Refrigeration – Storage of solidified or liquified gas – Including cryostat
Reexamination Certificate
1999-04-20
2001-01-16
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Including cryostat
C062S259200
Reexamination Certificate
active
06173577
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to methods and apparatus for cooling systems for use with cryogenic power conversion electronics. The invention more particularly relates to methods and apparatus for cooling systems for use with cryogenic power conversion electronics in which the cryogenic power conversion electronics are maintained in a temperature range of 90K to 236K within the cooling system.
BACKGROUND OF THE INVENTION
Cryogenics relates to the production and maintenance of very low temperatures, often using cryogenic fluids such as hydrogen, helium, oxygen, nitrogen, air or methane. Various discussions concerning cryogenic systems can be found in literature. See, e.g., Barron,
Cryogenic Systems,
2d Ed., Oxford University Press (1985); Bell, Jr.,
Cryogenic Engineering,
Prentice Hall, Inc. (1963); Vance,
Cryogenic Technology,
John Wiley & Sons, Inc. (1963); and Timmerhaus et al,
Cryogenic Process Engineering,
Plenum Press (1989).
U.S. Pat. Nos. 3,320,755; 3,714,796 and 3,728,868 disclose cryogenic refrigeration systems (i.e., cryostats). U.S. Pat. No. 4,237,699 relates to cryostats for producing cryogenic refrigeration by expansion of a working fluid through a Joule-Thomson orifice. The cryostat disclosed in U.S. Pat. No. 4,237,699 can be placed in a dewar so that the liquefied working fluid can be maintained to cool an object such as an infrared detector. U.S. Pat. Nos. 3,021,683 and 3,048,021 relate to gas liquefiers. U.S. Pat. No. 4,653,284 discloses a Joule-Thomson heat exchanger and cryostat. U.S. Pat. No. 4,781,033 discloses a heat exchanger for a fast cooldown.
Prior to and as a result of the discovery of high temperature superconductors (HTS), a significant amount of time and money has been spent to evaluate the operating characteristics of circuit components at low temperatures. For example, advantages have been observed when operating power MOSFETs at 77K. These advantages include a reduction of the on-resistance of the MOSFETs by as much as a factor of 30 at 77K.
The implementation of cryocooled electronic power conversion apparatus incorporating MOSFETs and HTS (high temperature superconductor) magnetics, however, has been directed at operational temperatures of 77K and lower. This is due in part to operational features of the HTS wire. A temperature of 77K has been achieved by operating the electronic circuitry in a bath of liquid nitrogen.
U.S. Pat. No. 5,347,168 to Russo discloses a high performance, cryogenically cooled circuit. The entire circuit, as opposed to for example only the superconducting portions of the circuit, are refrigerated to cryogenic temperatures. In addition to the improved operational characteristics of the superconducting based components, the diodes and the gating elements such as MOSFETs provide a circuit capable of operating a switching power supply at lower frequency using larger inductor values. The entire contents of U.S. Pat. No. 5,347,168 are incorporated herein by reference.
While the cryogenic electronics power supplies and power sinks disclosed in U.S. Pat. No. 5,347,168 represent a significant improvement over the prior art, the economics of large scale commercial products employing cryogenic power conversion electronics suggest that overall system and cost efficiency can be more easily met by operating the power electronics at temperatures in the range of 90K to 236K.
It would therefore be desirable to provide methods and apparatus for cryogenic cooling systems that allow operation in a temperature range of 90K to 236K, thereby overcoming the shortcomings associated with the prior art.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide methods and apparatus for cryogenically cooling electronic power conversion apparatus in the temperature range of 90K to 236K, and preferably in the range of 150K to 170K.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a chemically inert liquid cryogen.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a nontoxic liquid cryogen.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include an environmentally attractive liquid cryogen.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a dielectric cryogenic liquid heat transfer medium exhibiting acceptable heat transfer properties.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a non-flammable liquid cryogen.
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a non-ozone deleting liquid cryogen.
It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include octafluoropropane (perfluoropropane).
It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include fully fluorinated 5-carbon branched and straight-chain molecules (e.g., perfluoro n-pentane, 2-trifluoromethyl-1,1,1,2,3,3,4,4,4-nonfluorobutane (trifluoromethyl perfluoro n-butane), tetra(trifluoromethyl)methane (perfluoroisopentane), and other isomers).
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90 K to 236K and that include fully fluorinated 6-carbon branched and straight-chain molecules (e.g., tetradecafluorohexane (perfluoro n-hexane), trifluoromethyl perfluoro n-pentane, di(trifluoromethyl) perfluoro n-butane, and other isomers).
It is another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include fully fluorinated 7-carbon branched and straight-chain molecules (e.g., perfluoro n-heptane, trifluoromethyl perfluoro n-hexane, di(trifluoromethyl) perfluoro n-pentane, tri(trifluoromethyl) perfluoro n-butane, and other isomers).
It is yet another object of the present invention to provide cryogenic cooling systems that operate in the range of 90K to 236K and that include a mixture of fully fluorinated straight or branched-chain alkanes.
These and other objects of the invention are provided by methods and apparatus that include a cryogenic cooling system employing as a liquid cryogen an inert, dielectric, nonflammable, non-ozone depleting material operating in the temperature range of 90K to 236K. In preferred embodiments of the invention, the liquid cryogen is a fluorocarbon such as a fluoroalkane operating at temperatures between 90K and 236K. Preferably, the fluorocarbon does not contain any chlorine. Exemplary fluoroalkanes suitable for use in the invention include, but are not limited to, octafluoropropane (perfluoropropane), decafluoro n-butane (perfluoro n-butane), decafluoro isobutane (perfluoro isobutane), fluoroethane (e.g., between its boiling and melting points), hexafluoropropane, heptafluoropropane (e.g., 1,1,1,2,3,3,3-heptafluoropropane and 1,1,1,2,2,3,3-heptafluoropropane) and isomers and mixtures thereof. Preferably, the liquid cryogen is saturated and completely halogenated such that the formation of hydrogen fluoride (HF) in the event of an electrical arc is minimized. The temperature of the liquid cryogen can be maintained by a cryogenic refrigeration system with associated controls, cold heads, and heat exchangers.
The liquid cryogen may not exhibit nucleate boiling unless a critical heat flux is reached. Under normal conditions below boiling, the liquid will act as a thermal convective heat transfer medium for dissipative power electronic assemblies and components, and is suitable for a number of heat exchange configurations in which the average bulk temperature can be controlled. In
American Superconductor Corporation
Capossela Ronald
Choate Hall & Stewart
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