Thermoelectrochemical converter

Abstract

A thermoelectrochemical converter is provided. The converter includes a working fluid, first and second coupled membrane electrode assemblies (MEAs), first and second heat transfer members, a heat sink, and a heat source. Each MEA includes a first porous electrode operating at a first pressure, a second porous electrode operating at a second pressure higher than the first pressure, and an ionically conductive membrane sandwiched therebetween. The first MEA compresses the working fluid and the second MEA expands the working fluid. The first heat transfer member is coupled to and in thermal engagement with the low pressure electrode of the first MEA. The second heat transfer member is coupled to and in thermal interface with the low pressure electrode of the second MEA. The heat sink is coupled to the low pressure side of the first MEA and the heat source is coupled to the low pressure side of the second MEA.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Provisional Application No. 62 / 976,764, filed on February 14, 2020, the entire disclosure of which is incorporated herein. Background Art

[0003] The present invention relates to converting thermal energy into electrical energy or converting electrical energy into thermal energy using a heat engine having a pair of electrochemical cells.

[0004] The conversion of thermal or chemical energy into electrical energy, or vice versa, can be accomplished in a variety of ways. For example, known electrochemical cells or batteries rely on chemical reactions in which ions and electrons of an oxidized reactant are transferred to a reduced reactant via separate pathways. Specifically, the electrons are electrically transported via wiring through an external load, where they perform work, and the ions are conducted through an electrolyte separator.

[0005] However, battery-type electrochemical cells can only produce a limited amount of energy because the confines of the battery housing limit the amount of available reactants that can be contained therein. Although such cells can be designed to be charged by applying a reverse polarity current / voltage to the electrodes, such charging requires a separate power source. Another disadvantage of these conventional battery-type electrochemical cells is that the battery is generally unusable during the charging process.

[0006] To overcome the problems associated with battery-type electrochemical cells, fuel cells have been developed. In conventional fuel cells, chemical reactants are continuously supplied to and removed from the electrochemical cell. In a manner similar to batteries, fuel cells operate by conducting ionized species through a selective electrolyte that normally blocks the passage of electrons and non-ionized species.

[0007] The most common type of fuel cell is the hydrogen-oxygen fuel cell, which transfers hydrogen through one electrode and oxygen through the other. Hydrogen ions are conducted through an electrolyte separator to the oxygen side of the cell at the potential for the chemical reaction between hydrogen and oxygen. Porous electrodes on either side of the electrolyte separator couple the electrons involved in the chemical reaction through an external circuit to an external load. The electrons and hydrogen ions reform to form hydrogen, completing the reaction. The oxygen on the oxygen side of the cell produces water, which is then discharged from the system. A continuous supply of hydrogen and oxygen to the cell maintains a continuous current flow.

[0008] Mechanical heat engines have also been designed and used to generate electricity. These operate in a thermodynamic cycle, where a piston or turbine is used to perform shaft work to compress a working fluid. The compression process is performed at a low temperature, and after compression, the working fluid is raised to a higher temperature. At a higher temperature, the working fluid is allowed to expand against a load (such as a piston or turbine), thereby generating shaft work. The basic principle of all engines that use working fluids is that the work required to compress the working fluid at a low temperature is less than the work generated by expanding the working fluid at a higher temperature. This is true for all heat engines that use working fluids.

[0009] For example, a steam engine operates on the Rankine cycle, in which water is pumped to high pressure, then heated to steam and expanded by a piston or turbine to produce work. An internal combustion engine operates on the Otto cycle, in which cold ambient air is compressed by a piston and then heated to very high temperatures by the combustion of fuel in the cylinder. As the cycle continues, the expansion of the heated air against the piston produces more work than the work consumed in the cold compression process.

[0010] Stirling engines have been developed to operate on the Stirling cycle in an effort to provide an engine with high efficiency and greater versatility in the choice of heat source. An ideal Stirling thermodynamic cycle has equivalent efficiency to an ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine operating with high-temperature heat input and low-temperature heat output. However, like all mechanical engines, Stirling engines are subject to reliability issues and efficiency losses associated with their mechanical moving parts.

[0011] To avoid the problems inherent in mechanical heat engines, alkali metal thermoelectrochemical conversion (AMTEC) cells have been designed as thermoelectrochemical heat engines. AMTEC heat engines use pressure to generate a voltage potential and an electric current by forcing an ionizable working fluid (such as sodium) through an electrochemical cell at high temperature. Electrodes couple the current to an external load. Electrical work is performed when a pressure differential across an electrolyte separator forces molten sodium atoms through the electrolyte. The sodium is ionized upon entering the electrolyte, releasing electrons into the external circuit. On the other side of the electrolyte, the sodium ions recombine with the electrons upon exiting the electrolyte to reform sodium, in much the same way as the process occurs in battery and fuel cell-type electrochemical cells. The reconstituted sodium, at low pressure and high temperature, leaves the electrochemical cell as an expanding gas. The gas is then cooled and condensed back into a liquid. The resulting cryogenic liquid is then repressurized. The operation of an AMTEC engine approximates the Rankine thermodynamic cycle.

[0012] There are many publications on AMTEC technology. For example, see, Conceptual design of AMTEC demonstrative system for 100t / d garbage disposal power generating facility, Qiuya Ni et al. (Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing, China). Another representative publication is the Intersociety Energy Conversion Engineering Conference and Exhibit (IECEC), 35th, Las Vegas, Nevada (July 24-28, 2000), Technical Papers, Volume 2 (A00-37701 10-44). See also American Institute of Aeronautics and Astronautics, 190, pp. 1295-1299, Report No.-AIAA Paper 2000-3032.

[0013] AMTEC heat engines suffer from reliability issues due to the highly corrosive nature of the alkali metal working fluid. The utility of AMTEC engines is also very limited. Specifically, AMTEC engines can only operate at very high temperatures because the ionically conductive solid electrolyte reaches practical conductivity levels only at high temperatures. In fact, even the low-temperature pressurization process must be performed at relatively high temperatures compared to other types of heat engines because the alkali metal working fluid must remain above its melting temperature as it moves through the cycle. Mechanical pumps and even magnetohydrodynamic pumps have been used to pressurize the low-temperature working fluid.

[0014] To overcome the aforementioned shortcomings of conventional mechanical and thermoelectrochemical heat engines, the Johnson Thermoelectrochemical Converter (JTEC) system was developed (disclosed in U.S. Patent No. 7,160,639, filed on April 28, 2003). A typical JTEC system is a heat engine that includes a first electrochemical cell operating at a relatively low temperature, a second electrochemical cell operating at a relatively high temperature, a conduit system including a heat exchanger coupling the two cells, and a supply of an ionizable gas (e.g., hydrogen or oxygen) as a working fluid contained within the conduit system. Each electrochemical cell includes a membrane electrode assembly (MEA).

[0015] More specifically, a JTEC heat engine includes a first MEA coupled to a high-temperature heat source (i.e., a high-temperature MEA), a second MEA coupled to a low-temperature heat sink (i.e., a low-temperature MEA), and a recuperator connecting the two MEAs. Each MEA includes a non-porous membrane capable of conducting ions of the working fluid and a porous electrode capable of conducting electrons, located on opposite sides of the non-porous membrane.

