Abstract:
A thermal energy harvesting system employs a hot flow conduit and a cold flow conduit with a flow routing device interruptibly interconnecting the hot flow conduit and cold flow conduit with a flow casing. At least one shape memory actuator (SMA) tube is in fluid contact with the flow casing and fixed at a first end. The flow routing device sequentially supplies hot flow from the hot flow conduit and cold flow from the cold flow conduit inducing rotation of the at least one SMA tube at a second end. A generator or alternator is operably connected to the second end of the at least one SMA tube.

Description:
REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application is with a continuation-in-part of application Ser. No. 14/566,376 filed on Dec. 10, 2014 entitled SCALABLE MULTI-ELEMENT SHAPE MEMORY ALLOY ROTARY MOTOR having a common assignee with the present application, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND INFORMATION 
       [0002]    Field 
         [0003]    Embodiments of the disclosure relate generally to thermal energy harvesting for electrical power and more particularly to a system employing alternating flow control of hot and cold air streams through a shape memory alloy (SMA) tube providing rotation to an output shaft connected to a generator. 
         [0004]    Background 
         [0005]    There is a growing need for electrical power on modern aircraft. Everything from flight control surfaces to entertainment systems require more and more electrical power. At the same time, in the face of increasing demands for reduced emissions, engines are becoming more and more efficient and therefore have much less excess power available to be used for generating electrical power. Additionally the large number of electrical systems on modern aircraft also requires a significant amount of heavy and complex wiring. Existing methods for supplying energy to electrically powered systems, such as the environmental control system (ECS), rely exclusively on power generated by the engines or auxiliary power unit (APU). Ground power may be used for ground operations, however in flight electrical power is provided by the increasingly taxed engines. 
         [0006]    Shape memory actuator (SMA) systems have been employed in the prior art for energy harvesting by converting thermal energy to motion which is then employed for electrical energy generation. Typical prior art systems usually rely on two separated and stationary hot and cold zones and use a complex kinematic device that is powered by the shape change of the SMA component. As an SMA element heats up it literally moves itself out of the hot zone and into the cool zone, as it cools the subsequent shape change moves it back to the hot zone where the process is repeated. These methods often produce large motions, but very little work and only small amounts of electrical power are generated. 
         [0007]    It is therefore desirable to provide an efficient system for energy harvesting which can provide local power to reduce wiring runs and supplement engine and APU power. 
       SUMMARY 
       [0008]    Exemplary embodiments provide a thermal energy harvesting system having a hot flow conduit and a cold flow conduit with a flow routing device interruptibly interconnecting the hot flow conduit and cold flow conduit with a flow casing. At least one shape memory actuator (SMA) tube is in fluid contact with the flow casing and fixed at a first end. The flow routing device sequentially supplies hot flow from the hot flow conduit and cold flow from the cold flow conduit inducing rotation of the at least one SMA tube at a second end. A generator or alternator is operably connected to the second end of the at least one SMA tube. 
         [0009]    The embodiments disclosed provide a method for thermal energy harvesting by training at least one shape memory actuator (SMA) tube with a twist direction. The at least one SMA tube is constrained at a first end and a second end of the at least one SMA tube is supported with a bearing. The second end of the SMA tube is rotated to drive a generator/alternator through a gear box. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments, further details of which can be seen with reference to the following description and drawings. 
