The Stanford Solar Car Project has been working for two years on its latest all-solar vehicle, called Xenith, which later this year will travel across the Australian Outback powered by nothing but sunlight. The team says Xenith is the fastest solar car ever built.
The World Solar Challenge is held every two years and challenges teams to build ultra-efficient solar vehicles in a 3,000-kilometer (or 1,864-mile) race from Darwin to Adelaide. This year's race will take place October 16-23.
Based on the notion that a 1,000-watt car would complete the journey in 50 hours, the solar cars are allowed a nominal 5 kilowatt-hours of stored energy.
The rules, however, change for each event, giving teams just two years to conceptualize and build their vehicles. The idea is to constantly push the boundaries of what the vehicle can be, ranging in designs from ultraconceptual abstract ideas that may be less practical, and working toward real-world applications of energy efficiency that could be applied to future consumer products.
Based out of Stanford's 8,000-square-foot Volkswagen Automotive Innovation Lab, the Solar Car Project is a 30-student team working at the intersection of industry and academic research.
For the team members, who all have classes and jobs to attend to, this is basically a huge science fair project.
The team hopes to average about 60 mph during the race. They think they can finish the race in four days driving 8 hours a day, with drivers taking 4-hour shifts behind the wheel.
Most of the components for the Xenith solar car were designed and built by the Solar Car Project students in-house. Parts were machined here by the team, and software was designed by students to manage and optimize the vehicle's performance.
Xenith's 26 glass solar panels have brought multiple industrial prototype technologies together to make one of the most efficient arrays the world has ever seen.
The solar array, with ultra-high efficiency silicon solar cells donated by the SunPower Corporation, employs a proprietary antireflective coating and cutting-edge glass from Corning. An extremely thin thermoplastic urethane is used as the elastomer for the array encapsulation.
With modest resources at their disposal, the Solar Car Project relies largely on students' hard work and donations from corporate sponsors.
Unlike some teams, which have vast resources and build cars costing well over $1 million, Stanford's Solar Car Project has built their vehicle for the 2011 World Solar Challenge for only about $250,000. The team expects it to cost about $49,000 to ship the car to Australia, drive the support caravan during the race, provide basic housing and food for the team, and buy supplies and parts for the race.
Weight and aerodynamics play a critical role in a successful solar car
This front view of the Xenith solar car shows just how streamlined the vehicle is.
Once you get past about 1,500 watts of solar output, the Solar Car Project team members say it's far more important to focus on improving the body shape than anything else on the vehicle.
Weight and aerodynamics play a critical role in a successful solar car, the team says. They haven't yet been able to put their vehicle into a wind tunnel for more accurate aerodynamic testing, but this car has a very low profile. They hope to find someone willing to supply them with wind tunnel time in the future so they can further improve their design.
The driver's line of sight poses a significant design problem in that the upright seating design means that the driver's head can potentially cast a shadow onto the solar array, which can decrease solar energy capture.
An advantage of having the driver's bubble thermoformed is that it is completely seamless, which helps with aerodynamics. In addition, the entire bubble is clear, which results in less shading of the solar panels and allows the team to put the rear-view camera inside.
Shadows cast onto the array can drastically lower the car's power supply. If a shadow falls upon one cell, it will cause the entire panel to lose power. Therefore, the team has installed diodes to bypass that cell so that the rest of the panel continues to function.
An in-house-developed program called Shellpower simulates the sun's angle and the amount of power Xenith will generate at every point during the day while racing across Australia.
Strategy team leader Dan Posch imported Xenith's body design into Shellpower's 3D visualizer, and uses the program to look at the shadows cast on the car at different latitudes.
Using that shading information, the team ultimately decided on an efficient bypass scheme that would allow Xenith to generate the maximum amount of power throughout the entire race day.
Another new development on Xenith, seen here, is the introduction of a helmet-mounted display.
With hardware donated by Liteye Systems, the heads-up display gives the team a compact and efficient way of monitoring vital information in the car.
Using telemetry software designed by the team, Xenith is equipped with an easy-to-use interface that can be accessed from any mobile device within the telemetry network's range.
Improving both safety and strategy, the telemetry system allows anyone on the team to see the status of the car at any time, which means there are many eyes on any potential problems that might crop up along the race course.
A driver can control the vehicle with two foot pedals, in addition to buttons on the steering wheel that control lights, turn signals, and cruise control. The pedal on the right is pressed to the side to accelerate, and the brake is a standard forward-pressed foot pedal.
The thin and light carbon fiber body of Xenith is made of just two pieces that are hinged at the front and open like a clam shell, giving the driver a tiny space to slide into the upright seat.
The car is outfitted with both headlights and brake lights, making it street legal. Officially, the three-wheeled vehicle is classified as a motorcycle with a sidecar.
One of the rules for this year's race, which change from year to year, require the driver to be seated upright at 27 degrees, moving teams away from past designs in which drivers would recline or even lie down in order to maximize aerodynamics.
All three of Xenith's wheels turn, making it agile and giving it a tight turning radius.
A linear actuator on the third, rear wheel allows it to be angled. Set in dynamic mode, the third wheel can turn the opposite direction of the front wheels for a tighter turning radius.
In sailing mode the third wheel can be set at a specific degree, allowing it to be angled while still moving forward, like a crab, so the car can be angled into the wind for better aerodynamic performance.
Weighing just less than 400 pounds, stunningly light for a vehicle of this size, a single off-the-shelf mountain bike suspension is able to handle the rear wheel's shock absorbing needs.