Abstract:
A turbine engine includes a turbine driven by hot gas, a compressor rotating with the turbine to generate compressed air, an annular combustor coaxial with the turbine to combust fuel and compressed air to generate the hot gas, and an annular recuperator to recover heat from the turbine exhaust gas and heat the compressed air for combustion. The annular recuperator surrounds the turbine and includes two contiguous parts made from two materials having different thermal properties and joined to one another to form a single annular structure. One recuperator part is formed from a high-temperature material having a high thermal limit for exposure to high-temperature turbine exhaust gas, and the other recuperator part is formed from a material having a lower thermal limit than the high-temperature material for exposure to reduced-temperature turbine exhaust gas.

Description:
RELATED APPLICATIONS  
       [0001]    This patent application claims the priority of provisional patent application Ser. No. 60/260,964, filed Jan. 10, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    A turbogenerator generally includes, a turbine, an electrical generator, a compressor, a recuperator, and a combustor. The compressor impeller, the turbine impeller, and the generator rotor are mounted on a common shaft that is supported by journal and thrust bearings. Air is continuously compressed in the compressor, mixed with fuel, and injected into the combustor through fuel injectors. The air/fuel mixture is ignited in the combustor, such as by electrical spark, hot surface ignition, or catalyst. The heat energy released by the combustion reaction expands the combustion gas, which then impinges upon and rotates the turbine impeller and common shaft, thereby converting the heat energy released by the combustion reaction into rotary mechanical energy for driving the compressor impeller and the electrical generator rotor. After the combustion gas has passed through the turbine, it is typically vented to the atmosphere as exhaust gas.  
           [0003]    To increase overall system efficiency, the turbine engine is often recuperated, that is, a heat exchanger (recuperator) is utilized to recover waste heat from the exhaust gas prior to venting it to the atmosphere and to transfer the recovered heat to the compressed air prior to injection into the combustor. Employing heat recuperation can significantly reduce the amount of fuel required to sustain the combustion process in the combustor.  
           [0004]    The efficiency of a turbogenerator system increases at high temperatures. To withstand high temperatures, the recuperator must be fabricated from materials having high thermal limits. Such materials are generally expensive and therefore recuperators employing such materials may be impractical from an economic point of view. Therefore, what is now needed is a turbine engine with an improved recuperator that withstands high temperatures and that may be fabricated economically.  
         SUMMARY OF THE INVENTION  
         [0005]    In a first aspect, the present invention provides a turbine engine comprising a turbine disposed for rotation about an axis; a compressor coupled to the turbine for rotating therewith to generate compressed air; an annular combustor disposed coaxially with the turbine for combusting fuel and the compressed air to generate hot gas for rotating the turbine; and a plurality of cold cells annularly disposed about the turbine for conducting the compressed air from the compressor to the combustor, at least one of the cold cells including a hot part in fluid communication with the combustor and formed from a first material having a first temperature limit, and further including a cold part joined to the hot part and in fluid communication with the compressor, the cold part formed from a second material having a second temperature limit lower than the first temperature limit.  
           [0006]    In another aspect, the invention provides a method of operating a turbine engine, comprising driving a turbine rotationally with hot gas; coupling a compressor to the turbine to rotate therewith and generate compressed air; combusting fuel and the compressed air in an annular combustor disposed coaxially with the turbine to generate the hot gas for driving the turbine; and conducting the compressed air from the compressor to the combustor through a plurality of cold cells annularly disposed around the turbine, at least one of the cold cells including a hot part in fluid communication with the combustor and formed from a first material having a first temperature limit, and further including a cold part joined to the hot part and in fluid communication with the compressor, the cold part formed from a second material having a second temperature limit lower than the first temperature limit.  
           [0007]    In a further aspect, the cold part is connected to the hot part to form a continuous flow path for the compressed air from at least one cold inlet in the cold part in fluid communication with the compressor to at least one hot outlet in the hot part in fluid communication with the combustor. The plurality of cold cells may be disposed to define a plurality of hot cells therebetween for conducting exhaust gas from a turbine outlet to an exhaust vent to transfer thermal energy from exhaust gas flowing in the hot cells to compressed air flowing in the cold cells.  
