psig but preferably less than about 15 psig such that standard automotive cooling components can be used), and preferably at least 1 to 5 °C less than the operating temperature. This allows a portion, or all of the coolant to be vaporized while in direct or indirect thermal contact with the reactor. For example, for current fuel cells and fuel cell stacks, this operating temperature is in the range from about 50°C to about 95°C. For low temperature fuel cells, this operating temperature is from about room temperature to about 50°C. However, for next generation high temperature fuel cells and fuel cell stacks, the operating temperature will be in excess of about 95°C, extending up to about 125°C in the short term and about 200°C in the longer term; .

[0049] (d) Little or no capacity for poisoning of the reactor active area;

[0050] (e) At coolant pressures, a liquid viscosity, at -40 °C, of less than about 5 cPs, preferably less than about 2 cPs, most preferably less than about 1 cPs; and a liquid viscosity, at 80 °C„ of less than about 1.0 cPs, preferably less than about 0.5 cPs, most preferably less than about 0.3 cPs;

[0051] (f) A freezing point, at ambient pressure, of less than about -40 °C, preferably less than -50 °C;

[0052] (g) A relatively low atmospheric lifetime and Greenhouse Warming Potential to minimize potential environmental impact.

[0053] (h) Non-toxic and non-flammable characteristics.

[0054] The dielectric constant of a material is the ratio of the permittivity of a substance to the permittivity of free space. It represents the extent to which a material concentrates electric flux. As the dielectric constant increases, the electric flux density increases, if all other factors remain unchanged. This enables the material to hold its electric charge for long periods of time, and/or to hold large quantities of charge without surrendering said charge to surrounding media. .

[0055] For the present invention, the term 'extended periods' in reference to fluid resistivity refers to periods of time of practical use for electrochemical reactor operations, such that the deterioration of coolant resistivity over this time period, and the consequent removal, deionization or replacement of the coolant does not present a significant increase in system or component cost or complexity. Preferably, the resistance to dissolution of ions of the coolant will permit a maintained resistivity greater than 10³ Ohm-cm for more than 1000 hours, more preferably greater than 3000 hours, most preferably greater than 5000 hours of operation.

[0056] The coolant cools the reactor stack by absorbing excess stack heat via latent heat transfer to the coolant, and optionally by a combination of latent heat transfer and sensible heat transfer, so that a portion or all of the coolant is vaporized after absorbing the excess heat energy. By latent heat transfer, it is meant that the coolant absorbs heat from the reactor by vaporizing from the liquid phase to the vapor phase. By sensible heat transfer, it is meant that the coolant remains in the liquid phase throughout the cooling cycle.

[0057] In a second embodiment of the invention, the coolant is ciculated within a fuel cell by passing it through a cooling plate or cooling cell interspersed between the reactor plates of adjacent fuel cells within a fuel cell stack. The coolant is circulated in sufficient quantity to remove excess heat generated by the reactor via latent heat transfer, or a combination of latent and sensible heat transfer.

[0058] In a third embodiment of the invention, the coolant is circulated in a system such as that depicted in either US Patent No. 6,355,368 Bl or 6,146,779 in which the phase change of the coolant is used to provide the motive force for circulation. The 'thermal siphon' and 'heat pipe' concepts of these two patents, respectively, are particularly attractive for coolants such as those disclosed herein, as they allow for yet further reduction of parasitic losses associated with a fuel cell application via elimination of pumping equipment. Neither of the specified patents touches upon the electrical compatibility of the employed fluid and consequently does not benefit from elimination of deionizers within the system or from the control and minimization of shunt currents.

[0059] In a fourth embodiment of the present invention, the coolant is selected so that changes in the cooling system operating pressure result in a predictable and useful change in the coolant boiling point with respect to coolant pressure. That is, varying the operating pressure of the cooling system can alter the boiling point of the coolant within the cooling system. This ability to vary the coolant boiling point ensures that a portion or all of the coolant is vaporized while absorbing excess heat from the reactor. This variation of boiling point temperature can be achieved over a relatively modest pressure range (i.e., up to about 15 psig system pressure such as is commonly found in automotive coolant systems). Increasing cooling system pressure generally allows for the elevation of the coolant boiling point to ensure that it is maintained at the desired level below the operating temperature of the reactor stack.

