From Ford, a Plug-In That Tracks the Sun

Stonehenge, scientists tell us, took account of the sun’s changing path across the sky, using giant stone monoliths as seasonal alignment markers. Likewise, the C-Max Solar, a technology concept vehicle that Ford will display in Las Vegas, tracks the sun, though not for astronomical observation.

Rather, the car is intended to spend its days parked under a Fresnel lens, a kind of giant magnifying glass, and it is programmed to use its self-parking feature to maneuver as needed, staying aligned with the most concentrated area of sunlight. The car creeps a few feet over the course of the day to keep the solar panel under the lens’s focus point. And to compensate for the change of seasons, it lines itself up at a slightly different north-south positions from day to day when it is parked under the lens.

This is a bit like a youngster with a magnifying glass on a sunny afternoon trying to fry ants on the sidewalk, but in this case, the hand with the magnifying glass is stationary from sunup to sundown, and the ant, controlled by a computer, is seeking the warmth.

As a concept vehicle, there are no immediate plans for Ford to sell the C-Max Solar Energi, nor has there been any public announcement of what it would cost. Ford will introduce it at the gadget mecca formerly known as the Consumer Electronics Show. The show is now officially International CES and is increasingly car-centric; besides Ford, other car exhibitors will include Audi, BMW, Chrysler, General Motors, Kia, Mazda, Mercedes and Toyota. There will also be displays from some 125 makers of automotive technology, including Delphi, JVC Kenwood and Pioneer.

Ford’s tech showcase is a version of its small hybrid crossover that has had 1.5 square meters (about 16 square feet) of photovoltaic cells added to its roof, squeezed between the luggage-rack rails. On the road or parked at the curb, these cells, manufactured by Sunpower and already in wide commercial use, produce from 300 to 350 watts. So over an hour, they add about one mile of range.

That ratio, an hour of charging in sunlight for a mile of driving, illustrates why the surface area of the car’s body is too small to power the vehicle by solar power alone. Ford is using photovoltaic cells that are slightly more efficient than the average in use today, capturing the sun’s energy and converting it into electric current with an efficiency level of 21 percent.

But even if the cells were twice as efficient, and even if the hood and fenders were also covered with solar cells, they could not produce nearly enough energy to keep the car moving continuously.

Some cars can do that, but they are specialized competition vehicles and not likely to arrive at dealerships any time soon. The World Solar Challenge, an annual race from Darwin to Adelaide, Australia, draws entries from around the world that run at an average of about 50 miles per hour over an 1,864-mile course. But those cars each carry over 1,000 watts of cells, spread over six square meters or so, and last year’s winner, a Dutch entry called the Nuna 7, weighed a mere 350 pounds. It had a payload of one, and he was sweaty.

In fact, the entire electrical output of the C-Max is 300 to 350 watts, about the same as a single solar panel used in home installations. The typical home rooftop installation has about a dozen panels.

But there are ways to increase the output of a solar cell, including taking steps to compensate for the movement of the sun across the sky. Panels mounted on a roof or on the ground are often set up to pivot on a single axis, so they follow the sun across the sky. A few work on two axes, varying their angle by season of the year.

Ford’s idea, developed with the Georgia Institute of Technology, a frequent collaborator, is simpler. The lens is completely passive, standing in the same spot, supported by a framework, all year long. It creates a focus that traces a slightly different path from day to day. The trick is finding a cheap way to keep the solar panel under the focus.

Ford’s answer is to piggyback on existing abilities. The concept vehicle is a variant of the C-Max Energi plug-in hybrid, which is already on the market. In standard form, this crossover is almost smart enough to do the job, because it is available with blind-spot protection, adaptive cruise control and parking assist. In the concept vehicle version, the car moves itself.