Transcript Application of TAE to Rural (Score)

Thermo-acoustic technology in
low-cost applications
The Score-Stove™
Paul H. Riley
Score Project Director
How does Score-Stove™2 work?
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Uses Thermo-Acoustics (TAE)
 Exciting new technology
 No moving parts
» Stirling engine with no pistons
Relies on acoustic waves
» Making it cheap and reliable
 Difficult to design but low cost manufacture
 Used in Space probe
and a Natural Gas liquefying plant
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Wood or dung is burnt
 A specially shaped pipe gets red hot
 Another part of the pipe is cooled
 This generates sound at 100 Hz
» very noisy inside >170 dBA
» Outside whisper quiet hum
 Then a Linear Alternator turns the sound
into electricity
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The waste heat is used for cooking
Thermo-Acoustics
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Discovered by Byron Higgins (1777)
 demonstrated a spontaneous generation of sound waves in a pipe
A century later Lord Rayleigh [10]
 explained the phenomenon qualitatively
In the 1970s’ Ceperley [11]
 postulated an acoustic wave travelling in a resonator could cause the gas
to undergo a thermodynamic cycle similar to that in a Stirling engine
Used by Los Alamos (G Swift)
 space probe electrical generation
 Cooling 400 gallons per day methane
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Chinese Academy of Science
 Record of 1kWe 18% efficiency using pressurised Helium
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Aster Thermoakoestische Systemen (The Netherlands)
 Low-onset temperature TAE
 Waste heat recovery etc.
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Score
 Low-cost
 World record for wood burning Thermo-Acoustic Engine (TAE)
PV diagrams
Stirling Cycle
Power out = area
under curve
Pressure
Pressure
4 stroke petrol
Volume
Volume
Travelling wave TAE (pressure
in phase with velocity)
Volume
Pressure
Pressure
Standing wave TAE
Needs imperfect stack to get
power out (heat lag gives inphase component)
Smaller than 4 stroke
Smaller than Stirling.
Typically less than
10% mean pressure
Volume
Types of Thermo-acoustics
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Thermo-acoustic engines (TAE)
 Heat in results in sound in pipes
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Thermo-acoustic coolers (TAC)
 Sound in results in temperature difference
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Travelling wave (Both)
 Pressure and velocity in phase
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Standing wave (Both)
 Pressure and velocity nearly 90 degrees out of phase
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Only travelling waves carry power but
 Standing wave engines do work well, they always have a small
in phase component, i.e. always less than 90 degrees
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PSWR
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Pressure Standing Wave Ratio
PSWR= 1 is a pure travelling wave
PSWR = Infinity is a pure standing wave
PSWR less than 1.8 is a good travelling wave engine
Acoustic waves
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Each particle of gas moves to and fro through a
displacement smaller than the wavelength
The wavelength is determined by the pipe length and
speed of sound (frequency = 1/wavelength)
Power in the (travelling) wave is a function of
 mean pressure
 Dynamic Pressure amplitude
» (usually limited to << 10% mean)
 Diameter of pipe
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A travelling wave and standing wave is only
determined by the phase difference of the particles
Demo
Thermo-Acoustic waves
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A travelling wave TAE
 a Stirling engine without pistons
 The wave passes around the pipe replacing the pistons
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The regenerator acts as a velocity amplifier and
adds power to the wave
The wave passes to the alternator which then
extracts power
Velocity amplification is low, so significant power
must enter the regenerator.
Thermo-Acoustics Technology
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At first sight a TAE engine
looks simple.
Just a specially shaped pipe.
No moving parts needed to
generate sound
Linear Alternator turns sound
to electricity
Types of TAE
Scott Backhaus Los Alamos
The
Principle
of the ‘Standing-Wave’ ThermoBangkok
Nov 2009
acoustic Engine (Yu and Jaworski, 2009)
TAE performance
Power
Onset temperature
(when Oscillation starts)
1.Unloaded
2.With load
Ideal Engine
Real engine
Th-Tc
(temperature either
side of regenerator)
Typical single Looped TAE
Practical machines have travelling and
standing wave component. We use the term
PSWR (pressure standing wave ratio)
SW/TW. PSWR of 1 is a pure travelling wave
Linear
Alternator
Impedance miss-matches at
heat exchangers and alternator.
