Abstract:
A fluid-displacer engine which utilizes the thermodynamic Stirling cycle to extract energy from an external thermal gradient is disclosed. A working gas is disposed in each of two adjacent cylinders and is cycled from a hot region to a cold region of the respective cylinders by movement of a hot displacer fluid and a cold displacer fluid. Alternate heating and cooling of the working gas in each chamber causes the displacer fluid to flow from one cylinder to the other which, in turn allows one of the working gasses to expand and compresses the other. The flow of displacer fluids can be optimally controlled by the use of control valves. Energy can be extracted from the flow of the displacer fluids by the use of turbines in the displacer fluid flow paths.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 61/068,215 filed on Mar. 5, 2008 which is hereby incorporated by reference and U.S. Provisional Application No. 61/106,615 filed on Oct. 20, 2008 which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of energy conversion and more particularly to stirling engines for use in ocean thermal energy conversion systems. 
     BACKGROUND OF THE INVENTION 
     The stirling engine is a well known machine which utilizes the thermodynamic stirling cycle for converting thermal energy into mechanical or electrical energy. In a typical stirling engine, a working gas such as air, hydrogen or helium is alternately heated by a heat source and cooled by a heat sink. The expansion and compression of the working gas in response to the heating and cooling cycle is used to drive one or more pistons which in turn typically drive a shaft or drive gear system. 
     One well known type of stirling engine is the displacer-type stirling engine which is described with reference to  FIGS. 1A-1D . Referring first to  FIG. 1A , the displacer-type stirling engine  100  includes one power piston  102  and a displacer  104 . A working gas  106  moves in a chamber  108  from one side of the displacer  104  to the other side of the displacer  104 . Heating one side of the chamber  108  and cooling the other side of the chamber  108  causes repeated alternate expansion and contraction of the working gas  106  on alternate sides of the displacer  104  which in turn causes the displacer  104  to move alternately between the hot and the cold side of the chamber  108 . The working piston  102  is tightly sealed in a secondary chamber  110  in communication with the displacer chamber  108  and is forced upward during a power stroke as the working gas  106  on the hot side of the chamber expands. The working piston  102  may be mechanically linked by a crank shaft, for example, to the displacer  104  which times and coordinates their relative movements. The mechanical linkage, not shown, causes the working piston  102  to compress the working gas  106  and provides a downward movement ( FIG. 1B ) to the displacer  104 . Heat is extracted from the working gas  106  by a regenerator  112  which aids in the compression of working gas  106  on the cold side of the displacer and causes this gas to move around the displacer and re-fill the hot side of the chamber ( FIG. 1C ). The cool working gas is then heated by the hot side of the chamber ( FIG. 1D ) to drive the power piston  102  and move the displacer  104  downward. Energy is thereby extracted from the working gas in response to a temperature differential between the hot and cold sides of the chamber. 
     Various systems and methods have been known for extracting energy from the oceans and converting the oceans&#39; thermal energy to other forms of useful energy. The field of ocean thermal energy conversion (OTEC) holds much promise as a renewable energy source when certain technical barriers are overcome. In order to extract energy from the oceans, an OTEC system must include portions that extend from the warm ocean surface to much colder waters in the ocean depths. Disadvantageously, displacer-type stirling engines have been heretofore found to be impractical for use in OTEC systems due to the large sized displacement chambers that would be required and the various mechanical linkages which must span large distances. 
     SUMMARY OF THE INVENTION 
     The present invention provides a fluid-displacer engine which utilizes the thermodynamic stirling cycle to extract energy from an external thermal gradient. A working gas is disposed in each of two adjacent cylinders and is cycled from a hot region to a cold region of the respective cylinders by movement of a hot displacer fluid and a cold displacer fluid. Alternate heating and cooling of the working gas in each chamber causes the displacer fluid to flow from one cylinder to the other which, in turn allows one of the working gasses to expand and compresses the other. The flow of displacer fluids can be optimally controlled by the use of control valves. Energy can be extracted from the flow of the displacer fluids by the use of turbines in the displacer fluid flow paths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The foregoing and other features and advantages of the present invention will be more fully understood from the following detailed description of illustrative embodiments, taken in conjunction with the accompanying drawings in which: 
         FIGS. 1A-1D  are schematic diagrams of four operational states of a displacer-type stirling engine according to the PRIOR ART; 
         FIG. 2  is a schematic diagram of a fluid displacer engine according to an illustrative embodiment of the invention; 
         FIG. 3A-3D  are schematic diagrams of four operational states of a fluid displacer engine according to an illustrative embodiment of the invention; 
         FIG. 4  is a schematic diagram of a fluid displacer including a pump and nozzle system for spraying cold displacer fluid through a working gas to more efficiently exchange heat there-between; and 
         FIG. 5  is a schematic diagram of a fluid displacer engine including displacer fluid nozzles and heat exchangers for more efficient operation according to an illustrative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments of the present invention overcome the disadvantages of previously known stirling engines and previously known OTEC systems by replacing the solid displacer of a displacer-type stirling engine with hot and cold displacer fluids which cycle between pairs of hot and cold displacer chambers. 
