課程名稱 : 能量擷取技術 Energy Harvesting Technologies 授課教師 : 王東安 Lecture 1 1
Dung-An Wang Phone: 4-2284531 ext. 365 daw@dragon.nchu.edu.tw LECTURES: Wednesdays 1:1-4pm OFFICE HOURS: Wednesdays 11am-noon Course website: web.nchu.edu.tw/~daw/teaching/energy/energy_harvester.htm TEXTBOOK: GRADING: HOMEWORK: Shashank Priya and Daniel J. Inman, Energy Harvesting Technologies, Springer, 28. Homework (3%), Midterms (2%) Project(3%) Final (2%). Homework is due at the begging of the class. Late homework is not accepted. 2
COURSE OBJECTIVE Understand a systematic approach to analyzing energy harvesting problems. Study the techniques to design of energy harvesting system. Practice the systematic process on a semester-long project. 3
Lecture Outline Vibration energy harvesting Electrostatic harvester Electromagnetic harvester Piezoelectric harvester 4
Energy Harvesting Definition: The process of converting ambient energy into electrical energy. Characteristics Small amount of power for low-energy electronics Energy source for energy harvesters is ambient and free Little power generation, order of W/cm 3, need to be stored in a capacitor or battery. Use capacitors when huge energy spikes are needed. Batteries for a steady flow of energy supply. 5
Why Harvesting Energy? Independent sensor networks, mobile devices Autonomous No maintenance Free Low carbon print 6
Solar power Thermal energy Wind energy Salinity gradients Vibration energy The sources 7
Piezoelectric Electromagnetic Photovoltaic Thermoelectric Devices Electrostatic: Mechanical energy is converted into electrical energy by motion of the plates of an initially charged capacitor. Bio-energy harvesting Energy harvesting through the oxidation of blood sugars Tree metabolic energy harvesting 8
Market perspective The energy harvesting market has potential to approach the $25 million mark in 217 due to demand for wireless networks. (MEMS s trend, Oct 212) 9
Market perspective Piezoelectric energy harvesting market to reach $145 million in 218 (Aug 212, Energy Harvesting Journal. 1
Piezoelectric Stacks of layers of piezoelectric materials (PZT, PMN) 11
Magnetostrictive Material (MsM) 12
When the coil is activated, the Terfenol-D expands and produces output displacement. solenoid coil provides the magnetic field permanent magnets provide magnetic bias Inverse magnetostrictive effect (Villari effect) : change of the magnetic susceptibility of a material when subjected to a mechanical stress. 13
The Terfenol-D material has been shown to be capable of up to 2 ; however, its behavior is highly nonlinear in both magnetic field response and the effect of compressive prestress. optimize the internal prestress and magnetic bias to get a quasi linear behavior in the range of 75 1. 14
Magnetoactive model of MsM H is the magnetic field intensity, B is the magnetic flux density, is the magnetic permeability under constant stress, and d 33 is the piezomagnetic inducedstrain coefficient 15
A MsM energy harvester Wang and Yuan, 28. 16
MsM Upon dynamic or cyclic loading, this change in magnetization is converted into electrical energy using a pickup coil surrounding the MsM laminate according to Faraday s law. One drawback of the MsM, Terfenol-D, for energy harvesting is the requirement for a bias magnetic field. 17
Electromagnetic power generators Electromagnetic power generators (EMPGs) have the advantages of low output impedance and high output current levels 18
Design 19
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Piezoelectric Model S is the axial strain, T is the axial stress, E is the electric field (voltage per unit distance between electrodes), D is the electric displacement (charge per unit area), is the dielectric permittivity, and d 33 is the piezoelectric induced-strain coefficient. Compliance s is measured at zero electric field Permittivity is measured at zero mechanical stress 21
22 Piezoelectric fundamental k kij kl ijkl ij E e S C T k S ik kl ikl i E S e D 3 2 1 15 15 33 31 31 23 13 12 33 22 11 55 55 44 33 13 13 13 11 12 13 12 11 23 13 12 33 22 11 E E E e e e e e C C C C C C C C C C C C 3 2 1 33 22 11 23 13 12 33 22 11 33 31 31 15 15 3 2 1 E E E e e e e e D D D S S S Piezoelectric poling direction 3
23 Fundamental k kij kl ijkl ij E d T s S k T ik kl ikl i E T d D 3 2 1 15 15 33 31 31 23 13 12 33 22 11 55 55 44 33 13 13 13 11 12 13 12 11 23 13 12 33 22 11 E E E d d d d d s s s s s s s s s s s s 3 2 1 33 22 11 23 13 12 33 22 11 33 31 31 15 15 3 2 1 E E E d d d d d D D D T T T Piezoelectric poling direction 3
