Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 138

energy sources would then be harvested. Hence, it is not possible to
harvest energy from both energy sources simultaneously.

Type 3: HEH using an individual power converter for each EH source. Ac-
cording to Park et al. [35], each energy source has its own power
management circuit; hence, it allows energy to be harvested simulta-
neously from both energy sources. However, as the number of con-
verters increases, this scheme becomes more complex and bulky with
higher component counts, and the power loss in the individual con-
verter control circuitry becomes especially significant for micropower
EH sources.

•

Type 4: HEH by directly connecting the energy sources in parallel/series con-
figuration. Energy sources are directly connected together, and they
share a common power management circuit. Compared with the en-
ergy sources selection method used in Type 2, this scheme allows
energy to be harvested simultaneously from both energy sources.
In addition, only one power electronic-based converter with a sim-
ple and low-power control circuitry is required instead of dedicat-
ing each individual energy source with a power converter described
in Type 3. However, the challenge of this approach is that there
could be an impedance mismatch issue among the integrated energy
sources.

•

Of the four HEH methods mentioned, Types 1 and 2 are not adopted for the
HEH research of this chapter. The reason is that the Type 1 approach does not
have the advantage of harvesting from another energy source. As for the Type
2 approach, even though there are two different energy sources available for
harvesting, the HEH system does not have the capability to harvest from both
the energy sources at one time. Hence, in this chapter, two types of small-scale
HEH schemes are investigated: (1) hybrid of wind energy harvesting (WEH)
and solar energy harvesting (SEH) schemes for outdoor application and (2)
hybrid of indoor ambient light and thermal energy harvesting (TEH) schemes
for indoor application.
The first HEH system, illustrated in Section 5.2, is designed according to
the Type 3 approach of combining the two energy sources. In this Type 3
approach, each EH source is required to have its own unique power manage-
ment unit to condition the power flow from the energy source to its output
load. However, this is not the case for the fourth HEH method, which is uti-
lized by the latter HEH system and illustrated in Section 5.3. The proposed
HEH system requires only one power management unit to condition the com-
bined output power harvested from the solar and thermal energy sources. By
avoiding the use of different power management units for multiple energy
sources, the number of components used in the HEH system are decreased,
and the system's form factor, cost, and power losses are thus reduced. Before
proceeding into the details of the two HEH systems, the SEH system common
to both HEH systems is first investigated.