Johnson thermo-electrochemical converter

A converter includes a working fluid, a housing, a heat sink, a heat source that is at an elevated temperature relative to the heat sink, a first electrochemical cell disposed within the housing, and a micro/nano porous media disposed within the housing. The first electrochemical cell includes a first membrane electrode assembly across which the working fluid is configured to flow. The first membrane electrode assembly includes a first porous electrode and a second porous electrode and at least one ion conductive membrane sandwiched between the first and second porous electrodes. The first electrochemical cell is arranged between the heat source and the heat sink. The working fluid is contained within the micro/nano porous media. The micro/nano porous media is thermally coupled between the heat source and the heat sink, and creates a pressure differential across the first electrochemical cell by transpiration pumping of the working fluid.

BACKGROUND OF THE INVENTION

The conversion of heat energy or chemical energy to electrical energy, or vice-versa, may be accomplished in a variety of ways. For example, known electrochemical cells or batteries rely on chemical reactions wherein ions and electrons of a reactant being oxidized are transferred to the reactant being reduced via separate paths. Specifically, the electrons are transferred electrically via wiring through an external load where they perform work and the ions are conducted through an electrolyte separator.

However, battery type electrochemical cells can produce only a limited amount of energy because the confines of the battery casing limit the amount of available reactants that may be contained therein. Although such cells can be designed to be recharged by applying a reverse polarity current/voltage across the electrodes, such recharging requires a separate electrical source. Also, during the recharging process, the cell is typically not usable.

Fuel cells have been developed in an effort to overcome problems associated with battery type electrochemical cells. In conventional fuel cells, the chemical reactants are continuously supplied to the electrochemical cell and reaction products are continuously removed. In a manner similar to batteries, fuel cells operate by conducting an ionized species through a selective electrolyte which generally blocks passage of electrons and non-ionized species, the electrons having to pass externally through an electrical load to complete the reaction.

The most common type of fuel cell is a proton conductive membrane (PEM) hydrogen-oxygen fuel cell which passes hydrogen through one of the electrodes and oxygen through the other electrode. The hydrogen ions are conducted through a proton conductive electrolyte separator to the oxygen side of the cell under the voltage potential of the hydrogen-oxygen chemical reaction. Porous electrodes on either side of the electrolyte separator are used to couple the electrons involved in the chemical reaction through an external load via an external circuit. The electrons and hydrogen ions reconstitute hydrogen in a reaction with oxygen on the oxygen side of the cell for the production of water which is expelled from the system. A continuous electrical current is maintained by a continuous supply of hydrogen and oxygen to the cell.

Mechanical heat engines have also been designed and used to produce electrical power. Such mechanical heat engines operate on thermodynamic cycles wherein shaft work is performed using a piston or turbine 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 the high temperature, the working fluid is allowed to expand against a load, such as a piston or turbine, thereby producing shaft work. A key to the operation of all engines employing a working fluid is that less work is required to compress the working fluid at low temperatures than that produced by expanding it at high temperatures. This is the case for all thermodynamic engines employing a working fluid.

For example, steam engines operate on the Rankine thermodynamic cycle, wherein water is pumped to a high pressure, and then heated to steam and expanded through a piston or turbine to perform work. Internal combustion engines operate on the Otto cycle, wherein low-temperature ambient air is compressed by a piston and then heated to very high temperatures via fuel combustion inside the cylinder. As the cycle continues, the expansion of the heated air against the piston produces more work than that consumed during the lower temperature compression part of the cycle.

The Stirling engine has been developed to operate on the Stirling cycle in an effort to provide an engine that has high efficiency and offers greater versatility in the selection of the heat source. The ideal Stirling thermodynamic cycle is of equivalent efficiency to the ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine operating on heat input at high temperatures and heat rejection at low temperatures. However, as with all mechanical engines, the Stirling engine suffers from reliability problems and efficiency losses associated with its mechanical moving parts.

