Students from the Bochum University of Applied Sciences in Germany have been building solar cars since 1999. They are facing their most complex design challenge yet in creating a vehicle to race in the World Solar Challenge next year.
The 16-student team is seeking to establish a Guinness World Record in a 224-mile jaunt in the Simpson Desert in Australia. The record time for the journey from Purni Bore to Birdsville is 4 days, 21 hours and 23 minutes, and was set in 2017. The students are building a solar car that has to withstand desert temperatures that can reach 122 degrees, sandstorms, rugged terrain and the impact of elements on the car’s mechanical systems.
“We wanted to take solar mobility off-road and use solar energy in an area it is suited best: the desert,’’ said Birgit Reuter, one of the students on the team. “It was just a small step for us to go from an Australian road to the Australian desert. And, there are not exactly a lot of deserts in Germany.”
The desert destiny
Simpson Desert stretches south from Alice Springs towards the South Australia border and is host to some of the best four-wheel driving in the country. It is the fourth largest Australian desert, with an area of more than 68,000 square miles and home to the world’s largest sand dune desert.
Popular among tourists, the region is described as an erg, which is a flat area covered with wind-swept sand and little or no vegetative cover. No maintained roads cross the desert. The French line is the shortest and most direct route across the Simpson and is only a single lane. Simpson Desert averages less than six inches of rain per year, and gained notoriety in the 1950s as a major (but unsuccessful) search area for oil and gas.
The vehicle the students are building is a buggy that they have called “Froggee,” in that it looks like a frog. The students have been building solar cars for 19 years but built their first buggy in 2013. “Our first buggy was a conversion from a small, combustion buggy. Now it is an electric buggy built from scratch, much bigger and with more power,’’ Reuter said. “The old buggy was designed for a rocky desert, this one faces a much bigger challenge in a sand desert, so it needs more power and has gotten bigger in every way.”
Standing up under the sun
The vehicle the students are building is similar to the off-road cars used in the Dakar Rally. It includes two seats and is compact, but also must be able to stand up different surfaces such as dunes, rocks, mud, and erg.
The batteries are lithium ion, charged by a 40m² solar array that consists of 144 panels with 1052 silicon solar cells. “We don’t have any infrastructure in the desert, so we’re charging within three hours via a 40m² foldable solar array,’’ Reuter said. “This is basically our charging station to go. The energy is stored in a battery, just like any electric vehicle.” The output voltage is 650V at 6kW and 9.2A. With it, the buggy can be fully charged within 3 hours. Reuter believes that the buggy will be able to drive 100-150km on one charge. She added that, “Frankly, we will just have to test it to see how far it can go.”
Folding solar panels
The folding solar panel design is based on an origami folding technique called miura fold. The enables the array to be unfolded simply by pulling out the two outermost (top and bottom when folded) panels.
The team designed the folding panels, soldered and laminated them. They designed a frame to mount them on to get the right angle to the sun. Folding was a necessity as they need to carry the array around inside the buggy. Then every time the car stops, the team unfolds the array for a charge, and then refolds it before driving. This may be the first terrestrial foldable solar array, as before foldable arrays were only needed in space.
The more challenging assignment for the young engineers is preparing the vehicle for the climactic conditions in the desert. Besides heat, the components have to resist dirt and debris. The fine granules of sand could impact the performance of the vehicle’s components. “The desert is quite demanding, and we frequently couldn’t just take any parts that might fit the specs of the vehicle,’’ Reuter said.
The students’ vehicle includes two motors, one for each wheel axis. Each motor has a controller, and cables connect the controllers to the motors. Reuter said getting the controllers to guide the motors proved challenging and required the use of some specialty components. Among them are 10 cables manufactured by igus, a company based in Germany. The company, which runs its North American operations out of Providence, Rhode Island (US), offers “motion plastics” solutions that benefit a wide range of industries. For this application, students used the company’s chainflex cables.
“We needed a shielding in our motor cables because the motor controllers are inverters and bring out alternate current to the motors,’’ Reuter said. “In this field, it is essential to have good shielding to prevent electromagnetic compatibility (EMC) problems. We needed cables that are very robust and durable and at the same time conduct 400 volts. Since space is tight, they also needed to be bendable.”
Tough road ahead
Reuter said the team has some other concerns with the vehicle. They have gone to extreme lengths to ensure that the vehicle’s cooling system can withstand the soaring desert temperatures and have created barriers in components so that sand does not infiltrate other parts of the vehicle. Sand can act like sandpaper and cause abrasions in some components.
“The electric box will need to be tightly closed and all the outer parts need to be at least IP65,” Reuter said. IP65 is a standard established by the Electro-Technical Commission for Ingress Protection, and that figure means the assembly is totally protected against dust ingress. “We’re also worried about the vibrations, which will be even stronger when going off-road,’’ Reuter said. “We’re trying to make adjustments for that.”
Students designing the vehicle are on the path to degrees in mechanical and electric engineering, computer science, business, economics, and sustainability. In its last competition in 2017, the team struggled from the outset and needed to push the car across the finish line. While the vehicle for the 2019 race is still under construction, Reuter said the team is ready to challenge the existing record.
“Our vehicle is specifically made for these conditions, so we believe we will definitely be faster,’’ Reuter said.
Written by Thomas Renner, a freelance writer specializing in technology.