[0060] In this manner, a hydrofluorocarbon (HFC) coolant fluid such as Vertrel®, available from E.I. du Pont, can be used in the cooling system of the present invention. Vertrel® HFC boils at 54.6°C at a pressure of 14.7 psia (1 atm); by elevating the cooling system pressure by only 19.3 psi, the same Vertrel® coolant liquid now boils at about 80°C.

[0061] In a further embodiment of the present invention, the optimal reactor operating temperature and the control thereof may be achieved through manipulation of the properties of the coolant. By absorbing and removing heat from the reactor via the latent heat of vaporization of the coolant, the reactor temperature may be controlled by varying the pressure of the coolant. Thus, one can alter the operating temperature of the reactor to optimize performance and adapt to varying environmental or operational demands simply by adjusting the coolant system pressure.

[0062] In another preferred embodiment of the invention, the coolant is selected so as to have a relatively low rate of mass transfer with respect to the electrochemical reactor, cooling plate and cooling cell components with which it is in contact. Low mass transfer (i.e., low permeability or diffusivity) of the coolant through the reactor components corresponds to a low loss rate of coolant, thus allowing for longer coolant cell lifetime, less frequent charging of cooling liquid, and minimization or eleimination elimination of reactant/coolant crossover.

[0063] In a further embodiment of the invention, the coolant preferably has a low viscosity at reactor operating temperatures and coolant pressures. A lower coolant viscosity allows for a reduction in required pumping power, leading to increased efficiency of the reactor system. A lower coolant viscosity also corresponds to a reduced flow velocity required for turbulent flow for a given operating temperature and coolant pressure. Turbulent flow of the coolant when in direct or indirect thermal contact with the reactor is desirable because heat transfer occurs much more readily between surfaces exposed to a turbulent medium, unlike in a laminar flow scenario where a stagnant film can act as an insulator to heat transfer.

[0064] The present invention provides one or more of the following benefits:

[0065] (1) The coolant has an inherently high electrical resistivity coupled with a low dielectric constant, a low capacity for the attraction and dissolution of ions and compatibility with current fuel cell component materials to ensure that low conductivity is not compromised by the presence of conductive ions. Therefore, the coolant will not dissolve ions from the cooling loop and will not become conductive while in- use in the reactor stack for extended periods of time.

[0066] (2) The ability to control the reactor operating temperature to near- isothermal conditions through evaporative cooling (i.e., latent heat transfer in the absence of temperature change) within the fuel cell stack, which provides the potential to extend the useful operating life of the fuel cell components. [0067] (3) A coolant with permanently low conductivity will eliminate the possibility of stray electrical currents that cause corrosion within the reactor without having to regularly deionize the coolant.

[0068] (4) A coolant with permanently low conductivity eliminates the need for expensive non-corroding materials in the cooling loop and eliminates the need for a deioniser. The low conductivity coolant is compatible with commercial heat transfer materials and equipment in use in general automotive, refrigeration and HVAC applications. Thus, the present invention enables a lower cost cooling system, significantly reduces the size (volume) required by the cooling system components, and extends the operating temperature range which was previously constrained by the limited operating temperature range for deioniser resins.

[0069] (5)- The coolants of the present invention can withstand freezing in cold environments.

[0070] (6) Use of a coolant with relatively low viscosity, in both the liquid and gaseous state, at the reactor temperatures and coolant pressures, allows for a reduction in the required parasitic pumping power, leading ' to greater system efficiency. Lower fluid viscosity also allows for more facile introduction of turbulent flow, allowing for improvement of heat transfer to the coolant medium.

[0071] (7) The coolant is not a strong poison to the fuel cell active area and therefore greatly reduces the risk of contamination of the stack through cooling fluid leaks.

