M. SIMON | TECHNICAL CONTRIBUTOR
Battery harvesting versus energy harvesting
Should you use primary batteries or energy harvesting for your low power isolated from the grid application? Are primary batteries better
Power and energy
or worse than energy harvesting? Can you use one
or the other or do you need both? Neither? It is an
interesting question depending on the application,
the power required and the costs involved. So let us
explore the field and see if we can determine a set of
rules that will help us come up with a final answer. Stored energy (bat-
teries)? Harvested energy? Neither? Both?
Two things must be considered from the very beginning: power
required and energy required. They are not the same thing. For power,
look at the peak power required or “Volts” times “Amps”. For energy,
which is mainly a concern for batteries, you need to know the average
power for the required length of time or “Volts” times “Amps” times
“Time”. You can’t meet just the power requirements or energy requirements; both requirements must be satisfied. Energy harvesters only
need to be able to deliver the power (average and peak) required. A
harvester will deliver that power throughout its lifetime. The energy
stored in a battery does not limit the lifetime of a harvester-powered
device. That is a very powerful advantage.
If harvesting can be your sole source of energy, you can save the cost
of a battery. Alternatively, you can use the energy harvested to charge
a rechargeable battery (i.e. keeping a cell phone charged from mechanical energy produced by the human body at an outdoor rock concert.)
Logistics and labor can be a significant cost for a battery-powered
device, especially if your device is at the bottom of the ocean or in
space. Batteries work until they run out of energy. Harvesting works
until it fails and everything fails eventually. A battery backup for your
harvester is a good idea if the device is mission critical.
The energy harvester has temporary storage in the form of capacitors,
if its collection is intermittent, or if it must store small amounts of
energy in order to deliver pulses of power. Batteries often need the
same kind of storage in tandem with the batteries in order to deliver
pulses of power. Ceramic capacitors are preferred for storing small
amounts of energy. When you need more energy, electrolytic capacitors
can be used, but be aware of life limitations. For maximum energy storage, use super capacitors. Check the life specification on those, too. You
may need to use several types of capacitor to meet your requirements.
Light harvesters, commonly referred to as “solar cells”, have been
around for a very long time. The first widespread commercial use was
solar powered calculators, which were mostly powered by building
lighting. Garden lights have become quite common as costs have come
down for solar cells, white LEDs, and small rechargeable batteries.
Because of its intermittent nature, harvesting solar energy requires
storage, if its load needs to be serviced around the clock and in cloudy
weather. Some applications require three days of storage to get the
power delivery reliability required. That will vary with the applica-
tion, climate, and latitude.
Air motion and vibrations
A wind turbine collects energy from air motion. For megawatt power
devices, the grid makes up for peaks and dips. For off-grid and some
on-grid applications, batteries are used to get the energy delivered by
the generator to match the power required by the load. For low power,
you can use small turbines and generators or create devices that
vibrate in the wind and then collect the energy from the vibrations.
Vibration energy can be collected by a magnet and coil generator or
with a piezoelectric charge generator. Most vibration collectors are
tuned, which greatly amplifies the vibrations thus requiring less material for the collection mechanism. Tesla used the principle of resonance
(Q multiplication) to greatly “amplify” the high voltages his transformers produced, but resonance comes with a downside. The source needs
to be a fairly constant frequency because collection efficiency is greatly
reduced outside the resonance band.
Thermocouples, normally used as temperature sensors, can also be
used to capture energy from temperature differences. They have the
advantage of being able to handle high temperatures but they are not
very good when it comes to capturing large amounts of energy. At
lower temperatures thermoelectric heaters and coolers can be operated
in reverse allowing you to collect a current flow from a temperature
difference rather than using a current flow to create a temperature difference. Radioisotope thermoelectric generators use thermoelectrics to
develop continuous power on the order of watts to hundreds of watts
for decades. With the heat source built in, such generators are more
like batteries than harvesters. But the same principle can be used for
harvesting. Pyroelectric generators are another possibility.
Radio wave harvesting
Nowadays with rectennas collecting microwave energy the idea of
radio wave harvesting has gone mainstream. Generally this is not a
harvesting method because the microwaves are beamed to the receiver, but it could be if you are collecting AM broadcast station energy.
Depending on your distance from the station, station power, and the
power that you need a long wire antenna and a good ground may be
required. Don’t forget your lightning protection if you use such a setup.
The application area I find most interesting is energy harvesting for
biomedical applications, particularly for internal medicine applications
like pacemakers that can get enough energy from a beating heart to
keep the heart beating without requiring battery replacement surgery
every few years. Some other application areas are sensor networks,
energy control (light switches that communicate by radio and that are
powered by the flip of the switch is one neat application), machinery
monitoring, structural monitoring, surveillance, and no doubt more
will be developed as the technology becomes more pervasive.