WO2008044083A2

WO2008044083A2 - Thermally regenerative electrochemical converter and mehtod for converting thermal energy into electrical energy by the converter

Info

Publication number: WO2008044083A2
Application number: PCT/HU2007/000093
Authority: WO
Inventors: Attila HALÁCSY
Original assignee: E Konverzió Innovációs Kft
Priority date: 2006-10-11
Filing date: 2007-10-11
Publication date: 2008-04-17
Also published as: HU0600774D0; WO2008044083A3

Abstract

The invention relates to a thermally regenerative electrochemical converter and a method for converting thermal energy into electrical energy. The converter according to the invention comprises a working medium provided in the form of a metal hydride, a galvanic cell (12) comprising spaced-apart first and second solid electrodes (13, 14) and an electrolyte arranged at least partially between said electrodes (13, 14), wherein said electrolyte is chosen to selec- tively pass hydrogen ions of negative electrical charge and provided by a portion of the working medium. The converter also comprises means for decomposing the working medium into gaseous hydrogen (17) and a metal (46) in a molten state at a first temperature through thermal decomposition, means for supplying gaseous hydrogen (17) to the first electrode (13) and means for supplying the molten metal (46) to the second electrode (14). Said molten metal (46) recom- bines into the working medium through reacting with said gaseous hydrogen (17) the galvanic cell (12) at. a second temperature which is lower than said first temperature. An ancillary substance is added to the working medium, said ancillary substance causes the first temperature needed for decomposing the working medium to approach the second tempera-i ture needed for the recombination taking place in the galvanic cell (12). The con-i verter further comprises means for preventing the ancillary substance added to the working medium from getting into the galvanic cell (12).

Description

THERMALLY REGENERATED ELECTROCHEMICAL CONVERTER AND METHOD FOR CONVERTING THERMAL ENERGY INTO ELECTRICAL ENERGY BY THE CONVERTER

The present invention relates to a thermally regenerated electrochemical converter, as well as a method for converting thermal energy into electrical en- ergy by making use of said converter.

Thermally regenerated fuel cells and/or electrochemical converters are well-known. Processes for generating electrical energy from thermal energy by means of chemical energy are also known. Such means/systems and/or processes are disclosed e.g. in U.S. Pat. Nos. 4,818,638 and 4,692,390, respec- tively, wherein molten lithium hydride (LiH) or sodium hydride (NaH) is used as the working medium and also as the electrolyte of a thermally regenerated cell for converting thermal energy into electrical energy. In the process, having the thermal energy to be converted absorbed, the metal hydride applied as the working medium is decomposed at a suitable first temperature to metal and hy- drogen in a decomposition vessel. Then hydrogen is directed to a first electrode of a conveniently constructed electrochemical cell. The molten metal is simultaneously fed into a heat exchanger where it is cooled to a second temperature; said second temperature is significantly lower than said first one. At said lower second temperature, the molten metal thus cooled down is directed to a second electrode of the electrochemical cell. The second electrode is physically separated from the first electrode of the cell. Outside of the cell, the electrodes of the cell are coupled via electrically conducting elements. Inert hydrogen atoms fed to the first electrode of the cell transform into negatively charged hydrogen ions by receiving electrons from the first electrode, and then migrate to the second elec- trade of the cell through an electrolyte located between the electrodes. At the second electrode of the cell, the negatively charged hydrogen ions lose their charges, react with the molten metal fed to the second electrode and thereby combine into metal hydride at said lower temperature. All this means that an electric current flows within a circuit being electrically closed through the elec- trades and the electrolyte. The metal hydride formed at the second electrode is then pumped back into the decomposition vessel, wherein due to the input thermal energy it decomposes again. That is, by means of decomposing the metal hydride acting as the working medium/electrolyte into components and then combining said components, the input thermal energy is converted into electrical energy with a given efficiency.

The value of the second temperature is chosen so as to increase the volt- age of the electrochemical cell to a value as high as possible. To this end, a decrease in second temperature is required. The efficiency is basically determined by the difference of the first temperature (the so-called regeneration temperature) and the second temperature (i.e. the operation temperature of the cell) being lower than the first temperature; the greater the difference of the two values is, the larger is the power spent on cooling of the molten metal which arises as loss, that is, the lower is the efficiency of the conversion. For the solutions disclosed in the above cited U.S. patent documents, the temperature difference at issue is typically 500 to 600 degrees centigrade and 200 to 400 degrees centigrade for LiH and NaH electrolytes, respectively. The second temperature cannot be decreased arbitrarily; to assure pum- pability of the working medium/electrolyte and hence sustainability of the cyclical process, the second temperature should be kept continuously above the point of solidification of the metal hydride used. Accordingly, the working medium/electrolyte of such a converting system is usually provided in the form of various eutectics, that on the one hand have a cost-increasing effect and on the other hand spoil the efficiency of the energy conversion.

