Introduction
The
advances in low power electronics, and wireless sensor networks (WSNs) in
particular, have driven numerous researches in the field of energy harvesting
in the past decade. A wireless sensor node consists of low power
microcontroller unit, radio frequency transceiver and microelectromechanical-
(MEMS-) based sensor. The task of each node is to collect and transmit data to
the outside world via a radio link. Thousands of spatially distributed wireless
sensors can be developed which can be embedded virtually anywhere in civil
structures, bridges, or in the human body. WSN technology has gained increasing
importance in industrial automation, structural health monitoring, healthcare,
agriculture, and civil and military applications. Traditionally, batteries are
used as the electrical energy power sources to power wireless sensors and
embedded electronics. However, batteries have a limited life span and they are
expensive to maintain and hence they are not a long-term viable source of
energy for WSNs and embedded systems. Piezoelectric materials can be used to
harvest this energy since they have the unique ability of converting mechanical
strain energy into useful electrical energy.
Piezoelectric energy harvesting
devices—in the form of MEMS generators or nanogenerators—are a novel technology
that is a reliable alternative energy source for powering wireless sensor
devices. Unlike conventional MEMS generators, nanogenerators have an added
advantage of being flexible and foldable power sources which is ideal for applications
such as implantable biomedical sensors.
This
paper discusses the recent advances in micro- and nanoscale energy generation
using piezoelectric materials for ultra low power sensor applications.
Piezoelectric Energy Harvesting from Vibrations
Piezoelectric Materials
Piezoelectricity
stems from the Greek word “piezo” for pressure and the word “electric” for
electricity. When a force or stress is applied to a piezoelectric material, it
leads to an electric charge being induced across the material. This is known as
the direct piezoelectric effect. Conversely, the application of a charge or
electric field to the same material will result in a change in strain or
mechanical deformation. This is known as the indirect piezoelectric effect. It
is the direct piezoelectric effect that is employed in energy harvesting.
Examples of ceramics which exhibit the piezoelectric effect are
lead-zirconate-titanate (PZT), lead-titanate (PbTiO2),
lead-zirconate (PbZrO3), and barium-titanate (BaTiO3). To
date, the most commonly used piezoelectric ceramic is PZT mainly because it has
very high electromechanical coupling ability. However, PZT is an extremely
brittle material and hence this presents limitations to the strain that it can
safely withstand without being damaged . Polyvinylidenefluoride (PVDF) is
another common piezoelectric polymer which is more flexible and can be employed
in energy harvesting applications .
Piezoelectric MEMS Generator
System
In
piezoelectric energy harvesting from vibration, a mass is suspended by a beam,
with a piezoelectric layer on top of the beam. When the mass vibrates, the
piezoelectric lever is mechanically deformed and a voltage is generated. The
most common energy harvesting systems are cantilever structures that are mainly
designed to operate at their resonance frequencies. Such structures (unimorph
or bimorph cantilevers) are popular because they enable relatively high stress
levels on the piezoelectric material while minimizing the dimensions of the
devices .
Piezoelectric Nanogenerator System
The insulating properties of piezoelectric insulator
materials do not permit carrier transport from metal electrodes into the
insulating active materials. As a result, the nanogenerators fabricated from
these materials produce alternating (AC) power. On the other hand, the power
generation mechanisms of nanogenerators fabricated from piezoelectric
semiconductor materials produce both AC and direct power (DC). The coupled
semiconducting and piezoelectric properties are in essence responsible for the
DC and AC power, respectively. When piezoelectric semiconducting nanowires are
subjected to an external force perpendicular to the nanowires, a piezoelectric
potential is generated along the nanowires owing to the relative displacement
of cations with respect to anion under uniaxial strain . DC power from the
nanogenerators is attributed to the force exerted perpendicular to the axis of
superconducting nanowires; as a result, the nanowires bend laterally. The
generation of direct current (DC) from piezoelectric semiconductor
nanomaterials is attributed to the force exerted perpendicular to the axis of
the semiconducting nanowire—typically by subjecting a vertically grown nanowire
to the laterally moving tip. When a nanowire is subjected to an external force
by the moving tip, deformation occurs throughout the nanowire and an electric
field is created inside the nanostructure due to the piezoelectric effect. A
piezoelectric potential is generated along the width of NW owing to the
relative displacement of the cations with respect to anions. The stretched part
with the positive strain will exhibit a positive electrical potential, whereas
the compressed part with the negative strain will show a negative electrical
potential. As a result, the tip of the NW will have an electrical potential
distribution on its surface, whereas the bottom of the NW is neutralized
because it is grounded.
Energy Harvesting and WSN Operation: Challenges and Opportunities
Energy Harvesting and Wireless
Sensor Nodes Operation
Ambient
mechanical vibrations are harvested and converted to useful electrical energy
which is either stored in a storage element or is supplied directly to the
load. Energy storage is a key element of the energy harvesting system because
it is a bridge of stability between the energy source and the load that
provides a constant energy flow from an otherwise variable environmental source.
