Interview conducted by G.P. Thomas.
Fred Pimparel, Engineering & Development Manager at Morgan Advanced Materials, talks to AZoM about the recent advancements in piezoelectric energy harvesting and the potential benefits of using this as an energy source.
GT: Could you please give a brief overview of the concept of piezoelectricity and how it is produced?
FP: Piezoelectricity is the property possessed by some materials of becoming electrically charged when subjected to a mechanical stress. Such materials also exhibit the converse effect i.e. the occurrence of mechanical deformation on application of an electric field.
Certain compounds can be made piezoelectric by the application of a high electric field (polarisation) - these are termed ferroelectric materials. Another important group of piezoelectric materials are the piezoelectric ceramics, such as PZT. The PZT ceramics are solid solutions of lead Titanate (PbTiO3), and lead Zirconate (PbZrO3), modified by additives.
The PZT can be manufactured into components of almost any shape and size. As well as possessing strong piezoelectric properties, PZT is hard, strong, chemically inert and completely unaffected by humid environments. Before polarisation the dipoles in the ferroelectric material are randomly oriented. The polarisation process involves the application of an electric field across the ceramic, usually at an elevated temperature, causing switching or realignment of the dipoles.
GT: How is power generated from the piezoelectric effect and what key factors govern the performance of this power?
FP: In the case of piezoelectric generators, the energy conversion takes place via the direct piezoelectric effect where a voltage is induced or charge flows onto the electrodes of a piezoelectric material when a stress is applied to it.
The amount of electricity generated is dependent on the amount of force used in compressing or deforming the material, the amount and type of deformation of the material’s crystal structure and the speed or frequency of compressions or vibrations to the material.
Energy conversion is at its highest when a maximum deformation or strain of the piezoelectric material is applied within safe operating limits. An important parameter governing the transduction from mechanical energy to electrical energy is called the coupling factor, defined as keff=electrical energy OUT/mechanical energy IN, and is related naturally to the materials intrinsic piezoelectric and electromechanical properties.
High efficiency is related to a material having a high coupling constant, but there are other factors that need to be taken into account, for example the material and electrode geometry as well as frequency and amplitude of the mechanical stress.
GT: Why has piezoelectric energy traditionally been seen as potentially unreliable or unable to meet demands? How and why has that changed recently?
FP: An early challenge with piezoelectric materials was that outside of the optimal resonance they were prone to breaking relatively easily. Unless this is controlled by dampening, pre-biasing or the careful selection of protection materials, it can lead to rapid failure in the field.
As a result, they currently play a small part of the energy harvesting market today but they have been used in high numbers for simple barbecue lighters (where a small piezoelectric rod is rapidly compressed to generate a high voltage electrical arc) rather than being used to store the generated power to run electronic devices.
Other challenges with using piezoelectric materials include; matched impedance coupling of the energy source and transfer from the transducer to the recovery electronics. Impedance matching is required in electrical circuits to optimise the energy transfer efficiency and many approaches are available in the case of piezoelectric devices, but often our customers need guidance in this area.
The research motivation in this field is due to the reduced power requirement of small electronic components, such as the wireless sensor networks used in passive and active monitoring applications.
When considering the power duty cycle, enough power needs to be generated over a period of time to satisfy the demand on energy requirements. Furthermore, energy sources are rarely continuous which makes this task harder. Mechanical vibrations can vary in frequency, so the design of the transducer element and accompanying charge control electronics is critical to success. So whilst you can tune a harvester to work at a particular resonant frequency, you lose the efficiency if this frequency changes due to background interference.
The ideal solution is a broadband harvesting device which receives energy from a variety of frequencies simultaneously but at the moment no such product exists. I know this is currently the focus of a lot of academic research, where for example magnets are used to try and limit the vibrations within the main resonance of the device, which adds bulk and cost and does not enable true broadband ability.
Our ambition is to take some of this academic work and create a broadband piezo-solution that we can turn into a commercial product at low cost over the next two to three years.
GT: Could you highlight some recent innovations that utilise piezoelectric energy harvesting?
