Energy Harvesting and Applications

1. Energy Harvesting and Applications D. J. Inman A. Erturk, M.A. Karami, C. DeMarqui, S. Anton, B. Joyce, J. Hobeck and Y. Wang Center for Intelligent Material Systems and Structures NSF I/UCRC Center for Energy Harvesting Materials and Systems Virginia Tech Blacksburg, VA 24061, USA dinman@vt.eduwww.cimss.vt.edu and Institute for Smart Technologies University of Bristol Bristol, BS* 1TR UK redji@bristol.ac.uk

2. Outline • Introduction to the basics in energy harvesting • Self charging structures • Enhancements to vibration based energy harvesting using nonlinear dynamics (Erturk) • Piezomagnetic harvesting from a bistable beam • Piezoelectric harvesting from a bistable plate • Piezoaeroelastic energy harvesting (Erturk and DeMarque) • Harvesting • Simultaneous harvesting and flutter suppression • Minimum energy controllers to work with harvesters (Wang) • Application to Bridge Monitoring

3. Energy Harvesting as used here refers to: Capturing low levels of ambient waste energy to convert to useable electrical energy • The goals are: • To increase battery life or to replace batteries • To provide wireless sensor solutions to numerous problems • Most of the effort reported here focuses on harvesting mechanical vibration using the piezoelectric effect • Other important harvesting mechanisms are • Use of the Seebeck effect using thermoelectric materials to capture thermal gradients • Use of the photovoltaic effect to capture solar energy • Other effects useful in harvesting mechanical energy are the electromagnetic effect and the electrostatic effect

4. Some Sample Numbers • Mechanical Harvesting from ambient vibration produces values from milliwatts to microwatts. (eg 0.5 g at 8 Hz produces 10 mW) • Small thermal electrics and solar can produce up to a few watts • Small solar arrays can also produce of the order of a watt

5. Piezoelectric based Energy Harvesting Involves 4 Components Structural Dynamics Piezoelectric Constitutive laws Electric Circuits Storage and/or direct use Linear Cantilevers with tip masses well Studied and modeled Room here for advances in material science Batteries Li Ion Super caps Mechanical not much investigated Need adaptive and nonlinear circuits to address issues of conditioning and optimality Most studies focus on just optimal resistance

6. The piezoelectric effect couples mechanical strain to and electric field allowing for both sensing and actuation functions xi+1 xi Piezoceramic y t x y1 y2 b Host structure Neutral axis The constitutive equations are S is the strain, T is the stress, D is the electric displacement, E is the electric field, sE is the compliance measured at a constant electric field, eT is the permittivity measured at constant stress and d is the charge coefficient.

7. Harvesting vibration using the piezoelectric effect can be configured in a number of ways • Layered onto a structure • Cantilevered off of a vibrating mounting point (with tip mass) • Stacked between two moving surfaces • Flapping in the wind • ???? Proof mass PZT Cantilevered with tunable tip mass: Tip mass Moving base PZT Layered into a structural member: Bimorph vs. unimorph

8. Storage and Duty Cycles can extend the usefulness of harvested energy • Not all applications need continuous power • Storage devices can be used to enhance the usefulness of a harvesting system • Suppose telemetry of a sensor signal is required 1 sec out of every hour (following example illustrates) • Storage can be through batteries or capacitors • Batteries: limited cycles/hold charge well • Capacitors and super capacitors: large numbers of cycles, drain quickly

9. Most harvesting of vibration using the piezoelectric effect is based on the idea of resonance using a cantilever Response: Natural frequency of structure: PZT Tip mass Cantilevered with tunable tip mass: Moving base

10. Distributed Parameter Model (Erturk & Inman) External (air) damping Internal (strain rate) damping Electrical term External damping excitation Inertial excitation • Coupled electrical circuit equation: Internal capacitance of the piezoceramic Circuit excitation term

11. Closed form solution for both the mechanical and electrical response reveals backwards coupling Steady state voltage response: Steady state vibration response: Backward piezoelectric coupling in the beam response (clearly not an electrically induced viscous damping term)

12. Cantilever based energy harvesting in the linear region results in a single frequency device Power out versus frequency of disturbance for a linear harvester Issues: ambient energy is often broad band in the low frequency range, linear harvesting is narrowband, and frequency increases as the length of the cantilever decreases

13. Here we exploit nonlinear effects to increase the band width of the energy harvester • Nonlinear behavior is purposefully introduced using added magnets to encourage harvesting over a broader frequency range • Once completed the power generated by the linear and nonlinear system are compare for exactly the same ambient input signal

14. Limit Cycle Oscillations for Broad Band Harvesting • A magnetic field causes the equation of motion of the harvesting piezoelectric cantilever to be nonlinear • Spacing of the magnets results in: • 5 equilibrium (3 stable) • 3 equilibrium (2 stable) • 1 equilibrium (1 sable) • Limit cycle oscillation is the possible producing large amplitude periodic response over a range of input frequencies

15. Nonlinear effects: Bistable Beam Induces More Power Over Wider Frequency Range

16. Illustration of Limit Cycle Harvesting for low and high amplitude accelerations 8.5 mW at 0.35g an order of magnitude better then w/o magnets!

