Patent Publication Number: US-6709778-B2

Title: Electrochemical conversion system

Description:
REFERENCE TO RELATED APPLICATION 
     This is a continuation in part of U.S. patent application Ser. No. 09/546,654 filed Apr. 10, 2000. 
    
    
     TECHNICAL FIELD 
     This invention relates to the conversion of heat energy into electrical energy utilizing a hydrogen electrochemical cell. 
     BACKGROUND OF THE INVENTION 
     The conversion of chemical energy to electrical energy may be accomplished in a variety of ways. It is known that electrochemical cells or batteries rely on redox reactions wherein electrons from reactant being oxidized are transferred to a reactant being reduced. With the separation of the reactants from each other, it is possible to cause the electrons to flow through an external circuit where they can be used to perform work. 
     Electrochemical cells however have had a problem related to the exhaustion of the reactants. Although most cells can be recharged by applying a reverse polarity voltage across the electrodes, such recharging requires a separate electrical source. During the recharging of the cell the cell typically is not utilized as an electrical power source, thus rendering it unusable during the recharging period. 
     Fuel cells have been developed in an effort to overcome problems associated with electrochemical cells. Typically, fuel cells operate by passing an ionized species across a selective electrolyte which blocks the passage of the non-ionized species. By placing porous electrodes on either side of the electrolyte, a current may be induced in an external circuit connecting the electrodes. The most common type of fuel cell is a hydrogen-oxygen fuel cell which passes hydrogen through one of the electrodes while oxygen is passed through the other electrode. The hydrogen and oxygen combine at the electrolyte-electrode interface to produce water. By continuously removing the water, a concentration gradient is maintained to induce the flow of hydrogen and oxygen to the cell. 
     These types of fuel cells however suffer from a number of disadvantages. These cells must be continuously supplied with a reactant in order to continuously produce electricity. Additionally, these cells produce a continuous product stream which continuously must be removed, the removal of which may pose a problem. The porous electrodes of these fuel cells must allow the passage of the reactant entering the cell. However, over time these porous electrodes can become fouled or plugged so as to slow or even prevent the passage of the reactant. Such slowing of the reactant flow reduces the production of electricity. Lastly, the selection of an appropriate electrolyte is not always easy. The electrolyte must rapidly transport the ionized species in order to increase the current production. Frequently, the limited migration of the ionized species through the electrolyte is a limiting factor on the amount of current produced. 
     In an effort to avoid the problems inherent with the previously described fuel cells, thermoelectric conversion cells have be designed. These thermoelectric conversion cells utilize heat to produce a pressure gradient to induce the flow of a reactant, such as molten sodium, across a solid electrolyte. A current is generated as sodium atoms lose electrons upon entering the electrolyte and gain electrons upon leaving the electrolyte. These cell however also suffer from the plugging of the porous electrodes required to pass the sodium ions. Furthermore, the diffusion of the sodium ions through the solid electrolytes has proven to be slow, thereby limiting the amount of current produced by the cell. Lastly, these types of fuel cells operate at extremely high temperatures, typically in a range between 1,200-1,500 degrees Kelvin, making them impractical for many uses. 
     Another problem associated with thermoelectric conversion cells has been their dependency upon an heat source to enable the operation of the cell. As such, these fuel cells have typically been rather large in size. Furthermore, the requirement of these cells to include a heat source has oftentimes prevented their use in remote area where it would be impractical to locate and operate such. 
