Abstract:
According to the present invention, a method and apparatus for generating power aboard a marine vessel is provided. The method comprises the steps of: (a) providing a Rankine Cycle device that includes at least one of each of an evaporator, a turbo-generator that includes a turbine coupled with an electrical generator, a condenser, and a refrigerant feed pump; (b) disposing the one or more evaporators within an exhaust duct of a power plant of the marine vessel; (c) operating the power plant; and (d) selectively pumping refrigerant through the Rankine Cycle device, wherein refrigerant exiting the evaporator powers the turbine, which in turn powers the generator to produce power.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates to methods and apparatus for power generation using waste heat from a power plant in general, and to those methods and apparatus that utilize an organic Rankine cycle in particular.  
         [0003]     2. Background Information  
         [0004]     Marine and land based power plants can produce exhaust products in a temperature range of 350-1850° F. In most applications, the exhaust products are released to the environment and the thermal energy is lost. In some instances, however, the thermal energy is further utilized. For example, the thermal energy from the exhaust of an industrial gas turbine engine (IGT) has been used as the energy source to drive a Rankine cycle system.  
         [0005]     Rankine cycle systems can include a turbine coupled to an electrical generator, a condenser, a pump, and a vapor generator. The vapor generator is subjected to a heat source (e.g., geothermal energy source). The energy from the heat source is transferred to a fluid passing through the vapor generator. The energized fluid subsequently powers the turbine. After exiting the turbine, the fluid passes through the condenser and is subsequently pumped back into the vapor generator. In land-based applications, the condenser typically includes a plurality of airflow heat exchangers that transfer the thermal energy from the water to the ambient air.  
         [0006]     In the 1970&#39;s and 1980&#39;s the United States Navy investigated a marine application of a Rankine cycle system, referred to as the Rankine Cycle Energy Recovery (RACER) System. The RACER system, which utilized high-pressure steam as the working medium, was coupled to the drive system to augment propulsion horsepower. RACER could not be used to power any accessories because it as coupled to the drive system; i.e., if the drive system was not engaged, neither was the RACER system. The RACER system was never fully implemented and the program was cancelled because of problems associated with using high-pressure steam in a marine application.  
         [0007]     What is needed is a method and apparatus for power generation using waste heat from a power plant that can be used in a marine environment, and one that overcomes the problems associated with the prior art systems.  
       SUMMARY OF THE INVENTION  
       [0008]     According to the present invention, a method and apparatus for generating power aboard a marine vessel is provided. The method comprises the steps of: (a) providing a Rankine Cycle device that includes at least one of each of an evaporator, a turbo-generator that includes a turbine coupled with an electrical generator, a condenser, and a refrigerant feed pump; (b) disposing the one or more evaporators within an exhaust duct of a power plant of the marine vessel; (c) operating the power plant; and (d) selectively pumping refrigerant through the Rankine Cycle device, wherein refrigerant exiting the evaporator powers the turbine, which in turn powers the generator to produce electric power.  
         [0009]     The present method and apparatus provides significant advantages. For example, the range of a marine vessel that burns liquid fossil fuel within its power plant is typically dictated by the fuel reserve it can carry. In most modern marine vessels, a portion of the fuel reserve is devoted to running a power plant that generates electrical energy. Hence, both the propulsion needs and the electrical energy needs draw on the fuel reserve. The present method and apparatus decreases the fuel reserve requirements by generating electricity using waste heat generated by the power plant of the vessel rather than fossil fuel. Hence, the vessel is able to carry less fuel and have the same range, or carry the same amount of fuel and have a greater range. In addition, less fuel equates to lower weight, and lower weight enables increased vessel speed.  
         [0010]     If one considers the amount of fossil fuel that would be required to produce the electrical energy that can be created by the present method and apparatus via waste heat, it is clear that several other advantages are provided by the present invention. For example, the weight of the fuel required to produce “N” units of electrical power using the vessel&#39;s existing main or auxiliary power plants far exceeds the weight of a present ORC device capable of producing the same “N” units of electrical power via waste heat. In addition, consumption of liquid fuel changes the buoyancy characteristics of the vessel. The weight of the present ORC device remains constant, thereby facilitating buoyancy control of the vessel.  
         [0011]     For those embodiments that utilize a recuperator disposed within the condenser, the present inventor provides the additional benefits of an ORC device with increase efficiency disposed within a relatively compact unit.  