FP: MicroGen Systems Inc., a company located in USA has recently released its first piezoelectric MEMS product line to be used as a vibration energy harvester or micro-power generator (MPG) with 100, 120 Hz and custom resonant frequency response. The MPG is a small volume (1.0 cm3), wafer-level packaged device. MicroGen’s MPGs are potentially low cost, long lifetime (estimated > 20 years) devices that scavenge otherwise wasted ambient vibration energy able to replace or extend the lifetime of batteries in wireless sensor networks (WSN’s) and other microelectronic applications.
Arveni, a start-up company, based near Grenoble, France, has demonstrated a battery-less remote control. They also develop piezoelectric energy harvesting devices for a wide range of applications, from vibration to pulse harvesting. The innovation relies on mechanical structures of new piezoelectric materials being able to supply up to ten times more pulse energy harvesting compare with coil/magnet type solutions and innovative electronic architectures and design able to manage and recover up to fifteen times higher power in vibration energy harvesting compared with commercially available dedicated Energy Harvesting ICs, under similar conditions.
GT: Could you please go into more detail regarding the possible environmental benefits of piezoelectric energy harvesting?
FP: Energy harvesting has environmental benefits, for example the reduction of chemical waste produced by replacing batteries and potential monetary gains by reducing maintenance costs. If this can be achieved, the requirement of an external power source as well as the maintenance costs for periodic battery replacement and the chemical waste of conventional batteries can be reduced significantly and detoxify mainstream electronics.
This particularly applies to high volume applications and the potential to use a piezoelectric energy harvester for tyre pressure monitoring system (TPMS).
GT: How does piezoelectric energy harvesting compare to other energy harvesting technologies, such as thermoelectric or electromagnetic as a source of energy?
FP: The best harvesting technologies in terms of power density and/or efficiency, life and cost per watt are electromagnetic, photovoltaic, thermoelectric and piezoelectric. The reason for piezoelectric energy harvesting attracting so much new development includes its high power density, its superior efficiency as summarised in the below table and its suitability for vibration and other motion – ambient power forms that are omnipresent.
Power Density [mW/cm³]
*= Technologies with no moving parts
Thermal energy harvesting uses temperature differences or gradients to generate electricity. Efficiency of conversion is limited by the Carnot efficiency. The efficiency of thermoelectric generators is typically less than 1% for temperature gradient less than 40°C and it is hard to find such temperature gradient in the normal ambient environment.
Due to the low cost of photovoltaic modules, and the fact that light of sufficient intensity is present in most environments, this form of energy harvesting has become dominant. Photovoltaic modules may be used both outdoors (with sunlight) and indoors (with artificial lighting). While different types of module work better with certain types or intensities of light, the maximum efficiencies of commercial modules are generally in the region of 15%. Some modules with much higher efficiencies have been demonstrated in the lab, but the costs of these are prohibitive at present.
GT: Does piezoelectric energy harvesting have good energy efficiency and if so why?
FP: An important parameter governing the transduction from mechanical energy to electrical energy is called the coupling factor. High efficiency is synonymous to a material having a high coupling constant but there are other factors which play an important such as material and electrode geometry, frequency of operation and scale, electro-mechanical losses, frequency response, amplitude of the vibrations and acceleration and lastly the output load. For optimum efficiency, engineering design of the device must go through several iterations of analytical and functional modelling and careful consideration of piezoelectric material and the geometry must be observed.
GT: What properties must a material have to make it suitable for piezoelectric energy harvesting?
FP: A material suitable for piezoelectric energy harvesting, often referred to as a high-energy density material, is characterized by the large magnitude of product of the piezoelectric voltage constant (g) and the piezoelectric strain constant (d) given as (d.g). The condition for obtaining large magnitude of d.g has been shown to be as |d| = εn, where ε is the permittivity of the material and n is a material parameter.
The selection of a piezoelectric ceramic composition for a particular application is dependent on parameters such as operating temperature range (−20 ≤ T ≤ 80°C), operating frequency range (10–200Hz), external force amplitude (0.1–6N), and lifetime (>106 cycles).
The operating temperature range is determined by the Curie temperature of material which for most of the Pb(Zr, Ti)O3 ceramics is greater than 200 °C.
GT: The most commonly used piezoelectric material used for energy harvesting is ceramic lead zirconate titanate (PZT) – what key advantages does this have over other usable materials?
