P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Energy Harvesting for Pervasive Sensing Paul D. Mitcheson, Eric M. Yeatman Department of Electronic & Electrical Engineering Imperial College London

P. Mitcheson, Nov P. 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 P. 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 P. D. Mitcheson, IOM, March 2009 How Much Power? World electrical generation capacity4 terawatts Power station1 gigawatt House10 kilowatts Person, lightbulb100 watts Laptop, heart10 watts Cellphone power usage1 watt Wristwatch, sensor node1 microwatt Transmitted Cellphone signal1 nanowatt

P. Mitcheson, Nov P. 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 P. 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 P. D. Mitcheson, IOM, March cc wireless sensor node, IMEC

P. Mitcheson, Nov P. 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 P. D. Mitcheson, IOM, March 2009 Sensing Element Simple signals - temperature, pressure, motion – require electrical power above thermal noise limit. N T  W/Hz For most applications, this is negligible

P. Mitcheson, Nov P. 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 P. 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 Range (m) Transmit Power (dBm) Ideal free-space propagation Typical indoor Loss exponent (3.5) Figure: F. Martin, Motorola

P. Mitcheson, Nov P. 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  W  yr for 1 cm 3 battery feasible Not easy to beat! Useful energy reservoir for energy harvesting

P. Mitcheson, Nov Fuel-Based Power Sources Energy density much higher than for batteries,  10 kJ/ cm 3 Technology immature, fuel cells most promising Micro fuel cell, Yen et al. Fraunhofer Inst.

P. Mitcheson, Nov Energy SourceConversion Mechanism Light Ambient light, such as sunlight Solar Cells Thermal Temperature gradients Thermoelectric 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 Solar Cells highly developed suited to integration high power density possible:  100 mW/cm 2 (strong sunlight) but not common:  100  W/cm 2 (office) Need to be exposed, and oriented correctly Solar cell for Berkeley Pico-Radio

P. Mitcheson, Nov Solar Cells in Energy Harvesting Applications: Cost not the main issue Availability of light is key

P. Mitcheson, Nov Thermal need reasonable temperature difference (5 – 10  C) in short distance ADS device  10  W for 5  C even small  T hard to achieve Heat engine, Whalen et al, Applied Digital Solutions

P. Mitcheson, Nov Seiko Thermic (no longer in production)

P. Mitcheson, Nov Ambient Electromagnetic Radiation Graph: Mantiply et al.  10 V/m needed for reasonable power: not generally available

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Motion Energy Scavenging Direct force devices Inertial devices

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Direct Force: Heel Strike Heel strike generator: Paradiso et al, MIT

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Direct Force: larger scale East Japan Railway Co. Energy harvesting ticket gates

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 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 P. D. Mitcheson, IOM, March 2009 Peak force on proof mass F = ma = m  2 Y o Damper force < F or no movement Maximum work per transit W = Fz o = m  2 Y o z o Maximum power P = 2W/T = m  3 Y o z o /  Available Power from Inertial Harvesters

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 How much power is this? Plot assumes: Si proof mass (higher densities possible) max source acceleration 1g (determines Y o for any f) 10 x 10 x 2 mm 3 x 3 x 0.6 mm

P. Mitcheson, Nov Achievable Power Relative to Applications Sensor node watch cellphone laptop Plot assumes: proof mass 10 g/cc source acceleration 1g

P. Mitcheson, Nov P. 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 P. D. Mitcheson, IOM, March 2009 Typical Inertial Generators Piezoelectric Ferro solutionsWright et al, Berkeley

P. Mitcheson, Nov P. 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 P. D. Mitcheson, IOM, March 2009 Typical Inertial Generators Magnetic Southampton U.CUHK

P. Mitcheson, Nov P. 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 Typical Approach: Constant Charge Input phase Output phase

P. Mitcheson, Nov Assembled generatorDetail of deep-etched moving plate Prototype MEMS Device

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Device Operation Output > 2  W

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Other Options: Rotating Mass Example : Seiko Kinetic

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Large Inertial Generators Backpack: U Penn 7 watts!

P. Mitcheson, Nov P. D. Mitcheson, IOM, March 2009 Pervasive Sensing for Energy Generation

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