Tech Briefs Magazine - October 2021 - ST-4

Energy Harvesting
Ambient Energy
1μW
Autonomous
500%
200%
* Low TEG temp & vibration compatible only
** High temp TEG source unlikely
100%
50%
20%
10%
0%
Temp, light
sensor
RFID tag &
Pacemaker*
Low sense rate
Accelerometer
Hearing aid* &
CO2 detector
Smart**
Bandage
Asset tracking
tag
Figure 1. EH sweet spot for IoT edge devices/battery life extension impact. (Courtesy of EnABLES)
100mm³ TEG array 5°C gradient (10μW)
100mm³ TEG array 50 °C gradient (500μW)
5μW
IoT device power consumption
20μW
200μW
500μW
1mW
200mm³ Vibrational harvestor 0.1G (2μW)
200mm² Indoor solar panel (60μW)
EH entails using ambient energies that
are available - heat, vibrations, light -
for sources of power. There is a sweet
spot, from around one microwatt to a
few hundred microwatts, where there is
the 'double impact' of significantly less
drain on the existing power source and
increased viability for using ambient
energies from reasonably sized harvesters.
This can significantly increase
battery life, in some cases even leading to
complete power autonomy. (This is discussed
in a recent EU publication and is
illustrated in Figure 1.)
Figure 2.
Self-powered
diagnostics unit for condition
monitoring of machinery, can harvest PV,
vibrational, or thermoelectrical energy.
(Courtesy of EU MOSYCOUSIS project)
The approach we take to powering
the IoT is critical to enabling so many of
the technologies changing our world
every day. For example, connected and
autonomous vehicles (CAV), will be
dependent on reliable and ubiquitous
sensing with both high- and low-bandwidth
connectivity, all of which requires
increased power density and weightreduction
- two things that self-powered,
wireless sensor networks support.
Cost
A major added value of EH is to provide/supplement
system energy at the
point of consumption by capturing ambient
energy in the operating environment.
The justification and success of
4
Cov
EH implementations, particularly in
terms of total cost of ownership, are
highly dependent on the method of calculating
payback. For instance, adding
$3 - $5 to a system bill of material for EH
capabilities might seem crazy when comparing
it to a disposable coin cell costing
<<$0.25 at volume. Even neglecting environmental
and sustainability factors,
there is a lot to consider in the financial
analysis. If that battery ever needs to be
replaced, then the labor/access-logistics
costs alone can obliterate the coin cell
savings by orders of magnitude - heaven
forbid if that battery is in a harsh
and/or inaccessible environment, such
as a concrete wall, high ceiling, human
body, or deep oil well.
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ToC
A key challenge driven by integrating
EH into system design is dealing with
energy sources that can be quite sporadic
in nature. They need energy storage
and power management devices/
circuits to capture the energy and make
it available for later use. Not only are
there unique engineering efforts that
must be made to address power extraction
from ambient scavenging, but
many of those needs can be different
for each method of EH. In other words,
the capture of raw energy from the EH
transducer and power conversion/management/regulation
needs are different
for photovoltaics (PV) than for thermoelectric
generators (TEG) or vibrational
harvesting. Even powering different flavors
of PV cells, can vary greatly based
on the technology. The general
approach tends to be driven by the
nature of the raw, harvested energy, be
it DC (PV, TEG) or AC (vibrational, triboelectric,
RF).
PV cells directly convert light energy
from the sun and/or manmade sources,
whereas a TEG extracts energy from a
temperature differential to generate
electrical energy. Vibrational (electrodynamic
or piezoelectric) and triboelectric
sources are derived from physical movement.
RF capture typically involves the
use of a rectifying antenna (rectenna)
and balancing network, and then, as is
common, feeding it into a DC/DC conversion
block.
An optimal, EH-enabled system solution
may require maximum power point
tracking (MPPT) and/or carefully controlled
impedance matching to fully realize
its maximal energy potential. In addition,
many ambient energies are at very
low power and voltage levels. Most commercial-off-the-shelf
(COTS) power management
ICs (PMICs) are incapable of
converting energies below 10 μW and
100 mV to usable electricity. An example
Sensor Technology, October 2021
Battery life extension

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Tech Briefs Magazine - October 2021

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