Power plants – Motor operated by expansion and/or contraction of a unit of...
Reexamination Certificate
2002-06-20
2003-11-11
Nguyen, Hoang (Department: 3748)
Power plants
Motor operated by expansion and/or contraction of a unit of...
C060S517000, C060S721000
Reexamination Certificate
active
06644028
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to oscillating wave engines and thermoacoustic engines, including Stirling engines and thermoacoustic-Stirling hybrids.
BACKGROUND OF THE INVENTION
A variety of oscillating thermodynamic engines and refrigerators have been developed, including Stirling engines and refrigerators, Ericsson engines, orifice pulse-tube refrigerators, standing-wave thermoacoustic engines and refrigerators, free-piston Stirling engines and refrigerators, and thermoacoustic-Stirling hybrid engines and refrigerators. Much of the evolution of this entire family of oscillating thermodynamic technologies has been driven by the search for higher efficiencies, greater reliabilities, and lower fabrication costs.
Some combinations of one or more thermoacoustic engines and one or more thermoacoustic refrigerators or orifice pulse tube refrigerators, such as the thermoacoustic-Stirling hybrid engine driving three orifice pulse tube refrigerators shown in
FIG. 1
, have provided heat-driven refrigeration with no moving parts whatsoever. Such systems with no moving parts can yield the greatest reliability and lowest fabrication costs. As used herein, thermoacoustic engines mean both standing-wave thermoacoustic engines, in which stacks are used; thermoacoustic-Stirling hybrid engines, in which regenerators are used; and Stirling engines, in which regenerators are used.
FIG. 1
schematically shows one such prior art combined system
10
. This combined system comprises a chain of energy-conversion hardware: a natural-gas-fired burner
12
, which provides heat to a thermoacoustic-Stirling hybrid engine
14
, which in turn provides acoustic power to an orifice pulse-tube refrigerator
16
, which in turn cools and liquefies a purified natural gas stream. The conversion of heat to acoustic power occurs in regenerator
18
of engine
14
, which is a solid matrix smoothly spanning the temperature difference between hot heat exchanger
22
and main ambient heat exchanger
24
of the engine and containing small pores through which the gas oscillates. The conversion of acoustic power to refrigeration takes place similarly in regenerator
26
spanning a temperature gradient between ambient heat exchanger
28
and cold heat exchanger
32
.
Proper design of the resonator and total thermoacoustic system shown in
FIG. 1
ensures that the system oscillates spontaneously at a desired frequency, called the resonance frequency, when it is operating. Acoustic energy stored in the resonance, comprising kinetic energy of oscillating motion and compressional energy of oscillating pressure, acts like a flywheel so that acoustic power production in the engine and acoustic power consumption in the refrigerators can take place at arbitrarily different temporal phasing within each cycle of the wave. Proper design of the acoustic network in which the engine's regenerator and heat exchangers are imbedded causes the gas in the pores of the engine's regenerator
18
to move toward hot heat exchanger
22
while the pressure is high and toward main ambient heat exchanger
24
while the pressure is low, these oscillations occurring at the resonance frequency.
Thus, the oscillating thermal expansion and contraction of the gas in regenerator
18
, attending this oscillating motion along the steep temperature gradient in the pores, is temporally phased with respect to the oscillating pressure so that the thermal expansion occurs while the pressure is high and the thermal contraction occurs while the pressure is low. This expansion and contraction, properly phased with the oscillating pressure, is the thermodynamic work that produces acoustic power in engine
14
, maintaining the oscillation against consumption of acoustic power by the loads. The load comprises, e.g., the refrigerator
16
and also dissipative effects throughout the system.
The discussion above relies strongly on sufficient steepness of the temperature gradient in regenerator
18
along the direction of the oscillating motion, but is only weakly dependent on the amplitude of the oscillation. With ambient temperature fixed, the steepness of the temperature gradient is controlled by the temperature of hot heat exchanger
22
, henceforth called the hot temperature. Indeed, standing-wave thermoacoustic engines and thermoacoustic-Stirling hybrid engines can operate stably over a very broad range of oscillation amplitudes once the hot temperature exceeds a certain temperature, called the threshold temperature herein, with higher amplitudes associated with higher hot temperatures.
The threshold temperature depends on many details of the entire thermoacoustic system, including, in
FIG. 1
, the load provided by refrigerator
16
. A greater load (e.g. an additional refrigerator that might be connected in parallel) would cause a higher threshold temperature. Low oscillation amplitudes are encountered when the hot temperature is only slightly above the threshold temperature, while the higher amplitudes are achieved when the hot temperature is significantly hotter than the threshold temperature. High amplitude is desirable in order to achieve the highest acoustic power, and thermoacoustic systems are typically designed for routine operation at a high amplitude, called herein the design operating amplitude. In stable operation at any amplitude, a balance exists between the acoustic power produced by the engine and the acoustic power consumed by loads such as refrigerators and dissipative effects throughout the system.
Typically, to start such an engine, beginning from a state in which all parts of the engine are at ambient temperature, burner
12
is ignited and begins producing heat. At first, the heat from burner
12
simply warms the massive parts of hot heat exchanger
22
, burner
12
itself, and any hardware (not shown) associated with burner
12
, such as a counterflow recuperator that might pre-heat the fresh air delivered to burner
12
by capturing waste heat from the exhaust downstream of burner
12
and hot heat exchanger
22
. Hence, at first the temperatures of these parts of the system simply increase with time, and no thermoacoustic oscillations occur. The rate of temperature increase of these temperatures depends on the output from burner
12
and the heat capacity of these parts of the system.
When the hot temperature finally reaches the threshold temperature, the thermoacoustic oscillations begin spontaneously at the resonance frequency, typically at an amplitude that is much smaller than the design operating amplitude. Increases in burner
12
power then increase the hot temperature and the amplitude of the oscillations, with most of the additional burner power going into the thermoacoustic processes. Eventually the design operating amplitude is reached, and the oscillation amplitude stabilizes with burner
12
supplying a fixed amount of heat.
Typically, to effect a complete shutdown of such an engine, beginning from a state in which it is oscillating at high amplitude, such as its design operating amplitude, burner
12
power is reduced or eliminated, and the temperature of hot heat exchanger
22
begins to fall due to consumption of heat by the thermoacoustic processes in the engine and due to heat leak from hot heat exchanger
22
to ambient. As the hot temperature falls, the amplitude of the oscillations decreases, and hence the rate of fall of temperature may decrease. Eventually, the hot temperature falls below the threshold temperature, and the oscillations cease. Further decrease in the hot temperature toward ambient then occurs, usually caused by heat leak from hot heat exchanger
22
to ambient temperatures, but sometimes accelerated by circulation of air or water.
Some hysteresis between turn-on threshold temperature and shutdown threshold temperature may occur, but this does not affect the operation of the present invention as described herein.
The entire startup procedure can be as long as many hours, depending on the heat capacity of the parts that must be heated and on other factors such
Backhaus Scott N.
Gardner David L.
Swift Gregory W.
The Regents of the University of California
Wilson Ray G.
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