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Introduction

Beyond acoustic to electric power conversion limits

Kees de BlokAster Thermoacoustics

www.aster-thermoacoustics.com

Thermoacoustic engines

Acoustic to electric conversion

Conversion mechanismns

Current limitations

Bi-directional turbines

Principle

Experiments

Design approach

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Thermoacoustic engine

Basic geometry of a thermoacoustic engine

n Above onset temperature acoustic power gain exceeds losses and oscillation start

n Oscilllation frequency is set by (acoustic) length of the feedback tube

n At increasing input temperature (above onset) part of the acoustic loop power can be extracted as net output power

Acoustic output power can be converted to

n Electricity …

n Temperature lift

Acoustic resonance and feedback circuit

Acoustic to electric conversion

Conversion of (thermo) acoustic power to electricity

Acoustic pressure amplituden Periodic force (F) n Periodic displacement (s)

Conversion mechanismns•Piezo electric•Electro magnetic

n Magneto Hydro Dynamic (MHD) fluid pistonn Linear alternator, solid piston

•Others……….

•Solid or fluid characterized by a high acoustic impedance•Acoustic impedance matching by resonance

n Mass-spring systemn Gas springn Mechanical springn Gravity

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0

Meanpressure

Pressureamplitude

T

Alternator TAP Star-engine qdrive

The origin of limitations

Scaling up in power

• Induction (dφ/dt)n Force (pressure amplitude)n Velocity amplituden Frequencyn Magnetic field

• Stroken Limited by springs

• Moving massn Inertancen Gas springs

• Clearance sealn Tigth tolerancen Loss proportional with 1 / freq

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m

F, s

gap

Dp

pa

Helium, P0 = 0.6MPa, dr=5%, Dp= 120mm∅, Lp = 40mm

High power application example

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3m

Conversion of industrial waste heat into electricity

Thermoacoustic power (TAP)

•SBIR project phase2

nDesign and built of a TAP converting 100 kW waste heat at 160ºC into 10 kW electricity

nLocation: Smurfit Kappa Solid Board, Nieuweschans(Gr)

Other (industrial) applications•Heat transformer

nUpgrade waste heat above the pinch

•Gas liquefactionnStorage and transport of LNG

Balanced linear alternators

4 x 2 x 1.25kWe

High power application example

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Leassons learned from the TAP project in 2011 •Thermoacoustic energy conversion itself can be scaled up in power unlimited•Upscaling toward high power is limited by practical and by economic linear alternator issues

Practical issues• Piston stroke limited by stroke of the springs

• Sensitive to overload (varying load or input)

• Size and weigth of moving mass more than proportional with power (Larger TA system ⇒ lower frequency ⇒ less induction)

Economic issues • Cost > 3000 € / kW

• No mass production

• Per kW electrictricity relativelly large amont of magnetic materiaal

• Future availability and cost of raw materials for strong magnets (e.g. neodynium)

The TAP Linear alternator

Alternative acoustic to electric conversion

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Using the acoustic wave pressure component

• Convert periodic pressure variation into bi-directional linear motion (piston, membrane)

n Linear alternators

n MHD

n Piezo electric effect

n Others…….

Using the acoustic wave velocity component

• Convert bi-directional velocity into uni-directional rotation

n Bi-directional turbine

n Others……

0

Meanpressure

0

Acoustic wave motion

Pressureamplitude

Gas displacementamplitude

Bi-directional turbines

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Bi-directional turbines

Rotation is independent of flow direction

Know implementations •Lift based turbines

Wells turbine

Darrieus rotor (vertical axis wind turbine)

•Impulse based turbinesSavonious rotor

Axial impulse turbine

Radial impulse turbine

Existing technology used for oscillating water column (OWC) wave power plants (30-500kWe)

Bron: Limpet 500

Guide vanes

Rotor

Guide vanes

Air flow

Bi-directional turbines

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Rotor efficiencyn Fluid densityn Flow coefficient n Size (scale effect)

Power level• OCW plants

n Air at atmospheric pressure

n 5-50 kWe plants installed

• Acoustic power conversionn Gas at high elevated

pressuren Up to 1 kW tested (2013)n No scale or power limits

• Economics / productionn 130 €/kWe (# 1000)n Standard generatorn Plastic turbine

