Halogen-based Thermoelectrochemical Converters

Abstract

A method of generating electrical energy using an electrochemical direct thermal electric converter operating in a Rankine cycle. The converter includes a working fluid, a high temperature electrochemical cell including a first membrane electrode assembly, a low temperature electrochemical cell including a second membrane electrode assembly, an evaporator coupled to the first electrochemical cell, a condenser coupled to the second electrochemical cell, and an external load. The method includes introducing the working fluid as a liquid at the first membrane electrode assembly, expanding the working fluid through the first membrane electrode assembly and evaporating into a vapor, and cooling and condensing the vapor back into a liquid at the second membrane electrode assembly.

Description

[0001] Cross-references to related patent applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 028,125, filed on May 21, 2020, the entire contents of which are incorporated herein by reference. Background Art

[0003] The conversion of thermal or chemical energy to electrical energy, and 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, electrons are electrically transported through an external load via wires, where they perform work, while ions are conducted through an electrolyte separator.

[0004] However, battery-type electrochemical cells can only produce limited amounts of energy because the limitations of the battery housing restrict the number of available reactants that may be contained therein. While such cells can be designed to be charged by applying a reverse polarity current / voltage across the electrodes, such charging requires a separate power source. Furthermore, the battery is generally not usable during the charging process.

[0005] To overcome such problems associated with battery-type electrochemical cells, fuel cells have been developed. In a conventional fuel cell, chemical reactants are continuously supplied to an electrochemical cell and the reaction products are continuously removed. In a manner similar to a battery, a fuel cell operates by conducting ionized species through a selective electrolyte that normally blocks the passage of electrons and non-ionized species, so the electrons must pass externally through an electrical load to complete the reaction.

[0006] The most common type of fuel cell is the proton exchange membrane (PEM) hydrogen and oxygen fuel cell, which delivers hydrogen through one of the electrodes and oxygen through the other electrode. Under the voltage potential of the hydrogen oxidation chemical reaction, hydrogen ions are conducted through a proton-conducting electrolyte separator or PEM to the oxygen side of the cell. Porous electrodes located on either side of the electrolyte separator or proton exchange membrane are used to couple the electrons involved in the chemical reaction through an external load via an external circuit. The electrons and hydrogen ions recombine hydrogen in a reaction with oxygen on the oxygen side of the cell to produce water, which is exhausted from the system. Continuous current flow is maintained by a continuous supply of hydrogen and oxygen to the cell.

[0007] Mechanical heat engines have also been designed and used to produce electricity. Mechanical heat engines operate on a thermodynamic cycle in which 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 the high temperature, the working fluid is allowed to expand against a load (such as a piston or turbine), thereby producing shaft work. The key to the operation of all engines that use a working fluid is that the work required to compress the working fluid at a low temperature is less than the work produced by its expansion at a high temperature. This is true for all heat engines that use a working fluid.

[0008] For example, an evaporator operates on a Rankine cycle, where water is pumped to high pressure and then heated to vapor and expanded by a piston or turbine to perform work. An internal combustion engine operates on an Otto cycle, where cold, room-temperature 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 cold, compression portion of the cycle consumes.

[0009] 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. The efficiency of an ideal Stirling thermodynamic cycle is comparable to the ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine with heat input at high temperature and heat rejection at low temperature. However, like all mechanical engines, Stirling engines are subject to reliability issues and efficiency losses associated with their mechanical moving parts.

[0010] To avoid the problems inherent in mechanical heat engines, Alkali Metal Thermo-Electrochemical Conversion (AMTEC) cells have been designed as thermo-electrochemical heat engines. AMTEC heat engines use pressure to force an ionizable working fluid (such as sodium) through an electrochemical cell at high temperature, thereby generating a voltage potential and an electric current. 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 as it enters the electrolyte, releasing electrons into the external circuit. On the other side of the electrolyte, the sodium ions recombine with the electrons as they leave the electrolyte to reform the sodium in much the same way as the process occurs in battery and fuel cell type electrochemical cells. The recombined sodium leaves the electrochemical cell as an expanding gas at low pressure and high temperature. The gas is then cooled and condensed back into a liquid state. The resulting cryogenic liquid is re-pressurized. The operation of the AMTEC engine approximates the Rankine thermodynamic cycle.

[0011] There are many publications on alkali metal thermoelectrochemical conversion 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 "Intersociety Energy Conversion Engineering Conference and Exhibit (IECEC)", No. 35, Las Vegas, Nevada, July 24-28, 2000, "Collection of 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.