[0016] A JTEC operates like any other engine. For example, a jet engine consists of a compressor and a combustor. The compressor stage draws in air, compresses it, and supplies the compressed air to the combustor. The air is heated in the combustor and expanded by the power stage. The power stage couples shaft work back to the compressor stage, maintaining a continuous supply of compressed air. The difference between the work produced by the power stage and the work consumed by the compressor stage is the net work output of the jet engine. However, the key difference between a JTEC and a jet engine is that a jet engine's turbine is mechanical and operates on the Brayton thermodynamic cycle, while a JTEC heat engine is a fully solid-state engine and operates on the more efficient Carnot-equivalent Ericsson thermodynamic cycle.

[0017] During JTEC operation, the working fluid passes through each MEA by releasing electrons to the electrode on the inlet side, allowing ions to be conducted through the non-porous membrane to the opposite electrode. When the working fluid ions leave the membrane, the working fluid resupplies electrons to the working fluid ions, and the working fluid is reconstituted within the opposite electrode.

[0018] The low-temperature MEA operates at a lower voltage than the high-temperature MEA. The low-temperature MEA compresses the working fluid (e.g., hydrogen or oxygen) at a low voltage, while the high-temperature MEA stack expands the working fluid at a high voltage. The voltage difference between the two MEAs is applied to the external load. The working fluid circulates continuously within the JTEC heat engine and is never consumed. The current flowing through both MEAs and the external load is the same.

[0019] Specifically, in a JTEC heat engine, a pressure differential is applied to each MEA and a load is attached, generating voltage and current when the working fluid flows from high pressure to low pressure. When protons pass through the proton conducting membrane (PCM), electrons are stripped from the protons and the electron current is directed to the external load. The JTEC system utilizes the electrochemical potential of the working fluid pressure applied to the PCM. More specifically, on the high-pressure side of each MEA and the low-pressure side of each MEA, the working fluid is oxidized, resulting in the generation of protons and electrons. The pressure differential at the high-temperature end forces the protons to pass through the membrane, causing the electrode to conduct electrons through the external load, while applying an external voltage forces the protons to pass through the membrane at the low-temperature end. On the high-pressure side of each MEA and the low-pressure side of each MEA, the protons are reduced by electrons to reform the working fluid.

[0020] Unlike conventional fuel cells, where hydrogen leaving the MEA stack encounters oxygen and reacts with it to produce water, the JTEC system does not have either oxygen or water. This process can also be reversed. Specifically, voltage and current can be applied to pump the working fluid from low pressure to high pressure. The reverse process is very similar to using MEAs to electrolyze water, where water molecules are split and protons are conducted through the PCM, leaving oxygen on the water side. This process is typically used to supply hydrogen at high pressure to a pure hydrogen reservoir.

[0021] In a JTEC engine, using hydrogen as the ionizable gas (i.e., the working fluid), the potential generated due to the hydrogen pressure difference across the PCM is proportional to the natural logarithm of the pressure ratio and can be calculated using the Nernst equation:

[0022]

[0023] Where V OC is the open circuit voltage, R is the universal gas constant, T is the battery temperature, F is the Faraday constant, P H is the high pressure side pressure, P L is the low pressure side pressure, P H / P L is the pressure ratio. For example, Fuel Cell Handbook, JH Hirschenhofer et al., 4th edition, pp. 2-5 (1999).

[0024] The voltage is linearly related to the temperature and is a logarithmic function of the pressure ratio. Figure 1 As shown, Figure 1 This is a graph of the Nernst equation for hydrogen generation voltage versus temperature at several pressure ratios. For example, at a pressure ratio of 10,000, Figure 1 It is depicted that when the temperature is high, the voltage is high, and when the temperature is low, the voltage is low, so there is a linear relationship between these parameters.

[0025] The working fluid in a JTEC is compressed in a low-temperature electrochemical cell by supplying current at a voltage sufficient to overcome the Nernst potential of the low-temperature cell, thereby driving the working fluid from the low-pressure side of the membrane to the high-pressure side. Conversely, when current (power) is extracted at the Nernst potential of the high-temperature cell, the working fluid expands in the high-temperature electrochemical cell. When the working fluid expands from the high-pressure side of the membrane to the low-pressure side, an electric current is generated. As with any heat engine that uses a working fluid and conforms to the properties of a compressible gas, in a JTEC, the amount of work (electricity) extracted during high-temperature expansion exceeds the amount of work (electricity) input required for low-temperature compression. The difference between the heat energy input to the engine to maintain a constant temperature during high-temperature expansion and the heat energy removed to maintain a constant temperature during low-temperature compression is provided as the difference between the electrical energy output by the high-temperature expansion process and the electrical energy consumed by the low-temperature compression process.

[0026] Consistent with the Nernst equation, the high-temperature battery will have a higher voltage than the low-temperature battery. Since the current (I) through the two cells is the same, the voltage difference means that the power generated by the expansion of the working fluid in the high-temperature battery is higher than the power generated in the low-temperature battery. The high-temperature battery (V HT *I) output power is sufficient to drive the low temperature battery (V LT *I) compression process, and provides net power output to the external load ((V HT *I)-(V LT *I)). This voltage difference provides the basis for the JTEC engine.

[0027] The JTEC heat engine offers a practical approach using available high-barrier, low-conductivity membrane materials to provide a thermoelectrochemical heat engine that can approximate the Ericsson cycle, operate over a wide range of heat source temperatures, and eliminate the reliability and low efficiency issues associated with mechanical engines.

[0028] However, JTEC heat engines also have some disadvantages. For example, their design is complicated by the large membrane / electrode surface area required and the large number of cells electrically connected in series to achieve practical output voltage levels. Specifically, unlike conventional fuel cells, whose open-circuit voltage can exceed 1.0 volt, the Nernst voltage derived from the hydrogen pressure difference across the MEA is only in the range of approximately 0.2 volts. Therefore, many cells must be connected in series to achieve useful output voltage levels.

[0029] Furthermore, to achieve efficient energy conversion, the membrane must have good ionic conductivity and high diffusion barrier properties, as the diffusion of the working fluid (e.g., hydrogen) across the membrane under a pressure differential results in reduced electrical output and efficiency. However, known and available membrane materials with good ionic conductivity, such as Nafion manufactured by DuPont, are polymers and generally have very poor molecular diffusion barriers. Conversely, known and available membrane materials with high molecular diffusion barriers generally have relatively low ionic conductivity, so that using these materials will result in high system impedance and high polarization losses.

[0030] Therefore, the negative impact of the low ionic conductivity of available membrane materials remains a problem that limits the power output of JTEC heat engines. Specifically, the resistance to ionic conduction through the membrane is reflected in a reduction in output voltage, reduced efficiency, and reduced power density. In conventional implementations of JTEC heat engines, which focus on closely approximating isothermal expansion and compression processes to approximate the Ericsson cycle as a means of maximizing efficiency, the MEA activation energy and resistivity losses associated with membrane ionic conductivity overwhelm overall performance at the device level, especially when operating on low-grade, low-temperature heat sources.