U.S. Patent No. 5,139,895 teaches a possible further way of decreasing further said second temperature, that is the operation temperature of the cell. According to this, the LiH electrolyte of the cell comprises a mixture of lithium (Li) and sodium (Na) of a given ratio. In this way, the operating pressure and pressure difference being necessary for the migration of hydrogen ions within the system (i.e. for the operation of the cell) are provided on the one hand. On the other hand, a shift of the solidification point of the electrolyte formed by the applied metal hydride mixture to lower temperatures is realized, and hence the op- eration of the cell at a higher voltage is achieved. Due to said measure, however, at the set pressure value, the decomposition temperature of the applied electrolyte will increase which is disadvantageous in respect of the conversion efficiency in view of the above, because of e.g. the necessity of cooling to a larger extent. The beneficial effect of the usage of the Li/Na mixture on the operation temperature of the cell can be explained by the mutual dissolution of the two metals to a significant extent. However, as a consequence of the increase of the decomposition temperature, application of the Li/Na mixture and/or mixtures of similar type in a thermally regenerated electrochemical converter according to U.S. Patent No. 5,139,895 results in a decrease in the conversion efficiency, and hence cannot be eventually considered as advantageous.

In light of the above, an object of the present invention is to provide a thermally regenerated electrochemical converter and also a method for convert- ing thermal energy into electrical energy by means of said converter, wherein the difference between the regeneration and operation temperatures of an electrochemical cell made use of as part of the converter is significantly smaller than the same difference used in the case of traditional converters based on thermal regeneration. A further object of the present invention is to develop a thermally regenerated electrochemical converter of an enhanced conversion efficiency by means of decreasing the difference of the regeneration and operation temperatures of the electrochemical cell used and also a conversion method exploiting the converter for producing electric energy. A yet further object of the present invention is to ensure/facilitate an increase of the conversion efficiency of a regenerated electrochemical cell and a conversion method by suitably choosing the electrolyte/working medium of the electrochemical cell used.

The objects related to the provision of a thermally regenerated electro- chemical converter are achieved by a converter according to Claim 1. Possible further embodiments of the converter of Claim 1 are set forth in Claims 2 to 9. The objects related to the development of a method for converting thermal energy into electrical energy are accomplished by a method according to Claim 10. Possible further variants of the inventive method are set forth in Claims 11 to 16. Claims 17 to 20 related to the use of a metal hydride being mixed up with a metallic ancillary substance also helps with the accomplishment of the above objects. - A -

In particular, the above specified objects are accomplished by adding a metallic ancillary substance to the metal hydride used as the working medium/electrolyte of the thermally regenerated electrochemical converter according to the present invention, wherein said ancillary substance (i) has an electro- negativity being more positive than that of the metal component of the metal hydride, (ii) does not blend or blends up to only a negligible extent with the metal component of the metal hydride at the operation temperature of the cell, and (iii) forms a hydride which is unstable at the operation temperature of the cell and decomposes at a temperature that is significantly lower than the operation tem- perature of the cell; it decomposes preferably at most at 250 degrees centigrade, more preferably at most at 200 degrees centigrade.

As the electrolyte, preferentially LiH, NaH, magnesium hydride (MgH₂) or calcium hydride (CaH₂) is used in the thermally regenerated electrochemical converter according to the invention. Amongst these, LiH is the most preferred. As the metallic ancillary substance, preferably potassium (K), rubidium (Rb), cesium (Cs) or a blend of arbitrary ratio thereof is used in the thermally regenerated electrochemical converter according to the invention. An application of the metals enlisted here in combination with LiH electrolyte is extremely preferred, as lithium hardly blends with these metals and can easily be separated/purified from them due to its high boiling point (which is about 1330 degrees centigrade). For an ancillary substance of metallic potassium, in particular, this view is supported by Figure 4 representing the phase diagram of a K/Li system derived experimentally (for further details, the reader is referred to the paper of CW. Bale published in 1989). Without going into more detailed theoretical expounding, it is noted that the metallic ancillary substances added to the metal hydride within the converter function practically as catalysts and significantly decrease the decomposition temperature of the metal hydride used in the electrochemical cell; for conventional converters, this temperature is essentially identical with the regeneration temperature. That is, in view of the above, for the inventive solutions, the addition of an appropriate metallic ancillary substance induces a decrease in the regeneration temperature of the electrochemical cell. Moreover, as a consequence of this, the difference between the regeneration and the operation temperatures — O ~~ of the electrochemical cell used as part of the thermally regenerated electrochemical converter according to the invention also decreases significantly, which results in an increase in the conversion efficiency due to the decrease in the cooling losses. In particular, considering LiH electrolyte in our studies, the conclusion was drawn that the catalytic effect is exerted by the appropriate ancillary substance through modifying the mobility of negatively charged hydrogen ions (H^"-ions, from now on) in the molten metal hydride. In a molten metal hydride containing no ancillary substance the Li⁺-ions, due to their size, are highly mobile, and hence the hydrogen dissolved in lithium can hardly leave the molten phase. On the contrary, if a metallic ancillary substance, e.g. potassium is added to the metal hydride, hydrogen is retained dissolved to a lesser extent by the K⁺-ions and thus can go to a gaseous phase through the Li/K phase boundary more easily. The reason for this on the one hand is that Li and K cannot be alloyed with one an- other, and on the other hand, the K⁺-ions in a molten phase are less mobile than the Li⁺-ions due to their size.

According to literature and as is shown in Figure 5, LiH is a stable non- decomposing compound at its melting point (688 degrees centigrade) under standard pressure (1 atmosphere). If, however, potassium is added to the LiH under standard pressure and then is heated together with it, a portion of the hydrogen content of the LiH (about a half of it) can be released at the temperature of 688 degrees centigrade (that is, at the melting point of the LiH), as is proven by the experiments performed in this respect. This is also shown by the following Table, in which the amounts of gaseous H₂ (measured in units of millilitres) leaving from a given amount (0,189 g) of LiH salt heated together with a given amount (0,189 g) of potassium, as well as the ratios (expressed as percentages) of the released gas relative to the total amount of hydrogen to be released, calculated theoretically, from said amount of LiH salt are enumerated as a function of temperature.

