Download Report on the rectification of harvested energy in vibrational energy

Report on the rectification of harvested
energy in vibrational energy harvesting
materials technologies:
Materials, power systems design and
electronic engineering issues
Special
Interest
Group
Energy Harvesting
August 2012
The Energy Harvesting
Special Interest Group
The Technology Strategy Board has launched its Emerging Technologies and Industries programme and is working
with partners to build a UK programme in Energy Harvesting. The programme is intended to accelerate the development and commercial use of products, processes and services based on energy harvesting technology. The Energy
Harvesting Special Interest Group (SIG) brings together the
community along the value chain – from academia, materials technologies, devices, systems integration and through
to the user communities helping build a vibrant and productive Energy Harvesting community in the UK.
The information in this report will be used by the SIG to
widen the debate about where energy harvesting may have
a role to play. It will also provide input to briefings for funding agencies on challenge areas to be supported.
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Content
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The Energy Harvesting Special Interest
Group
Executive Summary
Energy Harvesting technologies
Electrical rectification
SImple circuits
Vibrational EH technology
Direct AC power utilisation
Optimisation strategies: Materials, geometry,
power systems design and electronics
engineering
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Electrostatic case study
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Piezoelectric case study
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Electromagnetic case study
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RF Harvesting
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Overview of design methodologies
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Recommendations for future research and
Roadmap
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Acknowledgements
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References
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Executive Summmary
Energy harvesting devices are widely regarded as an important technology in the future success of the wireless sensor
network, potentiality enabling almost infinite operating duration. To date, the vast majority of research on harvesters
(be they kinetic, thermal or solar) has concentrated on the
transduction mechanism. However, a complete energy harvester powered system requires suitable interface circuitry
to process the power output of the harvesting transducer
into a form which can be stored in a battery or capacitor to
power a low voltage, low power load, typically a sensor and
radio transceiver. This report discusses the state of the art
of such circuits, the features they are able to provide (above
that of simple AC to DC conversion) and illustrates this with
four case studies, one for each of the common types of
motion-driven energy harvester transduction mechanism
and an ambient RF harvester. It is shown that, whilst power
processing for harvesters is possible, significant gains need
to be made to allow operation of harvesters as they become
further miniaturised, and that the control circuit overhead
must also be reduced.
The report concludes with a suggested roadmap of research
in the area of micro and nano rectification and, because
the development of rectification and power processing interfaces are tied so closely to the transducer technologies,
system issues also feature in the roadmap. The main suggestions for future research fall into 5 areas, these being:
standards development, intelligent adaptive systems, nanoscale devices, systems integration and new materials
and hybrid devices. A suggested timescale for these developments is provided.
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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 voltage DC, 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, and
this specific challenge means that, in some cases, energy
harvester transducer design is modified away 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 developed for 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) is shown
schematically in Figure 1. Here, the piezoelectric material and mechanical structure provides energy in the form
of a charge separation (i.e. a charged capacitor) to the
interface circuit. The oscillation of the beam means that
the voltage developed on the piezoelectric capacitor contains purely AC components and thus some form of rectification is necessary if the system is to drive a low-power
DC load. Consequently the interface circuit in Figure 1
can, in its simplest form, be a diode rectifier. The generated energy is then stored (in a capacitor or battery) and
regulated before being supplied to a low-power load. As
energy is converted from a mechanical to electrical form
by the transducer and interface circuit, the mechanical
motion is damped, reducing the amplitude of the proof
mass. The control of the amount of damping applied is
critical to achieving high power densities for such systems
and is a key feature required of the rectifier interface.
The circuitry which implements the AC to DC conversion
process is, in its simplest form, a passive diode rectifier.
However, this may not be possible if the transducer output
voltage is low and so other solutions are required. In addition, the circuit which accomplishes the AC to DC conversion process can also perform other tasks, such as tuning
the resonant frequency of a kinetic harvester or increasing the available damping force. Both of these additional
functions can improve the system’s power density. These
and other issues related to the rectification and systems
control are discussed in this report.