In an effort to avoid the problems inherent with mechanical heat engines, Alkali Metal Thermo-Electrochemical Conversion (AMTEC) cells have been designed as a thermo-electrochemical heat engine. AMTEC heat engines utilize pressure to generate a voltage potential and electrical current by forcing an ionizable working fluid, such as sodium, through an electrochemical cell at high temperatures. The electrodes couple the electrical current to an external load. Electrical work is performed as the pressure differential across the electrolyte separator forces molten sodium atoms through the electrolyte. The sodium is ionized upon entering the electrolyte, thereby releasing electrons to the external circuit. On the other side of the electrolyte, the sodium ions recombine with the electrons to reconstitute sodium upon leaving the electrolyte, in much the same way as the process that occurs in battery and fuel cell type electrochemical cells. The reconstituted sodium, which is at a low pressure and a high temperature, leaves the electrochemical cell as an expanded gas. The gas is then cooled and condensed back to a liquid state. The resulting low-temperature liquid is then re-pressurized. Operation of an AMTEC engine approximates the Rankine thermodynamic cycle.

Numerous publications are available on AMTEC technology. See, for example,Conceptual design of AMTEC demonstrative system for100t/d garbage disposal power generating facility, Qiuya Ni et al. (Chinese Academy of Sciences, Inst. of Electrical Engineering, Beijing, China). Another representative publication isIntersociety Energy Conversion Engineering Conference and Exhibit(IECEC), 35th, Las Vegas, Nev. (Jul. 24-28, 2000), Collection of Technical Papers. Vol. 2 (A00-37701 10-44). Also see American Institute of Aeronautics and Astronautics, 190, p. 1295-1299. REPORT NUMBER(S)-AIAA Paper 2000-3032.

The heat rejected during cooling and re-condensation of the high temperature expanded gas leaving the electrode at low pressure represents a significant source of entropy loss and therefore AMTEC heat engine inefficiency. AMTEC engines also suffer from reliability issues due to the highly corrosive nature of the alkali metal working fluid. They also have very limited utility. Specifically, AMTEC engines can only be operated at very high temperatures because ionic conductive solid electrolytes achieve practical conductivity levels only at high temperatures. Indeed, even the low-temperature pressurization process must occur at a relatively high temperature, because the alkali metal working fluid must remain above its melt temperature at all times as it moves through the cycle. Mechanical pumps, wicks and even magneto-hydrodynamic pumps have been used to pressurize the low-temperature working fluid.

In an effort to overcome the above-described drawbacks of conventional mechanical and thermo-electrochemical heat engines, the Johnson Thermo-Electrochemical Converter (JTEC) system was developed, as disclosed in U.S. Pat. No. 7,160,639 filed Apr. 28, 2003, International Patent Application No. PCT/US2015/044435 filed Aug. 10, 2015, and International Patent Application No. PCT/US2016/21508 filed Mar. 9, 2016, the entire contents of all three documents being incorporated herein by reference.

A more recent development of the JTEC relates to an electrochemical direct JTEC having membrane electrode assemblies and a control circuit operate to maintain a constant prescribed pressure ratio within the converter. More particularly, extra hydrogen is pumped to the high pressure side of the converter, and the additional pumped hydrogen compensates for the normal pressure loss due to molecular hydrogen diffusion through the membranes of the membrane electrode assembly stacks. Diffusion of molecular hydrogen through the separator membrane represents a significant decrease in power density over time, because the diffusion reduces the pressure differential across the electrodes of the membrane electrode assemblies and thereby reduces output voltage. Diffusion of molecular hydrogen also causes a reduction in efficiency, since this diffusion from high pressure to low pressure occurs without the diffusing hydrogen molecules undergoing an electrochemical reaction to produce electrical power. The membrane electrode assemblies and control circuit therefore operate to maintain a constant prescribed pressure ratio within the converter.

A common challenge for the JTEC is the need for large membrane electrode assembly surface areas, because high levels of current are required to complement the small voltage levels available per each membrane electrode assembly, if useful levels of power are to be achieved. The need therefore remains for a JTEC that provides improved efficiency and power density per membrane electrode assembly pair.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention provide an improvement over a typical JTEC system. More particularly, embodiments of the present invention include features to promote transpiration pumping to achieve higher voltage output per membrane electrode assembly pair as compared with a conventional JTEC engine.

