All processes that involve energy conversion are, to some degree, inefficient. Motors
get
hot, as do power transistors, automobile engines, and light bulbs; in each case
energy
is wasted as heat. Radio stations put out megawatts of RF but their signals reach
antennas as microwatts. Energy harvesting devices capture some of this wasted
energy,
convert it to electricity, and put it to work.
The best known energy harvesting collectors are large solar panels and wind
generators,
which have become major alternative energy sources for the power grid. But small
embedded devices must rely on energy scavenging systems that can capture milliwatts
of
energy from light, vibration, thermal, or biological sources. Thanks to
ultra-low-power
MCUs these micropower energy harvesters can greatly extend the life of batteries in
consumer, industrial, and medical applications where battery replacement may be
difficult, expensive, or even impossible. With careful design, energy harvesting
devices
can even replace batteries altogether in some applications.
Since the output from energy harvesting devices is usually small and intermittent, a
system must be carefully designed that may include a boost converter, a charge
controller for a rechargeable Li-Ion or thin-film battery, a regulator for the MCU
and
other loads, an MCU, sensors, and a wireless connectivity module. The closer an
energy
harvesting device can come to supplying the overall demands of an embedded system,
the
closer that system can come to being battery free.