This means that a portion of the LiH content of a LiH/K system decomposes already at the melting point of the LiH, that is, the decomposition temperature of LiH is decreased by the ancillary substance of metallic potassium. The decomposition of LiH in the presence of an appropriate ancillary substance takes place in accordance with the relation of

LiH + Me = Li + Me + ¹/₂ H₂, wherein, according to our studies, Me stands for one of K, R, Cs or any blends thereof.

The released hydrogen (H₂) can be separated from the vapour of the an- ciliary substance added to the metal hydride by means of hydrogen permeable membranes made of metals constituting the iron-group of the periodic table of the chemical elements (preferably of e.g. nickel). Said separation is of fundamental importance when making use of a blend of said metal hydride/ancillary substance as the electrolyte of the electrochemical cell. In lack of separation, the ancillary substance would accumulate within the electrochemical cell and prohibit sooner or later the operation of the cell. The separation is based on the property that the above listed metallic ancillary substances cannot be alloyed with metals from the iron-group at all and/or the vapours thereof cannot diffuse through membranes made of such metals. For further use, the separated hydrogen can then be directed to an electrode of an electrochemical cell that forms part of a converter according to the invention. Lithium can also be separated from the metallic ancillary substances by suitable additional pieces of equipment. Lithium obtained in this way can be directed to the other electrode of the cell. The sepa- rated metallic ancillary substance can be used for decomposing further amounts of lithium hydride.

As in the inventive method the difference between the regeneration and the operation temperatures of the cell can be decreased to a very small value by adding the ancillary substance, cooling is required barely or none at all. A recuperation of a certain portion of the electric energy obtained by conversion is required merely to operate pumps as well as to cover natural losses of heat. Consequently, the efficiency of the conversion method according to the invention is significantly higher than that of known thermally regenerated cells or other known thermoelectric processes.

The invention is discussed in more detail with reference to the attached drawings, wherein

- Figure 1 is a schematic skeleton diagram of a possible exemplary embodiment of an energy conversion system based on a thermally regenerated electrochemi- cal converter in accordance with the invention;

- Figure 2 is a sectional view of a possible exemplary embodiment of the energy conversion system according to the invention in the uncharged state, that is, without a working medium/electrolyte;

- Figure 3 shows the energy conversion system of Figure 2 in its charged opera- tion state;

- Figure 4 illustrates the phase diagram of a potassium/lithium (K/Li) system which shows that molten metallic potassium and lithium mutually blend only at very high temperatures (i.e. at least at or above about 1500 degrees centigrade); and - Figure 5 is a plot of the hydrogen pressure (given in units of Torr's) within the closed volume portion above Li as a function of the dissolved/retained hydrogen content for various temperatures.

In what follows, the thermally regenerated electrochemical converter according to the invention and the conversion method achieved on the basis thereof will be discussed for an energy conversion system that makes use of lithium hydride as the electrolyte/working medium and potassium as ancillary substance, with no limitation on the scope of claims sought. Moreover, the converter to be detailed can also be made use of other metal hydrides and/or vari- ous ancillary substances; the modifications needed therefor are apparent to a person skilled in the relevant art, and hence, are not discussed here in more detail.

As is shown in Figure 1 , the energy conversion system based on a ther- mally regenerated electrochemical converter according to the invention converts the input thermal energy through given steps into electrical energy. Any process can serve as a source of thermal energy. In this respect, processes leading to a release of extremely large amounts of energy, e.g. heat producing processes concomitant with various nuclear reactions (nuclear fission, nuclear fusion, spal- lation, transmutation, etc.) are highly preferred. It is also clear from Figure 1 that, in the exemplary embodiment of the energy conversion system to be discussed here, said steps take place basically in five sub-units connected to each other by means of suitable tubings, as well as valves and pumps installed into the tubings. Accordantly, an embodiment of the energy conversion system based on a converter according to the invention is comprised of a sub-unit provided as the combination of a recovery unit 2 that is equipped with a heat exchanger and a melt separator 5; a vacuum distiller 19; a galvanic cell 12 for producing electric currents; a gas separator 18; a heat pump 28; as well as pressure tubes (illustrated only schematically in Figure 1) providing connections of said units with each other, and valves and pumps inserted into the tubes at the required locations.

Said sub-units, as well as their mutual relations will be discussed in detail with reference to an exemplary embodiment of the energy conversion system 1 , shown in Figure 2, based on a thermally regenerated electrochemical cell ac- cording to the invention.

