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Conversion of Energy

MICRO VIBRATIONAL ENERGY HARVESTING USING PIEZOELECTRIC TRANSDUCER JOSEPH M. RICHARDS, REZAUL KARIM NISHAT, AND SHAIKH AHMED

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERINGSOUTHERN ILLINOIS UNIVERSITY, CARBONDALE, IL 62901, USA

Motivation Energy harvesting systems have existed for decades in the

form of: Wind Turbines Hydro-electric generators Solar Power systems

Micro-energy harvesting has drawn increased interest as the need for a wireless electrical power supply increases. Virtually inexhaustible source Little or no adverse environmental effects

Energy harvesting that converts ambient energy available in the environment into electrical energy offers the potential of renewable power sources

Potential to directly replace the battery and ultimately dampen the environmental impact caused by the disposal of batteries. Challenges for Energy Harvesting

Micro-Energy Harvesting

Conclusions References1) Priya, S. Advances In Energy Harvesting Using Low Profile Piezoelectric Transducers.Journal of Electroceramics, March 2007, pp. 167-184.

2) Raju, M., & Grazier, M. ULP meets energy harvesting: A game-changing combination for design engineers. Texas Instruments: White Paper. April 2010

3) Beeby,S. O’Donnel, T. Roy, S. A micro electromagnetic generator for vibration energy harvesting. Journal of Micromechanics and Microengineering, June 2007, pp. 1257-1265.

4) Wright, P. K. A piezoelectric vibration based generator for wireless electronics. Smart Materials and Structures, Aug 2004, pp. 1131-1142

5) Priya, S. (2008). Energy harvesting technologies. New York: Springer.

Harvested power is derived from ambient sources so it tends to be unregulated, intermittent and small Energy storage is required (I.e thin-film batteries,

capacitors…) Energy cannot be stored indefinitely

Energy Harvesting Circuits vary in efficiency Energy harvesting circuits do not have rules for realizing

the best power efficiency in circuit design. In general, the design goal is to match the energy

harvesting circuit to the application of the circuit to achieve the best overall performance

The direct piezoelectric effect converts mechanical energy into electrical energy.

Pressure generates charges on the surface of piezoelectric materials.

The approximate mechanical power of a piezoelectric transducer vibrating assuming:

The mass of the vibration source is much greater than the seismic mass (m) in the generator

The vibration source is an infinite source

of power.

The piezoelectric generates maximum power when the operating frequency is also the natural frequency.

Transducer Design The transducer design consists of an energy

harvesting generator and an energy harvesting circuit

The energy harvesting circuit (LTC-3588) is a low-loss full-wave bridge rectifier with a high efficiency buck converter

The generator is a cantilevered beam with piezoelectric plates bonded on a substrate and a tip mass at one end

Energy Harvesting Circuit

A piezoelectric transducer is proposed which can generator usable electrical energy. The model provides some design intuition, which is summarized as follows: The system should be designed to resonate at the dominant

driving frequency of the target vibrations if possible. Power output is proportional to the proof mass attached to

the system. Conversion efficiency of the energy harvesting circuit can

be greatly increased through carful design of the circuit. As systems decrease in size and power consumption, the

possibility of powering these wireless micro-electronics by ambient energy harvesting increases.

Generator Configuration

Macro-scale energy harvesting generates megawatts of energy for reducing oil dependency.

Micro-energy harvesters scavenges milli-watts of ambient energy from surrounding sources. Ambient energy exists in various forms such as human activity, vehicles, and

environmental sources. The most prominent micro-energy harvesting technologies extract energy from vibration,

temperature gradients and light

Solar energy has proven to be capable of providing high power densities of 10,000 μW/cm3

Light intensity can drop the efficiency and power density significantly. Solar energy density drops down significantly inside buildings and is dependent on

weather

Temperature gradients are another alternative power source that has been proposed While power generated from temperature gradients is abundant in some applications, it is

unsatisfactory in many others.

Vibrational generators have been proposed using electromagnetic, electrostatic, and piezoelectric conversion methods. Each method for transforming energy has benefits for certain applications.

However, a quick comparison of methods is possible by considering the average energy density between each of the conversion methods.

It is clear that piezoelectric transducers have the greatest potential for producing higher power outputs than other conversion methods.

A few configurations for constructing piezoelectric transducers have been proposed a cantilevered beam with piezoelectric plates bonded on a substrate and a tip mass

at one end (a) a multilayer piezoelectric plates (b) equivalent lumped spring mass with external excitation (c)

The cantilevered beam configuration for the transducer is most favorable for a given force input.

The cantilever configuration results in the highest average strain in the piezoelectric. The power output is closely related to the average strain developed in the bender.

The cantilevered beam structure results in the lowest resonance frequency for a given size The target input vibrations are low frequency

The conversion from mechanical low frequency stress into usable electrical energy using a piezoelectric transducer can be outlined in three primary steps:

Firstly, the mechanical ac stress must be trapped from the piezoelectric source.

The mechanical energy is then converted into electrical energy with the piezoelectric transducer directly through the piezoelectric effect.

The processed electrical energy is then stored. An energy storage mechanism with significant energy density or power density is required.

Lithium ion batteries are generally chosen if high energy density is needed and ultracapacitors are chosen if high power density is needed.

Energy harvesting circuits do not have rules for realizing the best power efficiency in circuit design.

In general, the design goal is to match the energy harvesting circuit to the application of the circuit to achieve the best overall performance

A simple energy harvesting circuit consists of a diode rectifier (AC/DC) and a DC–DC converter. The addition of DC–DC converter has been shown to improve energy

harvesting by a factor of 7.

LT Spice IV Simulation LT spice IV is a high performance

simulator, schematic capture and waveform viewer

Using the simulator, it is possible to ensure the waveform of a circuit for a given input

In our simulation, we achieved the expected output waveform for the energy harvesting circuit.

Diagram of the piezoelectric plates bonded to cantilever beam

Block Diagram of energy harvesting circuit

Simulated waveform output for LTC-3588 using LT Spice IV

Waveform output for LTC-3588

Micro-energy Harvesting estimates. [2]

Micro-energy Harvesting estimates. [2]

Energy density estimates for three vibrational transducers. [4]

(a) Cantilever beam with tip mass, (b) multilayer PZT subjected to transverse vibration and (c) equivalent lumped spring mass system of a vibrating rigid body. [5]

A two-layer bender mounted as a cantilever. S is strain, Vis voltage, M is mass, and z is vertical displacement [4].

Schematic representation of the piezoelectric energy harvesting circuit [1].