Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 149

Eciency (%) vs. Load Resistance (Ω)

G = 80 W/m 2
G

100

= 130 W/m 2

= 200 W/m 2
G = 300 W/m 2
G

G

90

80

= 400 W/m 2
G = 500 W/m 2
G

70

= 650 W/m 2

60

0 1000 2000 3000 4000 5000 6000 7000

Load Resistance (Ω)

FIGURE 5.8
Efficiency of an MPPT boost converter for various solar irradiance.

of the DC-DC boost converter to attain high efficiency in a condition of very
low power rating.

5.2.2.3 Performance of SEH Subsystem
To evaluate the performance of the SEH subsystem with MPPT and without
MPPT, the electrical load was changed from a pure resistor to an energy stor-
age device (i.e., a supercapacitor). Unlike the resistor with constant resistance,
the impedance of the supercapacitor is always changing in accordance with
its energy storage level. During its charging process, the dynamic response of
the supercapacitor is of importance to be considered in the design of the boost
converter to ensure constant MPPT operation is achieved. Figures 5.9a and b
show two separate experiments being conducted on the SEH subsystem with
MPPT and without MPPT, respectively, at a solar irradiance of 80 W/m 2 .
Referring to Figure 5.9a , it is observed that the SEH subsystem with MPPT
charged the supercapacitor from a voltage level of 2.815 to 3.13 V in 500 s,
while in Figure 5.9b , the supercapacitor was charged from 2.84 to 2.975 V
in 500 s without any MPPT control. The total amount of energy stored in
the supercapacitor was determined to be around 4.68 J, while its counterpart
was only 1.96 J. It is obvious that SEH with MPPT control outperformed its
counterpart without MPPT control. The amount of energy harvested by the
SEH subsystem with MPPT was 2.39 times more than the amount of energy

20 S/s 50 s/div 20 S/s 50 s/div

<< Main : 10 k >> << Main : 10 k >>

3.13 V

2.84 V 2.975 V

2.815 V 3

2

3
2

1 1

0 0100 150 200 250 300 350 400 0 0100 150 200 250 300 350 400

Time/secs
(a) SEH Subsystem with MPPT

Time/secs

(b) SEH Subsystem without MPPT

FIGURE 5.9
Performance of an SEH subsystem with MPPT and without MPPT for charging a supercapacitor.