New contender for the World Solar Challenge

  • 09-Oct-2013 12:35 EDT

CUER's World Solar Challenge entry has sun-tracking solar cells under an aerodynamic canopy.

You’ve seen the Bridgestone World Solar Challenge (WSC) before. Extremely lightweight solar-powered cars, each “a big, flat tabletop of solar cells on wheels,” attempting to bisect the vast, empty aridity of the Australian Outback, driven by the sun’s rays alone. Top speeds can reach 50 or 60 mph (80 to 97 km/h).

Not only does the grueling 1882-mi (3029-km) race from Darwin through Alice Springs and down to Adelaide pose a formidable technical trial, the event has also been known to toss in random bush fires, vehicle fires, and even cloudy weather in addition to the relentless Aussie sun.

The 2013 version of this biennial torture-test runs from Oct. 6-13 with more than 46 teams, universities, corporations, and even high-school teams from 26 countries participating in three classes. Once the teams have left the starting line, they must travel as far as they can until 5 p.m. when they make camp in the bush where ever they happen to halt.

One car among this year’s premier Challenger field is taking a slightly different tack down the continent’s central Stuart Highway, hoping that a new idea just might tilt the contest in its favor. The team’s design will not only cut through air more efficiently because of its aerodynamic teardrop shape but also will maximize its share of the available sunlight by using a sun-tracking solar array to capture the most energy it can while it stays protected under a transparent canopy.

The builder, the Cambridge University Eco Racing (CUER) team, a U.K.-based team of 60 student volunteers, hopes that the extra 20% of power they collect by following the sun will give Resolution a leg up on some very tough competition, according to team manager Keno Mario-Ghae, a second-year undergraduate engineering student at Girton College. The ad hoc group includes mostly undergraduate engineering students with a leavening of graduate students.

“Last race we came in at the bottom third of the board so we need a different approach,” Mario-Ghae said.

In both 2009 and 2011 WSC races, Japan’s Tokai University, the Netherlands’ Nuon Solar Team (backed by the Delft University of Technology), and the University of Michigan Solar Car Team had finished first, second, and third, respectively.

“The narrowly beaten Dutch team has won the race four times in succession, so we are not underestimating the strength of the competition,” Mario-Ghae noted.

The key to these solar racer designs is to collect as much solar energy as possible while using as little as possible to travel at a certain speed. Beyond that, the big considerations come down to maximizing the efficiency of the energy transfer to the wheels while minimizing all sources of friction, including air resistance and rolling friction (which is related to vehicle weight). During the race, it’s all about energy management, the efficient balancing of power resources, and power consumption. At any moment, the optimal driving speed depends on the weather (and the forecast) and the remaining battery capacity.

(Note: On Oct. 5 the CUER team posted that it had withdrawn from the race due to an accident during final testing. No one was injured, but the car suffered “dynamic instabilities” that could not be repaired in time.)

A different road

“We didn’t have the funds or the resources available to the top teams, so we had to come up with a concept that would in some way be inherently better, something completely different that might give us a significant edge,” Mario-Ghae said.

The CUER engineers also knew that the conventional “tabletop” design had ruled the WSC for the last dozen years. Ever since the debut of the original Dutch Nuna and on to the Japanese successors, every winner has followed the same general plan, he noted. At this point, the competition has become a matter of the well-financed top teams building on existing, successful designs using ever more advanced photovoltaic technology and aerodynamics. And CUER could not compete on that basis.

The team also knew that “the margin between first and second place in the 2011 race was just 30 minutes, which is significant in a 3000-km race” because it indicates that it might not take much to tip the balance.

The key innovations started to come together a year ago, he recalled, when the team learned of a rule change that all the entries were to have four wheels.

“We had been planning a solar-powered motorbike at the time, so even though it was a dead-end, we were already thinking radically,” he explained.

Another change was that drivers must be seated in an upright position, unlike in the past where they could lie prone for better aerodynamics.

“We hit the drawing board with a clean sheet, but we did take from that earlier work the key idea of minimizing the size and weight of the design as much as possible,” Mario-Ghae said. “We went through 20 different sketches and finally hit upon the idea of decoupling the aerodynamic performance from the solar performance, which would allow us to improve them both without detriment to the other.”

CUER’s design philosophy, he explained, was to compress the car in all directions to make it as small, lightweight, and aerodynamic as possible and then to use solar-tracking plates housed beneath a transparent canopy that transmits 95% of the wavelengths that the solar cells use. The need to minimize the frontal area to cut drag led to the bullet nose, and the streamlined teardrop canopy at the back worked well to enclose the tilting solar panels.

Design trade-offs

Of course, “increased complexity is a risk of this approach, so it’s no surprise that we’ve been spending a lot of time getting the tracking system to operate correctly,” he reported. During the effort, the team developed the means to model the sunlight and the panel positions in any arbitrary geometry so members could simulate how the sun moves across the sky. That helped them produce software that triangulates the vehicle’s GPS position and direction of travel with the time of day to track the sun.

Mario-Ghae noted that CUER had opted to use satellite-grade gallium arsenide photovoltaic (PV) cells, which provide high (36%) conversion efficiencies. The rules allow teams to choose either 3 m2 of high-efficiency gallium arsenide or 6 m2 of silicon cells, which offer less efficiency (22%) but provide around a fifth more total solar power.

Resolution’s carbon-fiber monocoque structure, a one-piece shell that weighs only 35 kg (77 lb), was built in collaboration with the U.K.’s National Composites Centre. The solar array is connected to a lithium-ion battery pack and a 97%-efficient electric motor that is mounted in the hub of one of the rear wheels, thus avoiding gears or chains.

The CUER team captain acknowledged that his team benefited from considerable technical support from its U.K. industry sponsors, such as Jaguar and Land Rover.

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