09 July 2012

MIT chip captures power from multiple sources

A chip that could combine power harvested from light, heat and vibrations to run monitoring systems such as biomedical devices and environmental sensors has been developed by researchers at MIT.

"Energy harvesting is becoming a reality," said Professor Anantha Chandrakasan, head of MIT's department of Electrical Engineering and Computer Science. "Low power chips that can collect data and relay it to a central facility are under development, as are systems to harness power from environmental sources. But the new design achieves efficient use of multiple power sources in a single device, a big advantage since many of these sources are intermittent and unpredictable."

Key to the chip's design, according to Chandrakasan, is a sophisticated control system. "Typically, each energy source requires its own control circuit to meet its specific requirements," he explained. "For example, circuits to harvest thermal differences typically produce only 0.02 to 0.15V, while low power photovoltaic cells can generate 0.2 to 0.7V and vibration harvesting systems can produce up to 5V. Coordinating these disparate sources of energy in real time to produce a constant output is a tricky process."

So far, most efforts to harness multiple energy sources have simply switched among them, taking advantage of whichever one is generating the most energy at a given moment, Chandrakasan says, but that can waste the energy being delivered by the other sources. "Instead of that, we extract power from all the sources by switching rapidly between them," he explained. "At one particular instant, energy is extracted from one source by our chip, but the energy from other sources is stored in capacitors and later picked up, so none goes to waste."

Another challenge for the researchers was to minimise the power consumed by the control circuit itself, to leave as much as possible for the actual device it's powering. According to Chandrakasan, the team found a way for the control circuits to optimise the amount of energy extracted from each source. The researchers also integrated an innovative dual path architecture into the system.

"Typically, power sources would be used to charge up a storage device, such as a battery or a supercapacitor, which would then power an actual sensor or other circuit," Chandrakasan noted. "But in this control system, the sensor can either be powered from a storage device or directly from the source, bypassing the storage system altogether. That makes it more efficient. The chip uses a single time shared inductor, a crucial component to support the multiple converters needed in this design, rather than three separate ones."

David Freeman, chief technologist for power supply solutions at Texas Instruments, said: "The work being done at MIT is very important to enabling energy harvesting in various environments. The ability to extract energy from multiple different sources helps maximise the power for more functionality from systems like wireless sensor nodes. With innovations like these that combine multiple sources of energy, these systems can now start to increase functionality."

Laura Hopperton

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