     An illustrative embodiment of the invention is described with reference to  FIG. 2 . A fluid-displacer engine  300  includes a first hot fluid reservoir  302  connected to a second hot fluid reservoir  304  via a hot transfer pipe  310 . A first cold fluid reservoir  306  is connected to a second cold fluid reservoir  308  via a cold transfer pipe  312 . The first hot fluid reservoir  302  is connected to the first cold fluid reservoir  306  via a first gas conduit. The second hot fluid reservoir  304  is connected to the second cold fluid reservoir  308  via a second gas conduit  316 . 
     A hot displacer fluid  318  is disposed within the first hot fluid reservoir  302  and second hot fluid reservoir  304  and can pass there-between via the hot transfer piper  310 . A cold displacer fluid  320  is disposed within the first cold fluid reservoir  306  and the second cold fluid reservoir  308  and can travel there-between via the cold transfer pipe  312 . The hot displacer fluid  318  is prevented from passing through the first gas conduit  314  and the second gas conduit  316  to either of the first cold fluid reservoir  306  or the second cold fluid reservoir  308 . The cold displacer fluid  320  is prevented from passing through the first gas conduit  314  and the second gas conduit  316  to either of the first hot fluid reservoir  302  or the second hot fluid reservoir  304 . 
     In the illustrative embodiment, the first gas conduit  314  and second gas conduit  316  are vertically oriented. The top end of each gas conduit  314 ,  316  are disposed above the highest hot fluid level in their respective hot fluid reservoirs  302 ,  304 . The bottom end of each gas conduit  314 ,  316  are disposed above the highest cold fluid level in their respective cold fluid reservoirs  306 ,  308 . 
     A first working gas  322  is free to travel between the first hot fluid reservoir  302  and the first cold fluid reservoir  306  via the first gas conduit  314 . A second working gas  324  is free to travel between the second hot fluid reservoir  304  and the second cold fluid reservoir  308 . The thermal gradient between the hot fluid reservoirs  302 ,  304  and the cold fluid reservoirs  306 ,  308  alternately increases and decreases pressure in each working gas  322 ,  324  causing the hot displacer fluid  318  to cycle between the hot fluid reservoirs  302 ,  304  and causing the cold displacer fluid  320  to cycle between the cold fluid reservoirs  306 ,  308 . Flow control valves (not shown) may be used, for example, in transfer pipes  310 ,  312  to maintain the alternating directional flows of hot displacer fluid  318  and cold displacer fluid  320  and to allow time for a sufficient difference in pressure to develop between the first working gas  322  and the second working gas  324 . Energy can be extracted from the flow of the displacer fluids by providing an energy extracting device in communication with the hot fluid flow and or cold fluid flow for providing output energy from the fluid displacement engine  300 . Various energy extraction devices that are known in the art are suitable, for example a turbine generator (not shown) may be provided in the hot transfer pipe  310  and/or in the cold transfer pipe  312 . An illustrative embodiment of the invention, which includes a system of control valves and turbine generators, is described with reference to  FIGS. 3A-3D . 
     In  FIG. 3A , a fluid displacer engine  400  is shown wherein a first working gas  422  has been pushed into a first hot fluid reservoir  402  from the colder environment of a first cold fluid reservoir  406 . The hotter environment of the first hot fluid reservoir  402  adds heat to the first working gas  422 . A second working gas  424  has been pushed into a second cold fluid reservoir  408  from the warmer environment of the second hot fluid reservoir  304 . The colder environment of the second cold fluid reservoir  408  extracts heat from the second working gas  424 . A first valve  430  in the hot transfer pipe  410  is closed and a second valve  432  in the cold transfer pipe  412  is closed which prevents flow of the hot displacer fluid  418  and cold displacer fluid  420  and thereby maintains a constant volume for the first working gas  422  and second working gas  424 . Pressure in the first working gas  422  begins to increase as it warms and pressure in the second working gas  424  begins to decrease as it cools. 