           [0008]    In a still further aspect, the first material may comprise a single crystal metallic microstructure, or a superalloy comprising at least one of nickel and cobalt. Additionally, the second material may comprise an equiaxed metallic microstructure or stainless steel. The first material may also comprises a low-temperature material with a layer of high-temperature material deposited thereupon, such as a sintered ceramic. The first material may additionally comprise a layer of catalytic material deposited on a surface of the hot part in contact with the exhaust gas.  
           [0009]    In another aspect, the hot part may be joined to the cold part using any single one or combination of plasma welding, ultrasonic welding, friction welding, fusion welding, forge welding and laser beam welding. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1A is a perspective view, partially in section, of an integrated turbogenerator system;  
         [0011]    [0011]FIG. 1B is a magnified perspective view, partially in section, of the motor/generator portion of the integrated turbogenerator of FIG. 1A;  
         [0012]    [0012]FIG. 1C is an end view, from the motor/generator end, of the integrated turbogenerator of FIG. 1A;  
         [0013]    [0013]FIG. 1D is a magnified perspective view, partially in section, of the combustor-turbine exhaust portion of the integrated turbogenerator of FIG. 1A;  
         [0014]    [0014]FIG. 1E is a magnified perspective view, partially in section, of the compressor-turbine portion of the integrated turbogenerator of FIG. 1A;  
         [0015]    [0015]FIG. 2 is a block diagram schematic of a turbogenerator system as shown in FIGS.  1 A-E including a power controller having decoupled rotor speed, operating temperature, and DC bus voltage control loops;  
         [0016]    [0016]FIG. 3 is a diagram showing in cross-section the spacing and placement of cold and hot cells in the annular recuperator that may be used in the turbogenerator system of FIGS.  1 A-E;  
         [0017]    [0017]FIG. 4 is an enlarged view of the cold and hot cells in the annular recuperator of FIG. 3;  
         [0018]    [0018]FIG. 5 is a perspective view showing the joining of two heat transfer plates to form a cold cell as depicted in FIG. 4;  
         [0019]    [0019]FIG. 6 is a front view of a heat transfer plate of a cold cell depicted in FIG. 3 showing flow paths of compressed air and turbine exhaust gas;  
         [0020]    [0020]FIG. 7 illustrates joining of hot and cold parts of a heat transfer plate according to the invention for use with the turbogenerator system of FIGS.  1 A-E; and  
         [0021]    [0021]FIG. 8 illustrates welding of hot and cold parts of a heat transfer plate using a laser beam in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    With reference to FIG. 1A, an integrated turbogenerator  1  according to the present disclosure generally includes motor/generator section  10  and compressor-turbine section  30 . Compressor-turbine section  30  forms the engine of turbogenerator  1  and includes exterior can  32 , compressor  40 , combustor  50  and turbine  70 . A recuperator  90  may be optionally included.  
         [0023]    Referring now to FIG. 1B and FIG. 1C, in a currently preferred embodiment of the present disclosure, motor/generator section  10  may be a permanent magnet motor generator having a permanent magnet rotor or sleeve  12 . Any other suitable type of motor generator may also be used. Permanent magnet rotor or sleeve  12  may contain a permanent magnet  12 M. Permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein are rotatably supported within permanent magnet motor/generator stator  14 . Preferably, one or more compliant foil, fluid film, radial, or journal bearings  15 A and  15 B rotatably support permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein. All bearings, thrust, radial or journal bearings, in turbogenerator  1  may be fluid film bearings or compliant foil bearings. Motor/generator housing  16  encloses stator heat exchanger  17  having a plurality of radially extending stator cooling fins  18 . Stator cooling fins  18  connect to or form part of stator  14  and extend into annular space  10 A between motor/generator housing  16  and stator  14 . Wire windings  14 W exist on permanent magnet motor/generator stator  14 .  
         [0024]    Referring now to FIG. 1D, combustor  50  may include cylindrical inner wall  52  and cylindrical outer wall  54 . Cylindrical outer wall  54  may also include air inlets  55 . Cylindrical walls  52  and  54  define an annular interior space  50 S in combustor  50  defining an axis  50 A. Combustor  50  includes a generally annular wall  56  further defining one axial end of the annular interior space of combustor  50 . Associated with combustor  50  may be one or more fuel injector inlets  58  to accommodate fuel injectors which receive fuel from fuel control element  50 P as shown in FIG. 2, and inject fuel or a fuel air mixture to interior of  50 S combustor  50 . Inner cylindrical surface  53  is interior to cylindrical inner wall  52  and forms exhaust duct  59  for turbine  70 .  
