The Comparison Between Electromagnetic and Piezoelectric

Vibrational Energy Harvesters

Chi An Lu1 , Yu Tang Hu1 , Yu Chi Chang1 , and Weileun Fang1,2

1Power Mechanical Engineering dept., 2NEMS Inst., National Tsing Hua University, HsinChu, TAIWAN

ABSTRACT

In recent years, electronic components are starting to require low power

consumption, combined with electronic components getting tinier and tinier,

conventional batteries does not seem to be the ideal solution. The upcoming of

MEMS energy harvesters is a possible answer to the problem, and this paper

will attempt to determine if certain types of MEMS energy harvesters are up to

the demand. We find that batch produced vibrational piezoelectric energy

harvesters are applicable for low consumption devices, and manually

assembled large electromagnetic energy harvesters can be considered for larger

electronic devices.

KeywordsEnergy harvestElectromagnetic energy harvester

Piezoelectric energy harvester

IIntroduction

MEMS based energy harvesters are similar to

wireless battery chargers in the way that both do not

require a transfer cable to provide power to an electronic

component. However, unlike the limited life cycle of

batteries, a significant advantage the energy harvester has

is the possibility of allowing low power consumption

components to theoretically be able to operate infinitely,

which in turn allows continuous usage without worry of

power depletion.

The ability to operate components infinitely creates

possibilities of several new concepts, among them are

health monitoring devices [1], where energy harvesters

eliminates the need to replace the battery every now and

then. And another is reconnaissance cyborg insects [2],

with energy harvesters providing power to the sensors

and transmitters, it allows highly cost-effective

surveillance in harsh conditions over extended periods of

time.

In recent years, we have seen the rising of wearable

devices and the concept: Internet of Things (IoT), as well

as the rapid evolution of technology. We observed that

the overall design of electronic components leans

towards lowering the power consumption, a trend that

gives MEMS energy harvesters a chance to shine and

perform.

In general, the most common seen types of MEMS

energy harvesters are as follows: Photoelectric,

thermoelectric, electromagnetic, piezoelectric, and

electrostatic [3]. Amongst photoelectric harvesters, solar

harvesters are the most effective. Thermoelectric type

designs generate electricity by using temperature

differences, whereas the latter three mentioned takes in

kinetic energy and transforms it into electrical energy.

Considering most wearable devices are meant to be

used indoors, and not all devices are in direct contact

with our skin, we shall select the most commonplace

kinetic energy as the harvesting energy and chose

electromagnetic and piezoelectric harvester devices as

the focus points of this review.

1.1 Electromagnetic Energy Harvesters

According to D.P. Arnold [5], we can deduce the

scale law of electromagnetic devices. The scaling

relationship between power P, length L and acceleration

a0 as follows:

{ P 50

2

P 702 0

(1)

Arnolds paper divides energy harvesters into two

categories, rotational and vibrational devices, since (1)

has an acceleration factor embedded within, it makes

rotational type harvesters difficult to analyze; as for

vibrational harvesters, power scales down with length at

a magnitude between 10-5 ~ 10-7.

Power density graphs for the two harvesters are

shown in Fig.1 and Fig.2. These figures contains datas of

electromagnetic harvesters before 2007 [5], where it can

observed that the scale law is not completely applicable.

Our guess is that the given specifications does not reflect

the best possible performance of the harvesters, which

might be a result of lack of advanced technology for

optimization, but it would seem certain that power

density will decrease at a faster rate as the scale goes

even lower.

Fig.1 Summary of rotational generator (a) power density

(W/cm3) and (b) normalized power density

(W/cm3*krpm ) versus size. The data shows no

significant trend with device size.[5]

Fig.2 Summary of oscillatory generator (a) power

density and (b) normalized power density (W/cm3*g2)

versus size. Two reference lines are shown on (b),

indicating the theoretical scaling trend of normalized

power density with L2 and L4 [5]

Other than the two presented above, other

electromagnetic harvesters designs include the Rolling

Rod Microgenerator [6] and the Linear Vibration

Harvester [7], but both designs face the problem of

stiction which occurs during operation, which is why we

only select no-contact vibration design types of

harvesters for our review.

