The problem with depending on one source of power in the drive toward the battery-free operation of small biomedical devices, remote sensors and out-of-the-way gauges is that if the source is intermittent, not strong enough or runs out altogether, the device can stop working. A small MIT research team has developed a low-power chip design capable of simultaneously drawing power from photovoltaic, thermoelectric, and piezoelectric energy sources. The design also features novel dual-path architecture that allows it to run from either onboard energy storage or direct from its multiple power sources.
Previous research projects at the lab of MIT's Head of the Department of Electrical Engineering and Computer Science, Prof. Anantha Chandrakasan, have led to developments of super-low-power wireless communication and computer chips that have their power needs satisfied by either natural light, heat or vibrations. According to its designer, doctoral student Saurav Bandyopadhyay, the new energy combining circuit is capable of using all three ambient power sources at the same time.
Bandyopadhyay says that each source typically requires its own control circuit to meet its specific needs - thermal sources might only produce between 0.02 and 0.15 volts, low power PV cells can offer up to 0.7 volts and circuits can expect anything up to five volts from vibration harvesters. He points out that most efforts to draw power from multiple source have so far concentrated on simply switching between them, depending on which one is providing the most juice at any given moment.
For example, a sensor might initially get its power from a light source, which could then be abruptly cut off in favor of a piezoelectric harvester, then when the rumble dies down a thermal system might kick in. Rather than waste the energy available from blocked-off sources, the new design allows all three power sources to contribute by rapidly and continuously switching between them to harvest energy from multiple sources (almost) simultaneously.
The researcher has also optimized the control circuits to maximize the amount of power available to devices. Like other designs, the new chip routes energy to an onboard storage medium such as a battery or supercapacitor. Bandyopadhyay claims his development also allows the device to be powered directly from multiple sources, giving it the potential to bypass the storage system altogether.
The new design is claimed to result in 11 - 13 percent efficiency gains over the traditional two-stage approach, and be capable of handling input voltages from 20mV to 5V.
A paper entitled "Platform Architecture for Solar, Thermal, and Vibration Energy Combining With MPPT and Single Inductor" will shortly be published in the IEEE Journal of Solid-State Circuits. The project was funded by a collaboration of defense/semiconductor companies and DARPA.
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