         [0025]    Turbine  70  may include turbine wheel  72 . An end of combustor  50  opposite annular wall  56  further defines an aperture  71  in turbine  70  exposed to turbine wheel  72 . Bearing rotor  74  may include a radially extending thrust bearing portion, bearing rotor thrust disk  78 , constrained by bilateral thrust bearings  78 A and  78 B. Bearing rotor  74  may be rotatably supported by one or more journal bearings  75  within center bearing housing  79 . Bearing rotor thrust disk  78  at the compressor end of bearing rotor  74  is rotatably supported preferably by a bilateral thrust bearing  78 A and  78 B. Journal or radial bearing  75  and thrust bearings  78 A and  78 B may be fluid film or foil bearings.  
         [0026]    Turbine wheel  72 , bearing rotor  74  and compressor impeller  42  may be mechanically constrained by tie bolt  74 B, or other suitable technique, to rotate when turbine wheel  72  rotates. Mechanical link  76  mechanically constrains compressor impeller  42  to permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein causing permanent magnet rotor or sleeve  12  and the permanent magnet disposed therein to rotate when compressor impeller  42  rotates.  
         [0027]    Referring now to FIG. 1E, compressor  40  may include compressor impeller  42  and compressor impeller housing  44 . Recuperator  90  may have an annular shape defined by cylindrical recuperator inner wall  92  and cylindrical recuperator outer wall  94 . Recuperator  90  contains internal passages for gas flow, one set of passages, passages  33  connecting from compressor  40  to combustor  50 , and one set of passages, passages  97 , connecting from turbine exhaust  80  to turbogenerator exhaust output  2 .  
         [0028]    Referring again to FIG. 1B and FIG. 1C, in operation, air flows into primary inlet  20  and divides into compressor air  22  and motor/generator cooling air  24 . Motor/generator cooling air  24  flows into annular space  10 A between motor/generator housing  16  and permanent magnet motor/generator stator  14  along flow path  24 A. Heat is exchanged from stator cooling fins  18  to generator cooling air  24  in flow path  24 A, thereby cooling stator cooling fins  18  and stator  14  and forming heated air  24 B. Warm stator cooling air  24 B exits stator heat exchanger  17  into stator cavity  25  where it further divides into stator return cooling air  27  and rotor cooling air  28 . Rotor cooling air  28  passes around stator end  13 A and travels along rotor or sleeve  12 . Stator return cooling air  27  enters one or more cooling ducts  14 D and is conducted through stator  14  to provide further cooling. Stator return cooling air  27  and rotor cooling air  28  rejoin in stator cavity  29  and are drawn out of the motor/generator  10  by exhaust fan  11  which is connected to rotor or sleeve  12  and rotates with rotor or sleeve  12 . Exhaust air  27 B is conducted away from primary air inlet  20  by duct  10 .  
         [0029]    Referring again to FIG. 1E, compressor  40  receives compressor air  22 . Compressor impeller  42  compresses compressor air  22  and forces compressed gas  22 C to flow into a set of passages  33  in recuperator  90  connecting compressor  40  to combustor  50 . In passages  33  in recuperator  90 , heat is exchanged from walls  98  of recuperator  90  to compressed gas  22 C. As shown in FIG. 1E, heated compressed gas  22 H flows out of recuperator  90  to space  35  between cylindrical inner surface  82  of turbine exhaust  80  and cylindrical outer wall  54  of combustor  50 . Heated compressed gas  22 H may flow into combustor  54  through sidewall ports  55  or main inlet  57 . Fuel (not shown) may be reacted in combustor  50 , converting chemically stored energy to heat. Hot compressed gas  51  in combustor  50  flows through turbine  70  forcing turbine wheel  72  to rotate. Movement of surfaces of turbine wheel  72  away from gas molecules partially cools and decompresses gas  51 D moving through turbine  70 . Turbine  70  is designed so that exhaust gas  107  flowing from combustor  50  through turbine  70  enters cylindrical passage  59 . Partially cooled and decompressed gas in cylindrical passage  59  flows axially in a direction away from permanent magnet motor/generator section  10 , and then radially outward, and then axially in a direction toward permanent magnet motor/generator section  10  to passages  97  of recuperator  90 , as indicated by gas flow arrows  108  and  109  respectively.  