1.2 Piezoelectric Energy Harvesters

Based on the piezoelectric harvester review done by

H. S. Kim, Joo-Hyong Kim and Jaehwan Kim [8], the

review lists the attributes of all kinds of piezoelectric

materials, the point of our interest amongst the database

is the conversion coefficient, which induces the energy

conversion efficiency to grow 4 times the original value

when the coefficient doubles in value. As we can see

from Fig.3, piezoelectric membrane thickness is not a

contributing factor to conversion efficiency, indicating

the usage of membranes as the energy absorber is an

acceptable design for piezoelectric materials. Although

the effect of volume decrease on power density is not

mentioned in the review, we believe that the ability to

use membrane structure is an important factor to take

into consideration, especially when integrating MEMS

fabricating processes.

Piezoelectric harvest methods are divided into

impulse and vibration modes, impulse harvesters will

encounter stiction problems due to repeated contact

between structures. To get a clean comparison with all

our selected harvesters performing on an even footing,

we choose to take impulse harvesters out of the list,

which leaves vibration harvesters for piezoelectric

energy harvesters.

Fig.4 categorizes electrostatic, electromagnetic,

piezoelectric energy harvesters and their respective

power densities, where we find a relative low power

density for electromagnetic harvesting, which as

mentioned previously might be a problem with lack of

optimization. So we searched for some of the more

recent electromagnetic harvesters and piezoelectric

harvesters listed below, in hopes that technology has

advanced enough to get fully optimized energy

harvesters.

Fig.3 Energy conversion efficiency of a piezoelectric

PVDF nanogenerator[8]

Fig.4 Comparison of the energy density for the three

types of mechanical to electrical energy converters[8]

IIPaper Survey & Discussion

For this section, we will show both electromagnetic

and piezoelectric energy harvester designs in the past few

years, and present their differences in fabrication

process, design specifications and performance.

2.1 Vibrational Electromagnetic Energy Harvesters

2.1.1 S. P. Beeby et al. [9]

This team uses photolithography to define the shape

of the stainless steel cantilever beam, and manually

attaches the permanent magnet onto the beam as well as

the copper coil onto the stationary platform, Fig.5. The

sole MEMS fabricating process ends once the cantilever

beam is complete, and the paper focuses on effect of

number of coils has on electromagnetic harvesting,

shown in TABLE 2. Since the coil is stationary, its

effects on oscillating frequency is minimal to a few Hzs.

Something worthy of note is that in the three coil

numbers presented, generated energy all lands around 45

W, showing that generated power ceases to increase

after a certain number of coils. This type of vibrational

energy harvesting mostly use diodes to filter the

alternating electric current, since there will always be a

voltage drop after passing a diode [17], a higher generated

voltage means less energy loss, so a larger number of

coils for electromagnetic energy harvesters is optimal.

Fig.5 Micro cantilever generator[9]

Harvester specifications are shown in TABLE 3.

From the fabrication perspective, this design cannot put

the batch production of MEMS to use, but the

performance at low frequency and low acceleration

achieves 300W/cm3 output energy, which is acceptable

to put into commercial use for wearable devices.

2.1.2 Qian Zhang et al. [10]

This design defines an array pattern before

electroplating copper to form a coil, then uses ICP for

backside dry etching to form a cavity for the magnet

array. Finally, the magnets are assembled onto the

component as the stationary end, distance between beam

and magnet is approx. 100m, as shown in Fig.6. As was

mentioned during lectures, not fully supported structures

suffer from residual stress; and to obtain a low natural

frequency, the beam must be long in length. From the

specifications of this design, the beam shows minimal

bending, meaning that the beam must be rather thick,

which can only be achieved with bulk micromachining.

The authors of this paper made a large scale model

in addition to the MEMS scale model. The models are

tested on a shaker providing vertical vibration, and

measurement is done with a Laser Doppler Displacement

Meter to find the natural frequency, the component specs

are shown in TABLE 3.

Fig.6 Brief fabrication process of the micro-electromagnetic energy harvester with magnet and coil arrays[10]

TABLE 3. The comparison between four electromagnetic energy harvester.

Volume

(cm3)

Power

(W)

Power

Density(W/cm3)

Natural

Frequency (Hz)

Voltage

(mV)

Acceleration (g)

S. P. Beeby et

al.[9]

0.15 46 307 52 428

(Vrms)

0.06

Qian Zhang et

al.[10]

0.09 2.6 28.9 290 30

(Vp-p)

4

Qian Zhang et

al.[10]

26 26,300 10,120 65 28,800 12

K. Tao et al.[11] 0.02 ----- ----- 365 0.0209 1

The result is that in comparison with the MEMS

fabrication processed model, the manually assembled

large model has a much higher power rating, which

supports Arnolds assumptions of scale down effects. But

the big advantage this design has over the design by S.P.

