Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 176

Performance of Parallel S + T Energy Sources

800

DT10

P HEH,actual P HEH,MPPT

DT7

DT5

600

DT10

DT7

DT10

DT10

DT5

400

DT7

DT7

DT5

DT5

200

0

380 lux 425 lux 680 lux 1010 lux

Solar Irradiance (lux)

(a)

8

6%

6

4 3%

2

DT5 DT7 DT10 DT5 DT7 DT10 DT5 DT7 DT10 DT5 DT7 DT10

0

380 lux 425 lux 680 lux 1010 lux

Solar Irradiance (lux)

(b)

FIGURE 5.29
Performance of an HEH system in parallel configuration.

and ultralow-power fixed reference voltage method for the hybrid energy
harvester.

5.3.5.2 Power Conversion Efficiency of the HEH System
The power conversion efficiency of the HEH system is another important
investigation being carried out. Other than the regulating buck converter,
there are two main contributors of power losses to the HEH system; the boost
converter itself, which acts as an MPP tracker, and its associated sensing,
control, and PWM generation circuits. The efficiency of the boost converter
conv can be expressed as a function of its output load power P load over its input
DC power P dc under varying solar irradiance, temperature differences T ,

and loading R L conditions. Take, for example, at lux illumination, temperature
difference and output load resistance of 380 lux, 5 K, and 68 k ,respectively,

the efficiency of the boost converter is given by

V out / P out

P in ∗

R load
V in I in

conv = 100% = ∗ 100% (5.12)

95 V 2

4 . / 68 k

=

A ∗

100% = 91 . 8%

3 . 6 V ∗ 109

and the calculated efficiency of 91.8% is plotted in Figure 5.30 . For all other
solar irradiances, temperature differences, and loading conditions, the effi-
ciencies of the boost converter are calculated using Equation 5.12 , and the