Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 156

10 k Normal
20 S/s

2010/03/02 19:41:30
YOKOGAWA

50 s/div

CH1 10 : 1
0.500 V/div
DC Full
CH2 10 : 1
0.500 V/div
DC Full
CH3 10 : 1
1.00 V/div
DC Full
CH4 10 : 1
1.00 V/div
DC Full

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3.3 V 3.3 V

3.0 V 3.2 V 3.8 V

2.58 V

V cap

2.58 V

I wind, scaled

V solar

V solar

8.15 V

T

1.2 V

V wind, scaled

A
95 secs
SEH
@MPPT

B1
85 secs

C
110 secs
WEH
@MPPT

B2
90 secs

D
120 secs
SEH and WEH
@MPPT

FIGURE 5.15
Comparison between single ambient energy source harvesting and hybrid energy harvesting.

harvested by the SEH and WEH subsystems were computed to be 13 and
9.5 mW, respectively. In region D of Figure 5.15 , it can be seen that the HEH
system was harvesting simultaneously from both the solar and wind energy
sources, so the supercapacitor's voltage increased at a much faster rate, from
3.3 to 3.8 V in about 120 s. Based on the net energy accumulated in the su-
percapacitor, the electrical power harvested by the HEH system, operating in
MPPT mode, to charge the supercapacitor was calculated to be around 22.2
mW. This calculated output power of the HEH system is almost equivalent
to the summation of the electrical power output from the SEH and WEH
subsystems, that is, 12.65 and 9.85 mW, respectively, of 22.5 mW. It is clearly
demonstrated that HEH has tangible benefits over single renewable ambi-
ent source EH. The supercapacitor charges much faster compared to single
ambient source EH because of simultaneous charging from both solar and
wind energy. As a result, more energy is harvested in a shorter span of time,
allowing duty cycling operations of the wireless sensor node to increase.

5.2.4.2 Power Conversion Efficiency of the HEH System
The HEH system consists of two main EH subsystems, namely the WEH
and SEH subsystems. In each subsystem, there are stages of efficiency loss,
and these should be examined in greater detail to determine the amount of
power lost per stage and thus determine the performance of each stage. A
line diagram to illustrate the input and output power available for each stage
of both subsystems is provided in Figure 5.16 . It is intended to give a clearer
picture of understanding how power is distributed at each power conversion
stage.
Referring to Figure 5.16 , the line diagram of the WEH subsystem starts
from the input with a wind speed of 4 m/s where 18 mW of raw electrical