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Thermoacoustics for Liquefaction of Natural

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Thermoacoustics for Liquefaction of Natural R LNG TECHNOLOGY By Greg Swift Thermoacoustics for Los Alamos National Laboratory and John Wollan Liquefaction of Natural Gas Praxair A prototype device for liquefying natural gas using thermoacoustics is capable of producing 500 gallons per day of LNG, consuming 35 percent of the incoming gas in the process. Larger capacities operating at higher efficiencies are on the drawing board. ne ordinarily thinks of a sound In this article, we introduce the the half-wave resonance present in the wave as consisting only of basic principles of thermoacoustics apparatus illustrated by the schematic O coupled pressure and position and describe progress toward their in (c), where the engine is at the top oscillations. In fact, temperature use for liquefaction of natural gas. and the refrigerator is at the bottom. oscillations accompany the pressure Thermoacoustic natural-gas liquefiers Heat exchangers (HX) and a regenerator oscillations and when there are spatial are surprisingly simple: They use no in the engine convert some of the heat gradients in the temperature exotic materials, require no close power (QH) from burning natural gas at oscillations, oscillating heat flow tolerances, and are little more than a hot temperature (TH) into acoustic occurs. The combination of these welded pipe and heat exchangers power (W), rejecting waste heat power oscillations produces a rich variety of filled with pressurized helium. This (Q0) to a water stream at ambient “thermoacoustic” effects. In everyday simplicity, along with the reliability temperature (T0). Acoustic power is life, the thermal effects of sound are too and low maintenance inherent in consumed by the refrigerator, which small to be easily noticed; for example, thermoacoustic technology, suggests uses it to pump heat (QC) from a the amplitude of the temperature that thermoacoustic liquefiers could liquefying natural-gas load and rejects oscillation in conversational levels of enable economic recovery of marginal waste heat (Q´0 + Q˝0) to the ambient sound is only about 0.0001°C. gas resources such as associated water stream. Each of the heat However, in an extremely intense sound gas from offshore oil wells, gas exchangers may be of finned-tube or wave in a pressurized gas, these accumulations at remote locations, and shell-and-tube construction, as open to thermoacoustic effects can be even the recovery of landfill gas and helium flow as possible. Each harnessed to create powerful heat marginal coal seam gas accumulations. regenerator usually consists of a pile of engines and refrigerators. Whereas In addition, the technology could stainless-steel screens, supporting the typical engines and refrigerators rely on find an application in areas where smooth temperature profile between the crankshaft-coupled pistons or rotating smaller-scale gas liquefaction is two adjacent heat exchangers. turbines, thermoacoustic engines and needed: liquefaction at seasonal Thermodynamically, acoustic power refrigerators have no moving parts (or at peak shaving facilities and at fleet- is just as valuable as other forms of most only flexing parts without the need vehicle fueling stations. “work” such as electric power or for sliding seals). This simplicity, rotating-shaft power. The first law of coupled with reliability and relatively Thermoacoustic Basics thermodynamics determines that low cost, has highlighted the potential Many varieties of heat-driven W+Q0 = QH in the engine. The second of thermoacoustic devices for practical thermoacoustic refrigeration systems law shows that the engine efficiency use. As a result, thermoacoustics is exist, but in this article we consider W/QH is bounded by the Carnot maturing quickly from a topic of basic only a toroidal thermoacoustic-Stirling efficiency, 1 – T0/TH. The most efficient scientific research through the stages of hybrid engine driving a thermoacoustic thermoacoustic engine to date has applied research and on to important orifice pulse tube refrigerator (Figure achieved 40 percent of the Carnot practical applications. 1). Parts (a) and (b) of Figure 1 show efficiency, while the most powerful has Fall 2002 • GasTIPS 21 \ produced 17 kW of acoustic .. ........... ~~ .... , ....... ............ II power. Similarly, in the II refrigerator, the first law of (a) Position ., thermodynamics determines 1 that W + Qc = Qo'+ Q/; the second law shows that the efficiency Q&W,' known regenerator as the coefficient of performance, is bounded by the Carnot expression '-' TJ(T, - Tc). The most moving down, thermaUy / Resonator ,-i A expandim efficient thermoacoustic orifice pulse tube refrigerator to date has achieved 25 percent of this Carnot bound. One of the most important large dimensions in a I thermoacoustic device is the length of its resonator, which (together with the helium sound speed) determines the operating frequency, just as the length of an organ pipe determines its pitch. This I: ..l ....... length typically ranges ....... ......... ... from ....... ....... ... ... 10 cm for the simplest ............ ......... .... experimental systems to 10 ..... ....... m for today's most efficient and mature systems. The Figure 1: Schematic of a Heat-driven Thermoacoustic Refrigeration System resonator shown in Figure 1 uses a half-wavelength standing wave, How the Engine Mrks thermoacoustic engines and shown schematically in parts (a) and (b) To understand the conversion of heat to refrigerators, the amplitude of the (but without details of the wave within acoustic power by this simple engine, pressure oscillation is 10 percent of the the engine and refrigerator). This wave consider the magnified view of part of mean pressure, and the amplitude of appears spontaneously whenever the the regenerator shown in Figure 1, motion is a similar percentage of the temperature in the engine's hot heat part (d), which shows a typical parcel length of the regenerator. Thermal exchanger is high enough, and the of helium at four instants of time as contact between the oscillating helium amplitude of the wave increases as it oscillates in position, pressure, and the solid wall of the pore, plus the the heat supplied to the hot heat temperature, and density, exchanging externally imposed temperature exchanger increases. In parts (a) and heat with the nearby solid in the gradient, add a new feature to what (b), the pressure and position waves regenerator. The tiny pore of the would otherwise be a simple acoustic are shown at two times: the red curves regenerator is shown as a smooth-walled oscillation - oscillatory heat transfer show these variables when the helium channel for simplicity. between the helium and the solid. is at the uppermost extreme of its The wave carries the helium up and While the heliuq is moving downwards, position in the resonator, with density down along the pore, compressing and it encounters evdr warmer portions of and pressure highest at the top of the expanding it, with time phasing such the regenerator, so it absorbs heat and resonator and lowest at the bottom, that it is most pressurized while it is expands; while the helium is moving while the blue curves show them 180" moving down and most depressurized upwards, it rejects heat and contracts. later in the cycle. while it is moving up. In typical Figure 1, part (e), a pressure-volume 22 GasTlPS Fall 2002 ( p–V ) diagram for the parcel of helium density, cp is its isobaric specific heat low pressure time of the acoustic wave, illustrated in part (d), shows that the per unit mass, and f is the frequency of and the thermal contraction, attending ∫ ␦ helium does net work ( p dV) on its the acoustic oscillation; K is roughly its upward motion, occurs during the surroundings because expansion takes the distance heat can diffuse through high pressure time. The resulting during the high-pressure time of the the helium during a time 1/πf. In acoustic power absorbed by the helium ␦ cycle and the contraction during the today’s thermoacoustic systems, K is is supplied by the thermoacoustic low-pressure time. This process typically a fraction of a millimeter. engine, transmitted to the refrigerator depends on the correct time phasing (Pores too tight impose too much through the wave in the resonator. between motion and pressure, which is viscous drag on the helium.) maintained by inertial and compressive Development History effects in the ductwork near the How the Refrigerator Works Heat driven acoustic oscillators have regenerator. The net work that the The basic principle of operation of the been known for over a century — the helium does on its surroundings is thermoacoustic orifice pulse tube earliest and simplest was discovered produced at the resonance frequency. refrigerator is very similar to that of the accidentally by European glassblowers. Thus, the parcel of helium shown in (d), thermoacoustic engine. A magnified But an accurate theory applicable to and all others like it within the view of part of the refrigerator’s thermoacoustic phenomena was not regenerator, deliver acoustic power to regenerator in Figure 1, part (f), developed until the 1970s, through the the wave, while the wave sets the illustrates one typical parcel of helium efforts of Nicholas Rott and coworkers frequency of the power production. as it oscillates in position, pressure, at ETH-Zurich. Rott’s theory is based Each parcel of helium also deposits temperature, and density, exchanging on a low-amplitude linearization of the a little heat (not shown in Figure 1) at heat (dq) with the nearby solid in the Navier-Stokes, continuity, and energy one location in the regenerator while regenerator, moving that heat up the equations, with sinusoidal oscillations the pressure is rising and the parcel is temperature gradient. As the helium of all variables. relatively stationary near the upper oscillates along the refrigerator’s In the early 1980s, the thermal- extent of its motion. It absorbs that heat regenerator, it experiences changes in physics team at Los Alamos, supported near the lower extreme of its motion, at pressure. At the lower extreme of its by BES in DOE’s Office of Science, a warmer location in the regenerator, motion, the typical parcel of helium was frustrated by the large number when the pressure is falling.
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