     In  FIG. 3B  the next state of the fluid displacer engine  400  is shown wherein the first working gas  422 , having been heated in the first hot fluid reservoir  402 , has a higher pressure than the second working gas  424  which has been cooled in the second cold fluid reservoir  408 . The second valve  432  is then open and the first valve  430  remains closed. The pressure differential between the first and second working gasses  422 ,  424  causes cold displacer fluid  420  to flow through the cold transfer pipe  412  from the first cold fluid reservoir  406  to the second cold fluid reservoir  408  until the pressure of the first working gas  422  equals the pressure of the second working gas  424 . 
     In the illustrative embodiment, a first turbine  434  is disposed in the hot transfer pipe  410  and a second turbine  436  is disposed in the cold transfer pipe  412 . Flow of the cold displacer fluid  420  through the cold transfer pipe  412  causes rotation of the second turbine  436  which can be used to drive a generator or other mechanical device to extract energy from the fluid displacer engine  400 . 
     In  FIG. 3C , the next state of the fluid displacer engine  400  is shown wherein the first valve  430  and second valve  432  are both closed. The second working gas  424 , having been compressed and displaced from the second cold fluid reservoir  408  to the second hot fluid reservoir  404  by the flow of the cold displacer fluid  420 , is heated by the warmer environment of the second hot fluid reservoir  404  which increases the pressure of the second working gas  424 . The first working gas  422 , having been expanded and displaced from the first hot fluid reservoir  402  to the first cold fluid reservoir  406 , is cooled by the cooler environment of the second cold fluid reservoir  406  which decreases the pressure of the first working gas  422 . 
     In  FIG. 3D , the next state of the fluid displacer engine  400  is shown wherein the second working gas  424 , having been heated in the second hot fluid reservoir  404 , has a higher pressure than the first working gas  422  which has been cooled in the first cold fluid reservoir  406 . The first valve is then open and the second valve  432  remains closed. The pressure differential between the first and second working gasses  422 ,  424  causes the hot displacer fluid  418  to flow through the hot transfer pipe  410  from the second hot fluid reservoir  404  to the first hot fluid reservoir  402 . Flow of the hot displacer fluid  418  from through the hot transfer pipe  410  causes rotation of the first turbine  434  to extract energy from the fluid displacer engine  400 . 
     The cycle describe with reference to  FIGS. 3A-3D  continuously repeats, reversing direction for each new cycle. 
     In an illustrative embodiment, the inventive fluid displacer engine is employed in the context of an ocean thermal energy conversion (OTEC) system. In this embodiment, the hot fluid reservoirs are disposed at or near the ocean surface and the cold fluid reservoirs are disposed in the ocean depths, for example about 200 meters deep, where the ocean water is substantially cooler to take advantage of the natural temperature gradient of the ocean, and to thereby extract thermal energy from the ocean. The temperature gradient between the hot and cold fluid reservoirs can be further increased by maximizing the solar energy absorption of the hot fluid chambers. 
     The efficiency of a fluid displacer engine according to the various embodiments of the invention can be improved by increasing the heating and cooling rates of the working gas. The structure of the hot and cold fluid reservoirs and/or the first gas conduit and second gas conduit may be designed to efficiently exchange heat with the external environment by maximizing their surface area, for example, or by utilizing devices such as heat sinks and heat pipes in the interface between the working gases (and/or fluids) and the external environment. 
     A method of increasing the efficiency of a fluid displacer engine according to an illustrative embodiment of the invention is described with reference to  FIG. 4  wherein the cold working fluid  460  of the fluid displacer engine  450  is circulated within one or both of the cold fluid reservoirs  456 ,  458  by pumping the fluid through a nozzle  462 ,  464 . The spray of cold working fluid from the nozzle  462 ,  464  increases the rate of cooling of the working gas  466  through which it is sprayed. The nozzle  462 ,  464  is designed to spray droplets of the displacer fluid through the working gas without causing the displacer fluid to evaporate in the working gas. Circulator pumps  476 ,  478  are externally connected by means not shown to receive power and control signals as will be apparent to persons having ordinary skill in the art so that the appropriate circulator pump  476 ,  478  is energized at the time when it is desirable to cool the working gas in its respective cold fluid reservoir  456 ,  458 . Valves  468 ,  470  are operated as described hereinbefore with reference to the embodiment of  FIGS. 3A-3D . Turbines  472 ,  474  or other energy extraction devices can be used to extract energy from the working fluids as described hereinbefore with reference to  FIGS. 3A-3D . 