     Accordingly, it is seen that a need remains for an electrochemical conversion system that does not require a continuous source of reactant, which does not require an electrolyte which may become plugged over time, which may operate without notice, and which may be operated at relatively low temperatures. It is to the provision of such therefore that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     In a preferred form of the invention an electrochemical conversion system to be worn upon the skin of an animal, the system comprises a first mass of hydrogen absorbent material, a second mass of hydrogen absorbent material spaced from the fist mass of hydrogen absorbent material, a first electrode, a second electrode, a proton conductive membrane positioned between the first electrode and the second electrode, a housing, mounting means for mounting the housing to the skin of an animal, and a supply of hydrogen. The housing contains the first mass of hydrogen absorbent material, the second mass of hydrogen absorbent material, the first electrode, the second electrode and the proton conductive membrane. The housing has a first chamber containing the first mass of hydrogen absorbent material and adapted to thermally transfer heat between the first mass of hydrogen absorbent material and the skin of an animal wearing the electrochemical conversion system. The housing also has a second chamber containing the second mass of hydrogen absorbent material which is adapted to thermally transfer heat to and from ambience. With this construction, the first and second masses of hydrogen absorbent material are in fluid communication with each other through the first and second electrodes and through the proton conductive membrane, whereby the hydrogen may be desorbed by one mass of hydrogen absorbent material and absorbed by the other mass of hydrogen absorbent material while passing past and reacting with the electrodes and the proton conductive membrane so as to cause an electric potential difference between the first and second electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an electrochemical conversion system in a preferred form of the invention. 
     FIG. 2 is a schematic view of an electrochemical conversion system in another preferred form of the invention. 
     FIG. 3 is a schematic view of an electrochemical conversion system in another preferred form of the invention. 
     FIG. 4 is a view showing an electrochemical conversion system in yet another preferred form of the invention shown mounted to a person. 
     FIG. 5 is a partial cross-sectional side view of the electrochemical conversion system of FIG.  4 . 
     FIG. 6 is a bottom view of the electrochemical conversion system of FIG.  4 . 
    
    
     DETAILED DESCRIPTION 
     With reference next to the drawings, there is shown in FIG. 1 an electrochemical conversion system  10  in a preferred form of the invention. The system  10  has a housing  11  made of a non-reactive metal, such as stainless steel, in which is mounted an electrochemical or hydrogen concentration cell  12 . The combination of the housing  11  and the concentration cell  12  define a first chamber  13  and a second chamber  14  separated from each other by the concentration cell  12 . A first mass of hydride material  17 , such as LaNi 4.7 Al 0.3 , TiFe 0.9 Mn 0.1 , liquid hydride, or other similar material, is contained within the first chamber  13 . Similarly, a second mass of hydride material  18  is contained within the second chamber  14 . For clarity of explanation the terms hydride, metal hydride, hydrogen absorbent material, liquid hydride materials and hydrogen absorbent metal material are equivalent. 
     The electrochemical cell  12  has a first gas diffusion electrode  20 , a second gas diffusion electrode  21  and a proton conductive membrane  22 , such as Nafion made by E.I du Pont de Nemours, mounted between the first and second gas diffusion electrodes. This type of electrochemical cell  12  is available from E-Tek, Inc. of Somerset, N.J. The electrochemical cell electrodes  20  and  21  are electrically coupled to an external load or circuit  23 . 
     The system  10  also includes an alternating heat transfer system  25  which selectively transfers heat to one mass of metal hydride material while absorbing heat from the other mass of metal hydride material. The heat transfer system  25  may utilize any form of heat source such as electric heaters, gas burning heaters, heated air, radiation heat sources, radiant heaters or other conventionally known means for producing heat. The heat transfer system  25  may also utilize any form of heat absorbing or dissipating means (heat sink), such as cooled fluids. 
     Here, the heat transfer system  25  has a network of conduits coupled to control valves which control the flow of the heated and cooled fluids passing through the conduit network. In more detail, the heat transfer system  25  has a heating input conduit  26  extending between a heating fluid source  27  and a first control valve  28 , and a cooling input conduit  29  extending between a cooling fluid source  30  and a second control valve  31 . The first control valve  28  is coupled to a first heat transfer manifold  33  positioned to transfer heat to and from the first mass of metal hydride material  17 . Similarly, the second control valve  31  is coupled to a second heat transfer manifold  34  positioned to transfer heat to and from the second mass of metal hydride material  18 . A first bridging conduit  35  extends between the first control valve  28  and the second control valve  31 . A second bridging conduit  36  extends between the second control valve  31  and the first control valve  28 . 