[0012] The heat removed during the cooling and recondensation of the hot, expanding gases leaving the electrodes at low pressure is a significant source of entropy loss, and thus AMTEC heat engines are inefficient. AMTEC engines also suffer from reliability issues due to the highly corrosive nature of the alkali working fluids. Their utility is also very limited. Specifically, AMTEC engines can only operate at very high temperatures, since the ionically conductive solid electrolytes can only achieve practical levels of conductivity at high temperatures. In fact, even the cryogenic pressurization process must be carried out at relatively high temperatures, since the alkali working fluid must always remain above its melting temperature as it moves through the cycle. Mechanical pumps, cores, and even magnetohydrodynamic pumps have been used to pressurize the cryogenic working fluid.

[0013] In order to overcome the shortcomings of the above-mentioned traditional mechanical and thermoelectrochemical heat engines, the Johnson Thermo-Electrochemical Converter (JTEC) system was developed, as disclosed in U.S. Patent Application No. 7,160,639 filed on April 28, 2003, International Patent Application No. PCT / US2015 / 044435 filed on August 10, 2015, and International Patent Application No. PCT / US2016 / 21508 filed on March 9, 2016, the entire contents of which are incorporated herein by reference.

[0014] A newer development of the Johnson Thermo-Electrochemical Converter (JTEC) involves an electrochemical direct Johnson Thermo-Electrochemical Converter having a membrane electrode assembly and control circuitry that operates to maintain a constant specified pressure ratio within the converter. More specifically, additional hydrogen is pumped to the high pressure side of the converter, the additional pumped hydrogen compensating for the normal pressure loss due to diffusion of hydrogen molecules through the membranes of the membrane electrode assembly stack. Diffusion of hydrogen molecules through the separator membrane represents a significant reduction in power density over time because diffusion reduces the pressure difference across the electrodes of the membrane electrode assembly, thereby reducing the output voltage. Diffusion of hydrogen molecules also results in a reduction in efficiency because this diffusion from high pressure to low pressure occurs without the diffusing hydrogen molecules undergoing an electrochemical reaction to produce electricity. Therefore, the membrane electrode assembly and control circuitry operate to maintain a constant specified pressure ratio within the converter.

[0015] A common challenge with JTECs is the need for large membrane electrode assembly surface areas, since high levels of current are required to supplement the small voltage levels available to each membrane electrode assembly if useful power levels are to be achieved. As a result, conventional JTEC systems are difficult to manufacture. Thus, there remains a need for a JTEC that can provide a higher voltage (potential) to each membrane electrode assembly pair.

[0016] As energy demand continues to increase, fossil fuels still play a dominant role in energy resources. There is a great need for research in this area, especially with regard to the use of low-grade waste heat to reduce energy losses, which will reduce fuel consumption and ultimately achieve higher conversion efficiency. To this end, optimization of well-known systems can provide a way to generate electricity. It is well known that among the technologies available today, such as the organic Rankine cycle, the Kalina cycle, and the supercritical cycle, the organic Rankine cycle is the only technology that can produce 15-50% more power output for the same heat, and is mainly suitable for low-temperature heat recovery to generate electricity with low maintenance. See D. Huijuan Chen, Yogi Goswami, Elias K. Stefanakos, "A review of thermodynamic cycles and working fluids for the conversion of low-grade heat", "Renew Sustain Energy Rev." Vol. 14, pp. 3059-3067, 2010.

[0017] In fact, the Rankine cycle is widely used to recover various low-grade thermal energies, such as solar energy, biomass energy, and geothermal energy, and its performance is usually related to detailed working fluid characteristics. However, some researchers speculate that the working fluid has little to do with the efficiency of the cycle, which leads to the path of hybrid systems. See A. Perna, M. Minutillo, E. Jannelli, "Investigations on an advanced power system based on a high temperature polymer electrolyte membrane fuel cell and an organic Rankine cycle for heating and power production", "Energy", Vol. 88, pp. 874-884, 2015. However, hybrid systems involving fuel cells and Rankine cycles have not yet become basic technologies. In fact, phosphoric acid-based fuel cells are widely considered to be the most commercially available because they have high durability, structure, and a wider temperature operating range than Nafion-based fuel cells, although they have problems with low power density and high manufacturing costs. See H. Ito, “Economic and environmental assessment of phosphoric acid fuel cell-based combined heat and power system for an apartment complex,” Int'l. J Hydrogen Energy, Vol. 42, pp. 15449-15463, 2017; and S. Wang, S. P. Jiang, “Prospects of fuel cell technologies,” National Science Review, Vol. 4, pp. 163-166, 2017. When combined with the Rankine cycle, these performances are evaluated only using the first law of thermodynamics, which imposes limitations on practical operation and design. See “Doosan fuel cell to verify using ORC with PAFC,” Fuel Cell Bulletin, p. 1, 2019. The converter of the present invention avoids such problems BRIEF DESCRIPTION OF THE DRAWINGS

