4.1.1. Operational output current range
Energy harvesting devices do not operate at the MPP all the time. Circuits are available to operate solar modules at close to the MPP by actively modifying the load, but these require power to operate and so reduce the overall efficiency of an energy harvesting system.
In cases, such as use indoors under low levels of illumination, the available energy may be constrained to the extent that it is not
acceptable or practicable to accommodate the additional energy overhead associated with an MPP tracking circuit.
Instead it may be necessary to fix the operational point
such that the device is operating close to the MPP under
the most commonly encountered operational conditions.
In this case it is necessary to use a measure of how well
the energy harvester performs when it is not at the ideal
load. In this paper the range of output currents at which
the harvester provides at least 90% of the maximum power
density, under the light source of interest, is used to gauge
how robust a device is when deployed in different conditions.
The output current from each solar module was measured
from zero (open circuit) to the maximum current
(low load resistance) at which at the output power density
of solar cell is less than 10% of that at the MPP. Table 2
shows the range of output currents at which the solar cells
can generate at least 90% of the power density at the MPP
for the illumination source being tested at an illumination
level of 500 lx.