Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 158

5.2.5 Summary

In this section, a hybrid of WEH and SEH (HEH) scheme was proposed for
outdoor applications. The existing WEH subsystem was augmented with the
developed SEH subsystem to extend the operational lifetime of the wireless
sensor node. The proposed HEH topology used diodes to enable MPPT opera-
tions to be carried out in the WEH and SEH subsystems. The WEH subsystem
uses the RE technique, while the SEH subsystem uses the constant voltage
technique. This ensures simultaneous optimal charging of the supercapaci-
tor as well as powering of the wireless sensor node. Experimental tests were
conducted in accordance with the winter condition of the deployed ground
where the average wind speed and average solar irradiance level were given
as 4 m/s and 80 W/m 2 ,respectively. The electrical power harvested by the
SEH and WEH subsystems of the HEH system was 13 and 9.5 mW, respec-
tively. The total electrical power harvested by the optimized HEH system
was 22.5 mW, which was almost two times higher than the highest of the
single-source-based EH method.

5.3 Composite Solar, Thermal (S+T) Energy Sources
The concept of HEH from two readily available energy sources to augment
the life span of a wireless sensor node powered by single-source EH was
validated in Section 5.2. Many EH systems have already been developed, and
those reported in the literature are mostly for outdoor applications where
solar energy is plentiful. Very few research studies [143-144] have discussed
the indoor EH from an ambient light source, which has serious challenges to
resolve. In this section, the concept of EH from two readily available indoor
energy sources—ambient light energy from artificial lighting and thermal
energy from machine heat—to sustain low-power electronic remote sensors
for supervisory and alarm system is proposed.
There are two main challenges associated with indoor EH: (1) ambient
energy sources in the indoor environment are very weak, so the harvested
power is much lower than that of the outdoor condition; and (2) availabil-
ity of energy sources is dependent on the indoor environmental conditions.
According to Randall et al. [30], it has been shown that the light intensity of
artificial lighting conditions found in hospitals and office buildings is only
a small fraction of the light intensity for outdoor sun of 100 to 1000 W/m 2 .
As such, the power density of an amorphous solar cell with efficiency of less
than 10% under indoor light intensity of less than 10 W/m 2 is significantly

W/cm 2 [145]. This is even more challenging when indoor EH
is available for a limited period of time. The ambient energy sources can be in-
termittent and inconsistent at times depending on the indoor environmental
conditions. Take, for example, if indoor lighting is only available during office
hours and there is almost complete darkness for the rest of the day. Because

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