Abstract:
Apparatuses and methods for vaporizing a liquid cryogen and producing electric power, as well as devices and methods for improving the thermal contact between thermoelectric devices and heat transfer surfaces using positive and/or negative pressures. These teachings are applicable to a wide range of thermoelectric applications including thermoelectric vaporizers, thermoelectric generators and thermoelectric heaters/coolers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 09/877,781 filed Jun. 11, 2001, and claims the benefit of U.S. Provisional Application No. 60/376,412 filed May 1, 2002. The entire disclosures of the aforementioned applications are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A variety of thermoelectric transducers are known in the art for converting electric current into thermal energy and vice versa. In general, when an electric current passes through such a transducer, a temperature differential is produced across opposite sides or portions thereof. This phenomenon is known as the Peltier effect. Conversely, when two sides or portions of a thermoelectric transducer have different temperatures, the transducer produces an electric current. This opposite or reverse phenomenon is known as the Seebeck effect. Thus, a thermoelectric transducer can be used to produce thermal cooling (or heating) or electric power.  
           [0003]    In general, generating electricity thermoelectrically has been inefficient and therefore not cost effective, with thermoelectric devices transforming only about five percent of applied heat into electricity. This is due, in part, to the conductivity of heat through the p-type and n-type materials used in thermoelectric devices.  
           [0004]    Another problem encountered with thermoelectric devices is the poor thermal contact that can exist between a thermoelectric module and the hot and cold surfaces used to conduct heat and/or cold to or from the thermoelectric module. Existing thermoelectric coolers and thermoelectric generators frequently use springs, clamps and other mechanical devices for holding thermoelectric modules in contact with heat transfer surfaces. These mechanical devices tend to fail over time, however, including when subjected to severe vibrations. Additionally, poor thermal contact can arise from corrosion between a thermoelectric module and the mechanical devices intended to provide good thermal contact with heat transfer surface(s).  
           [0005]    In addition, electrolysis and oxidation of electrical wire connections to thermoelectric modules are among the leading causes of failures in thermoelectric modules. Further, foreign substances such as grease, soot, and dust often interfere with the operation of thermoelectric devices.  
         SUMMARY OF THE INVENTION  
         [0006]    The inventor hereof has succeeded at designing apparatuses and methods for improving the thermal contact between thermoelectric devices and heat transfer surfaces using positive and/or negative pressures. The inventor has also succeeded at designing apparatuses and methods for simultaneously vaporizing a liquid cryogen and producing electric power thermoelectrically. These teachings are applicable to a wide range of thermoelectric applications including thermoelectric vaporizers, thermoelectric generators, and thermoelectric heaters/coolers.  
           [0007]    A method according to one aspect of the present invention includes providing a thermoelectric module and a heat transfer surface, and using at least one of positive pressure and negative pressure to force the thermoelectric module against the heat transfer surface.  
           [0008]    An apparatus according to another aspect of the invention includes a biasing member for providing a biasing force, a thermoelectric module, and at least one rigid device positioned between the biasing member and the thermoelectric module for coupling the biasing force of the biasing member to one side of the thermoelectric module.  
           [0009]    A method according to another aspect of the present invention includes providing an apparatus having an inlet for receiving a cryogen in liquid form, an outlet for supplying vapor produced from the cryogen, at least one thermoelectric device for producing electric power, and electric terminals for supplying the electric power. The method further includes inputting a cryogen in liquid form into the inlet, and thermally coupling one side of the thermoelectric device to the cryogen and another side of the thermoelectric device to a heat source to produce a temperature differential across the thermoelectric device. The thermoelectric device produces electric power in-response to the temperature differential. The method also includes transferring heat to cryogen within the apparatus, at least a portion of the cryogen within the apparatus vaporizing in response to the transferred heat, and outputting the produced electric power via the electric terminals and the produced vapor via the outlet.  
