Energy harvesting is collecting small amounts of energy from the world around and converting it into a form that is usable by a device. By collecting ambient energy a device can operate much longer than a battery operated device without maintenance. This enables devices to be installed in remote locations, in large numbers, and with low installation costs.
One source of ambient energy is light. Light can be harvested using solar cells, but in indoor light settings the optimal solar cells will not be the same as in outdoor installations. Before we dig into the differences in solar cells, let’s compare indoor and outdoor lighting. Indoor light is fundamentally different from sunlight. Sunlight has a board spectrum ranging from ultraviolet light (UV), through visible, and into the infrared (IR). Indoor lighting has been moving toward high efficiency lighting types like LED and fluorescent. These lights emit primarily in the visible spectrum with little UV or IR, which leads to their high efficiency. The second major difference is the overall light intensity. Sunlight has an intensity of about 120,000 lux, while indoor lighting ranges from 50-1000 lux.
Now let’s define solar cell efficiency. Solar cell efficiency is a measure of the amount of power produced for a given area compared to the amount of optical power emitted by the sun. For example, the sun produces 1000W/m^2 of optical power over all wavelengths from UV to IR. If a solar cell were 100% efficient, a 1 m^2 panel would produce 1000W. Translating solar panel efficiency from outdoors to indoors is not straightforward. Both the limited light spectrum and the low optical power change the efficiency equation significantly.
Limiting the light spectrum to visual light will level the playing field for efficiency. For example, amorphous silicon only absorbs in the visual spectrum and crystalline silicon can absorb UV, visible, and IR; however when only the visual spectrum is present both technologies are capable of absorbing all wavelengths of light present.
In the presence of low light intensity, solar technologies with the least parasitic loss will operate most efficiently. Cell cutting and low quality silicon processing can lead to high material defect rates and low indoor efficiency. At high intensities, the parasitic losses can be a small fraction of the total power generated, so a technology could perform very well in high intensity outdoor light. However, when that same technology is brought indoors and the light intensity is only 200 lux (~1.7% of outdoor sun) parasitic leakage in the device can become quite significant, reducing overall efficiency.
Please join my session at Sensors Converge on September 21 at 9:50 a.m. to learn more about solar for energy harvesting. I will be talking about the differences between indoor and outdoor light, why a higher rated efficiency may not mean more power, how to properly size solar for your application, and what features to look for in a PMIC when pairing it with solar.
Daniel Stieler is president of PowerFilm, Inc., based in Ames, Iowa. Sensors Converge runs from Sept. 21-23 in San Jose with many sessions provided digitally as well. Register to attend here.