Outdoor vs Indoor Solar: The Key Differences
Posted on 08/10/2018 at 12:55 PM by Samuel Jones
Can I use my solar panel indoors?
We get that question a lot at PowerFilm. The answer isn’t as cut and dry as you might think.
To our eyes, indoor environments seem pretty bright. That’s only because our eyes are so good at adjusting.
In actuality, indoor lighting can be more than 1,000 times less intense than direct sunlight. That means there’s 1,000 times less power available for a solar panel to collect.
To compare indoor and outdoor intensity, we need to look at the Lux level, which measures how bright something is to the human eye. Literally, it’s a count of how many visible light rays are entering your eye per unit area.
Looking at the table below, we can see the drastic change in Lux level going from direct sunlight down to dimly lit indoor environments. Even in the same room, the power available can double or triple just by getting closer to a window or standing directly under a light.
Power Produced by PowerFim a-Si Solar Panels in Different Indoor and Outdoor Environments.
To answer the original question, only solar panels that are optimized for indoor/artificial light will work, such as PowerFilm’s Low Light Series OEM solar panels.
To see why, we need to look at the differences between sunlight and indoor/artificial light.
The two major differences are light intensity, as we have seen, and light spectrum or “color”.
During manufacturing, small defects can become incorporated into the solar material. These can occur when crystalline wafers are cut into smaller pieces or during the material fabrication process.
In outdoor environments these defects have little to no impact because of the large amount of available power. As the light level decreases, the impact of defects will become greater. At indoor light levels, small defects can render a solar panel inoperable.
Adjustments can be made to the manufacturing process to mitigate the impact of defects at low light levels albeit at the cost of outdoor performance.
The second major difference between outdoor and indoor light is their spectrums. Every light source, such as LEDs, Fluorescents, or the sun, is made up of a different combination of colors. The figure below shows a spectral comparison of the different light sources.
Light spectrum of the Sun, LEDs, Fluorescents, and Incandescents (Upper). Quantum efficiency of a-Si, c-Si, CIGS, and GaAs Solar Technology (Lower).
The Sun is a nearly continuous combination of colors from blue to red and even extends into the infrared (heat).
LEDs and Fluorescent lights, often used indoors, have more discrete light spectrums where a couple specific colors combine together to form white.
Unfortunately, solar panels aren’t colorblind and respond to each color of light differently. Spectral behavior is defined by Quantum Efficiency (QE). QE measures how well a solar panel can absorb and collect different colors, or wavelengths, of light. The QE of popular solar technologies are plotted in the above lower figure.
To collect the maximum amount of light possible from the sun, most technologies, such as c-Si, GaAs, and CIGS, are optimized to absorb visible and infrared light. Panels that use these technologies often perform poorly in indoor environments where there is no infrared light.
Amorphous silicon only absorbs light in the visible range and is therefore well matched to indoor light sources such as LEDs and Fluorescents.
Knowing your light source and having realistic power expectations will help you choose the right solar panel for your application.
For outdoor environments, PowerFilm’s Wireless Electronics Series OEM, Rollable, Foldable, and Soltronix products are well suited for medium and high power applications from 10mW up to 5kW+. These outdoor panels can power IoT devices, like cellular gateways and security cameras, or keep your electronics charged on your next outdoor adventure.
For indoor, artificial light environments, PowerFilm’s Low Light Series OEM panels are well suited for low power applications ranging from 10uW to 10mW. Many wireless sensors, small motors and actuators, and low power LEDs fall in this range. PowerFilm’s Indoor Development Kits are great tools for exploring the possibilities of indoor solar.
What can you power with solar?
When it comes to remote applications, portable power, and self-sustaining devices, the possibilities are endless.
Whether indoors or outdoors, PowerFilm can work with you to develop the best solar solution for your application.
Contact us and let's started on your solution today.
Categories: Solar Education