Figure 1: A typical energy harvesting system
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Electrical rectification
Simple Circuits
There are several possible mechanical architectures
of vibration based power generators [6]. In the case
of piezo-generators, the energy conversion takes
place via the direct piezoelectric effect. This is the
direct generation and delivery of charge onto the
electrodes of a piezoelectric material when a 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 the material.
Assuming the external driving force is sinusoidal (or
cyclical) in nature - as is the case for many vibrational sources of energy - then the charge generated by the piezoelectric material is also cyclical.
The charge developed depends on the piezoelectric characteristics, its geometry and the details of
the external mechanical vibration. The mechanical
vibrations, which are the source of 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 wave or full wave bridge
rectifier (a simple diode circuit) and a smoothing capacitor, Cs, with an an electrical load, RL connected (see Figure 2).
Figure 2: Standard rectification interface circuits for
energy harvesting, a) half wave rectifier and b) full
wave rectifier
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Assuming a single-mode external mechanical vibration
(the mechanical displacement u(t) is assumed to 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 affects the output waveform of the piezo-generator.
If the piezo-generator can develop sufficient voltage such
that the forward biased diodes in the bridge rectifier can
operate in their conducting mode (for silicon the switch on
voltage is about 0.6 V and for germanium diodes this is
about 0.3 V) then the piezo 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 connected to the circuit),
and with the addition of a smoothing capacitor this unipolar
cyclical voltage will appear as a DC voltage on the load resistor. More precisely, when the output voltage across the
load resistor exceeds the absolute value of the piezoelectric
device minus the 2 diode voltage drop then the piezo is in
an open circuit configuration and its voltage swings with its
displacement. When the absolute value of the piezo voltage
generated is equal to or greater than the storage capacitor
voltage plus the bridge rectifier voltage drop then electrical
energy is transferred to the capacitor and load. This rather simplistic explanation is sufficient for the needs of this
report and subsequent analysis 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 are generally based
on the reduction in the diode voltage drops associated with
semiconductor rectifier diodes (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 by active circuitry which is externally powered or powered directly from
the AC input signal [11]. Several other 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 switch harvesting on inductor). These circuit configurations intermittently switch the piezoelectric onto a resonating electrical
network (LCR) for a very short time, which has the effect
of increasing the voltage output and effectively increasing
the coupling coefficient of the piezomaterial. This has been
shown to accomplish gains of order times 8 in harvested
power compared to the standard bridge only configuration
[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
voltage drop and allows for a greater efficiency of harvesting power at much lower voltages. The series SSHI energy
harvesting circuit is shown in Figure 3 and one can see that
two digital switches are placed in series with the piezoelectric and rectifier. These switches are synchronised with the
piezo charge cycle, and when the latter is at a maximum the
switches close and energy is transferred through the rectifier to the storage capacitor. The switched voltage is actually
inverted through this process and losses can be significant.
Yet another variation on this approach uses a transformer
to further reduce the effect of the voltage drop [12] where
a transformer replaces the inductor in Figure 3 along with a
new diode in series with the load. In this report, a new technique, called single supply pre-biasing will be discussed,
which is superior to the SSH techniques.
More recent work has developed the synchronous switching
technology and coupled this with a voltage pre-bias to permit even greater power output of piezo energy harvesting
devices [8]. The method is particularly suited for undamped
and low frequency applications but with high excitation amplitude - 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 harvesters which use variable capacitor structures to couple kinetic energy into the
mechanical domain. An early 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 CMOS process, which
severely reduced the power density of the system.
7
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 Carlton House Terrace, London, was whether applications exist that do not require rectification of the cyclic
external energy source. For example, it is not necessary to
rectify an AC source to usefully power a light bulb. Various
interesting opportunities may exist with this specification,
which are briefly discussed below:
• Heat store: Here the AC power (rate at which the energy harvested is transferred, used or transformed)
is used to simply electrically heat a thermal heatsink.