Correct loop design needed
AHX
Regenerator
HHX
Thermal buffer tube
Secondary AHX
Tuning stub
Velocity increase
through regenerator
Power function of:
• Pipe mean pressure
• Drive ratio (< 10%)
Feedback pipe
Wave Direction
• Pipe area
Total pipe length ~ λ
• Gas used
Air, He most common
Looped tube travelling wave TAE
(a)
Left single regenerator
TAE,
(b)regenerator TAE
Right dual
Low onset temperature design
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Electrically powered rigs
 Omit parasitic heat losses
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Two ways to tackle
#2
Twin heat exchangers and regenerators
 Lower parasitic loss
 Lower TAE onset temp
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p_c
Field implementations
 Conductive heat loss
can dominate -> low efficiency
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#1
Multiple regenerators
 Can lower onset
 Aster 31K Th-Tc with 4 stage
 Useful for waste heat recovery
#4
#1
#3
#2
Quad TAE
Design Optimisation
Performance enhancement
Tuning
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Component matching
 Although there should be a
travelling wave at the regenerator,
some standing wave component
can help match devices with
different impedances
Area changes cause reflections
 Reflections cause standing waves
(SW)
 SW increase losses, due to
pressure anti-nodes
Reflections can be tuned out
 Use of ¼ or ¾ wave pipes
 Using tuning stubs
Component matching
Effect of matching on TAE
80
70
Figure of merit
60
50
LA performance
Regen performance
40
1/Acoustic Losses
30
Total
20
10
0
0
20
40
60
80
100
120
Frequency Hz
All parts of the engine have to be matched
as the operating margin is very narrow
Regenerator performance
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The regenerator has to transmit ~10 times more
power to the TA gas than the heat exchangers
It has to do it
 Twice per cycle
» During peak pressure from solid to TA gas
» During min pressure from TA gas to solid
 Without
»
»
»
»
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Friction losses
Heat conduction losses
Turbulence
Quickly (thermal penetration depth)
All the above are in conflict
 So proper design is essential
» Wire diameter (dependent on frequency and mean pressure)
» Porosity (typically 70%)
» Wire spacing
Prevention of Losses
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Inner (gas washed) surfaces must be smooth (polished)
 Undulations are OK as long as there is a smooth boundary
 No sharp corners, or rough surfaces
The area seen by the thermo-acoustic gas should be
constant, except where it is designed not to be
Any area transitions should be abrupt, not conical
Very Small filet to
prevent vortices
(on inside of the pipe)
Bend Losses
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Travelling wave mode 1
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Velocity increases on inner radius
 Not a problem if no vortex shedding
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Pressure increases at outer
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Mode 2
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Circulating flow cause losses
Demo
Bend Design
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Sharp corners are lossy
Even a 1mm radius can
eliminate vortex shedding
Gradual bends reduce friction
losses at the wall
Design Optimisations
Low cost is key:
System
Material
Labour
Optimisation: Cost
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Paradox
 Smoke free stove Nepalese
manufacture ~ £25
» Low labour costs
» Excludes profit and transport
 Gas stove (LPG) in UK
» £12.99 includes:
» Local tax and transport
» Profit (manufacturer and
retailer)
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Low material content is key
 Thin sections
 Strengthened by geometric
shape
Leads to low weight design
Optimisation: Cost Issues
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Optimisation examples
 Increased frequency
» Alternator efficiency
» Thermo-acoustic efficiency
 Increased pressure
» Mass of containment
» Power output per volume
 TAE topology
» Standing wave less complex, (Hence
lighter for given efficiency)
» Travelling wave more efficient
(Hence less weight per Watt)
 Working gas
» Air is cheapest
» Helium allows higher frequency
(hence lighter alternator and TAE)
Power to thickness ratio
1000
500
0
1 2 3 4 5 6 7 8 9
Bar
Optimisation:
System frequency / Alternator
Power versus Frequency for different alternator model sizes,
20mm maximum coil movement
500
Operation at higher
frequency increases cost of
electronics but dramatically
reduces alternator cost.
Watts
450
400
350
Model F = £20, 50% efficiency
However, noise then
becomes an issue.
300
Model F = £20, 85% efficiency
250
Model C = £4.2
50% efficiency
200
Required output power
150
Model A = £2.7
50% efficiency
100
50
Allowable range with simple electronics (Mains)
0
0
20
40
60
Hz
80
100
Low cost design range
120
Thermo-Acoustic Applications
Possible TAE Applications
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Electrical output
 Domestic stoves that also generate electricity (Score Stove) ~
100We (Air at 1-3 Bar)
 Community power generation 3k- 11kWe
(He at 4 to 30 Bar)
 Combined Heat and Power (CHP) 3kW – 15 kWe
(He at 4 to 30 Bar)
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Fuel
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Wood
Bio – gas
Agricultural waste
Fossil: Propane, Kerosene etc.