[0031] The heat engine of the present invention addresses these shortcomings. More specifically, the thermoelectrochemical converter of the present invention selectively implements a temperature gradient across each MEA to generate a local thermogalvanic voltage having a polarity opposite to that of the activation energy and resistivity loss voltages, thereby locally minimizing or eliminating their effects on output voltage, efficiency, and power density. Summary of the Invention

[0032] In one aspect, the present invention relates to a thermoelectrochemical converter comprising: a working fluid; a first membrane electrode assembly and a second membrane electrode assembly coupled to the first membrane electrode assembly, each of the first and second membrane electrode assemblies comprising: a first porous electrode operating at a first pressure, a second porous electrode operating at a second pressure higher than the first pressure, and an ion conductive membrane sandwiched therebetween, the first membrane electrode assembly being used to compress the working fluid, and the second membrane electrode assembly being used to expand the working fluid; a first heat transfer member coupled to the first porous electrode of the first membrane electrode assembly, the first heat transfer member being thermally bonded to a surface constituting a low-pressure side of the first membrane electrode assembly and promoting heat transfer from the surface; a second heat transfer member coupled to the first porous electrode of the second membrane electrode assembly, the second heat transfer member being thermally bonded to a surface constituting a low-pressure side of the second membrane electrode assembly and promoting heat transfer to the surface; a heat sink coupled to the low-pressure side of the first membrane electrode assembly; and a heat source coupled to the low-pressure side of the second membrane electrode assembly.

[0033] In another aspect, the present invention relates to a thermoelectrochemical converter comprising: an ionizable working fluid; at least one membrane electrode assembly comprising: a first porous electrode, a second porous electrode and at least one ion-conducting membrane, the ion-conducting membrane being configured to conduct ions of the ionizable working fluid sandwiched between the first and second porous electrodes; a first conduit containing the ionizable working fluid at a first pressure, and a second conduit containing the ionizable working fluid at a second pressure higher than the first pressure, the first conduit being coupled to the first porous electrode and corresponding to a low-pressure side of the at least one membrane electrode assembly, and the second conduit being coupled to the second porous electrode and corresponding to a high-pressure side of the at least one membrane electrode assembly; and a heat conductor coupled to the at least one membrane electrode assembly on the low-pressure side, the heat conductor coupling heat to and from substantially the entire surface of the membrane electrode assembly on the low-pressure side.

[0034] In another aspect, the present invention relates to a method for converting thermal energy into electrical energy. The method includes providing a thermoelectrochemical converter comprising: a working fluid; a first membrane electrode assembly (MEA) and a second MEA coupled to the first MEA, each of the first and second MEAs comprising: a first porous electrode operating at a first pressure, a second porous electrode operating at a second pressure higher than the first pressure, and an ion-conducting membrane sandwiched therebetween; a first heat transfer member coupled to the first porous electrode of the first MEA, the first heat transfer member thermally engaging a surface forming a low-pressure side of the first MEA and facilitating heat transfer from the surface; a second heat transfer member coupled to the first porous electrode of the second MEA, the second heat transfer member thermally engaging a surface forming a low-pressure side of the second MEA and facilitating heat transfer to the surface; a heat sink coupled to the low-pressure side of the first MEA; and a heat source coupled to the low-pressure side of the second MEA, the heat source being at an elevated temperature relative to the heat sink. The method also includes compressing the working fluid at the first MEA and expanding the working fluid at the second MEA. When the working fluid is compressed at the first membrane electrode assembly, the heat of compression generated is removed to a heat sink, wherein the first heat transfer member enhances the removal of the heat of compression, thereby generating a temperature gradient and a thermocouple voltage, wherein the temperature gradient increases toward the high-pressure side of the first membrane electrode assembly, and the thermocouple voltage causes the working fluid to move in the same direction as the voltage applied to the first membrane electrode assembly for pumping the working fluid, so that a reduced voltage is required to drive the compression of the working fluid at the first membrane electrode assembly. When the working fluid expands from the high-pressure side to the low-pressure side, electrical power is generated at the second membrane electrode assembly, wherein the second heat transfer member enhances the application of heat from the heat source to the low-pressure side of the second membrane electrode assembly, thereby generating a heat flux in a direction opposite to the direction of expansion of the working fluid through the second membrane electrode assembly, and the thermovoltaic potential generated by the heat applied to the low-pressure side pulls the working fluid toward the low-pressure side, thereby increasing the output voltage of the second membrane electrode assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The following detailed description of the preferred embodiments of the present invention will be better understood when read in conjunction with the accompanying drawings. For the purpose of illustrating the invention, there are shown in the accompanying drawings embodiments that are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the accompanying drawings:

[0036] Figure 1 is a Nernst equation plot of voltage versus temperature generated by an MEA using hydrogen as the working fluid for various pressure ratios;

[0037] Figure 2is a schematic diagram of a thermoelectrochemical converter according to an embodiment of the present invention;

[0038] Figure 3 yes Figure 2 Ideal temperature entropy diagram of the Ericsson engine cycle for converter operation;

[0039] Figure 4 is a schematic diagram of a heat engine including a plurality of stacked converters according to an embodiment of the present invention;

[0040] Figure 5 is a schematic diagram of a high-temperature MEA of a thermoelectrochemical converter according to an embodiment of the present invention;

[0041] Figure 6 is a schematic diagram showing the variation of power density with temperature difference for different thermoelectrochemical converter configurations; and

[0042] Figure 7 is a schematic diagram of a thermoelectrochemical converter according to an embodiment of the present invention. DETAILED DESCRIPTION

[0043] Certain terms are used in the following description for convenience only and are not limiting. The words "proximal," "distal," "upward," "downward," "bottom," and "top" designate directions in the accompanying drawings. In accordance with the present invention, the words "inwardly" and "outwardly" refer to directions toward and away from, respectively, the geometric center of the device and designated parts thereof. Unless otherwise specified herein, the terms "a," "an," and "the" are not limited to one element, but should be understood to mean "at least one." The terminology includes the above-mentioned words, their derivatives, and words of similar meaning.

[0044] It will also be understood that terms such as "first," "second," etc., are provided only for clarity, and the elements or components identified by these terms and their operations can be easily switched.

[0045] Referring to the drawings in detail, wherein like numerals represent like elements throughout the several views, Figure 1-7 Preferred embodiments of heat engines comprising one or more MEAs, or aspects thereof, are shown. The terms "electrochemical cell," "membrane electrode assembly," "membrane electrode assembly stack," "MEA," "MEA stack," "MEA cell," and "stack" are used interchangeably herein.

[0046] refer to Figure 7 , shows a thermoelectrochemical converter according to an embodiment of the present invention. The converter includes at least one MEA 200. The MEA 200 includes a membrane 220 sandwiched between a pair of electrodes 230, 232. It should be understood that the MEA 200 may include multiple overlapping layers of alternating electrodes 230, 232 and membranes 220 arranged in a high-density stacked configuration.

[0047] The membrane 220 is preferably an ion-conducting membrane or a proton-conducting membrane having a thickness of about 0.1 micrometers to 500 micrometers, more preferably about 1 micrometer to 500 micrometers. More specifically, the membrane 220 is preferably made of a proton-conducting material or an ion-conducting material, and more preferably made of a material that is ion-conducting to the working fluid passing through the MEA 200. In one embodiment, the membrane 220 is preferably formed of a material including polybenzimidazole, yttrium-doped barium zirconate, or titanium oxide, more preferably polybenzimidazole or yttrium-doped barium zirconate. However, those skilled in the art will understand that any material, preferably any polymer or ceramic material, that exhibits similar ionic conductivity over a wide temperature range can be used to form the membrane 220.

