Second day of Energy Harvesting Europe

The second day of the conference Energy Harvesting and Storage Europe was as rich with information and expert opinion as the first. Here are just few of the overall impressions given by the day's events. Wireless sensors were the primary focus but several useful general points were made.

There is near consensus that forces for adoption of energy harvesting are sharply increasing. We all experience the new mobile phones with batteries that last a few days instead of the week that was typical of their predecessors. Ken Ball of Microdul in Switzerland told us how he has the same thing with his hearing aids that are now so clever (noise cancellation, radio linking) that the batteries die in only 3 weeks vs 7 weeks before. This makes users very keen to see energy harvesting save the day. On the other hand, where new functionality more rarely seen as with notepad computers or WSN, power consumption is rapidly reducing with the new chips. This makes it more realistic to consider energy harvesting not least because - point number three - energy harvesting devices are delivering more power per unit of cost, volume and size. Impediments remain, with few believing that true plug and play broad band energy harvesting is available as yet. Compared with this, the arguments about whether a piezoelectric or an electrodynamic vibration technology is best is something of a sideshow.

Wireless sensor networks are finally gaining some traction and some, such as the Fraunhofer IIS patient/ blood bag tagging in two German hospitals involve using WSN for location, something previously considered to be very challenging. The main impediment to rollout of WSN in most potential applications was commonly agreed to be lack of appropriate, affordable energy harvesting however. In the meantime, there is consensus that thin film batteries have lower leakage current, faster charge discharge and other advantages, including energy density when compared with coin cells for example.

There is only limited progression from the materials traditionally used for thermoelectric and piezoelectric harvesting because the choices are so few but work with piezoelectrics Pb(MgNb)O3 and manganese doped PZT was described by Dr Nantaan Muensit of Prince of Songkia University in Thailand who focussed on the bandwidth problem with piezoelectrics. She also described her work on polyurethane magnetostrictive harvesting. Fraunhofer IKTS has silicon carbide thermoelectrics. Others were looking at capacitive harvesting, something usually reserved for micro scale in MEMS. MEMS were of interest to many speakers, from harvesting in MEMS to using MEMS type machining to create the unusually small Micropelt bismuth telluride thermoelectrics. The Holst Centre in the Netherlands reported impressive micromachined vibrational and thermal MEMS energy harvesters. Those enthusing about thermoelectric harvesting pointed to there being 50 billion machines in the world in which they could usefully be embedded to provide sensing. Progress here involves a number of ways of getting useful power from only a few degrees Kelvin temperature difference.

Clearly there is insufficient progress with the performance of any one form of energy harvesting to make it suitable for most applications, so the increasing problems with batteries have to be tackled by using much smaller thin film rechargeable batteries together with multiple forms of energy harvesting. Fraunhofer IZM even described silicon embedded microbatteries. EnOcean often uses two forms of harvesting in a single building control and Microstrain demonstrated a palm sized wireless sensor driven by no less than four types of energy harvesting at the same time.