Patent Publication Number: US-2009229291-A1

Title: Cooling System in a Rotating Reference Frame

Description:
BACKGROUND 
     Superconducting rotor field windings of a rotating machine must be cooled while in their superconducting state during operation. The conventional approach to cooling rotor field coils is to immerse the rotor in a cryogenic liquid pool. For example, a rotor employing conventional, low temperature superconducting (“LTS”) materials must be immersed in liquid helium. Similarly, rotors employing field coils made of high temperature superconducting (“HTS”) materials are typically cooled with liquid nitrogen or liquid neon. In either case, heat generated by or conducted in the rotor is absorbed by the cryogenic liquid which undergoes a phase change to the gaseous state. Consequently, the cryogenic liquid must be replenished on a continuing basis. 
     Another approach for cooling superconducting components is the use of a cryogenic refrigerator or cryocooler. Cryocoolers are mechanical devices operating in one of several thermodynamic cycles such as the Gifford-McMahon (“GM”) cycle and the Stirling cycle. More recently cryocoolers have been adapted for operation with rotors, such as in superconducting motors and generators. One example of doing so is described in U.S. Pat. No. 5,482,919, entitled “Superconducting Rotor”, and incorporated herein by reference. In this approach, a cryocooler system is mounted for co-rotation with a rotor. Mounting the cryocooler cold head for rotation with the rotor eliminates the use of a cryogenic liquid pool for rotor cooling and a cryogenic rotary joint. 
     Generally, the cold head portion (“cold head”) of a co-rotating cryocooler cools only a local thermal load. When a large thermal load such as a large rotor (e.g., a 36 MW-120 RPM Navy Drive Motor, or 8 MW-11 RPM wind power generator) needs to be cooled, a large cryocooler or a great number of cryocoolers are usually applied to the large thermal load in order to decrease the large thermal gradient generated between the thermal load and the cryocoolers. The additional coolers are typically mounted in the stationary frame, off the rotor, with the cooling power transferred via a helium gas circulation loop (such as described in U.S. Pat. No. 6,357,422) or a thermosiphon liquid cooling loop. Another traditional approach to reducing large thermal gradient is to use heat pipes between the cryocoolers and the thermal load. 
     SUMMARY 
     In one aspect, the invention features a cryogenic cooling system for cooling a thermal load disposed in a rotating reference frame. The cryogenic cooling system includes a cryocooler and a circulator, connected to each other, disposed in the rotating reference frame. The cryocooler has a cold head for cooling the thermal load. The circulator circulates a coolant to and from the thermal load. 
     Embodiments may include one or more of the following features. The cryocooler is radially positioned about a rotation axis of the rotating reference frame. The circulator is radially positioned about a rotation axis of the rotating reference frame. The thermal load is radially positioned about a rotation axis of the rotating reference frame. The cryogenic cooling system further includes a heat exchanger disposed in the rotating reference frame. The heat exchanger is thermally connected to the cold head. The cold head is a single-stage or a multi-stage device. The circulator circulates the coolant to the thermal load through the heat exchanger. The system further includes a compressor disposed in a stationary reference frame relative to the rotating reference frame. The compressor is in fluid communication with the cryocooler. The system further includes a gas coupling disposed between the rotating reference frame and the stationary reference frame. The gas coupling connects the cryocooler and the compressor. Two or more cryocoolers are disposed in the rotating reference frame. Two or more circulators are disposed in the rotating reference frame. The thermal load is a superconducting winding. 
     In another aspect, the invention features a rotating electric machine. The rotating electric machine includes a rotating reference frame having a rotation axis, a superconducting winding disposed in the frame, and a cryogenic cooling system disposed in the frame. The cryogenic cooling system includes a cryocooler having a cold head for cooling the superconducting winding, and a circulator connected to the cryocooler. The circulator can circulate a coolant to and from the superconducting winding. 
     In another aspect, the invention features a wind turbine. The wind turbine includes a rotating electric machine, which includes a rotating reference frame having a rotation axis, a superconducting winding disposed in the frame, and a cryogenic cooling system disposed in the frame. The cryogenic cooling system includes a cryocooler having a cold head for cooling the superconducting winding, and a circulator connected to the cryocooler, the circulator circulating a coolant to and from the superconducting winding. 
     Embodiments may include one or more of the following features. The cooling system is radially positioned about the rotation axis. The superconducting winding is radially positioned about the rotation axis. The superconducting winding is positioned in a plane parallel to the rotation axis. A plurality of the superconducting windings are equally spaced and radially positioned about the rotation axis within the frame. The cooling system further includes a heat exchanger thermally connected to the cold head. The circulator circulates the coolant to the superconducting winding through the heat exchanger. The cooling system includes two or more of the cryocoolers. The cooling system includes two or more of the circulators. The cooling system includes two or more of the circulators. The cooling system further includes a compressor connected to the cold head. The compressor can co-rotate with the cold head. The compressor receives electrical power through an electrically conducting slip-ring. 