In one embodiment, the invention relates to an electrochemical direct heat to electricity converter comprising a working fluid; a housing; a heat source and a heat sink, the heat source being at an elevated temperature above a temperature of the heat sink; a first electrochemical cell disposed within the housing and comprising a first membrane electrode assembly across which the working fluid is configured to flow, the first membrane electrode assembly of the first electrochemical cell including a first porous electrode and a second porous electrode and at least one ion conductive membrane sandwiched between the first and second porous electrodes, the first electrochemical cell being arranged between the heat source and the heat sink; and a micro/nano porous media disposed within the housing, the working fluid being contained within the micro/nano porous media, the micro/nano porous media being thermally coupled between the heat source and the heat sink and creating a pressure differential across the first electrochemical cell by transpiration pumping of the working fluid.

DETAILED DESCRIPTION OF THE INVENTION

Certain terminology is used in the following description for convenience only and is not limiting. The words “proximal,” “distal,” “upward,” “downward,” “bottom” and “top” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, a geometric center of the device, and designated parts thereof, in accordance with the present invention. Unless specifically set forth herein, the terms “a,” “an” and “the” are not limited to one element, but instead should be read as meaning “at least one.” The terminology includes the words noted above, derivatives thereof and words of similar import.

It will also be understood that terms such as “first,” “second,” and the like are provided only for purposes of clarity. The elements or components identified by these terms, and the operations thereof, may easily be switched. Also, MEA cell array and MEA cell stack may be used interchangeably

Referring toFIG. 1, there is shown a typical JTEC system (electrical connections not shown). JTEC is a heat engine that includes a first electrochemical cell100operating at a lower temperature, a second electrochemical cell110operating at a higher temperature relative to each other, a conduit system112including a heat exchanger114that couples the two cells100,110together, and a supply of ionizable gas (such as hydrogen, oxygen or alkali metal) as a working fluid contained within the conduit system112. Preferably, the working fluid is hydrogen. Each electrochemical cell100,110includes one or more membrane electrode assemblies.

More particularly, the first electrochemical cell100includes a first membrane electrode assembly (MEA)116coupled to a low temperature heat sink QL(i.e., the first MEA116is a low temperature MEA and the first cell100represents a low temperature side of the converter), the second electrochemical cell110includes a second MEA118coupled to a high temperature heat source QH(i.e., the second MEA118is a high temperature MEA and the second cell110represents a high temperature side of the converter), and a recuperative heat exchanger114connects the two MEAs116,118. Each MEA116,118includes a non-porous membrane120capable of conducting ions of the working fluid and porous electrodes122positioned on opposite sides of the non-porous membrane120that are capable of conducting electrons.

The membranes120are preferably ion conductive membranes or proton conductive membranes. The membranes120preferably have a thickness on the order of approximately 0.1 μm to 500 μm, and more preferably between approximately 1 μm and 500 μm. More particularly, the membranes120are preferably made from a proton conductive material, and more preferably a polymer proton conductive material or a ceramic proton conductive material. The membranes120of the MEAs116,118of the JTEC are not necessarily made of the same material. The material selected for a given MEA will depend on its intended operating temperature. In one embodiment, the membranes120are preferably formed of a material comprising a compound represented by the general formula NaxAlyTi3+x-yTi4+8-xO16, as disclosed in U.S. Pat. No. 4,927,793 of Hori et al., which is incorporated herein by reference, since this material exhibits high proton conductivity over a broad temperature range. However, it will be understood by those skilled in the art that any material, and preferably any polymer or ceramic material, which demonstrates a suitable proton conductivity over a desired temperature range may be used to form the membranes120. For example, in an alternate embodiment, the membranes120are formed of hydronium beta″ alumina.

The electrodes122of each MEA116,118are preferably thin electrodes having a thickness on the order of approximately 0.1 μm to 1 cm, and more preferably approximately 10 μm. The use of different materials for the various components of each MEA116,118(i.e., the electrodes122and the membranes120) could result in very high thermal stresses due to differences in the thermal expansion coefficients between the materials. Accordingly, the electrodes122of an MEA are preferably comprised or formed of the same material as the membranes120. However, the electrodes122are preferably porous structures, while the membranes120are preferably non-porous structures. Also, it will be understood that the electrodes122and the membranes120may be formed of different materials having similar thermal expansion coefficients.

