Wind Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 63

Another line diagram, shown in Figure 2.26 , is drawn to illustrate the power
distribution in the WEH system with an active rectifier and a resistance emu-
lation MPPT scheme. When an active rectifier is employed, the AC-DC con-
version efficiency has been improved from 50% to 72%. The WEH system
is further enhanced by using the closed-loop resistance emulator to perform
impedance matching, and it is shown in Figure 2.26 that there is about three
times more raw electrical power (AC) harvested from the wind turbine and
then converted into electrical power (DC) of 7.86 mW to charge the superca-
pacitor. The performance comparison between the line diagrams exhibited in
Figures 2.25 and 2.26 illustrates the significant contribution of the proposed
power management circuit in the overall WEH system incorporated into the
wireless sensor node.
For a 1.5-F, 5.5-V supercapacitor, the maximum amount of energy stored
in the supercapacitor is 22.69 J at 5.5 V. To fully charge the supercapacitor,
the required charging time t charge is computed to be 85 min when 7.86 mW of
electrical power is supplied. To make a fair comparison, the same charging
time of 85 min is used, and the WEH system with a standard power manage-
ment circuit is able to transfer 12 J of energy into the supercapacitor, which is
about half the maximum capacity of the supercapacitor. Comparing the two
line diagrams shown in Figures 2.25 and 2.26 , the operational lifetime of the
sensor node powered by the WEH system with an MPPT scheme is twice
that of the sensor node powered by the WEH system with a standard power
management circuit, hence making the WEH system with the MPPT scheme
a viable solution for extending the lifetime of the WSN.

2.1.4 Summary

There is a need for a paradigm shift from the battery-operated conventional
wireless sensor node towards a truly self-autonomous and sustainable en-
ergy harvesting wireless sensor node. Small-scale WEH through a micro
wind turbine generator is one of the options to power small autonomous
sensors deployed in remote locations for sensing under long-term exposure
to a hostile environment such as a forest fire. Two challenging problems
associated with a small-scale WEH, such as rectification of low-amplitude
AC voltage and impedance mismatch between source and load, have been
addressed. An efficient power management unit of the WEH system, con-
suming very little power (0.447 mW), has been designed to overcome these
challenges. It has been demonstrated with experimental results that with
the proposed active rectifier and resistance emulation-based MPPT scheme,
more electrical power (from 2.04 to 7.86 mW) is harvested from the wind
turbine with higher overall power conversion efficiency from 2.5% to 9.6%.
As such, it is more viable to achieve a truly self-autonomous and sustainable
wireless sensor node with optimal WEH using an efficient power manage-
ment circuit.