         [0012]     Yet another advantage of the present invention results from the thermal energy removed from the exhaust gases of the marine vessel power plant. The mass flow of the exhaust is a function of the volumetric flow and density of the exhaust. The present method and apparatus enables the exhaust gases to be significantly cooled and consequently the density of the exhaust gases increased. As a result, the mass flow is substantially decreased, and the required size of the marine power plant exhaust duct is substantially less.  
         [0013]     These and other objects, features and advantages of the present invention will become apparent in light of the detailed description of the best mode embodiment thereof, as illustrated in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a diagrammatic perspective view of an embodiment of the present invention ORC device, having a single turbo-generator.  
         [0015]      FIG. 2  is a diagrammatic perspective view of an embodiment of the present invention ORC device, having a pair of turbo-generators.  
         [0016]      FIG. 3  is a diagrammatic perspective view of an embodiment of the present invention ORC device, having three turbo-generators and a single condenser.  
         [0017]      FIG. 4  is a diagrammatic perspective view of an embodiment of the present invention ORC device, having three turbo-generators and a pair of condensers.  
         [0018]      FIG. 5  is a sectional planar view of a condenser.  
         [0019]      FIG. 6  is a diagrammatic perspective view of an evaporator.  
         [0020]      FIG. 7  is a schematic diagram of an ORC device that includes a single turbo-generator.  
         [0021]      FIG. 8  is a schematic diagram of an ORC device that includes a pair of turbo-generators.  
         [0022]      FIG. 9  is a schematic diagram of an ORC device that includes three turbo-generators.  
         [0023]      FIG. 10  is a schematic diagram of an ORC device that includes three turbo-generators and a pair of condensers.  
         [0024]      FIG. 11  is a diagrammatic pressure and enthalpy curve illustrating the Rankine Cycle.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Referring to  FIGS. 1-6 , the present method for utilizing waste heat includes an organic Rankine cycle (ORC) device  20  for waste heat utilization. The ORC device  20  includes at least one of each of the following: 1) a turbine coupled with an electrical generator (together hereinafter referred to as the “turbo-generator  22 ”); 2) a condenser  24 ; 3) a refrigerant feed pump  26 ; 4) an evaporator  28 ; and 5) a control system. The ORC device  20  is preferably a closed “hermetic” system with no fluid makeup. In the event of leaks, either non-condensables are automatically purged from the device  20  or charge is manually replenished from refrigerant gas cylinders.  
         [0026]     The ORC device  20  uses a commercially available refrigerant as the working medium. An example of an acceptable working medium is R-245fa (1,1,1,3,3, pentafluoropropane). R-245fa is a non-flammable, non-ozone depleting fluid. R-245fa has a saturation temperature near 300° F. and 300 PSIG that allows capture of waste heat over a wide range of IGT exhaust temperatures.  
         [0027]     Now referring to  FIGS. 1-4 , the turbo-generator includes a single-stage radial inflow turbine  30  that typically operates at about 18000 rpm, a gearbox  32  with integral lubrication system, and an induction generator  34  operating at 3600 rpm. The gearbox  32  includes a lubrication system. In some instances, the gearbox lubrication system is integral with the gearbox  32 .  
         [0028]     In one embodiment, the turbo-generator  22  is derived from a commercially available refrigerant compressor-motor unit; e.g., a Carrier Corporation model 19XR compressor-motor. As a turbine, the compressor is operated with a rotational direction that is opposite the direction it rotates when functioning as a compressor. Modifications performed to convert the compressor into a turbine include: 1) replacing the impeller with a rotor having rotor blades shaped for use in a turbine application; 2) changing the shroud to reflect the geometry of the rotor blades; 3) altering the flow area of the diffuser to enable it to perform as a nozzle under a given set of operating conditions; and 4) eliminating the inlet guide vanes which modulate refrigerant flow in the compressor mode. To the extent that there are elements within the 19XR compressor that have a maximum operating temperature below the operating temperature of the turbine  30 , those elements are replaced or modified to accommodate the higher operating temperature of the turbine  30 .  
         [0029]     In some embodiments, the turbo-generator  22  includes peripheral components such as an oil cooler  36  (shown schematically in  FIGS. 7-10 ) and oil reclaim eductor (not shown). Both the oil cooler  36  and the eductor and their associated plumbing are attached to the turbo-generator  22 .  