The recovery unit 2 of the energy conversion system 1 is provided in the form of a closed vessel with an inlet 51 being in fluid communication with a return tube 9 being recirculated from the galvanic cell 12, an outlet 7 and a recirculating means 6. The outlet 7 is in fluid communication with an inlet 52 of the melt sepa- rator 5, said inlet 52 being formed preferably in the upper region of the melt separator 5. The recirculating means 6 is in fluid communication with an outlet 53 of the melt separator 5, said outlet 53 locating preferably in the lower region of the melt separator 5. Between the recirculating means 6 and the outlet 53, a controlled pump 31 is arranged for transporting the melt from the melt separator 5 to the recovery unit 2. Within the vessel of the recovery unit 2, a heat exchanger 3 is arranged, said heat exchanger 3 having a gas inlet 24 at one end thereof and an other end which opens into a return tube 55 that exits from the in- ner volume of the recovery unit 2 through the wall thereof. The heat exchanger 3 has got an outer surface 23 spreading within the inner volume of the recovery unit 2. Furthermore, a gas off-take line 39 falls into the heat exchanger 3 at a location situated between the two ends of the heat exchanger 3 and preferably closer to the end which empties into the return tube 55. The other end of the gas off-take line 39, that also exits from the vessel of the recovery unit 2, is connected to an inlet 61 of the gas separator 18. The heat exchanger 3 is mounted into the inner volume of the recovery unit 2 in such a way that a significant portion of its outer surface 23 locate in the vicinity of the inlet 51. The points where the gas off-take line 39 and the return tube 55 exit from the recovery unit 2 are sealed hermetically. Moreover, in a position locating deeper than the mouth of the heat exchanger 3, the end of the return tube 55 being within the inner volume of the recovery unit 2 is equipped with a controlled melt pump 4 for transporting the melt.

The gas separator 18 is formed as a closed and hermetically sealed ves- sel provided with the inlet 61 , as well as a first outlet 62 and a second outlet 63. Within the gas separator 18, a hydrogen permeable membrane 25 is arranged that divides the inner volume of the gas separator 18 into a first region 66 and a second region 67 being separated from one another. The regions 66, 67 are preferably equal in size, although this is not necessary. The membrane 25 of po- rous structure is made of a material (for example of nickel) which is capable of transmitting exclusively gaseous hydrogen (H₂) from one side thereof to another. The membrane 25 is arranged within the gas separator 18 in such a way that the inlet 61 and the second outlet 63 open into one and the same of the regions 66, 67 defined by the membrane 25, while the first outlet 62 opens into the remaining other region. It is noted here that a controlled gas pump 30 is installed into the gas off-take line 39 in the vicinity of the inlet 61 for transporting gaseous substance. Furthermore, the return tube 55 exiting the recovery unit 2 is connected to the second outlet 63 of the gas separator 18. The vacuum distiller 19 is formed as a closed and hermetically sealed vessel made of a pressure-tight material and provided with an inlet 71 , a first outlet 72, a second outlet 73 and a third outlet 74. The inlet 71 is preferably in fluid communication with an outlet 54 formed in the upper region of the melt separator 5. The first 72 outlet of the vacuum distiller 19 is connected via a suitable tubing (not referred to in the drawings) into a section of the return tube 55 located outside of the recovery unit 2, while the third outlet 74 of the vacuum distiller 19 is connected via a suitable tubing (not referred to either) into the return tube 9 being recirculated from the galvanic cell 12. In between the outlet 54 of the melt separator 5 and the inlet 71 of the vacuum distiller 19, a controlled one-way transfer valve 32 is installed. The opening/closing of the third outlet 74 of the vacuum distiller 19 is performed by a controlled return valve 33 arranged in the outlet 74. A control of the opening/closing of the first outlet 72 of the vacuum distiller 19 is accomplished by a gas pump 20 for transporting gaseous substance from the vacuum distiller 19, said gas pump 20 being inserted into a tubing section that connects the outlet 72 into the return tube 55.

And finally, the galvanic cell 12 is provided in the form of an electrochemical cell built into a hermetically closed vessel, said cell comprising a cylindrical first electrode 13 closed from the bottom and a cylindrical second electrode opened from the bottom, wherein said first and second electrodes 13, 14 are apart from one another and arranged within the inner volume of said electrochemical cell. The first and second electrodes 13, 14 can also be prepared with a shape differing from the cylindrical one. The first electrode 13 is preferably a hy- drogen gas membrane, while the second electrode is provided by a lithium electrode. The electrode 14 possesses first and second surfaces 41 , 42, respective chemical reactions take place over these surfaces 41 , 42. The electrode 14 is preferably located within a volume portion that is surrounded by the electrode 13 and closed by the vessel of the galvanic cell 12 from the open direction (that is, from the above). The region contained within the electrode 13 is less than the volume of the vessel, preferentially to such an extent that surfaces 41 , 42 of suitable size be available for the taking place of the chemical reactions. When the galvanic cell 12 is charged (see Figure 3), an electrolyte (preferably molten Nth- ium) is present in the region within the electrode 14, while the metal hydride applied in the energy conversion system 1 (preferably molten lithium hydride) is present between the electrodes 13, 14. Furthermore, the electrode 13 is preferably provided with a spillway 26 in its upper region, the function of which will be- come apparent on the basis of a detailed discussion of the operation of the energy conversion system 1.

The vessel containing the electrochemical cell is provided with an outlet

81 , a gas inlet 82 and a feed entry 83. The outlet 81 is located preferably in the lower region of the vessel, more preferably at the bottom thereof, and is con- nected to the inlet 51 of the recovery unit 2 via the return tube 9. The gas inlet

82, which is preferably formed in the upper region of the vessel of the galvanic cell 12, is connected to the first outlet 62 of the gas separator 18 via a tubing 40. The feed entry 83 is connected to the second outlet 73 of the vacuum distiller 19 via a feed tubing 16. In the vicinity of the outlet 73, a controlled melt pump 36 is installed into the feed tubing 16. Furthermore, a controlled return pump 33' is arranged in the outlet 81.