[0048] The electrodes 230, 232 preferably each have a thickness of approximately 25 μm. The electrodes 230, 232 are preferably constructed or formed from the same material as the membrane 220, thereby eliminating or at least reducing the high thermal stresses that can occur during co-sintering or fusing to form the MEA 200 and during operation of the MEA 200 in many end-use applications at extreme temperatures. However, the electrodes 230, 232 are preferably porous, while the membrane 220 is preferably non-porous. It should be understood that the electrodes 230, 232 and the membrane 220 can be formed from different materials having similar coefficients of thermal expansion, thereby generating little or no thermal stress during co-sintering / fusing or use of the MEA 200.

[0049] In one embodiment, the porous electrodes 230, 232 may be doped or infused with additional materials to provide electronic conductivity and catalytic materials to facilitate oxidation and reduction of the working fluid.

[0050] The MEA 200 also includes a conduit system comprising at least one low pressure conduit 237 coupled to the first porous electrode 230 (at Figure 7 and at least one high-voltage conduit 238 coupled to the second porous electrode 232 (shown in dashed lines in FIG. Figure 7 denoted by solid lines in FIG. A supply of ionizable gas (preferably hydrogen) serving as a working fluid is contained within the conduit system. Those skilled in the art will appreciate that virtually any gas can be used as the working fluid (e.g., oxygen), as long as membrane 220 is formed from a material that is ion / proton conductive to the gas. In one embodiment, the working fluid is 100% oxygen. In another embodiment, the working fluid comprises 0.1% to 99.9% hydrogen, with the balance being an inert gas.

[0051] Low-pressure conduit 237 directs the flow of a working fluid (e.g., hydrogen) in the direction of arrow A, while high-pressure conduit 238 directs the flow of the working fluid in the direction of arrow B (i.e., the opposite direction of the flow of low-pressure conduit 237). Low-pressure conduit 37 and high-pressure conduit 38 define low-pressure and high-pressure electrodes 230 and 232, respectively, and the low-pressure and high-pressure sides of MEA 200.

[0052] The high pressure side of the MEA 200 can be at a pressure as low as 0.5 psi and as high as 3,000 psi. Preferably, the high pressure side of the MEA 200 is maintained at a pressure of approximately 300 psi. The low pressure side of the MEA 200 can be at a pressure as low as 0.0001 psi and as high as 0.3 psi. Preferably, the low pressure side of the MEA 200 is maintained at a pressure of approximately 0.03 psi. The preferred pressure ratio of the high pressure side to the low pressure side is 10,000:1 (see Figure 1 ).

[0053] The MEA 200 also includes at least one heat transfer member 240 coupled to the MEA 200 on the low-pressure side (i.e., the side corresponding to the low-pressure electrode 230 and the low-pressure conduit 237). The heat transfer member 240 effectively provides a thermal interface across the entire surface of the low-pressure side of the MEA 200, effectively coupling heat to and from substantially the entire surface of the low-pressure side of the MEA, for example, to a heat sink (not shown) or from a heat source (not shown).

[0054] The first terminal 233 and the second terminal 231 are connected to the low voltage electrode 230 and the high voltage electrode 232 , respectively.

[0055] In one embodiment, MEA 200 can be operated as a heat engine to expand the working fluid from high pressure to low pressure, thereby generating electricity. By connecting an electrical load to the first and second terminals 233, 231, electricity can be extracted from MEA 200. When the pressure difference between the high-pressure and low-pressure conduits 238, 237 forces the working fluid to pass through MEA 200, electrical power is generated. When under pressure, the working fluid is oxidized at the high-voltage electrode 232 connected to terminal 231, thereby releasing electrons to the high-voltage electrode 232 and causing the ions of the working fluid to enter the ion-conducting membrane 220 as shown by arrow 33. When the high-voltage electrode 232 is connected to an external load, the electrons flow through the load to the low-voltage electrode 230, where the ions leaving the membrane 220 are reduced to reconstruct the working fluid and the working fluid is coupled to a heat transfer member 240 so as to supply the expanded heat to the working fluid (e.g., from a heat source). When pressure forces the working fluid to flow through MEA 200, the converter supplies power to the external load.

[0056] In another embodiment, MEA 200 is configured to operate to pump a working fluid from a low pressure to a high pressure, thereby generating a compression process. The compression process consumes electricity. A power source is applied to a first terminal 233 and a second terminal 231. A voltage is applied at a potential sufficient to force current flow, overcoming the Nernst potential generated by MEA 200 at its operating temperature and pressure differential. The applied power strips electrons from the working fluid at the interface between low-voltage electrode 230 and membrane 220. The resulting ions are conducted through ion-conducting membrane 220 in the direction indicated by arrow 39. The power source supplies electrons to high-voltage electrode 232 to reconstitute the working fluid at the interface between high-voltage electrode 232 and membrane 220 when the ions leave membrane 220. This current, under the applied voltage, effectively provides the pumping power required to pump the working fluid from a low pressure to a high pressure. The removal of compression heat, for example to a heat sink (not shown), is facilitated by a heat transfer member 240 coupled to the low-pressure side of MEA 200.

[0057] refer to Figure 2 , shows a thermoelectrochemical converter according to another embodiment of the present invention. As described more fully herein, the converter operates on a combination of Nernst voltage and thermocouple voltage generated across two or more ion-conducting membrane electrode assemblies. Figure 3 Shown Figure 2 The ideal temperature entropy diagram of the Ericsson engine cycle of the converter. Electrical connections are Figure 2 Not shown.

[0058] refer to Figure 2 The converter includes a first MEA 10, a second MEA 28, a heat exchanger 26 connecting the first MEA 10 and the second MEA 28, a working fluid flowing in a continuous loop between the first MEA 10 and the second MEA 28, a first conduit 18 and a second conduit 20, all of which are housed in a monolithic co-sintered ceramic structure. Figure 2 The first MEA 10 and the second MEA 28 are Figure 7 The MEA 200 is the same as that of the MEA 200, so the above description of the various components of the MEA 200 will not be repeated herein because the description is also applicable to Figure 2 MEA 10, 28.

[0059] Briefly, the first MEA 10 includes a membrane 14 that conducts ions of the working fluid and is sandwiched between a first porous electrode 12 and a second porous electrode 16. The first MEA 10 is coupled to a heat sink 5 and is used to pump the working fluid from a low pressure to a high pressure (i.e., compress the working fluid), consuming electrical power and rejecting compression heat during the compression process (heat removal is performed by Figure 2 Arrow Q in Ldenoted by ). The second MEA 28 includes a membrane 24 that conducts ions of the working fluid and is sandwiched between a first porous electrode 30 and a second porous electrode 22. The second MEA 28 is coupled to a heat source 7 (heat supplied by Figure 2 Arrow Q in H The working fluid expands through the second MEA 28 to generate electricity.

[0060] The heat exchanger 26 is preferably a recuperative counter-flow heat exchanger that recovers heat by coupling the working fluid leaving the second MEA 28 to the working fluid flowing to the second MEA 28. Providing such a recuperative heat exchanger, in combination with heat sources and heat sinks coupled to the high and low temperature electrochemical cells (i.e., the MEA stack), can provide sufficient heat transfer for near constant temperature expansion and compression processes, thereby allowing the engine to approach a thermodynamic Ericsson cycle.