           [0011]      FIG. 1A  is a schematic block diagram of an aircraft employing a thermal energy harvesting system as disclosed herein; 
           [0012]      FIG. 1B  is a graph demonstrating SMA tube rotation based on temperature; 
           [0013]      FIG. 2A  is a block diagram of a first embodiment demonstrating hot flow actuation; 
           [0014]      FIG. 2B  is the block diagram of the embodiment of  FIG. 2A  demonstrating cold flow actuation; 
           [0015]      FIG. 3A  is an exemplary baffle plate for use in the first embodiment showing hot flow positioning; 
           [0016]      FIG. 3B  is the baffle plate of  FIG. 3A  fully rotated for cold flow; 
           [0017]      FIG. 4  is a graphic representation of SMA tube temperature and output rotation for the first embodiment; 
           [0018]      FIG. 5A  is an exemplary pictorial representation of the first embodiment; 
           [0019]      FIG. 5B  is a pictorial representation of the first embodiment with the flow casing removed to expose the SMA tube and baffle; 
           [0020]      FIGS. 6A-6D  show various rotation points of the SMA tube and baffle during operation; 
           [0021]      FIG. 7A  is a pictorial representation of a second embodiment with multiple SMA tubes having ratchets and gearing for continuous single direction output rotation; 
           [0022]      FIG. 7B  is a reverse pictorial representation of the second embodiment showing the features of the ratchets; 
           [0023]      FIG. 7C  is a block diagram of a control system for the second embodiment; 
           [0024]      FIGS. 8A-8D  demonstrate the operating sequence of an exemplary SMA tube and associated ratchet or sprag gear employed by the second embodiment; 
           [0025]      FIG. 9  shows an operational depiction the four actuator tubes of the second embodiment; 
           [0026]      FIG. 10  shows a sequence of operation by the four actuator tubes to provide constant rotary motion; 
           [0027]      FIG. 11  is a flow chart of a method for rotary actuation employing the first embodiment as disclosed herein; and, 
           [0028]      FIG. 12  is a flow chart of a method for rotary actuation employing the second embodiment as disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The embodiments and methods described herein provide a lightweight, compact, and rugged thermal energy harvesting system that will employ the available thermal energy in the Environmental Control System (ECS) of an aircraft and convert it into electrical energy. While described herein for use with respect to an aircraft, the embodiments disclosed herein are equally applicable to any vehicle, vessel or system having heat generating systems with available excess waste heat as a hot flow source and availability of a cold flow source. The electrical energy produced by the disclosed embodiments can be used to locally power the ECS at cruise conditions and can also generate extra power to be stored for use by the system on the ground, eliminating significant wire runs. An exemplary disclosed embodiment employs a stationary SMA tube that twists when it is heated and cooled. The tube is fixed at one end and allowed to rotate at the other. As the tube twists actuating air flowing around the tube is controlled by a flow routing device such as a baffle plate or valves operably connected to the tube, cycling between available hot and cold air flows extracted from the ECS such as hot engine bypass air and cold free stream air. As the SMA tube heats up, the baffle plate is rotated. When the tube reaches a rotated position based on a predefined hot temperature the baffle plate blocks the hot engine bypass air and opens up the cold free stream air flow. As the tube cools, the baffle plate rotates back and eventually the SMA tube reaches its fully cool rotation angle, the baffle plate closes off the cold flow and opens the hot flow and the process repeats. In alternative embodiments, control of the hot and cold air flow may be accomplished with solenoid valves controlled by rotational position of the tube. The SMA tube can generate significant torque, much higher that any known wire or spring designs. The large torque is geared in exemplary embodiments producing a very high RPM output to drive an electrical generator. Alternative embodiments incorporate multiple tubes for larger force or more rotation. Multiple tubes that use latching or ratcheting mechanisms are employed to produce rotation in just one direction. Flow paths that direct the heating and cooling air flow down the center of the SMA tube may be employed to eliminating some tubing or contained volume, reducing the weight, and increasing the multifunctional nature of the apparatus. 
         [0030]    Referring to the drawings, a thermal energy harvesting system  10  for use in an aircraft is shown in  FIG. 1A . An ECS  2  or other similar system in an aircraft  1  receives hot bypass airflow  3  from an engine or APU of the aircraft and cold airflow  4  from the air free stream around the aircraft through a heat exchanger or other means. Hot air and cold air extracted from the ECS are supplied to a SMA rotary motion generator  5  employing one or more SMA tubes. The amount of rotation of an SMA tube is a function of the length and outer diameter of the tube. The amount of force generated by the tube is a function of the outer diameter (OD) and wall thickness of the tube. The temperature at which the SMA tube rotates is a function of the alloy and the processing of the alloy. As seen in  FIG. 1B , the zones at which the hot and cold air flows around the tube would be switched are shown. Hot flow would pass over the tube as shown in trace segment  6  until it reaches a point  7   a  near the maximum hot rotation. The hot flow is then shut off and the cold flow would be opened as shown in trace segment  8 . The cold flow passes around the tube until the tube reaches a point  9   a  near its minimum cold actuation temperature and then the cold flow would be shut off and the hot flow would be opened and the process would repeat. The flow control may be accomplished such that in a range of rotation from point  7   b  to point  7   a  the hot flow is gradually reduced and cold flow gradually increased and in a range of rotation from point  9   b  to point  9   a  the cold flow is gradually reduced and the hot flow is gradually increased in anticipation of the rotation reversal. 
