Environmental Engineering Reference
In-Depth Information
so the WEH system charges the supercapacitor freely without much control
over the duty cycle of the boost converter. The controllable region only starts
when the supercapacitor voltage builds up to the MPPT voltage, V mppt = 1.15 V,
of the WEH system at a wind speed of 3.62 m/s. After this, the supercapacitor
continues to be charged with the maximum power harvested from the WEH
system. The whole process is illustrated by the source versus load voltage
diagram (left side) of Figure 2.19 . Based on the governing equation expressed
in Equation 2.5 , as the voltage of the supercapacitor charges, the effective
resistance of the supercapacitor increases; hence, the duty cycle is adjusted
according to Equation 2.6 , as shown in Figure 2.19 , to maintain MPPT at op-
timal resistance. By adding a supercapacitor, whose charging characteristics
are nonlinear in nature, as an energy buffer between the source and the load,
it is shown that the closed-loop resistance emulator of the boost converter is
still able to operate the WEH system at its MPPs.
2.1.2.4 Wireless Sensor Nodes
The WEH system is designed to power a commercially available wireless
sensor node supplied by Texas Instruments (TI) known as the wireless target
board, eZ430-RF2500T. The operations of the wireless sensor node deployed in
an application field are comprised of (1) sensing some external analog signals
such as voltage and current signals of the wind turbine generator or temper-
ature signal and (2) communicating and relaying the sensed information to
the gateway node every 1 s. On receiving the data at the base station, the col-
lected data is then postprocessed into usable information for any follow-up
action.
Using the readily available wireless development tool eZ430-RF2500T for
the TI wireless sensor node, which consists of an MSP430 microcontroller and
a CC2500 RF transceiver, all the hardware and software required to develop
the entire wireless project with the MSP430 is easily performed in a convenient
USB (universal serial bus) stick. The eZ430-RF2500T uses the MSP430F2274
16-bit ultralow-power microcontroller, which has 32-kB flash, 1-K RAM, 10-
bit analog-to-digital converter (ADC), and 2 op-amps and is paired with
the CC2500 multichannel RF (radio-frequency) transceiver designed for low-
power wireless applications. Since the MSP430 microcontroller is part of the
wireless sensor node, it is very convenient to make use of the onboard mi-
crocontroller, without incurring much extra overhead power, to achieve the
proposed MPPT scheme based on the resistor emulation approach rather than
implementing the MPPT scheme with the analog circuit [36, 70].
Long-term operation is an important goal of the WSN system. One attempt
to achieve this goal is to reduce energy consumption of the sensor node.
Energy reduction is carried out by improving hardware design and more in-
telligent power management, which entails turning off unused components
or slowing energy-hungry devices such as a microcontroller during idle pe-
riods. One approach taken is lowering the clock frequency from 1 MHz to
12 kHz, which uses the internal very-low-power, low-frequency oscillator
(VLO) without requiring a crystal. The active mode supply current at 3-V
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