To avoid electrical conduction/grounding, the electrodes 13, 14 are mounted into the vessel of the galvanic cell 12 by means of electrically insulating members 27. The electrodes 13 and 14 are connected electrically to a positive electrode 34 and a negative electrode 35, respectively, which electrodes 34, 35 are both arranged outside the galvanic cell 12. Electrical energy produced from thermal energy fed into the recovery unit 2 during operation of the energy conversion system 1 appears as a potential difference between the positive and negative electrodes 34, 35; when electrically closing the two electrodes 34, 35 outside the energy conversion system 1 , as a consequence of an electrical closing through the galvanic cell 12 a direct current will flow in the closed circuit due to said potential difference, which current can be used arbitrarily.

Preferably, the heat pump 28 providing a heat exchange of the feed tubing 16 and the return tube 9 and/or the media flowing therein also constitutes a part of the energy conversion system 1.

For the sake of simplicity, in the embodiment of the energy conversion system 1 , shown in Figure 2, based on a thermally regenerated electrochemical converter according to the invention only a single galvanic cell 12 is illustrated. To achieve higher voltages, of course, several galvanic cells 12 of the same type can be connected in series. The way of realizing said connection itself is apparent to a person skilled in the relevant art, and hence, is not discussed here. Consequently, for the sake of simplicity, from now on the term ,,galvanic cell 12" re- fers to a single galvanic cell or several galvanic cells being connected to one another appropriately.

In what follows, the operation of the energy conversion system 1 with the electrolyte/working medium of LiH and the ancillary substance of potassium added to the electrolyte/working medium will be described in detail with reference to Figures 2 and 3. However, after performing modifications in the energy conversion system 1 that are apparent to a person skilled in the relevant art, other metal hydrides, such as sodium hydride, magnesium hydride or calcium hydride can also be used as the electrolyte/working medium. Similarly, rubidium, cesium or a blend thereof can also be used as the ancillary substance. As a first step, the energy conversion system 1 , namely the region between the electrodes 13, 14 of the galvanic cell 12, the return tube 9 and the lower zone of the vacuum distiller 19 are filled up with the molten metal hydride, here with lithium hydride 45, which functions as the electrolyte of the galvanic cell 12 and, simultaneously, as the working medium of the energy conversion system 1. In particular, the filling-up can be done through the galvanic cell 12 or through a filling-up opening (not shown in the drawings) formed in the energy conversion system 1 at a suitable other location. Then, the recovery unit 2 is filled up to a level which corresponds to the position of the heat exchanger 3 with the molten ancillary substance, in this embodiment with potassium 37, through a filling-up opening (not shown in the drawings either) formed in the recovery unit 2 specifically for this purpose. As a next step, respective sub-units of the energy conversion system 1 are filled up with the pure molten metallic component of the metal hydride applied. In the present embodiment, in particular, metallic lithium is fed into the recovery unit 12 above the heat exchanger 3 and the potassium 37, as well as through the melt separator 5 above the lithium hydride 45 introduced into the vacuum distiller 19 earlier, and by operating the melt pump 36, through the feed tubing 16 and the feed entry 83 into the region defined by the electrode 14 of the galvanic cell 12 above the lithium hydride 45 already present in said re- gion. Due to evaporation of the media, vapours and/or blends of vapours of the media introduced into the system 1 will also appear in the energy conversion system 1. It is noted that the circulation of media present within the energy conversion system 1 that forms a closed system is achieved by means of the pumps being installed into the system 1.

To operate the energy conversion system 1 continuously, the thermal energy to be converted into electrical energy is fed into the system 1 through the heat exchanger 3 of the recovery unit 2. The source for thermal energy can be the coolant of a conventional nuclear reactor at critical state, a spallation reactor, a transmuting reactor (such as lead, bismuth, potassium or sodium) or the heat accumulator of a fusion reactor or any other source of thermal energy of high temperature. With respect to the rate of input of thermal energy, a condition should be met; namely, the input of thermal energy should be performed along with a heat transfer coefficient and a temperature difference in order that the thermal energy input could exactly cover the required decomposition heat of lithium hydride 45 entering the recovery unit 2. To put it in another way, the thermal energy input should be completed so as to retain the temperature of the recovery unit 2 constant throughout the input. If the recovery unit 2 cools down, the decomposition process of lithium hydride 45 falls off and the metal hydride applied might even solidify. If, however, the rate of input of thermal energy is higher than what is considered to be optimal, the temperature within the recovery unit 2, and hence, also the temperatures of the media leaving it get increased. This, eventually, induces an increase in the temperature of the galvanic cell 12, as a result of which the voltage of the cell 12 decreases. The present disadvan- tageous process can be compensated for by external cooling, however, when that is applied, a portion of the electrical energy obtained by conversion is used for the cooling, and hence, conversion efficiency of the energy conversion system 1 gets diminished.