Piezoelectric harvester A common material is lead zirconate-titanate, Pb(Zr,Ti)O 3 (PZT). PMN-PT is a relatively new piezoelectric material. The advantage of this material is the fourfold enhancement in the piezoelectric coefficients. However, the two main disadvantages of this material are the price and the fact that PMN-PT is still in its infant stage of development. 24
Piezoelectric harvester Mode of operation d 33 mode d 31 mode 25
Vibration energy harvester Vibration energy to electrical energy Sources: motors, rotating objects, subways, pipelines, HVAC systems, bridges, and even the human body 26
Vibrations of the Zigzag Microstructure for Energy Harvesting Effective energy harvesting devices typically consist of a cantilever beam substrate coated with a thin layer of piezoceramic material and fixed with a tip mass tuned to resonant at the dominant frequency of the ambient vibration. The fundamental natural frequency of a beam increases as its length decreases, so that at the MEMS scale the resonance condition occurs orders of magnitude higher than ambient vibration frequencies, rendering the harvester ineffective. 27
A new geometry for MEMS scale cantilever harvesters with low fundamental frequencies A zigzag geometry would be able to vibrate near resonance at the MEMS scale. 28
Device structure Each of the beams can bend out of the main plane and can twist. 29
Each of the beams is a uniform composite beam composed of a piezoelectric layer bonded to the substructure layer (unimorph) When the beams are deflected, some strain is generated in the piezoelectric layer, which generates electrical energy. 3
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Experimental Verification a macrosize model of the zigzag structure has been tested. 32
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Parasitic Power Harvesting in Shoes Wearable computing light weight, minimum effort, high power generation, convenient power delivery, and good power regulation. Up to 67 Watts of power are available from heel strikes during a brisk walk (68 kg person, 2 steps/sec, heel moving 5 cm). (IBM Systems Journal, Vol. 35, No. 3&4, 1996, pp. 618-629) Goal to unobtrusively collect energy for low-power applications. 34
Energy dissipating sole. Sport sneaker While walking in ordinary "hard" shoes, the foot is rapidly decelerated from its relatively high downward speed to zero velocity relative to the ground an action that requires the application of relatively large and sudden forces to the foot. Barring shock absorption in the feet, this can be simply modeled as a sudden step in velocity; the force applied to the foot to achieve this deceleration is an impulse 35
Dynamics 36
Dynamics Function of the insole and midsole in the sport sneaker is to work as a low-pass filter for this step in velocity, reducing the amount of force applied to the joints. This reduces any stress that the joints experience and also reduces the incidence of sports injuries. 37
The result is that the force and displacement values over time for the bottom and top of the midsole are not the same as in any passive filter, there is an energy loss in the sole while it performs this filtering function. The energy lost is in the higher harmonics of the step and is dissipated through internal losses in the sole. When the sole springs back after the step it does not exert as much force as before, returning less energy than was put into it, and it is this energy that we are trying to capture 38
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System Descriptions Parasitically tapping energy is to harness the bending of the sole A laminate of piezoelectric foil, shaped into an elongated hexagon 4
This stave is a bimorph built around a central 2-mm flexible plastic substrate, atop and below which are sandwiched 8-layer stacks of 28-micron PVDF (polyvinylidineflouride) sheets, epoxy-bonded 41
As the stave is very thin (under 3 mm), it can be easily molded directly into a shoe sole. When the stave is bent, the PVDF sheets on the outside surface are pulled into expansion, while those on the inside surface are pushed into contraction (due to their differing radii of curvature), producing voltages across silver-inked electrodes on each sheet through the dominant "3-1 longitudinal mode of piezoelectric coupling in PVDF. 42
