WO2005007771A1

WO2005007771A1 - Coolant liquids having a low dielectric constant and high resistivity for use in fuel cells & other electrochemical reactor stacks

Info

Publication number: WO2005007771A1
Application number: PCT/CA2004/001087
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: TW200509453A

Abstract

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 includes a coolant circulating in direct or indirect thermal contact with the reactor at a coolant pressure. The coolant is a liquid having a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10; a resistivity while circulated, in either direct or indirect thermal contact with the reactor of greater than 103 Ohm-cm; and a boiling point above the reactor temperature at the reactor pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy. 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. Preferably, the liquid is a composition having at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

Description

Coolant Liquids Having A Low Dielectric Constant And High Resistivity 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 coolant liquids having 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 system to maintain a stable operating temperature or isothermal system. With respect to fuel cells, the fact that heat is generated by the reactor stack 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 added to 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 the cooling water becoming the carrier for stray currents. Typical antifreeze solutions, such as ethylene glycol-water solutions, need deionization, as well. Deionisation resins are used to deionise the water, however commercial deionisation resins are limited to an upper temperature limit of 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] PCT published application WO/00/17951 filed by Ballard Power Systems Inc. / DBB Fuel Cell Engines GMBH discloses an invention entitled "Antifreeze Cooling Subsystem". The cooling system employs glycol-based coolants in conjunction with a de-ioniser in order to allow sub-freezing capability while avoiding shunt currents.

The system requires the use of a deioniser in the cooling loop; however, such a deioniser takes up too much volume, limits operating temperature ranges, requires periodic replacement, and is relatively expensive.

[0008] 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 —40 °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.

[0009] U.S. patent no. 6,432,566 issued to UTC Fuel Cells for an invention entitled "Direct Antifreeze Cooled Fuel Cell Power Plant". This patent discloses the use of an antifreeze solution for cooling fuel cells. In known fuel cells, a coolant or thermal management system is used that supplies a flow of cooling fluid through the fuel cell and other plant components to maintain the cell within an optimal temperature range and to efficiently distribute heat. If the cooling fluid is a water solution, it must be kept from freezing; typically an antifreeze solution such as ethylene glycol and water or propylene glycol and water is used as the cooling fluid in such coolant systems. A preferred antifreeze solution is alkanetriol selected from glycerol, butanetriol, and pentanetriol.

[0010] 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.

[0011] 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.

[0012] The present invention solves the problems of prior art systems by employing a coolant liquid that conducts little electrical current, that has a very low capacity for the dissolution of ions (i.e., very low tendency to act as a solvent for potentially conductive ions) and that may therefore remain non-conductive for extended periods. 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 the electrochemical reactor.

[0013] The disclosures of all patents/applications referenced herein are incorporated herein by reference.

Summary of the Invention:

[0014] 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 comprises a liquid having: [0015] (a) a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10;

[0016] (b) a resistivity while circulated in said thermal contact of greater than 10³ Ohm-cm; and

[0017] (c) a boiling point above the reactor temperature at the coolant pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy.

[0018] Preferably, the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

[0019] In a further aspect 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:

[0020] (a) operating the reactor at a reactor temperature;

[0021] (b) circulating a coolant in direct or indirect thermal contact within the reactor at a coolant pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy,

[0022] wherein the coolant comprises a liquid having a dielectric constant, at the reactor temperature and the coolant pressure, of less than about 10; a resistivity while in circulation, whether in direct or indirect thermal contact with the reactor, of greater than 10³ Ohm-cm; and a boiling point above the reactor temperature at the coolant pressure. [0023] Preferably, the electrochemical reactor is an electrochemical fuel cell such as a proton exchange membrane fuel cell.

[0024] 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:

[0025] (a) circulating in direct or indirect thermal contact with the fuel cell stack a coolant at a varying pressure;

[0026] (b) heating the coolant by absorbing and removing the excess heat energy from the fuel cell stack via sensible heat transfer to the coolant so that the coolant remains in its liquid phase after absorbing the excess heat energy; and

[0027] (c) varying the pressure of the coolant so that the coolant has a boiling point that remains above the desired operating temperature of the fuel cell stack;

[0028] wherein the coolant has a dielectric constant, at the operating temperature and pressure, of less than about 10; and a resistivity while in thermal contact with the fuel cell stack of greater than 10 Ohm-cm.