In one embodiment, the porous electrodes122may be doped or infused with additional material(s) to provide electronic conductivity and catalytic material, in order to promote oxidation and reduction of the working fluid.

On both the low temperature side100and high temperature side110of the converter, there may be present arrays or stacks of MEAs.116,118The MEAs116,118may be connected, for example, in series to achieve higher overall output voltage, or in parallel to achieve higher overall output current.

The electrical potential due to the ionizable gas (i.e., the working fluid) pressure differential across a MEA is proportional to the natural logarithm of the pressure ratio, and can be calculated using the Nernst equation:

where VOCis open circuit voltage, R is the universal gas constant, T is the cell temperature, F is Faraday's constant, PHis the pressure on the high pressure side, PLis the pressure on the low pressure side, and PH/PLis the pressure ratio. E.g.,Fuel Cell Handbook, J. H. Hirschenhofer et al., 4thEdition, p. 2-5 (1999). The voltage is linear with respect to temperature and is a logarithmic function of the pressure ratio.FIG. 2is a plot of the Nernst equation for hydrogen and shows the voltage vs. temperature relationship for several pressure ratios. For example, referring toFIG. 2, at a pressure ratio of 10,000, when the temperature is relatively high, the voltage is similarly relatively high and when the temperature is relatively low, the voltage is similarly relatively low. Thus, the voltage of a higher temperature MEA118will be higher than the voltage of a lower temperature MEA116under the same pressure differential.

As illustrated inFIG. 3, each MEA116,118can be represented as a voltage source and an internal resistance. In the example ofFIG. 3, hydrogen is the working fluid. An electron current is directed to an external load115as electrons are stripped from the protons, thereby allowing the protons to pass through the proton conductive membranes120. The higher voltage VHTof the higher temperature MEA118determines the direction of current flow by imposing a voltage across the lower temperature MEA116that exceeds the lower temperature MEA's Nernst potential VLT. The higher Nernst voltage VHTof the higher temperature MEA118forces reverse current flow through the low temperature MEA116, effectively charging the low temperature cell100, i.e. forcing hydrogen from low pressure to high pressure as hydrogen expands from high pressure to low pressure through the high temperature MEA118. In the low pressure side1of the low temperature MEA116and the high pressure side3of the high temperature MEA118, hydrogen gas is oxidized resulting in the creation of protons and electrons. On the high pressure side2of the low temperature MEA116and the low pressure side4of the high temperature MEA118, the protons are reduced with the electrons to reform hydrogen gas.

Ideally, the difference in voltage between the two MEAs116,118or two MEA stacks is applied across the external load115. The hydrogen enclosed in the JTEC heat engine circulates continuously inside the JTEC heat engine and is not consumed. Ideally, the hydrogen flow (electron current e−and proton current H+) through both MEA stacks116,118is the same. Representing the electron current flow as I, the power output by the high temperature cell (VHT*I) is sufficient to drive the compression process in the low temperature cell100(VLT*I) as well as supply net power output to an external load ((VHT*I)−(VLT*I)). The voltage differential provides the basis for the JTEC engine. As in any thermodynamic engine employing a working fluid and consistent with the nature of compressible gas, in the JTEC, a greater amount of work (electricity) is extracted during high temperature expansion than the work (electricity) input required for the low temperature compression.

The ideal JTEC operates on the Ericsson thermodynamic cycle, which is equivalent to the Carnot cycle. Referring toFIG. 4, there is shown the ideal temperature entropy diagram for the ideal Ericsson engine cycle. Reference numerals “1” through “4” inFIGS. 1, 3 and 4represent the corresponding different thermodynamic states of the working fluid as it progresses through the cycle. As shown inFIG. 1, beginning at the low-temperature, low-pressure state1, electrical energy Winis supplied to the low-temperature (first) MEA stack116in order to pump hydrogen from the low-temperature, low-pressure state1to the low-temperature, high-pressure state2. The temperature of the hydrogen is maintained nearly constant by removing heat QLfrom the proton conductive membranes120during the compression process. The membrane120is relatively thin (i.e., ideally on the order of 10 μm thick), and thus will not support a significant temperature gradient, so a near isothermal process is reasonable, provided adequate heat is transferred from the membrane120through its substrate.