The temperature of that heatsink will increase until the
losses (convection, radiation, conduction) match the
energy input. This heat can be used as another source
of energy.
Figure 3: Series SSHI circuit and typical waveforms - from
[12].
Vibrational EH technology
Of the various sources of ambient energy, mechanical energy in the form of vibrations is present in many environments, particularly where there is some form of machinery,
and is an alternative when light or thermal sources are not
sufficient. The most common method for scavenging this
energy source is to use resonant inertial devices. Typically,
this involves a resonant cantilever with a tip mass, where
accelerations arising from the vibrating source cause the tip
mass to oscillate. In order to 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 with electromagnetic transduction is
problematic because of the difficulty in producing compact
coils. In contrast, piezoelectric transduction has the potential for miniaturisation in MEMS scale devices.
8
• Clockwork wind up spring: Here, the rectification occurs through mechanical means such as ratchets 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 radiant RF (microwave) radiation from the environment. The issues here though
reside with precise matching 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 stress in a material or
liquid or gas, similar to the thermal heatsink approach.
• Composite systems providing anticlastic one way motion: Here we develop an approach that mechanically
rectifies the cyclical energy scavenged, whereby the
composite beam is only able to flex one way (which for
a piezo material would be in the same positive direction as its built in polarisation), thereby providing DC
rectified output. Half of the available energy is lost as
heat 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. There may be interesting ways
in which these materials may transduce the ambient
‘free’ energy into an energy that can be harnessed differently to piezo or electrostatic or EM harvesting
technologies.
• 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 energy into chemical forms.
• Artificial photosynthesis: The holy grail of energy conversion - that of photosynthesis - is a subject of great
academic and commercial interest with many applications outside of energy production. The utility of photosynthesis to create chemicals or to modify chemical
species through direct sunlight is the mainstay of all
plant life on earth.
• Hybrid - Solar/piezo: The combination of two or more
energy harvesting technologies may synergistically afford a direct AC utilisation of power scavenged from
the environment.
• Circuits that run off AC: There is current research
aimed at how one might directly power electronic circuitry with AC rather than DC (rectified AC) power. Notably, the work of Amirtharajah in development of AC
powered circuits has interesting potential applicability
to energy harvesting technology [14]
9
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 the transduction mechanism and the power conversion electronics. As these two
subsystems are closely linked (the very nature of a harvester is that the power extraction via a storage element must
influence the behaviour of the transducer, otherwise the
very little power can be extracted) the optimisation of the
whole system is of the greatest importance. Different types
of energy harvesters suffer from different 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 an
ambient RF harvester system, highlighting the requirements
of the power converter circuit and the methods that have
been identified thus far in the literature to improve system
performance.
Electrostatic case study
Electrostatic harvesters gained significant interest from
researchers involved in the initial MEMS energy harvester
work which took place in the late 1990s/early 2000s. The
main reasons for this interest in electrostatic devices were
probably the familiarity within the MEMS community of using electrostatic comb-drives as actuators, excellent MEMS
compatibility and the knowledge of the scaling of the electrostatic force at the micro-scale, which is clearly important
for harvesters to be miniaturised [15]. However, as has been
discussed here, the performance of the complete energy
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 been undertaken 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-charged when at maximum
capacitance in order to set up an electric field against which
mechanical work can be done in order to generate electrical
energy. In other words, a small quantity of charge is placed
on the electrodes before the motion of the generator drives
the plates apart, increasing the energy stored in the electric
10
field. This energy can then be transferred from the moving electrode capacitor into a separate energy store, which
could be another capacitor or a battery. There are two common methods of operating an electrostatic harvester, these
being constant charge mode and constant voltage mode.