Waste heat recovery
Solar
Lower cost Demo2
Housing
manufactured in town
LA (not shown) imported
Water tank made from ½ a 55
gallon drum. Pipes not shown.
Cooling via gravity circulation
Main carcase and hob sourced
locally (cement re-enforced
mud straw filled)
Bangkok, Nov 2009
Energy Flow Requirements
Heat to cooking Hob = 1.6kWth
Heat to Water (AHX) = 1.7kWth
TAE heat input (HHX) = 2kWth
Acoustic power = 300Wa
Alternator Loss = 150Wth
Losses
0.8kWth
Combustion = 4.4kWth
Storage Battery loss = 50Wth
Electrical Output to devices = 100We
Design for Low Cost [7]
User requirements
Eg 15W – 100We, 30 - €90 (5000 rupees)
System design
Eg 100Hz operating
frequency
Component design
Rigs that prove
performance
Work with large scale manufacture
Field tests
Cost evaluation
Market Evaluations
Where to manufacture?
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To make impact (100 Million pa)
needs mass manufacturing technology
India well placed for TAE technology manufacture
Linear Alternator: Dai-ichi Philippines, China
 High volume high quality speaker manufacturer
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Also needs route to market
 Training
 Sales and marketing
 Maintenance
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Transport cost
 Can dominate in remote areas, eg Nepal
(especially for heavy items)
 Current thinking is therefore to have some local assembly to
include heavy items, locally sourced
 Requires training in local areas
Optimisation: Alternator
Power versus Frequency for different alternator model sizes,
20mm maximum coil movement
500
Operation at higher
frequency increases cost of
electronics but dramatically
reduces alternator cost.
Watts
450
400
350
Model F = £20, 50% efficiency
However, noise then
becomes an issue.
300
Model F = £20, 85% efficiency
250
Model C = £4.2
50% efficiency
200
Required output power
150
Model A = £2.7
50% efficiency
100
50
Allowable range with simple electronics (Mains)
0
0
20
40
60
Hz
80
100
120
Back pocket slides
Excited loops
A Speaker exciting a loop
produces travelling wave in each
direction. When they combine the
loop has a standing wave.
A TAE exciting a loop when
correctly loaded with a linear
alternator produces a travelling
wave in mainly one direction.
Reflections at boundaries can
cause standing wave components
References
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9.
People with no electricity (millions) in 2008, Afghanistan = 23.3, Bangladesh = 94.9, India = 404.5,
Nepal =16.1,Pakistan = 70.4, Sri Lanka =4.7, Total for South Asia = 613.9,
http://www.iea.org/weo/electricity.asp
Backhaus, S., G. W. Swift, Traveling-wave thermoacoustic electric generator. Applied Physics
Letters, 85[6], pp. 1085-1087, 2004
Scott Backhaus, Condensed Matter and Thermal Physics Group, Los Alamos National Laboratory
“Thermoacoustic Electrical Cogeneration” ASEAN-US Next-Generation Cook Stove Workshop
K. De Blok Aster Thermoakoestische Systemen, Smeestraat 11, NL 8194 LG Veessen,
Netherlands“Low operating temperature integral thermo acoustic devices for solar cooling and
waste heat recovery, Acoustics 08 Paris.
K. De Blok Aster “Novel multistage traveling wave thermo acoustic power generators” ASME August
1 August 2010, Montreal
Yu Z, Jaworski A J, Backhaus S. In Press. "A low-cost electricity generator for rural areas using a
travelling wave looped-tube thermoacoustic engine". Proceedings of the Institution of Mechanical
Engineers - Part A: Journal of Power and Energy.
Catherine Gardner and Chris Lawn “Design Of A Standing-Wave Thermo-acoustic Engine”, The
sixteenth International Congress on Sound and Vibration, Krakow 5-9 July 2009.
Riley, P.H., Saha, C., and Johnson, C.J., “Designing a Low-Cost, Electricity Generating Cooking
Stove”, Technology and Society Magazine IEEE, summer 2010. Digital Object Identifier
10.1109/MTS.2010.937029, 1932-4529/10/$26.00©2010IEEE
http://www.score.uk.com/research/Shared%20Documents/TechnoSocial/Technology_Acceptance_PA.ppt