From state2, the hydrogen passes through the recuperative, counter flow heat exchanger114and is heated under approximately constant pressure to the high-temperature state3. Ideally, the heat needed to elevate the temperature of the hydrogen from state2to3is transferred from hydrogen flowing in the opposite direction through the heat exchanger114. At the high-temperature, high-pressure state3, electrical power is generated as hydrogen expands across the high temperature (second) MEA stack118from the high-pressure, high-temperature state3to the low-pressure, high-temperature state4. Heat QHis supplied to the thin film membrane120to achieve a near constant temperature expansion process. From state4to state1, the hydrogen flows through the recuperative heat exchanger114, wherein its temperature is lowered under constant pressure by heat transfer to hydrogen passing from state2to3. Upon leaving heat exchanger114, the hydrogen is pumped by the low-temperature MEA stack116from state1back to high-pressure state2as the cycle continues.

Referring toFIG. 5, there is shown an electrochemical direct heat to electricity converter in accordance with an embodiment of the present invention. Referring toFIG. 5, the converter comprises at least one electrochemical cell49which comprises at least one membrane electrode assembly51disposed within a housing or containment vessel56. The MEA51is comprised of an ion conductive membrane66sandwiched between a first electrode62and a second electrode64. The above discussion regarding the membrane and electrodes of the MEA represented inFIGS. 1-4also applies to the MEAs ofFIGS. 5-11.

The MEA51divides the containment vessel56into two separate volumes. Electrical terminals68extend into container56and are connected to the first and second electrodes62and64of the MEA51. A micro/nano porous media58,60is disposed within the containment vessel56. The micro/nano porous media58,60is thermally coupled between a heat source52and a heat sink54. More particularly, within the containment vessel56, the MEA51is sandwiched between a first volume of micro/nano porous media58and a second volume of micro/nano porous media60(hereinafter referred to as first porous media58and second porous media60for the sake of brevity). An elevated temperature heat source52is thermally coupled to the first porous media58on one side of the MEA51and a lower temperature heat sink54is thermally coupled to the second porous media60on the other side of the MEA51. The working fluid (e.g., hydrogen or other suitable ionizable gas) is contained within the first and second porous media58and60. The heat source52and the heat sink54produce a temperature gradient within the MEA51, and more particularly within the first and second porous media58and60. The temperature gradient, in turn, creates a transpiration pump effect resulting in a gas pressure differential across the MEA51, and more particularly across the first electrochemical cell49, in accordance with a Knudsen compressor effect.

The porous media58,60may comprise any inherently porous electrode material or even a non-porous electrode material that is made porous by any known technique (e.g., cast a slurry of carbon/polymer on the material and subsequently remove the polymer). Examples of materials that may be used to form the porous media58,60include, but are not limited to, carbon paper, fiber, cloth, foam and boards. Further examples of materials that may be used to form the porous media58,60include, but are not limited to, porous ceramics that are made conductive through a known process, such as sputtering, via a solution reduction process (e.g., soak a porous ceramic in a solution such as silver nitrate and then heat at 500° C. to decompose to silver), or carbonization of a polymer (e.g., usually by heating at elevated temperatures greater than 100° C. while flowing inert gas.

FIG. 6provides a visual depiction of the density, and thereby gas pressure gradients67and69, across the first and second porous media58,60resulting from the imposed temperature gradient across the micro/nano porous material of the first and second porous medial58,60of the MEA51. As illustrated by arrows55and57, the ionizable gas within the first and second porous media58,60tends to flow toward and create a higher pressure at the higher temperature regions of the first and second porous media58,60. Thus, the first and second nano/micro porous media58,60function as transpiration pump, such as a Knudsen pump.