The charge-voltage cycles of the transducer in each mode
is shown in Figure 4. In constant charge mode, the moving
electrodes separate with the electrodes in open circuit, i.e.
with the charge confined to the electrodes and unable to
flow in an external circuit. In constant voltage mode, the
electrodes are connected directly to a fixed voltage source
and as the plates separate, charge is driven from the electrodes into the voltage source, increasing the energy stored
in that source. In each case, the attractive force between
the electrodes should be set to an optimal value [18] which
maximises the mechanical work that can be done, given by
(1):
Q
Qopp
A
B
Vpc
Q
C
Qpre
Qres A
V
Vres
Vmax
(a) Constant change mode
B
C
Vopp
V
(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 MOSFETs
M3 and M4 are pulsed in order to charge Cint to a high voltage (the voltage which causes the force on the electrodes
to correspond to that given by 1). Then, pulsing M1 and
M2 in antiphase allows the variable capacitor to be charged
when at maximum capacitance. As the plates separate, M1
is held on, meaning that the large capacitor Cint holds the
voltage on the variable capacitor constant during 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 limits of this system. Firstly,
the devices must be designed to block the voltage which is
optimal for the capacitor to operate at and whilst increasing
this voltage can allow more work to be done against the
mechanical force, increases in voltage increase the specific
on-resistance of the devices. Secondly, there is a trade-off
in device area as an increased area will reduce conduction
loss but will increase off-state leakage and charge sharing
M1
Cpara
Rleak
when the devices are in the off-state.
Consequently, the strategy for optimising the system is to
firstly calculate the optimal voltage at which to operate the
electrodes, design the MOSFETs to block this voltage and
then perform an optimisation on the device area to maximise
the performance of the system. The results are shown in
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 the constant charge generator over the entire
operating envelope of size and accelerations, whilst the
constant voltage device can operate relatively well over a
large operating range. The reason for the poor performance
of the constant charge device is mainly due to charge sharing which occurs between the moving electrodes and the
attached semiconductors causing a significant reduction
in the mechanical work done. The constant voltage device
does not suffer from this problem as the voltage across the
electrodes remains constant during generation.
L
0.5
S1
Ron
CVAR
0.4
M2
Vsupply
0.3
0.2
0.1
0
1
10
(a) Constant charge mode
−2
10
0
10
0
10
−1
10
L1
L2
M1
CVAR
M2
Vint
M3
Cint
Vsupply
2
10
2
(a) Constant charge mode
M4
1
0.8
0.6
(b) Constant voltage mode
Figure 5: Basic circuits for electrostatic harvester operation (from [16] with permission).
0.4
0.2
Figure 6: System Effectiveness for constant
charge and constant
voltage generators (from
[16] with permission).
0
1
10
−2
10
0
10
0
10
−1
10
2
10
2
(b) Constant voltage mode
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In order to improve the performance of the electrostatic device types, better semiconductors are required with lower
leakage and lower on-state conduction loss when operated
at high voltages. It is possible that small silicon carbide devices and diamond devices may be able to allow the performance of these systems to be improved.
Piezoelectric case study
The piezoelectric transduction mechanism is attractive for
use in an energy harvester as it does not require a precharge to operate and tends to produce terminal voltages
in the range of hundreds of mV to a few volts. The output is
AC, but due to the voltage levels produced, this can usually be rectified using a simple full-wave rectifier, typically
using Schottky diodes. However, whilst such a scheme 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 from kinetic to electrical energy. Techniques to
increase the damping and maximise power generation can
be applied, by either modifying the geometry of the device
by providing an active power electronic interface 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 strain in the piezoelectric for it to be converted into electrical form. There have
been several reviews of piezoelectric 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 maximum at the root to zero at the end.
Through the cantilever thickness, the maximum strain is at
the points furthest from the neutral axis. These principles
have led to developments such as triangular cantilevers
with uniform strain along the length, and air spaced cantilevers to increase the distance from the neutral axis [22].
