Abstract:
A process employing a rotary axial turbine, and a nozzle, that includes providing a working fluid expansible from a characteristic supercritical region into the wet region, expanding the fluid from said supercritical region into the wet region via the nozzle and turbine blades, and providing an output shaft driven by the turbine.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to refrigeration cycle efficiency, and more particularly to recovery of energy in a process where a gas is expanded from a supercritical region into a wet region of the cycle. 
     The need for refrigeration systems using environmentally benign refrigerants has become greater in the last decade because of the dual drivers of ozone depletion and global warming. “Optimal” refrigerants that were engineered to maximize the cycle efficiency by minimizing the expansion throttling loss are no longer usable. The major air conditioning manufacturers have adopted refrigerants which either have a high vapor pressure and hence throttle loss (R- 134 A), or which have partial ozone depletion potential and high toxicity level (40 ppm Allowable Exposure Limit) and are due to be phased out (HFC- 123 ). 
     The choice of CO 2  as a refrigerant has many benefits. It is a natural, non-toxic substance with no ozone depletion potential. Additionally, there are volumetric and heat transfer advantages and the refrigerant has a low cost. A disadvantage is that the high pressure difference for compression and expansion result in a high throttling loss, leading to a poor cycle efficiency compared to current HCFC refrigerants. 
     However, a supercritical CO 2  refrigeration cycle with an expander was found to have the same or better performance as an HCFC cycle with expansion valve. The gain in CO 2  refrigerant cycle efficiency resulting from energy recovery with a 60% efficient expander was as much as 33% compared to a CO 2  cycle with a throttling valve, and 25% compared to the same throttling valve cycle with maximum internal heat exchange. 
     The need for an expander to recover energy from the expansion is clearly indicated if the CO 2  refrigeration cycle is to be widely deployed. However, the expansion, which starts in the supercritical region, enters the two-phase region, producing over 50% liquid. by mass. 
     No practical expanders have been developed for this range of operation from the supercritical region into the saturation region (transcritical expansion). Attempts to use radial inflow machines in the wet region have not been successful due to poor performance and erosion from the liquid centrifuging outward. Attempts to use positive displacement machines have not been, successful due to high cost and size and reliability issues. 
     Another requirement for expanders, to improve the efficiency of a refrigeration system, is a cost effective method to use the generated shaft power. 
     SUMMARY OF INVENTION 
     A primary objective of the invention is to provide an efficient, cost effective method of recovering energy in a process where a gas is expanded from the supercritical region into the wet region of the cycle. Another important objective is to provide an efficient, cost effective means and method to utilize the power generated by the above expansion to reduce the power required by the process, for example by a compressor in a refrigeration cycle. 
     Another object is to provide a method of operating an axial flow turbine having rotor blades rotating about an axis, employing a working fluid capable of two-phase flow, that includes: 
     a) vaporizing said fluid 
     b) compressing the vaporized fluid to a supercritical state, 
     c) cooling the compressed vaporized fluid while maintaining it in said supercritical state, 
     d) providing and operating a nozzle to receive and expand the compressed vaporized fluid to a pressure and temperature in the wet region of the cycle characterized by formation of liquid phase fluid droplets in the two-phase flow from the nozzle, 
     e) directing that two-phase flow toward the turbine blades whereby the flow is turned in the spaces between the blades and directed to flow axially of the turbine rotor as well as outwardly away from its axis, producing a swirl of the flow leaving the blades, which typically reduces, and operable to produce torque transferable to the rotor, and to act in the direction of rotor rotation. 
     f) and subsequently vaporizing said flow that leaves the blades pursuant to step a). 
     A further objective is to provide a turbine that comprises: 
     a) an expansion nozzle contoured to expand the high pressure, supercritical fluid to a lower pressure in the wet region, producing a high velocity directed flow of gas and sub-micron liquid droplets or of supersaturated gas, 
     b) axial flow blades, attached to a rotor, directed to receive the high velocity flow and turn it, generating torque acting on the rotor, 
     c) surfaces oriented to receive liquid from the turned flow and direct it away from the moving (rotating) rotor, 
     d) an exit duct to remove the gas and liquid flow from the flow directing surfaces and moving rotor, 
     e) and a shaft coupled to the rotor to transfer generated torque to a load. 