         [0030]     Referring to  FIG. 6 , a number of different evaporators  28  can be used with the ORC device  20 . A single pressure once-through evaporator  28  with vertical hot gas flow and horizontal flow of refrigerant through fin-tube parallel circuits serviced by vertical headers is an acceptable type of evaporator  28 . Examples of acceptable evaporator tube materials include carbon steel tubes with carbon steel fins, and stainless steel tubes with carbon steel fins, both of which have been successfully demonstrated in exhaust gas flows at up to 900° F. Other evaporator tube materials may be used alternatively. Inlet header flow orifices are used to facilitate refrigerant flow distribution. Different refrigerant flow configurations through the evaporator  28  can be utilized; e.g., co-flow, co-counterflow, co-flow boiler/superheater and a counterflow preheater, etc. The present evaporator  28  is not limited to any particular flow configuration.  
         [0031]     In all the evaporator  28  embodiments, the number of preheater tubes and the crossover point are selected in view of the desired hot gas exit temperature as well as the boiler section inlet subcooling. A pair of vertical tube sheets  38 , each disposed on an opposite end of the evaporator  28 , supports evaporator coils. Insulated casings  40  surround the entire evaporator  28  with removable panels for accessible cleaning.  
         [0032]     The number of evaporators  28  can be tailored to the application. For example, if there is more than one exhaust duct, an evaporator  28  can be disposed in each exhaust duct. More than one evaporator  28  disposed in a particular duct also offers the advantages of redundancy and the ability to handle a greater range of exhaust mass flow rates. At lower exhaust flow rates a single evaporator  28  may provide sufficient cooling, while still providing the energy necessary to power the turbo-generators  22 . At higher exhaust flow rates, a plurality of evaporators  28  may be used to provide sufficient cooling and the energy necessary to power the turbo-generators  22 .  
         [0033]     Referring to  FIGS. 1-5 , the condenser  24  is a shell-and-tube type unit that is sized to satisfy the requirements of the ORC device. The condenser  24  includes a housing  42  and a plurality of tubes  44  (hereinafter referred to as a “bank of tubes”) disposed within the housing  42 . The housing  42  includes a working medium inlet port  46 , a working medium exit port  48 , a coolant inlet port  50 , and a coolant exit port  52 . The coolant inlet and exit ports  50 , 52  are connected to the bank of tubes  44  to enable cooling fluid to enter the condenser  24  housing, pass through the bank of tubes  44 , and subsequently exit the condenser housing  42 . Likewise, the working medium inlet and exit ports  46 , 48  are connected to the condenser housing  42  to enable working medium to enter the housing  42 , pass around the bank of tubes  44 , and subsequently exit the housing  42 . In some embodiments, one or more diffuser plates  54  (see  FIG. 5 ) are positioned adjacent the working medium inlet  46  to facilitate distribution of the working medium within the condenser  24 . In the embodiment shown in  FIGS. 1-4 , the housing  42  includes a removable access panel  56  at each axial end of the housing  42 . In a preferred embodiment, one of the access panels  56  is pivotally attached to one circumferential side of the housing  42  and attachable to the opposite circumferential side via a selectively operable latch (not shown) so that the access panel  56  may be readily pivoted to provide access to the bank of tubes  44 .  
         [0034]     In some embodiments, a non-condensable purge unit  58  (shown schematically in  FIGS. 7-9 ) is attached to the condenser  24 . The purge unit  58  is operable to extract air and water vapor that may accumulate in the vapor region of a condenser housing  42  to minimize or eliminate their contribution to oil hydrolysis or component corrosion. The purge unit  58  is actuated only when the system controller thermodynamically identifies the presence of non-condensable gas.  
         [0035]     Referring to  FIG. 5 , in some embodiments, the ORC device  20  includes a recuperator  60  for preheating the working medium prior to its entry into the evaporator  28 . The recuperator  60  is operable to receive thermal energy from at least a portion of the working medium exiting the turbo-generator  22  and use it to preheat working medium entering the evaporator  28 . In the embodiment shown in  FIG. 5 , the recuperator  60  includes a plurality of ducts  62  disposed within the housing  42  of the condenser  24 . The ducts  62  are connected inline downstream of the working medium exit port  48  of the condenser  24  and upstream of the evaporator  28 . A partition  64  partially surrounds the recuperator ducts  62  to separate them from the remainder of the condenser  24 . Working medium enters the condenser  24  through the working medium inlet port  46  and passes through the recuperator  60  prior to entering the remainder of the condenser  24 . One or more diffusers  54  can be disposed within the recuperator to facilitate distribution of the working medium within the recuperator  60 . Placing the recuperator  60  within the condenser  24  advantageously minimizes the size of the ORC device  20 . A recuperator  60  disposed outside of the condenser  24  can be used alternatively, however.  