[0018] 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 present invention, the drawings show the presently preferred embodiments. However, it should be understood that the present invention is not limited to the precise arrangements and tools shown. In the drawings:

[0019] Figure 1 is a schematic diagram of a thermoelectrochemical converter according to an embodiment of the present invention;

[0020] Figure 2 yes Figure 1 Schematic diagram of the operating configuration of the thermoelectrochemical converter shown in;

[0021] Figure 3 yes Figure 1 Exploded view of the membrane electrode assembly and housing of the thermoelectrochemical converter shown in FIG. DETAILED DESCRIPTION

[0022] Certain terms are used in the following description for convenience only and are not intended to be limiting. The terms "proximal," "distal," "upward," "downward," "bottom," and "top" refer to directions as referenced in the figures. According to the present invention, "inwardly" and "outwardly" refer to directions toward and away from the geometric center of the device and designated parts thereof, respectively. 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 terms include the above-mentioned words, derivatives thereof and words of similar meaning.

[0023] It will also be understood that terms such as "first", "second", etc. are provided only for the sake of clarity. The elements or components identified by these terms and their operations can be easily switched. In addition, membrane electrode assembly (MEA) battery array and membrane electrode assembly battery stack can be used interchangeably.

[0024] refer to Figure 1 , an electrochemical direct thermal-electric converter 10 according to an embodiment of the present invention is shown. The electrochemical direct thermal-electric converter 10 includes a first electrochemical cell 100 operating at a first temperature, a second electrochemical cell 110 operating at a second temperature higher than the first temperature, a conduit system 112 coupling the first electrochemical cell 100 and the second electrochemical cell 110, and a supply of a working fluid contained in and passing through the conduit system 112. Each electrochemical cell 100, 110 includes one or more membrane electrode assemblies. In one embodiment, the converter 10 further includes a heat exchanger (not shown) that couples the low-temperature electrochemical cell 100 and the high-temperature electrochemical cell 110 together.

[0025] In one embodiment, the working fluid is selected from the group consisting of halogen and sulfur. For example, in one embodiment, the working fluid is preferably a halogen, such as bromine or iodine. In another embodiment, the working fluid is preferably sulfur.

[0026] More specifically, refer to Figure 1 The first electrochemical cell 100 includes a low temperature heat sink Q L The first electrochemical cell 100 is connected to the first membrane electrode assembly (MEA) 116 (i.e., the cold source). Therefore, the first membrane electrode assembly 116 is a low temperature membrane electrode assembly, and the first electrochemical cell 100 represents the low temperature side of the converter 10. The second electrochemical cell 110 includes a first membrane electrode assembly (MEA) 116 coupled to the high temperature heat source Q. H The second MEA 118 is coupled to the first MEA 116. Thus, the second MEA 118 is a high temperature MEA and the second electrochemical cell 110 represents the high temperature side of the converter 10. During operation, cooling is provided to the first MEA 116 and heat is provided to the second MEA 118.

[0027] There may be an array or stack of membrane electrode assemblies 116, 118 on the low temperature side 100 and the high temperature side 110 of the converter. The membrane electrode assemblies 116, 118 may be connected, for example, in series to achieve a higher overall output voltage, or in parallel to achieve a higher overall output current. In one embodiment, the first and second membrane electrode assemblies 116, 118 are arranged in series with an external load 115, such as Figure 2 In addition, the first and second membrane electrode assemblies 116, 118 are preferably connected in series in a closed loop.

[0028] Each membrane electrode assembly 116 , 118 includes a non-porous membrane 120 capable of conducting ions of a working fluid and a porous electrode 122 capable of conducting electrons located on an opposite side of the non-porous membrane 120 .