     An additional objective is to provide an assembly consisting of the above turbine elements and a load that includes a compressor to increase the pressure of a gas whereby power for the compressor is provided at least in part by the power generated by the turbine. 
     Yet another objective is to use the above assembly to increase the pressure of the flow leaving the turbine, after the liquid is evaporated in a refrigeration system or heat pump, thereby reducing the power required by the main compressor for the system. 
    
    
     These and other objects and advantages of the invention, as well as the details of an illustrative embodiment, will be more fully understood from the following specification and drawings, in which: 
     DRAWING DESCRIPTION 
     FIG. 1 is a schematic of supercritical refrigeration cycle with transcritical turbine; 
     FIG. 2 is a temperature entropy diagram for transcritical refrigeration cycle; 
     FIG. 3 is a schematic of supercritical. refrigeration cycle with throttling valve; 
     FIG. 4 is a section taken through the transcritical turbine and boost compressor; 
     FIG. 5 is a section taken through a transcritical nozzle; and 
     FIG. 6 is an enlarged fragmentary section taken through turbine impulse blades, and fluid flow paths. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a schematic diagram of a preferred refrigeration and heat pump process or system utilizing a transcritical turbine (TCT). The fluid is vaporized in an evaporator at  1 . The gas leaving the evaporator at  2 , is compressed in a compressor at  3 , to a higher pressure indicated at  3   a . The gas is further compressed by a main compressor  4  to a higher pressure shown at  6 . The main compressor is driven by a motor or other prime mover  5 . 
     The compressed gas  6  is in the supercritical region, i.e., having a temperature and temperature above the vapor dome for the substance. The supercritical gas is cooled in a heat exchanger  7  to conditions at  8 , which are also supercritical. The cooled supercritical gas is expanded in the nozzle  9  of the transcritical turbine to a pressure and temperature in the wet region of the fluid. The high velocity fluid leaving the nozzle drives the transcritical turbine  10  producing power that drives a shaft  12 . The fluid leaving the TCT at  11  enters the evaporator  1  where the liquid portion of the fluid is evaporated. 
     The heat source to evaporate the liquid can be a liquid or gas stream  14  to be cooled (refrigeration system), or a liquid or gas stream that is a source of heat to be raised in temperature (heat pump system). Similarly, the heat sink to cool the supercritical flow stream flowing via heat exchanger  7  from  6  to  8 , can be a liquid or gas stream  12   a , whose function is to remove and dissipate heat (refrigeration system), or which is to be heated (heat pump system). 
     An electric generator can be substituted for the compressor  3 , and the flow  2  routed directly to the main compressor  4  as via valve  24 . In this case the power generated by the electric generator can be used to reduce the electrical power required by the main compressor if it is driven by an electric motor also indicated at  5 . 
     A conventional refrigeration or heat pump process is shown for comparison in FIG.  3 . Fluid is vaporized in an evaporator  1   b . The gas produced, indicated at  2   b  flows into the compressor  4   b , and is compressed to a higher pressure at  6   b . The main compressor is driven by a motor or other prime mover,  5   b.    
     The compressed gas  6   b  is in the supercritical region, i.e., having a temperature above the vapor dome for the substance. The supercritical gas is cooled in a heat exchanger  7   b , to conditions at  8   b  which are also supercritical. The cooled supercritical gas is expanded in a throttling valve  10   b  to a pressure and temperature in the wet region of the fluid. The fluid leaving the throttling valve at  11   b , enters the evaporator  1   b , where the liquid portion of the fluid is evaporated. 
