By Bill Moore
Just how 'bright' an idea is Ford's sun-powered, fresnel-lens focused C-Max Solar Energi Concept car? Here are some back-of-the-envelope numbers to consider.
Photo: C-Max Solar Energi Concept on display at 2014 CES.
One of the oft-asked questions we've gotten over the 16 years of publishing EV World is, "Why can't we put solar on our cars and power them with the sun?"
The textbook answer is, "There's not enough surface area on the car to provide any meaningful amount of energy."
Ford's debut of the C-Max Solar Energi at the 2014 Consumer Electronics Show (CES) in Las Vegas this week doesn't meaningfully change that situation. It does, however, add an interesting new wrinkle.
Ford, working in cooperation with Georgia Tech, installed some 16 sq. feet (1.48 sq meter) of Sunpower high-efficiency solar cells on the roof of a standard C-Max Energi, the company's plug-in electric hybrid hatchback. Rated by the EPA at a combined MPG equivalent of 100 miles in EV-mode (44 city/41 highway in hybrid mode), the car's 7.6 kWh lithium-ion battery can propel it in electric drive up to 21 miles. Achieving that range usually requires driving at city speeds of 45 mph or less. The EPA's Fuel Economy.gov web site reports drivers getting from 31 to 148 mpg in the C-Max Energi.
Hybrid range is reportedly greater than 600 miles.
Now that 16 sq feet of photovoltaic area can generate an estimated 300 watts of electric energy per hour when the sun is directly overhead. That means that siting in the sun, say, eight hours, it will produce something less than 2,400 watts of electric power, or about one-third the capacity of the battery pack. Put another way and assuming the car consumes about 300 watt hours of energy per mile, that's about 8 miles of driving range or one mile per hour of solar charging. To improve this requires even more efficient solar cells, and Sunpower are some of the best in the world, at the moment.
Or… instead of extracting more energy out of sunlight, how about we turn up the the power of the sun?
That's what Ford and Georgia Tech have effectively done by using the magnifying power of a technology originally invented to improve the light output of costal lighthouses. French physicist Augustin-Jean Fresnel is credited creating the first multi-lensing system for the Cordouan lighthouse in 1823.
The relatively feeble whale oil light of the time was focused into a pair of opposing beams that could be seen 20 miles out to sea. Conversely, the diffuse light of the sun could also be focused and effectively magnified. That's what the C-Max Solar experiment does, except instead of using giant lens of glass, multiple small plastic lens are used. Embedded in an overhead lattice, the lens focus sunlight onto the solar array mounted on top of the car.
The result, says Ford, is that over the course of that same eight hour or so period, energy output of the PV cells increases to 8,000 watt hours (8kWh), more than three-times the original output.
But there is catch. The earth rotates, giving the illusion of the sun moving across the sky from East to West. As it does, the amount of energy striking the solar cells slowly rises and falls. Usually, engineers compensate for this by having the solar panels or concentrators track the sun's apparent movement.
In the Ford experiment, they decided to move the car, instead, as demonstrated in the video below. Because Ford has integrated self-parking technology in the car, which can move electrically without starting ICE-age engine, they could have it slowly creep along through the day, keeping the sun focused on the PV cells.
Of course, this means that some of that energy being collected by the roof is being used to also move the car electrically. How much? Ford isn't saying. Other issues, such as the amount of space and orientation of the system poses additional challenges. Instead of a single 375 square feet required to the average automobile parking spot, it would now have to nearly double to compensate for the movement of the car over the course of the day. Also, the parking stall would have to have an alignment that is pretty close to east-west. Additionally, the sun's apparent movement also shifts through the seasons because of the 23.5 degree tilt in the earth's axis. The more north you are, the more apparent this angular change. This too will effect the amount of energy falling on the fresnel array.
Another writer noted that there may also be potential safety concerns for pedestrians walking under one of these arrays when a car isn't there. And what happens to the finish of a car that doesn't have the panels on its roof or can't automatically move with the sun?
Which raises the question of why not just place the lens over the top of the PV cells? Presumably the answer brings us back to where we started: the surface area of the top of the C-Max roof versus the area of the canopy in which the fresnel lens are embedded. From the diagrams below, it would appear the canopy is at least as large as the car itself. That would be roughly 98 square feet (9.15 sq meters). That's six times the area of the solar array on the car's roof.
The real breakthrough here is the cost of cheap-to-make plastic fresnel lens versus high-efficiency solar cells. A 8.75 inch by 6.5 inch fresnel lens with frame is available on the web at $2.95 cents. Presumably in mass quantities they are even less. It would take about 250 to cover a canopy the size of the one Ford is discussing. Their diagram calls for 80 cells. That's probably around $500 in materials. Covering that same area with PV will cost about $1,380 (@ $230USD per panel).
Of course, Ford isn't saying how much that 300 watts of roof-top PV costs, so in the end, it could be a toss-up cost-wise.
But let's assume that the idea is feasible, what's the net result? What do that 8,000 watt hours of sunlight get you? Very possibly a fossil fuel-free drive home or to work any day the sun's shining and you live 20 miles or less from your place of work.
That's pretty neat, when you stop and think about it.
Originally published: 09 Jan 2014
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