[0061] First conduit 18 operates at a first pressure, and second conduit 20 operates at a second pressure, which is higher than the first pressure. Therefore, first conduit 18 is referred to herein as the "low-pressure conduit 18," and second conduit 20 is referred to herein as the "high-pressure conduit." Low-pressure conduit 18 couples first electrodes 12 and 30 of first MEA 10 and second MEA 28, respectively, to enable flow of a working fluid between first electrodes 12 and 30. Therefore, first electrodes 12 and 30 are low-pressure electrodes, the sides of MEAs 10 and 28 corresponding to first electrodes 12 and 30 are the low-pressure sides of the respective MEAs 10 and 28, and the sides of the converters corresponding to first electrodes 12 and 30 are the low-pressure sides of the converters. High-pressure conduits 20 couple second electrodes 16 and 22 of first MEA stack 10 and second MEA stack 28, respectively, to enable flow of a working fluid between second electrodes 16 and 22. Thus, second electrodes 16 , 22 are high voltage electrodes, the side of MEA 10 , 28 corresponding to second electrodes 16 , 22 is the high voltage side of the respective MEA 10 , 28 , and the side of the converter corresponding to second electrodes 16 , 22 is the high voltage side of the converter.

[0062] The high pressure side of each MEA 10, 28 can be at a pressure as low as 0.5 psi and as high as 3,000 psi. Preferably, the high pressure side of each MEA 10, 28 is maintained at a pressure of approximately 300 psi. The low pressure side of each MEA 10, 28 can be at a pressure as low as 0.0001 psi and as high as 0.3 psi. Preferably, the low pressure side of each MEA 10, 28 is maintained at a pressure of approximately 0.03 psi. The preferred pressure ratio of the high pressure side to the low pressure side of each MEA 10, 28 is 10,000:1 (see Figure 1 ).

[0063] The first MEA 10 and the second MEA 28 preferably each include at least one heat transfer member, also known as a heat conductor or heat sink. In one embodiment, the low-voltage electrodes 12, 30 of the first and second MEAs 10, 28 include heat transfer members. In one embodiment, the low-voltage electrodes 12, 30 themselves are configured to function as heat transfer members. In another embodiment, each low-voltage electrode 12, 30 is coupled to a separate heat transfer member.

[0064] exist Figure 2 In the embodiment of FIG. 1 , the low-voltage electrode 12 of the first MEA 10 is configured to function as a heat transfer member relative to the heat sink 5, and the low-voltage electrode 30 of the second MEA 28 is configured to function as a heat transfer member relative to the heat source 7. Therefore, no separate components are shown or designated as heat transfer members. Thus, in addition to functioning as electrodes, the low-voltage electrode 12 of the first MEA 10 facilitates heat transfer from the first MEA 10 to the associated heat sink 5, and the low-voltage electrode 30 of the second MEA 28 facilitates heat transfer from the associated heat source 7 to the second MEA 28.

[0065] In one embodiment, when the converter operates as a heat engine, the heat source 7 coupled to the second MEA 28 is preferably at an elevated temperature relative to the temperature of the heat sink 5 coupled to the first MEA 10. Thus, the first MEA 10 constitutes the low-temperature, compression cell and the low-temperature side of the converter, while the second MEA 28 constitutes the high-temperature, expansion cell and the high-temperature side of the converter, and has a higher Nernst voltage than the low-temperature cell 10. The operation of the low-temperature cell 10 is driven by electrical power input, with heat being rejected to the heat sink 5 by the first heat transfer member. The operation of the high-temperature cell 28 is driven by heat input from the heat source 7 to the second heat transfer member, generating electricity. As a result, the second MEA 28 generates an amount of electrical power sufficient to drive the operation of the first MEA 10 and provide a net power output.

[0066] In another embodiment, when the converter is used as a heat pump, the heat source 7 coupled to the second MEA 28 is preferably at a reduced temperature relative to the heat sink 5 to which the first MEA 10 is coupled. As the heat of expansion is extracted from the low-temperature heat source 7, the working fluid expands in the second MEA 28. The operation of the second MEA 28 is driven by the heat input from the heat source 7 to the second heat transfer member to generate electricity. Because the heat of compression is rejected at an elevated temperature, the working fluid is compressed at a high temperature in the first MEA 10. The operation of the first MEA 10 is driven by the input of electrical power, with the heat rejected to the heat sink 5 by the first heat transfer member. The amount of electrical power consumed by the first MEA 10 is greater than the amount of electrical power generated by the second MEA 28, requiring a net power input, as in the case of a heat pump.

[0067] The discussion in this paper focuses on the operating configuration where the converter operates as a heat engine.

[0068] Figure 3 Shown Figure 2 Ideal temperature entropy diagram of the Ericsson engine cycle for converter operation. Thermodynamic states 1 to 4 are Figure 2 and Figure 3 The points marked in are the same.

[0069] refer to Figure 2-3 The converter operates as follows, wherein the working fluid is hydrogen. However, it should be understood that another ionizable gas, such as oxygen, may be used as the working fluid instead. Starting from the low temperature and low pressure state 1, the electrical energy W IN The hydrogen is supplied to the low temperature MEA 10 to pump the hydrogen from the low temperature, low pressure state 1 to the low temperature, high pressure state 2. More specifically, the hydrogen is compressed in the low temperature MEA 10, thereby driving the hydrogen from the low pressure side to the high pressure side of the membrane 14. In the low temperature MEA 10, due to the heat of compression generated when the hydrogen transitions from the low pressure side to the high pressure side through the membrane 14, the temperature of the hydrogen (protons) increases by a limited amount, while the heat Q is transferred to the low temperature MEA 10. L Removed from the low pressure side of the membrane 14 to the heat sink 5. Heat dissipation to the heat sink 5 is enhanced by the low voltage electrode 12 comprising or coupled to a thermally conductive material. As a result, an increasing temperature gradient towards the high pressure side of the low temperature MEA 10 is maintained.

[0070] From the low-temperature, high-pressure state 2, the hydrogen passes through the recuperative countercurrent heat exchanger 26, where it is heated at a substantially constant pressure to reach the temperature of state 3. More specifically, state 3 is a high-temperature, high-pressure state. The heat required to raise the temperature of the hydrogen from the low-pressure, high-temperature state 2 to the high-temperature, high-pressure state 3 is transferred from the hydrogen flowing in the opposite direction through the heat exchanger 26. At the high-temperature, high-pressure state 3, as the hydrogen expands through the high-temperature, high-pressure MEA 28 from the high-pressure, high-temperature state 3 to the high-temperature, low-pressure state 4, electrical power W is generated. OUT In the high temperature MEA 28, when heat Q is supplied from the heat source 7 to the low pressure side of the membrane 24 H , the temperature of the hydrogen (protons) increases by a limited amount, facilitated by the low-pressure electrode 30 including or coupled to a thermally conductive material to overcome the temperature drop that occurs due to the heat of expansion generated when the hydrogen (protons) transitions from the high-pressure side to the low-pressure side across the membrane 24. As a result, an increasing temperature gradient toward the low-pressure side of the high-temperature MEA 28 is maintained. From the high-temperature, low-pressure state 4, the hydrogen passes through the recuperative countercurrent heat exchanger 26, where its temperature is reduced by transferring heat to the hydrogen flowing countercurrently from the low-temperature, high-pressure state 2 to the high-temperature, high-pressure state 3, until the hydrogen passing from the high-temperature, low-pressure state 4 reaches the temperature of the low-temperature, low-pressure state 1. As described above, as the cycle continues, the hydrogen is pumped back from the low-temperature, low-pressure state 1 to the low-temperature, high-pressure state 2 by the low-temperature MEA 10.

