Charging cell phones with the power of the sun

Solar cells charging an Android phone

When first asked to lead a hands-on project around renewable energy, the sheer volume of possibilities seemed intimidating. Energy source aside, I wanted something that would motivate our seventh graders to persevere through the inevitable challenges of their first week-long design project. What is it that a group of young adults with fairly reliable access to light and heat care most about powering? Their cell phones, of course.

Having identified the target device to power, there were still a multitude of renewable energy sources to choose between. Should we wire together a hundred pounds of potatoes? Build a hand crank generator using a DC motor or a magnet? Dive deep and build a Seebeck generator or a wind turbine?

In the end, I went with what I know – solar energy. Through two volunteer projects to install solar energy at orphanages in Africa, I’ve learned the basics of solar power and why it is incredibly important for the future of our planet. However, I also knew we didn’t need a (pricey) panel and inverter just to charge a low-end Android phone.

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My initial vision for the project was overly ambitious – students would start with solar cells and would wire them in different serial/parallel combinations, identify three or four independent variables to experiment with, and design a consumer charging device using what they discover. Our science teacher Ms. Chao, knowing that this would be our first week-long lab project, wisely helped scale the project back to focus on the key standards and topics we wanted to hit. With generous help from Mr. Williams, we carefully soldered the solar cells ahead of time, mounting them on cardboard to help prevent accidental breakage. Who knew unprotected solar cells were so brittle!

To successfully charge a phone, we needed to output somewhere around 1A at exactly 5V. To achieve this, we used at least three solar cells wired in series to reach the 1V minimum input of the 5V step up USB module we were using. The variables students manipulated were fairly straightforward – they measured the amperage and voltage output while adjusting distance from the lamp, changing the angle of incidence to the lamp, adding successive layers of tissue paper between the lamp and the solar cells, and with different numbers of solar cells wired in series.

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After an introductory lesson (available here) on solar energy and how solar panels work, students carefully captured data using Vernier LabQuests and worked collaboratively to analyze their results. They created research posters explaining the context of the experiment, their hypotheses and their results. They brainstormed modifications they would make to future iterations of their experiments and discussed how this technology would work as a consumer device. And despite the technical inefficiency of slowly charging a cell phone with a light bulb, hopefully at least a few were convinced of the importance of solar energy to combat global warming, even if only through the lens of having enough juice left to stay up browsing Instagram all night.

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