     The improvement provided by the TCT is illustrated by FIG. 2, a temperature entropy diagram for the processes. The numbers correspond to the conditions in FIG.  1  and FIG.  3 . In both processes the gas is compressed to a point in the supercritical region  6  and  6   b , and is cooled to a point  8  and  8   b , in the supercritical region. However the power generated by the TCT increases the gas pressure to point  3   a . The power required by the main compressor is then decreased since it only needs to increase the pressure from  3   a  to  6 , whereas the throttling valve process requires the compressor to increase the pressure form  2   b  to  6   b . In addition, since the expansion through the TCT is nearly isentropic, more liquid, as shown, is produced than in the throttling valve process. Thus, more cooling is also produced. 
     The TCT is shown in FIG.  4 . Supercritical working fluid such as CO 2  from the heat rejection heat exchanger enters a volute  1   d , which supplies one or more transcritical nozzles  12   d  in body structure  25 . The fluid is expanded in the nozzle or nozzles forming a two-phase mixture jetting at  26 , at high velocity. The flow is directed on the axial flow blades  2   d , transferring energy to the rotor  6   d . The flow swirls from the blades and from a blade shroud  3   d , the liquid being collected on the outlet pipe walls  4   d , as the flow is turned. The cooled two-phase mixture at  5   d  leaves through a vertical elbow  27  and flows to the evaporator as via path  11  seen in FIG.  1 . 
     The rotor power drives a boost compressor rotor  10   d  which may be on the same shaft  7   d . The shaft and rotors  6   d  and  10   d  may be supported by bearings  8   d  such as gas bearings, antifriction bearings or magnetic bearings associated with body structure  25 . The full flow from the evaporator enters the turbine rotor driven boost compressor  10   d  as via path  2  seen in FIG.  1 . The pressure is boosted by impeller blades  10   d ′ to a value at collection ring  11   d  above the evaporator pressure, reducing the power requirements of the main compressor. Reduction of the pressure ratio required for the main compressor has the secondary benefit of increasing the main compressor efficiency. The unit is typically hermetically sealed, and readily manufactured using castings. 
     This method has the potential for the lowest or very low manufacturing cost and highest or very high reliability, for smaller systems. A second option is the generation of electric power from the expansion, which can be used to reduce the net power to the compressor. Recent advances in high speed i.e. high angular velocity generators can make this option useful for larger systems. Typical angular velocity exceeds 90,000 RPM. 
     FIG. 5 shows a nozzle corresponding to nozzle  9  designed to expand the flow from supercritical conditions into the wet region. The supercritical fluid enters the nozzle at  9   a  at high pressure and low velocity, as at  1   e.    
     The flow is to be expanded, typically, to supersonic velocities. A convergent section at  1   e  is followed by a minimum area throat  2   e , from which the flow is expanded at  3   e  to a pressure at which liquid would form under equilibrium conditions. However, due to particle nucleation delay, further expansion occurs at  4   e  to a lower pressure, at which spontaneous condensations occur. The bulk of the droplets produced are very small, typically less than one micron (10 −6  meters) in diameter or cross section. The two-phase mixture continues to be accelerated until the exit pressure of the nozzle is reached, forming a high velocity jet at  5   e.    
     Depending upon the expansion pressure and length of the nozzle, the flow may leave the nozzle in the supersaturated condition as a gas, and reversion to form liquid droplets may occur after the flow leaves the nozzle. In either case the function of the nozzle is to convert the enthalpy of the supercritical gas to kinetic energy and to cause the formation of the liquid phase in the form of extremely small droplets which increases the efficiency of converting the kinetic energy to shaft power, in TCT  10 . 
     In FIG. 6 a cross section of the TCT blades  12  is provided. The jet  5   e  from the transcritical nozzle impinges upon concave surfaces  12   a  of axial flow turbine blades  12 . A fraction of the liquid droplets is centrifuged onto the blade by the angular turning of the flow, at  5   f  forming a liquid layer  13  on  12   a . Most of the small droplets  6   f  are turned with the bulk flow, transferring their energy through the gas flow to the blades. Separated liquid layer  13  leaves the blade at  14  with a decreased relative velocity due to the high friction loss from a liquid film. The gas flow is turned, flowing generally parallel at  15 , to the blade surface. The liquid droplets are centrifuged slightly towards the blades, transferring force to the gas which is transmitted to the blades. The largest fraction of the liquid droplets  6   f , are turned and leave the axial turbine blade with a much smaller friction loss than the liquid collected by the blade. 