     Embodiments may provide one or more of the following advantages. The invention provides alternative approaches to reducing large thermal gradients between a co-rotating cryocooler and a thermal load so as to improve the cooling efficiency of the co-rotating cryocooler, especially when the cryocooler is used to cool a large thermal load. By incorporating a circulator (e.g., a circulating fan or a pump) into the rotating reference frame of a cryogenic cooling system, along with the cryocooler, higher cooling power and efficiency can be achieved without requiring a large weight addition to the system. Additionally a cryogenic rotary coupling is not required. This results in less refrigeration costs and higher overall system reliability. 
     The details of one or more embodiments of the invention are set forth in the accompanying description below. Other features or advantages of the present invention will be apparent from the following drawings, detailed description of several embodiments, and also from the appending claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic representation of a cooling system in a rotating reference frame. 
         FIG. 2  is a schematic representation of the cooling system of  FIG. 1  in a superconducting rotor. 
         FIG. 3  is a schematic representation of another embodiment of the cooling system of  FIG. 1 . 
         FIG. 4  is a schematic representation of still another embodiment of the cooling system of  FIG. 1 . 
         FIG. 5  is a schematic representation of still another embodiment of the cooling system of  FIG. 1 . 
         FIG. 6  is a schematic of a wind generator having a rotating machine including the cooling system of  FIG. 1  configured to cool HTS rotors of the rotating machine. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a cryocooler  11  and a heat exchanger  15  are disposed in a rotating reference frame  10  of a cryogenic cooling system  100 . Heat exchanger  15  is connected to a cold head portion  12  of cryocooler  11 . Cryocooler  11  and heat exchanger  15  are used to maintain a coolant  18  (i.e., a cryogenic fluid) at cryogenic temperatures. A circulator  13  (e.g., a cryogenically adaptable fan or pump) is also disposed in frame  10  to move coolant  18  to and from a cryogenic cooling loop  21  (shown as the dotted line with arrows) that is located adjacent and in thermal communication with a thermal load  17  (e.g., a superconducting rotor winding). In essence, circulator  13  serves as the mechanical mechanism for providing the necessary force to move coolant  18  past heat exchanger  15 , which is connected to cryocooler  11 , and on to thermal load  17 . In this arrangement, cryogenic cooling system  100 , including cryocooler  11  and circulator  15 , helps maintain thermal load  17 , e.g., a superconducting winding, at cryogenic temperatures for it to operate properly and efficiently. The cryocooler  11  receives a high pressure working fluid from a compressor  23  through a line  19   a.  Lower pressure working fluid is returned to compressor  23  through a line  19   b.  Lines  19   a  and  19   b  are in fluid communication with cryocooler  11  through a rotary coupling or junction  25 . As illustrated, compressor  23  is disposed in a stationary reference frame  20 . As will be described in more detail below, it is generally preferable that an axis of symmetry of coupling  25  be coincident with the rotation axis of rotating reference frame  10 . 
     Referring now to  FIG. 2 , the cryogenic cooling system including the above-described cryocooler  11  and circulator  13  is used in a rotor assembly  200 . The rotor assembly  200  generally rotates within a stator assembly (not shown) of a rotating electric machine. The rotor assembly  200  includes a rotating vacuum vessel  38  in the form of a hollow annular member supported by bearings  30  on a shaft  32  that rotates about a rotation axis A. Within vessel  38 , a winding support  36  for holding a superconducting winding  17  is fastened to frame elements  34  at least one point to the surface of the vessel. Cryocooler  11  and circulator  13  of the cooling system are also fastened to frame elements  34  of vessel  38 . In operation, the superconducting winding is maintained at a cryogenic temperature level (e.g., below 77 Kelvin (K), preferably between 20 and 50 K or between 30 and 40 K) by use of the cryogenic cooling system. In this specific example, two cryocoolers  11  are used. A working gas  19  (e.g., helium) is conveyed to cryocoolers  11  through a coupling  25  which is disposed coaxially to the shaft  32  and between cryocoolers and a compressor  23 . As discussed above, circulator  13  forces coolant  18  to move past heat exchanger  15  connected to cryocooler  11  and on to the superconducting winding  17 . Coolant  18  decreases the thermal gradient between cryocoolers  11  and thermal load  17  and thus increases cooling efficiency of the cryocooler. Coolant  18  is preloaded in the vessel  38  before operation of the rotating electric machine. In certain applications, when some of the coolant turns into a liquid or solid phase due to overcooling, a make-up line  40  can supply gas-phase coolant (e.g., helium gas) as needed. Make-up line  40  is connected to a make-up gas source  42  (e.g., a gas bottle) through the supply line of the working gas  19 . 
