Thermo-electrochemical converter

Abstract

A thermo-electro-chemical converter direct heat to electricity engine has a monolithic co-sintered ceramic structure or a monolithic fused polymer structure that contains a working fluid within a continuous closed flow loop. The co-sintered ceramic or fused polymer structure includes a conduit system containing a heat exchanger, a first high density electrochemical cell stack, and a second high density electrochemical cell stack.

Claims

1. A heat to electricity converter comprising: at least three porous electrodes; a working fluid; at least two ion or proton conductive membranes, the at least three porous electrodes being stacked with the at least two ion or proton conductive membranes in an overlapping configuration whereby each ion or proton conductive membranes membrane is sandwiched between a pair of porous electrodes, the at least three porous electrodes and the at least two ion or proton conductive membranes comprising a co-sintered or fused monolithic structure; and a first conduit being a high pressure conduit containing the working fluid at a first pressure and a second conduit being a low pressure conduit containing the working fluid at a second pressure which is lower than the first pressure, wherein a first porous electrode of each sequential pair of the porous electrodes is coupled to the high pressure conduit for high pressure working fluid flow therethrough and a second porous electrode of each sequential pair of the porous electrodes is coupled to the low pressure conduit for low pressure working fluid flow therethrough, the first porous electrode being a high pressure electrode and the second porous electrode being a low pressure electrode, and wherein the converter generates electrical power when an electrical load is connected between the high and low pressure electrodes with the converter being coupled to a heat source, heat from the heat source being converted into electrical power as the working fluid expands from high pressure to low pressure with the heat of expansion being supplied by the heat source, or wherein the converter functions as a heat pump when electrical power is connected between the high and low pressure electrodes with the converter being coupled to a heat sink, a heat of compression being rejected to the heat sink as the electrical power drives compression of the working fluid from low pressure to high pressure.

2. The heat to electricity converter according to claim 1, further comprising an external power source, the power source being coupled between each sequential pair of porous electrodes whereby power applied to the at least three porous electrodes forces a flow of the working fluid flow from the low pressure electrode to the high pressure electrode as electron flow is forced by the external power source and induces ion or proton conductivity through the membrane sandwiched between the sequential pair of porous electrodes.

3. The heat to electricity converter according to claim 1, further comprising an electrical load, the electrical load being coupled between each sequential pair of porous electrodes whereby pressure forces flow of the working fluid from the high pressure electrode to the low pressure electrode by means of electrons being conducted through the external load and ions or protons being conducted through the membrane sandwiched between the sequential pair of porous electrodes.

4. The heat to electricity converter according to claim 1, wherein each of the at least two ion or proton conductive membranes is formed of a ceramic conductive material or a polymer conductive material.

5. The heat to electricity converter according to claim 1, wherein the at least three porous electrodes are electrically connected in series with each other.

6. The heat to electricity converter according to claim 1, further comprising a recuperative heat exchanger, the heat exchanger including the high and low pressure conduits configured to enable transfer of heat between the high pressure fluid and the low pressure fluid.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

(1) The following detailed description of preferred embodiments of the present invention will be better understood when read in conjunction with the appended drawing. For the purposes of illustrating the invention, there is shown in the drawing an embodiment which is presently preferred. It is understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. In the drawings:

(2) FIG. 1 is a plot of Nernst voltage versus temperature for several pressure ratios;

(3) FIG. 2 is a diagram of a Johnson Thermo-Electrochemical Converter including two membrane electrode assemblies connected back to back by a recuperative heat exchanger;

(4) FIG. 3 is a diagram of the Ericsson thermodynamic cycle;

(5) FIG. 4 is a schematic of a high density co-sintered or fused membrane electrode assembly stack in accordance with an embodiment of the present invention;

(6) FIG. 5 is a schematic of a co-sintered or fused heat engine in accordance with an embodiment of the present invention;

(7) FIG. 6 is a partial cross-sectional view of the co-sintered or fused heat engine shown in FIG. 5;

(8) FIG. 7 is a schematic of a heat engine using co-sintered or fused membrane electrode assembly stacks in accordance with an embodiment of the present invention;

(9) FIG. 8 is a schematic of a heat pump using co-sintered or fused membrane electrode assembly stacks in accordance with an embodiment of the present invention; and

(10) FIG. 9 is a cross-sectional view of a high density co-sintered or fused membrane electrode assembly stack in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(11) 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.

(12) 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.

