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