The plenary session of the inaugural nanoPower Forum presented a breadth of information including a review of potential applications for energy harvesting by Bradley J. Mitchell from Boeing Commercial Aircraft, a discussion of the challenges and possible solutions for power management when employing energy harvesting technologies by Peter Spies from the Fraunhofer Institute, followed by an overview of alternative energy harvesting technologies by Roy Freeland with Perpetuum. Lou Adams from Tadiran Batteries closed the session with a look at the benefits of battery powering for advanced low-power wireless networks.
Reducing weight, lowering installation costs, enabling rapid introduction of new features, and the rapid reconfiguration of passenger aircraft were the primary goals of the energy harvesting development programs discussed by Bradley Mitchell from Boeing. Numerous wireless sensor applications including monitoring corrosion, cracks, and impact damage and flight testing are envisioned for energy harvesting by Boeing. Improved passenger services systems (lighting, call buttons, and so on) are expected to be a significant opportunity for "finger-powered" electromagnetic energy harvesting in aircraft. Boeing is also developing a self-powered dimming window concept that may be powered by return air grill thermal energy harvesting. Distributed corrosion sensors in future air structures may be powered by vibration energy harvesting. Mitchell identified numerous research challenges, including, "understanding the energy available to be harvested, optimizing energy harvester performance and reducing the power requirements of wireless systems."
Peter Spies from the Fraunhofer Institute followed Mitchell’s presentation by discussing the importance of "power management for energy harvesting applications. According to Spies, "Besides the improvement of the energy harvesting transducers, high demands are made on power management. Power management can be the ’enabling technology’ for the use of energy harvesting power supplies. Improvements in the performance of power management will result in expanding the potential applications for energy harvesting."
According to Spies, there are several duties the power management is responsible for in energy harvesting power supplies. The first task he identified is matching the energy transducers voltage level with those of the electronic circuit or system to supply. The next function is the regulation of the supply voltage, to generate a constant voltage independent of source or load variations. Furthermore, the power consumption of the application devices has to be minimized by the power management so that a maximum of functionality, performance and operation time is achieved with the minimum of energy provided by the energy harvesting module. Another task for the power management is the management of the energy and the required storage units like capacitors or rechargeable batteries.
Perpetuum’s Roy Freeland warned delegates to "be aware of some wild claims out there" for various energy harvesting technologies and devices. Freeland provided detailed reviews of energy harvesting technologies and early adopter applications. He observed that while solar is widely used and available, machine vibration/motion energy harvesting is the "ideal source of energy." He went on to provide a comparison of various vibration energy harvesting approaches including piezoelectric, magnetostrictive, electromagnetic and electrostatic.
Freeland concluded that there are still reliability questions remaining about the long-term viability of systems employing magnetostrictive and piezoelectric technologies. He took the position that electromagnetic energy harvesting techniques offer the best combination of simplicity, low cost and reliable operation. He reviewed two applications for vibration energy harvesting using electromagnetic devices. Both involved condition monitoring of electric motor systems. One example was an installation at a "major international oil company" the second installation was at the Yorkshire Water utility. Both installations were designed to reduce maintenance costs and system failures.
Lou Adams from Tadiran Batteries closed out the plenary session with a discussion of Lithium/thionyl Chloride batteries as the "gold standard" of power supplies for long-term applications such as wireless sensor networks. He pointed out that the low self-discharge rate and high cell capacity of this chemistry combine to produce batteries with operating lives up to 20 years. Standard cells are cost-effective and used in numerous high-volume applications. According to Adams, "More than 25 million flat cells have been used for toll road and other automotive applications, more than 120 million small cylindrical cells have been used for metering, tracking, medical, and instrumentation applications, and more than 25 million big cells (C & D) have been used mostly for metering applications."
At the conclusion of the plenary session, most delegates were in agreement that energy harvesting was not a replacement for today’s batteries. Instead, energy harvesting can be used as an "enhancement" to battery power sources. The general consensus seemed to be that, further in the future, as energy harvesting is better characterized and power management techniques are improved, energy harvesting devices may enable new architectures for wireless sensor networks.