Solar or photovoltaic (PV) cells fixed to roofs convert sunlight into electricity. Bringing that technology indoors could further boost the energy efficiency of buildings and energize swaths of wireless smart technologies such as smoke alarms, cameras, and temperature sensors.
A straightforward approach was developed for capturing light indoors. Researchers tested the indoor charging ability of small modular PV devices made of different materials and then hooked up the lowest-efficiency module — composed of silicon — to a wireless temperature sensor. The results demonstrate that the silicon module, absorbing only light from an LED, supplied more power than the sensor consumed in operation. This suggests that the device could run continuously while lights remain on, which would do away with the need for someone to manually exchange or recharge the battery.
Most buildings are lit by a mix of both the Sun and artificial light sources during the day. At dusk, the latter could continue to supply energy to devices. However, light from common indoor sources, such as LEDs, spans a narrower spectrum of light than the wider bands emitted by the Sun and some solar cell materials are better at capturing these wavelengths than others.
To find out exactly how a few different materials would stack up, the team tested PV mini-modules made of gallium indium phosphide (GaInP), gallium arsenide (GaAs) — two materials geared toward white LED light — and silicon, a less efficient but more affordable and commonplace material. The researchers placed the centimeters-wide modules underneath a white LED housed inside an opaque black box to block out external light sources. The LED produced light at a fixed intensity of 1000 lux, comparable to light levels in a well-lit room, for the duration of the experiments. For the silicon and GaAs PV modules, soaking in indoor light proved less efficient than sunshine but the GaInP module performed far better under the LED than sunlight. Both the GaInP and GaAs modules significantly outpaced silicon indoors, converting 23.1% and 14.1% of the LED light into electrical power, respectively, compared with silicon’s 9.3% power conversion efficiency.
The rankings were the same for a charging test in which they timed how long it took the modules to fill a half-charged 4.18-volt battery, with silicon coming in last by a margin of more than a day and a half. The team was interested in learning if the silicon module, despite its poor performance relative to its top-shelf competitors, could generate enough power to run a low-demand Internet of Things (IoT) device.
The IoT device chosen for the experiment was a temperature sensor that was hooked up to the silicon PV module, placed once more under an LED. Upon turning the sensor on, the researchers found that it was able to feed temperature readings wirelessly to a computer nearby that was powered by the silicon module alone. After two hours, they switched off the light in the black box and the sensor continued to run, its battery depleting at half the rate it took to charge.
The researchers’ findings suggest that an already ubiquitous material in outdoor PV modules could be repurposed for indoor devices with low-capacity batteries. The results are particularly applicable to commercial buildings where lights are on around the clock.