Environmental Engineering Reference
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approach essentially consists of three main building blocks: (1) a boost con-
verter with MPP tracker and its control and PWM generation circuit that ma-
nipulates the operating point of the HEH scheme to keep harvesting power at
near-MPPs; (2) an energy storage element (i.e., supercapacitor) to buffer the
energy transfer between the source and the load; and (3) a regulating buck
converter to provide constant voltage to the wireless sensor node and other
electronic circuitries.
With reference to Figure 5.28 , the operation of the boost converter based on
the fixed voltage reference approach is given as follows: The MPPT voltage
reference signal V mppt of 3.6 V is compared with the feedback voltage signal
V fb from the output of the hybrid energy harvester. The resultant voltage er-
ror signal V err is fed into a PI controller to generate a low-frequency PWM
control signal, about 100 Hz, from a Texas Instruments microcontroller (TI
MSP430F2274). In order to miniaturize the HEH system by using smaller
passive components, the low-frequency PWM control signal generated from
the reduced clock speed microcontroller is transformed to a higher switch-
ing frequency of 10 kHz. This is achieved by designing an ultralow-power
PWM generation circuit made up of a micropower resistor set oscillator
(LTC6906) used for sawtooth generation and a micropower, rail-to-rail com-
plementary metal-oxide semiconductor (CMOS) comparator (LMC7215). The
low-frequency PWM signal, which represents the MPPT voltage reference, is
compared with the sawtooth signal to generate the high-frequency PWM gat-
ing signal to control the boost converter.
For an indoor environment, the ambient energy sources, such as a solar and
thermal gradient, are not always available at all times and at a steady level, so
there is a need to incorporate an energy storage device (i.e., supercapacitor)
in the HEH system to store the excessive energy harvested from the solar
panel or thermal energy harvester to buffer the indoor wireless sensor node
for those times when energy sources are unavailable. Moreover, by draw-
ing power simultaneously from both solar and thermal energy sources, the
throughput power of the HEH system is increased, which can enhance the per-
formance of the indoor wireless sensor node. A supercapacitor is employed
in this work because it has superior characteristics over batteries as described
by Simjee and Chou [34]. These characteristics include numerous full-charge
cycles (more than half a million charge cycles), long lifetime (10 to 20 years
operational lifetime), and high power density (an order of magnitude higher
continuous current than a battery). Unlike the discrete capacitors, which have
very small capacitance values in the picofarad-to-microfarad range, the su-
percapacitor has very large capacitance value farads in a range suitable for
energy storage purposes.
Last, the switched-mode voltage regulator (LTC1877) obtained from Linear
Technology is inserted after the supercapacitor to provide a constant operating
voltage of 2.8 V DC to the wireless sensor node and other electronic circuitries.
The efficiency of the regulating buck converter was experimentally tested to be
around 80% to 90%, consuming an operating current of 12
A. In this work,
the operation of the wireless sensor node deployed in an application field
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