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Follow the Sun Designing a solely solar-powered car that can travel long distances is a daunting task that calls for power management understanding, and all the trade-offs required ensure that the vehicle runs safely and reliably. Students at the University of California, Berkeley, have been designing and refining solar cars for over 20 years. Their shared goal has been to gain knowledge of sound engineering principles and put that knowledge to the test through a variety of solar car challenges. CalSol, the UC Berkeley solar vehicle team, used ANSYS fluid dynamics software extensively to determine the optimal shape for their most-recent one-seater, challenger-class solar vehicle called Zephyr. Fluid dynamics simulation performed on the outer carbon fiber shell of the car allowed the team to create the most aero- dynamic shape within design specifications and constraints, which include solar cell placement and optimal cells curvature, and dimensional requirements. Simulation enables the students to change a single design element and determine how the car will perform, without physical testing. Because solar vehicles are electrically powered, energy generated from solar cells is stored in lithium ion batteries — in this case, 420 units. These batteries are quite durable and perform well under normal conditions, but above a certain temperature, it is unsafe to charge the batteries. To ensure that this cut-off temperature is never reached, battery system ventilation is paramount. The team uses simulation with ANSYS CFD software to test ventilation for different battery layout designs, saving cost, time and manpower. Developing a solar vehicle for speed and endurance requires energy efficiency. CalSol optimizes as many vehicle elements as possible with simulation to reduce overall power consumption. By minimizing the drag coefficient for the shape of the solar vehicle, the team is able to maximize the output-to-input ratio of power. In addition, an overheated battery pack means that the car is unable to charge, and energy coming from the solar cells will be lost. Simulating airflow in the pack allows the team to target the temperatures of certain battery cells, aiding in creating a safe monitoring system and helping minimize energy loss from overheated battery cells. Zephyr has traveled over 70 miles per hour during preliminary road testing. It is expected to have a break-even speed (speed that the vehicle could travel with a net-zero power loss under perfect solar conditions) of 43 mph to 45 mph. Simulation has contributed in making all this possible. The team is now working on a more-practical two-seat solar vehicle. Information courtesy Holly Ubellacker and Ellande Tang. ACADEMIC ^ CalSol team “ Developing a solar vehicle for speed and endurance requires energy efficiency. ” ^ Battery pack temperature