         [0031]    A first exemplary embodiment the SMA rotary motion generator  5  of the thermal energy harvesting system  10  is shown in notional form in  FIGS. 2A and 2B . A SMA tube  12  is supported for fluid contact within a flow casing  14  with a fixed end  16  rigidly constrained at an end cap  18 . A rotating end  20  of the tube  12  is supported in a bearing  22  and provides a rotating output that may be connected to a gearbox and generator/alternator, as will be described in greater detail subsequently. A flow routing device such as a baffle plate  24 , to be described in greater detail subsequently, is connected to or driven by the tube  12  and incorporates a hot aperture  26  and a cold aperture  28  which alternately align with a hot flow conduit  30  receiving hot gas from the ECS or other source and a cold flow conduit  32  receiving cold gas from the ECS or other source. A seen in  FIG. 2A , flow of hot gas, as represented by arrow  34 , from the hot flow conduit  30  through the hot aperture  26  in the baffle plate  24  and through the flow casing  14  heats the tube  12  resulting in rotation of the tube in a first direction. Rotation of the tube displaces the hot aperture  26  from the hot flow conduit  30  interrupting the flow from the hot flow conduit and opens the cold aperture  30 , as seen in  FIG. 2B , introducing cold gas flow, as represented by arrow  36 , from the cold flow conduit  32  through cold aperture  30  resulting in rotation of the tube  12  in an opposite direction, closing the cold aperture  28  to interrupt flow from the cold flow conduit and reopening the hot aperture  26 . The process then repeats. 
         [0032]    The hot and cold apertures  26 ,  28  may be shaped to provide a flow and temperature profile of gas flow to the tube  12  for desired rotational speed. As seen in  FIGS. 3A and 3B , the apertures have a reducing area as the tube  12  rotates from a maximum flow to a reduced flow prior to cutoff. As seen in  FIG. 3A  for an initial position, the full diameter of hot flow conduit  30  is exposed in hot aperture  26  providing the greatest hot gas flow for initiating rotation of the tube  12  in the heating direction as shown by arrow  38 . The cold flow conduit  32  is closed by the baffle plate  24 . As the tube  12  and concentrically attached baffle  24  rotate in the heating direction, the area exposed by the hot aperture  26  reduces to a cutoff point. Simultaneously the cold aperture  28  opens and exposes the cold flow conduit  32  with increasing area. As the tube  12  reaches a maximum rotation as seen in  FIG. 3B , the diameter of the cold flow conduit  32  is fully exposed through the cold aperture  28  and the hot flow conduit  30  is blocked by the baffle plate  24  reversing the rotation of the tube  12  to a cooling direction as shown by arrow  40 . As the tube  12  and concentrically attached baffle  24  rotate in the cooling direction, the area exposed by the cold aperture  28  reduces to a cutoff point. As in the prior rotation direction, simultaneously, the hot aperture  26  opens and exposes the hot flow conduit  30  with increasing area. Alternating rotation of the tube  12  is maintained as long as hot and cold gas flow is available through the hot and cold flow conduits. 
         [0033]    The resulting sequence of heating and cooling with resulting rotational motion is shown in  FIG. 4  with temperature of the tube  12  induced by the hot and cold flows is shown by trace  42  and rotational angle of the tube  12  is shown in trace  44 . The point and which the flow around the SMA tube  12  transitions from hot to cold or cold to hot, how quickly it transitions, and the volume of flow can all be controlled by the pattern of apertures  26 , 28  in the baffle plate  24 . 
         [0034]    An exemplary physical implementation of the thermal energy harvesting system  10  employing first embodiment of the SMA rotary motion generator is shown in  FIGS. 5A and 5B . The flow casing  14 , seen in  FIG. 5A , houses the tube  12 , seen in  FIG. 5B . The end cap  18  constrains the tube  12  at the first end  16  and is attached to the flow casing  14 . Exhaust apertures  46  in the end cap  18  allow the flow of gas to exit the flow casing  14 . The rotating end of the tube  12  extends through the bearing  22  and is attached through a gear box  48  to a generator or alternator  50  to produce electrical power. The gear box  48  may include ratchets or other rotational direction conversion systems to provide single directional rotation from the gear box to the generator/alternator. 