The simple rectangular cantilever comprising a piezoelectric layer laminated to an elastic layer is the simplest and
most cost-effective design, and is therefore widely used.
However, it is not necessarily the most effective in terms
of the energy harvested. Although many workers do not
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electrode the 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 internal loss mechanism due to charge
redistribution within the cantilever. Charge flows from the
highly strained root of the cantilever to the unstrained tip,
and energy is lost in this process, reducing the effectiveness of the harvester. These internal losses can be significant and through reducing the electrode 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 of
exactly 2/3 of the beam length from the root. These results
have been experimentally confirmed [23].
(a)
(b)
Figure 7: a) schematic of a piezoelectric energy harvester,
with the piezoelectric layer electroded top and bottom, on
top of a passive substrate (grey), b) the voltage distribution
along a beam for infinitesimally small piezoelectric elements
and the schematic charge flow from high to low voltage regions.
Described in a little more detail, Figure 7 shows a typical
piezoelectric energy harvesting cantilever structure. 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, we
consider two limiting cases: a) each element of the piezoelectric material is electrically isolated from the others i.e.
open circuit, the dielectric displacement, D =0; b) all the ele-
ments are elec- trically connected in parallel so that charge
can flow to maintain an equipotential, V. Under open circuit
conditions a piezoelectric voltage is generated proportional
to the beam curvature. Figure 7b shows the distribution of
the open circuit voltage, V(x), along the beam, x , which can
be written as:
V (x) = 2 l − x Vave
(2)
l
where l is the length of the beam and Vave is the average
voltage. The energy stored over the whole of the beam, EV
is given by:
Ev = 1 1 CV (x)2 dx = 2 CVave2 l
0
2
(3)
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 . In this case the stored energy, EQ is:
EQ = 1 CVave2 l done through charge modification schemes, such as piezoelectric pre-biasing [9]. Such schemes have been shown
to increase the useful generated power by more than 10
times over what is achievable with a bridge rectifier. As will
be shown, the most 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 is done actively using
MOSFETs, diode drops do not occur in the current path [9]
causing losses to be minimised.
(4)
2
The difference between these two represents a 25% loss
in the stored energy before any external circuit is attached.
This energy is dissipated in the movement of charge along
a gradient of high to low potential From this work, it is clear
that the areas at the end of the beam contribute little energy
to the load and only serve to lower the average voltage and
therefore the stored energy. This model is readily extended
to partial coverage of the beam by changing the integration
limits in Equation 3. 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 more details please refer
to the published work [23].
In addition to selective electroding of the beam in order to
increase energy yield (ultimately because the 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
system by increasing increasing the damping force. This is
Figure 8: Simple model of a piezoelectric element with low
transduction factor (from [9] with permission).
A simple model of a piezoelectric energy harvester with
poor electromechanical coupling (i.e. a case where more
power can be extracted if damping can be increased) is
shown in Figure 8 where the piezoelectric transducer is represented as a current source in parallel with a capacitor.
The current from 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 capacitance of 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 the rectifier, but the conduction time,
and hence the total charge that moves through the rectifier,
is reduced. Therefore, there is an optimal output voltage at
which to operate the rectifier. This then also corresponds
to achieving the maximum available damping force on the
13
L is a physical inductor and Cp is the clamped capacitance
of the piezoelectric material. The closing of the switches
causes a pre-bias voltage to be applied to the piezoelectric
material with a polarity
(a) Piezo harvester with full-wave rectifier.
Figure 10: Single Supply Piezoelectric Prebiasing Circuit
(from [9] with permission).
(b) Waveforms for piezo with full-wave rectifier.
piezoelectric material.