FP: There are two extreme cases of the high-energy density material, PVDF piezoelectric polymer (typical d33 = 30 pC/N, ε33/εo = 13, g33 = 280×10−3 m²/C), and single crystals such as PMN – PT30 (typical d33 = 2200 pC/N, ε33/εo =6250, g33 = 40×10−3 m²/C).
It can be seen from this data that piezoelectric polymer has the highest piezoelectric voltage constant, g33, of 280 × 10−3 m²/C and single crystals have the highest product (d33.g33) of the order of 88×10−12 m²/N. However, the synthesis of both single crystal materials and polymers in large volume is challenging and expensive.
Thus, for mass applications, current focus is on improving the properties of polycrystalline PZT ceramics. Furthermore, PZT has proven its robustness in industrial applications for more than a half century, and it has several key advantages in the creation of compact, solid-state piezoelectric ceramic devices without moving parts and offering long-term reliability even in harsh environments. Relative to other piezoelectric materials, PZT has a high conversion efficiency and exceptional temperature stability. Moreover, the design versatility of PZT is also an important advantage: it can be developed into almost any shape and combined into composite structures with polymer filler.
GT: How has Morgan Advanced Materials been involved in the field of piezoelectric energy harvesters?
FP: I have had a personal interest since I joined the business 13 years ago. I remember reading the first paper from MIT featuring a piezoelectric patch that could go onto a shoe and send a small wireless signal to a computer.
Interest in energy harvesting within the organisation started following the acquisition of a Netherlands based business from Phillips in 2000, which manufactured tyre pressure monitoring systems. This presented the first potential high volume commercial product that could generate an attractive return on our investment. The business was relocated to the UK in 2008 and following the move we started developing the first prototypes.
GT: What does the future hold for piezoelectric energy harvesters?
FP: The traditional inorganic lead zirconate titanate PZT is still the most commonly used material for piezoelectric harvesting but many alternatives are receiving some attention, usually where efficiency and temperature performance of the material itself is not the primary consideration but factors such as flexibility and light weight come to the fore.
Micromechanical electromagnetic systems (MEMS) involve micron-scale mechanical and electrical components with moving parts. When used for piezoelectric energy harvesting, these parts are made to dimensions that resonates at the target dominant frequency – much like a tuning fork.
The simplest configuration for these device is a cantilever beam, where induced vibration causes mechanical strain and deformation in the cantilever arms leading to a generation of electric charges which are harvested from the device. These devices have the dual dimensional advantages of MEMS type sensors as well as being sensors in their own right, and being manufactured in the same fabrication environment and processes as with silicon technology, leading to easier integration and subsequently market adoption and growth.
Piezoelectric cantilever energy harvesting devices also have integrated proof mass attached; this helps to tune the vibration to the required resonant frequencies. The proof mass and subsequent beam shapes can and do have an effect on the damping and amount of energy harvested, which can be challenging as low frequencies are the most dominant in the natural environment. Such MEMS type devices may be created in thin film, then arrayed in either series or parallel to create the same effect as a proof mass, with the parallel systems reported to be favoured because it requires lower matched load resistances for maximum power output.
There are many other methods of creating piezoelectric materials and a lot of research effort is being invested into alternative methods; some of which are thin film piezoelectric (dependent on definitions MEMS type devices may also fit into this category)which have roles both as an energy harvesting device and as an electro-active polymer when electrically reversed.
Screen-printing thin and thick films is an interesting method of depositing piezoelectric materials. Thin films piezoelectric and ferroelectric films are being investigated at the Southampton University by Dr. Steve Beeby for vibration energy harvesting.
In a bid to get the most of PZT, new dimensional properties are being investigated such as the use of circular and multi-layered structures, also new forms are being researched such as the use of PZT ribbons that are created to be introduced into non-rigid materials such as clothing or biocompatible substrates; able to conform not just to irregular curved surfaces, but also possess stretch-ability and flexibility.
About Fred Pimparel
Fred has worked in the field of manufacturing of piezoelectric ceramics industry and applications for the last 13 years, in a variety of project management roles, product development, and lead technical and engineering positions.
He specialises in the fields of piezoelectric energy harvesting, hydrophones, piezoelectric actuators, ultrasonic transducers, electronic driving circuits and software programming using .NET for IEEE GPIB and RS232
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