23 juni 2014 1

Introduction

Acoustic to electric power conversion

Kees de Blok, Aster ThermoacousticsPawel Owczarek, Future energy management-University of Wraclow

Maurice Francois, Hekyom

Brief introduction

Thermoacoustic engine

Multistage traveling wave themoacoustics

High power applications

Acoustic to electric conversion

(movie)

Full scale design

Conclusions

Introduction

What is thermoacoustics?

•A key enabling energy conversion technology based on "classic"

thermodynamic cycles in which compression, displacement and

expansion of the gas is controlled by an acoustic wave rather then by

pistons and displacers.

•Characteristicsn No mechanical moving parts in the thermodynamic processn Maintenance freen Simple constructionn Large freedom of implementationn Low noisen High efficiency (>40% of the Carnot factor)n Large temperature rangen Scalable from Watt’s to MegaWatt’sn Inert gas like helium, argon or even air as working medium

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Introduction

What can we do with thermoacoustics?

Converting heat into acoustic energy (= mechanical energy)⇒ Heat engine

n Heat supply at high temperature from arbitrary heat sourcen Onset temperature difference ≈ 30ºCn Operating temperature differerence >100ºC

Converting the acoustic output power into electricityn Linear alternator (loudspeaker)n Bi-directional turbine

Converting acoustic energy into a temperature lift (By reversal of the thermodynamic cycle)

⇒ Heat pump or refrigeratorn Temperature lift: > 80ºCn Temperature range: -200ºC up to 250ºC

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TAEC

Heat supply at high

temperature

Heat sink at a low

temperature

Acoustic output power

TAEC

Heat taken at low

temperature

Heat sink at a high

temperature

Acoustic power

Introduction

Typical operating characteristics

•Low onset and operation temperaturen No wear and mechanical friction

•Large temperature rangen No phase change working gas

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Thermoacoustic heat pump

Thermoacoustic Heat Engine

Thermoacoustic cooler

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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 ton electricity …

n or drive a termoacoustic heat pump

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Multi-stage traveling wave thermoacoustics

Utilizing low and medium temperature heat sources

•Waste heat

•Solar (vacuum tube collectors)

•Geothermal

•…….

•.

Multi stage traveling wave thermoacoustic engine

n Increase of acoustic power gain proportional with

number of stages

n Less acoustic loop power relative to the net acoustic

output power (more compact design)

n Oscillation frequency set by the acoustic length

n Onset temperature difference < 30°C

n Operating temperature difference > 100 °C

4-stage thermoacoustic traveling wave engine (THATEA project)

High power applications

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100 kWT Thermo Acoustic Power generator

3m

Thermoacoustic power (TAP)Conversion of industrial waste heat into electricity

•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

High power applications

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Conclusions of the TAP project in 2011•Thermoacoustic energy conversion itself can be scaled up in power succesfully•Upscaling toward high power applications is blocked by the linear alternators

Practical issuesn Piston stroke limited by stroke of the springs

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

n Sensitive for overload

n Vibration

Economic issues n Cost > 3000 € / kW

n No mass production

n Per kW electrictricity relativelly large amont of magnetic materiaal

n Availability and cost of raw materials for strong magnets (neodynium)

The TAP Linear alternator

Acoustic to electric conversion

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1) Using the acoustic wave pressure component

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

n Linear alternators

n MHD

n Piezo electric effect

2) Using the acoustic wave velocity component

Convert periodic bi-directional velocity into uni-directional rotation

n Bi-directional turbine

0

Meanpressure

0

Acoustic wave motion

Pressureamplitude

Gas displacementamplitude

Acoustic to electric conversion

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Bi-directional turbines

Rotation is independent of flow direction

Know embodiments •Lift based turbines

Wells turbine

Darrieus rotor (wind turbine)

•Impulse based turbinesSavonious rotor (ventilation)

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

Acoustic to electric conversion

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Acoustic experiments on scale models

• Radial impuls turbine (100mm∅)• Axial impuls turbine (72mm∅)

Both manufactured in SLA-SMS 3-D printing.

brushless DC elektromotor used as generator

Observations:• Radial turbine

n Higher torque at lower rotational speed

• Axiale turbinen Lower torque at higher rotational speed

• Better efficiency for AC flow• Output power and efficiency observed to be

hardly dependent of acoustic frequency

Axiale impuls turbine

Relation rotor efficency and frequency

Acoustic to electric conversion

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Scaling experiment at the 100 kW TAP SKSBLinear alternator replaced by radial bi-directional inpulse turbine

•Measured rotor efficiency of 75% at 0.8MPa

•Efficiency proportional with fluid density

Radiale impuls turbine voor de TAP (Drotor =300 mm)

Radiale impuls turbine in position in engine stage #2

Acoustic to electric conversion

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Test axial turbines in the 100kW TAPManufactured by AGAN italy

Axial turbine :Rotor diameter: 200mm

Rotational speed : 2700rpm

Power: 2 kW

Generator : Outer runner permanent magnet motor

Aim of this experiment•Validate turbine model

•Acoustic impedance

•Avoid radial induced streaming

•Confirm feasible turbine effciency

•Starting point for manufacturing and turbine optimization

Efficiency in air at 0.8MPa of this axial bi-directional turbine is measured to be 80%

Turbine in preparaton

Turbine position inside the TAP

Full scale design 1MWT

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• Basic thermoacoustic engine stage

Low temperature heat exchanger

High temperature heat exchanger

Regenerator

Acoustic power in

Acoustic power out

Bi-directional turbine + generator

Low temperature cooling circuit15-40°C

(Waste) heat in(140-250°C)

Electricity out

Full scale design 1MWT

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Flue gas heat exchanger

Roof section or mounting platform

2 m

Looped heat-pipe circuits

Heat sink terminals

Conclusions

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•The TAP concept is theroretical and experimentally validated and recognized as a compatitive technology for converting waste heat into electricity.

•Upscaling in power toward industrial levels however was blocked by the increasing cost, mass and complexity of linear alternators

•As a practical and economic viable alternative for linear alternators at increasing power levels the concept of a bi-directional turbine, converting acoustic power into rotation and from there into electricity, is introduced and tested succesfully

•Rotor efficiency defined as shaft output power over acoustic input power is a function of fluid density, and is measured to raise from about 30% at atmospheric pressure up to 80% for air at 0.8MPa.

•As a major achievement, the initial limitation in upscaling the thermal and electric power levels is abrogated, paving the way for full scale application of thermoacoustic waste heat recovery in industry up to MW scale