     The first control valve  28  is operable between a first position wherein fluid flowing through the heat input conduit  26  flows to the first heat transfer manifold  33  and a second position wherein the heated fluid flowing through the heat input conduit  26  flows through the first bridging conduit  35  to the second control valve  31 . The second control valve  31  is operable between a first position wherein cooling fluid flowing through the cooling input conduit  29  flows to the second heat transfer manifold  34  and a second position wherein the cooling fluid flowing through the cooling input conduit  29  flows through the second bridging conduit  36  to the first control valve  28 . With the first and second control valve  28  and  31  in their second positions, cooling fluid flowing into and through the second bridging conduit  36  continues into the first heat transfer manifold  33 . Similarly, with the first and second control valves  28  and  31  in their second positions fluid flowing into and through the first bridging conduit  35  continuous into the second heat transfer manifold  34 . In short, with the first and second control valves in their first positions the heated fluid from the heat input conduit  26  is conveyed into the first heat transfer manifold  33  while the cooling fluid from the cooling input conduit  29  is conveyed into the second heat transfer manifold  34 . With the control valves repositioned to their second positions the flows of the fluids are alternated so that the heated fluid flowing from the heat input conduit  26  is now conveyed to the second heat transfer manifold  34  and the cooling fluid flowing from the cooling input conduit  29  is now conveyed to the first heat transfer manifold  33 . 
     In use, the control valves  28  and  31  are positioned in their first position so that a heated fluid passing through the heat input conduit  26  is directed into the first heat transfer manifold while a cooling fluid passing through the cooling input conduit  29  is directed into the second heat transfer manifold  34 . The heated fluid within the first heat transfer manifold  33  heats the first mass of metal hydride material  17  within the first chamber  13 . The heating of the metal hydride material causes the metal hydride material to release hydrogen gas into the first chamber  13 , thereby causing a pressure increase within the first chamber  13 . Conversely, the cooling fluid within the second heat transfer manifold  34  cools the second mass of metal hydride material  18  within the second chamber  14 . The cooling of the metal hydride material causes it to absorb any hydrogen gas within the second chamber  14 , thereby causing a pressure decrease within the second chamber. 
     As the pressure differential between the first and second chambers  13  and  14  increases an electrical potential across the cell is created and progressively increased, for as a current is passed through the external load  23  hydrogen gas at the higher pressure first chamber adjacent the first electrode  20  is oxidized into protons. These protons are conducted through the proton conductive membrane  22  to the second electrode  21  at the lower pressure second chamber  14 , where it is reduced into hydrogen gas and spontaneously absorbed by the second mass of metal hydride material  18 . The oxidation of the hydrogen gas causes the release of electrons which are then passed to the first electrode while the reduction of protons into hydrogen gas causes the acceptance or receiving of electrons from the second electrode, thereby inducing a current. The absorption of the hydrogen by the second mass maintains the second chamber  14  at a low pressure state. The second mass continues to absorb hydrogen gas until the metal hydride material approaches its saturation point. The net result is that the high pressure hydrogen gas is transferred from the first chamber  13  to the second chamber  14  through the electrochemical cell  12  until hydrogen gas in chamber  13  is depleted or the circuit  23  is turned off. 
     Once the first mass of metal hydride material  17  has released its associated hydrogen and the second mass of metal hydride material  18  has become substantially saturated with hydrogen, i.e. an equilibrium is reached, the heat transfer process is reversed. The heat transfer process is reversed by actuating the first and second control valves  28  and  31  to their second positions. Thus, the heating fluid flowing from the heat input conduit  26  is directed into the first bridging conduit  35  so as to flow to the second control valve  31  which then directs the heating fluid into the second heat transfer manifold  34 . Similarly, the cooling fluid flowing from the cooling input conduit  29  is directed into the second bridging conduit  36  so as to flow to the first control valve  28  which then directs the cooling fluid into the first heat transfer manifold  33 . As such, the second mass of metal hydride material  18  is now heated so as to release the hydrogen gas into the second chamber  14  thereby increasing the pressure therein. Conversely, the first mass of metal hydride material  17  is now cooled thereby causing it to absorb hydrogen gas and thereby decrease the pressure within the first chamber  13 . The pressure differential between the first and second chambers once again causes the hydrogen gas to pass through the electrochemical cell, albeit now in the opposite direction, thereby creating electricity as previously described. 
