Hybrid Energy Harvesting System - Energy Harvesting Autonomous Sensor Systems - page 141

method of obtaining the MPP. It measures the PV characteristics and then per-
turbs the operating point of the PV module towards the maximum point when
dP pv / 0isreached. IncCond is an alternative to the P&O method pro-

posed by Hussein et al. [135]. It is based on differentiating the PV power P pv
with respect to voltage V pv and setting the result to zero. The maximum point
is reached when the opposite of the instantaneous conductance, G

dV pv =

= I pv / V pv ,

is equal to its incremental conductance dI pv / dV pv . According to Esram et

al. [133], the P&O and IncCond techniques are the most effective MPPT tech-
niques, harvesting the most energy in comparison. This is because both MPPT
techniques have the ability to track the true MPP of the PV module accurately
under any solar irradiance level. However, the implementation of these MPPT
techniques becomes complex and expensive. They require the use of energy-
hungry devices like microcontrollers and some sensory circuitries, that is,
voltage and current to compute, process, and track the desired output power
in every processing iteration. In addition, at steady state, the operating point
of the PV module tends to oscillate around the MPP, thus giving rise to wasting
some harvesting energy [74].
In contrast, the CV technique is by far the simplest MPPT technique that can
be implemented. The operating voltage of the PV module V pv is kept near the
PV's MPP by matching it to a predetermined reference voltage. The reference
voltage is chosen to be close to the MPPT voltage V mppt . However, according
to Faranda et al. [134], the CV technique is mentioned to be the least-effective
MPPT technique. This is because the MPPT voltage tends to shift with the
varying solar irradiance and temperature. Hence, there is a small voltage
range where MPP occurs for the different operating conditions. Intuitively,
the CV technique will only yield an approximate MPP. The FOCV is an alter-
native to the CV method that is also a simple and cheap solution. It is based
on the voltage of the PV module at the MPP, V mppt = kV oc , which is approxi-

mately linearly proportional k to its open-circuit voltage V oc . During the SEH
process, the normal operation of the PV system is interrupted to measure the
open-circuit voltage V oc of the PV module. This is done by disconnecting the
PV module from the electrical load with an additional electronic switching
circuitry operating at a certain frequency. Once the open-circuit voltage is ob-
tained, it is multiplied with a predefined factor k to get the measured MPPT
voltage V mppt . The drawback with the FOCV method is that the interrupted
system operation yields power losses when scanning the entire control range.
To overcome this drawback, a pilot PV cell was proposed by Brunelli et
al. [136]. The pilot cell is supposed to share similar characteristics as the EH
PV module to obtain the open-circuit voltage of the PV module through the
pilot PV cell. Therefore, it is not necessary to disconnect the PV module from
the load in order to obtain the open-circuit voltage. However, the challenge
with this approach is that it is difficult to source for a pilot cell that has the exact
characteristic as the EH PV module. In short, each MPPT technique has its own
pros and cons. Hence, for small-scale EH, it is crucial to choose an appropriate
MPPT technique so the energy harvesting system is not overloaded but at the
same time achieves the MPPT effect.