[0072] (8) The coolant is not flammable, is non-toxic and has a relatively short atmospheric lifetime and low Global Warming Potential (GWP). [0073] (9) By employing azeotropic or azeotrope-like mixtures of HFEs, HFCs or HFEs and HFCs, the present invention allows for control over key coolant fluid characteristics such as: Global Warming Potential (addition of HFEs to HFCs generally leads to a lower, more favourable Global Warming Potential for the resulting mixture), boiling point, electrical and thermal conductivity, heat capacity and atmospheric lifetime, among other properties.

[0074] (10) A further reduction in the size (volume) of the waste heat exchanger due to the fact it is a condenser and not simply transferring sensible heat.

[0075] (11) By eliminating components such as the deioniser filter and allowing for a more efficient radiator, the total weight of the system would be reduced as well.

[0076] Materials that can be used as coolants in the present invention include compositions of one or more of the hydrofluoroethers (HFEs) listed in Table 1 and/or one or more of the hydrofluorocarbons (HFCs) listed in Table 2, where the composition is preferably, but not necessarily, azeotropic or azeotrope-like.

[0077] The specific composition of the mixture, azeotropic or azeotrope-like coolant is determined by the operating temperature of the electrochemical reactor. The boiling point of the coolant should be below the operating temperature of the reactor and preferably be at least 1 to 5°C less than the operating temperature at the coolant pressure.

[0078] The chemical formulas and boiling points of hydrofluoroethers (HFEs) suitable for use as coolants in the present invention are provided in Table 1. These compounds are believed to be non-flammable and non-toxic, with relatively short atmospheric lifetimes and relatively low greenhouse warming potentials. Of these, the preferred HFEs are HEE-7100, 7200 and 7500. [0079] Table 1: Hydrofluoroethers (HFEs) Suitable For Use As Coolants In The Present Invention, In Descending Order With Respect To Boiling Point At Atmospheric Pressure.

[0080] The chemical formulas and boiling points of hydrofluorocarbons (HFCs) suitable for use as coolants in the present invention are provided in Table 2. These compounds are believed to be non-flammable and non-toxic. In instances where listed compounds have relatively high global warming potentials, it is believed that mixing with hydrofluoroethers from Table 1 will significantly reduce the global warming potential of the mixture. It should be noted that the relatively high GWPs of some of the listed HFCs are still only a fraction of the GWPs associated with many CFCs and HCFCs which they were designed to replace. Of these listed in Table 2, the preferred HFC is Vertrel®.

[0081] Table 2: Hydrofluorocarbons (HFCs) Suitable For Use As Coolants In The Present Invention, In Descending Order With Respect To Boiling Point At Atmospheric Pressure.

[0082] Also useful as coolant liquids in the present invention are mixtures of refrigerants. Examples of mixtures of refrigerants are given in U.S. patent nos. 5,185,094; 5,232,618; 5,234,613; 5,236,611; 5,248,433; 5,277,834; 5,290,466; 5,387,357; 5,447,646; 5,589,098; 5,616,276; 5,635,099; 5,643,492; 5,700,388; 5,788,877; 5,800,730; and 6,416,683 Bl. All of these patents are hereby incorporated by reference.

[0083] With reference to azeotropic mixtures of the listed HFCs and HFEs, a phase study shows the compositions in Table 3 are azeotropic and have a boiling point that is higher or lower than either pure component in the azeotropic mixture. Table 3 shows the occurrence of the binary azeotropes for the approximate temperature range from 60 °C to 100 °C. The present invention can also be applied to mixtures featuring 1-99% of at least one of the components A or B listed in Table 3.

[0084] Table3: Binary Azeotropes Of HFEs And HFCs Listed In Tables 1 And 2 Suitable For Use As Coolants In The Present Invention, In Descending Order With Respect To Boiling Point At Atmospheric Pressure.

[0085] Thus, in the preferred embodiment of the present invention, a coolant is used to cool an electrochemical reactor stack that possesses a low dielectric constant (less than about 10), a boiling point at or less than the reactor operating temperature and a low freeze point temperature (less than about -40 °C), in contrast to typical deionised water coolant that has a very high dielectric constant (about 80), a boiling point above most stack operating temperatures and a freeze point temperature of only 0 °C (lower if an antifreeze is added).

