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Energy Harvesting for Pervasive Sensing

Learn about the concept of energy harvesting for pervasive sensing, which utilizes power sources in the local environment to power devices and systems. Discover various power sources and their applications in wireless sensor networks.

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Energy Harvesting for Pervasive Sensing

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  1. Energy Harvestingfor Pervasive Sensing Paul D. Mitcheson, Eric M. Yeatman Department of Electronic & Electrical Engineering Imperial College London

  2. 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”)

  3. 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

  4. 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

  5. Cost example: • Mains electricity: consumer price 15¢ / kWhr • Alkaline AA battery: 1 € / 3 Whr • Factor of 2,000

  6. 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

  7. 1 cc wireless sensor node, IMEC

  8. Sensor Node Power Requirements – How much power does our harvester need to supply? • Sensing Element • Signal Conditioning Electronics • Data Transmission

  9. 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

  10. Signal Conditioning Likely principal function: A/D Converter Recent results: Sauerbrey et al., Infineon (’03) Power < 1 mW possible for low sample rates!

  11. 5 0 4 0 Typical indoor Loss exponent (3.5) 3 0 2 0 1 0 Transmit Power (dBm) 0 - 1 0 - 2 0 - 3 0 Ideal free-space propagation - 4 0 - 5 0 1 1 0 1 0 0 1 0 0 0 Range (m) Data Transmission: Required Power Conclusions: Power independent of bit-rate for low bit-rate -30 dBm (1 mW) feasible for room-scale transmission range Figure: F. Martin, Motorola

  12. Estimated Total Power Needs • Peak power 1 – 100 uW • Average power can be below 1 uW • Batteries: Present Capability • 10 mWyr for 1 cm3 battery feasible • Not easy to beat! • Useful energy reservoir for energy harvesting

  13. Fuel-Based Power Sources • Energy density much higher than for batteries, 10 kJ/ cm3 • Technology immature, fuel cells most promising Fraunhofer Inst. Micro fuel cell, Yen et al.

  14. Energy Scavenging : Sources

  15. Solar Cells • highly developed • suited to integration • high power density possible: • 100 mW/cm2 (strong sunlight) • but not common: • 100 mW/cm2 (office) • Need to be exposed, and oriented correctly Solar cell for Berkeley Pico-Radio

  16. Solar Cells in Energy Harvesting Applications: • Cost not the main issue • Availability of light is key

  17. Thermal • need reasonable temperature difference (5 – 10C) in short distance • ADS device 10 mW for 5C • even small DT hard to achieve Heat engine, Whalen et al, Applied Digital Solutions

  18. Seiko Thermic (no longer in production)

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

  20. Motion Energy Scavenging • Direct force devices • Inertial devices

  21. Direct Force: Heel Strike Heel strike generator: Paradiso et al, MIT

  22. Direct Force: larger scale • East Japan Railway Co. • Energy harvesting ticket gates

  23. 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

  24. Available Power from Inertial Harvesters • Peak force on proof mass F = ma = mw2Yo • Damper force < F or no movement • Maximum work per transit W = Fzo = mw2Yozo • Maximum power P = 2W/T = mw3Yozo/p

  25. How much power is this? 10 x 10 x 2 mm 3 x 3 x 0.6 mm • Plot assumes: • Si proof mass (higher densities possible) • max source acceleration 1g (determines Yo for any f)

  26. Sensor node watch cellphone laptop Achievable Power Relative to Applications • Plot assumes: • proof mass 10 g/cc • source acceleration 1g

  27. Implementation Issues: Transduction Mechanism • Piezoelectric? • Difficult integration of piezo material • Reasonable voltage levels easy to achieve • Suitable for miniaturisation

  28. Typical Inertial Generators Piezoelectric Wright et al, Berkeley Ferro solutions

  29. Implementation Issues: Transduction Mechanism • Electromagnetic? • Dominant method for large scale conversion • Needs high df/dt to get damper force (f = flux) • df/dt = (df/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

  30. Typical Inertial Generators Magnetic Southampton U. CUHK

  31. 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)

  32. Typical Approach: Constant Charge Input phase Output phase

  33. Prototype MEMS Device Assembled generator Detail of deep-etched moving plate

  34. Device Operation Output > 2 mW

  35. Other Options: Rotating Mass Example : Seiko Kinetic

  36. Large Inertial Generators • Backpack: U Penn • 7 watts!

  37. Pervasive Sensing for Energy Generation

  38. 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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