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 energy from the mechanical system, or the open circuit voltage on the piezoelectric
material is insufficient to overcome the turn-on voltage of
the diodes, the pre-biasing method can be used. The circuit
which implements this is shown in Figure 10 and this simple
circuit, if operated correctly, can both increase the damping
on the piezoelectric material and rectify the output at the
same time, and in an 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
14
that causes the force on the piezoelectric material to resist
the motion of the cantilever on the next half-cycle. Thus,
controlling the pre-bias voltage allows the damping to be
controlled and allows the 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:
(5)
where m is the value of harvester proof mass, Ainput is
the base excitation, Γ is the transduction factor, Cp is the
clamped capacitance of the piezoelectric element and Zl is
the maximum amplitude of the mass within the package.
Clearly, for large acceleration inputs, the magnitude of the
pre-bias voltage increases to increase the damping.
A prototype of this pre-biasing system has been constructed using low power components [27] and has been shown
to give a significant performance increase over the bridge
rectifier and other charge modification techniques, such as
SSHI (synchronous switched harvesting on inductor). As
can be seen in Figure 11, the performance of the SSPB circuit is around 20% better than SSHI and around 12 times
better than a simple bridge rectifier interface.
Electromagnetic systems
Of all three types of transduction mechanisms for motion-driven harvesters, the electromagnetic is probably the
most recognisable to most engineers, as this mechanism is
used to generate electrical power in power stations and is
regularly used across the macro scale as a motor. The main
difficulty with this type of harvester is typically that the level
of the voltage output from the transducer tends to be quite
low or if increased, by increasing the number of turns on
the transducer, the output impedance of the transducer can
be very high. However, for larger energy harvesting devices,
passive rectification is possible on these devices, typically
using Schottky diodes either as a standard half or full-wave
rectifier, or as a single or multiple-stage voltage multiplier
[28].
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 with the 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 force and velocity)
only carry real power when at the same frequency and
when they have an in phase component. This means that
if only real power is being transferred from the mechanical
to the electrical 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 the case as current though a simple passive rectifier only occurs at the peak of the AC waveform,
often approximated as a rectangular pulse. However, as
only the fundamental current in this pulse carries real power, the optimal force can still be set with a bridge rectifier by
controlling the output voltage of 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-on voltage of even a
Schottky diode, a boost rectifier topology can be used [30].
Such a system, often operated in discontinuous conduction
mode, can be used to boost the voltage from the rectifier
and to 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 be provided, as will now be described.
So far, with all the transducer types, we have only considered the delivery of real power from the source to the energy
storage element, through some means of rectification, with
step-up or step-down capability. However, with the electromagnetic transducer, it is possible to also adjust the resonant frequency of the harvester by adding an active rectifier.
The basic idea behind this can be seen in Figure 12. Any
simple mass-spring-damper system can be represented in
the electrical domain by a parallel 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 and the
15
secondary side components are the electrical components
connected to the terminals of the coils. In order for a harvester to operate optimally, the resonant frequency of the
mass and spring should be set to the same frequency as
the driving frequency. If the mechanical mass and spring do
not 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 13: Inertial harvester with active rectifier capable of
tuning resonant frequency and damping
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 storing it in a battery, is to
use discrete passive components, as in Figure 12, possibly
switching in different values 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 made fully active, as shown
in Figure 13. This simple power electronics topology (effectively a full-wave rectifier where the diodes are replaced
with MOSFETs), known as an H-bridge, allows power to be
transferred from the mechanical system to the battery, and
the battery to the mechanical system for either polarity of
generated voltage, i.e. it is able to mimic any complex load
impedance, within practical limits set by the on-state resistance of the active devices.
16
Figure 14: Power output of electromagnetic harvester with
active rectification and with control of active and reactive
power (from [31] with permission)
As can be seen, if the bridge interface is set to behave with
a capacitive input impedance, this capacitor parallels with
the capacitor representing the mass, reducing the resonant
frequency. If the 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 shown in Figure 14. The typical resonant peak
in power output of the system can be seen. When the active
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 look slightly 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 by control 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.
