Environmental Engineering Reference
In-Depth Information
Equation 2.4
(under unity power factor operation as illustrated in
Figure 2.7
)
and then plotted in
Figure 2.10
for comparison.
V
dc,meas
/
R
load
P
dc
P
ac
∗
rect
=
100%
=
1)
∗
100%
(2.4)
V
ac,meas
∗
I
ac,meas
∗
(
cos
Over the span of wind speeds from 2.3 to 8.5 m/s, the efficiency of the
MOSFET-based active rectifier is on average 15% to 25% higher than the diode-
based passive rectifier (see
Figure 2.10
). This improvement in the efficiency
of the AC-DC conversion process is mainly due to the replacement of diodes
with very low on-state voltage drop MOSFETs and its associated ultralow-
power current sensing and control circuit. It is worth noting that the power
loss incurred in the current sensing and control circuit is only 90
W, which
is a small fraction of the total harvested power as seen in
Figure 2.9
, so it
does not pose any significant electrical loading to the main WEH system.
In addition, even though the components used in active rectifiers such as
MOSFETs and operational amplifiers are more expensive than a simple diode
rectifier, the surplus in harvested power is very crucial in a small-scale WEH
system. Hence, the proposed active rectifier holds great importance in the
design of the power management unit of the WEH system.
2.1.2.2 Boost Converter with Resistor Emulation-Based
Maximum Power Point Tracking (MPPT)
Unlike standard voltage-regulating boost converters, the main functions of
the boost converter in the power management unit of the WEH system are
(1) to step up the low DC voltage output of the wind turbine
V
dc
to charge
the energy storage device and (2) to perform MPPT so that maximum power
transfer takes place. Depending on the energy storage level of the supercapac-
itor, the output voltage of the wind generator
V
dc
is manipulated to transfer
maximum power to the supercapacitor by adjusting the duty cycle of the
pulse width modulation (PWM) gate signal of the boost converter such that
V
dc
is as close as possible to
V
mppt
, the voltage at which the harvested power
is at its maximum.
There are different algorithms proposed to date for seeking the MPP for
stand-alone photovoltaic systems [74] as well as for large-scale wind turbines
[66]. According to Salas et al. [74], the MPPT algorithms can be grouped
into direct and indirect methods. The direct method involves an iterative
and oscillating search for the MPPs, resulting in excessive energy loss during
the search process that is very undesirable for small-scale WEH systems.
With respect to the commonly used indirect methods, refer to the electrical
power harvested curves against the generated voltage and current as shown in
and current (
I
mppt
)atwhich the harvested power is maximum are different
for different incoming wind speeds; hence, variables such as
V
mppt
and
I
mppt
cannot be set as references for the indirect MPPT method.
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