[0029] 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:

[0030] The preferred embodiments of the present invention will be described with reference to the accompanying drawing:

[0031] Fig. 1 is schematic representation of one embodiment of the cooling system of the present invention. Detailed Description of the Preferred Embodiments:

[0032] The preferred embodiments of the present invention will now be described with reference to the accompanying figures.

[0033] 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 reactor 10. Within reactor 10 , there exists a means of circulating a coolant such that it is in direct (i.e., coolant channels in the reactor body) or indirect (i.e., a separate cooling cell inserted between elements of the reactor) thermal contact with the reactor. The coolant is a liquid that is caused to circulate through the cooling channels by use of a pump 12, inlet pipe 14 and outlet pipe 16. The coolant liquid, after circulating tlirough the cooling channels in reactor 10, has absorbed excess heat generated by the reactor 10. The warmer coolant liquid exits reactorlO via outlet 16 and is directed to heat exchanger 18 where the coolant liquid is cooled by a secondary fluid which may include ambient air. The cooled liquid exits heat exchanger 18 through conduit 20 and is directed to pump 12 where it is caused to circulate again through the cooling channels in reactor 10.

[0034] Preferably, coolant liquid used in the present invention has the following properties:

[0035] (a) A dielectric constant, at the reactor operating temperature and 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 10³ Ohm-cm for extended periods; [0036] (b) An initial electrical resistivity of greater than about 10⁷ Ohm- cm, preferably greater than about 10⁸ Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm;

[0037] (c) A fluid or fluid composition boiling point that is at least 5 °C greater than the electrochemical reactor operating temperature, at the 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). 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;

[0038] (d) Little or no capacity for poisoning of the reactor active area;

[0039] (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;

[0040] (f) A freezing point, at ambient pressure, of less than about -40 °C, preferably less than -50 °C;

[0041] (g) A relatively low atmospheric lifetime and Greenhouse Warming Potential to minimize potential environmental impact.

[0042] (h) Have non-toxic and non-flammable characteristics. [0043] 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.

[0044] 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.

[0045] The coolant liquid cools the reactor stack by absorbing excess stack heat via sensible heat transfer to the coolant liquid so that the coolant liquid remains in its liquid phase after absorbing the excess heat energy. By sensible heat transfer, it is meant that the coolant liquid remains in the liquid phase throughout the cooling cycle. That is, the coolant liquid absorbs heat during a change of temperature of the liquid, which is not accompanied by a change of state of the liquid.

[0046] 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.

[0047] In a third 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 liquid within the cooling system. This ability to vary the coolant liquid boiling point ensures that the coolant liquid remains a liquid throughout the whole cooling cycle.

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 cooling liquid boiling point to ensure that it is maintained at the desired level above the operating temperature of the reactor stack.

[0048] In this embodiment of the present invention, the method allows the operation of the fuel cell stack at a desired operating temperature while the fuel cell stack generates electrical energy and excess heat energy. This method comprises the steps of:

[0049] (a) circulating in direct or indirect thermal contact with the fuel cell stack a coolant at a varying pressure;

[0050] (b) heating the coolant by absorbing and removing the excess heat energy from the fuel cell stack via sensible heat transfer to the coolant so that the coolant remains in its liquid phase after absorbing the excess heat energy; and

[0051] (c) varying the pressure of the coolant so that the coolant has a boiling point that remains above the desired operating temperature of the reactor or fuel cell or fuel cell stack;

[0052] wherein the coolant has a dielectric constant, at the operating temperature and pressure, of less than about 10; and a resistivity while in thermal contact with the fuel cell stack of greater than 10³ Ohm-cm.

[0053] Preferably, the pressure of the coolant is varied so that the boiling point of the coolant is at least 5 °C above the desired operating temperature of the fuel cell stack.

[0054] 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), a property that precludes its use for the present application in electrochemical reactors operating at temperatures above about 50°C. By elevating the cooling system pressure by only 19.3 psi, the same Vertrel® coolant liquid now boils at about 80°C, thereby allowing for its use in reactors operating at temperatures up to about 75°C.

[0055] In a further embodiment of the present invention, the pressure of the coolant is manipulated so as to alter the temperature difference between the boiling point of the coolant and the desired operating temperature of the reactor or fuel cell or fuel cell stack. With respect to liquid coolants, an increase in said temperature difference generally corresponds to a decrease in the coolant vapour pressure experienced by the system and thus a reduction in the observed coolant loss, or leakage, with time.

[0056] In another preferred embodiment of the invention, the coolant liquid is selected so as to have a relatively low rate of mass transfer, in the liquid state, 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 fluid 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.

