p. mitcheson, nov. 2008 p. d. mitcheson, iom, march 2009 energy harvesting for pervasive sensing...

P. Mitcheson, Nov. 2008P. D. Mitcheson, IOM, March 2009

Energy Harvesting for Pervasive Sensing

Paul D. Mitcheson, Eric M. Yeatman

Department of Electronic & Electrical EngineeringImperial College London


Energy Harvesting: what is it?

• Taking useful advantage of power sources already present in the local environment

• This energy would otherwise be unused or wasted as e.g. heat

• “local” being local to the powered device or system

• Extracted power levels generally not limited by source, but by size and effectiveness of generator (“harvester”)


Energy Harvesting: what is it for?

• Normally not as a primary source of power, but for applications where mains power is not suitable, because of:

• Installation cost

• Mobility

• Remote/inaccessible/hostile location

• Usual alternative is batteries:

• Avoid replacement/recharging

• Avoid waste from used batteries


How Much Power?

World electrical generation capacity 4 terawatts

Power station 1 gigawatt

House 10 kilowatts

Person, lightbulb 100 watts

Laptop, heart 10 watts

Cellphone power usage 1 watt

Wristwatch, sensor node 1 microwatt

Transmitted Cellphone signal 1 nanowatt


Cost example:

• Mains electricity: consumer price 15¢ / kWhr

• Alkaline AA battery: 1 € / 3 Whr

• Factor of 2,000


Energy Harvesting Applications

• Key application is wireless sensor networks

• Sensors can be very low power

• Small size often important

• Minimal maintenance crucial if many nodes

• Implementation of WSNs could lead to higher energy efficiency of buildings etc


1 cc wireless sensor node, IMEC


Sensor Node Power Requirements – How much power does our harvester

need to supply?

•Sensing Element

•Signal Conditioning Electronics

•Data Transmission


Sensing Element

Simple signals - temperature, pressure, motion – require electrical power above thermal noise limit.

NT 10-20 W/Hz

For most applications, this is negligible


Signal Conditioning

Likely principal function: A/D Converter

Recent results: Sauerbrey et al., Infineon (’03)

Power < 1 W possible for low sample rates!


Data Transmission: Required Power

Conclusions:

Power independent of bit-rate for low bit-rate

-30 dBm (1 W) feasible for room-scale transmission range

1000100101

50

40

30

20

10

0

-10

-20

-30

-40

-50

Range (m)

Tra

ns

mit

Po

we

r (d

Bm

)

Ideal free-space propagation

Typical indoorLoss exponent(3.5)

Figure: F. Martin, Motorola


Estimated Total Power Needs

• Peak power 1 – 100 uW

• Average power can be below 1 uW

Batteries: Present Capability

•10 Wyr for 1 cm3 battery feasible

•Not easy to beat!

•Useful energy reservoir for energy harvesting

P. Mitcheson, Nov. 2008

Fuel-Based Power Sources

• Energy density much higher than for batteries, 10 kJ/ cm3

• Technology immature, fuel cells most promising

Micro fuel cell, Yen et al.Fraunhofer Inst.


Energy Source Conversion Mechanism

Light

Ambient light, such as sunlight Solar Cells

Thermal

Temperature gradientsThermoelectric or Heat Engine

Magnetic and Electro-magnetic

Electro-magnetic waves

Magnetic induction (induction loop)

Antennas

Kinetic

Volume flow (liquids or gases)

Movement and vibration

Magnetic (induction)

Piezoelectric

Electrostatic

Energy Scavenging : Sources


Solar Cells

• highly developed

• suited to integration

• high power density possible:

100 mW/cm2 (strong sunlight)

• but not common:

100 W/cm2 (office)

• Need to be exposed, and oriented correctly

Solar cell for Berkeley Pico-Radio


Solar Cells in Energy Harvesting Applications:

• Cost not the main issue

• Availability of light is key


Thermal

• need reasonable temperature difference (5 – 10C) in short distance

• ADS device 10 W for 5C

• even small T hard to achieve

Heat engine, Whalen et al,

Applied Digital Solutions


Seiko Thermic (no longer in production)


Ambient Electromagnetic Radiation

Graph: Mantiply et al.

