energy harvesting report

RepoRt on the Rectification of haRvested eneRgy in vibRational eneRgy haRvesting mateRials technologies:mateRials, poweR systems design and electRonic engineeRing issues

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Technology Strategy Board

Driving Innovation

Report on the rectification of harvested energy in

vibrational energy harvesting materials technologies:

Materials, power systems design and electronic engineering

issues

Markys G Cain and Paul D Mitcheson

August 2012


Driving Innovation

1 Executive Summary

Energy harvesting devices are widely regarded as an important technology in the future success ofthe wireless sensor network, potentiality enabling almost infinite operating duration. To date, thevast majority of research on harvesters (be they kinetic, thermal or solar) has concentrated on thetransduction mechanism. However, a complete energy harvester powered system requires suitableinterface circuitry to process the power output of the harvesting transducer into a form which can bestored in a battery or capacitor to power a low voltage, low power load, typically a sensor and radiotransceiver. This report discusses the state of the art of such circuits, the features they are able toprovide (above that of simple AC to DC conversion) and illustrates this with four case studies, onefor each of the common types of motion-driven energy harvester transduction mechanism and anambient RF harvester. It is shown that, whilst power processing for harvesters is possible, significantgains need to be made to allow operation of harvesters as they become further miniaturised, andthat the control circuit overhead must also be reduced.

The report concludes with a suggested roadmap of research in the area of micro and nano rectificationand, because the development of rectification and power processing interfaces are tied so closely tothe transducer technologies, system issues also feature in the roadmap. The main suggestions forfuture research fall into 5 areas, these being: standards development, intelligent adaptive systems,nanoscale devices, systems integration and new materials and hybrid devices. A suggested timescalefor these developments is provided.

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Report contents

1 Executive Summary 1

2 Energy Harvesting technologies 3

3 Electrical rectification 4

3.1 Simple circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43.2 Vibrational EH technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.3 Direct AC power utilisation - negating the need for rectification . . . . . . . . . . . 6

4 Optimisation strategies: Materials, device geometry, power systems design and

electronics engineering 8

4.1 Electrostatic case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84.2 Piezoelectric case study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114.3 Electromagnetic systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.4 RF Harvesting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.5 Overview of design methodologies for harvesters . . . . . . . . . . . . . . . . . . . 19

5 Recommendations for future research and Roadmap 20

6 Acknowledgements 22

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2 Energy Harvesting technologies

The purpose of this report is to discuss the state of the art and future directions for nano-rectification,which is the processing of the AC outputs of energy harvesting systems into regulated low voltageDC, suitable for powering an ultra-low power sensor node, for example [1], [2], [3]. As will be seen,the challenges of AC to DC conversion at low voltages and low power levels can be significant, andthis specific challenge means that, in some cases, energy harvester transducer design is modifiedaway from the optimal configuration in order to make passive rectification easier [4].

This report takes its steer from the simple fact that high performance solutions may be developedfor energy harvesting applications only if the complete system is considered holistically [5].

By way of introduction, a typical motion-driven energy harvesting system (of the piezoelectric type) isshown schematically in Figure 1. Here, the piezoelectric material and mechanical structure providesenergy in the form of a charge separation (i.e. a charged capacitor) to the interface circuit. Theoscillation of the beam means that the voltage developed on the piezoelectric capacitor containspurely AC components and thus some form of rectification is necessary if the system is to drive alow-power DC load. Consequently the interface circuit in Figure 1 can, in its simplest form, be adiode rectifier. The generated energy is then stored (in a capacitor or battery) and regulated beforebeing supplied to a low-power load. As energy is converted from a mechanical to electrical form bythe transducer and interface circuit, the mechanical motion is damped, reducing the amplitude ofthe proof mass. The control of the amount of damping applied is critical to achieving high powerdensities for such systems and is a key feature required of the rectifier interface.

Figure 1: A typical energy harvesting system

The circuitry which implements the AC to DC conversion process is, in its simplest form, a passivediode rectifier. However, this may not be possible if the transducer output voltage is low and soother solutions are required. In addition, the circuit which accomplishes the AC to DC conversionprocess can also perform other tasks, such as tuning the resonant frequency of a kinetic harvester orincreasing the available damping force. Both of these additional functions can improve the system’spower density. These and other issues related to the rectification and systems control are discussedin this report.