[0057] In a further embodiment of the invention, the coolant liquid preferably has a low viscosity at reactor operating temperatures and at coolant pressures. A lower coolant viscosity allows for a reduction in required pumping power, leading to increased efficiency of the recator system. A lower coolant liquid 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 liquid within the cooling channels 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.

[0058] The present invention provides one or more of the following benefits: [0059] (1) The coolant liquid has 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 in the coolant. Therefore, the coolant liquid will not readily dissolve ions from the cooling system and will not become conductive while in-use in the reactor stack over extended periods of time.

[0060] (2) A coolant liquid with permanently low conductivity will eliminate the possibility of stray electrical currents that cause corrosion within the reactor stack without having to regularly deionize the cooling fluid as is currently being done.

[0061] (3) A coolant liquid with permanently low conductivity eliminates the need for expensive non-corroding materials in the cooling system and eliminates the need for a deioniser. The low conductivity coolant liquid 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.

[0062] (4) Use of a coolant liquid with relatively low liquid viscosity, at the reactor operating 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 turbulent flow, allowing for improvement of heat transfer to the coolant. [0063] (5) The cooling liquid is preferably not a strong poison to the reactor active area and therefore greatly reduces the risk of contamination of the stack through cooling fluid leaks.

[0064] (6) The coolant liquids of the present invention preferably can withstand freezing in cold environments.

[0065] (7) The cooling liquids of the present invention preferably are not flammable, are non-toxic and have a relatively short atmospheric lifetime and low Global Warming Potential (GWP).

[0066] (8) By preferably 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.

[0067] (9) 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.

[0068] (10) The radiator size can be reduced due to the fact that the deioniser filter is eliminated. As such, the coolant temperature can operate above 60 °C, thereby increasing the temperature difference with the ambient resulting in a more effective and smaller radiator.

[0069] Liquids that can be used as coolant liquids 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. [0070] The specific composition of the mixture, azeotropic or azeotrope-like liquid is determined by the operating temperature of the electrochemical reactor. The boiling point of the coolant fluid should preferably be at least 5°C above the operating temperature of the reactor.

[0071] 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.

[0072] Table 1: Hydrofluoroethers (HFEs) Suitable For Use As Coolants In The Present Invention, In Descending Order With Respect To Boiling Point At Atmospheric Pressure.

[0073] 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.

[0074] Table 2: Hydrofluorocarbons (HFCs) Suitable For Use As Coolants In The Present Invention, In Descending Order With Respect To Boiling Point At Atmospheric Pressure.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] Thus, in a preferred embodiment of the present invention, a coolant liquid is used to cool an electrochemical reactor stack. The coolant liquid possesses a low dielectric constant (less than about 10) and a low freeze point (less than about -40 °C) in contrast to typical deionised water coolant that has a very high dielectric constant (about 80) and a freeze point temperature of only 0 °C (lower if an antifreeze is added).

[0079] Thus, in the specific context of fuel cell stacks, the present invention provides one or more of the following advantages:

[0080] (1) It allows for minimization of thermal gradients within the fuel cell and/or fuel cell stack and maintains fuel cell operating temperature at a desired point or within a desired range.

[0081] (2) By using low-viscosity liquids 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.

[0082] (3) It reduces the cost and size of the fuel cell cooling system . because the use of deionisers is eliminated.

[0083] (4) Stack shunt currents, which cause premature degradation of the fuel cell stack, are eliminated.

[0084] (5) The coolant liquids of the present invention extend the useful ambient operating temperature range of the fuel cell stack.

[0085] (6) 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.

[0086] (7) A further advantage is that the use of benign coolant fluids such as those listed in Tables 1 and 2 significantly reduces costs associated with shipping, receiving, handling and storage of coolant fluids.

[0087] 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 comprises a liquid having:

(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 10 Ohm-cm; and

(c) a boiling point above the reactor temperature at the coolant pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy.

2. The cooling system of claim 1, wherein the liquid has an initial electrical resistivity of greater than about 10⁷ Ohm-cm, preferably greater than about 10⁸ Ohm-cm, most preferably greater than about 10¹⁰ Ohm-cm.

3. The cooling system of claims 1 or 2, wherein the boiling point of the liquid is at least 5°C greater than the reactor temperature at the coolant pressure.

4. The cooling system of any one of claims 1-3 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.

5. The cooling system of any one of claims 1-3 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.

The cooling system of any one of claims 1-5 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.

The cooling system of any one of claims 1-6 wherein the liquid has a freezing point, at ambient pressure, of less than about-^-0°C, preferably less than about -50°C.