Lithium hydride 45 fed into the recovery unit 2 reacts with the ancillary substance, here potassium 37, over the surface 23 of the heat exchanger 3 and during blending, and gaseous hydrogen 22 containing potassium vapour and lithium/potassium blend 38 (in the form of an emulsion) are produced along with the removal of the combination heat, as well as of the evaporation heat of the al- kali vapour. The removed combination heat is covered partially by the thermal energy input coming from the outside and partially by the pressure-volume work performed within the system (by consuming a portion of the electrical energy produced by conversion). The removed evaporation heat is covered on the one hand by the condensation of the alkali content of the gaseous hydrogen 22 containing potassium vapour that flows into the heat exchanger 3 through the gas inlet 24 due to the pressure conditions prevailing within the energy conversion system 1. On the other hand, the removed evaporation heat is covered by the condensation of alkali vapour due to the compression between the melt pump 4 and the gas pump 30. Potassium vapour condensing within the heat exchanger 3 flows to the melt pump 4 through a melt plug 29 of molten potassium that forms in the heat exchanger 3. Then, when molten potassium 37 overruns the melt pump 4, the potassium 37 containing no molten lithium flows back to the recovery unit 2 from here. Gaseous hydrogen 22 containing potassium vapour flows from the heat exchanger 3 through the gas off-take line 39 and the gas pump 30 into the first region 66 of the gas separator 18, and then its hydrogen content leaves for the second region 67 of the gas separator 18 by diffusing through the hydrogen permeable membrane 25. In this way, the separation of the potassium vapour and the gaseous hydrogen takes place. Simultaneously, the lithium/potassium blend (emulsion) 38 exits through the outlet 7 of the recovery unit 2 into the melt separator 5.

The gas separator 18 attends to the separation of potassium vapour and gaseous hydrogen and/or to the continuous feeding in through a tubing 40 into the galvanic cell 12 of gaseous hydrogen 44 diffused into the region 67 through the membrane 25. The increase of pressure needed for the condensation at the melt pump 4 is realized by the gas pump 30.

In line with the above, gaseous hydrogen 44 diffused through the membrane 25 flows from the gas separator 18 through the tubing 40 into the galvanic cell 12. Simultaneously, potassium vapour accumulating within the first region 66 of the gas separator 18 flows to the melt pump 4 through the outlet 63 and the return tube 55. After having been condensed, potassium is recycled to the recovery unit 2 by the melt pump 4. The slight amount of potassium vapour (sev- eral molar percentages relative to the lithium) extracted from lithium 46 in the purification process of lithium 46 performed within the vacuum distiller 19 is also recycled to the melt pump 4 by the gas pump 20.

Separating the lithium/potassium blend 38 coming from the recovery unit 2 through the outlet 7 into various components is performed within the melt separator 5. Potassium (and if applied, rubidium and/or cesium) used as the ancillary substance blends with lithium even at high temperatures (i.e. at temperatures close to the decomposition temperature of LiH) only to a limited extent; according to the literature data such a blend contains only about five percentages of lith- ium. As specific weights of metallic potassium and lithium differ significantly, said metals can be easily separated from one another by means of e.g. a gravitational concept; the melts of the two metals arrange in layers on top of each other with respect to the specific weights, wherein in this case potassium can be found at the bottom. Potassium 37 separated from lithium/potassium blend 38 in this way is recycled from the melt separator 5 to the lower zone of the recovery unit 2 through the outlet 53 by the controlled pump 31 at the required flow rate. Liquid lithium 46 containing a slight amount of potassium dissolved therein, as well as lithium hydride and dissolved hydrogen also present accidentally therein are allowed to exit into the vacuum distiller 19 through the outlet 54 of the melt sepa- rator 5 via the transfer valve 32.

In the vacuum distiller 19, purification of potassium-containing lithium 46 - being fed into the distiller 19 and, optionally, also containing lithium hydride and dissolved hydrogen - from potassium (and/or from further ancillary substances, if such substances are used) takes place through evaporation in the presence of vacuum. The alkali vapour being evaporated is directed to the melt pump 4 through the outlet 72 and the return tube 55 by means of the gas pump 20, where it condenses. Hydrogen content, if any, enters the galvanic cell 12 through the gas separator 18, as discussed above. Hydrogen retained dissolved incidentally when potassium has been separated combines with lithium to form lith- ium hydride 45 that separates from lithium 46 within the vacuum distiller 19. Lithium hydride retained incidentally in lithium 46 does not disturb the electrochemical reactions taking place between the electrodes 13, 14 within the galvanic cell 12. On the contrary, the ancillary substance (in this case potassium) fed into the energy conversion system 1 should be separated from lithium 46 to the highest possible extent, as an incidental accumulation of the alkali metal within the galvanic cell 12 might lead to a decline of conversion efficiency, and eventually to an interrupt in the operation of the cell. Lithium hydride 45 obtained by separation is discharged from the vacuum distiller 19 into the return tube 9 by means of a return valve 33; said return tube 9 recycles lithium hydride 45 to the recovery unit 2 in order that it could take part in further thermal decomposition cycles. Lithium 46 purified from potassium (which, optionally, can also contain a slight amount LiH) is transferred from the vacuum distiller 19 into the galvanic cell 12 by means of the melt pump 36. To avoid the solidification of lithium hydride 45, a portion of the thermal energy to be converted can be used to heat the vacuum distiller 19.