         [0035]    The rotational sequence of the baffle plate  24  described with respect to  FIGS. 3A and 3B  is shown in detail for the physical implementation in  FIGS. 6A-6D . As seen in  FIG. 6A , with the tube  12  in an initial position, the hot aperture  26  fully exposes the hot flow conduit  30  and blocks the cold flow conduit  32 . Hot gas flows from the hot flow conduit through the flow casing heating the SMA tube causing it to rotate in a first direction (clockwise for the example in the drawings). As the tube  12  and baffle plate  24  rotate the profile of the hot aperture progressively restricts the area of the hot flow conduit  30  and the cold flow conduit  32  is exposed by the cold aperture  28  with a reduced area as seen in  FIG. 6B . Further rotation of the tube  12  and baffle plate  24  as seen in  FIG. 6C  causes the hot aperture  26  to further reduce the area of the hot flow conduit  30  while the cold aperture  28  is increasing the area of the cold flow conduit  32 . Finally at full rotation, approximately 90° for the exemplary embodiment, the cold aperture  28  exposes the entire area of the cold flow conduit  32  while the baffle plate  24  has closed the hot flow conduit  30 . The flow of cold gas from the cold flow conduit  32  through cold aperture  28  reverses the rotation of the tube  12  reversing the sequence of cold and hot gas exposure in the flow casing  24 . While shown in the drawings of the exemplary embodiment as concentric to and directly connected to the tube, the baffle plate may be driven by a gear train operably connected to the tube with appropriate conduits for routing the hot and cold flows into the flow casing. 
         [0036]    A continuous output rotation in a single direction for driving a generator/alternator is achieved in a second embodiment as shown in  FIGS. 7A and 7B . A plurality of SMA tubes, four tubes  12   a ,  12   b ,  12   c  and  12   d  for the exemplary embodiment shown in the drawings, are supported in a frame  52 . Rotation of each tube drives a gear  54   a ,  54   b ,  54   c  and  54   d  which, in turn drives a central drive gear  56 . The first end  16   a ,  16   b ,  16   c  and  16   d  of each tube is fixed to a ratchet gear  58   a ,  58   b ,  58   c  and  58   d  constrained against reverse rotation by a ratchet pawl  60   a ,  60   b .  60   c  and  60   d . The hot flow conduit  30  and cold flow conduit  32  are routed through a manifold  61  which incorporates a solenoid valve  64   a .  64   b ,  64   c  and  64   d  for each tube controlling flow into a respective inlet conduit  65   a ,  65   b ,  65   c  and  65   d . A central bore  13  (shown in phantom for example in tube  12   a ) in each SMA tube provides a flow casing connected to the respective inlet conduit to provide gas flow to heat and cool each SMA tube  12   a ,  12   b ,  12   c  and  12   d . The gas exhausted from the tubes flows into an exhaust manifold  62  through outlet conduits  63   a ,  63   b ,  63   c  and  63   d . While the flow casing is shown as the internal bore of the tubes for the exemplary embodiment, an external flow casing surrounding each tube and interconnected to the inlet and outlet conduits may be employed in alternative embodiments. 
         [0037]    Each of the solenoid valves  64   a ,  64   b ,  64   c  and  64   d  provides three positions, a cold flow position in which the cold flow conduit  32  is connected to the respective inlet conduit  65   a ,  65   b ,  65   c  or  65   d , a hot flow position in which the hot flow conduit  30  is connected to the respective inlet conduit  65   a ,  65   b ,  65   c  or  65   d , and a closed or off position in which no flow is provided through the valve. In the example usage in association with an aircraft ECS, the hot and cold flows may be hot bypass air from the engines or APU and cold free stream air. Hot and cold liquid flow sources may also be used in alternative embodiments. As seen in  FIG. 7C , a control processor  66  receives an angle position output from an angle sensor  68  adapted to determine the angle of the output shaft  49 . The processor  66  provides an output to sequentially control the solenoid valves  64   a ,  64   b ,  64   c  and  64   d  to heat or cool or block flow to the associated SMA tubes  12   a ,  12   b ,  12   c  and  12   d  to allow a continuous rotation of the output shaft  49 . In advanced systems, the solenoid valves may also provide a range of flow for each of the hot flow position and cold flow position and control system may be adapted to alter the hot and cold flows, similar to the effect of profiling the hot and cold apertures in the first embodiment, for maintaining constant rotation rates consistent with the characteristics of the SMA tubes. 