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3 Electrical rectification

3.1 Simple circuits

There are several possible mechanical architectures of vibration based power generators [6]. In thecase of piezo-generators, the energy conversion takes place via the direct piezoelectric effect. Thisis the direct generation and delivery of charge onto the electrodes of a piezoelectric material whena stress is applied to the material. The energy conversion is maximised by a maximum deformation(strain) of the piezoelectric material. This usually occurs at the electro-mechanical resonance of thematerial. Assuming the external driving force is sinusoidal (or cyclical) in nature - as is the casefor many vibrational sources of energy - then the charge generated by the piezoelectric material isalso cyclical. The charge developed depends on the piezoelectric characteristics, its geometry andthe details of the external mechanical vibration. The mechanical vibrations, which are the sourceof energy that is harvested from the environment, are not always periodic, uniform or continuous,however. The simplest electronic interface [7] for harvesting cyclical voltages consists of a half waveor full wave bridge rectifier (a simple diode circuit) and a smoothing capacitor, Cs, with an anelectrical load, RL connected (see Figure 2).

a

D1

Cs RL

(a)

D1

+

D2 D3

D4

Cs RL

−

(b)

Figure 2: Standard rectification interface circuits for energy harvesting, a) half wave rectifier and b)full wave rectifier

Assuming a single-mode external mechanical vibration (the mechanical displacement u(t) is assumedto be purely sinusoidal), then the open circuit voltage delivered by the piezo-element will also be si-nusoidal. However, the electrical circuit that connects the piezo-generator to the load resistor affectsthe output waveform of the piezo-generator. If the piezo-generator can develop sufficient voltagesuch that the forward biased diodes in the bridge rectifier can operate in their conducting mode (forsilicon the switch on voltage is about 0.6 V and for germanium diodes this is about 0.3 V) then thepiezo cyclical voltage will be rectified such that the voltage across the load resistor will be unipolar(positive going only or negative going only - depending on how the piezo-generator is connectedto the circuit), and with the addition of a smoothing capacitor this unipolar cyclical voltage will

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appear as a DC voltage on the load resistor. More precisely, when the output voltage across theload resistor exceeds the absolute value of the piezoelectric device minus the 2 diode voltage dropthen the piezo is in an open circuit configuration and its voltage swings with its displacement. Whenthe absolute value of the piezo voltage generated is equal to or greater than the storage capacitorvoltage plus the bridge rectifier voltage drop then electrical energy is transferred to the capacitorand load. This rather simplistic explanation is sufficient for the needs of this report and subsequentanalysis of alternative rectification strategies [8], [9]. More details are presented in section 4.2.

Improved methods for efficiently harvesting this type of mechanical-electrical energy conversion aregenerally based on the reduction in the diode voltage drops associated with semiconductor rectifierdiodes (or bridge rectifiers). The simplest way of achieving this is to use a synchronous rectifier,where diodes are replaced with MOSFETs [10]. Such synchronous rectifiers can be commutated byactive circuitry which is externally powered or powered directly from the AC input signal [11]. Severalother ways in which diode drops have been overcome involve using more sophisticated techniques,such as those reported in [7] and [12] which are based on the parallel SSHI (synchronized switchharvesting on inductor). These circuit configurations intermittently switch the piezoelectric ontoa resonating electrical network (LCR) for a very short time, which has the effect of increasing thevoltage output and effectively increasing the coupling coefficient of the piezomaterial. This has beenshown to accomplish gains of order times 8 in harvested power compared to the standard bridge onlyconfiguration [12]. An extension of the parallel SSHI method has been developed [12], and others,that is called series SSHI based upon rectification of the piezo voltage without significant voltagedrop and allows for a greater efficiency of harvesting power at much lower voltages. The series SSHIenergy harvesting circuit is shown in Figure 3 and one can see that two digital switches are placedin series with the piezoelectric and rectifier. These switches are synchronised with the piezo chargecycle, and when the latter is at a maximum the switches close and energy is transferred through therectifier to the storage capacitor. The switched voltage is actually inverted through this process andlosses can be significant. Yet another variation on this approach uses a transformer to further reducethe effect of the voltage drop [12] where a transformer replaces the inductor in Figure 3 along witha new diode in series with the load. In this report, a new technique, called single supply pre-biasingwill be discussed, which is superior to the SSH techniques.

More recent work has developed the synchronous switching technology and coupled this with avoltage pre-bias to permit even greater power output of piezo energy harvesting devices [8]. Themethod is particularly suited for undamped and low frequency applications but with high excitationamplitude - such environments are typically found in foot-fall and engine vibrations for example.

Some of the original work on harvester interface circuits was in relation to electrostatic harvesterswhich use variable capacitor structures to couple kinetic energy into the mechanical domain. Anearly example of such work is presented in [13]. In this paper, the upper limits on voltages for op-erating the transducer was set by the power processing electronics interface, limited by the CMOSprocess, which severely reduced the power density of the system.

