Abstract:
Thermoelectric devices, intended for placement in the exhaust of a hydrocarbon fuelled combustion device and particularly suited for use in the exhaust gas stream of an internal combustion engine propelling a vehicle, are described. Exhaust gas passing through the device is in thermal communication with one side of a thermoelectric module while the other side of the thermoelectric module is in thermal communication with a lower temperature environment. The heat extracted from the exhaust gasses is converted to electrical energy by the thermoelectric module. The performance of the generator is enhanced by thermally coupling the hot and cold junctions of the thermoelectric modules to phase-change materials which transform at a temperature compatible with the preferred operating temperatures of the thermoelectric modules. In a second embodiment, a plurality of thermoelectric modules, each with a preferred operating temperature and each with a uniquely-matched phase-change material may be used to compensate for the progressive lowering of the exhaust gas temperature as it traverses the length of the exhaust pipe.

Description:
[0001]    This invention was made with U.S. Government support under Agreement No. DEAC050000R22725 awarded by the Department of Energy. The U.S. Government may have certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention pertains to devices and methods for efficiently extracting at least a portion of the thermal energy in an internal combustion engine exhaust stream and converting the extracted energy to electrical energy. 
       BACKGROUND OF THE INVENTION 
       [0003]    Advances in the efficiency of both spark-ignition and compression-ignition internal combustion engines have contributed significantly to improved fuel economy of vehicles. Despite these improvements in engine efficiency an appreciable fraction of the available energy in the fuel is discharged as waste heat, a sizeable portion of which is discharged at high temperature in the vehicle exhaust. 
         [0004]    Yet further increases in vehicle fuel economy could be achieved if at least a portion of the waste heat contained in the exhaust stream could be efficiently captured. 
       SUMMARY OF THE INVENTION 
       [0005]    Many automotive vehicles have a gasoline-fueled or diesel-fueled engine in the front of the vehicle with a combustion gas exhaust conduit that connects to the exhaust manifold of the engine and leads under the vehicle to the rear where the exhaust is discharged. This exhaust passage comprises sections of high temperature resistant alloy steel pipes that typically carry the hot engine exhaust to and from a catalyzed exhaust gas treatment container, an exhaust resonator and a muffler for removing pollutants from the exhaust and managing its noise. The steel exhaust pipes are usually round with an internal diameter of about 70 mm to about 100 mm The temperature of the exhaust gas exiting the exhaust manifold at any time is a function of how long the engine has been running and how much fuel it is consuming, and may range from high temperatures of about 500° C. to a hundred degrees or more lower. Oxidation reactions in a catalytic converter often increase exhaust temperatures downstream of the converter. 
         [0006]    In accordance with embodiments of this invention, the exhaust conduit system also includes one or more thermoelectric modules, each comprising a suitably-packaged assembly comprising a plurality of thermoelectric elements with integral electrical interconnections and connectors for extraction of electrical energy. The designated high temperature sides of the thermoelectric elements (within the modules) are heated by hot exhaust gas and the lower temperature sides are cooled. So the thermoelectric module generates electrical energy from the exhaust and conducts it to a nearby storage battery for use in components on the vehicle. Often the lower temperature side (the cold side) of the thermoelectric module is cooled, for example, by ambient air or by engine coolant. A thermoelectric device comprises an array of thermoelectric modules are arranged around the circumference (or perimeter) of the exhaust gas path (or a conduit member for the exhaust) and along the flow axis of the exhaust to provide a desired heat transfer contact area between the high temperature side of the module and the flowing gas. 
         [0007]    Thermoelectric compositions are selected for effective operation at a temperature (or narrow temperature range) within typical expected upper levels of exhaust gas temperatures for a vehicle engine-exhaust system. In embodiments of this invention, one or more volumes of phase-change material are placed co-extensively between the hot exhaust gas and the high temperature side of thermoelectric modules. The composition of the phase-change material is selected to undergo melting and solidification cycles at the desired high-temperature side of the thermoelectric elements. Thus, the amount of phase-change material, its melting temperature, and its specific latent heat serve as a stabilizing temperature moderator for heat transfer between the thermoelectric material and the often continually-varying temperature of the exhaust gas. Similarly, a lower temperature phase-change material may be used between the low temperature side of the thermoelectric material and a medium used in heat exchange with the designated low temperature side of the thermoelectric module. Depending on the length of the thermoelectric device along the exhaust path and the typical temperature drop of the exhaust gas within such length, additional different phase-change materials and thermoelectric materials may be used in the hot and cold sides of down-stream modules. These downstream modules may be adapted for more efficiently extracting energy from the lower, downstream temperature engine exhaust. 
         [0008]    If, during infrequent sustained periods of elevated exhaust temperature, for example when climbing a long grade or when towing a heavy load, the entire volume of phase-change material is transformed to its high temperature phase, it will be incapable of storing additional heat at constant temperature and the temperature of the phase-change material, and correspondingly, the temperature of the thermoelectric module, will increase. If a thermoelectric module overheats and exceeds its preferred operating temperature the thermoelectric elements might be oxidized or decompose or otherwise degrade. 