         [0038]    Operation of each of the tubes shown in  FIGS. 7A and 7B  under control of the processor  66  is represented in  FIGS. 8A-8D . Each SMA tube is trained in a twist direction. Using tube  12   a  as an example, with the tube at a base temperature as shown in  FIG. 8A , first end  16   a  is constrained by ratchet gear  58   a  and pawl  60   a  as represented by blocked arrow  69 . Upon heating of the tube  12   a  as shown in  FIG. 8B , the tube twists through second end  20   a  in the twist direction driving gear  54   a  in a first rotational direction represented by arrows  70 . A rotation of second end  20   a  of  900  represented by twist line  34   a  is shown as an example. Ratchet gear  58   a  maintains the constraint on first end  16   a  resulting in the twist or rotation being induced at the second end  20   a . Cooling of tube  12   a  as represented in  FIG. 8C  results in gear  54   a  constraining second end  20   a  as represented by blocked arrow  71  while first end  16   a  rotates opposite to the constraining direction of ratchet  58   a  as represented by arrow  72 . Upon completion of the cooling of tube  12   a , first end  16   a  has assumed a new zero position as represented by twist line  34   a  and is again constrained by the ratchet  58   a  as shown in  FIG. 8D . 
         [0039]    As shown in  FIGS. 7A and 7B  four SMA tubes  12   a ,  12   b ,  12   c  and  12   d  each constrained at a first end  16   a ,  16   b ,  16   c  and  16   d  by a ratchet  58   a ,  58   b ,  58   c  and  58   d  are employed. A cover (not shown) may be employed to enclose the tubes as a self-contained unit. The second end  22   a ,  22   b ,  22   c  and  22   d  of each SMA tube  12   a ,  12   b ,  12   c  and  12   d  is connected to a gear  54   a ,  54   b ,  54   c  and  54   d . The gears  54   a ,  54   b ,  54   c  and  54   d  drive the output gear  56  attached to rotary output shaft  49  integrated in a gear box  48  which in turn drives the generator/alternator  50 . 
         [0040]    Operation of the tubes  12   a ,  12   b ,  12   c  and  12   d  is represented in  FIGS. 9 and 10 . As seen in  FIG. 9  with reference to elements as shown in  FIGS. 7A and 7B , in an initial state at a base temperature, gears  54   a ,  54   b ,  54   c  and  54   d  engage drive gear  56  at a first rotational position represented by index  80 . Continuous rotation of the drive gear  56  is achieved by sequential heating of the SMA tubes. Any desired sequence may be employed but an example is shown in  FIG. 10 . In a first rotation sequence  74   a , tube  12   a  is heated (represented by stipling of the gear in the drawing) by transitioning solenoid  64   a  for flow from the hot flow conduit  30  resulting in rotation of gear  54   a  as represented by arrow  76   a  (the first end  14   a  of tube  12   a  being constrained by ratchet gear  58   a ) which rotates drive gear  56  as represented by arrow  78   a . Tubes  12   b ,  12   c  and  12   d  remain at the base temperature and have a rigid body rotation with gears  54   b ,  54   c  and  54   d  which freely rotate with drive gear  56 . At the completion of the first rotation sequence, tube  12   a  is cooled (represented by cross hatching of the gear in the drawing) by transitioning solenoid  64   a  to the cold flow conduit  32  and tube  12   d  is heated by transitioning solenoid valve  64   d  for flow from the hot flow conduit  30  as shown in second rotation sequence  74   b  (the first end  16   d  of tube  12   d  constrained by ratchet gear  58   d ). Gear  54   d  driven by tube  12   d  rotates as represented by arrow  76   d  which continues the rotation of drive gear  56  as represented by arrow  68   b . Tubes  12   b  and  12   c  remain at the base state and operate in rigid body rotation with gears  54   b  and  54   c . Tube  12   a , cooling with first end  16   a  freely rotating with ratchet gear  58   a  and constrained by gear  54   a  which rotates with drive gear  56 , returns to the base state. The sequence of continuous rotation is propagated as shown in third rotation sequence  74   c  where tube  12   c  is now heated by transitioning solenoid  64   c  for flow from the hot flow conduit  30  resulting in rotation of gear  44   c  as represented by arrow  76   