Introduction - Energy Harvesting Autonomous Sensor Systems - page 22

2. Intermittent power supply from energy sources means EH is not fea-
sible in certain situations (e.g., zero solar energy at night)
3. Susceptibility to fluctuating energy sources may affect sensor node
operation
4. Expensive in terms of from well to tank as compared to fossil fuels
and causes lack of attractiveness

Clearly, it can be deduced from the list of benefits that EH technology is
a viable solution to power wireless sensor networks and mobile devices for
extended operation with the supplement of the energy storage devices, if not
completely eliminating storage devices such as batteries. EH works hand in
hand with batteries/supercapacitors to extend devices' operational lifetime.

1.3.2 Energy Harvesting System

In an EH system, there are generally four main components: energy collec-
tion and conversion mechanism (energy harvester), electrical power manage-
ment/conditioning circuit, energy storage device, and electrical load (wire-
less sensor node) ( Figure 1.16 ). Power output per unit mass or volume (i.e.,
power/energy density) is a key performance unit for the energy collection
and conversion mechanisms. The harvested power must be converted to elec-
tricity and conditioned to an appropriate form for either charging the system
batteries or powering the connected load directly. Proper load impedance
matching between the EH source and the electrical load is necessary to max-
imize the usage of the harvested energy. Appropriate electronic circuitry for
power conditioning and load impedance matching may be available com-
mercially or may require custom design and fabrication.
Referring to Figure 1.16 , it can be seen that the function of the energy har-
vester is to convert energy harnessed from environmental energy sources into
electrical energy. Some typical examples of the energy harvesters, as shown
in Figure 1.15 , include the lead zirconate titanate (PZT) ceramic material that
converts mechanical (strain or stress) energy into electrical energy due to the
piezoelectric effect, the photovoltaic (PV) cell that converts solar energy into
electrical energy, the thermoelectric generator (TEG) output electrical voltage

1 2 3 4

Environmental
Energy
Sources: Solar,
Vibration, hermal

AC-DC
Converter
(optional)

Energy
Harvester

Energy
Storage

Electrical
Load

DC-DC
Converter

Power
Management/
Conditioning

Controller

FIGURE 1.16
General block diagram representation of an EH system unit.