     The transfer of hydrogen across the electrochemical cell creates an energy (work) in the form of electrical energy through the external load. The voltage across the electrochemical cell may be calculated by utilizing the following equation: 
     
       
           E= ( RT/ 2 F )(ln)( P   h   /P   l ) 
       
     
     where R is the gas constant, T is the cell temperature, F is Faraday constant, and it is assumed that the temperature across the electrochemical cell is uniform. With this formula it can be calculated that the open circuit voltage for a single cell is 59.2 mV and that the electrical energy the concentration cell can provide is approximately 11.4 kJ for transferring one mole of hydrogen gas through the cell at 25 C. if the pressure ratio between the first and second chambers is 100:1. To generate a hydrogen pressure ratio, the temperature difference required between the heat source and the heat sink can be estimated utilizing the following equation: 
     
       
         Δ T=T   l (( RT   l  ln( P   h   /P   l ))/(−Δ H ))/1+(( RT   l  ln( P   h   /P   l ))/Δ H ) 
       
     
     where ΔH is the hydrogen desorption enthalpy. In deriving this equation, a linear relationship between logarithm of hydrogen pressure and the reciprocal of temperature is used. Thus, assuming that the heat sink temperature is 25 C. and the hydrogen desorption enthalpy is equal to −30 kJ/mol, the temperature difference required for generating a pressure ratio of 100:1 can be estimated to be 183 C. Accordingly, it can be understood that the hydrogen pressure can be greatly increased by a relatively low heating of the metal hydride material. 
     It should be understood that except for minor issues due to gas diffusion, the hydrogen gas cannot penetrate through the electrochemical cell at open circuit. However, when the electrochemical cell is under load hydrogen gas is oxidized and conducted through the membrane as protons where it is then reduced back to hydrogen gas, thereby providing an electric potential energy to the external load. 
     With reference next to FIG. 2, there is shown an electrochemical conversion system  40  in another preferred form of the invention. The system  40  has a first chamber  41  thermally insulated from ambience and includes heat sink/source material  44 , a second chamber  42  exposed to ambience, and an electrochemical cell  43  therebetween. This system may be utilized within a home to convert ambient heat to electricity or to utilized heat released by or absorbed by the reacting metal hydrides. As such, the heating and cooling means is provided by the environment. 
     In use, during the heat of the day the second chamber  42  is exposed to ambient heat while the insulated, first chamber  41  remains cool. The heating of the second chamber while the first chamber remains coo causes the release of hydrogen within the second chamber so as to create a pressure differential. The release of hydrogen increases the pressure within the second chamber. The pressure differential between the first and second chambers causes the hydrogen to pass from the second chamber to the first chamber through the electrochemical cell  43 , thereby creating electricity as previously described. The absorption of hydrogen in the metal hydride of the first chamber elevates its temperature to that of the second chamber. Equilibrium is eventually achieved between the two chambers at which time hydrogen transfer and the associated current flow stops. As the day turns to night, the ambient temperature assumably drops below that of the insulated first chamber  41 . The temperature difference, and thereby the pressure difference, causes the hydrogen gas within the first chamber&#39;s metal hydride material to be released, thereby increasing the pressure within the first chamber. This pressure difference causes the hydrogen to pass from the first chamber  41  to the second chamber  42 , i.e. reversing the process. Heat sink/source material  44  (water, parafin phase change material, or other high heat capacity material) is included in the first chamber to absorb heat from the metal hydride during the exothermic hydrogen absorption process. This helps to thermally stabilize the hydride allowing it to absorb more hydrogen than it otherwise would be able to without the material  44 . Similarly material  44  acts as a heat source when hydrogen is being released from the hydride in chamber  41  to support the indothermic desorption process. The amount of hydrogen that can be absorbed or desorbed during a given cycle has a direct relationship with the amount of heat that can be stored and released by material  44 . 