         [0009]    Thus, the exhaust system may be modified by incorporation of a bypass (exhaust) pipe which branches off the exhaust pipe upstream of the thermoelectric device and rejoins the exhaust pipe downstream of the thermoelectric module. By means of a suitable valve, the exhaust gas may be directed through the thermoelectric module or through the bypass pipe. When the phase-change material can no longer absorb heat without increase in temperature, the exhaust gas may be directed though the bypass pipe. The capabilities of the phase-change material may be assessed by measuring its temperature and this signal may be used to control and actuate the valve to direct the exhaust gas as appropriate. 
         [0010]    A variant of this concept is to admit ambient air into the exhaust stream in suitable proportion to maintain the air and exhaust gas mixture near or in a preferred temperature range. This temperature range may be selected so that the heat absorption capabilities of the phase-change material will not be exceeded and no overheating of a thermoelectric module will occur. Preferably the vent valve would be controlled using at least an integral controller and opened and closed with an actuator incorporating a sensor to report actuator position. More preferably, proportional-integral or proportional-integral-derivative control may be employed. 
         [0011]    Other objects and advantages of the invention will be apparent from a description of preferred embodiments which follows in this specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows, in cutaway plan view a vehicle engine, exhaust system and thermoelectric device arranged for efficient extraction of thermal energy from a vehicle exhaust. 
           [0013]      FIG. 2  is a plot of the variation of the dimensionless figure of merit for thermoelectric materials (ZT) as a function of the average of the hot and cold junction temperatures. 
           [0014]      FIG. 3  is a cross-section of one embodiment of an exemplary thermoelectric device and incorporating a plurality of thermoelectric modules in thermal communication with a high temperature phase-change material at their hot junction and a low temperature phase-change material at their cold junction. 
           [0015]      FIG. 4  shows, in partial cutaway, a perspective view of a representative thermoelectric module. 
           [0016]      FIG. 5  shows a longitudinal section of the exemplary thermoelectric device shown in  FIG. 3 . 
           [0017]      FIG. 6  shows in composite a second embodiment of a thermoelectric device incorporating a plurality of thermoelectric modules in thermal communication with a high temperature phase-change material at their hot junction and a low temperature phase-change material at their cold junction. Two configurations of low temperature phase-change materials are illustrated. 
           [0018]      FIG. 7  is a schematic representation of the variation of engine exhaust gas temperature with time, resulting from a series of engine operating modes. Also shown is the resulting temperature of the hot junction of the thermoelectric device which does not employ a phase-change material at the hot junction and which employs an exhaust gas diversion system to protect against overheating of the thermoelectric device. The maximum temperature which can be sustained by the thermoelectric device is indicated as T TE   max  and the corresponding exhaust gas temperature as T EX   max . 
           [0019]      FIG. 8  is a schematic representation of the variation of engine exhaust gas temperature with time resulting from a series of engine operating modes, and is identical to that shown in  FIG. 7 . Also shown is the resulting temperature of the hot junction of the thermoelectric device which employs a phase-change material at the hot junction. The maximum temperature which can be sustained by the thermoelectric module is indicated as T TE   max . 
           [0020]      FIG. 9  shows, in view substantially identical to that of  FIG. 1 , modification of a vehicle exhaust to incorporate a diverter pipe to enable the exhaust gas to by-pass the thermoelectric device. 
           [0021]      FIG. 10  shows in view substantially identical to that of  FIG. 1 , modification of a vehicle exhaust to incorporate a vent pipe to admit diluent air into the vehicle exhaust gas stream to reduce the gas temperature. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  shows, in plan view cutaway, the engine  2  and exhaust system  5  of an automobile  1  as well as a portion of the drive-train by which the engine power is delivered to the rear wheels to propel the vehicle. The exhaust system may comprise major elements and devices such as a catalytic convertor  4 , a resonator  7 , a muffler  8  and a tailpipe  9  each adapted for passage of exhaust from engine  2  and each serially interconnected by a plurality of hollow pipes or pipe segments identified as  3 ,  81 ,  82 ,  84 ,  80  and  6 . Also shown in exhaust system  5  is a thermoelectric device  10 , adapted for extraction of electrical energy from the thermal energy of the engine exhaust stream. 
         [0023]    To enhance catalytic activity on cold engine start, catalytic convertor  4  is preferably positioned as close to the engine as possible. Thermoelectric device  10  is then preferably positioned downstream of but close to, catalytic convertor  6  where the exhaust gas is hottest. This will generally result in the thermoelectric device being located between catalytic convertor  4  and the resonator  7 , if the vehicle is so equipped, or between the catalytic convertor  4  and muffler  8  in vehicles without a resonator or vehicles in which the resonator and muffler are commonly packaged. 
         [0024]    Thermoelectric modules employ two different (but complementary) thermoelectric materials, compactly packaged and with suitable electrical interconnects for producing an electrical current when separated junctions are subjected to a suitable temperature differential. Such power generation thermoelectric modules exploit the Seebeck effect, a phenomenon in which a temperature gradient is applied across a body and as a result an open circuit voltage, co-linear to the temperature gradient, is established. 