           [0010]    Additional aspects and features of the invention will be in part apparent and in part pointed out below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a sectional view of a thermoelectric device employing negative pressure for providing good thermal contact with a heat transfer surface;  
         [0012]    [0012]FIGS. 2A and 2B are sectional views of thermoelectric devices employing biasing members for providing good thermal contact with a heat transfer surface;  
         [0013]    [0013]FIG. 3 is a block diagram of a thermoelectric vaporizer/generator according to another embodiment of the invention;  
         [0014]    FIGS.  4 A- 4 C illustrate a tubular thermoelectric vaporizer having a wall formed of alternating layers of p-type and n-type materials;  
         [0015]    [0015]FIG. 5 illustrates a vacuum insulated thermoelectric heat exchanger according to another embodiment of the invention;  
         [0016]    [0016]FIGS. 6 and 7 are sectional views of vacuum insulated thermoelectric heat exchangers according to additional embodiments of the invention;  
         [0017]    [0017]FIGS. 8A and 8B depict a thermoelectric vaporizer having a solar heat collector according to another embodiment of the invention;  
         [0018]    FIGS.  9 A- 9 D illustrate a thermoelectric vaporizer according to yet another embodiment of the invention; and  
         [0019]    [0019]FIG. 10 is a sectional view of a thermoelectric heat exchanger according to still another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0020]    A method for improving the thermal contact between a thermoelectric device and a heat transfer surface according to one aspect of the present invention includes providing a thermoelectric module and a heat transfer surface, and using positive pressure and/or negative pressure to force the thermoelectric module against the heat transfer surface. In this manner, good thermal contact between the thermoelectric module and the heat transfer surface can be attained.  
         [0021]    An exemplary device for practicing the above-described method using negative pressure is illustrated in FIG. 1 and referred to generally by reference character  100 . As shown in FIG. 1, the device  100  includes a thermoelectric module  102  positioned between a heat transfer surface  104  and a pliable material  106 . Negative pressure is established in a region  108  between the pliable material  106  and the heat transfer surface  104  using, e.g., a vacuum pump (not shown). The negative pressure draws the pliable material  106  against the thermoelectric module  102  which, in turn, forces the thermoelectric module  102  against the heat transfer surface  104 . In this manner, good thermal contact is established between the thermoelectric module  102  and the heat transfer surface  104 , as well as between the thermoelectric module  102  and the pliable material  106 .  
         [0022]    Additionally, as the pressure drops within the region  108 , the higher pressure surrounding environment (when applicable) forces the pliable material  106  against the thermoelectric module  102  and thus the thermoelectric module  102  against the heat transfer surface  104 , thereby further contributing to the good thermal contact between the pliable material  106  and the thermoelectric module  102  and between the thermoelectric module  102  and the heat transfer surface  104 .  
         [0023]    In certain applications of the invention, the pliable material  106  is a thermally conductive material (e.g., a pliable metal foil), and the heat transfer surface  104  is a rigid surface.  
         [0024]    The vacuum region  108  shown in FIG. 1 can also be used to seal the thermoelectric module  102  in an oxygen-free, dust-free and moisture-free environment, thereby protecting the thermoelectric module from such elements. Further, the vacuum region  108  can be used to reduce or eliminate lateral heat loss from the thermoelectric module  102  by requiring all heat transfer to occur across the module&#39;s two thermal contact surfaces  110 ,  112 .  
         [0025]    Although the thermoelectric module  102  is depicted in FIG. 1 as directly contacting the heat transfer surface  104  and the pliable material  106 , it should be understood that the module  102  may be thermally coupled to the heat transfer surface  104  and/or the pliable material  106  through one or more intervening thermally conductive devices or materials.  
         [0026]    [0026]FIG. 2A illustrates a device  200  that employs positive pressure to improve the thermal contact between a thermoelectric module  202  and a heat transfer surface  204 . As shown therein, a rigid device  206  is positioned between the thermoelectric module  202  and a biasing member  208 . The biasing member  208  is preferably supported (directly or indirectly) on one side thereof by a rigid support surface  209 . The biasing member  208  provides a biasing force  210  which is coupled to the thermoelectric module  202  through the rigid device  206  therebetween. As a result of this force  210 , good thermal contact is established between the thermoelectric module  202  and the heat transfer surface  204 , as well as between the rigid device  206  and the thermoelectric module  202 .  