A Knudsen pump or compressor exploits thermal transpiration of a rarefied gas. The principle of thermal transpiration can be described in terms of an example of two volumes of gas at different temperatures T1and T2connected by a tube with a radius smaller than the mean free path (λ) of gas molecules. The behavior of this system depends on the Knudsen number (Kn≡λ/L, where L is a characteristic linear dimension of the tube). For Kn less than about 0.01 λ/L, the gas flows as a continuum; for Kn between about 0.01 and 10, the flow behavior of the gas is transitional between the continuum and free-molecular regimes; for Kn of about 10 or more, the flow regime is free-molecular. In the free-molecular regime, simple balancing of the equilibrium molecular fluxes leads to the following equation for the equilibrium pressures in the two volumes:
p1/p2=(T1/T2)1/2  Equation 1

The mean free path of a gas depends upon its temperature and pressure as well as its molecular diameter. It is given by Equation 2:

λ=R⁢T2⁢⁢π⁢⁢d2⁢NA⁢PEquation⁢⁢2
where P is pressure, T is temperature, d is the molecular diameter, NAis Avogadro's number and R is the universal gas constant. For hydrogen, d is approximately 2.89×10−10m.

A Knudsen compressor can be operated as a micro-scale pump or compressor over a pressure range from several atm down to about 10 mTorr. The critical components of Knudsen compressors are gas transport membranes, which can be formed from materials with randomized (porous) flow channels to densely packed parallel arrays of multiple individual flow channels. In the electrochemical direct heat to electricity converter according to the present invention, the first and second porous media58,60serve as the gas transport membranes. An applied temperature gradient across a transport membrane creates a thermal creep pumping action. Porous membranes have been formed from aerogels, and arrays of individual microspheres. Multiple parallel flow channels, with lateral dimensions in the 100's of micrometer diameter range have also been investigated. These are formed by precision machining of aerogel for the larger diameters to in-situ assemblies of carbon nano-tubes. (Han, Yen-Lin; Alexeenko, Alina A.; Young, Marcus; and Muntz, Eric Phillip, “Experimental and Computational Studies of Temperature Gradient Driven Molecular Transport in Gas Flows through Nano/Micro-Scale Channels” (2007). School of Aeronautics and Astronautics Faculty Publications. Paper 5. http://dx.doi.org/10.1080/15567260701337209)

Referring toFIG. 6, as represented by the density of the dot patterns in the first and second porous media58,60, pressure gradients67and69result in a pressure differential across the MEA51. The net result is a voltage output at the terminals68in accordance with the applied pressure differential and the temperature of the MEA51as defined by the Nernst equation.

FIG. 7shows an example embodiment of another electrochemical direct heat to electricity converter according to the present invention, wherein the MEA51is shifted to a location closer to the heat source52and farther from the heat sink54(i.e., the distance between the MEA51and the heat source52is smaller than the distance between the MEA51and the heat sink54), in order to provide higher MEA output voltage per the linear relationship with temperature, in accordance with the Nernst equation. This locational shift is achieved, for example, by the first volume of micro/nano porous media82being smaller dimensioned than the second volume of micro/nano porous media88. Also, the containment vessel96is augmented in comparison to the vessel56shown inFIGS. 5-6, and more particularly is larger dimensioned, in order to include a return gas flow space or conduit86. Due to the transpiration pumping effect generated within the first and second porous media82,88, the gas which flows under increased pressure into the region near the heat source52by the transpiration effect is free to flow to the low temperature region near the heat sink54through the open non-transpiration flow conduit86as illustrated by arrows85.

In one embodiment, the first and second volumes of micro/nano porous transpiration media58,60and82,88may have graduated pore structures, wherein the pores are smaller in the regions of high pressure where the gas mean free paths are shorter and are larger in the regions of low pressure where the gas mean free paths of the gas are longer, so as to provide minimal restrictions to the gas flow. Ideally, the graduated pore structures provide a consistent Knudsen number relationship between pore size and gas molecule mean free path as the gas transitions across the porous media toward increasing pressure. Ideally, the transition in pore size would be a continuous gradient. Such an approach maintains Knudsen numbers (Kn) above about 10 λ/L to meet the transpiration requirements, but not by additional orders of magnitude so as to not unnecessarily restrict gas flow. Molecules with mean free paths on the order of microns are not constrained to flow through porous media having pore sizes on the order of nanometers, and thus the transition in pore size across the porous media is a continuous gradient and there is minimal resistance to the flow of gas across the porous media.