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

The cooling system of claim 8, 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:

10. The cooling system of claim 8, wherein the coolant is selected from HFE- 7100, HFE-7200, HFE-7500 and Vertrel® HFC.

11. The cooling system of any one of claims 1-10, wherein the electrochemical reactor is an electrochemical fuel cell.

12. The cooling system of claim 11, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

13. The cooling system of any one of claims 1-12 further comprising a pump for circulating the coolant and a heat exchanger for cooling the coolant after it has absorbed heat from the reactor.

14. 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 a coolant in direct or indirect thermal contact with the reactor at a coolant pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy, wherein the coolant comprises a liquid having a dielectric constant, at the reactor temperature and coolant pressure, of less than about 10; a resistivity while circulated in the cooling channels of greater than 10³ Ohm-cm; and a boiling point above the reactor temperature at the coolant pressure.

15. The method of claim 14, 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.

16. The method of claims 14 or 15, wherein the boiling point of the liquid is at least 5°C greater than the reactor temperature at the coolant pressure.

17. The method of any one of claims 14-16 wherein the reactor temperature is in the range from about 50°C to about 95°C and the coolantpressure is in the range from a slight vacuum to 60 psig, preferably less than about 15 psig.

18. The method of any one of claims 14-16 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.

19. The method of any one of claims 14-18 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.

20. The method of any one of claims 14-19, wherein the liquid has a freezing point, at ambient pressure, of less than about-40°C, preferably less than about -50°C.

21. The method of claim 14, wherein the liquid comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

22. The method of claim 21, wherein

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

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

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

23. The method of claim 21, wherein the coolant is selected from HFE-7100, HFE-7200, HFE-7500 and Vertrel® HFC.

24. The method of any one of claims 14-23, wherein the electrochemical reactor is an electrochemical fuel cell.

25. The method of claim 24, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

26. 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 temperature and the coolant is at a coolant pressure, the coolant comprises a liquid 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 above the reactor temperature at the reactor pressure so that the coolant absorbs and removes excess heat energy from the reactor via sensible heat transfer to the coolant and the coolant remains in its liquid phase after absorbing the excess heat energy.

27. The use of claim 25, 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.

28. The use of claims 25 or 26, wherein the boiling point of the liquid is at least 5°C greater than the reactor temperature at the coolant pressure.

29. The use of any one of claims 25-27 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.

30. The use of any one of claims 25-27 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.

31. The use of any one of claims 25-29 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.

32. The use of any one of claims 25-30 wherein the liquid has a freezing point, at ambient pressure, of less than about-40°C, preferably less than about -50°C.

33. The use of claim 25, wherein the liquid comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

34. The use of claim 32, wherein

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

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

35. The use of claim 32, wherein the coolant is selected from HFE-7100, HFE- 7200, HFE-7500 and Vertrel® HFC.

36. The use of any one of claims 25-34, wherein the electiochemical reactor is an electrochemical fuel cell.

37. The use of claim 35, wherein the electrochemical fuel cell is a proton exchange membrane fuel cell.

38. 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 sensible heat transfer to the coolant so that the coolant remains in its liquid phase after absorbing the excess heat energy; and

(c) varying the pressure of the coolant so that the coolant has a boiling point that remains above the desired operating temperature of the fuel cell stack; wherein the coolant has a dielectric constant, at the operating temperature and pressure, of less than about 10; and a resistivity while in thermal contact with the fuel cell stack of greater than 10³ Ohm-cm.

39. The method of claim 38, wherein varying the pressure of the coolant maintains the boiling point of the coolant at least about 5 °C above the desired operating temperature of the fuel cell stack.

40. The method of claims 38 or 39, wherein the coolant 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.

41. The use of any one of claims 38-40, wherein the desired 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.

42. The method of any one of claims 38-40, wherein the desired 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.

43. The method of any one of claims 38-42, 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.

44. The method of any one of claims 38-43, wherein the coolant has a freezing point, at ambient pressure, of less than about-^-0°C, preferably less than about -50°C.

45. The method of claim 38, wherein the coolant comprises at least one hydrofluoroether, at least one hydrofluorocarbon, or an azeotrope or azeotrope-like mixture of hydrofluoroethers and/or hydrofluorocarbons.

46. The method of claim 45, wherein

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

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

44

47. The method of claim 45, wherein the coolant is selected from HFE-7100, HFE-7200, HFE-7500 and Vertrel® HFC.