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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