Professor Ray Baughman from the University of Texas at Dallas discussed in his keynote presentation the bi-scrolled yarns developed by his team. They are used, among other applications, for making woven flexible supercaps and batteries (ratio of materials: 90-98% LiFePO4, 10-2% multi-walled nanotubes) for electronic textiles. These yarns can also be used for a variety of other applications, such as superconducting cables, accessible contained hydrogen storage powder for fuel cells, heat storage textiles and electrodes for electronic textiles.
Another topic that Professor Baughman covered was the newly developed torsional artificial muscles: characterized by a simple structure (nanotube yarn, counter electrode, electrolyte and battery) actuating yarns can be made by UTD's biscrolled yarn process as before. The artificial muscle was demonstrated to be able to twist a paddle 1800 times heavier and 830 times larger in diameter than the muscle itself (which has a 12 micron diameter). The peak power reached 61W/kg (for reference, high power electric motors are approx. 300W/kg).
The one drawback of these artificial muscles is that they do not seem to be scalable in terms of thickness but they could potentially replace micro motors, not requiring any complex parts but just a yarn would be sufficient to perform the necessary tasks.
John Prater from the US Army Research office focused his talk on the fact that research for the US Army needs to be responsive for all system demands, large and small. Examples of the activities the Army Research Office is involved with included powering and communication of micro robotics and ground sensors and the related challenges, but also research to engineer a variety of materials: nano-thermoelectrics, metamaterials for cloaking and camouflage, etc. Also, powering MEMS, micro-turbine technology and Li- battery coatings are some more of the topics the office is involved with, activities ranging from basic and applied research all the way to technology transitions.
Vinod challa, from the University of Florida discussed energy harvesting for wireless power transmission (WPT). Its obvious advantage is that it doesn't have the variability or intermittency that characterizes energy harvesters but on the other hand it does require a transmitter and isn't zero- input power like harvesters are. Florida university research focuses on short and medium range WPT, meaning distances less than 10m all the way to contact of the device with the transmitter.
Short range contactless transmission is usually based on inductive coupling which has low efficiency at high frequencies. Vinod's research is focused on electrodynamic coupling, where a magnetic field forces a magnet to vibrate. Electrostatic or piezoelectric coupling can also be induced albeit with smaller efficiencies which may be suitable for smaller devices.
John Blottman, from the Naval Undersea Warfare Centre works on making "jellyfish" shaped devices, equipped with sensors and systems that allow it to emulate the jellyfish behaviour patterns: research there is focused not only on the biomechanics of swimming but also on bio-sensing and bio- energy in order to create prototype devices that are as jellyfish-like as possible. Sensing is important: mapping and understanding the environment around the device helps with its self-preservation. To that purpose, researchers use turtles, jellyfish, sharks, electric fish (they use magnetic fields, electromagnetic induction, electric charge to understand environment) as models. In terms of power and energy the effort is to emulate the way jellyfish eat in order to power jellyfish devices. (e.g. algae growing on a host, providing amino-acids for the host to consume) Capture, ingestion and conversion are the stages that need to be replicated.
Matthias Hunstig from Paderborn University discussed a method of increasing the power of piezoelectric energy harvesters through the use of magnetic stiffening: the mass at the end of the unilever is magnetic instead of passive, another magnet opposite it creates additional stiffness, increases natural frequency and leads to increased mechanical power. The increased deflection is not as significant as the increase in power and there are no issues relating to breaking the cantilever. The result is that the stiffened setup generates more than 7 times more power than the passive tip mass set-up.
Daniel Song from Hanyang University in South Korea presented recent work on the application of piezo-harvesting on the vibration environment of a train. The main result of this work was that the increase of vibration frequency from 11 to 28 Hz lead to an increase in output power from mV to V levels.
Other interesting presentations included John Turner from Virginia Tech who presented his work in impedance matching for piezoelectric generators. Aditya Rajapurkar from Siemens discussied design rules for piezoelectric energy harvesting and Laurent Pilon from UCLA focussed on pyroelectricity and waste heat harvesting.
Overall an event with a largely academic focus, it gave a very thorough overview mainly on piezoelectric technologies which still attract the largest academic interest, without leaving out other harvesting sources.