[0086] Thus, in the specific context of fuel cell stacks, the present invention provides one or more of the following advantages:

[0087] (1) By using a coolant that vaporizes at or below the operating temperature of the fuel cell or fuel cell stack, it uses the inherent advantage of latent heat transfer (that is, isothermal) and provides the ability to operate the stack with much smaller temperature gradients across the stack cells. The ability to maintain the fuel cell operating temperature at a desired point or within a desired range leads to better stack performance and longer component lifetime

[0088] (2) By using low-viscosity fluids and liquid/vapour or fully vaporized coolants, and by avoiding the use of complex or multi-stage cooling loops, it reduces parasitic power requirements that act to lower the overall fuel cell system efficiency.

[0089] (3) It reduces the size of the fuel cell cooling system because the use of deionisers is eliminated. [0090] (4) It reduces the size of the heat exchanger needed for the fuel cell system.

[0091] (5) Stack shunt currents, which cause premature degradation of the fuel cell stack, are eliminated.

[0092] (6) The coolants of the present invention extend the useful ambient operating temperature range of the fuel cell stack.

[0093] (7) It significantly reduces the cost of the fuel cell cooling subsystem by eliminating the need for a deioniser.

[0094] (8) By using coolants that are non-toxic, non-flammable, with relatively short atmospheric lifetimes and low Greenhouse-warming potentials, it eliminates safety concerns associated with cooling systems that employ toxic or flammable coolants such as ethylene glycol or propanediol, or potential environmental impact associated with the use of PFC or CFC-based coolants.

[0095] (9) The use of benign coolants such as those listed in Tables 1 and 2 significantly reduces costs associated with shipping, receiving, handling and storage of coolant fluids.

[0096] (9) By using the latent heat of vaporization of the coolant to absorb and remove heat energy from the fuel cell or fuel cell stack, the optimal stack operating temperature may be controlled through manipulation of the coolant system (i.e., coolant plate, loop or cell) pressure. This allows for greater control over stack operating temperature, resulting in optimized performance and extended component lifetime.

[0097] Although the present invention has been shown and described with respect to its preferred embodiments and in the examples, it will be understood by those skilled in the art that other changes, modifications, additions and omissions may be made without departing from the substance and the scope of the present invention as defined by the attached claims.

Claims

What is claimed is:

1. A cooling system for an electrochemical reactor that generates electrical energy and heat energy, wherein the reactor operates at a reactor temperature, the cooling system comprising a coolant circulating in direct or indirect thermal contact with the reactor at a coolant pressure, wherein the coolant has:

(a) a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10;

(b) a resistivity while circulated in said thermal contact of greater than 10³ Ohm-cm; and

(c) a boiling point below the reactor temperature at the reactor pressure, wherein the coolant is in its liquid phase as it first comes into said thermal contact with the reactor and a portion of the coolant is vaporized to a vapor by absorbing and removing excess heat energy from the reactor via sensible heat transfer and latent heat transfer to the coolant.

2. The cooling system of claim 1, wherein all of the coolant is vaporized to the vapor.

3. The cooling system of claim 1 or 2, wherein the coolant has an electrical resistivity of greater than about 10 Ohm-cm, preferably greater than about 10 Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm.

4. The cooling system of any one of claims 1-3, wherein the boiling point of the liquid is at least 1 to 5°C less than the reactor temperature at the coolant pressure.

5. The cooling system of any one of claims 1-4, wherein the reactor temperature is in the range from about 50°C to about 95°C and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

6. The cooling system of any one of claims 1-4, wherein the reactor temperature is in the range from about 20°C to about 200°C, preferably from about 95°C to about 200°C, most preferably from about 95°C to about 125°C, and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

7. The cooling system of any one of claims 1-6, wherein the coolant has a viscosity, at -40°C, of less than about 5 cPs, preferably less than about 2 cPs, most preferably less than about 1 cPs; and a viscosity, at 80 °C, of less than about 1.0 cPs, preferably less than about 0.5 cPs, most preferably less than about 0.3 cPs.