     A key feature of the impulse turbine design is the provision of an axial path (i.e. in a direction or directions having an axial component or components parallel to the turbine rotor axis) for the two-phase flow. As discussed previously, prior radial inflow turbine machinery will centrifuge liquid in a direction counter to the flow. This liquid can collect between the nozzles and rotor blades producing severe erosion. In the axial design of the TCT, the bulk of the liquid leaves the rotor with a swirl path extending about and lengthwise of the turbine axis to enable collection on the casing wall. The small fraction that is centrifuged toward blade tips is collected on a shroud surrounding the blade tips as at  3   d  in FIG. 4, the shroud also directing the flow to the case wall. 
     The advantages of a transcritical turbine refrigeration cycle using carbon dioxide as the working fluid are illustrated below. For purpose of the analysis and in an example, the heat exchanger outlet conditions were selected to be: 
     T 1 =104 F 
     p 1 =1400 psia 
     m=1440 lb/hr (where “m” is mass flow rate). 
     The expansion conditions chosen were: 
     p 2 =525 psia 
     T 2 =34.7 F 
     A transcritical nozzle efficiency of 94% gives a spouting velocity of: 
     V b =562 ft/s 
     The liquid fraction at the exit of the nozzle 
     1−x 2 =0.552 (where x 2  is the gas fraction at the exit of the nozzle). 
     The nozzle exit diameter is: 
     
       
           d   2 =(1.27 m/ρ 2m V b ) ½ =0.102 inch 
       
     
     where ρ 2m  is the density of the mixture and γ b  is nozzle exit velocity. 
     The axial rotor diameter and speed will depend upon the characteristics of the compressor wheel it is driving. For purposes of this illustration a speed of 110,000 rpm. is selected. The rotor diameter is 0.6 in. at the mean line. The outer diameter is 0.9 in. 
     Analysis of the turbine, assuming 100% of the liquid is collected on the blades gives a power of 1952 watts at a net rotor efficiency of 74%. The net turbine efficiency considering the nozzle efficiency is 69%. 
     Assuming an isentropic compressor efficiency of 80%, the power from the TCT, 4.64 Btu/lb, results in a pressure boost of the full vapor flowrate to 665 psia. The enthalpy at this point is 189.94 Btu/lb, the temperature is 66 F and the entropy is 0.4417 Btu/lbdegR. 
     The main compressor power (for 80% isentropic efficiency) required to increase the boost pressure of 665 psia to 1400 psia is 16.70 Btu/lb. 
     For the prior flash valve expansion, the main compressor power required to increase the pressure from the 525 psia evaporator pressure 20.88 Btu/lb. 
     The liquid fraction leaving the TCT unit is greater than that leaving the flash valve due to the energy removed from the process. The liquid fraction was calculated for the TCT to be 0.5348. The liquid fraction leaving the flash valve was calculated to be 0.4873. The increase in cooling capacity for the evaporator is 1.098. 
     Thus the increase in cooling per unit of power input is: 
     
       
         Δ COP/COP =(1.098)(20.88/16.70)=1.37, 
       
     
     where COP=coefficient o performance, Btu/kwhr 
     The above example provides approximately 6 ton of cooling. Increasing the size will improve the performance of the TCT because of partial admission effects and the decrease in the ratio of windage loss to shaft power output. The increase in COP will vary depending on the final cycle conditions. However, the above increase is significant and representative of the cycle efficiency advantages that can be realized with a transcritical CO 2  turbine and boost compressor utilized in place of the expansion valve. 
     Working fluid may include one or more of the following: 
     CO 2    
     isobutene 
     propane 
     butane 
     ammonia