In order to lower the impedance, the electrodes from all foil sheets are connected in parallel (switching polarities between foils on opposite laminate surfaces to avoid cancellation), resulting in a net capacitance of 33 nf. 43
An actual stave 44
the PVDF and PZT elements were mounted between the removable insole and rubber sole 45
The piezoelectric generators, being high-impedance devices, were terminated with 25KW resistors, which approximated their equivalent source resistance at the excitation frequencies, hence yielded maximum power transfer 46
the voltages produced across the load resistor by the piezo elements during a brisk walk, with the same foot hitting the ground at roughly 1 Hz. 47
the resulting power delivered to the load 48
A Self-Powered RF Tag System A batteryless, active RF tag, which transmits a shortrange wireless ID code to the vicinity while walking. This has immediate application in active environments, enabling the user to transmit their identity to the local neighborhood while passing through and allowing the building to locate its inhabitants and dynamically channel any relevant resources or information to them. 49
A lowpower RF transmitter We can mount it in the sneaker and use the energy extracted from walking to power it without the need for a battery. 5
Overhead view 51
The power-conditioning electronics for the tag circuit 52
Harvesting circuit The voltage from the piezo element (e.g., unimorph or stave) is full-wave rectified in the bridge D1, then charge is accumulated on the electrolytic capacitor C1. The Q1,Q2 circuit acts like an SCR with supercritical feedback. It was adapted from a similar circuit designed for powering motors off solar cells, revised to function with the very high impedance of the piezoelectric sources. 53
A MEMS piezoelectric vibration energy harvesters Based on AlN and making use of wafer-level vacuum packaging Power a small prototype (1 cm 3 volume) of a wireless autonomous sensor system The average power consumption of the whole system is less than 1 μw, which is continuously provided by the vibration energy harvester. 54
Fabrication The piezoelectric capacitor is formed by consecutive deposition, lithography and etching steps of a platinum bottom electrode, an AlN piezoelectric layer and an aluminum top electrode. The silicon mass and beam are shaped by subsequent front- and backside etching Glass-capping wafers with 4 μm deep cavities are etched with hydrofluoric acid and contact holes are obtained with powder blasting. 55
wafer-scale vacuum package process 56
(a) The AlN piezoelectric capacitor is located on top of the beam. The Si mass and beam are fabricated by subsequent front- and backside etching. (b) The SU-8 bonding layer is applied with a wafer-scale roller-coating process on the glass wafers. (c) The two glass wafers and the silicon wafer are vacuum bonded in two consecutive bonding steps. (d) After dicing, single devices are obtained with the movable mass and beam in the vacuum cavity. 57
After dicing through the bonded wafers, individual devices are obtained 58
Devices of different dimensions are designed to cover a broad frequency range. The devices have a beam thickness of 42 ± 3 μm (fabricated from Si wafers) or 5 ±.5 μm (fabricated from SOI wafers) the resulting operating frequencies range from 285 Hz to 11 Hz. 59
A record maximum power of 85 μw has been measured on an unpackaged device at an acceleration of 1.75 g and a resonance frequency of 325 Hz. The tip of the mass displaced with an amplitude of 675 μm. 6
Packaging The package of MEMS harvesters is essential for the reliability of the fabricated devices as it prevents excessive mass displacements and protects the silicon beam from external influences. However, the package introduces additional parasiticdamping mechanisms. 61
MEMS-based piezoelectric harvesting Piezoelectric material comparison: AlN versus PZT Lead zirconate titanate (PZT) a superior performance in generation of force and torque for actuators, and also of sensors with current detection. 62
A higher dielectric constant is generally preferable in energy harvesting applications as it lowers the source impedance of the device. Piezoelectric materials are generally high impedance devices resulting in the generation of high voltage and low current outputs 63
