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Heat-storing mediumRelated Patent Categories: Refrigeration, Gas Compression, Heat Regeneration And Expansion, E.g., Stirling CycleHeat-storing medium description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060201163, Heat-storing medium. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] The invention relates to a heat-storing medium for a low-temperature range, to a regenerator for low-temperature refrigerators, and to a low-temperature refrigerator. [0002] Low-temperature refrigerators are usually multistage gas refrigerators with the aid of which temperatures in the range below 15 Kelvin can be generated. Such gas refrigerators operate according to various principles, for example according to the Gifford-McMahon, the Stirling or the pulse tube principle. Independent of the operating principles, these refrigerators have in common that they comprise, in the area of a so-called cold head between the hot side and the cold side, a volume through which a working fluid flows, said volume being filled with the heat-storing medium and referred to as regenerator. The working fluid flows alternately in both directions through the regenerator and serves as an intermediate storage for heat absorbed or dissipated by the working fluid. The regenerator thus serves for thermally separating the working fluid in the cold chamber from that in the compressor-side hot chamber. For this purpose, the regenerator must have as high a heat capacity as possible as compared with the fluid flowing through the regenerator. While for temperatures of up to 15 Kelvin high-grade steel, bronze, lead or other metal bodies can be used, this is not possible at temperatures lying considerably below the aforementioned temperature since the specific heat capacity of these metals as compared with that of helium drastically decreases as from 30 Kelvin and below, and approaches zero in the range below 5 Kelvin. Therefore, in very low temperature ranges, i.e. in the range below 15 Kelvin, a fill of rare earth compounds is used as heat-storing medium in the regenerator, as is, for example, described in U.S. Pat. No. 5,186,765. A drawback encountered when using rare earth compounds is their magnetism which poses a problem when the compounds are employed in strong magnetic fields, for example in magnetic resonance tomographs. Further, rare earth compounds are susceptible to oxidation, tend to break due to their partial brittleness when vibrations occur, and are expensive. [0003] Helium and other low-boiling gases are also suitable storing media for very low temperature ranges. For example, helium has, in the range below 15 Kelvin, a high specific heat capacity with a pressure-dependent maximum at approximately 9 Kelvin, thus in this temperature range said heat capacity lies far above the heat capacity of metals. From DE-A-199 24 184 a regenerator is known in which helium is used as a heat-storing medium, wherein helium, like in a heat exchanger, is stationarily stored in a helically wound tube or a tube bundle in the regenerator housing. Alternatively, the regenerator housing may be filled with helium as the storing medium, while the working fluid flows in tubes through the regenerator housing. [0004] Tests on regenerators of such a configuration showed however that a targeted temperature of 4.2 Kelvin cannot be reached, which is due to the high heat input from the metallic helix and tube material and the too small contact surface. [0005] U.S. Pat. No. 4,359,872 describes a fill composed of helium-filled glass spheres as heat-storing medium. The wall thickness of the glass spheres must be relatively large to present an adequate strength at the required internal pressure and the low temperature. [0006] It is an object of the invention to provide a heat-storing medium with a high heat capacity in a very low temperature range, a regenerator and a low-temperature refrigerator comprising a heat-storing medium with a high heat capacity for very low temperatures. SUMMARY [0007] The heat-storing medium according to the invention destined for a low-temperature range, i.e. for temperatures below 15 Kelvin, is composed of a set of gastight sealed hollow bodies which is permeable to the working fluid, wherein each hollow body comprises a fill of low-boiling gas as heat-storing medium. Low-boiling gases are gases with a boiling point below 30 Kelvin. This holds true, e.g., for the gases hydrogen, helium and neon, and in fact to all their isotopes. Low-boiling gases have by their nature a relatively high specific heat capacity at low temperatures and are thus well suited as storing medium at temperatures below 30 Kelvin. Low-boiling gases are relatively inexpensive and may be enclosed in a hollow body comprising a hollow body wall of nonmagnetic, mechanically suited, non-oxidizing and inexpensive material. The heat-storing medium can thus be constructively adapted, in terms of its chemical, mechanical and magnetic properties, to any use thereof. Further, as compared with tubes and/or helices, the gastight sealed hollow bodies offer a considerably larger surface via which the heat exchange is effected. This considerably promotes the heat transfer. [0008] Preferably, the storing medium is a hollow body helium fill. A helium fill is a fill with a helium isotope, for example, with .sup.3He or .sup.4He. The storing medium helium has a relatively high specific heat capacity at temperatures below 15 Kelvin and is thus well suited as a storing medium at temperatures down to the range of 2 Kelvin. Further, helium is obtainable at a low price. [0009] Preferably, at a temperature of 4 Kelvin the helium fill has a pressure of more than 0.5 bar (7.25 psi), in particular a pressure above the critical pressure. At a helium fill pressure of more than 0.5 bar an absolute heat capacity is realized which allows the produced heat quantities to be stored in a relatively small regenerator. Such a regenerator is of very compact configuration as compared with metallic heat accumulators. [0010] Preferably, the material and the wall thickness of the hollow body wall are selected such that the thermal penetration depth equals at least once the wall thickness. The thermal penetration depth p is represented by the following equation .mu. = 2 .times. a f mod wherein a is the temperature conductivity of the selected hollow body wall material at the working temperature (for example, 2 Kelvin), and f.sub.mod is the modulation frequency at which the working gas cyclically alternately flows through the heat-storing medium. The working frequency f.sub.mod shall be assumed to amount to 1.0 to 10.0 Hz for low-temperature refrigerators. [0011] The wall of the hollow body is made of metal. Metals and metal alloys offer a good heat conductivity and good mechanical properties, which allow a small hollow body wall thickness to be realized. The hollow body wall can be made of copper, aluminium, silver, brass, steel or other metals or metal alloys. Alternatively, the hollow body wall can be made of ceramic material. [0012] By selecting non-ferromagnetic metals as material for the hollow body wall, a heat-storing medium can be provided which is suitable for use in strong magnetic fields, for example, for use in magnetic resonance tomographs and the like, without the need to take any further measures. [0013] According to a preferred embodiment, each hollow body has a diameter of less than 3.0 mm. At diameters of less than 3.0 mm a set of hollow bodies has such a large volume-specific surface that a sufficiently rapid heat absorption or dissipation is ensured. Typical diameters range from 0.2 to 0.7 mm. [0014] Preferably, each hollow body is of approximately spherical configuration. Selection of the spherical shape ensures, in the fill composed of hollow bodies, an approximately constant defined ratio between the hollow body surface, overall hollow body volume and fill material volume across the overall fill material volume. [0015] A regenerator according to the invention comprises a housing which is filled with the heat-storing medium described above. A low-temperature refrigerator according to the invention comprises the aforementioned regenerator and is configured as a regenerative cycle, preferably as a Gifford-McMahon, Stirling or pulse tube refrigerator, wherein helium is used as a working fluid. Thus helium is used both as a storing medium and, separately, as a working fluid. [0016] Still further advantages of the present invention will be appreciated to those of ordinary skill in the art upon reading and understanding the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS [0017] The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. [0018] FIG. 1 shows a schematic representation of a refrigerator, [0019] FIG. 2 shows a sectional view of a refrigerator regenerator with a fill composed of a set of helium-filled hollow bodies, and [0020] FIG. 3 shows a sectional view of a helium-filled hollow body. DETAILED DESCRIPTION Continue reading about Heat-storing medium... Full patent description for Heat-storing medium Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Heat-storing medium patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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