Toyota's Solar Car - How Practical?
By Bill Moore
Japan's Nikkei news service reports that Toyota is working on a solar-powered electric car, though the company hasn't confirmed it.
Since most solar-powered vehicles to date have been low-slung, highly-aerodynamic, single passenger, three-wheeled university experiments with top speeds approaching 50-60 mph, just how practical would Toyota's car be?
Could the company that developed the ground-breaking Prius, leap-frog the auto industry again with a viable, solar-powered electric car?
Actually, solar-powered electric cars already exist. Overlooking for the moment exotic academic experiments that compete in various solar challenges, the most famous being the 4,000 km race between Darwin and Adelaide across the outback of Australia every-other-year, there are scores of "real-world" vehicles running on solar power, a fair share of them built by Toyota a decade ago.
By Plugin America co-founder Paul Scott's estimate, about a quarter of the owners of Toyota RAV4 EVs recharge them from solar panels mounted on the homeowner's roof, including Scott. As long as the vehicle is parked at home during daylight hours, 100% of the energy being stored in the small SUV's NiMH battery pack comes from photons striking the silicon panels, generating a steady flow of free electrons. If the vehicle is in use, the electric power from the panels goes into the local grid, where it is, in essence, "banked" for later "withdrawal" at night.
This could be one approach that Toyota uses to power any future solar-electric car. Since it owns a significant share of its battery joint venture with Panasonic, who just acquired solar panel maker Sanyo, it will have access to this rapidly improving technology. Toyota's first generation "solar car" could come with a 100% solar power "option" that includes a complete solar equipment install on the owner's home, including panels and inverter to make the vehicle completely autonomous, as well as provide supplemental power to the home.
Of course, this isn't the same as having the car generate its own solar power from cells embedded in its skin, a long cherished dream, but one fraught with challenges and trade-offs.
The first challenge Toyota engineers have to confront is the energy efficiency of the solar cells. An electric car like the original GM EV1 or RAV4 EV consumes roughly 200-250 Watt hours of electric energy per mile. A really good silicon-based solar panel measuring about 12 square feet -- roughly the area of the roof of a car -- and operating at around 20% efficiency produces about 200 Watts of power at 42 volts on a bright, sunny day. 200 Watts is about one-quarter horsepower.
A solar-electric car developed to run on this amount of energy would be, by necessity, very small and light-weight, operating on the voltage of a typical electric golf car and propelled by a tiny, quarter-horsepower motor, hardly a muscle car, barely a motor scooter. By comparison, an Apollo-era Lunar Rover had four one-quarter horsepower motors, one in each wheel. Its top speed was about 10 mph, operating in one-sixth gravity.
To provide any meaningful mobility here on earth, you'll need batteries to store solar-generated electricity while the car is parked a good share of the day. With that stored energy, you could increase the horsepower rating of the motor, improving its speed, acceleration or extend its range. You'd also increase the weight of vehicle.
So let's assume the car is parked in the sunlight for 8 hours at work. Since the sun continually shifts in the sky during the day, the car's panels will likely only get sufficient sun to generate its full 200 Watts just five hours a day, depending on the time of year -- winter will be less, summer will be more. Five hours times 200 Watts means the car's solar cells will produce between 1000-1,500 Watt hours of energy or about 1-1.5 kWh on average while parked. That's enough to drive the car four or five miles, at most, assuming the system is 100% efficient, which it never is.
The average American commutes about 30 miles a day. So you will have to either increase the amount of solar surface area by six-fold, improve the efficiency of the solar collectors or reduce the amount of energy the car consumes per mile, and preferably all three. As you begin to do this, however, your car will begin to morph into one of those alien-looking university solar cars.
This isn't to say that Toyota engineers can't build a somewhat more practical vehicle. Venturi Automobiles in Monaco developed the Astrolab, a solar powered, tandem, two seater that it proved could run on the freeways of Los Angeles, surely the ultimate test of practicality, at least for balmy, Mediterranean-like climates. The car's practicality in Chicago or Seattle might be limited, though Marcelo de Luz demonstrated this summer that his Power of One solar car could traverse the rugged Canadian wilderness -- including encounters with wolves, moose and a possible alien spaceship -- to reach the Arctic Circle, the first solar car to do so.
Whatever Toyota comes up with -- and we may get a glimpse of it at this month's North American International Auto Show in Detroit -- it will have to conform with the know laws of physics, so don't expect to see any time soon a solar-powered Prius. It's a very long way from a small patch of cells on the roof running a cooling fan to a freeway-capable, solar-autonomous sport ute.
Some Solar-powered Cars
PowerofOne (Xof1) solar car not only drove across Canada but was the first solar vehicle to reach the Arctic Circle.
Globe-girdling SolarTaxi literally drove around the world on solar-power only, but its solar trailer could only provide half the energy it needed. The other half came from solar panels in Switzerland that put power into the local grid. Louis Palmer then 'withdrew' energy -- ATM-like -- from whatever electric grid was available.
Venturi Astrolab solar-hybrid merges onto LA freeway in 2007.
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