Beeby et al. is the lack of the final assembly process,

which is crucial for mass production.

2.1.3 K. Tao et al [11]

This team uses LIGA process to make the copper

coil, macromolecule coating is coated afterwards as an

insulation layer, copper and nickel is then electroplated

as structure layer. And finally, integrates magnetic

macromolecule composites to the component, and a lift-

off process to remove the sacrificial layer, the process is

as shown in Fig.7. The most significant aspect of this

energy harvester is the fact that it is produced using only

MEMS fabrication processes, easily achieving batch

production.

Fig.7 Process flow for fabrication of fully-integrated

energy harvesters[11]

Measurement of this harvester is done on a shaker

as well, shown in TABLE 3. The paper does not provide

the power rating nor power density, but we could use

Power equation (2)[17] to estimate, giving us a power

density graded in degrees of nanoWatt/cm3.

Power = 2 (2)

The energy output of this design is significantly

smaller than the first two presented, but the possibility of

batch production taken into consideration gives this

design a slight edge if the manufacturing department.

2.2 Vibrational Piezoelectric Energy Harvesters

2.2.1 D. Shen et al. [12]

This team proposed in 2008 Journal of

Micromechanics and Microengineering, to use silicon

and piezoelectric materials for a cantilever beam and a

pure silicon vibrating mass together forming the

harvesting device. The beam length and silicon mass is

tweaked to operate at a lower frequency. Fabrication

process is shown in Fig.8, thermal oxide is first grown on

silicon wafer before depositing electrical layer and

piezoelectric PZT layer. A lift-off process difines the

electrode pattern, and double-side dry etching forms the

beam and mass. The main fabrication processes used are

thermal growth, sputtering, lift-off, and dry etching, all

of which were mentioned during lectures, indicating a

high probability to be batch produced.

Fig.8 Fabrication flow chart: (a) multilayer

deposition; (b) top electrode patterning by liftoff (mask

1); (c) bottom electrode opening via RIE (mask 2); (d)

cantilever patterning by RIE (mask3); (e) proof mass

patterning and cantilever release via backside RIE

(mask4).[12]

Measurements are done using the same methods as

electromagnetic energy harvesters using a shaker to

determine the natural frequency. The additional testing in

this paper is changing acceleration magnitudes to

simulate different energy input, calculate the damping

ratio of the beam, and record the output power and

voltage under different loadings (RLoad). We therefore

choose the maximum power output in this paper as a

comparison basis, listed in TABLE 4.

2.2.2 E. E. Aktakka et al. [13]

Aktakkas team bonds PZT bulk materials onto SOI

wafer and applies mechanical thinning to get a

piezoelectric membrane. Parylene is used as the

insulation layer, and the electrodes are patterned via

sputtering. Double-sided dry etching is used to create the

cantilever beam, and a Transient Liquid Phase (TPL)

bonding completes the fabrication process.

Fig.9 Process steps of thinned-PZT on SOI process

The fabrication processes involved is a lot more

complicated than the previous design by Shens team,

and is likely to have a lower yield. The main point of

selecting this paper is for discussion of the effects of

packaging. Since PZT is affected by residual stress,

which in turn changes piezoelectric performances, this

paper offers insight into System in Packaging (SIP).[13]

The authors of this paper made two types of energy

harvesting devices using different materials, silicon and

tungsten, as the vibrating mass. Measurement on a

shaker gets the data shown in TABLE 4. In comparison

with the previous harvester at Fn = 462 Hz, 1.5g; and this

design at Fn = 415 Hz, 1.5g; the power density for this

energy harvester is only half of the previous Shen et al.

design, which we predict to be a result of too many

fabrication procedures causing accumulation of residual

stress.

2.2.3 R. Elfrink et al. [14]

Elfrink et al. purposes using vacuum packaging to

decrease aerial dampening and achieve better power

density. The fabrication process is shown in Fig.10. The

team made 11 different components and took them all to

a shaker for measurement, the results are in Fig.11. We

can see that power density increases significantly by

100~200 times with vacuum packaging.

