Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 167

power the load. The electrical power throughput of the hybrid energy har-
vester P HEH ( V R L )asafunction of its output voltage V R L is thus given by

= P pv ( V R L ) + P TEG ( V R L )
≈ V R L ∗

P HEH ( V R L )

+

I o exp I sc,pv −

V R L

V R L ∗ V oc,TEG −

V R L
n s kT c /

V R L ∗

q R s,TEG

(5.11)

Based on the technical specifications of the solar panel and the thermal
energy harvester given in Sections 5.3.2 and 5.3.3, respectively, the harvested
power expression of the hybrid energy harvester, as expressed by Equation
5.11 , is simulated over a range of output voltages V R L for solar irradiance
and temperature differences that correspond to the solar panel's short-circuit
current I sc,pv and the thermal energy harvester's open-circuit voltage V oc,TEG .A
set of the simulation and experimental results extracted under the minimum
(380 lux and

T =5K) and the maximum (1010 lux and T =10K)power

harvesting conditions is shown in Figure 5.23 .
Referring to Figure 5.23 , it can be observed that the measured power curve
(Measured S+T) of the hybrid energy harvester is the result of summing the
individual power curves, that is, solar panel [Solar (S)] and thermal energy
harvester [Thermal (T)], being superimposed into Figure 5.23 minus the neg-
ligible small power loss in the Schottky diodes. At MPPT voltage V R L ,mppt of
3.6 V, the output voltages of the solar panel and thermal energy harvester are
slightly higher than V R L ,mppt such that the two isolation diodes are conducting
in the forward bias condition, hence with reference to Figure 5.23 , it can be
seen that the hybrid energy harvester can generate power at the minimum of
252 W( P pv = 167 W, P TEG =85 W) and at the maximum of 693 W( P pv =

466 V pv ,

the solar panel operates in the open-circuit mode; therefore, no solar power
is harvested. This situation happens to the thermal energy harvester as well
if V R L

W, P TEG = 227 W). In addition, Figure 5.23 shows that when V R L ≥

≥

V TEG (3.6 V).
Another observation seen in Figure 5.23 is that the simulated waveforms
(Simulated S+T) based on the model expressed by Equation 5.11 and the mea-
sured waveforms (Measured S+T) obtained from experiments are quite sim-
ilar. The positive outcome of this observation verifies the expression model
derived in Equation 5.11 , which can then be used to determine the electrical
power throughput of the hybrid energy harvester P HEH to sustain the opera-
tional lifetime of the wireless sensor node. More analysis and characterization
works were conducted on the hybrid energy harvester to evaluate the perfor-
mance of the HEH system in powering the wireless sensor node. Figures 5.24
and 5.25 show the power curves of the HEH system at fixed solar irradiance
of 380 and 1010 lux, respectively, for varying thermal differences in the range
of 5 to 10 K. Conversely, the HEH system is also subjected to fixed thermal
differences of 5 and 10 K, as shown in Figures 5.26 and 5.27 , respectively, for
various solar irradiances between 380 and 1010 lux.