         [0027]    In some embodiments, the rigid device  206  includes one or more fluid passages for conveying a working fluid (e.g., a liquid cryogen or a low-boiling-point liquid), as illustrated by arrows  212 - 214  in FIG. 2A, and the device  200  is configured for thermally coupling the working fluid to the thermoelectric module  202 . For example, a fluid passage through the rigid device  206  can be located adjacent the thermoelectric module  202  such that the working fluid directly contacts a portion of the thermoelectric module  202 . Alternatively (or additionally), the rigid device  206  may be thermally conductive such that the working fluid is thermally coupled to the thermoelectric module  202  through the rigid device  206 . By thermally coupling the working fluid to the thermoelectric module  202 , the thermoelectric module can be used to heat (or cool) the working fluid, and/or the working fluid can be used to apply heat (or cooling) to the thermoelectric module  202 .  
         [0028]    In one embodiment, the rigid device  206  is a heat sink having fins across which the working fluid flows. The heat sink thermally couples the working fluid to the thermoelectric module  202  while, at the same time, couples the biasing force  210  of the biasing member  208  to the module&#39;s two thermal contact surfaces  216 ,  218 . It should be understood, however, that a variety of other devices can be employed as the rigid device  206 .  
         [0029]    The biasing member shown in FIG. 2A may be, for example, a spring, a pressurized air bladder, a resilient rubber material, or any other device capable of providing the biasing force  210 .  
         [0030]    As an alternative to the embodiment shown in FIG. 2A, the biasing member  208  may contact the thermoelectric module  202  directly such that the rigid device  206  can be eliminated. In such a case, the biasing member  208  may be provided, if desired, with one or more slots or channels through which a working fluid can flow with the working fluid thermally coupled to the thermoelectric module directly via direct contact with the thermoelectric module  202 , indirectly via the biasing member, and/or otherwise.  
         [0031]    The device  200  may also be configured with the thermoelectric module  202  sandwiched between multiple biasing members, as further described below.  
         [0032]    [0032]FIG. 2B illustrates an embodiment of the device  200  shown in FIG. 2A in which the rigid device  206  takes the form of several rigid transfer rods  252  for coupling the biasing force  210  of the biasing member  208  to the thermoelectric module  202 . The device  250  shown in FIG. 2B also employs a pliable heat transfer plate  254  positioned between the rigid transfer rods  252  and the thermoelectric module  202 . In this manner, the biasing force  210  of the biasing member  208  can be more evenly applied across the thermoelectric module, ensuring good thermal contact at numerous points across the module&#39;s thermal contact surfaces.  
         [0033]    [0033]FIG. 3 illustrates a device  300  for vaporizing a liquid cryogen (or a low-boiling-point liquid) and producing electric power according to another aspect of the present invention. As shown therein, the device  300  includes an inlet  302  for receiving a cryogen in liquid form, an outlet  304  for supplying vapor produced from the liquid cryogen, a thermoelectric module  306  for producing electric power, and electric terminals  308  for supplying the produced electric power.  
         [0034]    As used herein, “liquid cryogen” refers to substances in liquid form having temperatures at or below −150° C., including, e.g., liquid hydrogen, liquid nitrogen, and liquid oxygen.  
         [0035]    To use the device of FIG. 3, a liquid cryogen is fed into the device  300  via the inlet  302 . The input cryogen  310  is thermally coupled to one side of the thermoelectric module  306  and a heat source  312  is thermally coupled to another side of the thermoelectric module  306 , as indicated generally by arrows  314 ,  316  in FIG. 3. As a result, a temperature differential is produced across the thermoelectric module  306  from which the thermoelectric module produces electric power. Preferably at the same time, heat is transferred to the cryogen  310  within the device  300  which causes at least some of the cryogen  310  to vaporize. The produced vapor is output from the device  300  via the outlet  304 , and the electric power produced by the thermoelectric module  306  is output via the terminals  308 . In this manner, electric power and cryogen vapor can be produced simultaneously (if desired) from a liquid cryogen (e.g., liquid oxygen).  
         [0036]    In some embodiments, the heat source  312  is ambient heat from the environment external to the device  300 . It should be understood, however, that a variety of other heat sources may be advantageously employed. Further, while the heat source  312  is positioned external to the device  300 , the device  300  may be provided with the heat source  312  therein.  
         [0037]    Preferably, the heat transferred to the cryogen  310  is heat conducted through the thermoelectric module  306 , as indicated generally by arrow  318  in FIG. 3. In this manner, the heat loss inherent in thermoelectric power generation due to thermal conduction is advantageously used to vaporize the cryogen  310 . Thus, the heat  316  provided to the thermoelectric module is either converted to electricity thermoelectrically or is conducted through the module and absorbed by the cryogen  310  to vaporize liquid cryogen and/or increase the internal energy of cryogenic vapor within the device  300 . Alternatively, the device  300  can be provided with a separate heat source (i.e., in addition to the heat source  312 ) for transferring heat to the cryogen  310 .  