8. . The cooling system of any one of claims 1-7, wherein the coolant has a freezing point, at ambient pressure, of less than about-^-0°C, preferably less than about -50°C.

9. The cooling system of claim 1, wherein the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

10. The cooling system of claim 9, wherein

(a) the hydrofluoroether is selected from the group consisting of:

(b) the hydrofluorocarbon is selected from the group consisting of:

(c) and the azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons is selected from the group consisting of:

11. The cooling system of claim 9, wherein the coolant is selected from HFE- 7100, HFE-7200, HFE-7500 and Vertrel® HFC.

12. The cooling system of any one of claims 1-11, wherein the electrochemical reactor is an electrochemical fuel cell.

13. The cooling system of claim 12, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

14. The cooling system of any one of claims 1-13 further comprising a pump for circulating the coolant and a heat exchanger/condenser for cooling and condensing the coolant after it has absorbed heat from the reactor.

15. A method of cooling an electrochemical reactor that generates electrical energy and heat energy, the method comprising the steps of:

(a) operating the reactor at a reactor temperature;

(b) circulating in direct or indirect thermal contact with the reactor a coolant at a coolant pressure;

(c) heating the coolant by absorbing and removing excess heat energy from the reactor via sensible heat transfer to the coolant; and

(d) vaporizing a portion of the coolant to a vapor by absorbing and removing excess heat energy from the reactor via latent heat transfer to the coolant, wherein the coolant has a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10; a resistivity while circulated in said thermal contact with the reactor of greater than 10 Ohm-cm; and a boiling point below the reactor temperature at the coolant pressure.

16. The method of claim 15, wherein all of the coolant is vaporized to the vapor.

17. The method of claim 15 or 16, wherein the liquid has an electrical resistivity of greater than about 10⁷ Ohm-cm, preferably greater than about 10⁸ Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm.

18. The method of any one of claims 15-17, wherein the boiling point of the liquid is at least 1-5°C less than the reactor temperature at the coolant pressure.

19. The method of any one of claims 15-18 wherein the reactor temperature is in the range from about 50°C to about 95°C and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

20. The method of any one of claims 15-18 wherein the reactor temperature is in the range from about 20°C to about 200°C, preferably from about 95°C to about 200°C, most preferably from about 95°C to about 125°C, and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

21. The method of any one of claims 15-20 wherein the liquid has a viscosity, at - 40°C, of less than about 5 cPs, preferably less than about 2 cPs, most preferably less than about 1 cPs; and a viscosity, at 80 °C, of less than about 1.0 cPs, preferably less than about 0.5 cPs, most preferably less than about 0.3 cPs.

22. The method of any one of claims 15-21, wherein the liquid has a freezing point, at ambient pressure, of less than about-40°C, preferably less than about -50°C.

23. The method of claim 15, wherein the liquid comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

24. The method of claim 23, wherein

(a) the hydrofluoroether is selected from the group consisting of:

(b) the hydrofluorocarbon is selected from the group consisting of:

25. The method of claim 23, wherein the coolant is selected from HEE-7100, HFE-7200, HFE-7500 and Vertrel® HFC.

26. The. method of any one of claims 15-25, wherein the electrochemical reactor is an electrochemical fuel cell.

27. The method of claim 26, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

28. The use of a coolant to absorb and remove excess heat energy generated by an electrochemical reactor that generates electrical energy and heat energy, wherein the reactor operates at a reactor temperatureand the coolant is at a coolant pressure, the coolant having:

(b) a resistivity while circulated in direct or indirect thermal contact with the reactor of greater than 10³ Ohm-cm; and

(c) a boiling point below the reactor temperature at the coolant pressure wherein the coolant is in its liquid phase as it first comes into said thermal contact with the reactor and a portion of the coolant is vaporized to a vapor by absorbing and removing excess heat energy from the reactor via sensible heat transfer and latent heat transfer to the coolant.

