Indoor Lights Can Act as Solar for IoT Devices

Naomi R. 

We have been dependent on electricity to power our homes, and as time passes, recent developments have been created to further aid environmental sustainability. This includes Solar cells, also known as photovoltaic (PV) cells, are fixed to roofs to convert sunlight into electricity. Bringing the same technology indoors has the potential to enhance the energy efficiency of buildings. In addition, PV cells are capable of energizing IoT devices such as smoke alarms, cameras, and temperature sensors.

According to a study by the National Institute of Standards and Technology (NIST), a straightforward approach for capturing light indoors may be available very soon. NIST researchers tested the indoor charging ability of small modular PV devices made of different materials and then hooked up a silicon module to a wireless temperature sensor.

The team’s findings, published in the Energy Science & Engineering journal, demonstrate that the silicon module, which absorbs light from an LED, supplied more power than the sensor consumed while in operation. Thus, the device could run continuously while lights remain on, doing away with the need to manually exchange or recharge the battery.

“People in the field have assumed it’s possible to power IoT devices with PV modules in the long term, but we haven’t really seen the data to support that before now, so this is kind of a first step to say that we can pull it off,” Andrew Shore, a NIST mechanical engineer and lead author of the study, stated.


Let there be light

During the day, most buildings are lit by a mix of both the sun and artificial light sources. 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. Some solar cell materials are better at capturing these wavelengths than others.

To find out exactly how different materials would stack up, Shore and his colleagues tested PV mini-modules made of gallium indium phosphide (GaInP), gallium arsenide (GaAs), and silicon. GaInP and GaAs are two materials geared toward white LED light.

The researchers placed the modules underneath a white LED, housed inside an opaque black box to block out external light. 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 same results were observed when the team conducted a charging test. The researchers 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.


Singling out silicon

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

Their IoT device of choice for the next experiment was a temperature sensor that they hooked up to the silicon PV module, placed once more under an LED. Upon switching the sensor on, the researchers found that it was able to feed temperature readings wirelessly to a computer nearby, solely powered by the silicon module. They switched off the light in the black box after two hours and the sensor continued to run, its battery depleting at half the rate it took to charge.

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

The researchers’ findings suggest that an already commonplace 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 constantly switched on. 

However, the team wanted to know how well would PV-powered devices run in spaces that are only lit intermittently throughout the day or shut off at night. Another factor in consideration was the ambient light from outside.

The researchers set up light-measuring devices in NIST’s Net-Zero Energy Residential Test Facility to understand how much light is available throughout the day in an average residence. Afterwards, they would replicate the lighting conditions in the lab to find out how PV-powered IoT devices perform in a residential scenario.

Feeding the data into computer models is an important factor to predict how much power PV modules would produce indoors given a certain level of light, a key capability for cost-effective implementation of the technology.

“We’re turning on our lights all the time and as we move more toward computerized commercial buildings and homes, PV could be a way to harvest some of the wasted light energy and improve our energy efficiency,” Shore said.

As NIST continues to break new ground in energy efficiency studies, technology can be leveraged further not just to add convenience to our lives–it can also help sustain the world that we live in.

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