When talking about energy harvesting, the discussion tends to focus on photovoltaics
or piezo beams, electrothermal devices, and other ways of turning stray
energy into electrons. However, other technologies also can make energy harvesting useful.
Let’s look at a few:
• Wireless mesh networks: These have made it possible to place low-cost radios (the jargon
is “motes”) and receptive gateways into tough-to-monitor environments. Despite their
low power, these networks are robust. Each mote not only transmits but also receives
and retransmits signals from its neighboring radios. Each radio then supports the network
dynamically—if one radio drops out, others pick up and support the signals it was conveying
in a self-healing fashion. The mesh network is also relatively cheap. The motes themselves
are low-cost, easy to install, and wiring-free, and with energy harvesting, there’s no periodic
maintenance cost for battery replacement.
• MEMS sensors: We’ve reached a point in micromachining where the latest-generation sensors
are smaller, more sensitive, more rugged, and more accurate than ever before. Paralleling that,
available amplifiers and analog-to-digital converters (ADCs) can achieve acceptable accuracy
and dynamic range with lower input and operating voltages than previous generations.
• Small, efficient microcontrollers: Similar to the above developments, microcontrollers
(MCUs) have followed their own path to low power and decent performance. Indeed, for data
conversion, the standard peripherals on many basic MCUs are plenty “good enough.”
• Thin-film batteries: Their slimness is crucial for energy-harvesting applications, because they
can be implanted in circuit boards. Perhaps even more significant is that they have virtually
no equivalent series resistance (ESR), so they’re almost as easy to get power into as supercaps
are to get energy into . On top of that, they don’t self-discharge, so they remain ready for
use virtually forever. Coupled with supercapacitors, as in the EnerPak applications described
in the main article, they round out the energy-storage picture for most mesh-network applications.
It is also possible, as Front Edge Technology discussed at the NanoPower Forum, to
integrate a photovoltaic cell with a thin-film battery to make a self-charging battery.