The power interface circuits condition the harvested energy to enable the
charging of low capacitor batteries or supercapacitors and also provide
compatibility with the load requirements. For a sensor node fully powered by
ambient energy, the generated mean power must be greater than or equal to
the mean consumed power.
Duty Cycling and Advanced Power
Management Techniques
As
discussed earlier in sections, most embedded sensor systems support sleeping
modes, making a direct approach to duty cycling an attractive choice to power
management. This direct implementation of the duty cycling technique, though
popular in energy harvesting for wireless sensor networks, is not always the
best choice. Other than being a too simplistic approach, a fixed duty cycle
implies that if the energy source is supplying more energy than is being
consumed, the system will waste excess energy once the storage reservoir is
fully charged. To get over some of these challenges, advanced power management
techniques with strict hardware specifications have been proposed. These
techniques are dynamic voltage scaling (DVS), dynamic frequency scaling (DFS),
and a combination of DVS and DFS.
The
operation principle of DVS technique is that increasing a circuit’s voltage
allows it to switch faster, but with an increase in energy consumption, and
conversely the decrease in circuit voltage causes the circuit to have a low
switching time with an accompanied decrease in energy consumption. A similar
phenomenon is observed when the clock frequency is increased or decreased and
is the basic premise of the DFS technique. Unlike duty cycling where all tasks
stop during the sleeping mode, the advanced power management techniques allow
the system to keep running at a low pace and with reduced energy consumption,
without compromising the execution of important tasks . Besides DBS, DFS, and
duty cycling, the maximum power point tracking (MPPT) technique may be applied
in the energy harvesting system. In the MPPT technique, the object is to
transfer maximum power to the load. This technique is traditionally used in
solar systems which have a dynamic voltage-current characteristic where the
optimal load for maximum power depends on the operating point. The technique
requires constant monitoring of incoming energy, to determine the optimal
operating point, and an adaptive load.
Fundamental Material Issues
The
performance of a piezoelectric energy harvesting systems primarily depends on
the piezoelectric properties used to fabricate the generators. Generally, thin
film piezoelectric materials show better piezoelectric properties compared to
bulk piezoelectric materials. The use of single crystals and nanomaterials
(nanowires) has, in principle, improved the power density and energy conversion
efficiency hence the advance the miniaturization of device size while
maintaining a reasonable power output. Despite great research efforts on these
nanomaterials, there is lack of fundamental scientific understanding of and
experimental research on piezoelectric and flexoelectric effects in single
crystalline nanowires. This lag in research at this fundamental level
compromises fidelity of the mathematical algorithms used in modeling and
predicting the piezoelectric potential, mechanical to electrical energy conversion
efficiency and device material optimization.
The
other challenge relates to the coupling of piezoelectric and semiconducting
effect—resulting in the so-called piezotronic effect. The scientific
understanding of the interaction of electron distribution and semiconductor
band structures requires additional research efforts. The research will
potentially present an opportunity to facilitate in situ rectification of the
potential output by making use of the Schottky barrier formed between ZnO and
metal electrodes. While single crystal materials offer better piezoelectric
performance and give better power density compared to their bulk material
counterparts, costs of these materials are still very high and at times very
inhibitive. The current fabrication methods and the associated device
integration techniques at nanoscale are not yet suited for large scale
processing, and research efforts along this line will substantially reduce
fabrication costs and help translate piezoelectric energy harvesting from mere
experimental curiosity into real engineered device realisations to power
wireless sensors.
Design and Power Management
Issues
The
design of piezoelectric micropower generators and nanogenerators is in itself a
multidisciplinary area with challenges based in fundamental physics, material
science, mechanical engineering, and electrical engineering. Different
researchers from different discipline and background have reported several
researches in the area of piezoelectric energy harvesting. The multidisciplinary
approach and a holistic paradigm is perhaps the most promising way of designing
piezoelectric energy harvesting device.
As can
be observed from the review, there is still a need to improve the power output
of
piezoelectric generators to match the requirements of wireless sensor
devices. This challenge can be addressed by using piezoelectric material with
the best piezoelectric properties, the best device geometries, and the best
power electronics to condition and manage the power output. This is arguably
calls for a holistic design and optimization regime, together with an
established international metrology standard of piezoelectric energy harvesting
(which currently does not exist). Latest advances in synchronised switching
techniques have been reported as the latest achievements in power conditioning
interface circuits to date.
Conclusion
Vibration
energy harvesting using piezoelectric generators was discussed and its
potential as an alternative energy source for wireless sensor devices
overviewed. The maturity of piezoelectric energy harvesting as technology
entails that WSNs are energy efficient and their dependence on batteries is
limited. With advancement in ultralow microelectronics and ultra-low power
wireless microcontroller units, power consumption of sensor nodes is getting
lower and hence the harvested ambient energy may be sufficient to eliminate
batteries completely. In addition, piezoelectric nanogenerators open new
avenues for ambient power harvesting through foldable power options and
miniaturization of power packages thus enabling implantable medical sensing
capabilities. Energy harvesting using piezoelectric generators is an attractive
alternative energy source that has the potential to provide energy autonomy to
wireless sensor devices.
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