(13) Referring to the drawings in detail, wherein like numerals indicate like elements throughout the several views, FIGS. 4-9 show preferred embodiments of a high density co-sintered MEA stack 10. The terms electrochemical cell, membrane electrode assembly stack, MEA stack, and stack are used interchangeably herein.

(14) In one embodiment, where components of the high density MEA stack are made of ceramic materials, the MEA stack 10, and more particularly each of the flat components of the stack 10 (as described in detail hereinafter), is produced by co-sintering. Co-sintering is a known, low cost procedure for shaping ceramic materials, and more particularly for the fabrication of thin (i.e., from 20 m up to 500 m) flat components. Co-sintering technology can be used to produce a wide variety of controlled morphologies, from highly porous to fully dense microstructures. The co-sintering process is well known to those skilled in the art.

(15) Generally, starting powders of different natures, and more particularly starting ceramic powders, are incorporated and mixed together with an aqueous medium to form a slurry, and the slurry is then cast into green tapes using a tape casting method. Tape casting also allows for stacking the cast green tapes to obtain a multilayered final product (i.e., the MEA stack 10). More particularly, multiple coating layers of green ceramic material may be cast or screen printed onto each other to form a layered structure which is ultimately sintered to form a MEA stack.

(16) For a given powder, the sintering behavior of the cast green tapes, and hence the final microstructure of the sintered layers, depends on the arrangement and particle sizes, dispersion and homogeneity of the starting ceramic powder particles in the slurry. Consequently, the slurry formulation is a very important step in the shaping process.

(17) Preferably, the slurry is composed of a mixture of several organic and inorganic compounds. The organic components preferably include a binder, a dispersant, a plasticizer, and, in the case of organic tape casting, a solvent. Other additives, such as wetting agents, defoamers, and pore formers (if porosity is desired in the final microstructure) may also be used to form the slurry. The inorganic compounds include the ceramic powders to be shaped, sintering additives, and water as the medium/solvent for the aqueous tape casting. An example of a ceramic powder for formation of a high temperature MEA stack (as discussed in more detail hereinafter) 10 is yttrium doped barium cerate (Y:BaCeO.sub.3). An example of ceramic powder for formation of a low temperature MEA stack (as discussed in more detail hereinafter) 10 is a composite of 95% LiH.sub.2PO.sub.4 with 5% H.sub.3PO.sub.4.

(18) After casting, the stacked cast tapes are allowed to dry. The tapes may be allowed to air dry for a predetermined period of time or may be passed through a drier to accelerate drying. Select organic components may remain in the green tapes after drying. The tapes are then heated to elevated temperatures to effect sintering of the cast green tapes. The organic components which remained after drying are sacrificial materials that are removed when the tapes are heated for sintering. As such, the remaining organic components give rise to pores and flow passages which remain during the subsequent sintering treatment. The sintered layers of the MEA stack 10 are thus formed.

(19) In another embodiment, where components of the high density MEA stack are made of polymeric materials, the MEA stack 10, and more particularly each of the flat components of the stack 10 (as described in detail hereinafter), is produced by a fusing process. The various types of fusing processes are well known to those skilled in the art. For example, in one type of the fusing process, the polymeric materials may be softened using a solvent and/or glued together using a polymer/solvent solution. In another type of fusing process, the polymeric components may be assembled together in a series of hot pressing steps wherein a layer is added and hot pressed in place with each step.

(20) Referring to FIG. 4, there is shown the internal configuration 6 of a high density monolithic MEA stack 10 in accordance with a preferred embodiment of the present invention. The MEA stack 10 comprises overlapping layers of alternating electrodes 23 and membranes 22 arranged in a high density stacked configuration. That is, each membrane 22 is sandwiched between a pair of electrodes 23, such that the electrodes 23 are stacked in alternating sequence with the membranes 22, thus forming MEA stacks 10 through which a working fluid, preferably hydrogen, can pass by undergoing an electrochemical oxidation/reduction process.

(21) The membranes 22 are preferably ion conductive membranes or proton conductive membranes having a thickness on the order of approximately 0.1 m to 500 m, and more preferably between approximately 1 m and 500 m. More particularly, the membranes 22 are preferably made from a proton conductive material, and more preferably a polymer proton conductive material or a ceramic proton conductive material. In one embodiment, the membranes 22 are preferably formed of a material comprising a compound represented by the general formula Na.sub.x Al.sub.y Ti.sup.3+.sub.x-y Ti.sup.4+.sub.8-x O.sub.16, 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 similar proton conductivity over a broad temperature range may be used to form the membranes 22. For example, in an alternate embodiment, the membranes 22 are formed of hydronium beta alumina. The polymer or ceramic membrane material 22 preferably forms a high barrier to molecular working fluid flow and provides for effective containment of the working fluid.