[0029] The membrane 120 is preferably an ion conducting membrane or a proton conducting membrane. The membrane 120 is preferably about 0.1 micrometer to 500 micrometers thick, preferably between about 1 micrometer and 500 micrometers. More specifically, the membrane 120 is preferably made of a proton conducting material, preferably a polymer proton conducting material or a ceramic proton conducting material. The membranes 120 of the membrane electrode assemblies 116, 118 are not necessarily made of the same material. The material selected for a particular membrane electrode assembly 116, 118 will depend on its expected operating temperature. In one embodiment, the membrane 120 is preferably made of the general formula Na x Al y Ti 3+ x-y Ti 4+ 8-x O 16The membrane 120 is formed of a material composed of a compound represented by Hori et al., as disclosed in U.S. Pat. No. 4,927,793, which is incorporated herein by reference, because such materials exhibit high proton conductivity over a wide temperature range. However, it will be appreciated by those skilled in the art that any material, preferably any polymer or ceramic material, that exhibits suitable proton conductivity over a desired temperature range can be used to form the membrane 120. For example, in an optional embodiment, the membrane 120 is formed of hydrated β-alumina (hydronium beta"alumina).

[0030] The electrode 122 of each membrane electrode assembly 116, 118 preferably has a thickness of about 0.1 microns to 1 centimeter, more preferably about 10 microns. Due to the different thermal expansion coefficients between the materials, the use of different materials for the various components of each membrane electrode assembly 116, 118 (i.e., the electrode 122 and the membrane 120) may result in very high thermal stress. Therefore, the electrode 122 of the membrane electrode assembly 116, 118 is preferably composed or formed of the same material as the membrane 120. However, the electrode 122 is preferably a porous structure, while the membrane 120 is preferably a non-porous structure. In addition, it is understood that the electrode 122 and the membrane 120 can be formed of different materials with similar thermal expansion coefficients.

[0031] In one embodiment, the porous electrode 122 may be doped or infused with additional materials to provide electronic conductivity and catalytic materials to facilitate oxidation and reduction of the working fluid.

[0032] In one embodiment, each membrane electrode assembly is formed as follows. A solid electrolyte material capable of conducting ions of a working fluid (preferably bromine) is formed into a disk. The ion-conducting disk is impermeable to gases and liquids. Each side of the solid electrolyte disk is coated with an electrode material and a current collector to promote electrochemical reactions on the surface of the solid electrolyte disk, thereby forming the membrane electrode assemblies 116, 118.

[0033] refer to Figure 3 Each membrane electrode assembly 116, 118 is preferably disposed within a housing 140, such as Figure 3 Each housing 140 is preferably composed of glass or other materials provided with an internal glass backing layer, such as metal (e.g., stainless steel). However, it will be appreciated by those skilled in the art that other materials known or discovered to be stable when in contact with halogens or sulfur may also be used to form the housing 140.

[0034] The converter 10 preferably further includes an evaporator 142 and a condenser 144. More specifically, the high temperature side of the converter 10 includes or is coupled to the evaporator 142, which corresponds to the high temperature membrane electrode assembly 118, and the low temperature side includes or is coupled to the condenser 144, which corresponds to the low temperature membrane electrode assembly 116.

[0035] The converter 10 of the present invention operates in a gas phase cycle in which the working fluid is evaporated and condensed. More particularly, the converter 10 operates in a Rankine cycle. It is widely accepted in the art that only a limited number of fluids have the combination of chemical, environmental, thermodynamic and safety properties required for viable utilization in a Rankine cycle. See, for example, McLinden et al., "Nat. Commun.", 8 (2017), article No. 14476. This is because although some working fluids exhibit excellent thermodynamic properties in cooling cycles, their environmental or safety characteristics hinder their use, forcing the use of other fluids with lower performance for power generation cycles. Bromine, in particular, has generally never been considered for use in a Rankine cycle because it has a high ozone depletion potential. However, it has now been found that in the converter of the present invention, the risks associated with the high ozone depletion potential of bromine are negligible because the converter is operated as a closed system.

[0036] During operation, one side of the converter 10 is completely immersed in the working fluid as a liquid, while the other side of the converter 10 is only partially immersed in the working fluid as a liquid. At the high-temperature membrane electrode assembly 118, the liquid phase working fluid is initially in a high-temperature and high-pressure state. When the working fluid expands from the high-temperature and high-pressure state to the high-temperature and low-pressure state on the high-temperature membrane electrode assembly 118, electrical energy is generated. Heat Q H is provided to the thin film membrane 120 to achieve a near constant temperature expansion process. On the high temperature side of the converter 10, the liquid phase working fluid is also evaporated into a gas phase by the evaporator 142. The working fluid is then pumped or otherwise transported as a vapor to the low temperature membrane electrode assembly 116. At the low temperature membrane electrode assembly 116, electrical energy is provided to pump the gas phase working fluid from a low temperature, low pressure state to a low temperature, high pressure state. During the compression process, the temperature of the working fluid is reduced by removing heat Q from the proton conductive membrane 120. L On the low temperature side of the converter 10, the gas phase working fluid is condensed back into a liquid, more particularly a high pressure liquid, by the condenser 144. As the cycle continues, the high pressure liquid phase working fluid is pumped back into the high temperature membrane electrode assembly 118. Therefore, the converter 10 of the present invention is a closed loop engine and utilizes the Rankine cycle to convert thermal energy into electrical energy.