RF Harvesting
RF energy is available in the environmental ambient across
most areas in the developed world (and in many regions in
developing nations) due to the existence of TV and radio
transmission and the use of mobile phones and wifi networks. In all of these applications, energy is transmitted
as a means for communication, rather than for transferring
power. However, the transmitted power can be collected
and, if this can be done with a high enough efficiency and
accumulated, can be used to power a wireless sensor. A
typical RF harvesting system comprises an antenna, an impedance matching 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 energy collected by the antenna is transferred to the output storage element. Clearly,
the diode conducts for only half the AC cycle in the simple
topology of Figure 15, and it is important that the voltage
developed across the diode is high enough to turn on the
junction. In addition, the diode must have minimal reverse
recovery loss at RF frequencies (otherwise it will look capacitive rather than displaying a non-linear characteristic)
Figure 15: RF harvester system
in order that it may rectify properly. The amount of ambient
energy available is highly variable across different locations,
for instance the ambient RF power density is higher in urban areas than rural areas and high close to wifi access
points and mobile base stations, low levels of input power
cause low voltages at the input of the rectifier. For input
power levels that are too low, the diode will not commutate
and power is not harvested, although a low bias 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, the amount of energy is sufficient to allow DC power to be harvested. However, in semi-urban and
rural areas, the available input power drops below the level
that allows the diode to turn on, or for any power processing circuitry to start up. This is a clear application where low
input voltage capability is required of a power converter.
Overview of design methodologies
for harvesters
As has been demonstrated by the case studies above, traditional rectification is only one part of the feature set that
can and should be included in the power electronic interface to an energy harvesting device, be it a motion-driven or
other type of harvesting device. Features such as ultra-low
voltage start-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 of parameters (e.g.
maximum size of harvester and the vibration conditions)
should be considered and a transducer type chosen. This
choice, which is critical to maximising power density, is difficult and still has not been completely understood as there
are so many design decisions to be taken into account,
such as capability of the semiconductors, the amount of
energy storage required etc. The detailed discussion of
these decisions is beyond the scope of this report but it
is hoped that the reader has gained a flavour of the complexity of the problem and the possible features that can be
included in the interface circuit, other than performing the
rectification function.
17
Recommendations
for future research and Roadmap
A target roadmap for nano rectification is shown in Figure
16. There are 4 themes which have been identified, in addition to the agreement of measurement standards. The timeline and links between these themes is shown in Figure 16
and explained below:
1. Standards: Development of International standards for
definition and measurements of efficacy of energy harvesting devices - pan European to international with industrial
support. 2015 delivery. [Several de facto standards exist
and users quote from different standards, thus materials,
systems and devices can not be readily compared. Set
up community debate on the adoption of an appropriate
standard parameter to enable easy comparison of technologies.]
2. Intelligent adaptive systems: Development of self tuning
electronic systems control to account for broadband excitation sources. Rectifier technology: go beyond the ’passive’ rectifier to an active rectifier (system) for on the fly
operational optimisation of EH devices - first small scale
demonstrators by 2016. [Challenge in scaling. Active rectification for transfer of real and reactive power between transducer and storage element.]
18
3. Nanoscale Devices: Development of ‘zero’-control overhead synchronous rectifier for operation with 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 ZnO nano 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 improved material systems, or alternative approaches,
such as solid state techniques to the coupling of 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 and their integration - electronics systems control and power processing hardware as an
inte- grated structure with the active material (piezo, electrostatic, magnetic). Systems integration demonstrated:
2014
Figure 16: Roadmap for energy harvesting rectification strategies
19
Acknowledgements
The Materials Knowledge Transfer Network (KTN), Director
- Dr Robert Quarshie and Technology Manager - Smart and
Emerging Technologies of the Materials KTN, Dr Steve Morris, for supporting this 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.
Report written by Markys G Cain and Paul D Mitcheson
August 2012
20
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