         [0030]    In an alternate embodiment of the present disclosure, low pressure catalytic reactor  80 A may be included between fuel injector inlets  58  and recuperator  90 . Low pressure catalytic reactor  80 A may include internal surfaces (not shown) having catalytic material (e.g., Pd or Pt, not shown) disposed on them. Low pressure catalytic reactor  80 A may have a generally annular shape defined by cylindrical inner surface  82  and cylindrical low pressure outer surface  84 . Unreacted and incompletely reacted hydrocarbons in gas in low pressure catalytic reactor  80 A react to convert chemically stored energy into additional heat, and to lower concentrations of partial reaction products, such as harmful emissions including nitrous oxides (NOx).  
         [0031]    Gas  110  flows through passages  97  in recuperator  90  connecting from turbine exhaust  80  or catalytic reactor  80 A to turbogenerator exhaust output  2 , as indicated by gas flow arrow  112 , and then exhausts from turbogenerator  1 , as indicated by gas flow arrow  113 . Gas flowing through passages  97  in recuperator  90  connecting from turbine exhaust  80  to outside of turbogenerator  1  exchanges heat to walls  98  of recuperator  90 . Walls  98  of recuperator  90  heated by gas flowing from turbine exhaust  80  exchange heat to gas  22 C flowing in recuperator  90  from compressor  40  to combustor  50 .  
         [0032]    Turbogenerator  1  may also include various electrical sensor and control lines for providing feedback to power controller  201  and for receiving and implementing control signals as shown in FIG. 2.  
         [0033]    The integrated turbogenerator disclosed above is exemplary. Several alternative structural embodiments are disclosed below. In one alternative embodiment, air  22  may be replaced by a gaseous fuel mixture. In this embodiment, fuel injectors may not be necessary. This embodiment may include an air and fuel mixer upstream of compressor  40 .  
         [0034]    In another alternative embodiment, fuel may be conducted directly to compressor  40 , for example by a fuel conduit connecting to compressor impeller housing  44 . Fuel and air may be mixed by action of the compressor impeller  42 . In this embodiment, fuel injectors may not be necessary.  
         [0035]    In another alternative embodiment, combustor  50  may be a catalytic combustor.  
         [0036]    In still another alternative embodiment, geometric relationships and structures of components may differ from those shown in FIG. 1A. Permanent magnet motor/generator section  10  and compressor/combustor section  30  may have low pressure catalytic reactor  80 A outside of annular recuperator  90 , and may have recuperator  90  outside of low pressure catalytic reactor  80 A. Low pressure catalytic reactor  80 A may be disposed at least partially in cylindrical passage  59 , or in a passage of any shape confined by an inner wall of combustor  50 . Combustor  50  and low pressure catalytic reactor  80 A may be substantially or completely enclosed with an interior space formed by a generally annularly shaped recuperator  90 , or a recuperator  90  shaped to substantially enclose both combustor  50  and low pressure catalytic reactor  80 A on all but one face.  
         [0037]    An integrated turbogenerator is a turbogenerator in which the turbine, compressor, and generator are all constrained to rotate based upon rotation of the shaft to which the turbine is connected. The methods and apparatus disclosed herein are preferably but not necessarily used in connection with a turbogenerator, and preferably but not necessarily used in connection with an integrated turbogenerator.  
         [0038]    Referring now to FIG. 2, a preferred embodiment is shown in which a turbogenerator system  200  includes power controller  201  which has three substantially decoupled control loops for controlling (1) rotary speed, (2) temperature, and (3) DC bus voltage. A more detailed description of an appropriate power controller is disclosed in U.S. patent application Ser. No. 09/207,817, filed Dec. 8, 1998 in the names of Gilbreth, Wacknov and Wall, and assigned to the assignee of the present application which is incorporated herein in its entirety by this reference.  
         [0039]    Referring still to FIG. 2, turbogenerator system  200  includes integrated turbogenerator  1  and power controller  201 . Power controller  201  includes three decoupled or independent control loops.  