         [0036]     Referring to  FIGS. 1-4 , the ORC device  20  includes one or more variable speed refrigerant feed pumps  26  to supply liquid refrigerant to the evaporator  28 . In one embodiment, the refrigerant feed pump  26  is a turbine regenerative pump that supplies liquid refrigerant to the evaporator  28  with relatively low net pump suction head (NPSH). This design, combined with the relatively low system pressure difference, allows the feed pump  26  and condenser  24  to be mounted at the same elevation and obviates the need for separate condensate and feed pumps. In alternative embodiments, the refrigerant feed pump  26  may be a side channel centrifugal pump or an axial inlet centrifugal pump. The refrigerant feed pump  26  is equipped with an inverter to allow fully proportional variable speed operation across the full range of exhaust conditions. Other pump controls may be used alternatively. Applications using two or more refrigerant feed pumps  26  offer the advantage of redundancy. In some embodiments, the piping  74  disposed immediately aft of each of the feed pumps  26  are connected to one another by a cross-over piping segment  76 . Multiple refrigerant feed pumps  26  and the cross-over segment  76  enhance the ability of the ORC device  20  to accommodate a marine environment having significant pitch and roll by collecting working medium at different locations in the condenser  24 . ORC configurations having more than one turbo-generator  22  and more than one refrigerant feed pump  26  are provided with valves  66  (see  FIGS. 7-10 ) that enable each turbo-generator  22  or feed pump  26  to be selectively removed from the working medium flow pattern. Alternatively, a feed pump  26  may be associated with each turbo-generator  22 , and selective actuation of the associated feed pump  26  can be used to engage/disengage the associated turbo-generator  22 .  
         [0037]     The ORC device  20  configurations shown in  FIGS. 7-10  each includes a cooling circuit  68  used in marine applications, wherein a cooling medium (e.g., seawater) is accessed from a cooling medium source  70  (e.g., the body of water in the environment surrounding the marine vessel) and circuitously passed through the condenser  24  (via the coolant inlet and exit ports  50 , 52 ) and returned to the cooling medium source  70 . In alternative embodiments, the cooling circuit  68  includes a heat exchanger (e.g., a cooling tower) to remove thermal energy from the cooling medium.  
         [0038]     ORC device  20  configurations are shown schematically in  FIGS. 7-10 . These configurations represent examples of ORC device  20  configurations and should not be interpreted as the only configurations possible within the present invention. Arrows indicate the working medium flow pattern within each configuration.  
         [0039]     Referring to a first configuration shown in  FIG. 7 , beginning at a pair of refrigerant feed pumps  26 , working medium is pumped toward an evaporator  28 . In the embodiment shown in  FIG. 7 , prior to entering the evaporator  28 , the working medium passes through a recuperator  60 , wherein the working medium is preheated. In a marine application, the evaporator  28  is disposed within an exhaust duct that receives exhaust products from the vessel&#39;s power plant. Working medium exiting the evaporator  28  subsequently travels toward the turbo-generator  22 . A bypass valve  72 , disposed between the evaporator  28  and the turbo-generator  22 , enables the selective diversion of working medium around the turbo-generator  22  and toward the condenser  24 . An orifice  73  is disposed downstream of the bypass valve  72  to produce a flow-restriction. As will be discussed below, the bypass valve  72  is operable to fully bypass working medium around the turbo-generator  22 . Alternatively, the bypass valve  72  can operate to selectively vary the amount of working medium that is introduced into the turbo-generator  22 . Assuming some, or all, of the working medium has not been diverted around the turbo-generator  22 , the working medium enters the turbine  30  portion of the turbo-generator  22  and provides the energy necessary to power the turbo-generator  22 . Once through the turbo-generator  22 , the working medium travels toward the condenser  24 . Working medium that is diverted around the turbo-generator  22  also travels toward the condenser  24 . A perspective view of this configuration of the ORC device  20  is shown in  FIG. 1 , less the evaporator  28 .  
