thermo-acoustic technology in low-cost applications the score-stove™ paul h. riley score project...
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Thermo-acoustic technology in low-cost applications
The Score-Stove™
Paul H. RileyScore Project Director
How does Score-Stove™2 work?
Uses Thermo-Acoustics (TAE) Exciting new technology No moving parts
» Stirling engine with no pistonsRelies 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 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
The waste heat is used for cooking
Thermo-Acoustics
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
Chinese Academy of Science Record of 1kWe 18% efficiency using pressurised Helium
Aster Thermoakoestische Systemen (The Netherlands) Low-onset temperature TAE Waste heat recovery etc.
Score Low-cost World record for wood burning Thermo-Acoustic Engine (TAE)
PV diagrams
Volume
Pre
ssu
re
4 stroke petrol
Volume
Pre
ssu
re
Stirling Cycle
Power out = area under curve
Volume
Pre
ssu
re
Travelling wave TAE (pressure in phase with velocity)
Volume
Pre
ssu
re
Standing wave TAE
Needs imperfect stack to get power out (heat lag gives in-phase component)
Smaller than 4 stroke
Smaller than Stirling.Typically less than 10% mean pressure
Types of Thermo-acoustics
Thermo-acoustic engines (TAE) Heat in results in sound in pipes
Thermo-acoustic coolers (TAC) Sound in results in temperature difference
Travelling wave (Both) Pressure and velocity in phase
Standing wave (Both) Pressure and velocity nearly 90 degrees out of phase
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 PSWR
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
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
A travelling wave and standing wave is only determined by the phase difference of the particles
Demo
Thermo-Acoustic waves
A travelling wave TAE a Stirling engine without pistons The wave passes around the pipe replacing the pistons
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
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
Bangkok Nov 2009The Principle of the ‘Standing-Wave’ Thermo-acoustic Engine (Yu and Jaworski, 2009)
TAE performance
Power
Th-TcOnset temperature(when Oscillation starts)
1.Unloaded
2.With load
Ideal EngineReal engine
(temperature either side of regenerator)
Typical single Looped TAE
Linear Alternator
Feedback pipe
AHX
Regenerator
HHX
Thermal buffer tube
Secondary AHX
Tuning stub
Wave Direction
Total pipe length ~ λ
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
Impedance miss-matches at heat exchangers and alternator. Correct loop design needed
Velocity increase through regenerator
Power function of:
• Pipe mean pressure
• Drive ratio (< 10%)
• Pipe area
• Gas used Air, He most common
Looped tube travelling wave TAE
(a) (b)(a) (b)Left single regenerator TAE, Right dual regenerator TAE
Low onset temperature design
Electrically powered rigs Omit parasitic heat losses
Field implementations Conductive heat loss
can dominate -> low efficiency Two ways to tackle
Lower parasitic loss Lower TAE onset temp
Multiple regenerators Can lower onset Aster 31K Th-Tc with 4 stage Useful for waste heat recovery
p_c
#2
#1
Twin heat exchangers and regenerators
#1
#2
#3
#4
Quad TAE
Design Optimisation
Performance enhancement
Tuning
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
0
10
20
30
40
50
60
70
80
0 20 40 60 80 100 120
Frequency Hz
Fig
ure
of
mer
it
LA performance
Regen performance
1/Acoustic Losses
Total
All parts of the engine have to be matched as the operating margin is very narrow
Regenerator performance
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 » 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
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
Travelling wave mode 1
Velocity increases on inner radius
Not a problem if no vortex shedding
Pressure increases at outer
Mode 2
Circulating flow cause losses
Demo
Bend Design
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:SystemMaterialLabour
Optimisation: Cost
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)
Low material content is key Thin sections Strengthened by geometric
shape Leads to low weight design
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)
Optimisation: Cost Issues
Power to thickness ratio
0
500
1000
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
0
50
100
150
200
250
300
350
400
450
500
0 20 40 60 80 100 120
Hz
Wat
ts Model F = £20, 50% efficiency
Model A = £2.750% efficiency
Model C = £4.250% efficiency
Required output power
Model F = £20, 85% efficiency
Allowable range with simple electronics (Mains)
Operation at higher frequency increases cost of electronics but dramatically reduces alternator cost.
However, noise then becomes an issue.
Low cost design range
Thermo-Acoustic Applications
Possible TAE Applications
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) Fuel
Wood Bio – gas Agricultural waste Fossil: Propane, Kerosene etc. Waste heat recovery Solar
Lower cost Demo2
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)
Housingmanufactured in townLA (not shown) imported
Bangkok, Nov 2009
Heat to cooking Hob = 1.6kWth
TAE heat input (HHX) = 2kWth
Heat to Water (AHX) = 1.7kWth
Acoustic power = 300Wa
Alternator Loss = 150Wth
Storage Battery loss = 50Wth
Electrical Output to devices = 100WeCombustion = 4.4kWth
Losses0.8kWth
Energy Flow Requirements
Design for Low Cost [7]
System design
Component design
Rigs that prove performance
Field tests
Market Evaluations
Cost evaluation
User requirements
Design Iterations
Eg 15W – 100We, 30 - €90 (5000 rupees)
Eg 100Hz operating frequency
Work with large scale manufacture
Where to manufacture?
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 Also needs route to market
Training Sales and marketing Maintenance
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
0
50
100
150
200
250
300
350
400
450
500
0 20 40 60 80 100 120
Hz
Wat
ts Model F = £20, 50% efficiency
Model A = £2.750% efficiency
Model C = £4.250% efficiency
Required output power
Model F = £20, 85% efficiency
Allowable range with simple electronics (Mains)
Operation at higher frequency increases cost of electronics but dramatically reduces alternator cost.
However, noise then becomes an issue.
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
1. 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
2. Backhaus, S., G. W. Swift, Traveling-wave thermoacoustic electric generator. Applied Physics Letters, 85[6], pp. 1085-1087, 2004
3. Scott Backhaus, Condensed Matter and Thermal Physics Group, Los Alamos National Laboratory “Thermoacoustic Electrical Cogeneration” ASEAN-US Next-Generation Cook Stove Workshop
4. 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.
5. K. De Blok Aster “Novel multistage traveling wave thermo acoustic power generators” ASME August 1 August 2010, Montreal
6. 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.
7. 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.
8. 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
9. http://www.score.uk.com/research/Shared%20Documents/Techno-Social/Technology_Acceptance_PA.ppt