Gas pumpEnlarge Photo
While photovoltaic solar cells are steadily improving in efficiency and coming down in cost, they'd have to become one to two orders of magnitude more powerful than they are today to make a car covered in solar panels able to propel itself.
Consider the Solar Challenge entrants, which are fascinating but flimsy contraptions covered in photovoltaic cells that can accelerate one very light human very slowly to about 30 mph under the intense sunlight of the Australian outback.
University of Michigan's World Solar Challenge car (Photo: Flickr user Michigan Engineering)Enlarge Photo
Extrapolate that to, say, a mid-size sedan or utility vehicle that weighs up to 10 times as much and must carry five people and their gear, plus pass all global crash-safety tests.
It may happen one day, but it won't happen in the next decade or two.
There is a way to run your electric car purely on sunlight, however. It's simply putting enough solar panels on the roof of your home that you capture enough energy--on average--to cover your total household consumption plus recharging the plug-in car.
That most likely requires "net metering" contract with your power company (something electric utilities have begun to perceive as a serious threat to their business model).
And the "on average" part is important, since it probably only works if you feed excess power into the grid but can draw it back at night to recharge the car.
Airpod compressed-air cars, 2009Enlarge Photo
(5) Compressed-air cars are just around the corner.
This one erupts periodically as some blog or media outlet does a story on the odd-looking "air car" prototypes from some maker or another.
While it's possible that such vehicles may one day appear for very limited uses--low speeds, short distances, perhaps in warehouses--they're neither practical nor efficient.
The challenge is that compressed air isn't a very good energy carrier compared to other ways of storing energy--from batteries to hydrogen fuel--and to compress it enough to store the energy required to move a car weighing, say, a ton requires extremely robust and sturdy tanks.
Which are very heavy.
Worse yet, how do you compress the air into the tank in the first place? In most cases, you use a compressor driven by a electric motor or by a gasoline or diesel engine.
Their efficiencies vary--even the very best combustion engine is no more than 50 percent efficient at converting the fuel energy into torque--but this simply displaces emissions from the tailpipe to the generator or the electric power plant.
We've not seen any serious studies of the "wells-to-wheels" carbon required for compressed-air cars, but we're confident they'll emerge if such cars approach any kind of mass production.
While India's Tata was rumored to be putting such a car into production sometime after 2011, using technology licensed from a company called MDI, the Airpod has thus far not progressed beyond prototypes first shown in 2007.
Similar vehicles from smaller startup companies have also failed to appear.