         [0038]    FIGS.  4 A- 4 C illustrate a thermoelectric vaporizer  400  constructed of alternating layers of thermally and electrically conductive p-type and n-type materials according to another embodiment of the present invention. Similar to the device  300  of FIG. 3, the thermoelectric vaporizer  400  shown in FIG. 4 is capable of generating electrical power while performing as a cryogenic heat exchanger.  
         [0039]    In this embodiment, the thermoelectric vaporizer is constructed from a tube  402  having a wall constructed from alternating layers  404  of p-type and n-type materials. A liquid cryogen  406  (or a low-boiling-point liquid) flows through the center of the tube with atmospheric heat  408  surrounding the outside of the tube. As heat penetrates the alternating layers  404  of p-type and n-type materials, electricity is generated thermoelectrically to produce a positive electrical charge  410  and a negative electrical charge  412 . A portion of the heat that penetrates the alternating layers  404  is not converted into electricity, and is instead absorbed by the liquid cryogen  406  within the tube  402 . This causes the liquid cryogen to vaporize and form cryogenic vapor  414  which exits another end of the tube.  
         [0040]    A vacuum insulated thermoelectric vaporizer  500  that performs as a solid-state electric generator and as a cryogenic heat exchanger to vaporize a cryogen (or a low-boiling-point liquid) according to another embodiment of the present invention  500  is illustrated in FIG. 5. As shown therein, the vaporizer  500  is constructed of p-type and n-type materials in alternating layers with the direction of heat flow parallel to the p/n junctions. Each set  502  of alternating layers  502  is thermally coupled to a flowing liquid cryogen  504  on one side and a flowing heat source  506  on another side. In response to the temperature differentials across the sets  502  of alternating layers, the thermoelectric vaporizer  500  produces an alternating current output as a positive charge  508  and a negative charge  510 .  
         [0041]    According to another aspect of the present invention, the thermoelectric vaporizer  500  shown in FIG. 5 is surrounded by a vacuum insulation chamber  512  that isolates the sets  502  of alternating layers (as well as the heat source  502  and cryogen  504 ) from the external environment.  
         [0042]    [0042]FIG. 6 illustrates another embodiment of a vacuum insulated thermoelectric vaporizer  600  according to the present invention. As shown therein, the thermoelectric vaporizer  600  is tubular in shape. A heat source  602  flows through the center of the thermoelectric vaporizer  600  and is surrounded by alternating layers  604  of p-type and n-type materials. A liquid cryogen  606  (or a low-boiling-point liquid) flows across a side of the alternating layers  604  opposite the heat source  602  such that the layers  604  of materials are between the heat source  602  and the liquid cryogen  606 . The liquid cryogen flow chamber  606  is itself surrounded by a vacuum-insulation chamber  608  that isolates the alternating layers  604  (as well as the heat source  602  and cryogen  606 ) from the external environment. The alternating layers  604  of p-type and n-type materials generate a positive electrical current  610  and a negative electrical current  612  by converting a portion of the thermal energy from the heat source  602  into electricity. At least some of the heat from the heat source  602  that is not converted into electricity is absorbed by the cryogen  606 , thereby causing the cryogen to vaporize.  
         [0043]    [0043]FIG. 7 depicts another embodiment of a thermoelectric vaporizer  700 . In this embodiment, both an external heat source  702  and an inner heat source  704  are employed. A liquid cryogen  706  (or a low-boiling-point liquid) and the heat sources  702 ,  704  are separated by layers of p-type and n-type materials for producing electricity. The outer heat source  702  may be atmospheric heat and the inner heat source  704  may be, for example, the heat of compression, solar heat, geothermal water, hot exhaust gases of combustion, chemical heat, etc. Heat from the inner heat source  704  flows through the center of the thermoelectric vaporizer  700  and is surrounded by layers  708  of p-type and n-type materials. These layers  708  of material are surrounded by a liquid cryogen flow chamber  706  which itself is surrounded by another set of layers  710  of p-type and n-type materials, which are surrounded by the external heat source  702 . The alternating layers  708 ,  710  of p-type and n-type materials generate a positive electrical current  712  and a negative electrical current  714  by converting a portion of the thermal energy from the external heat source  702  and the inner heat source  704  into electricity. At least some of the heat from the external heat source  702  and the inner heat source  704  that is not converted to electricity is absorbed by the liquid cryogen  706 , causing the cryogen to vaporize.  