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Figure 3: Series SSHI circuit and typical waveforms - from [12].

3.2 Vibrational EH technology

Of the various sources of ambient energy, mechanical energy in the form of vibrations is present inmany environments, particularly where there is some form of machinery, and is an alternative whenlight or thermal sources are not sufficient. The most common method for scavenging this energysource is to use resonant inertial devices. Typically, this involves a resonant cantilever with a tipmass, where accelerations arising from the vibrating source cause the tip mass to oscillate. In orderto convert the kinetic energy to electrical, three methods have been used, electromagnetic, elec-trostatic and piezoelectric. Electrostatic, although well suited to Micro-ElectroMechanical (MEMS)scale devices, has been less studied recently due to low power levels, whilst miniaturisation withelectromagnetic transduction is problematic because of the difficulty in producing compact coils. Incontrast, piezoelectric transduction has the potential for miniaturisation in MEMS scale devices.

3.3 Direct AC power utilisation - negating the need for rectification

One of the basic questions asked of the ‘Intelligent energy harvesting - strategies for Utilising har-vested energy’, held on 5th May 2011 at the Institute of Materials, Minerals and Mining, 1 CarltonHouse Terrace, London, was whether applications exist that do not require rectification of the cyclic

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Driving Innovation

external energy source. For example, it is not necessary to rectify an AC source to usefully powera light bulb. Various interesting opportunities may exist with this specification, which are brieflydiscussed below:

• Heat store: Here the AC power (rate at which the energy harvested is transferred, used ortransformed) is used to simply electrically heat a thermal heatsink. The temperature of thatheatsink will increase until the losses (convection, radiation, conduction) match the energyinput. This heat can be used as another source of energy.

• Clockwork wind up spring: Here, the rectification occurs through mechanical means such asratchets and gears. This leads to only half the available energy from being utilised per cycle,however.

• RF: The development of nano-antennas or nantennas has been shown to harvest radiantRF (microwave) radiation from the environment. The issues here though reside with precisematching of the nantenna physical dimension with the wavelength of the background radiation.

• Fluid flow /pressure store: This is a method of storing energy in the form of pressure or stressin a material or liquid or gas, similar to the thermal heatsink approach.

• Composite systems providing anticlastic one way motion: Here we develop an approach thatmechanically rectifies the cyclical energy scavenged, whereby the composite beam is only ableto flex one way (which for a piezo material would be in the same positive direction as its builtin polarisation), thereby providing DC rectified output. Half of the available energy is lost asheat in this case, however.

• Phase change materials: A phase change material is one where one of its characteristic prop-erties (modulus, structure, resistivity) changes with applied force, load, light, field etc. Theremay be interesting ways in which these materials may transduce the ambient ‘free’ energyinto an energy that can be harnessed - differently to piezo or electrostatic or EM harvestingtechnologies.

• Electrochemical/biological: Storage of energy in a chemical form pervades society (oil, petrol,gasoline etc) and there may be ways of using the scavenged energy to directly transfer energyinto chemical forms.

• Artificial photosynthesis: The holy grail of energy conversion - that of photosynthesis - is asubject of great academic and commercial interest with many applications outside of energyproduction. The utility of photosynthesis to create chemicals or to modify chemical speciesthrough direct sunlight is the mainstay of all plant life on earth.

• Hybrid - Solar/piezo: The combination of two or more energy harvesting technologies maysynergistically afford a direct AC utilisation of power scavenged from the environment.

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• Circuits that run off AC: There is current research aimed at how one might directly powerelectronic circuitry with AC rather than DC (rectified AC) power. Notably, the work ofAmirtharajah in development of AC powered circuits has interesting potential applicability toenergy harvesting technology [14]

4 Optimisation strategies: Materials, device geometry, power

systems design and electronics engineering

The performance of any energy harvesting system is highly dependent on the performance of thetransduction mechanism and the power conversion electronics. As these two subsystems are closelylinked (the very nature of a harvester is that the power extraction via a storage element must influencethe behaviour of the transducer, otherwise the very little power can be extracted) the optimisationof the whole system is of the greatest importance. Different types of energy harvesters suffer fromdifferent bottlenecks in technology and so here the design of harvesters and power processing cir-cuitry will be discussed for four types of harvester: the three common motion-driven devices and anambient RF harvester system, highlighting the requirements of the power converter circuit and themethods that have been identified thus far in the literature to improve system performance.

