Study of the Applicability and Limitations of the FOCV Technique for Indoor Low-Area PV Cells

In the context of energy harvesting for autonomous sensors placed indoors, an evaluation of the fractional open-circuit voltage (FOCV) as a tracking technique for low-area photovoltaic (PV) cells is carried out in this article. The applicability and limitations of the FOCV are experimentally demonstrated by means of four commercial FOCV-based chips (BQ25570, ADP5092, SPV1050, and MAX20361) and two low-area PV cells of different technology (amorphous and monocrystalline). The experimental results show that the efficiency of the tracking is quite high (i.e., more than 98%) in the range between 250 and 1500 lux and, therefore, the overall efficiency is practically equal to the efficiency of the ensuing power management unit (PMU). However, at low levels of illuminance (e.g., 100 lux), the tracking efficiency depends on the limitations of both the FOCV technique itself (i.e., the fraction of the open-circuit voltage to be applied does not remain constant) and the FOCV implementation (e.g., in the charging process of the capacitors involved at the input stage). These two limitations can partially compensate for each other in some scenarios so that high efficiency can also be obtained at 100 lux. With the knowledge of the previous limitations, methods for increasing efficiency are suggested and proven.