     The cryocooler forming a part of the present invention may be a single-stage or a multi-stage device. Suitable cryocoolers include those that can operate using any appropriate thermodynamic cycle such as the Gifford-McMahon cycle and the Stirling cycle, a detailed description of which can be found in U.S. Pat. No. 5,482,919. Preferably, a Helix Technologies Cryodyne Model 1020 is used in this invention. The circulator is selected for suitability for operating in a cryogenic environment. Such circulator is manufactured by American Superconductor and a smaller version (e.g., Model A20) is manufactured by Stirling Technologies. Suitable coolants and/or working fluids for use with the circulator and cryocooler include, but are not limited to, helium, neon, nitrogen, argon, hydrogen, oxygen, and mixtures thereof. The superconductor material forming the superconducting winding may be conventional, low temperature superconductors such as niobium-tin having a transition temperature below 35 K, or a high temperature superconductor having a transition temperature above 35 K. Suitable high temperature superconductors for the field coils are members of the bismuth-strontium-calcium-copper oxide family, the yttrium-barium-copper oxide system, mercury based materials and thallium-based high temperature superconductor materials. The rotary coupling  25 , in one example, includes a gas-to-gas inner seal and a ferrofluid outer seal. Details of the coupling have been described in U.S. Pat. No. 6,536,218, the content of which is herein incorporated by reference. 
     Referring to  FIG. 3 , in another embodiment, more than one cryocooler  11  are used to help maintain each superconducting winding at cryogenic temperatures. In this embodiment, three cryocoolers  11  are disposed in close proximity to superconducting winding  17 . One circulator  13  is used to move coolant  18  to and from the winding. In this specific example, the cryocoolers and the circulator have their axes of symmetry perpendicular to the rotation axis A of rotating reference frame  10 . 
     Among other advantages, using more than one cryocooler  11  increases efficiency and ease of maintenance. In particular, employing more than one cryocooler  11  arranged in series reduces the work load of each cryocooler, so that each cryocooler works less to lower the temperature of coolant  18 . Also, if one cryocooler malfunctions, the redundancy in the system overcomes any loss. Further, if one cryocooler does malfunction, it can be isolated from the system by proper valving to allow maintenance to be performed without shutting down the system and without introducing contaminants into the system. 
     Referring to  FIG. 4 , in still another embodiment, more than one circulator  13  is used together with one or more cryocoolers. For example, in this embodiment, two circulators  13  and three cryocoolers  11  are disposed in rotating reference frame  10 . The circulators and the cryocoolers have their axes of symmetry parallel to the rotation axis of the rotating reference frame. Similar to using multiple cryocoolers in the cooling system, using multiple circulators provides redundancy and facilitates maintenance in the event that one of the circulators requires maintenance or replacement. Appropriate valve and bypass conduits are required to allow each of circulator  13  to be isolated from the other while allowing continuous operation of the system. 
       FIG. 5  shows another embodyment of the invention in which both cryocooler cold head  11  and compressor  23  are mounted for rotation in rotating reference frame  10 . An electrically conducting slip-ring  43  allows electricity to be transported to compressor  23  from a non-rotating source of electrical energy  44 . The embodiment of  FIG. 5  obviates fluid rotary coupling  25  of the embodyment of  FIG. 1 . 
     In all embodiments, it is generally preferable that the superconducting windings are radially positioned about the rotation axis of the rotating reference frame to which it is attached, and have their longitudinal axes parallel to the rotation axis. It is also preferable that the cryocoolers as well as the circulators are also radially positioned about the rotation axis of the rotating reference frame. Their axes of symmetry are either parallel or non-parallel to the rotation axis. 
     There are many applications in which superconducting rotor field windings of a rotating machine must be cooled while in their superconducting state during operation. One example of such an application includes an HTS wind generator  300  employed in a wind turbine ( FIG. 6 ). Such generators  300  include rotors, here represented by rotating reference frame  310 . The rotors employ coils  317  made of high temperature superconducting (“HTS”) materials. As seen in the figure, the HTS coils  317  of the wind generator  300  are cooled using the above-described cooling system in which at least one cryocooler  311  and at least one circulator  313  are disposed in the rotating reference frame  310  of the rotor. In some embodiments, a compressor  323  may also be disposed in the rotating reference frame  310 . 
     Other Embodiments 
     All of the features disclosed in this specification may be combined in any combination. Each feature disclosed in this specification may be replaced by an alternative feature serving the same, equivalent, or similar purpose. For example, coolant  18 , instead of being preloaded in the cooling system before operation, can be supplied through make-up line  40  once operation starts. For another example, when a physical cryogenic cooling loop  21  may be absent, and coolant  18  (e.g., helium gas) is dispersed randomly within vessel  38 . In this case, circulator  13  moves the coolant to and from thermal load  17  to decrease the thermal gradient while cryocooler  11  cools the coolant to a suitable low temperature. In addition, rotating vessel  38 , in certain applications, does not require a vacuum condition. Thus, unless expressly stated otherwise, each feature disclosed is only an example of a generic series of equivalent or similar features. 
     From the above description, one skilled in the art can easily ascertain the essential characteristics of the present invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. Thus, other embodiments are also within the scope of the following claims.