         [0025]    Many thermoelectric material combinations may be employed, but semiconductor materials, which offer higher performance than many competing materials, are preferred. For such semiconductor materials the polarity of the voltage with respect to the applied temperature gradient is dependent on the nature of the majority charge carriers. When a temperature difference exists between ends of a thermoelectric element, heated charge carriers (electrons or holes) flow towards the cooler end. Where a pair of dissimilar thermoelectric semiconductor elements, that is a pair consisting of an n-type element and a p-type element, are suitably connected together to form an electrical circuit, a direct current flows in that circuit. 
         [0026]    Several families of semiconductor thermoelectric material compounds have been discovered and developed. Among these compounds, skutterudite (CoSb 3  or Co 4 Sb 12 ) is an example. Cubic Co 4 Sb 12  possesses two voids in a crystallographic unit cell. The voids may be filled, partially or completely, for example, with one or more rare-earth, alkaline-earth, or alkali metal elements. Such partial filling approaches may be used to adjust or tune thermoelectric properties of the crystalline material. The skutterudites display semiconductor properties and distinct compositions can be formed with p-type (hole charge carriers) and n-type (electron charge carriers) conductivity. Many other thermoelectric compositions are known and available. 
         [0027]    The capabilities of thermoelectric materials and modules are generally represented in terms of a figure of merit, Z, defined as: 
         [0000]        Z=σS   2 /κ,
 
         [0000]    where σ is the electrical conductivity, κ is the thermal conductivity, and S is the Seebeck coefficient or thermopower which has units of voltage per (degree) kelvin. More commonly, a dimensionless figure of merit ZT is used, where T is the average of the hot and cold temperatures to which the module is exposed. Greater values of ZT indicate greater thermodynamic efficiency. For thermoelectric modules, as opposed to elements, ZT may also be used as a figure of merit provided the two materials of the couple have similar Z values. 
         [0028]    As shown in  FIG. 2 , a plot of ZT versus temperature for a number of thermoelectric materials, the efficiency with which thermoelectric materials convert thermal energy to electrical energy depends both on the material and its temperature. Thus, optimal energy conversion results when the thermoelectric material exhibits its maximum value of ZT at the operating temperature of the module. As shown in  FIG. 2 , partially-filled p-type and n-type doped skutterudites based on the composition Co 4 Sb 12  and containing varying proportions of Ba, La and Yb exhibit a combined ZT of about 1.2 between about 800 and 900 K, a typical exhaust gas temperature range for both spark-ignition and compression-ignition engines. Thus, the partially-filled Co 4 Sb 12  doped skutterudites offer superior performance over some alternative thermoelectric materials including Bi 2 Te 3 , PbTe and SiGe whose figure of merit, ZT is also shown in  FIG. 2 . The anticipated operating temperature range of the device, however, encompasses the temperature of about 850 K at which Co 4 Sb 12  may decompose. It is therefore mandatory that the operating temperature of Co 4 Sb 12 -based skutterudite thermoelectric materials be well controlled. 
         [0029]    A section through a representative suitable device  10  for extracting electrical energy from engine exhaust gases is shown in  FIG. 3 . Exhaust gas  12 , contained by inner tube surfaces  13  flows in the interior of thin-walled octagonal tube  14  having a center  11 . Thin wall octagonal tubes  14 ,  18  and thin wall round tubes  30  and  38  are arranged in spaced-apart, nested relation and have centers generally aligned and coincident with center  11 . Thin-walled octagonal tube  18  is located in spaced-apart relation to tube  14  and the gap between outer wall  15  of tube  14  and inner wall  17  of tube  18  is filled with a high-temperature phase-change material  16 . A plurality of thermoelectric modules  50  are positioned between thin-walled octagonal tube  18  and thin-walled round tube  30 . 
         [0030]    The temperature of the exhaust gas exiting the catalytic converter of a diesel engine may range from about 190° C. to about 500° C. in normal operation, depending on load. As will be discussed more fully later, the role of the high-temperature phase-change material  16  is to moderate or damp these changes in exhaust temperature so that the hot side of thermoelectric module  50  is maintained at a more constant operating temperature. 
         [0031]    As best shown at  FIG. 4 , each thermoelectric module  50  comprises a plurality of dissimilar thermoelectric elements  22 ,  22 ′, here shown as semiconducting elements, where  22  represents a p-type element and  22 ′ represents an n-type element. The individual elements  22  and  22 ′ are electrically connected one to another by a plurality of electrical interconnects  24  and are mounted between first electrically non-conducting mounting plate  20  and second electrically non-conducting mounting plate  26 . Exposure of mounting plate  20  to an elevated temperature in conjunction with exposure of mounting plate  26  to a lower temperature enables heat flow  42  and results in module  50  generating electrical energy accessible through connectors  44  and  46 . Filled or partially-filled skutterudites based on Co 4 Sb 12  are preferably employed as the semiconducting elements with composition Ce x CoFe 3 Sb 12 , with x taking values of between 0 and 1, being most preferred for the p-type elements, and composition Ba 0.08 La 0.05 Yb 0.04 Co 4 Sb 12.05  being most preferred for the n-type elements. However, other filled or partially filled skutterudites based on Co 4 Sb 12  with ‘guest’ or filler atoms which may be one or more of Na, K, Ca, Sr, Ba, Ce, Pr, Nd, Sm, Eu, Gd, La and Yb in suitable proportion may also be used. 
