One cannot talk about energy harvesters without discussing wireless mesh networks, sensor batteries (particularly thinfilm batteries), and supercapacitors—along with concepts of power management—nearly in the same breath. Harvesting is a complex and evolving discipline that promises rewards and challenges for engineers who want to take existing skills in new directions.
Most of the technical background information in this report was derived from interviews with companies that presented papers at the NanoPower Forum put on by the Darnell Group, a market analysis organization, in June in Costa Mesa, Calif. For a top-down look at applications, see “Energy-Harvesting Critical Success Factors.”
QUESTIONS OF SCALE Before anything else, the terms “nanopower” and “harvesting” need to be sorted out. “Harvesting” gets applied indiscriminately to things as diverse as grid-tied solar systems and patient-powered heart monitors.
In one way, “harvesting” sounds like big combines and threshers working through vast fields, collecting tons of produce. Photovoltaic (PV) and geothermal energy harvesting fit that description. In another way, it’s more like gleaning—following after the threshers and collecting what’s been passed over.
Either way, collecting the energy is only a small part of the picture. You then have to store it, which involves power density versus energy density considerations in the storage medium, along with equivalent series resistance (ESR) and charge/discharge characteristics. That, in turn, leads to considerations of power management—not just in terms of how you run the application, but in terms of how you husband those electrons you’ve harvested or gleaned.
On the large scale, harvesting that power management would be something like maximum power-point tracking, But for this article, we’re focusing on the small-scale gleaning companies spotlighted at the NanoPower Forum.
A CASE HISTORY I don’t know of any explicit design examples of smallscale energy-harvesting systems that are as thorough as what Charles Lakeman of TPL’s Micropower Division presented at the forum, so I’ve adapted that here for its instructional value. Lakeman described a product called EnerPak that combines smart, ultra-low-power charge management circuitry and electrochemical energy storage (Fig. 1).
As Lakeman described the design problem that’s facing the engineer, any wireless sensing application, a class that embraces most of the things people are trying to do with small-scale energy-harvesting today, has three basic modes: data collection, data communication, and idle (sleep) modes. The power demands for each of these modes are significantly different. The default sleep mode draws perhaps a few microwatts. Sense and compute functions draw a few tens of milliwatts or less, while wireless data transmission can require several hundreds of milliwatts, but only in bursts.
To accommodate these disparate power needs, designers usually simply design for a battery that is capable of handling the system’s highest power demands—those for data transmission. That’s not such a good idea, Lakeman said, as it leads to selecting a battery that’s oversized for most of the operational lifetime and capabilities of the system. It’s better to combine a smaller battery with a supercapacitor.
In that synergistic pairing, the supercapacitor delivers energy efficiently. It exhibits high specific power, which allows it to supply the radio (or wireless mesh network node) when it needs to transmit, while the battery stores energy efficiently and provides backup when the harvester isn’t providing enough power. Using a low-impedance supercapacitor as the primary energy-delivery device is much more efficient than oversizing the battery.
Then there’s power management. In the EnerPak, an ultralow- power TI MSP43 microcontroller (MCU) monitors the state of charge of both the battery and supercapacitor. Simultaneously, it dynamically adjusts the operation of the charging module to accommodate any fluctuations in the level of energy delivered by the harvester. Should the incoming energy not be sufficient to recharge the supercapacitors (e.g., in a solar-powered system at night), the MCU switches in the battery. There’s also some IP in the MCU.
“Because energy harvesters only produce very small amounts of power, this circuitry has been designed to operate extremely efficiently to transfer as much of the available power as possible to the energy storage devices without wasting it in the charger,” Lakeman said.
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