The World Solar Challenge for cars powered only by sunshine is on this weekend in Darwin, and for the first time in its 20-year history, organisers are making it tougher, demanding that today's sun-fuelled contraptions conform to the designs of more conventional cars.
They have been years in the making. Now these solar prototypes are going through final adjustments before Sunday's race.
Cars of every weird and wonderful shape will showcase the latest in solar technology as they travel from Darwin to Adelaide.
But with solar power almost verging on mainstream, race director Chris Selwood says organisers have set a new challenge by redefining the car.
"We've taken 25 per cent of their solar cells away from them so their power generation capacity is cut by a quarter," he said.
"We've given them an aerodynamic disadvantage by telling them that the driver should sit in a normal, upright seating position and not laying down in the car as they have in the older cars, and certain practical things, like the driver has to get in and out of the car unaided."
The tougher rules make it unlikely this year's race will set any speed records, which is just as well because for the first time, drivers have to contend with the Northern Territory's new 130-kilometres-per-hour road limit.
And in keeping with the green theme, organisers have included a Greenfleet category for cars powered on alternative fuels like canola oil, hydrogen and ethanol derived from sugar cane.
One team is confident it won't have to refuel at all during the 3,000-kilometre trip. New Zealander Cam Feast says his team, Bios Fuel, has developed a revolutionary car that runs on waste oil and water
"If you just took waste oil and water and you poured it into an engine, it will blow up, basically. It will blow the injectors off," he said.
"What Steve has worked out is a way of bonding the structures of the waste oil and the water so that they actually bond together, and then can be used to burn and create energy in an engine."
He says he is literally burning up waste.
"What we're doing is taking this waste stream, taking the energy from that and converting it to fuel," he said.