         [0032]    Returning to  FIG. 3 , the first electrically non-conducting mounting plate  20  is placed in thermal contact with outer wall  19  of octagonal thin walled tube  18 , while the second-electrically conducting mounting plate  26  is embedded in a thermally-conductive, flowable paste  28  in thermal contact with inner wall  29  of round thin-wall tube  30 . Suitable thermally-conductive pastes include Omegatherm OT-201 Thermally Conductive Silicone Paste (Omega Engineering, Inc., Stamford, Conn.) and AOS Non-Silicone XT-3 Heat Sink Compound (AOS Thermal Compounds, Eatontown, N.J.). Typically electrically non-conducting mounting plates  22  and  26  are fabricated of aluminum oxide (Al 2 O 3 ) but other materials such as aluminum nitride (AlN) with better thermal conductance may be employed. 
         [0033]    Thermoelectric modules are generally square in plan view and may be obtained in a range of plan view dimensions from about 10 mm×10 mm to about 60 mm×60 mm, most commonly 30 mm×30 mm and 40 mm×40 mm, with a typical height, including the non-conducting mounting plates, of between 3 and 5 mm. For optimum output, the plan view dimensions of the module should be chosen to be consistent with the dimensions and geometry of the mounting surface. For example the dimension of the mounting surface of the octagonal tube  18 , shown in  FIG. 2 , is indicated as “d”. Modules of plan view dimension “d” or multiple modules whose plan view dimensions sum to “d” will enable placement of the maximum number of modules on the outer surface  19  of thermally-conductive tube wall  18  and thus, promote maximum electricity generation. 
         [0034]    The gap between the outer wall  31  of round thin wall tube  30  and the inner wall  37  of round thin wall tube  38  is filled with low temperature phase-change material  32  within which are positioned channels  34  for passage of coolant  36 . As will be made clearer subsequently, low temperature phase-change material  32  serves to moderate the impact of exhaust gas temperature changes on the cold junction temperature of thermoelectric module  50 . 
         [0035]    It will be appreciated that the specific geometry depicted is exemplary and not limiting, and that other configurations and arrangement may be adopted by the elements shown. For example, coolant channels  34 , and associated coolant  36 , may be omitted and cooling of the low temperature phase-change material accomplished by passage of ambient air over outer surface  39  of round thin wall tube  38 . Further, outer surface  39  of thin wall tube  38  need not be smooth, but could have fins or similar features to promote more efficient heat transfer to ambient air. Similarly, any of tubes  14 ,  18 ,  30  and  38  may exhibit a wide range of cross-sections including round, oval, polygonal or regular polygonal without restriction to the scope of the invention. Likewise a conformable thermally-conductive medium which does not deteriorate at the service temperatures of interest, analogous to the thermally-conductive paste  28  shown applied to inner surface  29  of tube  30  may be applied to outer surface  19  of tube  18 , to enhance thermal contact with mounting surfaces  20  and  26  of thermoelectric module  50 . Thermally conductive paste  28 , or analogous material, may also be used to advantage to fill any microscopic gaps, arising, for example due to surface roughness, between two nominally flat, contacting surfaces. 
         [0036]      FIG. 5  shows a longitudinal cross-sectional view of the thermoelectric device  10  incorporating attachment segments  82  and  84  and indicates how it might be inserted between segments of exhaust pipe  80 ,  81 , shown in fragmentary view, of a vehicle. Details of the method(s) of permanently or releasably securing and attaching segments  82  and  84  to exhaust pipes  80  and  81 , including welding, or bolted-together flange joints, or clamped slip-fit joints, are well known to those skilled in the art and are not shown. 
         [0037]    Thermoelectric device  10  is depicted as having a diverging section  82  and a converging section  84  and a larger inner dimension than that of exhaust pipes  80 ,  81 . Such geometry has several benefits. It enables a greater surface area inner surface  13  of octagonal tube permitting placement of increased area of thermoelectric modules and promoting greater electric power output. Also, it will result in a local reduction of exhaust gas flow rate, permitting longer dwell of the exhaust gas within thin wall tube  14 , and thereby enable more complete extraction of the thermal energy of the exhaust gases for increased electrical output. While preferred, such a configuration is not required. Various relative dimensions of exhaust pipes  80 ,  81  and inner tube  14  may be adopted, responsive to, for example, packaging constraints, without prejudice to the scope of the invention. 
         [0038]    The tube openings are closed substantially by annular endcaps  72  and  74  acting in conjunction with reservoirs  64  and  62 . Endcaps  72  and  74  are individually attached to each of tubes  14 ,  18 ,  30  and  38  by suitable means, for example by brazing, to form individual compartments to contain and segregate the phase-change materials and the thermoelectric modules. Thus, both of the low temperature phase-change material  32  and high temperature phase-change material  16  are contained within their respective volumes. Similarly the region between thin wall tubes  18  and  30 , occupied by thermoelectric modules  50  and thermally conductive paste  28  is sealed. This region may be evacuated or filled with an inert gas to preclude or minimize oxidation or volatilization of the thermoelectric elements  22 ,  22 ′ ( FIG. 4 ) and paste  28 . 
