Saturday, July 30 marked that start of the 2016 American Solar Challenge. The eight-day 1,975-mile road course passes through seven states starting from Brecksville, Ohio and ending up in Hot Springs, S.D. We got a chance to talk to several of the college teams fielding vehicles in this year’s competition, including two of the top winners from this year’s Formula Sun Grand Prix, an annual track race that is held on grand prix or road-style closed courses.
Most teams use silicon solar cells which mount conformally to the car body. They are quite similar to those conventional solar panels but lack the coating normally put over top the roof-top versions which is left off in the interest of higher efficiency. Many parameters on the cars are set by race regulations. For example, contestants are limited to about six square meters of cell area. This generally gives the cars about a max output of a kilowatt to work with. To make things more interesting, that figure will shrink to four square meters in next year’s contest because teams are getting extremely good at squeezing power out of their cells. There are also regulations governing the amount of battery capacity the cars can carry, about five kilowatt-hours.
So teams end up attacking some problems in similar ways. Cooling the driver is one example. Driver ventilation in most of the cars we saw consisted of a single small hole in the body, slightly forward of the cockpit windshield. The car fielded by the University of Kentucky put the tiny vent hole up near the nose of the car, then routed the air back to the driver via a plastic hose. None of cars put any juice into an electric fan. Team members from the University of Michigan car said the cockpit area ends up being about 20 degrees hotter than ambient. Drivers need to stay hydrated.
Still, teams exercise a lot of design latitude in attacking problems. For example, some of the cars ride on just three wheels in a reverse-trike configuration for the sake of aerodynamics. There are different approaches as well for configuring motors. Firms such as Gochermann and Mitsuba make electric motors specifically for solar cars said to be about 95% efficient, and several of the teams use them.
But a few took a different tack. The University of Kentucky team, for example, used a 7.5-hp hub motor to drive its three-wheeled entry. The University of Minnesota team built its own motor, as did the University of Missouri team with a dual-stator motor for its three-wheeler. Team members claim the car can hit 90 mph though competition rules limit its top speed on the course to 65 mph.
When it comes to steering and braking, several teams borrow gear from go-karts or motocross bikes, though a few have built their own. But the wheels and tires on the cars are specially made for solar races, sporting solid wheels and a narrow tread.
All in all, though, use of specific technologies don’t seem to be the differentiators when fielding a winning car. Nearly every team we spoke with said reliability would likely be the competitive edge. “Teams that are still assembling their car an hour before the race starts aren’t going to field an entry that’s reliable,” said one University of Iowa team member. “You can’t win this race if you’re car is broken down on the side of the road.”