4.1 Electrostatic case study

Electrostatic harvesters gained significant interest from researchers involved in the initial MEMSenergy harvester work which took place in the late 1990s/early 2000s. The main reasons for thisinterest in electrostatic devices were probably the familiarity within the MEMS community of us-ing electrostatic comb-drives as actuators, excellent MEMS compatibility and the knowledge ofthe scaling of the electrostatic force at the micro-scale, which is clearly important for harvestersto be miniaturised [15]. However, as has been discussed here, the performance of the completeenergy harvester power system module is far more important than the performance of just the en-ergy harvesting transduction mechanism in isolation. Recently, a comprehensive study has beenundertaken which analyses the performance of the complete electrostatic harvester system to de-termine the upper limits on such systems as a function of excitation level and device dimensions [16].

Unless an electret is included [17], electrostatic transducers used as generators must be pre-chargedwhen at maximum capacitance in order to set up an electric field against which mechanical workcan be done in order to generate electrical energy. In other words, a small quantity of charge isplaced on the electrodes before the motion of the generator drives the plates apart, increasing theenergy stored in the electric field. This energy can then be transferred from the moving electrodecapacitor into a separate energy store, which could be another capacitor or a battery.

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There are two common methods of operating an electrostatic harvester, these being constant chargemode and constant voltage mode. The charge-voltage cycles of the transducer in each mode is shownin Figure 4. In constant charge mode, the moving electrodes separate with the electrodes in opencircuit, i.e. with the charge confined to the electrodes and unable to flow in an external circuit. Inconstant voltage mode, the electrodes are connected directly to a fixed voltage source and as theplates separate, charge is driven from the electrodes into the voltage source, increasing the energystored in that source. In each case, the attractive force between the electrodes should be set to anoptimal value [18] which maximises the mechanical work that can be done, given by (1):

FoptCZres=

π

4mA0 (1)

Q

VA

BCQopp

Vpc Vmax

(a) Constant charge mode

Q

V

A

B

CQres

Vres

Qpre

Vopp

(b) Constant voltage mode

Figure 4: Idealised charge versus voltage (QV) generation cycles (from [16] with permission).

Two basic circuits which can be used to operate these QV cycles are shown in Figure 5. In Fig-ure 5a, the variable capacitor can be pre-charged at maximum capacitance by pulsing M1 and M2

in antiphase to charge Cvar to an optimal pre-charge voltage which sets the force to that given by(1). The plates then separate with the MOSFETs off and so the voltage on the plates increases.M1 and M2 are then pulsed again in antiphase to transfer the energy back to the storage element.For the constant voltage device, the circuit of Figure 5b can be used. In this circuit the MOSFETsM3 and M4 are pulsed in order to charge Cint to a high voltage (the voltage which causes the forceon the electrodes to correspond to that given by 1). Then, pulsing M1 and M2 in antiphase allowsthe variable capacitor to be charged when at maximum capacitance. As the plates separate, M1 isheld on, meaning that the large capacitor Cint holds the voltage on the variable capacitor constantduring plate separation. M3 and M4 then pulse to transfer energy back into the storage element.

The non-ideal properties of the MOSFET switches are the main cause of the performance limitsof this system. Firstly, the devices must be designed to block the voltage which is optimal for thecapacitor to operate at and whilst increasing this voltage can allow more work to be done against themechanical force, increases in voltage increase the specific on-resistance of the devices. Secondly,

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M1

M2Ron

CvarVsupply

L

Cpara

S1

Rleak


M2

Cvar

Vsupply

L1 L2

M1 M3

M4CintVopp


Figure 5: Basic circuits for electrostatic harvester operation (from [16] with permission).

there is a trade-off in device area as an increased area will reduce conduction loss but will increaseoff-state leakage and charge sharing when the devices are in the off-state.

Consequently, the strategy for optimising the system is to firstly calculate the optimal voltage atwhich to operate the electrodes, design the MOSFETs to block this voltage and then perform anoptimisation on the device area to maximise the performance of the system. The results are shownin Figure 6 and assume silicon is used as the semiconducting material. As can be seen, the max-imum system effectiveness (see [19] for details on the calculation of effectiveness) is poor for theconstant charge generator over the entire operating envelope of size and accelerations, whilst theconstant voltage device can operate relatively well over a large operating range. The reason forthe poor performance of the constant charge device is mainly due to charge sharing which occursbetween the moving electrodes and the attached semiconductors causing a significant reduction inthe mechanical work done. The constant voltage device does not suffer from this problem as thevoltage across the electrodes remains constant during generation.

In order to improve the performance of the electrostatic device types, better semiconductors arerequired with lower leakage and lower on-state conduction loss when operated at high voltages. It ispossible that small silicon carbide devices and diamond devices may be able to allow the performanceof these systems to be improved.