         [0039]    Exhaust gases flow along and through the passage formed by inner surface  13  of the wall of thin wall tube  14  in a direction indicated by arrow  60  and transfer heat through wall  14  to a first phase-change material  16 . The phase-change material is generally contained within the gap volume between tubes  14  and  18  bounded by end caps  72  and  74  but also including enclosed external volume  64 . The phase-change material is typically a solid at ambient temperature of about 25° C. and melts at a temperature comparable to but no greater than the maximum preferred operating temperature of the thermoelectric module. 
         [0040]    Phase-change materials function as efficient, substantially-constant temperature heat reservoirs and typically employ the phase change of a solid to a liquid and vice versa. When a solid is heated to its melting point and melted, it stores energy as latent heat and releases it when it cools and solidifies. Pure materials, congruently melting alloys or compounds, or eutectic compositions of alloys and compounds melt at a fixed temperature. Thus, a phase-change material consisting of one of these species of materials is capable of absorbing and releasing heat without increase or decrease in temperature at its melting point for as long as it contains both solid and liquid. As such it is capable of buffering the thermoelectric module from temporary variations in exhaust gas temperature. Thus, for example, a short-term increase in exhaust gas temperature will convey additional heat to the phase-change material and, in consequence, result in melting of some additional solid material. However, since melting occurs at constant temperature the thermoelectric module will continue to experience a generally constant hot junction temperature. Similarly, a short-term reduction in exhaust temperature will initiate freezing of some of the liquid and release of heat to again maintain the thermoelectric module hot junction at a generally constant temperature. 
         [0041]    At all other temperatures, that is, when the phase-change material is entirely solid or entirely liquid, the phase-change material will behave typically, increasing its temperature as it absorbs heat and decreasing its temperature when it releases heat. Also the quantity of heat which may be stored in a phase-change material will depend on its volume (or mass). Thus, the volume (or mass) of phase-change material should be preselected in accordance with both the magnitude of the anticipated variability in exhaust gas temperature and the duration of such variation. A convenient basis for such a determination may be the exhaust gas temperature variation encountered during dynamometer test procedures conducted for establishing comparative fuel economy data. An example may be the U.S. FTP-72 (Federal Test Procedure) cycle which is intended to be representative of an urban driving cycle. 
         [0042]    For maximum effectiveness and utility in this application, the phase change temperature of the selected material should closely correspond to the desired operating temperature of the thermoelectric device. Exemplary materials and binary alloys which melt at temperatures of between about 350° C. and 415° C. and would therefore be compatible with the filled skutterudite thermoelectric compositions mentioned earlier include (Melting Points in parentheses): Al 0.33 Zn 0.67  (382° C); Ca 0.22 Zn 0.78  (385° C); Ba 0.65 Mg 0.35  (358° C); Cu 0.14 Sn 0.86  (415° C.); KOH (360° C.); ZnBr 2  (394° C.); and InI (351° C.). It will be appreciated that an additional requirement is that the phase-change material should be compatible with its containment materials and should at least not react with, alloy with, corrode or embrittle these containment materials. 
         [0043]    The volume of phase-change material required will vary based on a number of factors including the engine efficiency, the latent heat/unit volume of the phase-change material and the volume of exhaust gas emitted. Of course the exhaust gas volume will generally also vary with engine capacity. 
         [0044]    As an example, consider a large SUV with a 5.3 L, V-8 engine and a curb weight of about 2700 kilograms. Such a vehicle is expected to generate recoverable exhaust energy of about 350 watts averaged over the entire urban FTP cycle, but generate a peak output of about 750 watts for a maximum of about 180 seconds. Thus, if the thermoelectric system is sized and adapted to accommodate the 350 watt average output, the quantity of phase change material should be selected to temporarily sequester the short-term power excess of about 20 watt-hours (750−350=400 watts excess over 180 seconds= 1/20 hour). If the selected phase-change material is the Cu 0.14 Sn 0.86  alloy which has a heat of fusion of about 20 watt-hours per kilogram, then about 1 kilogram of phase change material would be required. While the specific example cited is illustrative, and not limiting, it will be appreciated that the described procedure may readily be adapted and applied to other vehicles and operating conditions. 
         [0045]    By considering  FIGS. 4 and 5  in conjunction, it is clear that heat transferred through high temperature phase-change material  16  will raise the temperature of the hot side, that is the side corresponding to electrically non-conducting mounting surface  22 , of thermoelectric module  50 . Second electrically non-conducting mounting surface  26 , in good thermal contact with thin wall tube  30  through thermally conductive paste  28  is maintained at low temperature by low temperature phase-change material  32 , here shown as cooled by passage of engine coolant  36  in channels  34  in direction indicated by arrows  35 . Thus, a temperature gradient is established across the individual thermoelectric elements  22  and  22 ′ producing electrical energy which may be extracted by conductors  44 ′ and  46 ′ connected to connectors  44  and  46 . 
