Energy Harvesting Technologies for Wireless SensorsPMG17 PMG17 from Perpetuum Ltd Resonant generator tuned to 2nd harmonic of mains frequency – 100 or 120 Hz 55 mm diameter x 55

Energy Harvesting Technologiesfor Wireless Sensors

Andrew S Holmes

O ti l d S i d t D i GOptical and Semiconductor Devices GroupDepartment of Electrical and Electronic Engineering

Imperial College London

17th World Micromachine Summit1

Wireless Sensor Applications

Wireless sensors very well established in certain market sectors e.g. domestic fuel monitoring

Huge opportunity for expansion in other areas such as:• Machine/process monitoring• Remote monitoring

- inaccessible/hostile environments• Intelligent buildings

HVAC lighting security- HVAC, lighting, security• Medical telemetry

- continuous, unobtrusive monitoring• Product tracking• Product tracking• Ubiquitous computing

- ad hoc sensor networks• Military surveillanceMilitary surveillance

- ‘smart dust’ concept 1 cc wireless sensor node [IMEC]


Power Sources for Wireless Sensors

Short term solutions inevitably based on chemical batteries• High energy density (~2000 J/cm3 or ~500 mA.hr/cm3 at 1V)• Limited life before recharging or replacement • Disposal/recycling problematic

Fuel-burning power sources• Very high energy density• Technologies still some way from maturityTechnologies still some way from maturity• Limited life before refuelling, as for batteries

E h ti

MeOH fuel cell[Fraunhofer Inst.]

MEMS gas turbine stage[MIT]

Energy harvesting• Long term storage capacity no longer an issue• Low power density in most cases

100 W/ 2 f l ll i ffi i te.g. 100 W/cm2 for solar cell in office environment• Intermittent supply in many cases so likely to

be used with battery/capacitor back-up Pico Radio solar cell[UC Berkeley]

Vibration-driven generator1 mW @ 0.25g rms [Perpetuum Ltd]


[UC Berkeley] [Perpetuum Ltd]

Energy Harvesting Technologies

Energy Source Conversion Mechanism

Electromagnetic radiationAmbient lightRadio waves (ambient or targeted)

Photovoltaic cellAntenna / Induction loopRadio waves (ambient or targeted) Antenna / Induction loop

HeatTemperature gradients Thermoelectric device or

H t iHeat engineKinetic energyMovement and vibration ElectrostaticVolume flow (of liquids or gases) Magnetic (induction)

Piezoelectric

Technology of choice will depend strongly on application environment, average power and duty cycle requirements


Motion-driven Microgenerators


Inertial Energy Harvesters

• Single point of attachment to moving “host” e.g. machine, person…• Peak inertial force on proof mass: F = ma = m2Yop o

where a is the peak acceleration applied by the host

• Damper force < F or no internal movement

M i k t it W F 2Y Maximum work per transit: W Fzo = m2Yozo

Maximum harvested power: P = 2W/T m3Yozo/

zo

y = Y cos( t)m

y = Y cos( t)o

damper implements energy conversion


How Much Power is Available?

100000

Plot assumes: • cubic device with mass occupying half of

volume the other half allowing movement

1000

10000

100000 volume, the other half allowing movement• const. source acceleration amplitude (2Y0)

of 10 m/s2 (equiv to Y0 = 25 cm at 1 Hz)• proof mass with density 20 g/cc

1

10

100

pow

er (m

W)

f = 1 Hzf = 10 Hz sensor node *

p y g

0.01

0.1

1p

watchcellphonelaptop0.001

0.01 0.1 1 10 100 1000

volume (cc)

laptop

* For the sensor node, we are assuming a simple physical sensor (e.g. temp, pressure or motion) with short-range (e.g. within room) wireless link and low data-rate


Comparison of Architectures

c = t f

excitation frequencyNormalised axes:

c resonant frequency

(resonant devices)

Zl/Y0 = mass travel rangeexcitation amplitude

Power = P (Watts)

m3Y02

cZl/Y0

• Resonant devices better for large generators / small displacements, operated near resonance

• Non-resonant good for large displacements, wide input frequency ranges

Mitcheson P.D., Green T.C., Yeatman E.M., Holmes A.S., “Architectures for vibration-driven micropower generators”, IEEE/ASME J. Microelectromechanical Systems 13(3), (2004), 429-440.