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10−2

100

10210

−1

100

1010

0.1

0.2

0.3

0.4

0.5

Acceleration [m/s2]Length of cube [mm]

Sys

tem

Effe

ctiv

enes

s


10−2

100

10210

−1

100

1010

0.2

0.4

0.6

0.8

1

Acceleration [m/s2]Length of cube [mm]

Sys

tem

Effe

ctiv

enes

s


Figure 6: System Effectiveness for constant charge and constant voltage generators (from [16] withpermission).

4.2 Piezoelectric case study

The piezoelectric transduction mechanism is attractive for use in an energy harvester as it does notrequire a pre-charge to operate and tends to produce terminal voltages in the range of hundreds ofmV to a few volts. The output is AC, but due to the voltage levels produced, this can usually berectified using a simple full-wave rectifier, typically using Schottky diodes. However, whilst such ascheme is advantageous in terms of simplicity, robustness and low component count, it can be dif-ficult to obtain the necessary electrical damping forces to achieve maximum power conversion fromkinetic to electrical energy. Techniques to increase the damping and maximise power generation canbe applied, by either modifying the geometry of the device by providing an active power electronicinterface to the system, or in combination, which will now be described.

For an efficient piezoelectric energy harvester the vibrational energy must be transferred into a strainin the piezoelectric for it to be converted into electrical form. There have been several reviews ofpiezoelectric energy harvesters [1] [20], [21], [6] with many proposed methods, but the most popu-lar because of its simplicity is the fixed-free cantilever, vibrating at its fundamental flexural mode.The strain energy in the cantilever in this mode varies linearly along the length from the maximumat the root to zero at the end. Through the cantilever thickness, the maximum strain is at thepoints furthest from the neutral axis. These principles have led to developments such as triangularcantilevers with uniform strain along the length, and air spaced cantilevers to increase the distancefrom the neutral axis [22].

The simple rectangular cantilever comprising a piezoelectric layer laminated to an elastic layer is thesimplest and most cost-effective design, and is therefore widely used. However, it is not necessarilythe most effective in terms of the energy harvested. Although many workers do not electrode the

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piezoelectric in regions of zero strain, such as below the tip mass or fixed end, few have investi-gated the electrode coverage of the beam. In this case study we show that there is an internalloss mechanism due to charge redistribution within the cantilever. Charge flows from the highlystrained root of the cantilever to the unstrained tip, and energy is lost in this process, reducing theeffectiveness of the harvester. These internal losses can be significant and through reducing theelectrode coverage of the beam we can increase power output by up to 18%! For the simple can-tilever arrangement discussed here, the harvested energy is maximised with an electrode coverage ofexactly 2/3 of the beam length from the root. These results have been experimentally confirmed [23].

x

yw l

F

(a)

x x=l

V

Vave

Charge Equalisation

(b)

Figure 7: a) schematic of a piezoelectric energy harvester, with the piezoelectric layer electrodedtop and bottom, on top of a passive substrate (grey), b) the voltage distribution along a beam forinfinitesimally small piezoelectric elements and the schematic charge flow from high to low voltageregions.

Described in a little more detail, Figure 7 shows a typical piezoelectric energy harvesting cantileverstructure. The curvature of the beam, and therefore the strain developed in the ceramic, is pro-portional to the distance from the loaded end of the cantilever [24], [25]. In this case study, weconsider two limiting cases: a) each element of the piezoelectric material is electrically isolatedfrom the others i.e. open circuit, the dielectric displacement, D =0; b) all the elements are elec-trically connected in parallel so that charge can flow to maintain an equipotential, V. Under opencircuit conditions a piezoelectric voltage is generated proportional to the beam curvature. Figure 7bshows the distribution of the open circuit voltage, V(x), along the beam, x , which can be written as:

V (x) = 2l − x

lVave (2)

where l is the length of the beam and Vave is the average voltage. The energy stored over the wholeof the beam, EV is given by:

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Ev =∫ l

0

1

2CV (x)2

dx =2

3CVave

2l (3)

where C is the capacitance per unit length of the beam. In case b) the charge is allowed to flow(e.g. in an electrode covering the whole of the beam) until the voltage everywhere equals Vave . Inthis case the stored energy, EQ is:

EQ =1

2CVave

2l (4)

The difference between these two represents a 25% loss in the stored energy before any externalcircuit is attached. This energy is dissipated in the movement of charge along a gradient of high tolow potential From this work, it is clear that the areas at the end of the beam contribute little energyto the load and only serve to lower the average voltage and therefore the stored energy. This modelis readily extended to partial coverage of the beam by changing the integration limits in Equation3. This shows that the maximum power output is obtained when only 2/3 of the beam is covered,and the harvested energy at this optimum is 18% higher than a fully electroded beam. For moredetails please refer to the published work [23].

