Abstract:
A cooling system for high temperature superconductor equipment comprising a cryocooler in heat exchange relationship with the high temperature superconductor equipment, and an air separation unit in heat exchange relationship with the cryocooler, the system arranged such that heat from the high temperature superconductor equipment is rejected to said air separation unit via the cryocooler.

Description:
[0001]     This invention was made with Government support under contract number DE-FC36-02GO11100 awarded by U.S. Department of Energy. The Government has certain rights in the invention. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to the cooling of equipment utilizing superconductors and more specifically, to the linking of a cyrocooler for high temperature superconductors with an air separation unit in a power generation plant.  
         [0003]     One of the fundamental problems presented by various equipment that utilize superconductors is that the superconductors must be kept within a strict cryogenic temperature range so that the superconductors remain in a superconducting state. If, for example, the temperature is increased above the critical range even briefly, heat is generated within the superconducting wire that could cause further increases in temperature and perhaps lead to equipment failure.  
         [0004]     Cryocoolers capable of cooling at temperatures between 4.2 K and 77 K have long been available. However, it is insufficient to simply achieve the operating temperature range. The cryocooler must also be capable of removing heat for a given application (its cooling capacity in watts). In this regard, removing 10 watts at 30 K is much easier than removing 500 watts at the same temperature. Moreover, depending on the thermodynamic cycle being used, a 500 watt heat load could be merely difficult or practically impossible to remove.  
         [0005]     Users of power equipment expect that equipment to be extremely reliable. Typical allowances for unreliability for a complete turbine-generator limit the generator to only eight hours downtime each year. Each component within the generator must be even more reliable so that the entire generator achieves the stated goal. As applied to a cryocooler, the reliability budget for the equipment forces the use of redundant systems and equipment that allows online maintenance to avoid unnecessary downtime. As a result, reliability brings both complexity and cost to the cryocooler.  
         [0006]     It is now generally known that superconducting equipment can be used in power stations. The equipment presently includes power cables, transformers, generators, fault current limiters and the like. Given that each of these components employs superconducting materials at some cryogenic temperature, and that production of coolants at cryogenic temperatures can be expensive and perhaps unreliable, a means is desired whereby cooling capacity at temperatures between, for example, liquid helium and liquid nitrogen is readily available at an economical cost.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0007]     In an exemplary embodiment of this invention, a cryocooler for high temperature superconductors (HTS) is used that links into the basic process for creating relatively pure oxygen in an integrated gasification combined cycle (IGCC) power plant.  
         [0008]     Coal gasification processes convert solid coal into synthetic gas, primarily CO and H 2 . Typically, O 2  is used as the oxidizing medium. In an 1GCC plant, a cryogenic air separation unit (ASU) is often used to provide pure oxygen to the gasification reactor, often using or supplemented by, post-compression air bleed from the gas turbine. The ASU typically produces nitrogen and oxygen in the range of 63-90 K, depending on the point within the cycle being considered, and at mass flow rates that are very high compared to the cooling requirements of HTS equipment. The typical cryocooler for HTS applications operates between room temperature (25° C.) and the HTS operating temperature which may be between 30 K and 77 K. For example, in a generator, the HTS field winding may operate at 30 K while in an underground power cable, the HTS wires could be bathed in liquid nitrogen at 77 K. The key technology in known cryocoolers is the transfer of heat from the very cold cryogenic region to ambient air or other heat sinks at room temperature.  
         [0009]     In accordance with this invention, however, the HTS cryocooler is modified so that the thermodynamic cycle operates between the desired HTS wire temperature and a heat sink much closer in temperature to the wire compared to room temperature. This is done by linking the cryocooler into the air separation process, reducing the complexity and capital cost of the cryocooler without sacrificing operating reliability.  
         [0010]     Compared to existing cryocoolers that operate between an ambient temperature of 25° C. and a working temperature of 30 K, the heat sink for the cryocooler in the example embodiment is approximately 77 K. The reduction in the “apparent” ambient temperature allows the cryocooler to be simpler, less expensive and more reliable. In addition, it consumes less power, thereby improving the efficiency advantage of the HTS equipment.  
