Patent Publication Number: US-2009229268-A1

Title: Cogeneration system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2005-303488, filed on Oct. 18, 2005, the contents of which are hereby incorporated by reference into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cogeneration system. In particular, the present invention relates to a cogeneration system that includes a Stirling engine. 
     2. Description of the Related Art 
     A cogeneration system has been developed in which a generator is driven by means of fuel such as combustible gas, and heat which is produced during the driving is recovered for use in air conditioning, heating water supplies(prefer this: water heating), or the like. 
     PCT publication No. WO 01/90656 discloses a cogeneration system including a Stirling engine. This cogeneration system comprises, in addition to the Stirling engine, a burner for heating a heated head of the Stirling engine and a heat exchanger for recovering heat from gas combusted in the burner. In the cogeneration system, the gas combusted in the burner is directed through a duct to the heat exchanger. 
     BRIEF SUMMARY OF THE INVENTION 
     While gas combusted in the burner is being transported through the duct into the heat exchanger in the above-described conventional system, the combusted gas heat can escape to the outside atmosphere. Then, exhaust heat (which refers to heat released into the atmosphere without itself being used) is wastefully released, resulting in a decline in efficiency of the cogeneration system. 
     The present invention, which solves the above issue, provides a technology capable of reducing exhaust heat by preventing the combusted gas heat from escaping to the outside atmosphere. 
     A cogeneration system of the present teachings is provided with a Stirling engine, a burner, a first exhaust passage, a second exhaust passage that continues from the first exhaust passage, and a first heat exchanger. The Stirling engine has a pillar-shaped heated head. The burner faces an end surface of the heated head of the Stirling engine. The first exhaust passage extends along the side surface of the heated head of the Stirling engine. The second exhaust passage extends along a side of the first exhaust passage opposite to the Stirling engine. The first heat exchanger is arranged on a side of the second exhaust passage opposite to the Stirling engine. 
     In the cogeneration system, combusted gas from the burner is initially directed to the first exhaust passage. Since, in the first exhaust passage, the combusted gas from the burner flows along the side surface of the heated head in the Stirling engine, the combusted gas heat is used for heating the Stirling engine. After passing through the first exhaust passage, the combusted gas is then directed toward the second exhaust passage. In the second exhaust passage, because the combusted gas from the burner flows along the first heat exchanger, heat is recovered from the combusted gas by the first heat exchanger. 
     The cogeneration system has the first exhaust passage and the second exhaust passage disposed in a laminated form between the Stirling engine and the first heat exchanger. In this manner, the escape of combusted gas heat to the outside atmosphere is prevented, and thereby efficient heating of the Stirling engine and exhaust heat recovery by means of the first heat exchanger is realized. In addition, high-temperature heating of the heated head of the Stirling engine is realized since the first exhaust passage is maintained at elevated temperatures due to isolation of the first exhaust passage from the first heat exchanger via the second exhaust passage. 
     According to the cogeneration system, the combusted gas heat escaping to the outside atmosphere is prevented, and thereby the exhaust heat is preferably reduced. 
     In the aforementioned cogeneration system, it is preferable that the first exhaust passage surrounds the side surface of the heated head of the Stirling engine, the second exhaust passage surrounds the first exhaust passage on its outside, and the first heat exchanger surrounds the second exhaust passage on its outside. 
     Since the first exhaust passage and the second exhaust passage are surrounded by the Stirling engine and the first heat exchanger in the cogeneration system, the combusted gas heat escaping to the outside atmosphere from the burner is further suppressed. 
     The cogeneration system may include a second heat exchanger that is connected to a downstream end of the second exhaust passage. 
     It enables recovery of heat remaining in the combusted gas discharged out of the second exhaust passage, and thereby further reduces exhaust heat. 
     It is preferable that the second heat exchanger be a latent heat exchanger that can extract latent heat by condensing vapor from combusted gas. 
     Since the combusted gas from the burner contains high volumes of vapor, latent heat can be extracted from the combusted gas by condensing the high volumes of vapor. When the second heat exchanger is the latent heat exchanger, a greater amount of heat can be further recovered from the combusted gas whose temperature has already been lowered by the first heat exchanger, resulting in reduction of exhaust heat. 
     According to the present invention, preventing combusted gas heat from escaping to the outside atmosphere reduces the exhaust heat, and thereby energy efficiency in the cogeneration system can be remarkably enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows a structure of a cogeneration system; 
         FIG. 2  is a cross-sectional view showing a schematic configuration of a thermal combustor; 
         FIG. 3  is a perspective view showing the exterior of the thermal combustor; 
         FIG. 4  is a perspective view showing a partial configuration of the thermal combustor, and 
         FIG. 5  is a graph showing changes in temperature of combusted gas that is flowing through an exhaust passage. 