         [0040]    A first control loop, temperature control loop  228 , regulates a temperature related to the desired operating temperature of primary combustor  50  to a set point, by varying fuel flow from fuel control element  50 P to primary combustor  50 . Temperature controller  228 C receives a temperature set point, T*, from temperature set point source  232 , and receives a measured temperature from temperature sensor  226 S connected to measured temperature line  226 . Temperature controller  228 C generates and transmits over fuel control signal line  230  to fuel pump  50 P a fuel control signal for controlling the amount of fuel supplied by fuel pump  50 P to primary combustor  50  to an amount intended to result in a desired operating temperature in primary combustor  50 . Temperature sensor  226 S may directly measure the temperature in primary combustor  50  or may measure a temperature of an element or area from which the temperature in the primary combustor  50  may be inferred.  
         [0041]    A second control loop, speed control loop  216 , controls speed of the shaft common to the turbine  70 , compressor  40 , and motor/generator  10 , hereafter referred to as the common shaft, by varying torque applied by the motor generator to the common shaft. Torque applied by the motor generator to the common shaft depends upon power or current drawn from or pumped into windings of motor/generator  10 . Bi-directional generator power converter  202  is controlled by rotor speed controller  216 C to transmit power or current in or out of motor/generator  10 , as indicated by bi-directional arrow  242 . Rotor speed controller  216  receives the rotary speed signal from measured speed line  220  and a rotary speed set point signal from a rotary speed set point source  218 . Rotary speed controller  216 C generates and transmits to generator power converter  202  a power conversion control signal on line  222  controlling generator power converter  202 &#39;s transfer of power or current between AC lines  203  (i.e., from motor/generator  10 ) and DC bus  204 . Rotary speed set point source  218  may convert to the rotary speed set point a power set point P* received from power set point source  224 .  
         [0042]    A third control loop, voltage control loop  234 , controls bus voltage on DC bus  204  to a set point by transferring power or voltage between DC bus  204  and any of (1) Load/Grid  208  and/or (2) energy storage device  210 , and/or (3) by transferring power or voltage from DC bus  204  to dynamic brake resistor  214 . A sensor measures voltage DC bus  204  and transmits a measured voltage signal over measured voltage line  236 . Bus voltage controller  234 C receives the measured voltage signal from voltage line  236  and a voltage set point signal V* from voltage set point source  238 . Bus voltage controller  234 C generates and transmits signals to bi-directional load power converter  206  and bi-directional battery power converter  212  controlling their transmission of power or voltage between DC bus  204 , load/grid  208 , and energy storage device  210 , respectively. In addition, bus voltage controller  234  transmits a control signal to control connection of dynamic brake resistor  214  to DC bus  204 .  
         [0043]    Power controller  201  regulates temperature to a set point by varying fuel flow, adds or removes power or current to motor/generator  10  under control of generator power converter  202  to control rotor speed to a set point as indicated by bi-directional arrow  242 , and controls bus voltage to a set point by (1) applying or removing power from DC bus  204  under the control of load power converter  206  as indicated by bi-directional arrow  244 , (2) applying or removing power from energy storage device  210  under the control of battery power converter  212 , and (3) by removing power from DC bus  204  by modulating the connection of dynamic brake resistor  214  to DC bus  204 .  
         [0044]    Referring again to FIG. 1E, recuperator  90  receives, channels, and transfers heat from hot fluid stream  110  (formed by the turbine exhaust gas) to cold fluid stream  22 C (formed by the compressed air from the compressor). Ideally, recuperator  90  maximizes the thermal intermixing of the two streams while keeping the streams physically separate and also minimizing the flow resistance encountered by the two streams. Recuperator  90  may include a plurality of low temperature, high pressure “cold” cells disposed adjacent to high temperature, low pressure “hot” cells in an alternating pattern repeated over the entire diameter of the recuperator.  
         [0045]    Referring to FIG. 3 and FIG. 4, recuperator core  41  is shown in greater detail as formed of alternating cold cells  380  and hot cells  382  disposed in an annular pattern. Hot cells  382  may be flow channels defined by neighboring cold cells  380 , outer diameter  384  as defined by annular housing  340  of annular recuperator  90 , and inner diameter  386 . Cold cells  380  may be formed with a generally rectangular cross section and thereafter may be molded into a generally arcuate configuration. This arcuate configuration allows both cold and hot cells to maintain a relatively constant cross section along their radial length. The upper edges of cold cells  380  abut annular housing  340  but are typically not connected to the housing so as to be able to move with respect to the housing as may be necessitated by thermal expansion and contraction.  