17. Comparison of voltage vs velocity vs time of linear and nonlinear harvesters for increasing frequency

18. Power Output Comparison of Linear vs Nonlinear Linear Resonance Note that at linear resonance the linear system will always win, however it is narrow band and falls off quickly away from resonance and that the nonlinear has higher values overall

19. Such nonlinearities can also be induced by using a bistable plate Bistable carbon-fiber plate with piezoceramic patches The plate is clamped to a seismic shaker from its center point. Arrieta, A.F., Erturk, A., Hagedorn, P., and Inman, D.J., 2010, A Piezoelectric Bi-stable Plate for Nonlinear Broadband Energy Harvesting, Applied Physics Letters (in press).

20. Various nonlinear phenomena can be observed in the bistableplate, enhancing harvesting. High-energy LCO (8.6 Hz) Voltage history samples Chaos (12.5 Hz) Intermittency (9.8 Hz) Arrieta, A.F., Erturk, A., Hagedorn, P., and Inman, D.J., 2010, A Piezoelectric Bi-stable Plate for Nonlinear Broadband Energy Harvesting, Applied Physics Letters (in press).

21. Large-amplitude oscillations generate very high power outputovera range of frequencies. Average power vs. Load resistance Average power vs. Frequency (98.5 kohm) Arrieta, A.F., Erturk, A., Hagedorn, P., and Inman, D.J., 2010, A Piezoelectric Bi-stable Plate for Nonlinear Broadband Energy Harvesting, Applied Physics Letters (in press).

22. Application of energy harvesting to Structural Health monitoring • The goal is to provide power for remote monitoring systems so that battery life can be extended and or batteries can be removed • Three applications are under way • One is monitoring in flows • One is the monitoring of a bridge • The last is the monitoring of a wind turbine blade

23. Piezoelectric Grass for Harvesting Energy from a Flow for Running Remote Sensors In line configuration Staggered configuration

24. Acceleration data of the bridge has been simplified to a harmonic function for simulations in the lab. Acceleration signal measured on the bridge 3-span steel girder bridge (08/18/09 - Roanoke) Approximation as a persistent single harmonic (0.05g at 7.7 Hz) Experimental setup Accelerometer Piezoelectric and electromagnetic generators Acceleration measured on the shaker Seismic shaker

25. Piezoelectric and electromagnetic power outputs have been measured for an acceleration input of 0.05g (RMS: 0.035g) at 7.7 Hz. Combined piezoelectric-electromagnetic generator configuration Piezoceramic patches Accelerometer Rare earth magnets Coil Seismic shaker Electromagnetic part : 0.22 V for 82 ohms = 0.6 mW(per coil) Piezoelectric part : 11.2 V for 470 kohms = 0.3 mW Power output of a single generator (for 0.05g) = 0.9 mW

26. Increased base acceleration amplitude results in a larger power output. (0.1g, RMS: 0.07gat 7.7 Hz yields 2.7 mW). Electromagnetic part : 0.42 V for 100ohms = 1.8 mW (from a single coil) Piezoelectric part : 21 V for 470 kohms = 0.94 mW Power output of a single generator (for 0.1g) = 2.7 mW [click on the movie]

27. Piezoelectric Low Speed Wind Harvester • Compact Contact-less prototype has been fabricated and is waiting to be tested • Magnet rotor needs to be matched with blade rotor output profile • Multiple blade geometries have been printed Hope to gather wind speeds at a few kmh

28. Example of how Energy harvesting enables other technologies: Monitoring of wind turbines Impact Impact or cracking detected via Acoustic Emission (AE) Signals activated periodically and/or by impact. Sensor Data Sleep NO Fatigue YES Blade Energy Impedance activated via AE and periodically to see if significant damage exists Sleep NO Energy harvesting from blade vibration and centripetal force Broadcast damage state YES

29. Uses the interplay between gravity and centripetal force to harvest energy Path of magnet

30. Sample of energy harvesting capability for wind turbine blades Voltage versus time

31. Prospects of using harvested energy to perform Control • The first example is harvesting energy from low induced wing vibrations in an aircraft wing • This action of harvesting automatically induces a shunting effect which acts a a vibration suppression system • The result is an increase in flutter speed whilst simultaneously harvesting energy • The second is a look at performing active control using only harvested energy to provide vibration suppression. • As a first step, we examine which control laws for vibration suppression will use the least amount of energy

