Environmental Engineering Reference
In-Depth Information
uncertain indoor energy sources pose significant challenges for EH from a
single energy source for sustaining the operation of the wireless sensor nodes
over the entire lifetime.
To enhance the performance of the wireless sensor node in an indoor en-
vironment, HEH from solar and thermal energy sources is proposed in this
section. Among the various artificial energy sources, solar and thermal en-
ergy sources share similar high power densities, as can be seen in Table 5.3 .
In addition, the undesirable intermittent solar energy sources found in offices
or factory lighting can be supplemented by the continuous thermal energy
supply from waste heat generated by machinery. Whenever solar energy is
not available, instead of solely relying on the energy stored in the onboard
energy storage devices proposed by Nasiri et al. [143] and Hande et al. [144],
an alternate approach is to harvest from the readily available thermal en-
ergy source to continue powering the operation of the wireless sensor node
before exhausting the energy stored in the energy storage device. Another
key characteristic of the proposed HEH system is that the HEH system is
able to harvest simultaneously from both energy sources whenever they are
available instead of harvesting from an individual energy source at one time
[126-129], hence the performance of the indoor wireless sensor node can be
enhanced.
5.3.2
Indoor SEH Subsystem
The solar panel used in this section is specially selected for use under indoor
conditions (i.e., artificial lighting from fluorescent lamps) and at room tem-
perature (relatively less variation of temperature in an indoor environment
than outdoors [30]). The physical size of the chosen solar panel is 55
×
30
×
1mm(2.1
.04) and its cross-sectional area A can be calculated to
be around 16.5 cm 2 .Atvery low light illumination, for example, G = 380 lux
(
×
1.18
×
380/120 = 3.17 W/m 2 [30]), the open-circuit voltage V oc and short-circuit
current I sc are measured to be 4.14 V and 60
A, respectively. The output volt-
age and current obtained at MPPT points are V mppt = 3.5 V and I mppt = 51.44
A, respectively. For the given technical characteristics of the solar panel,
the corresponding solar panel's efficiency can also be determined using the
following equation:
P pv
G
=
A
100%
(5.7)
Based on Equation 5.7 , the efficiency of the solar panel can be calculated
to be around 3.4%, which is relatively lower than the outdoor solar panel
[145]. Due to the low efficiency of the solar panel in the indoor condition, the
power harvested is also low; hence, it is necessary to optimize the indoor SEH
subsystem in order to maximize the power harvested from the solar panel.
Further investigations were carried out on the solar panel to investigate its
performance in different lighting conditions. Figures 5.17 and 5.18 show the
indoor solar panel's P-V and P-R curves at different lux illuminations. Both
 
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