         [0040]     A second ORC device  20  configuration is schematically shown in  FIG. 8  that includes a pair of turbo-generators  22 . The turbine inlets are connected to a feed conduit from the evaporator  28 . A turbine inlet valve  66   a  is disposed immediately upstream of each turbo-generator  22 . In some embodiments, a turbine exit valve  66   b  is disposed immediately downstream of each turbo-generator  22 . In those embodiments, a safety pressure bleed is provided connected to the low pressure side of the ORC device. The second ORC device  20  configuration also includes a plurality of evaporators  28 . An evaporator inlet valve  78  is disposed immediately upstream of each evaporator  28 . In some embodiments, an evaporator exit valve  80  is disposed immediately downstream of each evaporator  28 . A perspective view of this configuration of the ORC device  20  is shown in  FIG. 2 , less the evaporator  28 .  
         [0041]     A third ORC device  20  configuration is schematically shown in  FIG. 9  that includes three turbo-generators  22 . A perspective view of a portion of this configuration of the ORC device  20  is shown in  FIG. 3 , less the evaporator  28 .  
         [0042]     A fourth ORC device  20  configuration is schematically shown in  FIG. 10  that includes three turbo-generators  22  and a pair of condensers  24 . A perspective view of a portion of this configuration of the ORC device  20  is shown in  FIG. 4 , less the evaporator  28 .  
         [0043]     In all of the configurations, the ORC controls maintain the ORC device  20  along a highly predictable programmed turbine inlet superheat/pressure curve through the use of the variable speed feed pump  26  in a closed hermetic environment. An example of such a curve is shown in  FIG. 11 .  
         [0044]     The condenser load is regulated via the feed pump(s)  26  to maintain condensing pressure as the system load changes. In addition to the primary feed pump speed/superheat control loop, the ORC controls can also be used to control: 1) net exported power generation by controlling either hot gas blower speed or bypass valve  72  position depending on the application; 2) selective staging of the generator  34  and gearbox  32  oil flow; and 3) actuation of the purge unit  58 . The ORC controls can also be used to monitor all ORC system sensors and evaluate if any system operational set point ranges are exceeded. Alerts and alarms can be generated and logged in a manner analogous to the operation of a commercially available chillers, with the control system initiating a protective shutdown sequence (and potentially a restart lockout) in the event of an alarm. The specific details of the ORC controls will depend upon the specific configuration involved and the application at hand. The present invention ORC device  20  can be designed for fully automated unattended operation with appropriate levels of prognostics and diagnostics.  
         [0045]     The ORC device  20  can be equipped with a system enable relay that can be triggered from the ORC controls or can be self-initiating using a hot gas temperature sensor. After the ORC device  20  is activated, the system will await the enable signal to begin the autostart sequence. Once the autostart sequence is triggered, fluid supply to the evaporator  28  is ramped up at a controlled rate to begin building pressure across the bypass valve  72  while the condenser load is matched to the system load. When the control system determines that turbine superheat is under control, the turbine oil pump is activated and the generator  34  is energized as an induction motor. The turbine speed is thus locked to the grid frequency with no requirement for frequency synchronization. With the turbine at speed, the turbine inlet valve  66   a  opens automatically and power inflow to the generator seamlessly transitions into electrical power generation.  
         [0046]     Shutdown of the ORC device  20  is equally straightforward. When the temperature of the exhaust products passing through the evaporator(s)  28  falls below the operational limit, or if superheat cannot be maintained at minimum power, the ORC controls system begins an auto-shutdown sequence. With the generator  34  still connected to the grid, the turbine inlet valve  66   a  closes and the turbine bypass valve  72  opens. The generator  34  once again becomes a motor (as opposed to a generator) and draws power momentarily before power is removed and the unit coasts to a stop. The refrigerant feed pump  26  continues to run to cool the evaporator  28  while the condenser  24  continues to reject load, eventually resulting in a continuous small liquid circulation through the system. Once system temperature and pressure are adequate for shutdown, the refrigerant feed pump  26 , turbine oil pump, and condenser  24  are secured and the system is ready for the next enable signal.  
         [0047]     When the autostart sequence is complete, the control system begins continuous superheat control and alarm monitoring. The control system will track all hot gas load changes within a specified turndown ratio. Very rapid load changes can be tracked. During load increases, significant superheat overshoot can be accommodated until the system reaches a new equilibrium. During load decreases, the system can briefly transition to turbine bypass until superheat control is re-established. If the supplied heat load becomes too high or low, superheat will move outside qualified limits and the system will (currently) shutdown. From this state, the ORC device  20  will again initiate the autostart sequence after a short delay if evaporator high temperature is present.  
         [0048]     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and the scope of the invention.