We tend to think of solar power as something that’s generated outside through solar panels and then used to power electricity indoors through some type of conversion system. But researchers also have been working to figure out ways to capture light from indoor sources to power small devices, such as those for the Internet of Things (IoT).
Researchers at the National Institute of Standards and Technology (NIST) have created small, modular photovoltaic (PV) devices made of different materials and then tested their indoor charging ability. They successfully used the lowest efficiency module, comprised of silicon, to generate electricity from an LED to power a wireless temperature sensor, they said.
While scientists already have believed it possible to use small PV modules to power IoT devices, “we haven't really seen the data to support that before now,” said Andrew Shore, a NIST mechanical engineer who worked on the project. The team’s work, then, “is kind of a first step to say that we can pull it off,” he said in a press statement.
Light from common indoor sources, such as LEDs, spans a narrower spectrum of light than the wider bands emitted by the sun. However, some solar cell materials can still capture these wavelengths, though some are better than others at doing so.
Finding the Right Materials
The team set out to find out exactly which ones would perform the best at doing this. They tested PV mini-modules about a few centimeters wide made of three different materials--gallium indium phosphide (GaInP), gallium arsenide (GaAs), and silicon. GaInP and GaAs are known to capture white LED light, while silicon is less efficient at soaking in this type of light but well known for its ubiquitous use in outdoor solar panels, and thus less expensive, researchers said.
Researchers placed the modules underneath a white LED and then housed the system inside an opaque black box to block out external light sources. The LED produced light at a fixed intensity of 1000 lux for the duration of the tests; this is comparable to light levels in a well-lit room, they said.
To no one’s surprise, the GaInP and GaAs modules performed far better at soaking up LED light than silicon, converting 23.1% and 14.1% of the LED light into electrical power, respectively. These numbers are compared with silicon’s 9.3% power conversion efficiency, researchers said.
The silicon and GaAs modules also still showed a less efficient performance with indoor light than sunshine, while the GaInP module performed far better under the LED than sunlight, they found.
The performance 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. The silicon PV module took the longest, taking more than a day and a half longer than the others, researchers said.
Putting Silicon to the Test Inside
However, even at this poor performance relative to the other modules in generating electricity and charging a battery, researchers still wanted to know if the silicon module, could generate enough power to run a low-power IoT device, Shore said.
The team chose a temperature sensor that researchers hooked up to the silicon PV module and once again placed under an LED. They turned the sensor on and found that it could feed temperature readings wirelessly to a computer nearby while being powered by the silicon module alone.
Moreover, even after two hours, researchers switched off the light in the black box and discovered that the sensor continued to run, its battery depleting at half the rate it took to charge, they said.
“Even with a less efficient mini-module, we found that we could still supply more power than the wireless sensor consumed,” Shore said in a press statement.
Researchers published a paper on their work in the journal Energy, Science & Engineering.
The findings indicate that even though it didn’t perform generally as well as its more expensive counterparts, silicon can be repurposed from its use in outdoor PV modules to develop indoor devices with low-capacity batteries, researchers said. These modules would be particularly useful in commercial buildings where lights are on around the clock, they said.
The team also plans to see how well PV modules could generate electricity to buildings that are lit only intermittently throughout the day through future research, first by setting up light-measuring devices in NIST’s Net-Zero Energy Residential Test Facility to gain an understanding of what light is available throughout the day in an average residence, Shore said.
They then plan to replicate the lighting conditions of the net-zero house in the lab to find out how PV-powered IoT devices perform in a residential scenario, he said.
Researchers then will feed the data they collect into computer models to predict how much power PV modules could produce indoors given a certain level of light. This type of data is key to one day implementing the technology on a broad scale cost-effectively, Shore said.
“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,” he said in a press statement.
Elizabeth Montalbano is a freelance writer who has written about technology and culture for more than 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.