29. The use of claim 28, wherein all of the coolant is vaporized to the vapor.

30. The use of claim 28 or 29, wherein the liquid has an electrical resistivity of greater than about 10⁷ Ohm-cm, preferably greater than about 10⁸ Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm.

31. The use of any one of claims 28-30, wherein the boiling point of the liquid is at least 1 to 5°C less than the reactor temperature at the coolant pressure.

32. The use of any one of claims 28-31 wherein the reactor temperature is in the range from about 50°C to about 95°C and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

33. The use of any one of claims 28-31 wherein the reactor temperature is in the range from about 20°C to about 200°C, preferably from about 95°C to about 200°C, most preferably from about 95°C to about 125°C, and the coolant pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

34. The use of any one of claims 28-33 wherein the liquid has a viscosity, at - 40°C, of less than about 5 cPs, preferably less than about 2 cPs, most preferably less than about 1 cPs; and a viscosity, at 80 °C, of less than about 1.0 cPs, preferably less than about 0.5 cPs, most preferably less than about 0.3 cPs.

35. The use of any one of claims 28-34 wherein the liquid has a freezing point, at ambient pressure, of less than about-40°C, preferably less than about -50°C.

36. The use of claim 28, wherein the liquid comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

37. The use of claim 36, wherein

(a) the hydrofluoroether is selected from the group consisting of:

(b) the hydrofluorocarbon is selected from the group consisting of:

38. The use of claim 36, wherein the coolant is selected from HFE-7100, HFE- 7200, HFE-7500 and Vertrel® HFC.

39. The use of any one of claims 28-38, wherein the electrochemical reactor is an electrochemical fuel cell.

40. The use of claim 39, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

41. A method of operating a fuel cell stack at a desired operating temperature, wherein the fuel cell stack generates electrical energy and excess heat energy, the method comprising the steps of:

(a) circulating in direct or indirect thermal contact with the fuel cell stack a coolant at a varying pressure;

(b) heating the coolant by absorbing and removing the excess heat energy from the fuel cell stack via latent heat transfer to the coolant so that a portion or all of the coolant is vaporized to a vapor; and

(c) adjusting the pressure of the coolant so that the desired operating temperature is obtained, wherein the coolant has a dielectric constant, at the operating temperature and pressure, of less than about 10; a resistivity while in said thermal contact with the fuel cell stack of greater than 10 Ohm-cm; and a boiling point below the reactor temperature at the reactor pressure.

42. The method of claim 41, wherein all of the coolant is vaporized to the vapor.

43. The method of claim 41 or 42, wherein the coolant has an electrical resistivity of greater than about 10 Ohm-cm, preferably greater than about 10 Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm.

44. The method of any one of claims 41-43, wherein the boiling point of the coolant is at least 1 to 5°C less than the operating temperature at the operating pressure.

45. The method of any one of claims 41-44 wherein the operating temperature is in the range from about 50°C to about 95°C and the pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

46. The method of any one of claims 41-44 wherein the operating temperature is in the range from about 20°C to about 200°C, preferably from about 95°C to about 200°C, most preferably from about 95°C to about 125°C, and the pressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

47. The method of any one of claims 41-46 wherein the coolant has a viscosity, at - 0°C, of less than about 5 cPs, preferably less than about 2 cPs, most preferably less than about 1 cPs; and a viscosity, at 80 °C, of less than about 1.0 cPs, preferably less than about 0.5 cPs, most preferably less than about 0.3 cPs.

48. The method of any one of claims 41-47, wherein the coolant has a freezing point, at ambient temperature and pressure, of less than about-40°C, preferably less than about -50°C.

49. The method of claim 41, wherein the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

50. The method of claim 49, wherein

(a) the hydrofluoroether is selected from the group consisting of:

(b) the hydrofluorocarbon is selected from the group consisting of:

51. The method of claim 49, wherein the coolant is selected from HFE-7100, HFE-7200, HFE-7500 and Vertrel® HFC.

52. The method of any one of claims 41-51, wherein the fuel cell is a proton exchange membrane fuel cell.