         [0046]    With reference now to FIG. 5 and FIG. 6, a typical high-pressure cold cell  380  is shown to include two heat transfer plates  150  spaced apart from each other. The plate  150  may have a heat transfer surface  152  and a lip  156  may extend along the entire perimeter of the heat transfer surface  152 . The two heat transfer surfaces  152  of a cold cell  380  may be spaced apart by having lip  156  of one heat transfer surface  152  abutting lip  156  of other heat transfer surface  152 . The cold cell  380  may be formed by welding lips  156  of the two heat transfer plates  150  along the edge of the cold cell  380 . The cold cell  380  has a generally trapezoidal shape defined by a longer inner edge  160 , a shorter outer edge  162 , and angled edges  163  and  164  extending between the inner and outer edge. The lip  156  is interrupted at the two opposite ends of the inner edge  160  to form air inlet  170  and air outlet  172 .  
         [0047]    As described previously, cool compressed air enters the air inlet  170 , is heated while flowing along the axial length of the cell  150 , and exits as hot air through outlet  172 . To encourage the even distribution of air flow, flow channels are defined within the cell  380 , including directional channels  174  and convolute channels  175 . The purpose of directional channels  174  is to radially distribute air flow between inner edge  160  and outer edge  162 . Convolute channels  175  are designed to maximize thermal intermixing of compressed air  22 C with counter-flowing exhaust gas  110 , and extend from both sides of each heat transfer surface. Angled edges  163  and  164  serve to direct the flow of air and aid in maintaining relatively constant velocity throughout the cell.  
         [0048]    As may be appreciated from FIG. 5 and FIG. 6 and the previous description, the part of cell  380  and heat transfer plates  150  bordered by angled edge  163  is in contact with hot exhaust gas  102  from the turbine and relatively hot compressed air  114 , whereas the part of the cell  380  bordered by angled edge  164  is in contact with cold exhaust gas  104  (that has transferred a significant amount of its heat energy to counterflowing cold compressed air  115 ) and cold compressed air  115 . Thus, there is a significant difference in the temperature of the cell  380  and heat transfer plates  150  near cold air inlet  170  and the temperature near hot air outlet  172 . Hot exhaust gas  102  is generally limited in conventional stainless steel recuperators to a temperature of no more than 1200° F. due to the thermal limit of the stainless steel. However, the exhaust gas temperature may decrease significantly along the axis of the recuperator from the hot exhaust side to the cold exhaust side, and thus only the recuperator material near the hot exhaust side would typically experience temperatures near the thermal limit.  
         [0049]    An embodiment of the invention takes advantage of this decreasing temperature profile along the length of the recuperator and provides a recuperator fabricated from different materials having different thermal limits at various axial locations along the axial length of the recuperator. The materials may be selected in accordance with a predicted temperature profile for the recuperator, such as the predicted temperature profile for full-load operation of the turbogenerator at the highest rated TET. Generally, a high-temperature material, that is, a material having a high thermal limit, is used to fabricate the part of cold cell  380  near the inlet of hot exhaust  102  , and a lower-cost low-temperature material having a lower thermal limit is used to fabricate the part of cold cell  380  near the outlet of cold exhaust  104 . Controller  200  may be used to control the TET not to exceed a predetermined temperature and ensure the recuperator operates within allowable temperature limits.  
         [0050]    Thus, and with continued reference to FIG. 6, in one embodiment of the invention each heat transfer plate  150  of cold cell  380  has high-temperature part  240  formed from a high-temperature material, and low-temperature part  250  formed from a low-temperature material. The high-temperature part may be joined to the low-temperature part along seam  260  by any practicable method as known to those skilled in the art. The joining of the high-temperature and the low-temperature parts provides a continuous flow path for the exhaust gas from hot exhaust  102  to cold exhaust  104 . The seam  260  may be located at an axially displaced location sufficiently removed from the hot exhaust inlet to ensure that it will not experience temperatures in excess of the low-temperature material thermal limit. The portion of the heat transfer plate  150  that experiences high temperatures may be limited to a reduced area near hot exhaust  102  defined by a curvilinear boundary shown in FIG. 6 as line  259 . However, for ease of fabrication, a generally straight seam  260  may be provided. The size of hot part  240  may be greater than that required for normal operations to allow for operational temperature fluctuations.  
