Post on 05-Aug-2018
1. ENERGY HARVESTING: STATE OF THE ART
The ever-increasing number of advances in technology have shaped the world into a
place where we are more and more accustomed to being always connected; from our
phones to our homes and even cities, as electronic hardware circuitries become
cheaper and smaller and wireless communication more efficient, paradigms such as
Wireless Sensor Networks or the Internet of Things are now a reality. As they rely
heavily on wireless, “smart” devices, there is an emerging need to overcome the
energy problem: a lot of these devices are embedded and need to continuously run so
it is not efficient to use the connection to the power grid as it would be unpractical,
nor use batteries, for they would require proper maintenance. A possible solution that
has gained momentum in the past years is to use ambient energy sources and convert
them into electrical energy to recharge the batteries.
Large-scale ambient energy (e.g. solar, wind and tide), and large-scale technologies
are already widely used and constantly updated, especially in the light of the growing
concern for the environment, a hot topic right now. At the other end of the scale,
energy harvesting, although is not spread as much, has been the recurring object of
research efforts.
Energy Harvesting is a technique which aims to scavenge or recover the small
amounts of ambient energy that would otherwise be lost, with a consequent
significant economic and environmental impact. The captured energy is generally
very small (of the order of mJ) especially compared with the amount large scale
plants using ambient energy sources such as wind farms can achieve (order of several
hundreds of MJ). However, unlike the large-scale power stations that must be fixed at
a given location, Energy Harvesting devices for small-scale electronics are portable
and in the case of low power applications (such as nodes in Wireless Sensor
Networks, or medical devices embedded in the human body) could not only decrease
the dependency on conventional batteries, but even replace them.
It is easy to see that the environment is full of waste and unused ambient energy. Of
course, not all of it is suitable to all types of applications: for example, even if solar
energy is widely studied and there are a lot of technologies available to capture it, it
would not be optimal to use it in an indoor environment, as we can see from the table
1.1 [1], which reports the main available choices for ambient energy sources.
The best strategy would be to base the solution on whichever energy source is
available at the specific application areas.
Energy Source Performance Conversion Efficiency
Ambient Light 100mW/cm3 (outdoor)
10µW/cm3 (indoor)
15-20%
Thermal 60-135µW/cm3 <1%
Vibration 4µW/cm3 (human motion)
800µW/cm3(industrial
machines)
25-50%
Wind 1mW/cm2 0.5-10%
RF Wide Range, varies with
distance from RF source
50%
Table 1.1: ambient energy sources.
Adopting an Energy Harvesting based solution can bring many advantages:
1- Improve Efficiency: this technique recovers energy that otherwise would be
wasted and eliminates the need for operations such as battery maintenance;
2- Reduce Environmental Impact;
3- Enable new technology.
Given that these types of systems rely on renewables, they are affected by the same
problems that are characteristic of the large-scale plants; particularly, the main
concern is that the energy source is of fluctuating nature. This can cause damage to
the electrical load in correspondence of peak energy values and impair its ability to
continuously run when the energy is not available, as is the case of a solar energy
based system at night. These aspects must be taken in consideration in the design
phase of an EH system, for example inserting an appropriate solution for the energy
storage.
In an EH system, there are generally four main components: energy collection and
conversion mechanism (the actual energy harvester), electrical power
management/conditioning circuit, energy storage device, and electrical load (for
example, a wireless sensor node able to transmit the collected data). In this thesis, the
focus will be on the first element. In fact, in order to optimize the design of the
application, the choice of the EH technique is crucial: a summary of the main options
is reviewed in the following section.
1.1 TYPES OF AMBIENT ENERGY SOURCES
1.1.1 Solar Energy
This is certainly the most known of renewable energy sources. Energy can be
extracted to power electronic devices using tiny photovoltaic cells. Commercial off-
the-shelf solutions already exist and can be classified into two categories: single
crystal, with a conversion efficiency ranging from 15% up to 40%, and the cheaper
thin film, whose efficiency is 13% at the maximum. Form table 1.1 it can be seen
that this technique is attractive because of its high-power density (100mW/cm3)
available for direct sunlight at noon; however, this number drops dramatically when
indoor. Hence, to guarantee the requested value of extracted power, either (or both)
high efficiency conversion solutions and proper energy storage devices must be
adopted.
