Special section: Polymers in energy harvesting
Energy harvesting is the scavenging of energy from sources that would normally be lost to the environment, for example solar, thermal, wind and kinetic energy. Despite more than 10 years of academic and industrial interest, it is still an emerging technology. Dr Steve Morris from the Materials KTN takes a closer look.
Public and media expectations can be heightened by high-profile reports of exciting applications of scavenging power to charge up mobile phones, laptops or even cities. And while it is perfectly reasonable to dream, it is equally important to disseminate a more widely trusted source of information that supports energy harvesting technology in perhaps less adventurous applications. To this aim, there are various national and international research programmes, networks and institutes focused on developing technologies, materials and systems that may eventually provide a unique solution to real industrial requirements that would benefit from locally sourced energy.
One of the most challenging issues for energy harvesting is the development of energy sources able to supply power to devices capable of communication of sensed data, for example wearable electronics or wireless sensor networks. These devices are often based on microelectromechanical systems (MEMS) technologies. MEMS are often very small and require little power, but their applications are limited by their reliance on battery power. Scavenging energy from ambient vibrations, wind, heat, light or biomechanical movement could enable smart sensors to function indefinitely. Several academic and commercial groups have been involved in the analysis and development of vibration-powered energy harvesting technology.
To date, most applications in energy harvesting have used materials such as metals, ceramics and composites. There has been less effort in development of polymer-based harvesting devices, despite the fact that they can be produced in huge volumes, with a wide variety of material characteristics, and MEMS devices can be made from polymers using processes such as injection moulding, embossing or stereo lithography.
One of the candidate polymers is polyvinylidene difluoride (PVDF), but there is little academic or industrial demonstration of such materials for scavenging applications. There are many reasons for this, including the material’s inherently low output compared to its ceramic counterparts. However, there are some very interesting developments that might provide opportunities for polymer deployment in energy harvesting. PVDF is lightweight, flexible, low cost, lead-free and can be produced in large sheets. The films have a high sensitivity to strain, are highly manufacturable, have a high dynamic range and inherently high bandwidth (non-resonant). The method of electrode attachment (sputtering or screen printing) is paramount to the device function.
Typical power densities available from energy harvesting devices are highly dependent upon the specific application (affecting the generator’s size) and design of the harvesting generator. In general, for motion-powered devices, typical values are a few µW/ cm3 for human body-powered applications and hundreds of µW/cm3 for machinepowered generators. Most energy scavenging devices for wearable electronics generate very little power.
Among several power generation options, piezoelectric vibration-based PVDF systems appear to be an attractive and practical solution, combining good power density and availability of primary source. Depending on mechanical vibration amplitude and material characteristics, piezoelectric devices should produce alternating voltages ranging from hundreds of millivolts to dozens of volts. The challenging task is converting small amounts of energy to effectively charge a battery or a capacitor, and then to use it for powering electronic circuits. It is clear that many parts of the flow of energy from ambient source to doing useful work presents us with a systems problem and this means a holistic approach is really the preferred development guide for future energy harvesting devices.
For piezoelectric materials, if the direction of the applied force is the same as that of the piezoelectric element’s polarisation, the corresponding power generated is always positive. However, generally, mechanical energy can only partially be converted into electrical energy, and if this electrical energy is simply rectified using one diode, then during a cycle of the stress exertion on the piezoelectric element, only half the available mechanical energy is harvested. Using full-wave rectification, however, a complete cycle of mechanical energy may be converted into electricity.
Richard Brown of Measurement Specialities explained that their 28μm thick piezo PVDF film, when stretched to 1%, gives 5mJ/cm2 per cycle electrical energy. While this only represents a 0.25% efficiency, Richard said, ‘It is just about possible to flex piezo film to power up Bluetooth for a single message’. The technology is scalable in that multiple lay-ups using conventional polymer processing may achieve even higher outputs for a given mechanical excitation.