         [0011]     In one exemplary embodiment, the cryocooler is based on a Reverse Brayton cooling cycle. Specifically, cold fluid from the ASU enters a reservoir available to the cryocooler and cools a separate fluid circulating between the cryogenic reservoir and a recuperative heat exchanger in the cryocooler. A separate fluid circulates between the recuperative heat exchanger and the HTS equipment. By rejecting heat from the HTS equipment to the cryogenic reservoir at a temperature of 63-90 K, instead of to a traditional heat sink at room temperature, i.e., 25° C. (or 298 K), the complexity of the cryocooler can be reduced along with capital cost.  
         [0012]     In a second exemplary embodiment, the ASU may be linked with an otherwise conventional Gifford-McMahon (GM) cryocooler. In this embodiment, a pair of auxiliary heat exchangers is inserted in the links from the GM cryocoder compressors. One side of these heat exchangers is fed from the compressor and the other side from nitrogen lines from the ASU.  
         [0013]     In a third exemplary embodiment, nitrogen (gaseous or liquid) or liquefied air, which is to a large extent a by-product of the ASU cycle, is simply supplied as the primary coolant to the HTS equipment. The connection between the ASU and HTS equipment can be through insulated piping or via dewars (in the case of liquid coolants) that are filled by the ASU and moved as needed to the HTS equipment.  
         [0014]     Accordingly, in one aspect, the present invention relates to a cooling system for high temperature superconductor equipment comprising a cryocooler in heat exchange relationship with the high temperature superconductor equipment, and an air separation unit in heat exchange relationship with the cryocooler, said system arranged such that heat from the high temperature superconductor equipment is transferred to said air separation unit via the cryocooler.  
         [0015]     In another aspect, the invention relates to a cooling system for high temperature superconductor equipment comprising a cryocooler in heat exchange relationship with the high temperature superconductor equipment, and an air separation unit in heat exchange relationship with the cryocooler, the system arranged such that heat from the high temperature superconductor equipment is transferred to the air separation unit via the cryocooler, wherein the cryocooler includes a first heat exchanger and wherein a cryogenic fluid utilized in the air separation unit passes in heat exchange relationship with gaseous helium or neon from the high temperature superconductor equipment in the first heat exchanger, wherein the air separation unit includes a second heat exchanger, and wherein the cryogenic fluid passes in heat exchange relationship with said gaseous helium or neon in the second heat exchanger, and further wherein the gaseous helium or neon is compressed in a compressor upstream of the first heat exchanger and expanded in an expansion turbine downstream of the first heat exchanger.  
         [0016]     In still another aspect, the invention relates to a method of cooling high temperature superconductor equipment comprising (a) integrating cooling hardware of the high temperature superconductor equipment with an air separation unit of an integrated gasification combined-cycle power plant, and (b) transferring heat from the high temperature superconductor equipment to fluid in the air separation unit.  
         [0017]     The invention will now be described in connection with the drawings identified below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a schematic diagram of a Reverse Brayton-type cryocooler connected between a cryogenic reservoir of an air separation unit in an IGCC plant and equipment utilizing high temperature superconductors in accordance with a first exemplary embodiment;  
         [0019]      FIG. 2  is a schematic diagram of a Gifford-McMahon cycle cryocooler connected between an air separation unit in an IGCC plant and equipment utilizing high temperature superconductors in accordance with a second exemplary embodiment; and  
         [0020]      FIG. 3  is a schematic diagram of an arrangement where the equipment utilizing high temperature superconductors is cooled directly by fluid from an air separation unit in accordance with a third exemplary embodiment. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The exemplary embodiments describe different arrangements for using a cryocooler for high temperature superconductors that links into the basic process for creating relatively pure oxygen in an IGCC power plant.  FIG. 1  illustrates an arrangement  10  utilizing a Reverse Brayton cooling cycle cryocooler. This arrangement includes an otherwise conventional cryocooler  12  fluidly connected to a cryogenic reservoir  14  of an air separation unit (ASU)  16  that is incorporated into an IGCC plant  17  and that supplies pure oxygen ( 02 ) thereto. In this arrangement, cold fluid enters the reservoir  14  via line  18  and exits through the reservoir  14  via line  20  for return to the ASU. The fluid in this circuit (AB) is typically liquid nitrogen or liquid air at a temperature of between 63-92 K. The fluid in line  20  is slightly higher in temperature than in line A because of the heat rejected (i.e., transferred) from the cryocooler to the ASU, and at a slightly lower pressure because of the pressure losses within the reservoir  14 .  
