hours are short. The cost of overestimating source energy is system
As do many applications, the parking-kiosk example benefits from the
happy accident that most of its in-use hours coincide with daylight hours.
Transactions typically take a minute or less and, although the system
cannot control the time between transactions as it can in many data-collection applications, one can derive typical and worst case numbers
from traditional meter-use data. Between uses, the system can shut down
leaving only a low-power microcontroller to look for a user key press and
the power subsystem to charge the battery.
The choice is yours
At present, power management ICs are available for a wide variety of
transducer types and some accommodate multiple transducers, increasing
the opportunity for systems to operate without battery backup. For
those that can, eliminating batteries reduces system cost, eliminates one
source maintenance expense, and increases system reliability. For those
that do require a secondary electro-chemical energy source, battery
chemistries have been improving their cost, density, and charge-cycle
Potential ambient-energy transducers for harvesting or scavenging
include kinetic, thermo-electric, solar, mechanical stress, and RF. If
triboelectric transducers and their power converters become practical,
we may see a resurgence of corduroy in the wearables market but, until
then, your wardrobe remains safe.
Of course, not all ambient energy sources are created equal.
Available energy, be it for harvesting or scavenging, is available
in quantities that, across all source types, vary over six orders of
magnitude, as figure 2 shows. ECN
The author thanks Tony Armstrong of Linear Technology and
Peter Dipo-Ajayi of Texas Instruments for sharing their insights.
Figure 2: A comparison of ambient energy sources (before conversion). (Data
source: CEA-Leti; graphic source: Linear Technology; used by permission)