     Referring next to FIG. 3, there is shown an electrochemical conversion system  50  in yet another preferred form of the invention. The system  50  has a first chamber  51  and a second chamber  52  in fluid communication with each other through a transfer conduit  53  having a liquid pump  54  which pumps liquid from the first chamber  51  to the second chamber  52 . The system  50  also has a reaction chamber  57 , containing an electrochemical conversion cell  58 , in fluid communication with the second chamber  52  through a first gas conduit  59 . The system also has a liquid conduit  62  extending from the second chamber  52  to the first chamber  51  and a second gas conduit  63  extending from the reaction chamber  57  to a junction  64  in the liquid conduit  62 . A heating source  66  is positioned to transfer heat to the second chamber  52  while a cooling source or heat exchanger  67  is coupled to remove heat at the junction  64  of the liquid conduit  62  and second gas conduit  63 , or thereafter within the first chamber. A supply of hydrogen gas concentrated hydride liquid is contained within the system  50 . The hydride liquid may be HySorb organometallic liquid such as organometallic dihydrogen complexes containing dihydrogen ligands a specific example of which is cyclopentadienyl dihydrogen. 
     In use, the liquid hydride within the second chamber  52  is heated by the heat source  66  to a temperature to cause the release of hydrogen gas from the liquid hydride. The hydrogen gas passes through the first gas conduit  59  and into the reaction chamber  57  wherein the pressure differential therein causes the hydrogen gas to pass through the conversion cell  58  to create electricity as previously described. The hydrogen gas then continues through the second gas conduit  63  to the junction  64 . Simultaneously, the hydrogen depleted liquid passes from the second chamber  52  through the liquid conduit  62  and through the junction  64  whereby the cooling source  67  extracts the heat from the hydrogen depleted liquid and hydrogen gas so that the hydrogen gas mixing with the liquid at the junction recombines with the liquid to form a hydride liquid. The recombined hydride liquid and any remaining hydrogen gas and hydrogen depleted liquid passes into the first chamber  51  wherein the remaining hydrogen depleted liquid may continue to recombine with the remaining hydrogen gas. The hydride liquid may then be pumped through the transfer conduit  53 , by the liquid pump  54 , into the second chamber  52  wherein it may be cycled through the system again. It should be noted that the system may be used continuously or in batched sequence. 
     It should also be understood that the heating of the metal hydride material provides for a much greater fluctuation in gas pressure within the chambers than the heating of gas alone. For it should be noted that the desorption enthalpy is very large for these materials, typically in the order of −30 kJ/mole. As such, a temperature change of merely 150° C. may change the pressure by two orders (100). For example, the hydrogen pressure equilibrium with the metal hydride may increase from 0.42 atm to 42 atm with a temperature change from 25° C. to 175° C. Comparably, a temperature change of 150° C. for hydrogen gas merely increases the gas pressure from 0.42 atm to 0.60, an increase of only 42%. As such, it can be seen that utilizing metal hydride is an effective means of converting thermal energy (related to temperature) to mechanical energy (related to pressure) through a pressure differential. Furthermore, this system converts energy without any mechanically moving part except for the control valves. 
     Lastly, it should be understood that by controlling the passage of the electrical current one may control the flow of the hydrogen across the electrochemical cell. As such, the rate of heat removed from the high temperature side can be controlled precisely based on the hydrogen desorption enthalpy and the hydrogen flow rate. It should be noted that when a metal hydride releases hydrogen it absorbs heat from its environment, i.e. an endothermic reaction, while the adsorption of hydrogen gas causes the release of heat to its environment, i.e. an exothermic reaction. Consequently, the temperature at the heat source side can be controlled in this manner for refrigeration purposes. 
     With reference next to FIGS. 4-6, there is shown an electrochemical conversion system  70  in another preferred form of the invention. The system  70  has an extremely thin, thermally conductive housing  71  and a central thermally insulative separator  72  which in combination with the housing forms a first chamber  73  and a second chamber  74  in fluid communication with the first chamber  73  through a flow channel  75 . The housing  71  may contain a mounting substrate  77  which also acts as a heat sink. The flow channel  75  contains a previously described electrochemical cell  79  through which hydrogen must pass in order to pass completely through the flow channel  75 . The electrochemical cell  79  is shown electrically coupled to electronics  81  mounted to the substrate  77 , which in this embodiment includes a sensor  82 , a data transmitter  83 , a power conditioning circuit  84 , a data processing circuitry  85 , and a thin film battery  86 . The system  70  also includes a layer of adhesive  88  or other suitable means, such as a pliable, peripheral sheet having an adhesive such as that similar to a band-aid, of mounting the system to the skin of person or other animal. 