         [0044]    [0044]FIGS. 8A and 8B depict a thermoelectric vaporizer  800  having a solar heat collector for vaporizing liquid air (or any other cryogen or a low-boiling-point liquid) while producing electricity. Solar radiation  802  is preferably concentrated by a fresnel lens  804  positioned on a top side of the thermoelectric vaporizer  800 . The upper surface  806  of the vaporizer is preferably painted black to absorb heat. The bottom side of the thermoelectric vaporizer is preferably provided with insulation  808  to prevent heat from penetrating a bottom surface of the vaporizer. One or more tubes  810  are provided with walls constructed from alternating layers of p-type and n-type materials that generate a positive electrical current  812  and a negative electrical current  814  when a liquid cryogen flows therethrough by converting a portion of the thermal energy from the solar radiation  802  (which is a heat source) into electricity. At least some of the heat that is not converted into electricity by the alternating layers is conducted through the alternating layers and absorbed by the cryogen to produce cryogen vapor.  
         [0045]    The produced cryogen vapor may be used to perform mechanical work. In one preferred application, the thermoelectric vaporizer  800  of FIG. 8 is located on the roof of a cryogenic vapor powered vehicle for supplying the produced vapor thereto.  
         [0046]    FIGS.  9 A- 9 D depict a thermoelectric vaporizer  900  according to another embodiment of the invention. As shown therein, the thermoelectric vaporizer  900  is constructed of thermoelectric modules  902  that generate DC electric power thermoelectrically. The vaporizer  900  also includes a vessel  904  for containing liquid cryogen  906  (or a low-boiling-point liquid), as well as on/off level sensors  908  and an inlet valve  910  for controlling the level of liquid cryogen  906  within the vessel  904 .  
         [0047]    Ambient temperature air  912  is preferably drawn into a blower housing  914  of the thermoelectric vaporizer by a fan motor  916  having fan blades  918 . Alternativley, other air moving means may be employed, such as an air compressor. The forced air flows though the blower housing  914  and across heat fins  920  that transfer heat from the air  912  to the thermoelectric modules  902  within the vessel  904 . A portion of the heat is converted to DC current thermoelectrically by the thermoelectric modules  902 . At least some of the remaining heat conducts through the thermoelectric modules  902  and is absorbed by the cryogen  906  to produce cryogen vapor. The forced air, having heat removed, is cooled and is allowed to exit the thermoelectric vaporizer as cold air  922 . The cryogen vapor is output from the thermoelectric vaporizer  906  for any desired use (e.g., gaseous oxygen needed by hospitals for their patients).  
         [0048]    As shown in FIGS.  9 B- 9 D, the thermoelectric modules  902  are positioned inside the vessel  904  against a rigid inner wall  924  and are covered by a metal foil  926  that is sealed at edges of the vessel&#39;s wall by overlap strips  928  screwed to the inner wall  924 . Heat transfer fins  930  are located on the outside of the rigid inner wall  924  in order to conduct heat to the thermoelectric modules  902 . The area  932  between the rigid inner wall  924  and the metal foil  926  forms a vacuum chamber to protect the thermoelectric modules from corrosion, dirt, moisture, and other harmful effects.  
         [0049]    [0049]FIG. 9D in particular details the mounting and vacuum seal formed for the thermoelectric modules  902 . The vessel&#39;s housing is preferably a rigid material that readily conducts heat. The thermoelectric modules are mounted against the housing on the inside of the vessel with insulation material  934  filling in spaces between the modules and the housing. Heat transfer fins are attached to the outside of the housing with an outer wall  936  trapping flowing air (containing heat) between the inner wall  924  and the outer wall  936 . The thermoelectric modules mounted on the inner wall  924  are covered by the pliable metal foil  926  that is allowed to draw tightly against the thermoelectric modules  902  when a vacuum is formed between the inner wall  924  and the metal foil  926 . This forms a vacuum pack which seals the thermoelectric modules  902  and provides good thermal contact between the thermoelectric modules  902 , the metal foil  926 , and the inner wall  924 . The thermal contact is enhanced by outward pressure exerted by the liquid cryogen  906  within the vessel, which applies pressure against the pliable metal foil  926  and thus the thermoelectric modules  902 , causing the modules  902  to press more firmly against the rigid inner wall  924  of the vessel housing  904 .  