Water turbines• High density

Acoustic turbines• Large scale• high pressure

Acoustic turbines• Small scale• Atmospheric pressure

Oscillating water columnPower (OWC) plant• Large scale• Atmospheric pressure

Initial experiments

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Acoustic experiments on scale models

• Radiale impuls turbine • Axiale impuls turbine

Both manufactured in SLA-SMS 3-D printing.

brushless DC elektromotor used as generator

Observations:•Radial turbine

•Higher torque at lower rotational speed

•Axiale turbine•Lower torque at higher rotational speed

•Output power and efficiency of both embodiments not sensitive to acoustic frequency

Axial impuls turbine

Relation rotor efficency and frequency

Scaling experiment performed in the 100 kW TAP

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Linear alternator set replaced by radial bi-directional inpulse turbine

•Measured rotor efficiency of 75% for air at 0.8MPa and 16Hz oscillation frequency •Confirms that efficiency increase with fluid density

Radial impuls turbine(Drotor =300 mm)

Radial impuls turbine mounted in engine stage #1

Axial impuls turbine(Drotor =200 mm)

Design approach

Turbine act as a complex acoustic impedancen Inertance by acceleration in blade sections n Real part represent shaft power and losses

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20 40 60 80 1000

0.5

1

1.5

2

2.5

freq [Hz]

|Z| /

ρC

20 40 60 80 1000

10

20

30

40

50

60

70

80

90

freq [Hz]

angl

e Z

[deg

]

0000 Rpm1000 Rpm2000 Rpm3000 Rpm4000 Rpm4500 Rpm

Guide vanes

Rotor

Generator

Measured impedance of a 72mm∅ axial 4-stage turbine

Zin

2-stage turbine

Design approach

Coupling with the TA engine

• In series Inserted in the traveling wave feedback loopNo additonal tubingLow velocityLow rotational speed

• In Parallel n Connected by a T junction to the traveling

wave feedback loopn Additional tubingn Hellmholts type resonatorn High velocity n High rotational speed

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Bi-directional turbine Terminated with complex impedance

Bi-directional turbine

Terminated with Z ≈ ρ.c

Design approach

Turbine design using analytic expresions

• Flow coefficient φφ = gas velocity (va) / circumferencial speed (Ur)

• Torque coefficient CTCT = f (Re, shape, angle)

• Input coefficient CACA = f (Re, shape, angle, surface)

• Rotor efficiency ηR

• Typical Reynolds numbersOWC 104 < Re 5.104

High power TA turbine Re > 2.105

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0 1 2 3 4 50

2

4

6

8

10

φ = va/ UR

CTCA

Design approach

• Example of an input file for an axial

turbinen T0 = 300;

n P0 = 29e5;

n GasType = 'He';

n D0 = 0.050; % TAEC reference diameter

n V0 = 8; % input flow velocity amplitude at D0

n R0 = 0.040; % outer radius rotor

n R1 = 0.031; % inner radius rotor

n Lt = 0.015; % blade chord length

n B1 = 0.0012; % rotor blade thickness

n B2 = 0.0025; % distance between rotor blades

n z = 30; % number of blades

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24-Oct-2015 12:09:50

Data_file: STAGE axial turbine, v3

GasType = He

v0 = 8.00 m.s-1 (effective value)

Solidity = 1.02 -

P0 = 2.90 MPa

RP = 2400 Rpm

|Z| = 0.11 ρc

P_shaft = 48.21 W

P_ac = 67.85 W

Eff = 71.0 %

Conclusions (1)

• Thermoacoustic energy conversion can be scaled up in power to industrial power levels n Increase dimensionsn Lower frequency

• Linear alternators can not be scaled up accordingly for practical and economic reasons

• Search for an alternative approach by adapting bi-directional turbines from OWC power plants and converting the velocity component of the acoustic wave into rotation n Bi-directional turbines are proven technology for Oscillating Water Column power plants n Coupled to standard generators n No vibration issuesn Scalable

Experiments are performed to validated this approach and it is observed that • Turbine efficiency raise with fluid density (mean pressure) • Performance is maintained over a large frequency range (non-resonant)• No strict tolerances• Scalable• Ready for mass production (plastic moulding)

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