Every time you turn on a light in your home or office, you are wasting energy. But what if the flip of the light switch also meant power generation?

We usually think of solar cells, or photovoltaics (PV), fixed to the roof, converting sunlight into electricity, but bringing that technology indoors can further increase the energy efficiency of buildings and activate parts of smart wireless technologies, such as smoke alarms, cameras and temperature sensors, also called Internet of Things (IoT) devices. Now, a study by the National Institute of Standards and Technology (NIST) suggests that a direct approach to capturing light indoors may be achievable. NIST researchers tested the internal charging capability of small modular PV devices made of different materials and then connected the lowest efficiency module – consisting of silicon – to a wireless temperature sensor.

Team results, published in the journal Energy Science & Engineering, demonstrate that the silicon module, absorbing only light from an LED, supplied more power than the sensor consumed at work. This result suggests that the device can operate continuously while the lights remain on, which will eliminate the need for someone to exchange or recharge the battery manually.

“People in the field have assumed that it is possible to power IoT devices with PV modules in the long run, but we have not really seen the data to support them before, so this is a kind of step. first to say we can pull the eraser, “said Andrew Shore, a NIST mechanical engineer and lead author of the study.

Most buildings are illuminated by a mixture of sunlight and artificial light sources during the day. At dusk, the latter can continue to supply power to the equipment. However, light from common internal sources, such as LEDs, involves a narrower spectrum of light than the broader bands emitted by the sun, and some solar cell materials are better at capturing these wavelengths. than others.

To find out exactly how several different materials would be collected, Shore and colleagues tested mini PV modules made of indium gallium phosphide (GaInP), gallium arsenide (GaAs) – two materials directed at white LED light. – and silicone, a less efficient material, but more affordable and common.

The researchers placed the modules centimeters wide under a white LED, placed inside a dark black box to block external light sources. The LED produced light with a fixed intensity of 1000 lux, comparable to light levels in a well-lit room, for the duration of the experiments. For silicon and GaAs PV modules, indoor light soaking proved to be less efficient than sunlight, but the GaInP module performed much better under LEDs than in sunlight. Both GaInP and GaAs modules significantly exceeded the silicon inside, converting 23.1% and 14.1% of the LED light into electricity, respectively, compared to the 9.3% silicon power conversion efficiency.

Unsurprisingly for the researchers, the rankings were the same for a charging test in which they determined how long it took the modules to charge a semi-charged 4.18 volt, ultra-silicon battery with a margin of more than a day half.

The team was interested in learning if the silicon module, despite its poor performance compared to its top competitors, could generate enough power to run a low-demand IoT device, Shore said.

Their IoT device chosen for the next experiment was a temperature sensor they connected to the PV silicone module, placed once again under an LED. By turning on the sensor, the researchers found that it was able to power wireless temperature readings on a nearby computer, powered only by the silicon module. After two hours, they turned off the light in the black box and the sensor continued to work, its battery depleted to half the speed it needed to charge.

“Even with a less efficient mini module, we found that we could still supply more power than the wireless sensor consumes,” Shore said.

The researchers’ findings suggest that an already ubiquitous material in outdoor PV modules could be reused for indoor equipment with low-capacity batteries. The results are especially applicable to commercial buildings where the lights are on all the time. But how well would PV power appliances work in spaces that are only lit intermittently during the day or closed at night? And how much factor would be the ambient light coming from outside? Homes and office spaces are not black boxes.

The team plans to address both questions, first by setting up light metering devices at NIST’s Net-Zero Energy Residential Test Center to gain an understanding of what light is available all day in an average residence, Shore said. . They will then repeat the zero-mesh home lighting conditions in the lab to discover how PV-powered IoT devices work in a residential scenario.

Feeding their data into computer models will also be important to predict how much power PV modules will produce indoors given a certain level of light, a key capability for implementing cost-effective technology.

“We are turning on our lights all the time and as we move more towards computerized commercial buildings and homes, PV can be a way to reap some of the lost light energy and improve our energy efficiency,” he said. Shore.