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

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

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 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

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

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

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

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

Looped tube travelling wave TAE 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 #1 p_c #2 Twin heat exchangers and regenerators #1 #2 #3 #4 Quad TAE

Performance enhancement 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 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

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

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: 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)

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

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 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)

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

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

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 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 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/Techno-Social/Technology_Acceptance_PA.ppt