In addition to selective electroding of the beam in order to increase energy yield (ultimately becausethe QV product is raised by only forming a capacitance on the high stress parts of the structure),other techniques can be employed in the electronics in order to increase the work done by the systemby increasing increasing the damping force. This is done through charge modification schemes, suchas piezoelectric pre-biasing [9]. Such schemes have been shown to increase the useful generatedpower by more than 10 times over what is achievable with a bridge rectifier. As will be shown, themost efficient circuit for implementing the pre-biasing scheme, known as single-supply pre-biasing,automatically rectifies the output of the piezoelectric transducer and, as all the commutation isdone actively using MOSFETs, diode drops do not occur in the current path [9] causing losses tobe minimised.

Figure 8: Simple model of a piezoelectric element with low transduction factor (from [9] withpermission).

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A simple model of a piezoelectric energy harvester with poor electromechanical coupling (i.e. a casewhere more power can be extracted if damping can be increased) is shown in Figure 8 where thepiezoelectric transducer is represented as a current source in parallel with a capacitor. The currentfrom the source is proportional to the velocity of the tip of the piezoelectric cantilever with a coeffi-cient known as the transduction factor, and the shunt capacitor represents the clamped capacitanceof the transducer. With a simple bridge rectifier interface added, as shown in Figure 9(a), the volt-age on the electrodes is shown in Figure 9(b). Clearly, as the rectifier output voltage is increased,charge displaced by the piezoelectric effect is pushed into a higher voltage at the output of therectifier, but the conduction time, and hence the total charge that moves through the rectifier, isreduced. Therefore, there is an optimal output voltage at which to operate the rectifier. This thenalso corresponds to achieving the maximum available damping force on the piezoelectric material.

(a) Piezo harvester with full-wave rectifier. (b) Waveforms for piezo with full-wave rectifier.

Figure 9: Piezo with full-wave rectifier (from [9] with permission).

If this damping force achievable with the bridge rectifier is insufficient to extract maximum energyfrom the mechanical system, or the open circuit voltage on the piezoelectric material is insufficientto overcome the turn-on voltage of the diodes, the pre-biasing method can be used. The circuitwhich implements this is shown in Figure 10 and this simple circuit, if operated correctly, can bothincrease the damping on the piezoelectric material and rectify the output at the same time, and inan efficient way.

The basic principle of operation of this circuit is that at maximum deflection of the cantilever,opposite pairs of switches are fired for one resonant half period of the LC circuit, where L is aphysical inductor and Cp is the clamped capacitance of the piezoelectric material. The closing ofthe switches causes a pre-bias voltage to be applied to the piezoelectric material with a polarity

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Figure 10: Single Supply Piezoelectric Prebiasing Circuit (from [9] with permission).

that causes the force on the piezoelectric material to resist the motion of the cantilever on the nexthalf-cycle. Thus, controlling the pre-bias voltage allows the damping to be controlled and allowsthe power density of the system to be maximised. The optimal pre-bias voltage can be determined(as a function of the mechanical parameters) and is given (from [26]) by:

VPB =

(

−

π

4mAinput +

ΓZl

Cp

)

1

Γ(5)

where m is the value of harvester proof mass, Ainput is the base excitation, Γ is the transductionfactor, Cp is the clamped capacitance of the piezoelectric element and Zl is the maximum amplitudeof the mass within the package. Clearly, for large acceleration inputs, the magnitude of the pre-biasvoltage increases to increase the damping.

A prototype of this pre-biasing system has been constructed using low power components [27] andhas been shown to give a significant performance increase over the bridge rectifier and other chargemodification techniques, such as SSHI (synchronous switched harvesting on inductor). As can beseen in Figure 11, the performance of the SSPB circuit is around 20% better than SSHI and around12 times better than a simple bridge rectifier interface.

4.3 Electromagnetic systems

Of all three types of transduction mechanisms for motion-driven harvesters, the electromagnetic isprobably the most recognisable to most engineers, as this mechanism is used to generate electricalpower in power stations and is regularly used across the macro scale as a motor. The main difficultywith this type of harvester is typically that the level of the voltage output from the transducer tendsto be quite low or if increased, by increasing the number of turns on the transducer, the outputimpedance of the transducer can be very high. However, for larger energy harvesting devices, passiverectification is possible on these devices, typically using Schottky diodes either as a standard half orfull-wave rectifier, or as a single or multiple-stage voltage multiplier [28].

