Electric cars offer freedom from the gas pump, but they tie drivers to the task of charging up. To truly cut the power cord, it would help if you could carry your generator on board, and the fuel would be free.

That's the allure of the solar car, in many ways the holy grail of clean energy transport. It came one step closer to reality this week with Ford Motor debuting its C-MAX Solar Energi Concept car at Consumer Electronics Show (CES) 2014 in Las Vegas. (Related Quiz: What You Don't Know About Cars and Fuel)

Ford used the biggest consumer electronics show of the year to showcase not-yet-ready-for-mass-production technology and the possibilities for renewable energy transportation. (See related, "Pictures: Cars That Fired Our Love-Hate Relationship With Fuel.")

The solar plug-in hybrid crossover, based on Ford's C-MAX Energi plug-in hybrid, has 16 square feet (1.5 square meters) of photovoltaic panels built into the roof.

Even with a full day of blazing sunlight, the eight-kilowatt-capacity panels wouldn't capture enough energy to fully charge the car's lithium-ion battery. So Ford, along with the Georgia Institute of Technology, developed a flat acrylic lens to stand over the car in a canopy that can marshal sunlight from a larger area. The lens would act like a magnifying glass and concentrate sunlight onto the car, boosting solar uptake eightfold. It would take about seven hours of sunlight to fully charge the battery, which could then power the car for an estimated 21 miles (33.8 kilometers) before a gas engine kicks in. (See related, "Pictures: A Rare Look Inside Carmakers' Drive for 55 MPG.")

That means the C-MAX Solar Energi is still tethered to conventional fuel, but integrating solar energy into vehicles poses special challenges. Engineers—both professional and student—have been grappling for years with the issues of capacity, aerodynamics, and energy storage, in their zeal to design and build vehicles that run on the sun. (See related, "Wireless Power May Cut the Cord for Plug-In Devices, Including Cars. ")

—Josie Garthwaite

Follow Josie Garthwaite on Twitter.

Pictures: Ford Solar Car at CES 2014, Past Sun-Power Vehicles

With Ford debuting a solar concept car at the 2014 Consumer Electronic Show, we take a look at past efforts to harness the power of the sun to put vehicles into motion.