(22) The electrodes 23 are preferably thin electrodes having a thickness on the order of approximately 10 m to 1 cm, and more preferably approximately 50 m to 1,000 m. The use of different materials for the various components (i.e., the electrodes 23 and the membranes 22) could result in very high thermal stresses due to differences in the thermal expansion coefficients between the materials. Accordingly, the electrodes 23 are preferably comprised or formed of the same material as the membranes 22. However, the electrodes 23 are preferably porous structures, while the membranes 22 are preferably non-porous structures. Because the same basic material composition is preferably used for the electrodes 23 as for the bulk membrane 22 material structure, the high thermal stresses that would otherwise occur under the extreme temperatures encountered during co-sintering or fusing to form the MEA stacks 10 and in many end-use applications during operation of the MEA stacks 10 are eliminated or at least reduced. However, it will be understood that the electrodes 23 and the membranes 22 may be formed of different materials having similar thermal expansion coefficients, such that there would be little or no thermal stress generated during co-sintering/fusing or use of the MEA stack 10.

(23) In one embodiment, the porous electrodes 23 may 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.

(24) The length 28 of the MEA stack 10 is preferably between approximately 0.25 cm and 10 cm. The width (depth into the drawing) of the MEA stack 10 is preferably between approximately 1 cm and 100 cm. However, it will be understood by those skilled in the art that the dimensions of the MEA stack 10 may vary and be selected as appropriate depending on the application in which the MEA stack 10 is to be used.

(25) Given the low ion conductivity of known and available ceramic materials which may be used to form the membranes 22 and the low Nernst voltage levels generated at reasonable operating temperatures and pressures by these ceramic membrane materials, high membrane surface areas are desirable within the MEA stack 10. Resistive losses associated with high current density, as protons are conducted through the membranes, could otherwise represent a significant reduction in output voltage and thereby efficiency.

(26) Accordingly, the MEA stack 10 has a high density of overlapping electrodes 23 and membranes 22, which yields a very high membrane to electrode interface area within a relatively small stack volume, with the ion conductive material of the membranes 22 comprising the bulk structure of the MEA stack 10. More particularly, the bulk area of the MEA stack 10 is occupied by a plurality of the membranes 22. It will be understood that the bulk area within a particular stack 10 will depend on the number of membrane 22 and electrode 23 layers, as well as the respective thicknesses of such layers, within a given unit of stack height. For example, a representative stack 10 having membranes 22 with a thickness of 20 m sandwiched between 40 m porous electrodes 23, will have a total membrane area of 166 cm.sup.2 per cm.sup.3 of stack volume. In one embodiment, the plurality of membranes 22 are surrounded by an external housing 21, which may be made of the same or of a different material as the membranes 22.

(27) The MEA stack 10 further comprises a conduit system including at least one low pressure conduit 37 (represented by dashed lines in FIG. 4) and at least one high pressure conduit 38 (represented by solid lines in FIG. 4). Preferably, the conduit system includes a plurality of low pressure conduits 37 and a plurality of high pressure conduits 38. A supply of an ionizable gas, preferably hydrogen, is contained within the conduit system as the working fluid.

(28) The low pressure conduits 37 direct the flow of the working fluid (e.g., hydrogen) in the direction of arrow A, while the high pressure conduits 38 direct the flow of the working fluid in the direction of arrow B (i.e., the opposite direction of the low pressure conduits 37 flow). The low pressure conduits 37 and high pressure conduits 38 define low and high pressure sides of the MEA stack 10. The high pressure side of the MEA stack 10 may be at a pressure of as low as 0.5 psi and as high as 3,000 psi. Preferably, the high pressure side of the MEA stack 10 is maintained at a pressure of approximately 300 psi. The low pressure side of the MEA stack 10 may be at a pressure of as low as 0.0001 psi and as high as 0.3 psi. Preferably, the low pressure side of the MEA stack is maintained at a pressure of approximately 0.03 psi. A preferred pressure ratio of the high pressure side to the low pressure side is 10,000:1. The electrodes 23 in each MEA stack 10 are alternatingly coupled to the high pressure and low pressure conduits 38, 37, respectively, such that each membrane 22 is sandwiched between a first electrode 23 supplied by a high pressure conduit 38 and a second electrode 23 supplied by a low pressure conduit 37. Accordingly, each membrane 22 is preferably situated between a high pressure electrode 23b and a low pressure electrode 23a, such that each membrane 22 has a high pressure side and a low pressure side.