It is worth stating that solar energy can be exploited also by means of heat carried by
the solar radiation.
1.1.2 Wind Energy
Like solar energy, wind energy is a subject that has been (and still is) widely studied
in relation to large-scale technologies, but unlike the former, despite its relatively
high power electric density, it has not been developed much for small-scale
applications. The few attempts that can be found in literature are oriented towards
design of small sensors that are able to work in remote and/or hostile environments,
more than for being embedded in the devices we use every day.
The investigated methods for wind energy conversion up to now take inspiration
from their large-scale counterparts, and consists of miniaturizing structures such as
windmills and anemometers in order to exploit the near linear relationship between
wind speed and frequency of rotation. The advantage of this choice is that it can rely
on years of research: this does in fact yield to an increased operational lifetime and
harvested power for devices that employ this solution; however, it comes at the cost
of a large size, higher energy conversion loss and a high overall cost. For this reason,
more recent research efforts [2] have been focused on integrating the existing
architectures with piezoelectric materials in order to obtain the same quantity of
harvested power but reducing dimensions and increasing conversion efficiency. The
main problem with this approach is that the robustness that usually is associated with
wind harvesting solution is reduced due to the great strain applied on the
piezoelectric, which deploy easily with stress and age.
1.1.3 Thermal Energy
Theoretically, thermal energy would seem like the better solution when designing
devices targeted to the industrial environment as wasted heat is present in abundance
in all types of system. Conversion can be based on Seebeck effect and
thermocouples, or exploiting the Pyroelectric effect (i.e. the property of certain
materials to present a temporary voltage following a temperature change). The
conversion efficiency of a thermal energy harvesting device is in any case related to
Carnot’s law:
� =���� − ����
����
Where ���� and ���� are the extreme values of the temperature gradient to which
the device is subjected, expressed in Kelvin.
Application of Carnot Law usually yields higher values of efficiency compared to the
actual ones, because of the characteristic efficiencies of the single devices that must
be taken into account that are well below the simple Carnot’s rule. Hence, at least for
now, the attainable electrical power density turns out to be a small fraction of the one
the material can offer with Carnot efficiency.
1.1.4 Radio Frequency
In the modern environment, there are multiple wireless sources of different
frequencies radiating in all directions, such as TV, mobile phone, WLAN, etc. So, it
is easy to think that they would made a good source for energy harvesting purposes.
For the conversion, an antenna or an array of antennae must be used: hence,
frequency selection is an important consideration to be made in the design phase.
The common choice would be GSM, as mobile phone signals are prevalent and
propagate well both in and out of buildings. Also, the distance from the RF source is
a very important parameter, since the antenna dimension depend on it. The extracted
power range is wide as it is dependent on the aforementioned parameters. As an
example, GSM900, yields about 7µW/cm3.
1.1.5 Vibration
Mechanical vibrations are attractive to use as a power source for low-power
electronics thanks to their power density and abundance in real environments. Since
the device object of this work exploits this type of energy, its characteristics will be
analysed more in detail in the following section.
1.2 ENERGY HARVESTING FROM VIBRATION
Mechanical vibration can come in various forms: seismic noise, human and vehicle
motion, vibrating systems such as electrical machinery. They can be frequency-
specific, like the case of rotating machines but they are more frequently random.
Vibrational Energy Harvesting is the conversion of kinetic energy present in the
environment into electrical energy. Actually, this conversion is articulated in two
steps: first, vibrations coming from the source are converted into the relative motion
between two elements, using systems such as mass-spring or cantilever beam: this is
needed because ambient vibrations are generally low in amplitude, so through this
step we exploit the resonance phenomenon of the oscillating mass-spring or
cantilever system, increasing the output harvested power. Then, there is the
conversion into electrical energy via electro-mechanical transducer. A traditional
mechanism of this type can be schematized as it shown in figure 1.1:
Figure 1.1: a simple energy harvesting system
In terms of energy balance, the input kinetic energy contained in the base vibration is
transmitted to the mechanism part of the energy is stored thanks to the mass-spring
dynamics, part is dissipated and part is converted into electrical energy. There are
different kinds of dissipative effects that can be relevant for a vibration harvester,
such as internal friction of the elements in the system and viscous friction due to the
mass moving within a gas, which contrast the mass movement; these phenomena are
taken into account with the presence of the damper.
