“Circuits for energy harvesting” (2009 Annual Report)
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Energy harvesting for self-powered sensors
Embedded systems researchers are working with mechanical engineers from the Center for Intelligent Material Systems and Structures (CIMSS) to develop energy harvesting technology for embedded devices. Their goal, according to Dong Ha, is to develop self-powered sensors for hard-to-reach places, such as bridges and helicopter turbine engines.
The group is exploring a variety of techniques for harvesting energy from the environment — including miniature windmills, mechanical vibrations, thermal, and solar power. The big challenge, Ha says, is that “our circuit should adapt to the changing environment to harvest the maximum amount of energy. A simple example is sunlight, which changes in intensity and direction.” This maximum power point tracking presents its own challenge: if the circuit expends too much energy improving its own efficiency, it may consume more than it gains. “We’re dealing with milliwatts here,” Ha says. “We have to not just make our circuit efficient at transferring power, but also make sure it doesn’t use too much itself.”
Bridge sensor nodes
Ha’s team is partnering with Dan Inman, director of CIMSS to develop embedded bridge sensor nodes as part of a $14 million grant from the National Institute of Standards and Technology (NIST). Physical Acoustic Corp. is developing the nodes, which will combine power from miniature windmills and mechanical vibrations.The miniature windmills produce low-voltage dc power, which must be stepped up to 7.5V dc yielding only about 60mW after conversion, explains Ha. The vibration is harnessed via both piezoelectric patches and electromagnetic coils. Piezoelectric patches generate 20-40V ac, which must be converted to 7.4V dc to charge the battery.
Helicopter engine sensors
Energy harvesting may also prove useful in the close confines of helicopter turboshaft engines. The ECE/CIMSS team is collaborating with Prime Research and Pratt & Whitney to develop wireless engine sensors that will power themselves through airflow, thermoelectric, and vibration energy harvesting.
High engine temperature plays both good and bad roles. The good part is that it provides an energy source. CIMSS experiments demonstrated the ability to harvest up to 40mW when a thermal harvester is placed on a 200°C surface and passively cooled with a heat sink. The high temperature inside the jet engine severely degrades the performance of circuits, posing a design challenge. Bulk CMOS technologies fail at temperatures greater than 200°C, so the group plans to employ silicon-on-insulator technology and use various compensation schemes to mitigate adverse effects of high temperature.
Ha’s team is also designing a power management system that will produce about 200mW, on average, regulated power under California sunlight from combined solar and thermal sources. Ha is working with Accellent Technologies, which is developing a structural health monitoring sensor node for infrastructure, and the power management system will provide 18.5V dc to power the sensor node.
The ECE team is investigating all aspects of the power management system including design of a thermal harvester possibly with active cooling, a battery charging circuit, and voltage boosters and regulators.