10 V/m needed for reasonable power: not generally available


Motion Energy Scavenging

• Direct force devices

• Inertial devices


Direct Force: Heel Strike

Heel strike generator: Paradiso et al, MIT


Direct Force: larger scale

East Japan Railway Co.

• Energy harvesting ticket gates


m

z o

y = Y cos( t)o

dam per im plem ents energy conversion

Inertial Harvesters• Mass mounted on a spring within a frame

• Frame attached to moving “host” (person, machine…)

• Host motion vibrates internal mass

• Internal transducer extracts power


m

z o

y = Y cos( t)o

dam per im plem ents energy conversion

• Peak force on proof mass F = ma = m2Yo

• Damper force < F or no movement

• Maximum work per transit W = Fzo = m2Yozo

• Maximum power P = 2W/T = m3Yozo/

Available Power from Inertial Harvesters


0.1

1

10

100

1 10 100

frequency (Hz)

pow

er (

uW)

How much power is this?

Plot assumes:

• Si proof mass (higher densities possible)

• max source acceleration 1g (determines Yo for any f)

10 x 10 x 2 mm

3 x 3 x 0.6 mm


Achievable Power Relative to Applications

0.001

0.01

0.1

1

10

100

1000

10000

100000

0.01 0.1 1 10 100 1000

volume (cc)

pow

er (

mW

)

f = 1 Hz

f = 10 Hz

Sensor node

watch

cellphone

laptop

Plot assumes:

• proof mass 10 g/cc

• source acceleration 1g


Implementation Issues: Transduction Mechanism

Piezoelectric?• Difficult integration of piezo material• Reasonable voltage levels easy to achieve• Suitable for miniaturisation


Typical Inertial Generators

Piezoelectric

Ferro solutionsWright et al, Berkeley


Implementation Issues: Transduction Mechanism

Electromagnetic?• Dominant method for large scale conversion• Needs high d/dt to get damper force ( = flux)• d/dt = (d/dz )(dz/dt )• Low frequency (low dz/dt) needs very high flux gradient• Hard to get enough voltage in small device (coil turns)• Efficiency issues (coil current)

Variant: magnetostrictive


Typical Inertial Generators

Magnetic

Southampton U. CUHK


Implementation Issues: Mechanism

Electrostatic?• Simple implementation, no field gradient problem• Suitable for small size scale• Damping force can be varied via applied voltage• But needs priming voltage (or electret)


Typical Approach: Constant Charge

Input phase Output phase

inputinputVCQ outputoutputVCQ

inputoutput

inputouput V

C

CV

222

2

1

2

1

2

1outputoutputinputinputouputoutput VCVCVCE

inputoutput VV


Assembled generator Detail of deep-etched moving plate

Prototype MEMS Device


Device Operation

po

siti

on

tim e

tim e

trajectory of m oving plate

vo

ltag

e

t2 t3t1

voltage on moving plate

upperlim it

lower lim it

moving plate/ proof mass

fixed plate

discharge contact

charging contact

Output > 2 W


Other Options: Rotating Mass

Example : Seiko Kinetic


Large Inertial Generators

Backpack: U Penn

• 7 watts!


Pervasive Sensing for Energy Generation


Conclusions

• Power levels in the microwatt range are enough for many wireless sensor nodes

• Small energy harvesters can achieve these levels

• Help enable pervasive sensing by eliminating maintenance burden

Contact: [email protected]

Review Paper: Mitcheson, Yeatman et al., “Energy Harvesting From Human and Machine Motion for Wireless Electronic Devices”, Proceedings of the IEEE 96(9), 1457-1486 (1998).

p. mitcheson, nov. 2008 p. d. mitcheson, iom, march 2009 energy harvesting for pervasive sensing...

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