The task of the heat pump 28 is to ensure that the galvanic cell 12 constituting a part of the energy conversion system 1 based on a thermally regener- ated electrochemical converter according to the invention could operate at the least possible temperature, namely at the melting point of LiH (that is, at 688 degrees centigrade), as the electrical potential of the cell after all will be the highest under this condition. As a portion of the chemical energy - besides enabling conversion into electrical energy - transforms into heat, said optimal operating temperature increases due to the heat released. This can be prevented by pre- cooling lithium 46 fed into the galvanic cell 12. Pre-cooling involves solidification of a portion of lithium hydride 45 used as the electrolyte; the system of a two- phase solid/molten lithium hydride at issue, however, maintains the galvanic cell 12 at the melting point of lithium hydride, and hence, no external cooling is re- quired. The heat pump 28 cools lithium 46 flowing therethrough towards the galvanic cell 12 in a heat exchanger 21 and transfers the heat obtained from lithium 46 in a heat exchanger 11 to lithium hydride 45 transported by the return pump 33' from the galvanic cell 12 into the recovery unit 2. In this way, the heat released in the galvanic cell 12 is partially used as a supply for the decomposition heat needed in the recovery unit 2. Lithium 46 pre-cooled by the heat pump 28 is directed into the galvanic cell 12, wherein it takes part in electrochemical reactions needed for the commencement of the electrical current. A proper operation of the galvanic cell 12 requires a periodical filling-up of the galvanic cell 12 with lithium. Pre-cooled lithium 46 enters the galvanic cell 12 at the feed entry 83. If more than one cells are applied, the galvanic cells 12 are filled up intermittently with metallic lithium 46 and during the filling-up the gal- vanic cell 12 being just filled is switched off from the production of electricity. Thereby, grounding by molten metal of the other galvanic cells 12 connected optionally in series with the one being just filled is avoided. When lithium is fed into the cell 12, the level of lithium 46, as well as the level of lithium hydride 45 forming the electrolyte between the electrodes 13, 14 get raised and a portion of lith- ium hydride 45 pours through the spillway 26 into the vessel containing the electrodes 13, 14. As during the filling-up with lithium 46 the galvanic cell 12 being just filled is switched off from the series of cells producing electricity, the remaining cells are not grounded by the electrolyte of lithium hydride 45 flowing out through the spillway 26 either. Lithium hydride 45 which flowed over is trans- ferred by the return pump 33' from the vessel of the galvanic cell 12 into the heat exchanger 11 and then from the heat exchanger 11 into the recovery unit 2. The electrical energy produced from the input thermal energy by means of the energy conversion system 1 can be withdrawn from the system through the external electrodes 34, 35. Over the surface of the galvanic cell's electrode 14 made of metallic lithium, at the phase boundary of lithium and lithium hydride the process of Li = Li⁺ + e^" takes place, which is accompanied by a loss of electron. Over the surface of a membrane forming the electrode 13 of the galvanic cell 12 which extends adjacent to gaseous hydrogen (i.e. over the surface that faces to the in- ner wall of the vessel), an interstitial metal hydride formation takes place in accordance with the reaction equation of ^ΛA H₂ + Xn = XnH. This process is electrically neutral. Over the surface of the galvanic cell's electrode 13 which extends adjacent to the electrolyte (i.e. over the surface that faces to the electrode 14), the process of XnH + e^" = Xn + H^" takes place, that is, an electron is gained. Then, hydrogen ions produced by the reaction migrate through lithium hydride 45 to the surface 41 , where they take part in the reaction of the interstitial metal hydride formation defined by H^' + Xn = XnH + e^", which is accompanied by a loss of electron. Simultaneously, the process of XnH + Li = LiH takes place over the surface 42, which is electrically neutral. In a single galvanic cell 12 constructed as described here, a voltage of about 0.15 to 0.16 V can be achieved between the electrodes 34, 35 if it is operated at the melting point of lithium hydride.

The energy conversion system 1 based on a thermally regenerated elec- trochemical cell according to the invention can be constructed and operated besides the working medium/electrolyte of LiH with further electrolytes, such as NaH, MgH₂ or CaH₂, and besides the ancillary substance of K with Rb and/or Cs (or an arbitrary blend thereof), as it was discussed earlier. Modifications/changes of the energy conversion system 1 needed for this, however, fall within the com- mon general knowledge of a person skilled in the relevant art.

It is also apparent to a person skilled in the art that a decrease of the regeneration (or decomposition) temperature of a metal hydride through the addition of a metallic ancillary substance with the above-detailed properties to the metal hydride, which is the basis for the solutions according to the present inven- tion, can also be applied to a plurality of further solutions that are suitable for converting thermal energy into electrical energy, optionally even without making use of a galvanic cell.

Claims

1. Thermally regenerated electrochemical converter, comprising

- a working medium provided in the form of a metal hydride;

- a galvanic cell (12) comprising spaced-apart first and second solid electrodes (13, 14) and an electrolyte arranged at least partially between said electrodes

(13, 14), said electrolyte being chosen to selectively pass hydrogen ions of negative electrical charge and provided in the form of a portion of the working medium;

- means for decomposing the working medium into gaseous hydrogen (17) and a metal (46) in a molten state at a first temperature through thermal decomposition;

- means for supplying gaseous hydrogen (17) to the first electrode (13); and

- means for supplying the molten metal (46) to the second electrode (14), wherein said molten metal (46) recombines into the working medium through re- acting with said gaseous hydrogen (17) at the second electrode (14) of the galvanic cell (12) at a second temperature lower than said first temperature, meanwhile hydrogen ions passed from the first electrode (13) to the second electrode (14) induce an electrical potential difference between the electrodes (13, 14), characterized in that an ancillary substance is added to the working medium, said ancillary substance being capable of causing the first temperature needed for decomposing the working medium to approach the second temperature needed for the recombination taking place in the galvanic cell (12), and that the converter is provided with means for preventing the ancillary substance added to the working medium from getting into the galvanic cell (12).

2. The converter according to Claim 1 , wherein the ancillary substance added to the working medium is a metal with the properties of (i) being more positive in terms of its electronegativity than the metallic component of the working medium; (ii) being non-blending or blending up to only a negligible extent with the metal component of the working medium at the second temperature; and

(iii) forming a hydride being an unstable chemical compound at the second temperature.