Fig.10 Wafer-scale vacuum package process flow. a. The

piezoelectric capacitor is formed by consecutive

deposition, lithography and etching steps of the Pt

bottom electrode, the AlN piezoelectric layer and the Al

top electrode. The Si mass and beam are shaped by

subsequent front- and backside etching. b. The cavities in

the glass wafer are etched with HF and the contact holes

are made with powder blasting. The SU-8 bonding layer

is applied with a wafer scale roller-coating process. c.

The glass substrates are bonded to the Si device wafer in

two consecutive wafer scale vacuum bonding steps. d.

Single devices with the movable mass and beam in the

vacuum cavity are obtained after dicing.[14]

Fig.11 Resonance curves of packaged devices type 3 at

0.1 g and 1.0 g at atmospheric pressure and at vacuum,

showing a 200 and 140 fold increase in power.[14]

We selected the best power density component and

placed it into TABLE 4 to compare with the other two

designs, where we find the selected component to be

slightly weak in power density, but this might be due to

different piezoelectric materials [8]. After reconfiguration

[8] of each designs power rating, the Elfrink et al.

harvester is approximated at 340 microWatt/cm3, around

100 times the Shen et al. design, showing just how big an

effect vacuum packaging has for vibrational piezoelectric

energy harvesters.

TABLE 4 The comparison between four piezoelectric energy harvester

Volume

(cm3)

Power

(W)

Power

Density(W/cm3)

Natural

Frequency (Hz)

Voltage

(mV)

Acceleration (g)

D. Shen et al.[12] 0.000625 2.15 3,272 462.5 150

(Vp-p)

2

E. E. Aktakka et

al.[13]

0.1462 205 1,404 154 ----- 1.5

E. E. Aktakka et

al.[13]

0.1462 160.8 1,099 415 ----- 1.5

R. Elfrink et

al.[14]

2.5 85 340 325 ----- 1.75

IIIConclusion

We have now separately compared different

vibrational electromagnetic energy harvesters and

vibrational piezoelectric energy harvesters. For the

electromagnetic devices, it is observed that as the

manufacturing becomes MEMS based, the power density

goes down. Since the three designs we choose have

volume differences within an order, scale down effects

can be neglected, and the main factors to power density

drop are the two following reasons: One, multiple

MEMS processes means lower yield, so the coil number

is limited, and from the results of S. P. Beeby et al., coil

amount could lower the output voltage and power

density Two, the magnets are made using composites, the

energy product is 2 to 3 orders weaker than bulk

permanent magnets. Though MEMS production makes

electromagnetic energy harvesters capable of batch

producing, the problem remains when implementing

magnetic materials into MEMS [16], and the rapid

decrease of power rating due to scaling down is also an

extremely negative factor.

As for vibrational piezoelectric energy harvesters,

we selected three cantilever beam vibration designs, and

observed a more stable power output from these

cantilever piezoelectric energy harvesters. The main 4

factors affecting output are: Natural frequency, decided

by structure distortion; Residual stresses, resulting from

multiple MEMS fabricating procedures and packaging

procedure; Application of vacuum packing, which

decreases the effect of aerial dampening and increases

power density; Piezoelectric material, due to power

rating being the piezoelectric converting coefficient

squared, making PZT a much more suitable material

rather than A1N

At the end of our review, we draft a graph of Power

density-Volume containing all the energy harvesters we

selected, Fig.12.

Fig.12 The comparison of power density between

piezoelectric & electromagnetic energy harvester

In conclusion, scale effects are neglect able for

vibrational piezoelectric energy harvesters, and

microscale piezoelectric harvesters have power densities

2~3 orders higher than vibrational electromagnetic

energy harvesters, plus the advantage of being

compatible with existing MEMS fabricating

technologies, if considering energy harvesting to power

low consumption wearable devices, piezoelectric

harvesters are much more suitable. Another aspect that

Qian Zhang et al.[10] provides is observed at larger scales

of >10cm3, where assembled electromagnetic energy

harvesters have a power density than piezoelectric

energy harvesters, so larger devices should consider

using electromagnetic energy harvesters for power.

IVAcknowledgement

A thank you to Professor Fang, for introducing us

to the world of MEMS, and providing us the knowledge

to understand and delve further into micro processes. A

deep gratitude to National Tsing Hua University for

providing access to databases of publications around the

world.

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