Solar-powered sensors designed to improve energy efficiency for Internet of Things

MIT researchers have developed an ultralow-power circuit that improves efficiency of energy harvesting to more than 80% and can be used to power the Internet of Things (IoT) for extended periods of time.

By Larry Hardesty, MIT News Office July 4, 2015

Researchers at MIT presented a power converter chip that can harvest more than 80% of the energy trickling into it at the Symposia on VLSI Technology and Circuits. Previous experimental ultralow-power converters had efficiencies of only 40 or 50%. The plan is to use this technology for the Internet of Things (IoT) to increase efficiency and longevity for projects. The researchers’ chip achieved those efficiency improvements while assuming additional responsibilities. The solar cell is able to charge a battery while directly powering a device.

All of those operations also share one inductor—the chip’s main electrical component—which saves on circuit board space, but increases the circuit complexity even further. Even with all of those considerations, the chip’s power consumption remains low.

"We still want to have battery-charging capability, and we still want to provide a regulated output voltage," said Dina Reda El-Damak, an MIT graduate student in electrical engineering and computer science and first author on the new paper. "We need to regulate the input to extract the maximum power, and we really want to do all these tasks with inductor sharing and see which operational mode is the best. And we want to do it without compromising the performance, at very limited input power levels—10 nanowatts to 1 microwatt—for the Internet of things."

Ups and downs

The circuit’s chief function is to regulate the voltages between the solar cell, the battery, and the device the cell is powering. If the battery operates for too long at a voltage that’s either too high or too low, for instance, its chemical reactants break down, and it loses the ability to hold a charge.

To control the current flow across their chip, El-Damak and her advisor, Anantha Chandrakasan, the Joseph F. and Nancy P. Keithley Professor in Electrical Engineering, used an inductor. When a current passes through an inductor, it generates a magnetic field, which is designed to resist any change in the current.

Throwing switches in the inductor’s path causes it to alternately charge and discharge, so that the current flowing through it continuously ramps up and then drops back down to zero. Keeping a lid on the current improves the circuit’s efficiency since the rate at which it dissipates energy as heat is proportional to the square of the current.

Once the current drops to zero the switches in the inductor’s path need to be thrown immediately. Otherwise, current could flow through the circuit in the wrong direction, which would diminish its efficiency. The complication is that the rate at which the current rises and falls depends on the voltage generated by the solar cell, which is highly variable. This means the timing of the switch throws has to vary, as well.

Electric hourglass

To control the switches’ timing, El-Damak and Chandrakasan used an electrical component called a capacitor, which can store electrical charge. The higher the current, the more rapidly the capacitor fills. When it’s full, the circuit stops charging the inductor.

The rate at which the current drops off, however, depends on the output voltage, whose regulation is the very purpose of the chip. Since that voltage is fixed, the variation in timing has to come from variation in capacitance. El-Damak and Chandrakasan equipped the chip with a bank of capacitors of different sizes. As the current drops, it charges a subset of those capacitors. The capacitor selected is determined by the solar cell’s voltage. When the capacitor fills, the switches in the inductor’s path are flipped. 

Massachusetts Institute of Technology (MIT)

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– Edited by CFE Media. See more Control Engineering energy efficiency stories.