[0037] Because pumping liquids requires very little work compared to compressing gases, the work return ratio of the Rankine cycle is considered favorable because it allows for higher overall device voltages. Additionally, because the boiling point of the working fluid is relatively low, device voltages can be higher even at waste heat temperatures (i.e., 100°C to 200°C).

[0038] Those skilled in the art will appreciate that modifications may be made to the above embodiments without departing from the broad inventive concept thereof. It is therefore to 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. An electrochemical direct thermoelectric converter, comprising: a working fluid selected from the group consisting of halogens and sulfur; a heat source and a heat sink, wherein the temperature of the heat source is higher than the temperature of the heat sink; a first electrochemical cell comprising a first membrane electrode assembly, the working fluid being configured to flow through the first membrane electrode assembly, the first membrane electrode assembly comprising a first porous electrode, a second porous electrode, and at least one ion-conducting membrane sandwiched between the first porous electrode and the second porous electrode, the first electrochemical cell being coupled to the heat source; a second electrochemical cell comprising a second membrane electrode assembly, the working fluid being configured to flow through the second membrane electrode assembly, the second membrane electrode assembly comprising a first porous electrode, a second porous electrode, and at least one ion-conducting membrane sandwiched between the first porous electrode and the second porous electrode, the second electrochemical cell being coupled to the heat sink; an evaporator coupled to the first electrochemical cell; a condenser coupled to the second electrochemical cell; as well as an external load connected in series with the first electrochemical cell and the second electrochemical cell; wherein the working fluid is introduced into the first membrane electrode assembly as a liquid and expands through the first membrane electrode assembly, thereby generating a voltage in the first membrane electrode assembly, and the working fluid is evaporated into a gas phase, and The working fluid is introduced into the second membrane electrode assembly in the gas phase and is cooled and condensed back into the liquid phase.

2. The electrochemical direct thermal electric converter according to claim 1, wherein the working fluid is one of bromine and iodine.

3. The electrochemical direct thermal electric converter of claim 2, wherein the working fluid is bromine.

4. The electrochemical direct thermal-electric converter of claim 1, wherein the converter is configured to operate in a Rankine cycle.

5. The electrochemical direct thermal-electric converter of claim 1, wherein each of the first membrane electrode assembly and the second membrane electrode assembly is disposed within a housing comprising glass.

6. A method of generating electrical energy using an electrochemical direct thermoelectric converter, The converter comprises: A working fluid, wherein the working fluid is selected from the group consisting of halogen and sulfur; a heat source and a heat sink, wherein the temperature of the heat source is higher than the temperature of the heat sink; a first electrochemical cell comprising a first membrane electrode assembly, the working fluid being configured to flow through the first membrane electrode assembly, the first membrane electrode assembly comprising a first porous electrode, a second porous electrode, and at least one ion-conducting membrane sandwiched between the first porous electrode and the second porous electrode, the first electrochemical cell being coupled to the heat source; a second electrochemical cell comprising a second membrane electrode assembly, the working fluid being configured to flow through the second membrane electrode assembly, the second membrane electrode assembly comprising a first porous electrode, a second porous electrode, and at least one ion-conducting membrane sandwiched between the first porous electrode and the second porous electrode, the second electrochemical cell being coupled to the heat sink; an evaporator coupled to the first electrochemical cell; a condenser coupled to the second electrochemical cell; as well as an external load connected in series with the first electrochemical cell and the second electrochemical cell; The method includes operating the converter in a Rankine cycle: fully immersing one side of the converter with the working fluid as a liquid and partially immersing the other side of the converter with the working fluid as a liquid, introducing the working fluid as a liquid into the first membrane electrode assembly, the liquid expanding through the first membrane electrode assembly so that the first membrane electrode assembly generates a voltage, the working fluid being evaporated into a gas phase by the evaporator, and The working fluid is introduced into the second membrane electrode assembly as a vapor, and the vapor is cooled and condensed back into a liquid state.

7. The method of claim 6, wherein the working fluid is one of bromine and iodine.

8. The method of claim 7, wherein the working fluid is bromine.

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