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