         [0046]    An important characteristic of thermoelectric materials is that they possess a low thermal conductivity so that the temperature difference between the hot and cold junctions of the device may be maintained. However, even low thermal conductivity materials will allow passage of some heat and establish a temperature gradient based on a balance between the rate at which heat is conducted by the thermoelectric elements and the rate at which the heat is lost at the cold junction. Generally the rate of heat loss by the cold junction depends on the temperature of the junction and the thermal environment in the immediate vicinity of the junction. Under a stable thermal environment, the temperature of the cold junction may remain relatively stable and a low temperature phase-change material in thermal communication with the low temperature junction may not be necessary. 
         [0047]    However, if the thermal environment is not adequately stable there may be significant variation in cold junction temperature. In this situation it may be therefore preferred to maintain a more stable cold junction by using a second, low temperature phase-change material as a heat sink for the cold junction. 
         [0048]    The low temperature phase-change material is selected for compatibility with the expected operating temperature of the cold junction of the thermoelectric module. In the circulating engine coolant configuration shown, a material undergoing a phase change at about normal engine coolant temperature, typically between 90° C. and 100° C. should be used. Numerous low melting point phase-change materials are available commercially with phase change temperatures from about 10° C. to 100° C. Some examples suitable for use with 90-100° C. coolant include (melting points indicated in parentheses): E89 (89° C.) and E83 (83° C.)—both available from EPS Limited, Yaxley, UK; H 89 (89° C.)—available from TEAP Energy, Wangara Dc, Australia; and RT 90 (90° C.)—available from RubiTherm GmbH, Berlin, Germany. In addition numerous low melting point metallic alloys based on eutectic or near-eutectic binary, ternary or quaternary alloys of bismuth, tin, lead, cadmium and indium are known. The melting points of numerous of these alloys lie in the range of from about 35° C. to 100° C. 
         [0049]    Thus, any of a plurality of available phase-change materials having a phase change temperature substantially equal to the temperature of the low temperature junction may be selected. In turn the temperature of the low temperature junction will be dictated by the temperature of the cooling medium and its ability to extract all the heat conducted through the elements. Thus, the engine coolant may be passed through a separate reservoir to further lower its temperature, or liquid cooling may be dispensed with entirely and forced air cooling employed. These strategies may result in a lower cold junction temperature than achievable using 90-100° C. engine coolant and necessitate a choice of phase-change material which differs from some of the examples cited. 
         [0050]    Like the hot junction phase-change material, cold junction phase-change material  32  ( FIG. 5 ) is fully contained within the gap between thin wall tubes  30  and  38  and bounded on one end by segments of endcap  72  and  74  and reservoir  62 . Channels  34  which are embedded within phase-change material  34  are sealed as they penetrate endcaps  72 ,  74 . 
         [0051]    Reservoirs  62  and  64  serve to accommodate the volume changes attendant on both thermal expansion and, more importantly, on the transformation of the solid phase-change material to liquid. It is preferred that the entire available volume for retention of the phase-change material be filled when the phase-change material is liquid. Thus, reservoirs  62  and  64  will be filled with liquid elevated above the level of the remaining phase-change material. On cooling, as the phase-change material contracts during solidification the liquid stored in reservoirs  62  and  64  will be gravity fed to compensate for the shrinkage. It may be noted that the reservoir is positioned adjacent to the hot incoming exhaust gas and should therefore be the last region to solidify. Also the first region of the phase-change material to solidify is expected to be where the exhaust gas is coolest, that is adjacent to endplate  74 . Thus, reservoirs  62  and  64  are well positioned to feed liquid to compensate for the liquid to solid volume change throughout the solidification process. It will be appreciated that in the absence of such reservoirs the resulting uncompensated volume contraction of the material could lead to shrinkage cavities which would interfere with heat flow through the phase-change material. 
         [0052]    From inspection of  FIG. 5  it is apparent that the thermoelectric modules  50 , in addition to being disposed around tube  14  also extend along the length of the thermoelectric device.  FIG. 6  shows, in composite view, two aspects of a second embodiment of the invention which seeks to further enhance the efficiency of such an extended thermoelectric device. 
         [0053]    The heat stored in the exhaust gas is finite so that as it progresses along the length of tube  14  the exhaust gas will lose heat to the thermoelectric modules and the exhaust gas temperature will progressively decrease. The effect of some degree of temperature decrease on thermoelectric efficiency may be mitigated by the ability of the phase-change material to maintain a constant temperature. However, if the temperature decrease is significant it will exceed the capability of the high temperature phase-change material to moderate it. In this case, the choice of the thermoelectric element material will be a compromise since the ‘peaked’ thermoelectric efficiency versus temperature response will necessarily result in only some portion of the modules operating in their optimum temperature range. To counter this decrease in efficiency, two separate groups of thermoelectric modules  50 ,  50 ′ of different composition may be employed as shown in  FIG. 6 . Preferably each group of thermoelectric modules is selected and positioned in a temperature zone within tube  14  for operation at its maximum efficiency. Further, each group is associated with its individual high temperature phase-change material  16 ,  16 ′ selected for phase change temperature compatibility with the temperature at which the thermoelectric modules operate most efficiently. 
