Introduction - Energy Harvesting Autonomous Sensor Systems - page 23

when there is a thermal gradient across it, and the wind turbine that converts
kinetic energy from wind flow into electrical energy. The harvested electrical
energy from the energy harvester needs to be conditioned by some form of
power conditioning circuit before supplying it to the load. The main objective
of the power electronics technology in the power conditioning circuit as seen
in Figure 1.16 is to process and control the flow of electrical energy from the
source to the load in such a way that energy is used efficiently. This matching
process is a crucial step to ensure that maximum power is transferred from
the source to the electrical load. Another function of the power conditioning
circuit involves the conversion and regulation of electrical voltage at higher
levels into suitable levels for the loads.
To ensure continuity in the load operation even when the external power
source is weak or temporarily unavailable, the excess energy already being
harvested has to be stored in either the rechargeable battery or supercapac-
itor as shown in Figure 1.16 . Depending on the environmental condition of
the ambient energy source, the characteristic of the energy harvester, and
the power requirement of the load (i.e., wireless sensor node and control cir-
cuitry), each individual EH system is designed and optimized accordingly in
order to sustain the operation of the wireless sensor node.

1.3.3 Review of Past Works on Energy Harvesting Systems

There is a significant amount of research works in the literature on harvest-
ing or scavenging small-scale environmental energy for powering wireless
sensor nodes. One important point to note is that in order to make the sensor
node truly autonomous and self-sustainable in a WSN, the choice of the EH
technique is crucial. As such, review of the past works on EH systems is a
necessity.

1.3.3.1 Solar Energy Harvesting System
The solar energy of an outdoor incident light at midday holds a power density
of roughly 100 mW per square centimetre and indicates that in a small volume
of 1 cm 2 , 100 mW of electrical power can be harvested from the sun by using
a solar panel. Conversely, the lighting power density in indoor environments
such as illuminated offices drops tremendously to almost 100 cm 2 [30].

Commercially off-the-shelf, single-crystal solar cells offer efficiencies of about
15% and up to 20% to 40% for the state-of-the-art expensive research PV cells
recorded by Green et al. in a PV progress report [31]. Thin-film polycrystalline
and amorphous silicon solar cells are also commercially available and cost
less than single-crystal silicon, but also have lower efficiency of only 10% to
13% [30].
Recently, a number of solar EH prototypes have been presented that per-
form increasingly efficient energy conversion. Two of the first prototypes
were Heliomote [32] and Prometheus [33]. In both systems, the solar panels
are directly connected with the storage device. The system Prometheus is de-
picted in Figure 1.17a . In this case, the solar panel is directly connected to a

W /