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Figure 11: Power output of piezoelectric harvester using different interface circuits

In order for an electromagnetic generator to achieve maximum power density, as is the case withthe other transducer types, the force produced by the transducer must be set to an optimal value.In the case of the electromagnetic harvester, the force on the transducer can be set by control-ling the current in the pick up coil. Conjugate power variables (e.g. voltage and current, or forceand velocity) only carry real power when at the same frequency and when they have an in phasecomponent. This means that if only real power is being transferred from the mechanical to theelectrical domain, the current through the coil must be in phase with the developed voltage, i.e.with the relative velocity between the magnet and coil. With a simple bridge rectifier, this is not thecase as current though a simple passive rectifier only occurs at the peak of the AC waveform, oftenapproximated as a rectangular pulse. However, as only the fundamental current in this pulse carriesreal power, the optimal force can still be set with a bridge rectifier by controlling the output voltageof the rectifier and this controlling the pulse width and height, and hence its fundamental current [29].

If the voltage from the output of an electromagnetic harvester is too low to overcome the turn-onvoltage of even a Schottky diode, a boost rectifier topology can be used [30]. Such a system, oftenoperated in discontinuous conduction mode, can be used to boost the voltage from the rectifier andto modify the damping of the harvester in order to keep the power density a maximum. However,once the rectifier becomes active rather than passive, significant additional functionality can beprovided, as will now be described.

So far, with all the transducer types, we have only considered the delivery of real power from thesource to the energy storage element, through some means of rectification, with step-up or step-downcapability. However, with the electromagnetic transducer, it is possible to also adjust the resonantfrequency of the harvester by adding an active rectifier. The basic idea behind this can be seen inFigure 12. Any simple mass-spring-damper system can be represented in the electrical domain by aparallel RLC circuit (representing the mass, spring and damper) with a current source excitation,representing the vibration. The transducer in the circuit represents the transduction mechanism

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and the secondary side components are the electrical components connected to the terminals of thecoils. In order for a harvester to operate optimally, the resonant frequency of the mass and springshould be set to the same frequency as the driving frequency. If the mechanical mass and spring donot resonate at the driving frequency, passive reactive components can be added to the load which,when paralleled with the reactive components, can modify the resonant frequency of the system.

Figure 12: Inertial harvester with passive load (from [31] with permission).

One way of achieving this tuning, and at the same time rectifying the harvester output and storingit in a battery, is to use discrete passive components, as in Figure 12, possibly switching in differentvalues from a bank of components. However, in order to make the system infinitely tuneable (i.e.not being reliant on a finite number of passive components), the rectifier interface can be madefully active, as shown in Figure 13. This simple power electronics topology (effectively a full-waverectifier where the diodes are replaced with MOSFETs), known as an H-bridge, allows power to betransferred from the mechanical system to the battery, and the battery to the mechanical systemfor either polarity of generated voltage, i.e. it is able to mimic any complex load impedance, withinpractical limits set by the on-state resistance of the active devices.

Figure 13: Inertial harvester with active rectifier capable of tuning resonant frequency and damping

As can be seen, if the bridge interface is set to behave with a capacitive input impedance, this

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Figure 14: Power output of electromagnetic harvester with active rectification and with control ofactive and reactive power (from [31] with permission).

capacitor parallels with the capacitor representing the mass, reducing the resonant frequency. Ifthe bridge behaves inductively, the inductance parallels with the inductance representing the spring,reducing the effective inductance and increasing the resonant frequency. The results of this tech-nique, applied to a pendulum harvester intended to generate power in a rocking boat [31], are shownin Figure 14. The typical resonant peak in power output of the system can be seen. When theactive rectifier interface is configured to behave with a capacitive element to the input impedance,the power generated at low frequency is increased and when the interface is configured to lookslightly inductive (approximated here with a negative capacitance), the power generated at frequen-cies above the natural resonant frequency of the system increases. It should also be noted that bycontrol of the resistive input impedance of the bridge, the level of damping can also be controlled,allowing the power density of the system to be maximised.

4.4 RF Harvesting

RF energy is available in the environmental ambient across most areas in the developed world (andin many regions in developing nations) due to the existence of TV and radio transmission and theuse of mobile phones and wifi networks. In all of these applications, energy is transmitted as ameans for communication, rather than for transferring power. However, the transmitted power canbe collected and, if this can be done with a high enough efficiency and accumulated, can be usedto power a wireless sensor. A typical RF harvesting system comprises an antenna, an impedancematching circuit, a rectifier and a storage element, as shown in Figure 15.