Machine Powered Applications

• Resonant vibration-driven generators aimed at machine/process monitoring are the most highly developed

• Synchronous electrical machines have predictable vibration frequency, making them ideal for resonant energy harvesters

• Several commercial offerings, e.g.Several commercial offerings, e.g.

PMG17 PMG17 from Perpetuum LtdResonant generator tuned to 2nd harmonic of mains frequency – 100 or 120 Hz

55 mm diameter x 55 mm length55 mm diameter x 55 mm length

4.5 mW output power (rectified DC) at 0.1g acceleration


Human Powered ApplicationsExcitations are slow, large in amplitude and irregular compared to those generally encountered in machine applicationsencountered in machine applications

• Non-resonant device can win at small generator sizes• Data obtained in collaboration with ETH Zurich (T. von Buren)

von Büren T., Mitcheson P.D., Green T.C., Yeatman E.M., Holmes A.S., Tröster G., “Optimization of inertial micropower generators for human walking motion”, IEEE Sensors Journal, 6(1), (2006), 28-38.


Non-resonant Device developed at Imperial

Discharge contact on top plate

Model: MEMS parallel plate capacitor implementation:

on top plateMoving capacitor plate / mass

Fixed capacitorFixed capacitor plate on baseplate

Pre-charging contact

Generation cycle:

• Capacitor is pre-charged when mass is at bottom (max capacitance)

• Under sufficiently large (downward) frame acceleration, capacitor plates separate at constant charge, and work is done against electrostatic force stored electrostatic energy and plate voltage increase

• Charge is transferred (at higher voltage) to external circuit when moving plate reaches position of max displacement


Energy Yield per Cycle

SeparationInput phase Output phase

inputinputVCQ outputoutputVCQ

inputoutput

inputouput V

CC

V output

Generated energy:

222

21)(

21

21

21

outputoutputinputoutputoutputoutputinputinputouputoutput VCVVVCVCVCE

Generated energy:

2222inputoutput VV


Measured Performanceshakergenerator

V lt b h i t i d• Voltage probe has input impedance >1012 and dynamically measures voltage on capacitor

voltage probe

• Net power in this experiment: 2.2 μW


Motion-driven Harvesters – are they any good?

1.6%

1.8%

EMESPZ

Volume Figure of Merit defined as:

Useful Power Output

1%

1.2%

1.4%

V

PZFoMV = Useful Power Output

AuVol4/3Y03116

Represents ratio of output

0.6%

0.8%

1%

FoM

VRepresents ratio of output power to that of idealised generators on slide 7

As of 2008 best devices

0

0.2%

0.4%As of 2008 best devices achieved only about 2%

Better devices have emerged since but there is still a way

2000 2002 2004 2006 20080

Publication Year

since, but there is still a way to go...

Main issues are: (1) damping/transduction – need to implement stronger dampers; (2) power

Mitcheson P.D., Yeatman E.M., Kondala Rao G., Holmes A.S., Green T.C., “Energy harvesting from human and machine motion for wireless electronic devices”, Proc. IEEE 96(9), (2008), 1457-1486.

( ) p g p g p ; ( ) pconversion electronics – difficult to make efficient; (3) adaptive operation


, ( ), ( ),

Flow-driven Microgenerators


Energy Scavenging from Air Flow

Basic concept: wind turbines on a smaller scale (cm-scale or smaller)

E t t ki tiExtract kinetic energy from air flow

K.E. per unit vol in flow = ½V2

K.E. per sec crossing swept area is:P il = ½V2xAV = ½AV3 100

1000

10000

100000

(mW

)

CP = 0.1Betz limit (CP = 0.59)

For 1 cm-dia disc:

Pavail ½V xAV ½AVActual output power is:

P = ½AV3CP0 001

0.01

0.1

1

10

Out

put p

ower

Land vehicle

Flight vehicle

where CP = power coefficient 0.0001

0.001

0.1 1 10 100 1000

Flow speed (m/sec)

HVAC duct


2-cm dia. Device developed at Imperial

• Ducted turbine with integrated axial-flux permanent magnet generator

• mW output power levels

• Starts at low flow speeds (~3 m/s)

• Applications in HVAC duct sensing and gas pipeline monitoring

5

6

7

er (m

W)

Tunnel

2

3

4

5

or o

utpu

t pow

Tunnel speed

10.0 m/s

0

1

2

0 1000 2000 3000 4000 5000 6000

Gen

erat

o

7.0 m/s

8.0 m/s

9.0 m/s

6.0 m/s


Rotation speed (RPM)

Comparison with other Flow-driven Harvesters

• Small flow-driven devices are expected to perform relatively poorly because of high viscous losses

• Small turbines also suffer from relatively large clearances and bearing losses

10000 B li iC l t t d i t

10

100

1000

10000

W/c

m^2

)

Betz limitCp = 0.1Federspiel (2003), A = 81 sq.cmRancourt (2007), A = 13.9 sq.cmMyers (2007), A ~ 317 sq.cmHolmes (2009), A = 3.14 sq.cm

Cm-scale prototype devices to date have struggled to reach Cp ~ 0.1

N th l f l ( W)

0.1

1

10

r den

sity

(mWNevertheless, useful (mW)

power levels can be generated because available power in flow is significant

0.0001

0.001

0.01Po

wer

po e o s s g caeven at modest flow speeds

Duct sensing applications look quite viable even with

0.1 1 10 100Flow speed [m/sec]

qcurrent devices

Bansal A., Howey D.A., Holmes A.S., “Cm-scale air turbine and generator for energy scavenging from low speed flows” Proc Transducers 2009 Denver Colorado USA 21 25 June 2009 pp 529 532


low-speed flows , Proc. Transducers 2009, Denver, Colorado, USA, 21-25 June 2009, pp. 529-532.

HVAC Duct Sensor Concept

“Spider” mountedi id d tinside duct

Sensor arrayDistributed network of wireless sensors with peer-to-peer communication to relay data to control centre

Generator / Transceiver

data to control centre

Monitoring of:• Air flow and temp for HVAC controlAir flow and temp for HVAC control• Air-quality e.g. RH; CO2, Ammonia, VOCs


SSummary

M ti d i h t till f i t l lMotion-driven energy harvesters are still performing at a level some way below what is theoretically achievable

Current performance is adequate for some important applications such p q p ppas machine monitoring, and commercial solutions are available

Improvements in performance will be required before harvesting power from human body motion can become viablefrom human body motion can become viable

Flow-driven devices at cm-scale also have relatively low conversion efficiencies, but the available power in the flow is such that duct sensing applications appear viable

17th World Micromachine Summit

Acknowledgements

Motion-driven Generators:Eric YeatmanEric YeatmanPaul MitchesonTim GreenPeng Miao (now with Oxford Instruments)Bernard Stark (now with University of Bristol)

Flow-driven Generators:Keith Pullen (now with City University, London)Guodong Hong (now with Microsaic Systems plc)Guodong Hong (now with Microsaic Systems plc)Anshu BansalDavid Howey


Contact

Andrew S HolmesProfessor of Micro Electro Mechanical Systems

Optical and Semiconductor Devices GroupDepartment of Electrical and Electronic EngineeringImperial College LondonExhibition Road, London SW7 2BT, UKExhibition Road, London SW7 2BT, UK

Tel: +44 (0)20 7594 6239Fax: + 44 (0)20 7594 6308Fax: + 44 (0)20 7594 6308

Email : [email protected]

Web: http://www3.imperial.ac.uk/opticalandsemidev


Energy Harvesting Technologies for Wireless SensorsPMG17 PMG17 from Perpetuum Ltd Resonant generator tuned to 2nd harmonic of mains frequency – 100 or 120 Hz 55 mm diameter x 55

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