(29) First and second terminals 31 and 32 are connected to the electrodes 23 of the MEA stack 10. Each terminal 31, 32 is preferably connected to the electrodes 23 in an alternating sequence, such that the high pressure electrodes 23 are connected to each other and to one of the terminals (e.g., the first terminal 31) and the low pressure electrodes 23a are connected to each other and to the other terminal (e.g., the second terminal 32).

(30) In one embodiment, the MEA stack 10 may be configured to expand the working fluid from high pressure to low pressure so as to generate electricity. Still referring to FIG. 4, power may be extracted from the MEA stack 10 by connecting an electric load to the first and second terminals 31 and 32. Electric power is produced as the pressure differential between the high and low pressure conduits 38, 37 forces the working fluid through the MEA stack 10.

(31) Referring to FIG. 1, using a preferred pressure ratio of 10,000:1, where the MEA stack 10 is a high temperature stack, operating at a temperature of 625K, the high temperature MEA stack 10 would have a Nernst voltage of approximately 250 mV. On the other hand, if one maintains operation of the MEA stack 10 at a relatively low temperature of 325K, the low temperature MEA stack 10 would have a Nernst voltage of approximately 125 mV. In this case, the open circuit voltage of the converter would be approximately 125 mV.

(32) Referring again to FIG. 4, while under pressure, the working fluid is oxidized at the high pressure electrodes 23b connected to common second terminal 32, thereby releasing electrons to the electrodes 23b and causing ions of the working fluid to enter the ion/proton conductive membranes 22 as indicated by arrows 33. With the electrodes 23b connected to an external load, electrons flow through the load, through common first terminal 31 and then to the low pressure electrodes 23a, where ions/protons exiting the membranes 22 are reduced to reconstitute the working fluid. The converter supplies power to the external load as pressure forces the working fluid to flow through the MEA stack 10. In one embodiment, a heat source (not shown) may be coupled to the MEA stack 10 to supply heat of expansion to the working fluid so as to maintain a continuous and nearly isothermal expansion process.

(33) In another embodiment, the MEA stack 10 is configured to operate to pump the working fluid from low pressure to high pressure creating a compression process. Electrical power is consumed by the compression process. A power source is applied across the first and second terminals 31 and 32. Voltage is applied at a potential that is sufficient to force current flow by overcoming the Nernst potential generated by the MEA stack 10 at its operating temperature and pressure differential. The applied power strips electrons from the working fluid at the interface of each low pressure electrode 23a and membrane 22. The resulting ions are conducted through the ion conductive membranes 22 in the direction indicated by arrows 39. The power source supplies electrons to the high pressure electrodes 23b via the first terminal 31, so as to reconstitute the working fluid at the interface of each high pressure electrode 23b and membrane 22 as ions exit the membrane 22. This current flow under the applied voltage, in effect, provides the pumping power needed for pumping the working fluid from low pressure to high pressure. In one embodiment, a heat sink (not shown) may be coupled to the MEA stack 10 to remove the resulting heat of compression, so as to maintain a continuous compression process.

(34) Referring to FIG. 5, there is shown a co-sintered/fused high density direct heat to electricity converter or heat engine 11, and more particularly a monolithic JTEC 11, in accordance with a preferred embodiment of the present invention. The monolithic structure of the JTEC 11 includes a heat exchanger 13, a first high density MEA stack 14, and a second high density MEA stack 16. A first interface 12 is provided for connection of one of the MEA stacks 14, 16 to a heat sink (thereby forming a low temperature MEA stack) and a second interface 18 is provided for connection of the other one of the MEA stacks 14, 16 to a heat source (thereby forming a high temperature MEA stack). The first and second high density MEA stacks 14, 16 generally have the same configuration and structure as described above for the co-sintered MEA stack 10. However, it will be understood that the particular material employed as the membranes (i.e., ion conductors) in the high temperature stack may be different form that employed in the low temperature stack. For example, in one preferred embodiment, the high temperature MEA stack 14, 16 is formed of ceramic materials, while the low temperature MEA stack 14, 16 is formed of polymer materials.

(35) Referring to FIGS. 6-8, the first high density MEA stack 14 includes a plurality of porous electrodes 25, 41 and an ion or proton conductive membrane 24 sandwiched between each pair of adjacent electrodes 25, 41. The second high density MEA stack 16 includes a plurality of porous electrodes 23, 42 and an ion or proton conductive membrane 22 sandwiched between each pair of adjacent electrodes 23, 42. In each stack 14, 16, the porous electrodes 25, 41 and 23, 42 are stacked in alternating sequence with the membranes 24, 22.