As for the transducer, it is simply represented by a block with an input (mechanical,
from the movement of the mass) and two output terminals accompanied by signs plus
and minus, thus indicating the existence of a voltage difference V (electrical output).
It can be implemented according to a particular conversion technique: these
VIBRATING BODY
k
d
m
OSCILLATOR
TRANSDUCER
+ -
techniques can be classified into three groups, basing on the physical principle they
exploit:
1. Electrostatic: they use a variable capacitor so to generate charges from the
relative motion between the capacitor’s plates. They are well suited for
MEMS (Micro Electro Mechanical System) level type of application, have a
good output voltage and are long-lasting. However, they need an external
power source and yield relatively low power density at small scale.
2. Electromagnetic: based on electromagnetic induction, they exploit the
relative motion between a coil and a magnetic mass; as opposing to precedent
group, these devices do not need an external power source, but they are
inefficient at MEMS scale for they require a large mass displacement
3. Piezoelectric: based on the use of piezoelectric materials, they are well
adapted to miniaturization without the need of an output voltage source, but
they can be very expensive and are affected by ageing.
In particular, we will focus on the third category. Before going into detail with a
possible configuration, let us illustrate the basic characteristics of piezoelectric
materials.
1.2.1 Piezoelectric Materials
Piezoelectric transducers are based on the property of piezoelectric materials of
accumulating charges when stressed (direct effect) and to strain in case of the
application of an electrical signal (inverse effect). This property arises from the
presence of asymmetry in the crystalline lattice-like ionic structure of the material,
resulting in the presence of polarized dipoles: when an electrical field is applied, they
are displaced by electrostatic forces, with a subsequent mechanical deformation of
the whole piece.
This effect occurs naturally in quartz crystals, but can be induced in other materials,
such as asymmetric polymers and specially formulated ceramics consisting mainly of
Lead, Zirconium, and Titanium. Regarding the latter case, it must be stated that these
types of materials show an initially random orientation of dipoles leading to a null
polarization. Hence, in order to obtain a preferential polarization axis, a polarization
process is required.
The material is first heated to its Curie temperature. Then, a voltage field of a
sufficient strength is applied in the desired direction, forcing the ions to realign along
the excitation field axis. Finally, keeping the field constant, the material is cooled
down to its original temperature.
Figure 1.2: domain alignment in LZT [3]
In a piezoelectric material, mechanical and electrical quantities are related according
to the following equations:
� = �� + ���
� = ��� + ��
Where:
o D is the Electric Flux Density;
o T is the Stress;
o S is the Strain;
o E is the electric field;
o �� is the electrical permittivity for a constant stress T;
o �� is the compliance (inverse of Young module) for an applied field E;
o d is the piezoelectric constant, expressed in [C/N].
In real devices, the above relations must be considered for the six possible axis: 3 for
stress due to compression/expansion and 3 for torsional stress.
Figure 1.3: stress axes.
[��] = ���,��[��] + ���,��[��]
[��] = [��,�][��] + [��,�][��]
With
��, � = 1, … ,6
�, �, � = 1,2,3 ��,� = ��,� ; ��,� = 0 ��� � ≠ �
Where the first index indicates the polarization axis and the second the stress
direction.
So, the two equation model the polarization and the strain of the material,
respectively. As we can see, both are the result of the sum of mechanical and
electrical contribution. For a non-piezoelectric material, of course, the strain would
depend only on mechanical quantities while the polarization only on electrical ones.
In particular, d, which ties the material polarization with the applied stress, is a
parameter whose evaluation is crucial in order to choose the right material for the
desired energy harvesting application. Other important characteristic material
quantities are:
o ��, the piezoelectric voltage coefficient, defined as �
�� , which ties the voltage
V across the electrodes (separated by gap l) with the stress, as shown by the
following equation:
� = ����
o k, coupling factor, which describes the conversion between mechanical and
electrical energy, and can be defined in terms of the other parameters, as
���
����
Some properties for common piezoelectric material are reported in table 1.2.
