Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2002-02-12
2003-07-22
Capossela, Ronald (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C062S003100, C062S457800, C091S042000, C060S636000
Reexamination Certificate
active
06595006
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to devices for cooling, generating power, compressing, pumping, and evacuating to pressure below ambient.
There are numerous references in the literature to miniaturized devices whose recent appearance is coincident with the application of microfabrication processes to the production of mechanical systems. Microfabrication processes are those typically associated with integrated circuit production, but more generally include processes capable of producing components and assemblies with micron-sized features and producing a plurality of assemblies or components simultaneously or in “batches”. The fine dimensional tolerances of microfabrication processes means that entire classes of novel miniaturized machines can be realized. The ability to produce multiple parts simultaneously means that in many cases these novel machines may be produced efficiently and in great numbers; batching leads to economy-of-scale reduction in the production costs.
The realization that many macroscopic machines can be fully miniaturized has led to a class of devices; for examples see “Silicon Micromachining”, Cambridge University Press, 1998 and “Handbook of Microlithography, Micromachining, and Microfabrication”, SPIE Press, 1997, incorporated herein by reference to the extent not inconsistent herewith.
One way to enhance microelectronic system efficiency as well as increase reliability is to cool electronic devices to temperatures that substantially reduce power consumption/generation. This however, implies a substantial cooling system. The large heat density produced by present generation electronics is one of the main problems facing present cooling systems. Present heat densities are now exceeding 30 W/cm
2
. However, if these same circuits could be cooled to cryogenic temperatures, they could operate at higher frequencies, with reduced power, more reliably, and at lower voltages as indicated in the references by E. Simoen and P. Ghazavi (citations below).
Three of the most important considerations facing any novel active cooling technology are cost of implementation, reliability, and efficiency. If the cooling device is prohibitively expensive and/or unreliable or if the cooling device uses substantially more power than is saved, many of the benefits derived from cooling vanish. Efficiency is especially important when cryogenic cooling is considered, since most common cryogenic systems operate at only a few percent of Carnot efficiency.
Present cooling and refrigeration technologies are unable to satisfy present demands. For example, as CMOS electronics have approached the 0.18 &mgr;m gate size, they have begun to generate heat densities that require active cooling if these electronic devices are to operate efficiently and reliably. To date there have been many proposed solutions to the problem of cooling very dense microelectronics, but very few of these proposals provide substantial sub-ambient cooling power, and none do so efficiently.
There are also many applications for cryogenic cooling, which extend beyond the needs of conventional electronics, such as superconducting electronics and infrared imaging sensors cooled to temperatures of 35 K and below. There are many applications for which superconducting electronics, operating at both high (70-35 K) and low (35-4 K) temperatures, provide the only feasible solution. Likewise there are many high-precision long-wavelength remote sensing applications, which can only be realized if the sensing detector is maintained at very low temperatures. Often, however these applications have limited space available for the cryogenic system and limited power with which to drive such a system. These two requirements greatly increase the cost and difficulty of realizing present cryogenic support systems.
Present cryogenic cooling technologies suffer from one or more of the following limitations: limited lifetime, high cost, large size, excessive weight, vibration, and ineffective integration with the objects to be cooled. Commercial and tactical cryocoolers that operate at liquid nitrogen temperatures cost on the order of tens of thousands of dollars, generally have lifetimes of less than two years, have limited heat lift, and do not incorporate effective vibration control. Low-temperature cryocoolers, such as Gifford-McMahon cryocoolers, weigh several hundred pounds, have high vibration, and require several kilowatts of power input for a few watts of heat lift at 10 K and below. Aerospace cryocoolers that have long lifetimes and vibration control can cost over one million dollars each. These cryocoolers have high efficiency for temperatures above 50 K; however, as their operating temperature decreases, their efficiency gets much worse, and their practical minimum temperature is about 30 K. Further, all present cryocoolers require complex and expensive assembly procedures that do not readily lend themselves to mass production; therefore, they are limited in their capacity to enjoy economy of scale cost reductions.
For all of these reasons, there has been a need in developing miniaturized and highly efficient cryogenic systems using Micro-Electro-Mechanical Systems (MEMS) technologies. For example, see U.S. Pat. Nos. 5,932,940, 5,941,079, and 5,457,956. Unfortunately, applying the common principles of refrigeration and cryogenic design to systems with dimensional scales of microns and millimeters has posed substantial problems. For example, U.S. Pat. No. 5,932,940 proposes a reverse Brayton cycle refrigerator, but to extract useful amounts of heat the proposed system must operate at very large rotational speeds, 300-1000 krpms. Among the many technical challenges presented in reducing such a design to practice is the fact that this turbine speed requires complex load bearing assemblies; refer to K. S. Breuer et al. “Challenges for High-Speed Lubrication in MEMS”. To operate properly, these bearings must be fabricated with such precision that present MEMS process technologies are not capable of satisfying the requirements.
U.S. Pat. Nos. 5,457,956 and 5,941,079 appear to require a material that has thermal properties outside those of known materials. In addition, the frequency at which these proposed MEMS compressors must operate to produce a useful cooling effect is too high for an efficient resonant system.
Chemical charge storage batteries have provided the majority of the portable energy sources for powering portable electronics. Such batteries, however, are limited in both power density and lifetime, particularly when it is desired that the power source be reusable. For these reasons and since the power demands of portable electronic devices have been steadily increasing, chemical batteries have become increasingly inadequate. By comparison to batteries, heat engines, such as internal combustion engines, generate large amounts of power, but are typically massive, making them incompatible with portable applications. One of the reasons that heat engines produce large amounts of power results from the large energy density of liquid fuels, much larger per unit mass than any known charge storage device. Thus, if liquid fuels or pressurized gases can be used to drive a miniaturized electrical power generator, the result would be a revolution in portable energy technology which would enjoy increased operating times at higher levels of power consumption, reduced operating expense, higher levels of reusability, and a more environmentally benign operational effect.
Of the proposed solutions to portable power generation, two examples are found in U.S. Pat. Nos. 5,932,940 and 6,109,222. U.S. Pat. No. 5,932,940 proposes a microscale gas turbine operating at very large rotational speeds, which can collect the energy released during the gas-phase combustion of a fuel and oxidizer, and convert it into electrical energy. The miniaturization of the gas turbine provides many technical advantages. However, the very large rotational speed required to produce useful effects has presented severe difficulties in producin
Mohling Robert A.
Thiesen Jack H.
Willen Gary S.
Capossela Ronald
Greenlee Winner and Sullivan P.C.
Technology Applications, Inc.
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