(36) Referring to FIGS. 5-6, the thermo-electrochemical converter 11 also includes a plurality of conduits 37, 38 and an ionizable working fluid contained within the conduits 37, 38. As discussed above with respect to the MEA stack 10, conduits 38 are high pressure conduits and conduits 37 are low pressure conduits. Preferably, one electrode 41, 42 of any sequential pair of electrodes 25, 41 and 23, 42 is coupled to a high pressure conduit 38 for high pressure flow of the working fluid and the other electrode 23, 25 of the sequential pair of electrodes 25, 41 and 23, 42 is coupled to a low pressure conduit 37 for low pressure flow of the working fluid. Each of the sandwiched membranes 22, 24 is thereby subjected to the pressure differential between a high pressure porous electrode 41, 42 and a low pressure porous electrode 23, 25. The high pressure conduits 38 couple high pressure working fluid flow between the high pressure electrodes 42 of the second MEA stack 16 and the high pressure electrodes 41 of the first MEA stack 14. Similarly, the low pressure conduits 37 couple low pressure working fluid flow between the low pressure electrodes 23 of the second MEA stack 16 and the low pressure electrodes 25 of the first MEA stack 14.

(37) Referring to FIG. 7, in one embodiment, the thermo-electrochemical converter 11 is attached to an external electrical load 56. The first MEA stack 14 is preferably coupled to an elevated temperature heat source 58 and the second MEA stack 16 is preferably coupled to a heat sink 60 which operates at a temperature below the elevated temperature of the first MEA stack 14 and the heat source 58. As such, the first MEA stack 14 is a high temperature stack and the second MEA stack 16 is a low temperature stack. Preferably, the high temperature stack 14 is formed of ceramic materials and the low temperature stack 16 is formed of polymer materials.

(38) The low temperature stack 16 may operate in the range of 50 C. to 1,500 C., preferably approximately 55 C. However, the operating temperature of the low temperature stack 16 must be sufficiently high so as to have heat efficiently removed from it by ambient temperature air, water or other suitable heat sink in its environment. The high temperature stack 14 may operate at temperatures between ambient to as high as 1,500 C., preferably approximately 550 C. Preferably, the high temperature stack 14 operates at a higher temperature than the low temperature stack 16. It will be understood that, for a heat engine generating power, the higher the temperature difference between the two stacks, the greater the engine's theoretical conversion efficiency. Total load 50, which consists of the external load 56 and the second MEA stack 16 connected in series, is coupled to the first MEA stack 14 by first and second terminals 52 and 54.

(39) Still referring to FIG. 7, the first MEA stack 14 supplies power to the total load 50 as pressure forces flow of the working fluid from the first set of high pressure electrodes 41, which are connected to a terminal 35, to the first set of low pressure electrodes 25, which are connected to a terminal 34. The power is supplied at the Nernst voltage of the first MEA stack 14 based on the applied pressure differential and its temperature less the voltage loss due to the internal impedance of the stack 14. Under the force of pressure, electrons are conducted through total load 50 and ions 36 are conducted through the ion conductive membranes 24 of the first MEA stack 14.

(40) The voltage produced by the first MEA stack 14 is divided between the second MEA stack 16 and the external load 56 of total load 50. As configured, a portion of the power produced by the first MEA stack 14 is supplied to the second MEA stack 16 by connection to a second set of high pressure electrodes 42 (i.e., the electrodes connected to the terminal 31) of the second MEA stack 16 and a second set of low pressure electrodes 23 (i.e., the electrodes connected to the terminal 32). Working fluid flow is forced from low pressure to high pressure as the electron flow forced under the applied power induces ion conductivity through the ion conductive membranes 22 of the second MEA stack 16. The remaining power produced by the first MEA stack 14 is supplied to external load 56.