     
    
    
     REFERENCE NUMERALS IN THE DRAWINGS 
     
         
           10 : cogeneration system 
           20 : thermal combustor 
           24 : burner 
           26 : first heat exchanger 
           40 : second heat exchanger 
           70 : Stirling engine 
         A: first exhaust passage 
         B: second exhaust passage 
         C: third exhaust passage 
         D: fourth exhaust passage 
         E: fifth exhaust passage 
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Main Features of an Embodiment of the Invention 
     Feature 1: A cogeneration system comprises a free piston type Stirling engine. 
     Feature 2: The cogeneration system comprises a burner for burning combustible gas. 
     Feature 3: The cogeneration system comprises a blower that draws in combusted gas from the burner to discharge the gas to the outside. 
     Embodiment of the Invention 
     A preferred embodiment of the present invention will be described with reference to the drawings. 
       FIG. 1  is a schematic diagram showing a structure of a cogeneration system  10  according to the present embodiment. The cogeneration system  10  comprises a Stirling engine  70  and a thermal combustor  20 . The Stirling engine  70  is equipped with pillar-shaped (more specifically, cylindrical pillar-shaped) heated heads  94   a  and  94   b  as well as a cooler  96 . In view of vibrations of the Stirling engine  70  under operating conditions, the Stirling engine  70  is supported via an elastic member of a frame (not illustrated) of the cogeneration system  10 . However, the thermal combustor  20  is fixed to the frame. Accordingly, the thermal combustor  20  and the Stirling engine  70  are not fixed to each other, which ensures that, even if the Stirling engine  70  vibrates during its operation, the vibrations of the Stirling engine  70  are not transferred to the thermal combustor  20 . 
     The thermal combustor  20  includes a burner  24  for burning combustible gas to heat the heated heads  94   a  and  94   b  of the Stirling engine. Further, the thermal combustor  20  also includes heat exchangers  26  and  40  for recovering heat produced while the Stirling engine is being heated. The Stirling engine  70  is driven when the heated heads  94   a  and  94   b  are heated by the thermal combustor  20  while being cooled by the cooler  96 , to thereby generate and provide alternating current power to external electrical machinery or apparatuses. The cogeneration system  10  is a combined heat and power supplying system in which combustible gas is burned to supply both electricity and heat. 
     Referring to  FIG. 1 , the structure of the Stirling engine  70  is described. The Stirling engine  70 , which is a so-called beta type Stirling engine, comprises a housing  94  formed in the shape of a pillar outline, a displacer  78  installed in working spaces  100  and  102  inside the housing  94 , and an output piston  80  which faces the working space  100  in the housing  94 . The heated heads  94   a  and  94   b  in the Stirling engine  70  are located at an upper end of the housing  94  in the drawing. 
     The displacer  78  is housed in a cylinder  104  installed in the working spaces  100  and  102 , and is slidably supported by a shaft  72  which is fixed to the housing  94 . The displacer  78  is connected to the shaft  72  via two spring members  76 , thereby allowing the displacer  78  to reciprocate along an inner surface  104   a  of the cylinder  104  at a predetermined frequency (a characteristic frequency). The characteristic frequency of the displacer  78  can be adjusted by means of spring constants of the two spring members  76 . In this connection, a plate spring or the like may be used for each of the spring members  76 . On the other hand, the shaft  72  is secured to the housing  94  via a fixed plate  79  in which a plurality of holes  79   a  is formed. 
     The working spaces  100  and  102  are filled with a working medium of helium gas. Regarding the working spaces  100  and  102 , the working space  102  located above the displacer  78  in the drawing is a heated-part working space to be heated by the thermal combustor  20 . The heated-part working space  102  is disposed in an internal area between the heated heads  94   a  and  94   b.  On the other hand, the working space  100  located below the displacer  78  in the drawing is a cooled-part working space to be cooled by the cooler  96 . The heated-part working space  102  communicates with the cooled-part working space  100  by a space between an outer surface  104   b  of the cylinder  104  and an inner surface of the housing  94 , and a regenerator  74  is installed in the communicating space. 
