Solar Electric Charging

Our solar charge controllers cost less than \$1, consisting only of over voltage protection provided by a diode chain that dumps excess heat into a solar electric cooker being developed by others on our team.

Research Students

Jojo Fleischman, Brandon Dela Cruz

This work is part of our research toward radically inexpensive solar electricity. Please meet our research team.  Additionally, this work has been addressed in projects in Schwartz’s Appropriate Technology Classes in Spring of 2017. Lastly, a group of 4 electrical engineering students wrote a Report on Solar Charging for EE413, an advanced design class.

Objective

We wanted to develop a system that functions like a charge controller, so that the voltage outputted by a photovoltaic device can be easily regulated and tweaked for the user’s interests. This system also needed to be cheap, cost effective, and sustainable.

Why solar?

Currently, residents in rural Africa do not have a reliable, cheap way to charge their cell phones. They are forced to run a trek to the nearest shop connected to the electrical grid, usually hours away from their homes. With our research in a charge controller that is extremely inexpensive, these people will be able to charge their devices using solar panels they can set up right at home.

Solution

The solar electric cookers use strings of diodes to cook food, and each diode in the diode chain has a near constant voltage drop of ~0.7 V when sufficient current (1-6 Amps) is driven through them. Thus, a string of diodes can perform the job of voltage regulation and follows the criteria listed above. By utilizing a set number of diodes, we can define the right amount of voltage for a cell phone or a LiFePO4 battery to power LED lights at night (see image below). We use rectifier diodes(1N5408) costing about \$0.01 each that are rated up to 150°C. We have found that the diode functions for a junction temperature of up to about 300°C.

A reliable, maximum power point tracking (MPPT) charge controller can cost up to \$100 or more, while our string of diodes can be made for less than a dollar. That price difference makes our system much more pratical for communities in Africa who are looking to save on everything they have to buy.

Charging Setups: With different solar panel arrangements, we wanted to confirm that charging via diodes was achievable and to create characteristic curves to find the maximum power point.

We added solar panels to an electric-assisted bicycle pedicab so it can run on sunlight.