MULTI-STAGE TRAVELING WAVE THERMOACOUSTICS IN PRACTICE
Kees de Blok
Aster Thermoakoestische Systemen,
Introduction
Acoustic resonance and feedback circuitry
Multi-stage engines
Practical applications and economics
Summary and conclusions
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Acoustic resonance and feedback circuitry
General performance of practical thermoacoustic engines and heat pumps
Acoustic (power) gain depends on
� Heat input temperature difference (∆Tin)
� ∆T across heat exchangers ( ∆Thex)
� # stages (N)
Acoustic attenuation is caused by
� Regenerator - heat exchangers (αT)
� Resonance and feedback circuitry (αA)
Acoustic loss (αA) is proportional with acoustic (loop) power in the resonance and feedback circuitry
� Loss minimum (fundamental) at small amplitude by thermal and viscous boundary layer losses
� Loss maximum at high amplitude by turbulence
Design approach:
•Minimize acoustic losses relative to the acoustic power transfered
•Maximize the ratio between acoustic output power and acoustic loop power
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−
−
∆−∆≈ AT
Hexinloopacoutac
T
TTNPP αα
)(..
__
Acoustic resonance and feedback circuitry
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The function of the acoustic resonance and feedback circuitry in a thermo acoustic system is:
� set the resonance frequency.
� store acoustic energy to be amplified by the thermo acoustic engine (loop power)
� transfer net acoustic output power of the engine to an acoustic load
� set the proper phasing between pressure and velocity in the regenerator
� act as a pressure vessel
Analyzing acoustic losses and ranking should be made independent of the thermo acoustic core!
� What belongs to what?
TA engine TA heat pump
Resonator
TA equivalent source TA equivalent load
Acoustic resonance and feedback circuit
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Acoustic resonance and feedback circuitry
Equivalent "source – load" models for
existing and alternative acoustic
circuits
1. ¼ λ resonator with alternator
2. ¼ λ resonator with heat pump
3. ½ λ resonator with heat pump
4. Mechanical resonator
5. Traveling wave loop resonator
sourceac
Lossac
sourceac
loadac
CouplingP
P
P
P
_
_
_
_1−==η
0 1 2 3 40
50
100
150
200
length [m]
pa
[kP
a),
va
[m
.s-1
]
pa
va
0 0.5 1 1.5 2 2.5 30
50
100
150
200
length [m]
pa
[kP
a),
va
[m
.s-1
]
pa
va
Acoustic loadAcoustic power sourceS L
1
2
3
4
5
Pressure and velocity amplitudeconstant and
independent of position
Typical amplitude distributions
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Coupling efficiency of the various configurations
1.60.8640
Traveling wave
6.30.8837*
Mechanical
28.00.8157
21.40.6994
Standing wave
internal
gas volume [dm3]
ηηηηCouplingTotal loss [W
Configuration
normal operating condition(5% dr)
* clearance seal losses not included
Acoustic resonance and feedback circuitry
5
Input parameters
Gas: Helium
Mean pressure: 4 MpaFrequency: 120 HzSource power: 300 W Source drive ratio: 5%
0 0.02 0.04 0.06 0.08 0.10
100
200
300
400
500
pa /P
0
Dis
sip
atio
n [W
]
He @ 40 bar, freq = 120 Hz
thermal
viscous
total
0 0.02 0.04 0.06 0.08 0.10
50
100
150
200
pa /P
0
Dis
sip
atio
n [W
]
He @ 40 bar, freq = 120 Hz
thermal
viscous
total
Multi-stage engines
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Multi-stage traveling wave thermoacoustic engine
•Multiple stages� Increase acoustic power gain
� Enlarge the in- and output heat transfer surface
� High power density
•Traveling wave feedback� Low acoustic loss
� Reduced loop power relative to the net acoustic output power
� Relatively small internal (gas)volume
� Self-matching (4-stage only)
•As a result� Onset temperature difference < 30 K
� Operational temperature difference ≥ 100 K
4-stage thermoacoustic traveling wave engine
Multi-stage engines
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THATEA low temperature integral system
• Multistage thermoacoustic engine and
thermoacoustic refrigerator designed
and build by different partners
• Themoacoustic refrigerator driven by a
3 stage low temperature
thermoacoustic engine
• Measurement setup
� Heat supplied by thermal oil circuit
� Heat removed by water circuit
� Working fluid is helium at 2.7 Mpa
� Pressure amplitude measured near cold hex of each stage
� Acoustic power measured at cooler input (#1) and at engine input (#4)
Multi-stage engines
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Measured performance of the THATEA low temperature integrated system Measurements setup
Amplitude distribution
Engine
TH_EHot hex input temperature ºC 169 211 239
TC_ECold hex input temperature ºC 12 13.2 13
QENet thermal input power W 739 903 1224
QstatStatic heat loss W 235 296 340
TH_regRegenerator high temperature ºC 138 178 199
TC_regRegenerator low temperature ºC 32.1 38.8 47
Pac1Acoustic power at refrigerator input (#1) 134 192 274
Pac2Acoustic power at engine input (#2) W 73.0 91.4 121
WfbAcoustic loss feedback W 21.4 30.8 44
Wout_EAcoustic output power (Pac1 – Pac2 + ¾ .Wfb ) W 76.6 124 187
ηT_EThermal efficiency (Wout_E / QE ) - 0.10 0.14 0.15
