1. Thermo-acoustic technology in low-cost applications The Score-Stove™
Paul H. Riley Score Project Director

2. How does Score-Stove™2 work?
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
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

3. 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)

4. PV diagrams Stirling Cycle Volume Pressure 4 stroke petrol Volume
Smaller than 4 stroke
Power out = area under curve
Travelling wave TAE (pressure in phase with velocity)
Standing wave TAE
Volume
Pressure
Needs imperfect stack to get power out (heat lag gives in-phase component)
Volume
Pressure
Smaller than Stirling. Typically less than 10% mean pressure

5. 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

6. 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

7. 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.

8. 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

9. Types of TAE
Scott Backhaus Los Alamos

10. The Principle of the ‘Standing-Wave’ Thermo-acoustic Engine (Yu and Jaworski, 2009)
Bangkok Nov 2009 10

11. TAE performance Ideal Engine Real engine Power Th-Tc
Onset temperature (when Oscillation starts)
Unloaded
With load
(temperature either side of regenerator)

12. Typical single Looped TAE
AHX
Regenerator
HHX
Thermal buffer tube
Secondary AHX
Tuning stub
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
Power function of:
Pipe mean pressure
Drive ratio (< 10%)
Pipe area
Gas used Air, He most common
Velocity increase through regenerator
Wave Direction
Total pipe length ~ λ
Feedback pipe

13. Looped tube travelling wave TAE
Left single regenerator TAE, Right dual regenerator TAE

14. 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 #1
p_c #2
Twin heat exchangers and regenerators #1 #2 #3 #4
Quad TAE

15. Performance enhancement
Design Optimisation
Performance enhancement

16. 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

17. Component matching
All parts of the engine have to be matched as the operating margin is very narrow

18. 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

19. 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)

20. 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

21. Bend Design Sharp corners are lossy
Even a 1mm radius can eliminate vortex shedding
Gradual bends reduce friction losses at the wall

22. Low cost is key: System Material Labour
Design Optimisations
Low cost is key: System
Material Labour

23. 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

24. Optimisation: Cost Issues
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)

25. Optimisation: System frequency / Alternator
Operation at higher frequency increases cost of electronics but dramatically reduces alternator cost.
However, noise then becomes an issue.
Low cost design range

26. Thermo-Acoustic Applications

27. 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

30. 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)

31. Energy Flow Requirements
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 = 100We
Combustion = 4.4kWth
Losses
0.8kWth

32. Rigs that prove performance
Design for Low Cost [7]
User requirements
Eg 15W – 100We, 30 - €90 (5000 rupees)
System design
Rigs that prove performance
Eg 100Hz operating frequency
Work with large scale manufacture
Component design
Design Iterations
Field tests
Cost evaluation
Market Evaluations

33. 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

34. Optimisation: Alternator
Operation at higher frequency increases cost of electronics but dramatically reduces alternator cost.
However, noise then becomes an issue.

35. Back pocket slides

36. 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

37. References
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,
Backhaus, S., G. W. Swift, Traveling-wave thermoacoustic electric generator. Applied Physics Letters, 85[6], pp , 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 Digital Object Identifier /MTS , /10/$26.00©2010IEEE