[0071] Figure 4 A converter assembly according to another embodiment of the present invention is shown and details a specific preferred configuration of low voltage electrodes 12, 30 as thermally conductive electrodes that function as heat sink and heat source diffusers, respectively, to transfer heat from and to the respective MEAs 10, 28.

[0072] exist Figure 4 In the embodiment shown, the heat engine comprises two identical converters 100 arranged in a stacked configuration, but it will be appreciated that more than two converters 100 may be included in the assembly. The converters 100 are identical to each other and are similar to those described above with respect to FIG. Figure 2 and Figure 2 Therefore, it is not necessary to describe in detail the configuration of each converter 100. Briefly, each converter 100 comprises a first low temperature MEA 110 coupled to a heat sink 115 (where heat removal is indicated by arrow Q). L ), the second high temperature MEA 128 is coupled to the heat source 117 (heat supply is indicated by arrow Q H , a heat exchanger 126 connects the low-temperature and high-temperature MEAs 110, 128. Each low-temperature MEA 110 includes an ion-conducting membrane 114 sandwiched between a first electrode 112 and a second electrode 116. Each high-temperature MEA 128 includes an ion-conducting membrane 124 sandwiched between a first electrode 130 and a second electrode 122.

[0073] Each converter 100 also includes a first conduit 118 operating at a first pressure and a second conduit 120 operating at a second pressure higher than the first pressure. The low-pressure conduit 118 of each converter 100 couples the first electrode 112 of the low-temperature MEA 110 with the first electrode 130 of the high-temperature MEA 128, thereby enabling the flow of a working fluid between the two low-pressure electrodes 112, 130. The low-pressure electrodes 112, 130 constitute the low-pressure side of the respective MEAs 110, 128 and are respectively coupled to a heat sink 115 and a heat source 117. The high-pressure conduit 120 of each converter 100 couples the second electrode 116 of the low-temperature MEA 110 with the second electrode 122 of the high-temperature MEA 128, thereby enabling the flow of a working fluid between the high-temperature electrodes 116, 122. The high-temperature electrodes 116, 122 constitute the high-pressure side of the respective MEAs 110, 128.

[0074] The first and second converters 100 are stacked such that the low-voltage electrodes 112 and 130 of each converter 100 are arranged in a back-to-back configuration, heat is supplied to the back-to-back low-voltage electrodes 130 of the high-temperature MEA cells 128 by a heat source 117, and heat is removed from the back-to-back low-voltage electrodes 112 of the low-temperature MEA cells 110 by a heat sink 115. Thus, the low-temperature MEAs 110 constitute a low-temperature MEA stack, and the high-temperature MEAs 128 constitute a high-temperature MEA stack.

[0075] In another embodiment (not shown), the low-temperature MEAs 110 and the high-temperature MEAs 128 of the first and second converters 100 need not include different low-voltage electrodes 112, 130. Instead, the low-temperature MEAs 110 of the first and second converters 100 can be stacked on top of each other such that adjacent MEAs 110 share a common low-voltage electrode 112, and the high-temperature MEAs 128 of the first and second converters 100 can be stacked on top of each other such that adjacent MEAs 110 share a common low-voltage electrode 130. Similarly, the converters 100 need not have different low-pressure and high-pressure conduits 118, 120, but can instead share common conduits 118, 120.

[0076] A first heat transfer member 140, more specifically, a first heat conductor 140, is coupled to and disposed between adjacent low voltage electrodes 112 of the low temperature MEA stack 140. The first heat conductor 140 couples the low temperature MEA stack 140 to an associated heat sink 115. A second heat transfer member 142, more specifically, a second heat conductor 142, is coupled to and disposed between adjacent low voltage electrodes 130 of the high temperature MEA stack 160. The second heat conductor 142 couples the high temperature MEA stack 160 to an associated heat source 117. It should be understood that although the heat conductors 140, 142 are Figure 4 Although shown as a separate component from the low-voltage electrodes 112 , 130 , the thermal conductors 140 , 142 may alternatively be an integral component of the low-voltage electrodes 112 , 130 .

[0077] The first heat conductor 140 effectively provides a thermal interface across the entire surface of the low-pressure side of the low-temperature MEA 110 (i.e., the side corresponding to the low-voltage electrode 112), and facilitates heat transfer from the low-temperature MEA 110 to the heat sink 115. The second heat conductor 142 effectively provides a thermal interface across the entire surface of the low-pressure side of the high-temperature MEA 128 (i.e., the side corresponding to the low-voltage electrode 130), and facilitates heat transfer from the heat source 117 to the high-temperature MEA 128.

[0078] The operation of each converter 100 is similar to that described above with respect to Figure 2-3 Proceed in the same manner as described for states 1 to 4.

[0079] The first and second heat conductors 140, 142 are preferably made of a porous material, but may include non-porous portions to provide enhanced heat transfer. The first and second heat conductors 140, 142 may also include portions formed of different materials that provide enhanced heat transfer. The first and second heat sinks 140, 142 may be formed of virtually any thermally conductive material. Examples of such thermally conductive materials include, but are not limited to, metals, diamond, graphite, silicon carbide, aluminum nitride, and any equivalents thereof. The first and second heat sinks 140, 142 may be formed of the same material or a mixture of materials, or of different materials or a mixture of materials.

[0080] refer to Figure 5 , shows a high temperature MEA 170 of a converter according to an embodiment of the present invention. Figure 5 The high temperature MEA 170, heat input is achieved in the form of current and voltage generation to improve the overall converter performance. The MEA 170 includes an ion conductive membrane 172 sandwiched between a first porous electrode 174 and a second porous electrode 176, and more specifically a membrane having a thickness of T 172 The first electrode 174 is coupled to a heat source (not shown) such that the first electrode 174 is a heating electrode. The first electrode 174 operates at a first pressure and the second electrode 176 operates at a second pressure higher than the first pressure. In one embodiment, the low voltage electrode 174 comprises a material having a high thermal conductivity and thus acts as a heat sink or is coupled to a component comprising such a thermally conductive material. A mass flow 178 of hydrogen enters the high voltage electrode 176 and is oxidized. The resulting electrons are conducted through an external circuit (not shown) via the first and second electrode terminals 180 and 182, while the resulting protons 184 are conducted through the proton conducting membrane 172. The internal resistive heating 186 due to the proton current is Figure 5 Designated as Q R Under the pressure differential across the MEA 170, heat conducted from the high temperature side 176 into the MEA 170 is dissipated by the expansion of the hydrogen gas through the membrane 172 as heat of expansion 136, Q EXP When the circuit is complete, the protons and electrons are reduced back to hydrogen gas at the low voltage electrode 174.

[0081] The voltage generated across the MEA 170 is due to the high voltage P H and low pressure P L The difference between the Nernst voltage is the result. Therefore, it is desirable to maintain a constant temperature compression and expansion process in order to approximate the Ericsson cycle. Ideally, the heat output 150, Q on the low temperature side 176 OUTis zero because there is no heat sink on the inlet side of the MEA other than that associated with the incoming hydrogen. The voltage loss within the MEA 170 is related to the resistance of the membrane, the hydrogen activation energy, the voltage loss due to the pressure drop of the hydrogen flow in the conduits coupling the multiple MEAs back to back, the hydrogen permeation through the membrane, etc.

