[Editor’s Note: If you’re not sure what an IV Swinger does Chris has a detailed two part video explaining the background of this project, what it does and why he made it. You can watch the first video here.]
The IV Swinger is an IV curve tracer for photovoltaic (PV) solar panels. I built it for my friend Professor Gil Masters to use in the lab for his CEE176B course at Stanford (“Electric Power: Renewables and Efficiency”). It was used in the Spring 2015 and 2016 CEE176B labs. It was also used by the Stanford Solar Car project in 2015.
An IV curve plots the current and voltage generated by a PV panel for different load resistances ranging from short circuit to open circuit. The point on this curve that has the largest V * I product is the maximum power point (MPP). The shape of the IV curve and the MPP for a PV panel depend on factors such as its temperature and the intensity and uniformity of the sunlight falling on its surface. A typical IV curve of an unshaded PV panel looks like this:
But if single cell on the panel (1/60 of the panel) is shaded, the IV curve looks like this:
The maximum power has dropped by 40% even though less than 2% of the surface is shaded! Understanding IV curves for PV panels under different conditions such as shading is an important part of learning about the technology. The IV Swinger enables students to quickly and accurately generate real-world IV curves for PV panels.
The IV Swinger consists of a bank of loads and relays to select whether each load is included in the circuit or bypassed. It has an ammeter and voltmeter to measure the current and voltage at each point. Python code running on a Raspberry Pi controls the sequencing, takes the measurements, and generates the IV curve graphs. The graphs are written as PDFs to one or more USB thumb drives and the raw data is also written as CSV files.
I posted the following YouTube video demonstrating the IV Swinger in action:
It should be quite possible for others to replicate and hopefully improve on the design of the IV Swinger. My hope is that the design could be used for other college or even high school courses teaching PV technology. It’s not a “weekend” project, however, and step-by-step instructions would be too lengthy for an article like this. I posted another YouTube video giving an overview of the design and construction:
If you are interested in building an IV Swinger, the full design documentation (including Fritzing file) and software is posted on GitHub.
