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

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

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”)

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

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

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

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

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

Cost example:

• Mains electricity: consumer price 15¢ / kWhr

• Alkaline AA battery: 1 € / 3 Whr

• Factor of 2,000

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

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

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

1 cc wireless sensor node, IMEC

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

Sensor Node Power Requirements – How much power does our harvester

need to supply?

•Sensing Element

•Signal Conditioning Electronics

•Data Transmission

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

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

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

Signal Conditioning

Likely principal function: A/D Converter

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

Power < 1 W possible for low sample rates!

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

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

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

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.

P. Mitcheson, Nov. 2008

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

P. Mitcheson, Nov. 2008

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

P. Mitcheson, Nov. 2008

Solar Cells in Energy Harvesting Applications:

• Cost not the main issue

• Availability of light is key

P. Mitcheson, Nov. 2008

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

P. Mitcheson, Nov. 2008

Seiko Thermic (no longer in production)

P. Mitcheson, Nov. 2008

Ambient Electromagnetic Radiation

Graph: Mantiply et al.

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

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

Motion Energy Scavenging

• Direct force devices

• Inertial devices

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

Direct Force: Heel Strike

Heel strike generator: Paradiso et al, MIT

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

Direct Force: larger scale

East Japan Railway Co.

• Energy harvesting ticket gates

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

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

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

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

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

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

P. Mitcheson, Nov. 2008

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

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

Implementation Issues: Transduction Mechanism

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

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

Typical Inertial Generators

Piezoelectric

Ferro solutionsWright et al, Berkeley

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

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

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

Typical Inertial Generators

Magnetic

Southampton U. CUHK

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

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)

P. Mitcheson, Nov. 2008

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

P. Mitcheson, Nov. 2008

Assembled generator Detail of deep-etched moving plate

Prototype MEMS Device

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

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

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

Other Options: Rotating Mass

Example : Seiko Kinetic

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

Large Inertial Generators

Backpack: U Penn

• 7 watts!

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

Pervasive Sensing for Energy Generation

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

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: paul.mitcheson@imperial.ac.uk

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).