[0082] In the present invention, because the heat transfer member facilitates the supply or removal of heat, the temperature gradient generated by the heat flux through each MEA is in the direction opposite to the direction of proton conduction, and a thermocouple voltage (e.g., Seebeck voltage) is generated on each MEA, which has a polarity opposite to the resistive voltage loss associated with proton conduction. Therefore, the temperature gradient and the thermocouple voltage locally minimize or completely offset the losses, such as activation energy loss and resistive voltage loss. For example, referring to Figure 5 Because heat is applied to one side of MEA 170, i.e., low-pressure side 174, which is coupled to or includes a heat transfer member, the temperature gradient generated by the heat flux across MEA 170 is in a direction opposite to that of proton conduction, and a thermocouple voltage (e.g., a Seebeck voltage) is generated across MEA 170 that has an opposite polarity relative to the resistive voltage losses associated with proton conduction. Thus, the temperature gradient and thermocouple voltage locally minimize or completely cancel out losses, such as activation energy losses and resistive voltage losses.

[0083] Figure 6 The results of a modeling analysis of a complete thermoelectrochemical converter according to the present invention, including high and low temperature MEA cells showing the effect of heat sink location, are shown. A representative converter was operated at a 150°C heat source and a 50°C heat sink. Figure 6 The membrane of the converter analyzed in the diagram includes phosphate-based polybenzimidazole as the proton-conducting membrane material. This membrane has an ionic conductivity of 0.056 S / cm at 50°C and 0.355 S / cm at 150°C. The model allocates 70% of the theoretical Nernst open-circuit voltage of two MEAs arranged back-to-back as the output voltage to the load, with the remaining 30% consumed by internal losses. Under these conditions, the maximum possible output current is constrained to a level such that losses associated with resistance, hydrogen flow, activation energy, and other factors can be accommodated within 30% of the open-circuit voltage, thereby achieving a Carnot output efficiency of 70%. The model predicts the output power density at the 70% Carnot output efficiency constraint.

[0084] The configuration represented by line 64 (where the heat flux is in the opposite direction to the ion flux) has an increased power density compared to the configuration of line 62 (where the heat flux is in the same direction as the ion flux). Figure 6This reflects the significant change in power density with only a few degrees of temperature difference across the membrane. Note that when there is no temperature gradient, the power output is 9mW / cm 2 , and both models operate at the same power level. However, as a temperature gradient develops across the membranes of the two MEAs, the performance of the two configurations differs. Line 62 shows the output power density when the heat flux is in the same direction as the hydrogen or proton flux through each MEA. On the other hand, line 64 shows the heat input and extraction from each MEA in a manner that generates a heat flux in the opposite direction of the proton or hydrogen flux.

[0085] The change in power output occurs due to the additive or subtractive nature of the thermocouple voltage generated on the MEA, depending on whether the heat flux is in the opposite direction or in the same direction as the working fluid flow. In the configuration represented by the representative line 64 of the present invention, at the low-temperature MEA, heat is extracted on the low-pressure side and the low-pressure side is maintained at a relatively low temperature. When the working fluid (e.g., hydrogen) is moved through the MEA to the high-pressure side by the applied pumping voltage, compression heat is generated. The generated compression heat is conducted back to the low-pressure side in the opposite direction. The end result is that the thermocouple voltage tends to move the working fluid in the same direction as the voltage applied to the cell to produce pumping. The combination of the two voltages generated overcomes the Nernst potential defined by the temperature and pressure ratio of the MEA, which results in a lower input voltage being required to achieve the desired pumping.

[0086] On the other hand, in the configuration represented by line 64, in addition to the Nernst potential of the battery, a thermocouple voltage is generated at the high temperature MEA, which results in an output voltage higher than the output voltage achieved by the Nernst potential alone. At the high temperature MEA, heat is input on the low pressure side so that the resulting heat flux entering the MEA is opposite to the direction in which the working fluid expands through the MEA from the high pressure side to the low pressure side. The pressure forces the working fluid to pass through the MEA, and as protons are conducted through the membrane, electrons are stripped to the external circuit (load and low temperature MEA). The thermovoltaic potential generated by the heat applied to the low pressure side has the same effect. The thermovoltaic potential pulls the working fluid to the low pressure side. This voltage is added to the Nernst voltage. This combination results in a higher overall high temperature MEA output voltage.

[0087] The combination of these two effects at the low and high temperature MEAs, i.e., the lower voltage required to drive compression of the low temperature MEA and the higher output voltage produced by the high temperature MEA, results in a larger amount of system level voltage available to overcome internal losses, especially those related to membrane resistance.

[0088] In contrast to the present invention, operation of the converter when the heat flux is in the same direction as the proton (hydrogen) flux results in a lower overall system voltage output because in this case the thermovoltaic voltage operates in opposition to the Nernst voltage rather than adding to it. Figure 6 The falling voltage on line 62 in illustrates this configuration. Figure 6 In the configuration represented by line 62, heat is extracted from the low-temperature MEA from the high-pressure side rather than the low-pressure side. In this case, the low-temperature side of the MEA is the side where the working fluid is compressed. The resulting thermovoltaic potential tends to move the working fluid toward the higher-temperature, low-pressure side, in opposition to the applied voltage that pumps the working fluid to the high-pressure side. Therefore, the applied voltage must be high enough to overcome the thermovoltaic potential generated by the applied temperature gradient and the Nernst voltage during compression of the low-temperature MEA. Similarly, providing heat on the high-pressure side of the high-temperature MEA creates a thermovoltaic potential that tends to move the working fluid toward the high-pressure side, in opposition to the expansion of the working fluid from the high-pressure side. The net effect is to reduce the output voltage of the MEA.

[0089] It will be appreciated by those skilled in the art that changes may be made to the above embodiments without departing from the broad inventive concept thereof. Therefore, it will be understood that the present invention is not limited to the particular embodiments disclosed, but is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A thermoelectrochemical converter comprising: working fluid; a first membrane electrode assembly and a second membrane electrode assembly coupled to the first membrane electrode assembly, each of the first membrane electrode assembly and the second membrane electrode assembly comprising: a first porous electrode operating at a first pressure, a second porous electrode operating at a second pressure higher than the first pressure, and an ion conductive membrane sandwiched therebetween, the first membrane electrode assembly being configured to compress the working fluid, and the second membrane electrode assembly being configured to expand the working fluid; a first conduit containing the working fluid and operating at a first pressure, and a second conduit containing the working fluid and operating at a second pressure higher than the first pressure, the first conduit being coupled to the first porous electrode of each of the first membrane electrode assembly and the second membrane electrode assembly so as to allow the working fluid to flow between the first porous electrodes; and a second conduit being coupled to the second porous electrode of each of the first membrane electrode assembly and the second membrane electrode assembly so as to allow the working fluid to flow between the second porous electrodes; a first heat transfer member coupled to the first porous electrode of the first membrane electrode assembly, the first heat transfer member thermally engaging with and facilitating heat transfer from a surface comprising the low pressure side of the first membrane electrode assembly; a second heat transfer member coupled to the first porous electrode of the second membrane electrode assembly, the second heat transfer member thermally engaging with and facilitating heat transfer to a surface comprising the low pressure side of the second membrane electrode assembly; a heat sink coupled to the low-voltage side of the first membrane electrode assembly; and A heat source is coupled to the low pressure side of the second membrane electrode assembly.