3. The converter according to Claim 1 or 2, wherein the working medium is lithium hydride, sodium hydride, magnesium hydride or calcium hydride.

4. The converter according to Claim 3, wherein the ancillary substance is potassium, rubidium, cesium or a combination/blend of an arbitrary ratio thereof.

5. The converter according to Claim 4, wherein the working medium is lithium hydride and the ancillary substance is potassium.

6. The converter according to any of Claims 1 to 5, wherein the means for preventing the ancillary substance from getting into the galvanic cell (12) is a gas separator (18) provided in the form of a vessel having an inlet (61) and outlets (62, 63) and defining a closed inner volume, said gas separator (18) is adapted to purify hydrogen (17) fed to the first electrode (13) from the ancillary substance essentially to the full extent, a hydrogen permeable membrane (25) extends within said inner volume of the gas separator (18), said membrane (25) dividing said inner volume to separate first and second regions (66, 67), said first region (66) being in fluid communication with said means for decomposing the working medium and said second region (67) being in fluid communication with said galvanic cell (12), wherein gaseous hydrogen (22) containing said ancillary substance is fed into the first region (66) and said gaseous hydrogen (17) being purified from the ancillary substance leaves the gas separator (18) through the sec- ond region (67).

7. The converter according to Claim 6, wherein said hydrogen permeable membrane (25) is made of a metal, preferably of nickel, chosen from the iron- group of the periodic table of the chemical elements.

8. The converter according to any of Claims 1 to 7, wherein the means for preventing the ancillary substance from getting into the galvanic cell (12) is a vacuum distiller (19) provided in the form of a vessel having an inlet (71) and outlets (72, 73, 74) and defining a closed inner volume, said vacuum distiller (19) is adapted to purify the metal (46) fed to the second electrode (14) from the ancillary substance essentially to the full extent through evaporation in the pres- ence of vacuum.

9. The converter according to any of Claims 1 to 8, wherein said galvanic cell (12) comprises at least two galvanic cells electrically connected into series.

10. Method for producing electrical energy from thermal energy, comprising the steps of - adding an ancillary substance to a working medium provided in the form of a metal hydride;

- decomposing said working medium into components at a first temperature through thermal decomposition in the presence of said ancillary substance;

- purifying said components provided in the form of gaseous hydrogen (17) and a metal (46) in a molten state separately from said ancillary substance by means of separation; then

- directing said gaseous hydrogen to a first electrode (13) of a galvanic cell (12) comprising spaced-apart first and second solid electrodes (13, 14) and an electrolyte arranged at least partially between said electrodes (13, 14), said electrolyte being chosen to selectively pass hydrogen ions of negative electrical charge and provided in the form of the working medium;

- directing said molten metal (46) to the second electrode (14) of said galvanic cell (12) and reacting it with said gaseous hydrogen (17) at the second electrode (14) at a second temperature lower than said first temperature, thereby causing its recombination into said working medium, meanwhile inducing an electrical potential difference between said electrodes (13, 14) by means of hydrogen ions passed from said first electrode (13) to said second electrode (14); wherein

- said second temperature needed for the recombination taking place in the galvanic cell (12) is approached by said first temperature needed for decomposing the working medium via addition of said ancillary substance to said working medium.

11. The method according to Claim 10, wherein the ancillary substance added to the working medium is a metal with the properties of (i) being more positive in terms of its electronegativity than the metallic component of the working medium; (ii) being non-blending or blending up to only a negligible extent with the metal component of the working medium at the second temperature; and (iii) forming a hydride being an unstable chemical compound at the second temperature.

12. The method according to Claim 10 or 11 , wherein lithium hydride, sodium hydride, magnesium hydride or calcium hydride is used as the working medium.

13. The method according to Claim 12, wherein potassium, rubidium, cesium or a combination/blend of an arbitrary ratio thereof is used as the ancillary substance.

14. The method according to Claim 13, wherein lithium hydride is applied as the working medium and potassium is applied as the ancillary substance.

15. The method according to any of Claims 10 to 14, wherein separation of said ancillary substance from said components of the working medium is per- formed by a diffusion separation technique or by vacuum distillation.

16. The method according to any of Claims 10 to 15, wherein said thermal energy needed for the thermal decomposition of said working medium is provided by performing controlled nuclear reactions.

17. Use of a metallic ancillary substance mixed to a metal hydride in a process for converting thermal energy into electrical energy by means of a thermally regenerated electrochemical cell having definite regeneration and operation temperatures for causing said regeneration temperature to approach said operating temperature of the cell, and thereby, for enhancing conversion efficiency of the conversion process.

18. The use according to Claim 17, whereby the metal hydride is chosen from the group of lithium hydride, sodium hydride, magnesium hydride and calcium hydride, and the metallic ancillary substance is chosen from the group of potassium, rubidium, cesium and a combination/blend of an arbitrary ratio thereof.

19. The use according to Claim 18, whereby the metal hydride is lithium hydride and the metallic ancillary substance is potassium.

20. Use of a metallic ancillary substance mixed to a metal hydride in a process for converting thermal energy into electrical energy for decreasing the regeneration temperature needed for the decomposition of said metal hydride into its chemical components, wherein said metal hydride is chosen from the group of lithium hydride, sodium hydride, magnesium hydride and calcium hydride, and said metallic ancillary substance is chosen from the group of potassium, rubidium, cesium and a combination/blend of an arbitrary ratio thereof.