         [0054]    Two configurations of the low temperature junction are shown. In one aspect, a single volume of low temperature phase-change material  32 ′ spans the length of the device and is cooled by a flow of engine coolant  36  contained within channels  34  and flowing in the direction indicated by arrow  35 . In a second aspect, separate volumes, each containing differing low temperature phase-change materials  32 ′ and  32 ″ are associated with and positioned in alignment with each of thermoelectric modules  50  and  50 ′ and their respective high temperature phase-change material volumes  16  and  16 ′. Further, each low temperature phase-change volume is individually cooled by passage of engine coolant. The volume containing low temperature phase-change material  32 ′ is cooled by flow indicated by arrows  135  and entering at inlet  137  and exiting at exit  138 : the volume containing low temperature phase-change material  32 ″ is cooled by a flow indicated by arrows  135 ′entering at inlet  137 ′ and exiting at exit  138 ′. 
         [0055]    The embodiment of  FIG. 6  has been described and illustrated with reference to only two groups of thermoelectric devices. It will, however, be apparent that extension of the embodiment to incorporate yet additional thermoelectric modules and associated phase-change materials may be contemplated. 
         [0056]    The reservoirs for the high temperature phase-change materials differ from that shown in  FIG. 5  and indicated by  62  for the low temperature phase-change material. Reservoirs  66 ,  67  comprise expandable metal bellows  68 , which in an unloaded state are fully collapsed but expand and increase their storage volume in response to volume expansion of the liquid and contract and reduce their storage volume as the liquid solidifies and contracts. The reservoirs are positioned at the hottest location experienced by the respective phase-change material so that any material contained within the reservoir will be the first to melt and the last to solidify for the reasons previously detailed. Reservoir  66  may be positioned external to thin wall tube  14 , but reservoir  67  may be located in the exhaust flow as shown. 
         [0057]    The benefits of coupling phase-change materials with thermo electric devices may be readily appreciated by consideration of  FIGS. 7 and 8  which illustrate the effects of change in engine operating conditions on the operation and performance of thermoelectric devices with ( FIG. 8 ) and without ( FIG. 7 ) the phase-change materials. 
         [0058]    Consider first the behavior when the phase-change materials are absent.  FIG. 7  plots the temperature (T) of the exhaust gases, curve  105 , and the hot junction of the thermoelectric device, curve  115 , with time (t) over periods  102 - 110  representing varying engine operating conditions, starting from a cold engine. Initially, as the engine is started and begins to warm up, the exhaust temperature progressively increases as shown in time period  102 . The temperature of the hot junction of the thermoelectric device, curve  115 , likewise increases but is lower than and lags the temperature of the exhaust gas. When the engine has reached its normal operating temperature and is operated at some steady state, period  104 , the exhaust temperature  105  exhibits a temperature plateau. The thermoelectric device junction temperature  115  similarly shows a plateau but at a lower temperature. 
         [0059]    Also shown on  FIG. 7  are the maximum operating temperature of the thermoelectric device, T TE   max  and the corresponding exhaust temperature, designated as T EX   max . In time period  106  the engine is operated under heavy load, for example hard acceleration, and the exhaust gas temperature rapidly increases and exceeds T EX   max . Continued passage of the exhaust gas over the thermoelectric device would cause its hot junction temperature to exceed T TE   max . To avoid this, the exhaust gas is intercepted and diverted, by methods to be described later, and directed along another path which bypasses the thermoelectric device, so that during time period  112  the thermoelectric device is not exposed to exhaust gas and cools. At the end of time period  106 , the engine condition reverts to an idle condition and during time period  108  the exhaust temperature drops. As it drops below T EX   max , signaling the end of period  112 , the exhaust gas is again directed to flow over and heat the thermoelectric device hot junction so that its temperature begins to rise. In time period  110 , the engine is operated at steady state and maintains a generally constant exhaust gas temperature of less than T EX   max . Correspondingly the thermoelectric device temperature is maintained at a safe operating temperature of less than T TE   max . 
         [0060]      FIG. 8  shows the behavior of a similar thermoelectric module in which both the hot and cold junctions are maintained in thermal contact with an appropriate phase-change material subject to the same progression in engine operating conditions and exhaust gas temperatures illustrated in  FIG. 8 . Comparison of  FIGS. 7 and 8  indicates that the behavior of this configuration, represented by exhaust gas temperature curve  105  and thermoelectric hot junction temperature curve  125 , is substantially equivalent to that of the configuration without the phase-change materials. However, dramatic differences are apparent under the heavy load condition of time period  106 . With the ability of the thermoelectric material to absorb heat isothermally, no increase in thermoelectric junction temperature, curve  125  results during time period  106  despite the rapid increase in exhaust gas temperature manifested by curve  105 . With this absorption of heat by the phase-change material occurring during time period  122 , and represented by downwardly-pointing arrows  121 , it is unnecessary to divert the exhaust gas and maximum output may be derived from the thermoelectric device without concern for overheating the thermoelectric device. During engine idle, time period  108 , and the resultant decrease in exhaust gas temperature, the release of heat from the phase-change material during time period  124  and represented by upwardly-pointing arrows  123 , continues to maintain a generally constant thermoelectric device hot junction temperature. Again this constancy of temperature at the hot junction of the thermoelectric device will promote obtaining maximum power out from the device, in this case, even under low exhaust temperature engine operating regimes. Resumption of steady state operation during time period  110  yields a device response substantially identical to that shown in  FIG. 7  during the same time period. 
