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University challenge: solar technology
A team of students from the UK's Cambridge University set out to win the 2014 World Solar Challenge with a newly designed, super-efficient solar car
by Rachel Evans
The World Solar Challenge provides a platform for students to enhance their skills while also pushing energy-efficient technology to its boundaries. This annual competition is set in the outback of the Australian desert. Teams from all over the world work toward the same goal – to make it the full 3,000km from Darwin to Adelaide, in an all-electric vehicle that is powered by solar energy.
This year, the organizers altered the rules so that entrants are now allowed to enter only a four-wheeled vehicle. Using no more than 6 sq.m. of solar panels, cars are allowed a nominal 5kWh of stored energy. All other energy must come from the sun or be recovered from the kinetic energy of the vehicle. In addition to this, the drivers and members of their teams must battle the burning heat and lack of sleep, from start to finish.
A team from Cambridge University, consisting of members from several areas of academia such as engineering, physics, business and media, created ‘Resolution’, an all-new solar car for the 2013 competition.
Instead of the typical ‘table top’ design whereby the driver is fitted in between a silicone solar panel, Cambridge University Eco Racing (CUER) opted to use a tracking plate, which follows the sun to gather as much energy as possible. They also used galium arsenide solar panels, which are believed to be around 36% efficient as part of an assembled array.
Coupled with this, a carbon-fiber monocoque made in the style of an F1 car was built in collaboration with the UK’s National Composites Centre.
To design the car, CUER first drew it in SolidWorks, then put it into ANSA CAE software. Oliver Armitage, head of testing, says of the design, “The rules state that the driver has to sit with a certain back angle and that their heels are below their behind! The rules also state that in case of a crash, they have to be able swing forward.
“We designed the aerodynamic package around that volume of space. We took the curve and projected that forward to get the front shape of the car, which is a teardrop. It’s the most aerodynamic tail you can put on that and it is designed for the minimum coefficient of drag,” he says.
CUER received sponsorship in various forms from more than 25 parties, including suppliers such as SKF, Ansys, Millbrook Proving Ground and the National Composites Centre. Cambridge Precision and Jaguar Land Rover produced most of the parts.
Resolution is driven by an in-hub permanent magnet motor housed in the left rear wheel. It was produced to bespoke measurements by Australian supplier CSIRO, which designs them specifically for solar cars. The wheel had to be specially designed in order to house the motor, as Armitage explains: “We had to put a tower on the outside of the wheel rim and then inside we had to design it to hold those magnets and the load associated with that. The magnets are pulled together with the force of about 1.2 tons.
“We designed a dish with the tire on the outside, into which is bolted a ring of high-strength neodymium magnets. A second ring of magnets is located in a cap, which is lowered onto the main dish by a special rig. This allows the two magnet rings to be brought together in a controlled way despite the extremely strong forces of attraction between them. There are coils of wire located in the gap between these two magnet rings.”
Match Tech optimized this proposed wheel design in finite element modeling to make it thinner and lighter.
The battery is fan-cooled and the motor is air-cooled; in a previous race, during a stoppage, an incident occurred where a battery caught fire. CUER wanted to ensure this didn’t happen to Resolution. “Part of the design of the motor casing is to have the components that get hot, thermally connected to the outside walls so they can transmit the heat out,” says Armitage.
Student Alex Robinson, who has previously undertaken internships at McLaren Racing, designed a suspension system that features double-wishbones with a pushrod shock at the front and swing arms, one for each wheel, with a big anti-roll bar between the two at the rear. Armitage says this was the easiest lightweight system to fit in the space that was available. He adds, “For the back we were considering a cantilever design – this would allow us to change between the two. But it’s a much more complicated design.
“We gave ourselves the option to upgrade to a lighter system that does the same thing on the rear if needed.”
At the beginning of the solar car program, the team identified specific areas to test. Armitage reports, “There are several vehicle requirements; for example, we had to be able to do a maneuver at a certain speed, and we had to be able to change lane and back again in a certain time, at a certain speed.” Initial test runs featured drives of just a mile or so.
Many teams that have previously won the World Solar Challenge, Armitage notes, have benefited from access to a full-size wind tunnel. Only at a slight disadvantage, CUER did some wind tunnel testing with a quarter-scale model in the facilities at Cambridge. Complementary to this, models were run through CFD in Ansys Fluent software.
The team aimed to ensure that at least a race day’s time had been spent in the car by the drivers, before the race. “We put the car on a constant circuit around the high-speed bowl at Millbrook. We constantly logged our power, our speed, and all the temperatures of the car,” says Armitage. A total distance of 50km was covered including turning circle and brake tests.
“If we knew that when we went above a certain speed, certain parts of the car might overheat, we would be prepared,” he adds.
CUER also built the battery, loaded it, and then discharged it under a resistor. During testing, the battery induced a voltage in the battery box that resonated back into the motor box and caused the motor to cut out. This problem was quickly fixed with capacitors, which were fitted to the motor.
However, Armitage notes, “Rarely do individual components fail. It’s usually the interfaces between them.”
As part of its goal to promote young engineering talent, JLR lent its environmental test chamber to the team. This enabled CUER to run the car, with the driver inside, at full speed in temperatures of around 40°C and wind speeds of 80km/h or greater, on a rolling-road with temperature-representative heat-loading. “This allowed us to monitor the temperature of both the driver and the technical components of the car that are heat sensitive, such as the battery and solar cells,” says Armitage.
Next, CUER used the test track at JLR to constantly run the car and perform reliability testing. Armitage explains, “This was similar to what we had done at Millbrook, but we used the JLR track as it was next door to where the components were being made, so the car as a whole could be tested with new components as they were put on.”
Unfortunately, come October, during last-minute testing on a specifically designated road in Australia, the car had an accident and rolled onto its side. Further tests carried out in controlled conditions revealed new dynamic instabilities that the team was unable to fix. It was forced to retire.
Despite the fact that CUER did not compete this year, the team has gained several positives from the experience. “The car’s crash structure performed as designed in the event of an accident,” comments Keno Mario-Ghae, team manager. “Also, Resolution produced its best ever results while testing in Australia, indicating that CUER would have been on target to be competitive.”
Future plans haven’t yet been agreed, however Armitage comments, “We believe the design principles embodied in Resolution are sound. I personally would like to see some variation of this design run in a future solar car race, whether that is the World Solar Challenge or otherwise.”