Crystal
(Quartz)
Ceramic
(Barium
Titanate)
Polymer
(Polyvinylidene
Fluoride)
Density [Kg*m-3] 7.5 5.7 1.78
Relative Permittivity
��/��
1200 1700 12
Piezoelectric Constant
��� [C/N]
110 78 23
Voltage Constant
��� [V*m/N]
10 5 216
Electromechanical Constant ��� 30 21 12
Table 1.2 [3]: Piezoelectric properties.
1.3 PIEZOELECTRIC ENERGY HARVESTER: LINEAR VS NONLINEAR APPROACH
Going back to the system presented in figure 1.1, we can easily derive a
mathematical model of its mechanical behaviour referring to the classical Newtown
equation of motion:
�� +��(�)
��+ �� = �(�) (1.1)
Where:
o � is the mass;
o �, � and � are the mass displacement, velocity and acceleration, respectively;
o �(�) is the potential energy of the system;
o �� is the dissipated energy modelled by the damper;
o �(�) is the stochastic source modelling the external input mechanical
vibrations.
Particular attention must be focused on potential energy; its expression depends on
the geometry of the device and its dynamics. In this case, it is related to the presence
of the spring, hence is linear and equal to �
����. This makes the device fall into the
category of linear energy harvesters. Another device of this group, much more used,
is the so-called cantilever beam configuration, schematized below, in figure 1.4:
Figure 1.4: cantilever beam architecture for energy harvesting
Δx
+
-
PZT
Vibrating Wall
In this case, the bending of the beam is the equivalent of the movement of the spring:
in particular, the position of complete bending of the beam corresponds to a complete
extension/compression of the spring.
Being linear, both system show resonant behaviour: in short, resonance is a
phenomenon that occurs when the system is driven by an external force whose
frequency is equal to the resonance frequency of the system, and it maximizes its
response (figure 1.5).
This means the system would be efficient only when incoming energy is distributed
around its resonant frequency.
This is not a desirable feature when dealing with vibration energy harvesting
because, as previously stated, except for very specific cases (i.e. devices mounted on
rotating machines), vibrations are distributed over a wide frequency spectrum, with
significant predominance of low frequency components, which are very difficult to
deal with, especially if we want a small device.
Figure 1.5 Resonant EH frequency response.
100
101
102
103
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
Magnitude (
dB
)
System Response
Frequency (rad/s)
For this reason, research efforts have been aimed in two different directions:
1- design of linear devices whose resonant frequency is appropriately “tuned” to
better match the spectral distribution of the available energy in the application
environment, or
2- design of a nonlinear device.
As for the first approach, the idea is to tune the resonant frequency of the device
according to the specific application. This can be done by acting on device
characteristic parameters such as the length or the stiffness of the cantilever beam, or
of the spring. However, this is not a good solution because ambient condition might
change, thus the device would need to be retuned, and this is not easy to do, for
resonant frequency depends on its physical parameters, and changing them in most
cases would mean to completely redesign the harvester.
Based on the discussion made so far, it has become clear that linear resonant
vibration harvesters present drawbacks that limit seriously their application: they can
harvest energy only in a narrow frequency band, and to increase the efficiency, this
band would have to become narrower, as we would need to amplify the oscillator
frequency response. One of the proposed solution for this problem was to use
oscillator arrays in order to cover a large frequency range by overlapping their
responses. However, while this method does widen the frequency spectrum, the
overall harvested power is decreased. In addition, there are problems in maintaining
the resonant behaviour when scaling down the dimension. All of this implies that this
kind of device might not be the better solution for harvesting vibration, which bring
us to the second point of the above list: the nonlinear harvester.
This is not a well-defined category, but intuitively, we could say that the search for
the best solution in terms of non-resonant systems should start from the potential
energy function �(�) which can be considered one of the most important part of the
design for it acts as a dynamical energy storage facility (before transduction) for the
mechanical oscillator. We already know that the proposed potential function must
respect the condition:
�(�) ≠1
2���
Meaning nonlinear devices are all those whose potential energy is not quadratically
dependent on the displacement variable of interest. In this regard, various research
papers have investigated possible different solutions. Specifically, it emerged that the
better performance is given by oscillators which possess either a bistable or a
monostable behaviour.
Concerning the device topology and structure, nonlinearities can be introduced in
various ways: changing the geometry of the springs/cantilever beam, inserting
material nonlinearities, inducing stress or electromagnetic coupling with external
fields.