In 1987, when Australia’s annual World Solar Challenge first revved up its sun-powered engines, many of the cars in competition couldn’t harness or store enough energy from natural light to complete the journey across the country.

The idea of solar-powered cars seeing commercial adoption seemed like something out of science fiction.

As the race’s contestants take off again 20 years later, technological advancements in batterystorageand solar panel efficiency have made it a lot easier to drive thousands of miles across Australia in a car that is powered solely by solar.

Meanwhile, average car speeds in the race have jumped from around 40 mph to more than 60 mph. And the world’s first high-performance commercial car incorporating solar power is set to go into production soon.

Despite all those advancements, analysts say the idea that cars with solar panels on their roofs will ever become a common site on the highway is still a little dubious.

The race -- think NASCAR crossed with a multimillion dollar science fair -- kicked off on Sunday.

Thirty-eight solar cars are racing from Darwin, on the northern coast of Australia, to the southern harbor town of Adelaide over the span of a week. Along for the ride will also be 19 cars demonstrating various fuels, including waste mineral water, sugar-cane ethanol, fuel cells and canola oil.

"The challenge was always to cross the country on the power of sunlight, and when we first did that it was by no means a certainty that any particular team would finish," reflected event coordinator Chris Selwood, who has worked with the event for more than 10 years, last week as he checked in entrants in Darwin.

"In latter years, it’s become not only a certainty but an expectation that people would finish,” he said. “So we have to make it harder."

This year, event coordinators have made changes to encourage more realistic cars, including the requirement that drivers be able to get in and out of their cars without assistance, and be able to sit upright. And, most drastically, car designers will have a 25-percent smaller area to fill with solar cells -- down from 8 square meters to 6.

"That’s an indication that these things are getting far more efficient," Selwood said.

The teams from universities around the world have spent millions in cash and product donations for their prototype vehicles. Some have spent as much as $500,000 on solar cells alone, according to reports.

Much of those donations have come from such car companies as Ford Motor Co. and General Motors, and from solar manufacturers like SolarWorld. These companies see the event as an opportunity to showcase technological advancements and to earn some green points.

Monaco-based car manufacturer Venturi, meanwhile, plans in March to start commercial production on the $125,000 Astrolab, an electric-solar hybrid car that uses 3.6 square meters of photovoltaic solar cells that are 21 percent energy-efficient and that recharge the car when it’s in motion.

But widespread adoption of solar cars is still far from certain.

"Solar races can help spur development of more efficient and better solar cells," said ABI Research automotive analyst David Alexander, "but they still would have to come up with an order of magnitude improvement in the power you get per square inch to make it a viable possibility for practical car use.".

Solar cells would have to get a whole lot cheaper and a whole lot more efficient, said Alexander.

Right now, the amount of space on a car that could hold solar cells isn’t enough to power the kind of heavy vehicles that most people drive. Plus, cells may not be strong enough to withstand the wear and tear required from the vehicle, Alexander said.

Alexander predicted that mainstream incorporation of solar cells cars may still be 15 to 20 years away, and even then used only to supplement other electric power sources.

"It’s wonderfully green, having solar cells on the car -- you can’t get any cleaner than that," said Alexander. "But these cars have, at the most, two little seats and no carrying capacity, and when you look at how people use cars, it simply isn’t practical."

Downplaying any major commercial role for solar-powered cars, Selwood sees the races more as an opportunity to promote and advance technologies that can help spur the adoption of cars run on all sorts of alternative electrical power.

"It’s an imperative that we will be seeing zero-emission vehicles run on electricity," he said, "but whether that electricity is generated by a wind turbine fed into the car, or whether solar cells on the car or whether solar cells on roof of the garage … There are all sorts of possibilities."