The function of the impedance matching circuit is to ensure that a maximum amount of energycollected by the antenna is transferred to the output storage element. Clearly, the diode conductsfor only half the AC cycle in the simple topology of Figure 15, and it is important that the voltagedeveloped across the diode is high enough to turn on the junction. In addition, the diode musthave minimal reverse recovery loss at RF frequencies (otherwise it will look capacitive rather than

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Figure 15: RF harvester system

displaying a non-linear characteristic) in order that it may rectify properly. The amount of ambientenergy available is highly variable across different locations, for instance the ambient RF powerdensity is higher in urban areas than rural areas and high close to wifi access points and mobile basestations, low levels of input power cause low voltages at the input of the rectifier. For input powerlevels that are too low, the diode will not commutate and power is not harvested, although a lowbias detector, such as an SMS7630 may be used [32].

A survey or power levels across London has recently been undertaken (http://www.londonrfsurvey.org)and this shows that in many locations, using simple RF harvester topology shown in Figure 15, theamount of energy is sufficient to allow DC power to be harvested. However, in semi-urban and ruralareas, the available input power drops below the level that allows the diode to turn on, or for anypower processing circuitry to start up. This is a clear application where low input voltage capabilityis required of a power converter.

4.5 Overview of design methodologies for harvesters

As has been demonstrated by the case studies above, traditional rectification is only one part of thefeature set that can and should be included in the power electronic interface to an energy harvestingdevice, be it a motion-driven or other type of harvesting device. Features such as ultra-low voltagestart-up, achieving the optimal damping, modifying system resonant frequency and up and down-conversion of the generated voltage are all important factors.

A systems approach is required in the design of an optimised energy harvesting system. A set ofparameters (e.g. maximum size of harvester and the vibration conditions) should be considered anda transducer type chosen. This choice, which is critical to maximising power density, is difficult andstill has not been completely understood as there are so many design decisions to be taken intoaccount, such as capability of the semiconductors, the amount of energy storage required etc. Thedetailed discussion of these decisions is beyond the scope of this report but it is hoped that thereader has gained a flavour of the complexity of the problem and the possible features that can be

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included in the interface circuit, other than performing the rectification function.

5 Recommendations for future research and Roadmap

A target roadmap for nano rectification is shown in Figure 16. There are 4 themes which have beenidentified, in addition to the agreement of measurement standards. The timeline and links betweenthese themes is shown in Figure 16 and explained below:

1. Standards: Development of International standards for definition and measurements of efficacyof energy harvesting devices - pan European to international with industrial support. 2015delivery. [Several de facto standards exist and users quote from different standards, thusmaterials, systems and devices can not be readily compared. Set up community debate on theadoption of an appropriate standard parameter to enable easy comparison of technologies.]

2. Intelligent adaptive systems: Development of self tuning electronic systems control to accountfor broadband excitation sources. Rectifier technology: go beyond the ’passive’ rectifier toan active rectifier (system) for on the fly operational optimisation of EH devices - first smallscale demonstrators by 2016. [Challenge in scaling. Active rectification for transfer of realand reactive power between transducer and storage element.]

3. Nanoscale Devices: Development of ‘zero’-control overhead synchronous rectifier for operationwith sub-threshold AC input signals - techniques and systems integration of state of the art’rectification’ technologies (regular synchronous rectifiers and those with additional functions,such as pre-biasing etc) to accelerate the nano materials based EH structures (such as ZnOnano rods). First demonstration at the nanoscale: 2016.

4. Systems Integration and new materials: Miniaturisation and quality increase of passive elec-trical component technologies (inductors and transformers) through the the use of improvedmaterial systems, or alternative approaches, such as solid state techniques to the couplingof electrical to magnetic energy. [Improvement in passive electronic component technology].First demonstration of solid state solutions: 2018.

5. Hybrid Devices: Trade off between transducer complexity and electronics complexity andtheir integration - electronics systems control and power processing hardware as an inte-grated structure with the active material (piezo, electrostatic, magnetic). Systems integrationdemonstrated: 2014

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nFigure 16: Roadmap for energy harvesting rectification strategies

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6 Acknowledgements

The Materials Knowledge Transfer Network (KTN), Director - Dr Robert Quarshie and TechnologyManager - Smart and Emerging Technologies of the Materials KTN, Dr Steve Morris, for supportingthis study.Smart Materials and Systems Committee (SMASC), Institute of Materials, Minerals and Mining,London, UK.Dr Mark Stewart, and Dr Paul Weaver, National Physical Laboratory, Teddington, UK.EPSRC.

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