Test of Low-Area PV Cells Under Real Indoor Operating Conditions for Extended Periods of Time

In the context of powering indoor sensor nodes from the ambient optical energy, an experimental test of low-area photovoltaic (PV) cells under real indoor operating conditions for some weeks is proposed and carried out herein. The study involves PV cells of different technology (i.e., monocrystalline, amorphous, organic, and III-V type) and days with different weather (i.e., sunny, partly-cloudy, cloudy, and rainy). The experimental results show that, in a sunny day, the average (per day) power density can be up to 118 μW/cm
²
for the monocrystalline technology. This corresponds, for an active area of 10 cm
²
, to an average power of more than 1 mW, which is an acceptable value for a low-power sensor node. In a cloudy day, the average power density is around 25 μW/cm
²
for the monocrystalline, provided that the room under test is at the top floors of the building. However, under only the effects of artificial lighting, the average power density is really low, to be precise: less than 2 μW/cm
²
for the monocrystalline. This value increases up to 13 μW/cm
²
for the III-V cell, but this is an expensive technology. Therefore, the energetic survival of an indoor low-cost sensor node needs to leverage the natural light.