     In the Stirling engine  70 , helium gas in the heated-part working space  102  is continuously heated while helium gas in the cooled-part working space  100  is continuously cooled, causing the displacer  78  to reciprocate inside the cylinder  104 . The reciprocating motion of the displacer  78  achieves a change in the ratio between the volumes of the heated-part working space  102  and the cooled-part working space  100 . For example, as the displacer  78  moves to a heated-part working space  102  side, the heated-part working space  102  decreases, and the cooled-part working space  100  accordingly increases. At this time, the helium gas in the heated-part working space  102  moves through the regenerator  74  into the cooled-part working space  100 . Contrarily, as the displacer  78  moves to a cooled-part working space  100  side, the cooled-part working space  100  becomes smaller, and the heated-part working space  102  accordingly increases. Then, the helium gas contained in the cooled-part working space  100  moves through the regenerator  74  into the heated-part working space  102 . Since the change of the ratio between the volume of the heated-part working space  102  and the volume of the cooled-part working space  100  brings about a change in a proportion of helium gas at high temperatures and helium gas at low temperatures, pressure of the overall working spaces  100  and  102  varies. The pressure variation in the overall working spaces  100  and  102  is applied to the output piston  80  facing the space  100 . 
     The output piston  80 , having a shaft  82  connected via spring members  84  and  92  to the housing  94 , is supported so as to be reciprocatable along a vertical direction of the drawing at a predetermined frequency (a characteristic frequency). The characteristic frequency of the output piston  80  can be changed by means of spring constants of the spring members  84  and  92 . In this embodiment, the characteristic frequency of the output piston  80  is adjusted to be equal to the characteristic frequency of the displacer  78 . 
     The output piston  80  is forced to reciprocate by the above-described pressure variation that occurs in the overall working spaces  100  and  102 . Then, the frequency of the output piston  80  matches the frequency of the displacer  78 . Since the characteristic frequencies of the output piston  80  and the displacer  78  are set to an equal value, unnecessary damping of the output piston  80  or the displacer  78  is prevented. 
     As shown in  FIG. 1 , a plurality of magnets  86  is arranged on the shaft  82  of the output piston  80 . Further, on a housing  94  side, an iron core  88  and a coil  90  are disposed at a location facing the plurality of magnets  86 . In the Stirling engine  70 , a generator is constructed using a group of the magnets  86 , the iron core  88 , the coil  90 , and other components, so as to generate alternating current power by the reciprocating motion of the output piston  80 . Here, a frequency of the alternating current power to be generated is equal to a frequency of the reciprocating motion of the output piston  80 . Accordingly, the frequency of the alternating current power to be generated can be adjusted by changing the characteristic frequency of the displacer  78  or the output piston  80 . 
     Next, the thermal combustor  20  is described.  FIG. 2  is a cross sectional view showing a schematic configuration of the thermal combustor  20 , and  FIG. 3  is a perspective view showing the exterior of the thermal combustor  20 . 
     As illustrated in  FIGS. 2 and 3 , the thermal combustor  20  comprises a head plate  22  and a burner  24  attached to the head plate  22 . The burner  24  faces toward an end surface  94   a  of the heated head in the Stirling engine  70 . An air fuel mixture in which combustible gas and air are mixed is supplied from a gas supply port  25  to the burner  24 . The burner  24  burns the combustible gas to heat up the end surface  94   a  of the heated head and a side surface  94   b  of the heated head in the Stirling engine  70 . Then, the burner  24  heats up the end surface  94   a  of the heated head to approximately 650° C. and the side surface  94   b  of the heated head to approximately 600° C. 
       FIG. 4  is a perspective view showing the thermal combustor  20  from which the head plate  22  and other components are removed. As illustrated in  FIGS. 2 and 4 , the thermal combustor  20  includes a cylindrical bulkhead member  32  attached to the head plate  22 . The bulkhead member  32  extends along the side surface  94   b  of the heated head in the Stirling engine  70 . A gap is provided between the side surface  94   b  of the heated head in the Stirling engine  70  and an inner surface  32   a  of the bulkhead member  32 , to form a first exhaust passage A in which combusted gas from the burner  24  is introduced along the side surface  94   b  of the heated head in the Stirling engine  70 . The first exhaust passage A extends from the end surface  94   a  of the heated head in the Stirling engine  70  along the side surface  94   b  of the heated head in the Stirling engine  70 . The first exhaust passage A is formed by a space having a roughly cylindrical shape so as to surround the side surface  94   b  of the heated head. 
     In the thermal combustor  20 , while the combusted gas from the burner  24  is flowing through the first exhaust passage A, the Stirling engine  70  is heated by the combusted gas heat. In this way, output of the Stirling engine  70  is maintained. 
