Experimental Evaluation and Comparison of FOCV-Based PMU for Indoor Low-Power PV Cells

In the context of energy harvesting for low-power sensors located indoors, this work evaluates the applicability and limitations of the fractional open-circuit voltage (FOCV) as a tracking technique for low-area photovoltaic (PV) cells. The study is carried out experimentally using three commercial chips (BQ25570, ADP5092, and SPV1050) that rely on the FOCV technique. Further, these FOCV-based chips are applied to two PV cells of different technology (amorphous and monocrystalline) to better observe the limitations under different operating conditions. According to the experimental results, in the range between 250 and 1500 lux, the efficiency of the FOCV tracking is very high (99%) for the three chips and the two PV cells under test. However, at very low levels of illuminance (e.g., 100 lux), the efficiency of the tracking depends on both the chip (BQ25570 being the most efficient) and the cell. In addition, in some scenarios (e.g., with the monocrystalline cell), the limitations of the FOCV implementation (especially, in terms of the charging process of the capacitors) can partially compensate for the limitations of the technique itself. As a consequence of that, at low levels of illuminance, we can have values of efficiency higher than those expected.