A hybrid piezoelectric filament/fibre textile with the ability to convert mechanical energy as well as solar energy to electrical energy has been developed at the University of Bolton by Professor Elias Siores. Existing piezo fibres are based on ceramics, which are inflexible and fragile. Siores’ technology is based on extruded monofilaments or ribbons of PVDF that are formable and therefore less susceptible to mechanical breakage. A secondary spun-coat layer of organic photovoltaic material provides for solar capture yielding an energy harvesting hybrid structure with a weavable form suitable for clothing and other textiles. The material is lightweight, easy to produce and is ideal for use in in regions with little sunlight. The material is light source activated and has a lifetime of around three years. Under normal conditions the energy output from 20cm2 of piezo fibres is about 1W, but testing of the material under humid, wet and windy conditions is needed.
Working in with UK’s National Physical Laboratory, Dr Joe Briscoe from Queen Mary University of London is developing ZnO nanorods as a piezoelectric material. Forming orientated ZnO nanorods on PET film can produce a material that when bent generates current that can power micro devices or charge batteries. The technology is highly scalable and Dr Briscoe has developed processes to produce large area devices. The materials’ characteristics are highly non-linear, and the functional materials research team at NPL has provided metrological tools to better define the performance of these novel materials. Dr Briscoe states that energy densities of mW/cm2 may be achievable in the long run, making this technology suitable for niche energy harvesting applications such as sensor networks or applications demanding conformal coating processes.
As well as piezoelectric devices, electroactive polymers (EAPs) have been proposed for harvesting energy. These polymers have a large strain, elastic energy density, and high energy conversion efficiency. The total weight of systems based on EAPs is proposed to be significantly lower than those based on piezoelectric materials. EAPs are used for actuation and are typically acrylic or silicone membranes with thin films being used for lower voltage. Traditionally, research has been dominated by the materials’ unique muscle-like properties for robot artificial muscles or artificial organism applications. The theory behind elastomer-related energy harvesting is that you need to put energy in to get energy out and generate a voltage, as elastomers can stretch further than human muscle and 0.4J/g energy is possible from these EAPs. Conversely for actuation, (for example, robotics mimicking human muscle) the operating principles are based on an electric-field induced strain from forces set up between two parallel plates in a capacitor. The University of Bristol Robotics Laboratory printed an energy harvesting circuit onto a soft polymer, meaning its harvesting circuitry is now fully elastic – an exciting development. Ways to maximise performance by developing new materials and engineering systems pose several research challenges, including reducing resistive losses in electrodes and leakage currents in films, reducing EH circuit losses and developing processes to better control the quality of thinner membranes.
Piezo technology is good in terms of being compatible with electronic circuits (it inherently produces high voltages and low currents) but suffers from inefficiency and fatigue. The key to maximising energy generated is to control the damping of an oscillating device and the rectification that is needed to convert the AC to DC voltages. It is possible to optimise circuits by preloading them with a bias voltage. The extra energy generated when the piezo material is further strained can then be used to charge a battery or capacitor. Dr Paul Mitcheson of Imperial College London has led the development of complex switching circuits that more effectively harness the energy scavenged and deliver that to the load. Theoretical limits to the increase in delivered power has been shown to be of order 8Q/p, where Q is the quality factor of the circuit resulting in improvements of up to 10 fold compared to conventional rectification circuits. Work undertaken at Imperial College means more power output is retained from piezo devices than would usually be gathered. Improving the energy harvesting efficiency is achieved by producing optimal damping and rectifying signal simultaneously, however there is potential for further work in this area.
The forthcoming Framework 7 call relating to energy harvesting opportunities concerns materials solutions for durable energy harvesters. FP7 call: Innovative materials for advanced applications 2.2 NMP.2013.2.2-4 is a two-stage process and is focused on self or low power applications, the development of energy harvesting and storage materials. It will cover multi source harvesting, (excluding photovoltaics) and will be multi disciplinary, although industrial participation is essential.
This feature is based on information presented at a UK’s Materials KTN and IOM3 Smart Materials & Systems Committee event that took place in summer 2012.