     Although circulator pumps  476 ,  478  are shown as circulating displacer fluid entirely within a respective displacer fluid reservoir, various embodiments of the present invention (not shown) include a circulation path that extends out of the displacer fluid chamber, then optionally through a heat exchanger and back into the respective displacer fluid reservoir. In at least one embodiment, each displacer fluid reservoir is equipped with a circulator pump which passes the displacer fluid through a corresponding heat exchanger and then sprays the displacer fluid, via a nozzle, for example, back into the displacer fluid reservoir. 
     Another method of increasing the efficiency of a fluid displacer engine according to an illustrative embodiment of the invention is described with reference to  FIG. 5  in which the cold displacer fluid passes through a nozzle and is sprayed into the appropriate cold fluid reservoir to more efficiently cool the working gas. The fluid displacer engine  500  includes a first hot fluid reservoir  502  connected to a second hot fluid reservoir via a hot fluid transfer pipe  507  and hot fluid valve  510 . A first cold fluid reservoir  506  is connected to a second cold fluid reservoir  508  via a first cold fluid transfer pipe  512  and first cold fluid valve  513 . The first cold fluid reservoir  506  is also connected to the second cold fluid reservoir  508  via a second cold fluid transfer pipe  515  and a second cold fluid valve  517 . 
     The embodiment shown in  FIG. 5  performs in essentially the same manner as the embodiment described with reference to  FIG. 3  except that the valves  513  and  517  are controlled, by a controller for example, to enforce unidirectional flow from an input end of one cold transfer pipe to an output end of the same transfer pipe, while closing flow through the other cold transfer pipe. Each cold fluid transfer pipe  512 ,  515  is terminated at its output end with a nozzle  519  which causes the displacer fluid to be sprayed into its respective chamber and thereby more efficiently absorb heat from the working gas. The nozzle is designed to spray droplets of the displacer fluid through the working gas without causing the displacer fluid to evaporate in the working gas. 
     In the illustrative embodiment, a turbine  521  may be disposed in the hot transfer pipe  507 ,  510  and in each cold transfer pipe  512 ,  515 . Flow of the displacer fluid through an appropriate transfer pipe causes rotation of the respective turbine which can be used to drive a generator or other mechanical device to extract energy from the fluid displacer engine  500 . 
     The efficiency of the fluid displacer engine  500  may be further enhanced by preheating the hot displacer fluid and/or by pre-cooling the cold displacer fluid during transfer of the respective displacer fluid to its appropriate fluid reservoir. In an illustrative embodiment, a pre-heating coil  523  is disposed in the hot transfer pipe  507  and a pre-cooling coil  525  is disposed in each of the cold transfer pipes  512 ,  515 . Alternatively or in addition to the pre-heating coil  523  and pre-cooling coil  525 , various other pre-heating and/or pre-cooling mechanisms may be used to preheat and/or pre-cool the displacer fluids as they are transferred into their appropriate fluid reservoir. 
     The various valves disclosed herein may be controlled manually or by any number of different controller types such as a mechanical linkage, a micro controller or other microprocessor system, for example. Persons having ordinary skill in the art should appreciate that use of computer controlled valves and pumps can provide timing of displacer fluid transfer to optimize heat transfer between a working gas and the displacer fluid. In one embodiment, for example, the transfer of hot and cold displacer fluid is aided by the use of pumps to assist in the flow of displacer fluid from one displacer fluid reservoir to the other corresponding displacer fluid reservoir. In this embodiment computer controlled valves provide precise timing to effect optimal heat transfer between the working gas and the displacer fluid. In addition to the cold displacer fluid being sprayed through a nozzle in the working gas, the use of computer controlled valves and pumps can be combined with the use of nozzles in the hot displacer chamber for spraying hot displacer fluid through the working gas. 
     The hot and cold displacer fluids may be the same or different fluids and preferably have a high heat capacity. Examples of fluids that would be suitable for use as hot and cold displacer fluids according to the various illustrative embodiments of the invention include water or virtually any liquid which will not change state in the presence of the working gas given the working temperatures and pressures of the fluid displacement engine. 
     The first and second working gas may be the same or different types of gas and preferable have a high heat capacity. Examples of gasses that would be suitable for use as a working gas according to the various illustrative embodiments of the invention include air, nitrogen, hydrogen, and helium, for example. 
     While the invention has been described with reference to illustrative embodiments, it will be understood by those skilled in the art that various other changes, omissions, and/or additions may be made and substantial equivalents may be substituted for elements thereof without departing from the spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from the scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed for carrying out this invention, but that the invention will include all embodiments, falling within the scope of the appended claims. Moreover, unless specifically stated any use of the terms first, second, etc., do not denote any order of importance, but rather the terms first, second, etc. are used to distinguish one element from another.