2. The thermoelectrochemical converter according to claim 1, characterized in that in a first operating configuration of the converter functioning as a heat engine, the operating temperature of the first membrane electrode assembly is lower than the operating temperature of the second membrane electrode assembly, operation of the first membrane electrode assembly is driven by an input of electrical power, heat is rejected by the first heat transfer member to the heat sink, operation of the second membrane electrode assembly is driven by heat input from the heat source to the second heat transfer member to generate electricity, and the second membrane electrode assembly generates an amount of electrical power sufficient to drive operation of the first membrane electrode assembly and provide a net power output; as well as wherein, in a second operating configuration of the converter serving as a heat pump, the operating temperature of the first membrane electrode assembly is higher than the operating temperature of the second membrane electrode assembly, the operation of the first membrane electrode assembly is driven by electrical power input, heat is discharged to the heat sink by the first heat transfer member, the operation of the second membrane electrode assembly is driven by heat input from the heat source to the second heat transfer member to generate electricity, and the amount of electrical power consumed by the first membrane electrode assembly is greater than the amount of electrical power generated by the second membrane electrode assembly because the heat pump requires a net power input.

3. The thermoelectrochemical converter according to claim 1, characterized in that The first and second membrane electrode assemblies are coupled to each other such that the first membrane electrode assembly supplies compressed working fluid to the second membrane electrode assembly and the second membrane electrode assembly supplies expanded working fluid to the first membrane electrode assembly.

4. The thermoelectrochemical converter according to claim 3, characterized in that The compressed working fluid is supplied from the high voltage electrode of the first membrane electrode assembly to the high voltage electrode of the second membrane electrode assembly, and wherein the expanded working fluid is supplied from the low voltage electrode of the second membrane electrode assembly to the low voltage electrode of the first membrane electrode assembly.

5. The thermoelectrochemical converter according to claim 1, characterized in that Also included is an external power source connected to the first porous electrode and the second porous electrode, power is applied to the electrodes and drives the working fluid to flow because the external power source forces electron flow through each membrane in the first and second membrane electrode assemblies to induce ionic conduction.

6. The thermoelectrochemical converter according to claim 1, characterized in that At the first membrane electrode assembly, heat is extracted on the low-pressure side and compression heat is generated when the working fluid moves through the first membrane electrode assembly to the high-pressure side by the applied pumping voltage, wherein the compression heat is conducted in a direction toward the low-pressure side of the first membrane electrode assembly so that the thermocouple voltage tends to cause the working fluid to move in the same direction as the voltage applied to the first membrane electrode assembly to produce pumping of the working fluid from the low-pressure side to the high-pressure side.

7. The thermoelectrochemical converter according to claim 1, characterized in that At the second membrane electrode assembly, heat is applied to the low pressure side so that the direction of the resulting heat flux entering the second membrane electrode assembly is opposite to the direction of expansion of the working fluid from the high pressure side to the low pressure side through the second membrane electrode assembly, and wherein the thermovoltaic potential generated by the heat applied to the low pressure side pulls the working fluid toward the low pressure side, thereby increasing the output voltage of the second membrane electrode assembly.

8. A thermoelectrochemical converter comprising: ionizable working fluid; at least one membrane electrode assembly comprising: a first porous electrode, a second porous electrode, and at least one ion-conducting membrane configured to conduct ions of the ionizable working fluid sandwiched between the first porous electrode and the second porous electrode; operating a first conduit containing the ionizable working fluid at a first pressure, and a second conduit containing the ionizable working fluid at a second pressure higher than the first pressure, the first conduit being coupled to the first porous electrode and corresponding to a low pressure side of the at least one membrane electrode assembly, and the second conduit being coupled to the second porous electrode and corresponding to a high pressure side of the at least one membrane electrode assembly; and A heat conductor is coupled to the at least one membrane electrode assembly on the low pressure side, the heat conductor coupling heat to and from substantially the entire surface of the membrane electrode assembly on the low pressure side.

9. The thermoelectrochemical converter according to claim 8, characterized in that Also included is a heat sink coupled to the at least one membrane electrode assembly at the low pressure side.

10. The thermoelectrochemical converter according to claim 8, characterized in that Also included is a heat source coupled to the at least one membrane electrode assembly at the low pressure side.

11. The thermoelectrochemical converter according to claim 10, characterized in that A temperature gradient is generated by heat flux through the at least one membrane electrode assembly in a direction opposite to the direction of ionic conduction of the working fluid, thereby generating a thermocouple voltage across the at least one membrane electrode assembly having an opposite polarity relative to the resistive voltage losses associated with ionic conduction.

12. A method for converting thermal energy into electrical energy, the method comprising: A thermoelectrochemical converter is provided, comprising: working fluid; a first membrane electrode assembly and a second membrane electrode assembly coupled to the first membrane electrode assembly, each of the first membrane electrode assembly and the second membrane electrode assembly comprising: a first porous electrode operating at a first pressure, a second porous electrode operating at a second pressure higher than the first pressure, and an ion conductive membrane sandwiched therebetween; a first conduit containing the working fluid and operating at a first pressure, and a second conduit containing the working fluid and operating at a second pressure higher than the first pressure, the first conduit being coupled to the first porous electrode of each of the first membrane electrode assembly and the second membrane electrode assembly so as to allow the working fluid to flow between the first porous electrodes; and a second conduit being coupled to the second porous electrode of each of the first membrane electrode assembly and the second membrane electrode assembly so as to allow the working fluid to flow between the second porous electrodes; a first heat transfer member coupled to the first porous electrode of the first membrane electrode assembly, the first heat transfer member thermally engaging with and facilitating heat transfer from a surface comprising the low pressure side of the first membrane electrode assembly; a second heat transfer member coupled to the first porous electrode of the second membrane electrode assembly, the second heat transfer member thermally engaging with and facilitating heat transfer to a surface comprising the low pressure side of the second membrane electrode assembly; a heat sink coupled to the low-voltage side of the first membrane electrode assembly; and a heat source coupled to the low pressure side of the second membrane electrode assembly, the heat source being at an elevated temperature relative to the heat sink; compressing the working fluid at the first membrane electrode assembly; and expanding the working fluid at the second membrane electrode assembly, wherein heat of compression generated when the working fluid is compressed at the first membrane electrode assembly is removed to a heat sink, wherein the removal of the heat of compression is enhanced by a first heat transfer member such that a temperature gradient that increases toward the high pressure side of the first membrane electrode assembly is generated, and a thermocouple voltage is generated that moves the working fluid in the same direction as a voltage applied to the first membrane electrode assembly for pumping the working fluid such that a reduced voltage is required to drive compression of the working fluid at the first membrane electrode assembly, and wherein, when the working fluid expands from the high-pressure side to the low-pressure side, electric power is generated at the second membrane-electrode assembly, wherein the application of heat from the heat source to the low-pressure side of the second membrane-electrode assembly is enhanced by the second heat transfer member, so that a heat flux is generated in a direction opposite to the direction of expansion of the working fluid through the second membrane-electrode assembly, and the thermovoltaic potential generated by the heat applied to the low-pressure side pulls the working fluid toward the low-pressure side, thereby increasing the output voltage of the second membrane-electrode assembly.

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