Shining a light on solar racing: the power of a data driven car
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March 25, 2023
The Innoptus Solar Team is a group of 20 engineering students who design and build a solar car every two years to compete in international solar races. Multiple teams have to travel about 4000 km's throughout deserts, mountain ranges, coasts etc. in Morocco, South-Africa and Australia. Right now, the team is preparing to participate in the Bridgestone World Solar Challenge 2023, with their newest solar car.
The car does a lot of data logging. In particular, the data we collect is crucial to our battery management strategy, which is key to our success in these races. We need to ensure that our battery lasts the entire race and is not depleted too early on, while also driving the race in the most efficient way possible.
The solar car is designed, optimized and produced almost entirely by the student team. This includes the motor, battery and solar panel. All of those systems need to be monitored thoroughly during the race. This monitoring happens with a self-written monitoring program, that stores and visualizes data from the solar car constantly throughout the race. During each day of the race, a big database is built with the metrics of the car during that race day. Visualizing this database during and after a race day helps the team a lot with interpreting all of these signals. For example, we record the speed and motor temperature during a race day:
So why do we need these measurements?
The strategic aspect of a solar race, is mainly battery management. First, how can we drive a race, so that the battery rests the entire race, and isn't empty after day 1? Second, how can we do this in the most efficient way, aka how can we drive the race in the least amount of time and arrive at the finish line with not too much energy left?
In this data story, we want to explain how we use data to make decisions during the race. The data that we are using is the data log from the last day of the SASOL Solar Challenge 2022. We reached second place in this race, after an unfortunate incident with our battery, which will be explained in more detail later in this story.
Monitoring the solar panels
The solar panel that is mounted on top of the car, consists of 3 strings. These are a bunch of solar cells placed in a series, to collect as much energy as possible. These 3 strings are all passed to our MPPT system, which stands for Maximum Power Point Tracking system. The power is logged separately for each string, as well as the total power:
One thing immediately clear is that strings 1 and 3 are the smaller strings on the side of the canopy, yielding less power. You can also see an interesting phenomenon that is very specific to this race. Between 12h and 14h15, we were driving the loops of that day. It is part of the race format: in the SASOL Solar Challenge, every day there is an option to drive a certain part of the route (a loop) as often as you want. In the end, the team that has driven the most km’s wins the race.
During the time that we are driving in these loops, you can see that string 1 and 3 are kind of doing opposite things. While one goes up, the other one goes down. This is because of the shadow the canopy causes to fall on one of the two strings, depending on what direction we are driving. When one string is partly in the shadow, the other one will also be directed to the sun. You can see this pattern repeat itself four times, meaning we drove 4 loops that final day. If we zoom in on the plot above, we can see this more clearly:
In the incoming energy plot you can see a couple of spikes. These happen during the stops of the race day. Each day of the race, a control stop is organized, where we have to take a break that takes 30 minutes. After each loop, we are required to do a loop stop. This is an obligated break that takes 5 minutes. During these breaks, we can actually open our solar panel and direct it towards the sun. This is why there is such a big spike in incoming energy. Zooming in on one loop shows the 5 minute break clearly at the end:
Insights in the batteries
Managing the battery of the solar car is one of the most important tasks of the strategy department. It consists of a bunch of small battery cells, linked together. During the race, our battery gets drained by the motor.
During the last day of the SASOL Solar Challenge, we had an issue with our battery pack. There was unbalance between the cells in the pack. Some were a lot more empty than others, and a battery pack is only as good as its most empty cell. You can see in the last day that one hour after starting the day, one cell in the battery pack starts dropping significantly. For safety, we decided to slow down until the control stop, and perform a procedure there on the battery pack to remove/uncouple the affected cells. This procedure went as trained, and after a while we were back on the road: (click to expand the plot)
The SASOL Solar Challenge differs from the traditional Australian Solar Challenge by adding two special stages to the race. Basically, throughout the race there are two days where you get the route information the night before that race day. For our team it was important to find a way to quickly be able to go from an organization route file, to an overview of this special race day that can be easily interpreted. We want to be able to see where the biggest challenges on the road are, like traffic, slope and speed limits. Combining a map with a speed limit and altitude plot gives us exactly what we need:
In the plot above, you can interactively click the map and it will show you the exact moment in the graph below it. This helped us with identifying the exact location of steep mountain roads, and in general made it very easy for us to navigate the route file that the organization provides for the teams. We start off with a very dry csv file that is basically an information bomb, and with a few clicks we are able to visualize all of this in a clear way. This helped save a lot of time and also provided an easy way to do a sanity check on the route files, since they are also used in the strategy software.
If you read this far, thanks for your interest in how we use data of our solar panels, batteries and race strategy! Love from the Innoptus Solar Team.
Written by Tine Wildiers. Data is captured from a real race in September 2022, in South Africa. Visualisations done in Marple.
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