         [0022]     By means of a separate circuit (CD), fluid cooled in the reservoir  14  enters a heat exchanger  22  in the cryocooler  12  via line  24  and flow controller  25 , and returns to the reservoir  14  via line  26 . The fluid in line  24  is at a temperature slightly greater than the temperatures in line  18  or  20 , but less than the fluid temperature in line  26 . The fluid in this circuit could also be liquid nitrogen but the circuits AB and CD are separate and discreet circuits. A separate cooling loop (EF) in the cryocooler  12  cools the HTS equipment  28 , with cooling fluid from the heat exchanger  22  expanded in the turbine  30  via line  32  and returned to the heat exchanger via line  34 . A valve  36  in line  38  upstream of the HTS equipment  28  provides an optional bypass in the event flow to the HTS needs to be adjusted. In this way, the heat generated in the cryocooler  12  by the HTS equipment can be rejected to the cool fluid in the ASU rather than to a relatively high (room) temperature heatsink.  
         [0023]      FIG. 2  illustrates a second embodiment including an arrangement  40  where an air separation unit  42  for an IGCC plant  45  is linked to a Gifford-McMahon (GM) cryocooler  44  used to cool the HTS equipment  46 . More specifically, liquid nitrogen (or LN 2 ) or liquid air from the ASU  42  is circulated to a first auxiliary heat exchanger  50  via line  48  and flow controller  49 , and returned to the ASU via line  52 . Approximately half of the cold liquid in line  48  is diverted to a parallel, second auxiliary heat exchanger  54  via line  56  and returned to the ASU via lines  58  and  52 .  
         [0024]     The cryogenic fluid to be cooled (gaseous helium, hydrogen, liquid nitrogen or liquid neon) leaves the HTS  46  via line  60  and is circulated through a counterflow heat exchanger  62  and a compressor  64  before passing through the first auxiliary heat exchanger  50  via line  66 , and back through the counterflow heat exchanger  62 . An injection valve  68  permits some bleed off of fluid from line  70  before the fluid passes in heat exchange relationship with the GM cryocooler refrigerator  72 . From here, the fluid returns to the HTS equipment  46 .  
         [0025]     A separate closed loop is also established between the cryocooler  44  and the second auxiliary heat exchanger  54 . Specifically, fluid from the cryocooler refrigerator  72  flows via line  74  through the cryocooler compressor  76  and then through the exchanger  54  before returning to the cryocooler refrigerator  72  via line  78 . With this arrangement, heat from the HTS equipment  46  and cryocooler  44  is rejected to the ASU  42 , again gaining the benefit of using the cooler heat sink of the ASU.  
         [0026]      FIG. 3  discloses still another arrangement where heat from the HTS is rejected to the ASU. Here, nitrogen (liquid or gaseous) or liquid air from the ASU  80  for an IGCC plant  81  is supplied as the primary coolant to the HTS equipment  82 . More specifically, liquid N 2 , for example, flows out of the ASU  80  via line  84  through a pump and flow controller  86  in the otherwise conventional cryocooler  88  and into the HTS equipment via line  90 . The liquid is returned to the ASU via line  92 . This arrangement is particularly useful where the HTS equipment also uses liquid for cooling, and little effect is seen on the ASU where the liquid is returned at a slightly higher temperature.  
         [0027]     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. For example, a pulse-tube refrigerator or Sterling-cycle refrigerator may also be employed as the cryocooler in the described system.