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.
See photos of the solar-powered cars below.
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.”
A side view of the Principia College car. It’s evident that teams put a lot of work into aerodynamic analysis.
This view of the car designed by students at Principia College shows the small vent (red circle) providing the driver with outside air. The Principia car was also the only one we noticed that used tinted plastic for the cockpit to cut down on the radiant energy coming in. Designers of the car say they configured the strings of solar cells so cells on surfaces having the same angle to the sun connect on the same string. This helps the car’s MPPT tracking.
The University of Michigan team has won the Solar Challenge several times in the past. The U of M car was the only one we saw where the driver sits side saddle style rather than in the center of the car. This is because the car is configured a bit like a catamaran.
The underneath view of the U. of Mich. car makes it clear why the driver sits to the side. The catamaran-type configuration is done for aerodynamic reasons.
The University of Kentucky car got frsh air to its driver via a plastic tube (red circle) from a vent in the front. The team also put gallium arsenide solar cells (here outlined in red) in areas where silicon cells wouldn’t fit.
The powered wheel on the University of Kentucky entry: The three-wheeler also sits on an aluminum frame. Cars generally were on either aluminum or carbon fiber chassis. Carbon fiber versions were a bit lighter though more expensive.
The University of Minnesota fielded what’s called a cruise class car, so named because it seats two people. Team members say they scratch-built many components on the car, such as the electric motor, that other teams bought from outside vendors.
One thing the University of Minnesota team had to do was add a crumple zone (outlined in red) to its cruise-class car in order to compete in the Challenge.
Appalachian State University had the only entry we noticed to sport spoke wheels.
The Dunwoody College of Technology fielded this car built on a carbon-fiber chassis. It borrows lots of parts from go-karts and bikes and is powered by a special electric motor from Mitsuba. Team members say it cost about $500,000 to build.
This car from Iowa State configures its 391 solar cells into four subarrays for efficiency purposes. It weighs about 400 lb and will hit 70 mph, team members say.
Venting in the front of the cockpit cowling was a widely used technique for getting outside air to the driver. This is the three-wheeler from the University of Missouri.