University teams from around the world design and build solar-powered cars to compete in the World Solar Challenge. Held every two years, this competition is designed to promote research and spur interest in solar-powered vehicles. For the most recent competition, 40 graduate and undergraduate students on the University of New South Wales (UNSW) Solar Racing Team designed and built eVe, the team’s fifth-generation solar car (Figure 1). We earned Line Honors as the first to cross the finish line in the 3000 km race from Darwin to Adelaide. Just nine months later, eVe broke a record that had stood for 26 years by travelling 500 km (311 miles) on a single battery charge at an average speed of 107 km/h (66.5 mph).
Without MATLAB® we would not have been able to pursue the record so aggressively. To make the most of the power stored in eVe’s lithium ion batteries (Figure 2), we needed to know exactly how the batteries would perform in various states of charge and under various loads. A MATLAB model enabled us to predict battery performance and determine how fast eVe should go to maximize battery usage. We also used MATLAB to monitor performance while eVe was on the track.
Modeling the eVe Battery Pack with MATLAB
We needed a model that accurately captured the nonlinear behavior of the batteries. Our MATLAB battery model is based on a widely used set of equations for battery dynamics. To make the model accurate we had to tune the parameters used in these equations to match the specific properties of the batteries we were using. We identified these parameter values by running numerous experiments in which we discharged batteries under a range of loads. We used Data Acquisition Toolbox™ and a National Instruments DAQ to measure and record voltages and currents during these experiments, and then used MATLAB to fit the equations to the measured results.
We selected MATLAB for this task for several reasons. First, since MATLAB is used throughout the engineering curriculum at UNSW, most students on the Sunswift team had used it before to solve engineering problems. Second, MATLAB was readily available. UNSW has a Total Academic Headcount license, making it easy for students to access MATLAB for our projects. Third, MATLAB code is easy for engineers to understand, which meant that new members of the team could get started quickly.
Sunswift at the World Solar Challenge
UNSW is a world leader in photovoltaic research, and in the early years of the World Solar Challenge numerous competitors used UNSW technology in their vehicles. In 1996, a group of UNSW students decided to capitalize on the university’s technology leadership and form a team themselves. The vehicle they developed, Sunswift 1, placed ninth out of 46 participants in its first competition (Figure 3).
In 2013, the competition added a cruiser class to encourage the development of more practical solar cars capable of carrying a driver and passengers.
Sunswift eVe was designed to compete in this class. With seating capacity for two, eVe is equipped with four square meters of silicon solar cells capable of producing 800 watts (Figure 4). The solar array drives brushless DC motors inside the car’s rear wheels, enabling eVe to reach a top speed of 132km/h.
eVe posted the fastest time in the inaugural cruiser class competition, traveling the 3022 km in 38 hours and 35 minutes.
Monitoring Performance in Real Time
By using MATLAB to monitor eVe’s performance during the race, we were able to quickly troubleshoot problems. For example, MATLAB analysis of battery discharge rates showed that the brakes began rubbing after the hand brake was applied. We had the drivers minimize their use of the hand brake until we could fix the problem.
We used a similar technique to identify faulty photovoltaic cells. Before the race we measured and analyzed cell output under various sun conditions to establish a baseline in MATLAB. By comparing the telemetry data from eVe to this baseline during the race, we were able to pinpoint malfunctioning cells within seconds.
Preparing to Set the World Record
After the World Solar Challenge, the Sunswift team decided to attempt to break a world record, and we had a specific record in mind: the fastest electric vehicle over a distance of 500 km. We chose this record because it fit with one of the team’s overarching goals: making practical use of solar technology. Consumers are often reluctant to purchase electric vehicles because they have range anxiety, or fears that the vehicle will not have sufficient power to reach its destination. We believed that if eVe could complete the 500km at a typical highway speed of 100 km/h on a single charge, it would alleviate this concern.
Our challenge was to determine the maximum speed at which eVe could travel and have almost no power left in the batteries after 500 km. (eVe’s solar panels would be covered throughout the run.) Before setting out, we ran simulations using our MATLAB battery model to see how fast eVe would be able to go. To account for hills and banked curves on the track at the Australian Automotive Research Centre in Anglesea, Victoria we collected GPS data during several test runs. We processed this data in MATLAB to create an altitude profile for the entire track that we could include with the battery model in our simulations (Figure 5).
We were pleasantly surprised when the initial simulations indicated that eVe could travel faster than 100 km/h and still have enough power to complete the run.
Moving Beyond the Finish Line
We set the record by cruising at over 110 km/h for much of the time, averaging 106.966 km/h with pit stops. Crossing the finish line to set the world record has proved to be more of a starting point than an end, however. Recruiters at top engineering companies were impressed with the Sunswift team’s achievement and what it showed about our ability to systematically solve challenging problems. Many of us have now begun our engineering careers with those companies.
eVe will be competing in the 2015 World Solar Challenge, which starts in Darwin on 18 October and ends in Adelaide on 25 October. Meanwhile, the current Sunswift team has embarked on its own ambitious project to make practical use of solar technology. They are redesigning and rebuilding eVe to make it Australia’s first road-legal solar sports car. They are also continuing longstanding community outreach programs in which team members visit local schools, use eVe to get students excited about engineering and renewable energy, and inspire the next generation of Sunswift engineering students.