     As shown in  FIGS. 2 ,  3 , and  4 , the thermal combustor  20  comprises a first heat exchanger  26 . The first heat exchanger  26  includes a cylindrical-shaped heat transfer member  30  and a heat transfer tube  28  arranged along an outer surface  30   b  of the heat transfer member  30 . The heat transfer member  30  is disposed so as to surround the outer surface  32   b  of the bulkhead member  32 , and an inner surface  30   a  of the heat transfer member  30  faces toward the outer surface  32   b  of the bulkhead member  32 . A gap is provided between the outer surface  32   b  of the bulkhead member  32  and the inner surface  30   a  of the heat transfer member  30  to form a second exhaust passage B. A great number of slots are formed on the inner surface  30   a  of the heat transfer member  30  in order to provide the heat transfer member  30  with a greater area that borders the second exhaust passage B. 
     The second exhaust passage B communicates with the first exhaust passage A. The second exhaust passage B is located on a side of the first exhaust passage A opposite to the Stirling engine  70 , and extends along the first exhaust passage A. Combusted gas flowing from the first exhaust passage A changes direction and flows into the second exhaust passage B. The second exhaust passage B, which is formed by a substantially cylindrical-shaped space, surrounds the first exhaust passage A from outside over the bulkhead member  32 . 
     A plate member  31  for defining a junction area between the first exhaust passage A and the second exhaust passage B is attached to the lower end of the heat transfer member  30  in the drawing. The plate member  31  is fixed to the heat transfer member  30  and extends from the heat transfer member  30  toward the side surface  94   b  of the heated head in the Stirling engine  70 . The plate member  31  may be composed of a metal having a high thermal conductivity, such as, for example, copper. The plate member  31  is cooled by a heat carrier flowing through the heat transfer tube  28  with the plate member  31  acting like a part of the heat transfer member  30 . 
     As described above, the thermal combustor  20  and the Stirling engine  70  are not fixed to each other. Therefore, the plate member  31  is not fixed to any part of the Stirling engine  70 , which allows the plate member  31  to slide relative to the side surface  94   b  of the heated head. Since interstices between the plate member  31  and the end surface  94   b  of the heated head are not completely sealed, the combusted gas can slightly escape from the interstices between the plate member  31  and the side surface  94   b  of the heated head. 
     As shown in  FIG. 2 , a boot  27  formed of rubber, being an elastic member, is provided between the plate member  31  and the cooler  96  of the Stirling engine  70 . The boot  27  forms a space  29  isolated from the atmosphere in conjunction with the plate member  31  and the Stirling engine  70 . In this formation, the combusted gas, having escaped from the interstices between the plate member  31  and the side surface  94   b  of the heated head, is kept in the space  29  to thereby prevent the combusted gas from being released into the atmosphere. 
     Because the space  29  borders the plate member  31 , temperature increase of the space  29  is suppressed. In this way, even when the combusted gas flows into the space  29 , the space  29  is protected against high temperatures, which, in turn, prevents the possibility of the boot  27  rupturing due to the combusted gas heat. In other words, the combusted gas heat having escaped from the interstices between the plate member  31  and the side surface  94   b  of the heated head is also recovered. In addition, a problem of high-temperature combusted gas being ejected to the outside atmosphere from the ruptured boot  27  will not occur. 
     In the thermal combustor  20 , while combusted gas from the burner  24  is flowing through the second exhaust passage B, the combusted gas heat is recovered by the first heat exchanger  26 . Because the heat transfer member  30  of the first heat exchanger  26  has the large area that faces the second exhaust passage B, a greater amount of heat can be recovered from the combusted gas. 
     In the thermal combustor  20 , the first exhaust passage A is formed so as to surround the side surface  94   b  of the heated head in the Stirling engine  70 , while the second exhaust passage B is formed so as to surround the first exhaust passage A from outside, and the first heat exchanger  26  is disposed so as to surround the second exhaust passage B from outside. In this manner, escape of combustion heat produced by the burner  24  to the outside atmosphere is prevented, which enables utilization of a greater amount of heat for heating up the Stirling engine or enables a greater amount of heat recovery by means of the first heat exchanger. 
     The first exhaust passage A is isolated from the first heat exchanger  26  by the second exhaust passage B. The isolation inhibits the first heat exchanger  26  from recovering heat from the combusted gas flowing through the first exhaust passage A. The first exhaust passage A is maintained at a temperature higher than that of the second exhaust passage B, so that the heated heads  94   a  and  94   b  of the Stirling engine  70  can be heated to higher temperatures. 