     Here, the system  70  has a first mass of metal hydride material  90  positioned within the first chamber  73  and a second mass of metal hydride material  91  positioned within the housing second chamber  74 . 
     This system is designed to be worn upon the skin of a person. When worn on the skin the first mass of metal hydride material  90  is maintained at a near constant temperature as it is positioned in thermal communication with the person&#39;s skin through the substrate  77  which acts as a heat sink to minimize temperature variations. Simultaneously, the second mass of metal hydride material  91  is exposed to ambience. During operation, this system  70  converts ambient heat to electricity through the heat released by or absorbed by the reacting metal hydrides. As such, the heating and cooling means is provided by body heat and the environment. 
     In use, with the first chamber  73  and the first mass of metal hydride material  90  therein is in thermal contact with the person&#39;s skin through the heat sink substrate  77  the first mass of metal hydride material  90  is maintained at a generally constant temperature. However, during the heat of the day in a hot climate environment the second chamber  91  is exposed to ambient heat wherein the heating of the second chamber  74  and the cooler temperature of the first chamber  73  causes the release of hydrogen from the second mass of material  91  within the second chamber  74  so as to create a pressure differential. The pressure differential between the first and second chambers  73  and  74  causes the hydrogen to pass from the second chamber  74  to the first chamber  73  through the flow channel  75  and electrochemical cell  79 , thereby creating electricity as previously described. This electricity may be utilized to power the electronics  81  or may be stored within the thin film battery  86  for later use to power the remaining electronics. 
     As the day turns to night, the ambient temperature assumably drops below that of the thermally stable first chamber  73 . The temperature difference, and thereby the pressure difference, causes the hydrogen gas within the first mass of material  90  within the first chamber  73  to be released, thereby increasing the pressure within the first chamber  73 . This pressure difference causes the hydrogen to pass back from the first chamber  73  to the second chamber  74  through the flow channel  75  and electrochemical cell  79 , i.e. reversing the direction of the process while still creating electricity. 
     It should also be understood that the system may also incorporate the physical movement of the masses of material so that the positions of the first and second masses of metal hydride material are periodically exchanged. This may be accomplished simply by forming the housing  71  so that an operator simply inverts the positions of the first and second chambers by flipping the housing over to its opposite side. This method may be preferred when the wearer is exposed to ambient conditions wherein the day time temperature and the night time temperature do not vary substantially or where the range of ambience temperatures of the second mass of metal hydride material does not include the generally constant temperature of the first mass of metal hydride material, i.e. the ambient temperature remains either below or above that of the constant body temperature. 
     The just described embodiment allows the conversion system with electronics to be worn by a person without the need for a large battery to store enough power to provide multiple bursts of power in transmitting data. It should be understood that the electronic may be used to transmit data obtained through the sensors  82 , which may contain the wearer&#39;s vital statistics, global position, etc. As electricity is produced continually these devices provide the sensors with the ability to continually or intermittingly monitor without being restricted by the energy available by a single source such as a battery which has a limited lifetime. Obviously, the electricity produced by the system establishes certain parameters with regard to the electronics or the storage necessary to operate the electronics. However, any electronic device within such parameters may be utilized in conjunction with the conversion system. 
     With regard to all shown embodiments, it should also be understood that the system may utilize several electrochemical cells aligned parallel to each other so that the hydrogen gas passes through each cell in succession. Several electrochemical cells may also be positioned upon one membrane and connected in series or in parallel to each other. It should also be understood that as an alternative to the adhesive layer the system may include mounting means such as an article of clothing, an elastic band or other suitable device for maintaining the proximity of the first mass of metal hydride material to the skin of the wearer. 
     It thus is seen that an electrochemical conversion system is now provided which is efficient and which may be operated upon a person in remote locations. It should of course be understood that many modifications, in addition to those specifically recited herein, may be made to the specific preferred embodiments describe herein without departure from the spirit and scope of the invention as set forth in the following claims.