(41) The thermo-electrochemical converter 11 shown in FIG. 8 is also configured as a JTEC and operates as a heat pump. Essentially, the operation of the thermo-electrochemical converter 11 is in reverse to that of the engine 11 of FIG. 7. Referring to FIG. 8, the heat pump 11 is attached to an external power source 58. The first MEA stack 14 is coupled to a low temperature heat source 66 and operates to remove heat from the low temperature heat source 66, thereby creating a refrigeration effect. The low temperature heat source 66 may operate at a temperature of 50 C. to 100 C., and preferably operates at a temperature of approximately 15 C. More preferably, the low temperature heat source 66 operates at a temperature which is sufficiently low to have heat of working fluid expansion effectively transferred to the low temperature heat source 66 from ambient temperature air or water or other suitable heat source in its environment where cooling is desired. The first MEA stack 14 generates power as pressure forces working fluid flow from the first set of high pressure electrodes 41, which are connected to the terminal 35, to the first set of low pressure electrodes 25, which are connected to the terminal 34. Pressure forces working fluid flow through the first MEA stack 14 by forcing ion conduction through ion conductive membranes 22 of the first MEA stack 14, as electrons are conducted through the external power source 58 and the second MEA stack 16 in series.

(42) Still referring to FIG. 8, the external power source 58 and the first MEA stack 14 are connected in series and comprise a total power source 61. The total power source 61 supplies power to the second MEA stack 16 and is connected to the second set of high pressure electrodes 41 (i.e., the electrodes connected to terminal 31) and the second set of low pressure electrodes 23 (i.e., the electrodes connected to terminal 32) within the second MEA stack 16. The working fluid is transferred from the low pressure electrodes 23 to the high pressure electrodes 41 as electron flow forced by the power source 61 induces working fluid ion conduction through the ion conductive membranes 22 of the second MEA stack 16. The first MEA stack 14 is coupled to first and second terminals 62 and 64.

(43) Still referring to FIG. 8, the second MEA stack 16 is coupled to an elevated temperature heat sink 68 and thus operates at higher temperature than the first MEA stack 14. The second MEA stack 16 rejects the heat of compression of the working fluid to the high temperature heat sink 68 for effective operation of the engine as a heat pump 11. The Nernst voltage of the high temperature second MEA stack 16 is higher than the Nernst voltage of the low temperature first MEA stack 14. The additional voltage needed to overcome the higher Nernst voltage of the high temperature second MEA stack 16 is provided by the external power source 58.

(44) The monolithic structure of the high density direct heat to electricity converter or heat engine 11, and more particularly the co-sintered or fused monolithic structure of the MEA stacks 14, 16, results in a more efficient engine construction process, as compared with conventional converters. This is because the need to make many tedious interconnects and, more importantly, the need to construct thicker and more bulky standalone electrode and membrane layers are unnecessary. For example, a membrane of a conventional converter typically has a thickness on the order of 100 m in order to have sufficient integrity to survive the construction process. In the converter 11 of the invention, the membranes 22 may be thin coatings on the order of 10 m or less. The sequential coating of multiple thin layers onto each other or the sequential lamination of multiple thin layer to each other results in a monolithic multilayered structure, wherein the layers mechanically reinforce each other to provide structural integrity while at the same time providing high MEA surface area within a relative small MEA stack volume. The high surface area enables relatively low current density and thereby low resistive losses.

(45) The invention is equivalent to taking fixed source current and dividing it among many high impedance resistors connected in parallel, so that the net result is equivalent to a single low impedance resistor. However, the necessary structure and connections are constructed in a very efficient manner, using screen printing or other suitable techniques to apply multiple coating layers of green ceramic material onto each other and then sintering them into a solid state heat engine having single monolithic structure or using suitable techniques to fuse together multiple polymer foils, films or layers into a solid state heat engine having single monolithic structure.

(46) Referring to FIG. 9, there is shown a cross-sectional view of a monolithic MEA stack 80 including an outer casing 90, proton conductive membrane material 93, high pressure porous electrodes 94, low pressure porous electrodes 95, high pressure flow conduits 97 and low pressure flow conduits 96. The MEA stack 80 is configured such that high pressure working fluid flowing within the high pressure conduits 97 can freely flow into or out of the high pressure porous electrodes 94. The high pressure porous electrodes 94 are electrically connected to each other by an interconnect 98. Similarly, working fluid flowing within the low pressure conduits 96 can easily flow can into or out of the low pressure porous electrodes 95. The low pressure porous electrodes 96 are connected to each other by an interconnect 99. The interconnects 98 and 99 may or may not be porous. Non-porous electrical terminals 91 (see also FIG. 5) and 92 provide electrical contact points for external electrical connections to the MEA stack 80. Terminal 91 is connected to the low pressure porous electrodes 95 and terminal 92 is connected to the high pressure porous electrodes 94. As FIG. 9 illustrates, with constituent MEAs electrically connected in parallel, connections may similarly be made to connect individual MEA stacks in series for increased net output voltage.

(47) It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.