         [0061]    In discussion of the temperature profiles shown in  FIG. 7 , it was noted that under some engine operating conditions it may be necessary to divert the exhaust gases to avoid overheating and thereby degrading the thermoelectric device. A method for diverting the gases on demand is illustrated in  FIG. 9  which shows a portion of the exhaust system shown in  FIG. 1  and incorporating modified exhaust tube segments  80 ′ and  81 ′ to which is attached diverter pipe  136 . Diverter pipe  136  blends into tube segments  80 ′ and  81 ′ in a ‘Y’ configuration which minimizes fluid disturbances and minimizes creating additional back-pressure which might interfere with efficient engine operation. At the point of the ‘Y’ comprising diverter pipe  136  and tube  81 ′ is mounted flapper valve  132 , rotating about pivot  130  located at the point of the ‘Y’ and capable of being rotated about pivot  130  along the arc indicated by double-ended arrow arc segment  134 . When flapper valve  132  is positioned with its end at location  137 , the exhaust gas will flow through thermoelectric device  10 ; when flapper valve  132  is positioned with its end at location  139 , the exhaust gas will flow through diverter pipe  136 . Thus, in response to a temperature signal responsive to exhaust gas temperature, measured, for example by thermocouple  140 , a controller (not shown) could actuate, for example by an electric motor (not shown), flapper valve  132  to controllably direct the exhaust gases through thermoelectric device  10  or diverter pipe  136 . It will be appreciated that with this setup the exhaust gas flows through either the diverter pipe  136  or the thermoelectric device  10  so that a simple ON-OFF controller is adequate. When the exhaust gas temperature falls below T EX   max , the exhaust gas flow would be redirected through the thermoelectric device. Some hysteresis may be incorporated to avoid rapid cycling between the ON and OFF conditions. 
         [0062]    The above-described control strategy, responsive only to exhaust gas temperature, is conservative and takes no advantage of the temperature-stabilizing characteristics of the high temperature phase-change material. 
         [0063]    For devices  10  which incorporate phase-change materials it is preferred to employ the temperature of the phase-change material to initiate diversion of the exhaust gas stream and employ the phase-change material to accommodate short-term exhaust gas temperature deviations. Thus, initially, the thermoelectric device will be isolated from the effects of an exhaust gas temperature in excess of T EX   max  by the heat absorbing behavior of the high temperature phase-change material as illustrated in  FIG. 8 . However, the period during which the high temperature phase-change material will be effective in stabilizing the temperature and buffering the thermoelectric modules from exhaust gas temperatures of greater than T EX   max  is necessarily limited by the volume or mass of thermoelectric material used. Under some conditions, for example under sustained heavy load like that experienced when climbing a long steep grade or towing a heavy trailer, the entire volume of phase-change material could melt and thereby be rendered incapable of further heat absorption leading to overheating of the thermoelectric modules. Since, in this circumstance, no change in exhaust gas temperature is contemplated, measurement of the exhaust gas temperature alone cannot be used to trigger diversion of the exhaust stream. Here exhaust gas diversion may predicated on direct measurement of an excessive hot junction temperature of the thermoelectric module. More preferably excessive temperature of the high temperature phase-change material would initiate diversion of the exhaust stream to the diverter pipe. Thermocouples or resistance thermometers (not shown) incorporated within thermoelectric device  10  may be employed to sense these temperatures. 
         [0064]    An alternative approach to guard against excessive thermoelectric module temperatures is illustrated in  FIG. 10 . In this configuration an air inlet  156 , open to ambient air is incorporated in modified exhaust tube segment  81 ″ and the inflow of diluent air is controlled by a flapper valve  152  pivoting about pivot  150  along the arc indicated by double-ended arrow arc segment  154  may be employed to introduce diluent ambient air into the exhaust stream through inlet pipe  156  to reduce the temperature of the exhaust gas. In this configuration when the end of flapper valve  152  is positioned at location  157  no dilution of the exhaust will occur; when the end of flapper valve  152  is positioned at some location intermediate between locations  157  and its maximum setting, shown as  159 , the gas passing through thermoelectric device  10  will be a mixture of exhaust gas and air. In this configuration thermocouple  160  could be used, in conjunction with a controller (not shown) and valve actuator (not shown) to control the flowing gas temperature to approximate the maximum sustainable temperature of the thermoelectric device and employ the phase-change material to ‘fine tune’ the temperature of the device. Again, in implementation of such an approach it will be necessary to ensure that excessive back pressure is not generated. In this approach a more sophisticated controller and additional sensors will be required. The controller should be at least a proportional controller but yet more capable controllers such as proportional-integral (PI) or proportional-integral-derivative (PID) controllers may be employed. It will also be necessary to sense the position of flapper valve  152 , either by a sensor mounted directly on the valve or integrated with the actuator. 
         [0065]    While some practices of the invention have been illustrated, these embodiments are intended to illustrate the invention but not to limit its scope.