Dielectric constant The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. Technically, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum. Similarly, it is also the ratio of the capacitance of a capacitor using that material as a dielectric, compared to a similar capacitor which has a vacuum as its dielectric. The relative permittivity of a material for a frequency of zero is known as its static relative permittivity or as its dielectric constant 64
dielectric loss A loss of energy which eventually produces a rise in temperature of a dielectric placed in an alternating electrical field. 65
AlN Aluminum nitride (AlN) is a more preferable material for power generation since it has much lower dielectric constant compared with PZT and the power generation is quite comparable. An important advantage of AlN is that the fabrication of AlN devices by a standard sputter deposition technique makes it more favorable than the complex deposition of PZT. This comparison indicates that AlN, if not more suitable for energy harvesting than PZT, it is a strong competitor to PZT 66
Piezoelectric roads for California California Assemblyman Mike Gatto has proposed a new bill that will implement piezoelectric technology already in use in Italy and Israel to harness energy from road vibrations. 67
Principle When a car or truck passes over pavement, the pavement vibrates slightly. By placing relatively inexpensive piezoelectric sensors underneath a road, the vibrations produced by vehicles can be converted into electricity Used to power roadside lights, call boxes, and neighbouring communities. 68
According to one report, "When the technology was put to the test in 29, the Israeli government was able to generate 2, watt-hours of electricity simply by implementing the system on a 1-meter stretch of highway." 69
Micro-fabricated piezoelectric vibration energy harvesters To harvest energy from ambient vibration sources in a machine room of a large building Data were collected at a sampling rate of 248 Hz using a Microstrain G-link wireless 3-axis accelerometer magnetically mounted on the vibration source. All three axes were surveyed, but in most cases the z- axis (orthogonal to the vibrating surface) acceleration was an order of magnitude higher than x- and y-axes, so only the z acceleration is used 7
the dominant frequency peaks from the majority of the sources surveyed lie between 2 and 6 Hz, with another set of frequencies lying between 12 and 14 Hz. Some sources also contain peaks around 35 Hz. 71
fan belt cage vibration compressor base vibration 72
Frequency spectra (power spectral density (PSD)) 73
PSD in g 2 /Hz units Why do we square the sampled data? This is in order to obtain a positive quantity. Imagine the signal was a truly random one, containing all frequencies at all amplitudes between zero and the peak. This is known as white noise, the analogy being white light which contains all visible frequencies in equal amounts and noise describing no specific pattern of amplitude. If this were the case, then the mean value would be zero, because a sampled point would be equally as likely to have a positive value as a negative value and the net result after sampling many, many points would be zero. 74
PSD in g 2 /Hz units To give us an idea about the variation of the mean-square value of g² across the frequency range, we progressively filter the signal from Hz to some upper bound that we decide. For instance, first we could stop all frequencies above 1Hz and just look at the mean-square value of g² below 1Hz. If we then divide the mean-square value of g² up to 1Hz by what is called the bandwidth of the filter (1Hz in this case) we get the spectral density of the signal in g²/hz, up to 1Hz. Now we can increase the filter bandwidth to 2Hz and look at the mean-square value now. Again we divide this mean-square value by the filter bandwidth to get the spectral density up to 2 Hz. This is repeated this for 3Hz, 4Hz etc.. If we plot the quantity of mean-square divided by bandwidth (g²/hz) against frequency (Hz) we get a familiar looking PSD curve. 75
PSD in g 2 /Hz units Actually PSD (Power Spectral Density) is a bit of a mis-nomer. It should be correcly termed ASD for Acceleration Spectral Density. It has become known as PSD where the "power" originates from the output from the accelerometers during structural test. 76
Design 77