32. Flow Induced Energy Harvesting Typical section with piezoceramics Experimental setup

33. Flow Induced Energy Harvesting Results Piezoaeroelastic equations Model validation – piezoaeroelastic response at the flutter boundary

34. The time-domain piezoaeroelastic solution can predict the electromechanical response for airflow speeds below the linear flutter speed. Tip displacement Total damping vs. Airflow speed Electrical power De Marqui, Jr., C., Erturk, A., and Inman, D.J., 2010, Piezoaeroelastic Modeling and Analysis of a Generator Wing with Continuous and Segmented Electrodes, Journal of Intelligent Material Systems and Structures, 21(in press) doi: 10.1177/1045389 X10372261.

35. Predictions for wing based harvesting and passive control • The analytical model predicts that the piezoaeroelastic system will harvest 10.7 mW at an air speed of 9.32 m/s • The shunting effect of the energy harvester simultaneously adds damping to the system and predicts an increase in flutter speed of 5.5% (that is a reduction in vibration)

36. Examination of Low Power Control Laws • The goal is to find the feedback control law for vibration suppression the uses the least amount of energy • Fix the performance by fixing the settling time and overshoot and then computing several different control laws to obtain the desired response and then comparing the energy required for each

37. Proposed Hybrid Control Laws Use several common vibration suppression controllers, then use a switching algorithm over the top:

38. The response of four controllers and their hybrid implementation

39. Power consumption for each of the 8 controllers

40. Summary of the average power used by each controller (experimental): The best choice of controller for use with harvested energy

41. Self Charging structures and Combined Effects: piezoelectric, solar and thermal energy harvesting with flexible thin-film batteries. New generation self-charging structures with flexible piezoceramic, solar panel and battery layers Aluminum substructure components Piezoceramic patch Flexible solar panel Flexible thin-film battery Thermoelectric generator Efficient circuit design for battery charging Vibration power Heat sink Regulator circuit for impedance matching Thermoelectric generator Solar power Thin-film battery Heater Thermal power

42. Extending the concept of the self charging structure to solar and thermal energy harvesting Flexible solar panels Thin film Battery 70 mW Piezoelectrictransducer 30 mW/g2 10 mW Thermoelectricgenerator

43. 3.E/M CHARACTERIZATION: of the multifunctional system for different vibration, solar and thermal energy levels. Different stages of fabrication Base excitation using a shaker Assembly: 4” x 1” x 0.06” Aluminum (innermost) Thin-film battery Piezo layer Flexible solar panel (outermost) Solar power (outdoor) Thermal power Vibration power at resonance hot side cold side Temperature [oC] Power [mW] power output Power [mW] temperature difference Load resistance [ohms] Load resistance [ohms]

44. Circuits and Implementation Switching Impedance Matching Circuits Thin-film Battery #1 Thermoelectric Harvester Piezoelectric Harvester Thin-film Battery #2 Solar Harvester

45. UAV APPLICATION: embed piezoelectric and thin-film battery into the wing spar of a UAV to form a multifunctional spar capable of powering a local sensor • Incorporate energy harvesting devices and novel storage elements into UAVs. • Provide local power source for low-power sensors in aircraft • Flight endurance should remain unchanged with addition of harvesting Multifunctionality Load bearing + Power generation + Energy storage

46. Low Frequency MEMS Cantilever Harvesting • MEMS scale demands a cantilever of short length • The resonance frequency of a cantilever is inversely proportional to the square of its length: short beams mean high frequencies • Ambient energy in most systems are low • A potential solution is to use a zigzag arrangement of cantilevers

47. Reduction in frequency with member increase and experimental validation of model

48. 3 mm 3 mm ~ 550 Hz, 20 nw/g2 ~400 Hz 1.45 mm 300 um 300 um ~ 380 Hz, 35 nw/g2

49. Five popular piezoelectric ceramics have been considered (PZT-5A, PZT-5H and 3 single crystal types). Compliance |d31| • An order of magnitude difference for the d31 values of single crystals is not the case for their effective e31 values due to the effect of elastic compliance.

50. Comparison of these 5 Bimorph Cantilevers: • For resonance excitation, the peak power does NOT differ by an order of magnitude as d31 does. • For different dynamic flexibilities at resonance, the peak power for resonance excitation does not depend much on d31 . • When the dynamic flexibilities of the bimorphs are artificially made identical, the maximum power outputs for resonance excitation become very similar. • Larger power outputs of the single crystal bimorphs are due to their larger dynamic flexibilities (rather than their very large d31 constants). • Mechanical damping (hard to control due to clamping conditions and adhesive layers) can change the entire picture • Modifications in the harvester’s geometry might be much more effective than the type of active material used. Bimorph with larger d31 gave less power!