In a preferred embodiment of the configuration ofFIG. 7, the pore size of the layer of micro/nano porous media89in the minimum temperature region nearest the heat sink54is smaller than the pore size of the layer of micro/nano porous media87in the maximum temperature region nearest the heat source52. With rejection of heat Q as illustrated by arrow83, the cooled gas85will enter the second porous media88near the temperature of the heat sink54and undergo transpiration pumping therein. Since the pressure of the gas as it leaves the layer of porous media89going upward through the second porous media88will be increased above the pressure of gas85leaving the layer of porous media87, the net result is amplification of the pressure differential applied across the MEA51. The effect of including the return flow conduit86is equivalent to having a multistage Knudsen transpiration pump. The pressure ratio created by such a pump is given by: PH/PL=(TH/TL)n/2, where n is the number of pumping stages. In this case, the number of pump stages, n is 2. (see: Naveen K Guptal, Seungdo An and Yogesh B Gianchandani; A Si-micromachined 48-stage Knudsen pump for on-chip vacuum; J. Micromech. Microeng. 22 (2012) 105026 (8 pp) doi:10.1088/0960-1317/22/10/105026).

Referring toFIG. 8, there is shown another embodiment of an electrochemical direct heat to electricity converter according to the present invention. The MEA51shown inFIG. 8includes a plurality of recuperative heat exchanger layers90, in order to recover heat from gas leaving the high temperature region by transferring the heat to gas flowing back to the high temperature region. The recuperative heat exchanger layers90may be embedded within the porous media at spaced-apart positions, or may be arranged in an alternating manner with layers of the porous media. The porous media is not shown inFIG. 8for ease of illustrating the recuperative heat exchanger layers90. As opposed to rejecting the heat externally, as illustrated by arrow83inFIG. 7, the heat is transferred to the gas flowing within the transpiration media, as illustrated by arrows92inFIG. 8. The net effect is less waste heat rejected to the environment and, thereby, more efficient pumping.

FIG. 9illustrates the density and thereby gas pressure gradients102and104across the first and second porous media82,88resulting from the imposed temperature gradient in the MEA51ofFIG. 7. Flow arrows85show the direction of flow motivated by the Knudsen pumping effect. The pressure in return flow conduit86is uniform whereas the transpiration pumping within the first and second micro/nano porous media82and88(seeFIG. 7) creates well-defined gas pressure and density gradients. The density of gas in the return flow conduit86increases as its temperature is reduced as the gas gets closer to the low temperature heat sink54.

Referring toFIG. 10, there is shown an altered embodiment ofFIG. 7, wherein the electrochemical direct heat to electricity converter further includes a second MEA112. The second MEA112is embedded within the second micro/nano porous media88near the heat sink54, so that the second MEA112will operate at a lower temperature than the first MEA51proximate the heat source52, and thereby at a lower voltage. The second MEA112functions as a low temperature compressor and supplies gas at an increased pressure to a portion106of the second micro/nano porous media88that couples gas flow between the second MEA112and the first MEA51. The second MEA112is configured in the same manner as the first MEA51(an ion conductive membrane66sandwiched between a first electrode62and a second electrode64).

FIG. 11shows the pressure and density distribution for the JTEC configuration shown inFIG. 10. The pressure gradients resulting from the transpiration pumping effect increases the pressure differential across the high temperature (first) MEA51while at the same time decreases the pressure differential across the low temperature (second) MEA112. The configuration results in a higher net output voltage since the voltage difference between the low temperature MEA voltage and the high temperature MEA voltage will be greater than for a conventional JTEC that does not utilize the Knudsen pumping media. The pore structure of the electrodes of the MEAs can be designed for consistency with the micro/nano porous media for continuity of the transpiration pumping effect.