     As shown in  FIGS. 2 and 3 , the thermal combustor  20  comprises a second heat exchanger  40 , which is connected via an exhaust duct  38  to the second exhaust passage B. The combusted gas from the burner  24  is directed from the second exhaust passage B through the exhaust duct  38  (a third exhaust passage C) to the second heat exchanger  40 . In an inner region  40   a  of the second heat exchanger  40 , a heat transfer tube  42  is arranged. The heat transfer tube  42  is supplied, from an inlet port  42   a  thereof, with a heat carrier such as water, which is then discharged from an exhaust port  42   b.    
     The second heat exchanger  40  recovers heat from the combusted gas into the heat carrier in the heat transfer tube  42  while the combusted gas is flowing through the inner region (a fourth exhaust passage D)  40   a  of the second heat exchanger  40 . During the recovery, the second heat exchanger  40  condenses water vapor contained in the combusted gas in order to extract latent heat. The heat carrier, which circulates throughout the heat transfer tube  42  in the second heat exchanger  40 , is heated by the heat recovered from the combusted gas. Then, the heat carrier is supplied with an external device, such as air conditioning equipment, or the like. 
     As illustrated in  FIGS. 2 and 3 , the thermal combustor  20  comprises a blower  44  which draws in the combusted gas from the burner  24  to discharge the combusted gas to the outside. The blower  44  includes a fan  46  and a motor  48  for rotating the fan  46 . The blower  44  is connected to a downstream end of the second heat exchanger  40 . When the blower  44  rotates the fan  46 , the combusted gas from the burner  24  is sequentially sucked into the first exhaust passage A, the second exhaust passage B, the third exhaust passage C, the fourth exhaust passage D, and a fifth exhaust passage E (in the blower  44 ), and subsequently discharged from an exhaust port  50 . 
     The inside of the exhaust passages A to E is maintained at a negative pressure relative to atmospheric pressure by the blower  44  that draws in the combusted gas from the downstream end of the exhaust passages A to E. As a result, for example, in a case where the boot  27  ruptures, because atmospheric air flows into the exhaust passages A to E from a hole in the ruptured boot  27 , a situation in which high-temperature combusted gas is ejected from the ruptured boot  27  into air and the like is prevented. 
       FIG. 5  is a graph showing how the temperature of the combusted gas generated by the burner  24  decreases, while sequentially flowing through from the first exhaust passage A to the fifth exhaust passage E and then discharged from the exhaust port  50 . Reference letter A in  FIG. 5  represents a zone of the first exhaust passage A, reference letter B in  FIG. 5  represents a zone of the second exhaust passage B, reference letter C in  FIG. 5  represents a zone of the third exhaust passage C, reference letter D in  FIG. 5  represents a zone of the fourth exhaust passage D, and reference letter E in  FIG. 5  represents a zone of the fifth exhaust passage E. As depicted in  FIG. 5 , in the zone of the first exhaust passage A, the combusted gas heat is used for heating the Stirling engine  70 , thereby lowering the temperature of the combusted gas. In the zone of the second exhaust passage B, the combusted gas heat is recovered by the first heat exchanger  26 , thereby lowering the temperature of the combusted gas. In the zone of the fourth exhaust passage D, the combusted gas heat is recovered by the second heat exchanger  40 , thereby lowering the temperature of the combusted gas. Here, in the zone of the fourth exhaust passage D, water vapor contained in combustion gas is condensed, to thereby extract latent heat from the combusted gas. Accordingly, compared to the other zones, the amount of heat recovered from the combusted gas in the fourth exhaust passage D is greater relative to a lowered range of temperature of the combusted gas. In the final stage where the combusted gas is delivered from the exhaust port  50 , the temperature of the combusted gas is lowered to a level of approximate room temperatures. In the cogeneration system  10 , because exhaust heat is sufficiently reduced, energy efficiency is remarkably enhanced. 
     The example of employing the free-piston type Stirling engine has been described in the above embodiment, but the technology disclosed therein can be applied with any form or type of Stirling engine. 
     The example of using combustible gas for fuel has been illustrated in the above-described embodiment, but the technology disclosed herein can also be applied with any fuel type or the like. 
     A specific embodiment of the present invention is described above, but the embodiment merely illustrates some possibilities of the invention and does not restrict the claims thereof. The art set forth in the claims includes transformations and modifications to the specific examples set forth above. 
     Furthermore, the technical elements disclosed in the present specification or drawing may be utilized separately or in all types of conjunctions and are not limited to the conjunctions set forth in the claims at the time of filling of the application. Furthermore, the art disclosed in the present specification or figures may be utilized to simultaneously realize a plurality of aims or to realize one of these aims.