c  (the first end  16   c  of tube  12   c  being constrained by ratchet gear  58   c ) which rotates drive gear  56  as represented by arrow  78   c . Tube  12   d  is cooling by transitioning solenoid  64   d  for flow from the cold flow conduit  32  with first end  16   d  freely rotating in ratchet gear  58   d  and constrained by gear  54   d  which rotates with drive gear  56 , returning to the base state. Tubes  12   a  has now returned to the base state and tube  12   b  remains at the base state with both tubes operating in rigid body rotation with gears  54   a  and  54   c  At the completion of the third rotation sequence, tube  12   c  is allowed to begin cooling by transitioning solenoid  64   c  for flow from the cold flow conduit  30  and tube  12   b  is heated by transitioning solenoid  64   b  for flow from the hot flow conduit  30  as shown in fourth rotation sequence  74   d  (the first end  16   b  of tube  12   b  constrained by ratchet gear  58   c ). Gear  54   b  driven by tube  12   b  rotates as represented by arrow  76   b  which continues the rotation of drive gear  56  as represented by arrow  78   d . Tube  12   b , cooling with first end  16   b  freely rotating with gear  16   b  and constrained by gear  54   b  which rotates with drive gear  56 , returns to the base state. Tube  12   d  has now returned to the base state and with tube  12   a  in the base state both tubes rotate with gears  54   d  and  54   a  in a rigid body rotation with drive gear  56  and first ends  16   d  and  16   a  freely rotating with ratche gears  60   d  and  60   a . Rotation of the drive gear  56  through the various rotation sequences can be seen by the rotation of index  80  and the repetition of the sequences described with respect to  FIG. 10  allows continuous rotary motion of the drive gear. While four SMA tubes are shown, any desired number of two or more tubes may be employed. Additionally, while described herein as employing ratchet gears  58   a ,  58   b .  58   c  and  58   d  with pawls  60   a ,  60   b ,  60   c  and  60   d  to provide selected directional motion of the SMA tubes, the first ends  16   a ,  16   b ,  16   c  and  16   d  of the tubes may be fixed as in the initial embodiment and a sprag gear substituted for each of the gears  54   a ,  54   b ,  54   c  and  54   d  attached to the second ends  20   a ,  20   b ,  20   c  and  20   d  of the tubes. 
         [0041]    The first embodiment disclosed herein allows a method of thermal energy harvesting using SMA rotary actuation as depicted in  FIG. 11 . A SMA tube is trained with a twist direction, step  1100 , and constrained at a first end, step  1102 . A second end of the SMA tube is supported by a bearing, step  1104 , and rotation of the second end drives a generator/alternator through a gear box, step  1106 . A hot flow conduit is connected to receive hot bypass air from an ECS and a flow routing device such as a baffle plate with apertures connects for flow adjacent the tube, step  1108 . The tube rotates in a first direction responsive to heating by the hot flow, step  1110 . A cold flow conduit is connected to receive cold air from the ECS and upon reaching a predetermined twist in the tube, the flow routing device connects the cold flow conduit for flow adjacent the tube, step  1112 , and the tube rotates in a second direction responsive to cooling by the cold flow, step  1114 . 
         [0042]    The second embodiment disclosed herein allows a method of thermal energy harvesting using SMA rotary actuation with continuous rotation in a single direction as depicted in  FIG. 12 . A plurality of SMA tubes are trained with a twist direction, step  1200 , and constrained at first ends with ratchets, step  1202 . Second ends of the SMA tubes are engaged by gears, step  1204 . A drive gear is operably engaged by the gears for rotational output to a shaft, step  1206 , and an electrical generator/alternator is driven by the shaft, step  1208 . Sequential heating and cooling of the tubes in the plurality is accomplished to provide continuous rotation of the drive gear and output shaft, step  1210 . 
         [0043]    Having now described various embodiments of the disclosure in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present disclosure as defined in the following claims.