Introduction - Energy Harvesting Autonomous Sensor Systems - page 19

Finite
Energy
Source
Such as
Batteries

Sensor
Nodes
in WSN

Energy
Management
Circuit

Energy
Harvesting

FIGURE 1.14
Paradigm shift from a conventional battery-operated WSN.

the energy capacity and unpredictable lifetime performance of the battery,
energy harvesting (EH) technology has emerged as a promising solution
for a paradigm shift from a conventional battery-operated WSN to a self-
sustainable/autonomous WSN [25-26], as illustrated in Figure 1.14 .

1.3.1 Overview of Energy Harvesting

Energy harvesting is a technique that captures, harvests, or scavenges a va-
riety of unused ambient energy sources (e.g., solar, thermal, vibration, and
wind) and converts the harvested energy into electrical energy to recharge
the batteries. The harvested energy is generally very small (of the order of
millijoules) as compared to large-scale EH using renewable energy sources
such as solar farms and wind farms, which are on the order of several hun-
dreds of megajoules. Unlike the large-scale power stations that are fixed at a
given location, the small-scale energy sources are portable and readily avail-
able for usage. Various EH sources, excluding the biological type, that can be
converted into electrical energy are shown in Figure 1.15 .
Our environment is full of waste and unused ambient energy generated
from the energy sources seen in Figure 1.15 . These renewable energy sources
are ample and readily available in the environment, so it is not necessary to
deliberately expend efforts to create these energy sources, like the example
of burning nonrenewable fossil fuels to create steam, which in turn would
cause the steam turbine to rotate to create electrical energy. Unlike fossil fuels,
which are exhaustible, the majority of the environmental energy sources are
renewable and sustainable for almost an infinite period. Numerous studies
and experiments have been conducted to investigate the levels of energy that
could be harvested from the ambient environment. A compilation of various
EH sources and their power/energy densities is given in Table 1.3 .
Table 1.3 shows the performance of each EH source in terms of the power
density factor. It can be clearly observed that there is no unique solution suit-
able for all environments and applications. According to Table 1.3 , it can be
observed that a solar energy source yields the highest power density. How-
ever, this may not always be the case. Under illuminated indoor conditions,
the ambient light energy harvested by the solar panel drops tremendously.
The other EH sources could provide higher power density, depending on the