         [0051]    The high-temperature material may be any material known to those skilled in the art to possess the required physical properties, and in one embodiment include nickel and/or cobalt based superalloys. In another embodiment, the high-temperature material may have a single crystal metallic microstructure. A single crystal is a monocrystalline structure in which the casting is one single grain (crystal). The low-temperature material may have an equiaxed metallic microstructure. An equiaxed microstructure is a polycrystalline structure in which all of the grains (crystals) in a casting have approximately the same dimensions in all directions.  
         [0052]    Methods for forming the seam may include plasma welding, ultrasonic welding, friction welding, fusion welding, forge welding and/or laser beam welding. As shown in FIG. 7, the actual joint at which the seam  260  is formed may be fabricated by adjoining the edge of low-temperature part  250  to the edge of high-temperature part  240  and welding the edges together. In the embodiment of FIG. 7, the adjoining edges have reduced thickness to aid the welding process by presenting reduced areas to adjoin and weld together. This configuration would result in a channel  270  extending the length of the seam  260  on both sides of the heat transfer surface  152 . The channel  270  may be filled with a suitably high thermal-limit material  280  to preclude any pressure drop in the gases flowing through the cold and hot cells of the recuperator.  
         [0053]    With reference to FIG. 8, in another embodiment, the seam  260  would be formed by overlaying the edge of one of the parts (in FIG. 8, low-temperature part  250 ) over the edge of the other part (in FIG. 8, high-temperature part  240 ) and ‘spiking’ seam  260  through both edges by laser beam welding or a similar method. The width and depth of the spike would be dependent upon the power deposited by the weld and the period of time over which the weld is applied, and could therefore be generally formed in a desired shape. Thus, the spike (i.e. mass of low-temperature and high-temperature material melted together by the heat applied by the weld) may extend completely through both the low-temperature part  250  and the high-temperature part  240  as shown in FIG. 8, or alternatively may extend only partially through the bottom part (e.g. the high-temperature part in FIG. 8).  
         [0054]    Seam  260  may be formed on each individual heat transfer plate  150  after low-temperature part  250  and high-temperature part  240  have been formed into the desired configuration. Alternatively, the seam may be formed between two continuous sheets of low-temperature and high-temperature material, respectively, prior to shaping individual heat transfer plates  150 . In yet another embodiment, seam  260  may be formed by forge welding two such continuous sheets of low-temperature and high-temperature material prior to shaping individual heat transfer plates  150 . This structure would also result in a channel  270  extending the length of seam  260  on both sides of heat transfer surface  152 . Channel  270  may be filled with a suitably high thermal-limit material  280  to preclude any pressure drop in the gases flowing through the cold and hot cells of the recuperator.  
         [0055]    Although the embodiments described above focus upon an annular recuperator, those skilled in the art will understand that the invention is equally applicable to any recuperator, including primary surface recuperators, spiral wound recuperators, plate fin recuperators, and box type recuperators. The invention is also not limited to counter-flowing recuperators, but may be applied to recuperators in which the hot and cold streams flow in the same direction. Furthermore, the invention contemplates the use of any type of low- and high-temperature material that offers the requisite characteristics, including thermal limits, cost, ease of fabrication, compatibility with one another, and ability to join together with sufficient strength.  
         [0056]    Materials that may be utilized according to the invention include layered materials, e.g. sheets of various materials bonded to each other such as, for example, a low-temperature material with a layer of high-temperature material (e.g. a sintered ceramic) deposited on it. In another embodiment of the invention, high-temperature part  240  may include a layer of catalytic material deposited thereon to react with any unburned fuel or other hydrocarbons present in hot exhaust gas  102 , thus providing in essence a secondary catalytic reactor for turbogenerator system  1 . Further details of such a secondary catalytic reactor may be found in co-pending and co-owned U.S. patent application Ser. No. 09/933,663 filed on Aug. 22, 2001, entitled “INTEGRATED TURBINE POWER GENERATION SYSTEM HAVING LOW PRESSURE SUPPLEMENTAL CATALYTIC REACTOR” and incorporated herein in its entirety by reference thereto.  
         [0057]    Having now described the invention in accordance with the requirements of the patent statutes, those skilled in the art will understand how to make changes and modifications to the embodiments disclosed to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention, as defined and limited solely by the following claims.