Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 148

50 S/s 20 s/div

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Sudden Increase in
Irradiance

3

V solar

2

Sudden Decrease in
Irradiance

No MPPT

1

80
W/m 2

80 W/m 2 500 W/m 2 80 W/m 2 500 W/m 2

0 20 40 60 80 100 120 140 160 180

Time (sec)

FIGURE 5.7
Performance of a boost converter with CV-based MPPT under varying solar irradiance.

for all the other MPPs occurring at different solar irradiance. As can be seen
in Figure 5.7 , there were several sudden variations of irradiance levels at
different intervals. The closed-loop PI feedback control is able to correct the
output voltage of the solar panel back to the MPPT voltage of 2.58 V. This
shows that the changing operating conditions of the deployed environment
is well taken care of by the proposed closed-loop CV scheme of the designed
boost converter.
The designed boost converter with a CV-based MPPT approach has been
demonstrated to yield capability in extracting maximum power from the solar
panel, but this comes at the expense of additional power losses in the con-
verter. The efficiency of the boost converter

conv is determined based on the
function of its output load power P load over its input DC power P dc .Taking
the target deployment area with average solar irradiance of 80 W/m 2

as an

example, the efficiency of the converter is calculated to be as follows:

V out / P out

P in ∗

R load
V in I in

= 100% = ∗ 100% (5.6)

conv

98 V 2

3 . / 1200

=

6mA ∗

100% = 91%

2 . 58 V ∗ 5 .

For all other illuminations and resistance loadings, the efficiencies of the
boost converter are calculated using Equation 5.6 to be between 80% and
92%, and the computed results are shown in Figure 5.8 . It is observed from
Figure 5.8 that the efficiency of the boost converter generally drops as the
irradiance level increases. This is due to increasing current output from the
solar panel when light irradiance increases. The increased current contributes
to ohmic losses, hence causing a lower efficiency level to be attained. Referring
to Figure 5.8 , it can be seen that even for a low solar irradiance condition where
the power harvested is small (around 14.4 mW), the boost converter is still
able to achieve a reasonably good efficiency of 91%. This exhibits the ability