Refrigeration – Gas compression – heat regeneration and expansion – e.g.,...
Reexamination Certificate
2003-04-09
2004-04-27
Doerrler, William C. (Department: 3744)
Refrigeration
Gas compression, heat regeneration and expansion, e.g.,...
C060S520000
Reexamination Certificate
active
06725670
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to thermoacoustic devices and, more specifically, to thermoacoustic engines and refrigeration pumps. However, the present invention has applicability outside the field of thermoacoustics, and is therefore not limited to thermoacoustic devices.
BACKGROUND
During the past two decades, there has been an increasing interest in the development of thermoacoustical cooling engines (pumps) for a variety of commercial, military and industrial applications. Interest in thermoacoustic cooling has accelerated rapidly with the production ban of chlorofluorocarbons (CFC's). Thermoacoustic refrigerators can be constructed such that they use only inert gases, which are non-toxic and do not contribute to ozone depletion, nor to global warming. Exemplary prior art designs for thermoacoustic engines and refrigerators are shown in the following patents: U.S. Pat. Nos. 4,398,398; 4,489,553, 4,722,201, 5,303,555, 5,647,216, 5,953,921, 6,032,464, and 6,314,740.
For a complete appreciation of the present invention, an understanding of earlier heat engines is beneficial.
Commercial Failure of the Stirling Cycle
The idea of passing a gaseous working fluid back and forth through a porous medium of high heat capacity (a “regenerator”) to improve the efficiency of a heat engine can be traced back to the invention in 1816, by Rev. Robert Stirling in England, of the thermodynamic cycle that bears his name. Although that invention was concerned with the production of useful mechanical work from heat, it was subsequently recognized that the Stirling cycle could be reversed to produce useful cooling, if mechanical energy was provided to the system.
The Stirling cycle has been attractive both as an engine and as a refrigerator for nearly two centuries because it could, in principle, achieve the maximum efficiency allowed within the constraints of the First and Second Laws of Thermodynamics. This limit of thermodynamically perfect performance is called the Carnot limit. Although an ideal Stirling engine or refrigerator could (in principle) exhibit Carnot performance, neither Stirling engines nor refrigerators ever achieved large-scale commercial success. A few engines based on the Stirling cycle have been used as the primary power source in submarines and many small refrigerators based on the Stirling cycle have been used to cool infrared detection electronics for military applications such as night vision goggles.
There are several reasons why this efficient approach to power production or refrigeration has not yet become commercially viable in most applications. The fundamental reason is that the improved efficiency (and more recently, the reduced environmental impact) of Stirling cycle devices was not an adequate incentive for its widespread adoption because the additional complexity and associated capital cost of the heat exchangers required by the Stirling cycle was not economically justified. In engine applications, the internal combustion engine was favored over the Stirling engine because it could exploit the high-temperature combustion of the fuel without requiring the solid parts of the engine to reach the same high temperature as the combustion products. After the energy was extracted from the combustion process, the excess heat carried by the combustion products could be exhausted directly to the atmosphere. No separate heat exchanger was required to exhaust waste heat from the engine, as required in closed cycle engines.
In refrigeration applications, the vapor-compression (Rankine) cycle has been the dominant means for mechanical production of refrigeration. Although the Rankine cycle is less efficient than an ideal Stirling cycle, the additional mechanical complexity of a Stirling refrigerator and the cost of the heat exchangers needed for Stirling cycle refrigeration was, again, not economically justified. In a vapor-compression refrigerator, the vaporized working fluid could be used to extract the heat directly from the refrigeration load without requiring a secondary heat exchanger and a secondary heat exchange fluid. Because the phase-change of the working fluid exploited by the Rankine cycle was accompanied by a large latent heat, it was possible to produce vapor-compression refrigerators for cooling loads as small as a few tens of watts or as large as air conditioners with a cooling capacity equivalent to the energy absorbed by the melting of 2,000,000 pounds of ice per day (about 3.5 megawatts of useful cooling power).
Recent Developments
During the 20
th
century, many improvements to the Stirling cycle, for both refrigeration and for the conversion of heat to mechanical work, have been made. Thus far, none of these improvements have been sufficient to warrant the replacement of either the internal combustion engine or the vapor-compression refrigeration process by devices using a Stirling cycle. During the final quarter of the 20
th
century, an awareness of the environmental impact of both the internal combustion engine and the chlorofluorocarbons (CFCs) and other man-made chemicals used in most vapor-compression refrigerators and air conditioners became widespread. The global effects of stratospheric ozone depletion caused by CFCs, and the anthropogenic contributions to global warming produced by “greenhouse gases”, as well as other more localized effects such as “acid rain,” have stimulated a careful re-examination of both engine and refrigeration technology.
Beginning in the early 1980's, “thermoacoustics” has been one path that has been pursued to provide a new paradigm for production of environmentally friendly and energy-efficient alternatives to internal combustion engines and vapor-compression refrigerators. The thermoacoustic paradigm attempts to use the pressure oscillations and gas motions associated with sound waves to execute engine and refrigeration cycles with a minimum of mechanical moving parts. This is a conceptual break from the 19
th
century approach, in use to this day, that employs mechanical contrivances such as lubricated pistons moving in close-fitting cylinders, mechanically-actuated valves, flywheels, linkages, cams, etc., to impose the pressure changes and gas motions required to execute the cyclic processes that produce mechanical power or useful refrigeration. The first attempt to produce a practical “acoustical heat-pumping engine” (a thermoacoustic refrigerator) was patented by Wheatley, Swift, and Migliori in 1983 (see U.S. Pat. No. 4,398,398).
The Backhaus/Swift Engine
Since the invention of Wheatley, et al., there has been a continuous effort to produce thermoacoustic engines and refrigerators that would have the simplicity and robustness that came with the elimination of most mechanical parts, while achieving efficiencies that were comparable to or better than internal combustion engines and vapor-compression refrigerators. In 1999, Scott Backhaus and Greg Swift, both from Los Alamos National Laboratory in New Mexico, published the results of an experiment that used the thermoacoustic paradigm to produce a Stirling cycle engine that had a thermal efficiency of 30% [see “A thermoacoustic-Stirling heat engine,” Nature 399, 335-338 (1999)]. Their experimental device combined an acoustic phasing network and acoustic resonator to produce a one-horsepower Stirling cycle engine that was as efficient as a gas-powered automotive internal combustion engine but required no moving parts.
A version of the Backhaus/Swift engine that is suitable for refrigeration applications is shown in
FIG. 1
, which is taken from U.S. Pat. No. 6,032,464 (originally
FIG. 6
) patented by Swift, Backhaus & Gardner. The engine
1
included a driver or sound source
2
(in this case an intrinsically irreversible thermoacoustic engine) attached to a pressure vessel
3
. The engine includes a toroidal path defined by an inertance tube
4
, a secondary chamber (compliance)
5
, and a thermal buffer tube
6
. A flexible diaphragm
7
is attached to one end of the thermal buffer tubes
6
to act as a mass
Garrett Steven L.
Poese Matthew E.
Smith Robert W. M.
Wakeland Ray S.
Gifford Krass Groh Sprinkle Anderson & Citkowski PC
The Penn State Research Foundation
LandOfFree
Thermoacoustic device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Thermoacoustic device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermoacoustic device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3236289