η2_EExegetic efficiency relative to TH_E - 0.29 0.34 0.35
η2_E_regExegetic efficiency relative to TH_reg - 0.42 0.48 0.50
Refrigerator
dr Drive ratio at cold hex % 1.33 1.53 1.78
Win_RAcoustic input power (Pac1 – Pac2 – ¼.Wfb ) W 55.2 93.4 143
Tc_RCold hex temperature ºC -33.7 -40.5 -45.5
QC_RNet cooling power W 78.2 95.1 95.4
TH_RAfter refrigerator temperature ºC 19.2 24.2 18.8
QH_RHeat rejected W 135 182 253
COP ( QC_R / Win_R ) - 1.42 1.02 0.67
η2_RExegetic efficiency relative to TC_R - 0.32 0.29 0.19
Multi-stage engines
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Thermoacoustic power (TAP)
Conversion of industrial waste heat into electricity
SBIR Pilot installed at production plant of
Smurfit-Kappa solid boards (Netherlands)
• Configuration:
� 4-stage thermoacoustic engine
� 4 x balanced (2 x1.25 kW) linear alternators positioned near cold hex inside each vessel
• Design parameters
� Working gas: helium at 0.7 Mpa
� Oscillation frequency: 75-80 Hz
� Thermal input power: 100 kWT
� Heat input temperature: 155ºC
� Heat rejection temperature: 20ºC
� Electric output power: 10 kWe
Multi-stage engines
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Observations:
• Onset temperature ≈ 40 ºC
• ∆Pac_loop / ∆T ≈ 1000 W / ºC
• High temperature drop across the high
temperature heat exchangers
� ∆T≈ 45ºC at 39 kW thermal input power
• Low thermal conductivity thermal l oil
• Low flow rate thermal oil
• TAP runs effectively at 55ºC temperature
difference (design ∆T = 100ºC).
Consequently extracting only 39 kW heat
from the flue gas
• Linear alternators failed to handle the full
acoustic power
� Mechanical resonance ≈ 40Hz
� At the same pressure amplitude ¼ of the design power
� Mechanical issues
∆Tww_H
∆Treg
∆Tww_L
Multi-stage engines
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Layout of the loaded 3-stage engine
Thermoacoustic conversion efficiency measured by
using stage#3 as artificial acoustic load
� Disconnecting hot hex #3 from the heat supply
� Engine output power is acoustic dissipation in #3
• Viscous loss
• Temerature lift generated
•The conversion efficiency of the (in this case 3-stage
engine) is defined as:
•Measured values
� Dissipated power in #3 equals 1.64 kW
� Total heat supplied to #1,2,4 is 20.0 kW
•Thermal efficiency is 1.64 / 20.0 = 0.082
� The temperatures of the in- and output flows are respectively 99°C and 18°C yielding a Carnot factor of 99-18) / (273+99) = 0.22
Exegetic efficiency of the three stage engine then
equals 0.082 / 0.22 = 0.37
4,2,1_#
3_#
H
ac
Q
P∆=η
Practical applications and economics
The TAP pilot has demonstrated that multi-stage
traveling wave thermoacoustic systems:
•Operate well under realistic conditions
•Are scalable in power over multiple orders of
magnitude
•Can be implemented in industrial proceses
•Respond flexible to temperature and available heat
The TAP project also made clear that linear
alternators are critical both in terms of cost and
scalability
� Cost alternators TAP is more than half the cost of the complete thermoacoustic part
•Magnet size (cost) more than proportional with
thermal input power
� Acoustics become larger ⇒ frequency goes down
� Mechanical issues (high moving mass)
� Cost of Neodynium goes up rapidely
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Practical applications and economics
Other applications economic feasible with the multistage traveling wave concept
Heat transformer
•Raising waste heat above the pinch of an industrial process
Splitting the waste heat flow
�The first flow is used to power the three engine stages
�Temperature of the second flow is raised by the fourth (heat pump) stage
Gas liquefaction
Liquefying LNG or bio-gas for storage an transport
Splitting the gas flow
�The first flow is burned to power one or more medium temperature multi-stage TA engines
�The second flow is liquefied in one or more heat pump stages or pulse tubes
Solar powered cooling
Heat from vacuum tube collectors is used to power the three engine stages
The fourth stage is used as refrigerator
Estimated return of investment currently 6-7 year without subsidy
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Summary and conclusions
Overall performance of actual thermoacoustic systems is dominated by acoustic losses in the resonance
an feedback circuitry
� Traveling wave feedback yield the highest acoustic power transfer for a given drive ratio
� Traveling wave feedback allows for cascading an arbitrary number of regenerator units
The multi-stage traveling wave concept
� yield a low onset and operating temperature as required for actual applications
� is validated and implemented on both lab and on industrial scale
� shows an exegetic efficiency close to 40% even at low input temperatures (150-250ºC )
About linear alternators
� Complexity and cost remains a serious issue
� Could hardly be scaled up in power to multi-kW scale
Other economic feasible applications on industrial and domestic scale
� Heat transformer
� Gas liquefaction
� Solar powered cooling
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