About 90 people from seven countries attended this fifth annual energy harvesting workshop in Roanoke, VA, USA, which serves as an excellent platform to air the latest technical progress with energy harvesters. What was immediately apparent to IDTechEx, attending, was the effort on new forms of energy harvesters. Certainly the majority of talks focussed on piezoelectrics and few on the more mature photovoltaics and electrodynamics, but also covered were thermal energy harvesters, flow energy harvesters (developed by Prime Research LLC), small scale wind energy harvesting (Virginia Tech, which incorporates piezoelectrics) and biological energy harvesters (Virginia Tech).
Virginia Tech University is about 140 years old with approximately 30,000 students and will spend a huge $140 million this year on research activities. Virgina Tech has a range of energy harvesting technology development programs and appears to be the leading academic organization in this regard in energy harvesting. Don Leo from Virginia Tech opened the event by covering their work with biological energy harvesters, taking biological "fuel" such as proteins and using those as the active element for energy harvesting. Two years ago they were able to generate nanowatt outputs from small proteins harvested from plant cells. However, the system did not last for more than ten to fifteen minutes. Over the last 18 months they have improved it such that the devices can now last for weeks using bilayer devices. The currents generated are very small - picoamps, but so are the devices - the proteins are on the order of nanometers. They are now addressing scaling this up to sub micron sizes which would enable energy/area densities on the order of to 100mW/cm2.
Steve Arms of Microstrain presented on harvesting energy from multiple energy sources for wireless sensors. Microstrain make displacement sensors for robotic systems, orientation systems for unmanned systems and wireless sensors. Microstrain has applied the technology to helicopters. In a helicopter a pitch link to the rotor has a piezoelectric material integrated onto it (supplied by Smart Materials) - the strain on the material is converted into energy. The energy harvester patches bonded to the link generate approximately 2.5mW. Bell helicopter specified a lifetime of 1000 hours flying time. Piezoelectric harvesters work well at high frequencies but less so at low frequencies, where electrodynamic systems are usually used. For example, these are used to harvest energy from the shaking of the helicopter. In Greece, Microstrain has used photovoltaic cells to power sensors to monitor bridges. Steve described that for energy harvesting elements, batteries need to be trickle charged, need to have low leakage, and high charge/discharge cycles. Of the options available, only thin film lithium batteries are low leakage, e.g. such as the batteries from Infinite Power Solutions and Cymbet. Mor recently, Microstrain has developed a unit that has multiple energy harvesters. Multiple energy sources raise issues such as impedance matching, capability of the rest of the circuitry to work with different voltage inputs etc. Microstrain's "EH-Link" harvests power from multiple sources, which has been enabled by new energy harvesting power management circuitry. MicroStrain has also added a Capacitive Discharge Voltage, converting relatively high voltage pulses from 20-130V to lower voltages at higher currents, e.g. input from a piezoelectric device. Other aspects of the device have been optimized for energy harvesters, such as the use of FRAM for memory rather than EEPROM which saves energy. Sending the data quickly saves energy (energy is measured in milliwatts seconds). UWB radio energy use (e.g. from suppliers such as Decawave) is 200 times better in this regard than most 802.15.4 technologies.
Texas MicroPower (TMP) is a startup based in Texas. Dr Pradeep Shah, presenting, spoke of the three main limitations he sees for adoption of piezoelectrics in energy harvesting devices:
1. Energy harvester and power management efficiency to power end applications
2. Form factor - reducing the volume of the energy harvester device
3. Cost - reduction will be facilitated by scalable high volume compatible manufacturing for large scale deployment
TMP is looking at making more efficient piezoelectric transducers, optimizing the form factor and is addressing manufacturing processes. TMP is developing thin film piezoelectric materials in thin film layers which can be stacked if needed. The company is also looking to optimise the structures, building them in a format that is usable for a wide variety of applications, and is addressing building large arrays of these structures.
Thomas Daue from Smart Materials Corporation spoke of piezo ceramic macro fibre composite energy harvesters. Smart Materials make different types of piezoelectric materials. The materials can be optimised in different ways depending on the application requirements - such as versions which generate electricity when stretched horizontally or others when stretched in the vertical plane. MFC harvester is available in two different ways - one takes energy when compressed in the perpendicular plane. Thomas reported that they found the harvested energy is proportional to the size of the device. The output energy is a function of frequency, source impedance matching and deflection as a function of acceleration.
Andreas Schoenecker from Fraunhofer IKTS, Germany, said that there are over 200 piezoelectric materials that could be used for energy harvesting. They have limited this to just over 50 which they are now investigating more closely, in order to build up a profile of these materials and make best recommendations for a given application. Schoenecker reported that regardless of the material the energy density limit is 100mJ/cm3. However, different designs and materials achieve different voltage and charge constants - so one needs to find the right material depending on the application - proper material selection is key. Material selection will depend on energy density and the maximum stress level from the application. However, reliability over long periods of time is an issue.
Frederic Dynys from NASA spoke of thermoelectrics in space and vehicle applications. NASA is testing a variety of thermoelectric energy harvesters to power wireless sensors in remote locations such as engines.
Small scale wind turbines
Virginia Tech is also doing research into small scale wind turbines. Most wind turbines use electromagnetic turbines. These are inexpensive and have high power output, but for small devices they are bulky and require high torque (i.e. high speed winds) - being less effective for low speed applications. Virginia Tech is looking at using piezoelectric turbines, which require lower torque and are smaller in profile as no gearbox is necessary. However, the disadvantages are that output power is low and they are less effective at higher velocities. Applications could be structural health monitoring sensors. Virginia tech as developed a "contactless" piezoelectric wind turbine where the losses are mechanical friction on bearings and magnetic friction.
The IDTechEx view
Raghu Das, IDTechEx, presented on the markets and trends for energy harvesters. He summarised that there is a trend to multiple energy harvesters for a device. Key to reducing costs and opening markets is the development of new low power circuitry. Form factor for consumer applications (cellphones, laptops etc) is also important, and here the new thin, flexible forms of photovoltaics are of interest to consumer electronics companies. Over the next 5 years, there will be great interest in using energy harvesters for wireless sensors but ultimately the biggest opportunity by unit numbers and value is for consumer devices - which will include disposables ten years from now.
For more read : Energy Harvesting and Storage for Electronic Devices 2009-2019