         [0050]    The vacuum insulation is preferably formed by a vacuum pump (not shown) that draws a vacuum between the metal foil  926  and enclosed sections of the housing  904  to prevent heat transfer in areas of the housing at which heat transfer is undesirable. Apertures  938  extend through portions of the enclosed sections below the metal foil  926  to allow such areas to be vacuumed by the vacuum pump.  
         [0051]    [0051]FIG. 10 illustrates a thermoelectric heat exchanger  950  according to another embodiment of the present invention. As shown therein, the heat exchanger includes a rigid housing  952  preferably formed of two substantially identical housing members  954   a ,  954   b  detachably connected to one another (e.g., via flanges and threaded fasteners). Removably positioned within the housing are several thermoelectric modules  958  sandwiched between two movable assemblies. Each assembly includes an inflatable air bladder  960   a ,  960   b , an inner pliable plate  962   a ,  962   b , heat/pressure transfer rods  964   a ,  964   b , perforated alignment plates  966   a ,  966   b , and an outer pliable plate  968   a ,  968   b . As shown in FIG. 10, the transfer rods  964  extend through apertures in the alignment plates  966  to maintain the alignment of the transfer rods  964 . Preferably, the transfer rods  964  can freely slide within such apertures.  
         [0052]    By inflating the air bladders  960 , pressure is applied to the outer pliable plates  968 . This pressure is coupled from the outer pliable plates  968  to the thermoelectric modules  958  via the transfer rods  964  and the inner pliable plates  962 . In this manner, good thermal contact is established between the thermoelectric modules  958  and the inner pliable plates  962 , which are preferably thermally conductive. By applying equal gas pressure to the air bladders  960 , equal pressure can be applied against opposite sides of the thermoelectric modules  958 . Alternatively, differential pressures can be employed. Gas inlet valves  970   a ,  970   b  are provided for supplying pressurized gas to the air bladders  960 , as shown in FIG. 10.  
         [0053]    Each housing member  954   a ,  954   b  is preferably provided with inlet ports  955   a ,  955   b  and outlet ports  956   a ,  956   b  for receiving and discharging working fluids. Thus, a hot working fluid (gaseous or liquid) may flow through the upper half of the heat exchanger between the inner and outer pliable plates  962   a ,  968   a . Similarly, a cold working fluid (gaseous or liquid) may flow through the lower half of the heat exchanger between the inner and outer pliable plates  962   b ,  968   b.    
         [0054]    To create a temperature differential across the thermoelectric modules  958 , and thereby produce electric power, heat from the hot working fluid is transferred to the inner pliable plate  962   a  both directly through contact with the inner pliable plate  962   a , and indirectly through the thermally conductive transfer rods  964   a  across which the hot working fluid flows. Similarly, cold from the cold working fluid, which acts as a heat sink, is transferred to the inner pliable plate  962   b  both directly through contact with the inner pliable plate  962   b , as well as indirectly through the thermally conductive transfer rods  964   b.    
         [0055]    When the thermoelectric heat exchanger  950  of FIG. 10 is used as a vaporizer, a liquid cryogen or liquid state low-boiling-point-liquid flows through the lower half of the heat exchanger. Heat from the hot working fluid that conducts through the thermoelectric modules  958  is absorbed by the liquid cryogen or the liquid state low-boiling-point-liquid, thereby causing at least some of such liquid to vaporize into the gaseous state. The produced vapor is output from the heat exchanger via the outlet  956   a . Alternativley, electric power can be supplied to the thermoelectric modules for producing heat which can be transferred to the liquid cryogen or low-boiling-point-liquid for the purpose of producing vapor. Electric power can also be supplied to the thermoelectric modules  958  for heating and/or cooling working fluids without causing vaporization.  
         [0056]    Those skilled in the art will appreciate that many changes can be made in the above embodiments without departing from the spirit and scope of the invention. Therefore, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.