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Solar roofs for cars?
henry..12   1/8/2014 3:42:43 PM

I've lived "off the grid" for seven years. There's NO power lines within 3 miles of my house. And your premise is a very good one. As I've experimented with a variety of solar electric topologies, one thing is very clear: the storage media is a major, and short lived, component. The problem with a fixed solar mount on a car roof is that the efficiency just isn't there. Better to spend the money on more starge media and charge that media up during the day to be used to recharge the car's batteries during the night.

Fixed solar installations like some businesses in teh US Southwest have installed make sense for recharging a car at work, and the parking structures shade cars from the sun in any case.

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Good to see some sensible discussion
Bert22306   1/8/2014 3:45:48 PM
It's gratifying to see an article that debunks mindless hype. Efficient, sleek vehicles, with skinny tires, i.e. vehicles with far less frontal area than the oafish C-Max, require roughly 12 HP to travel at 50 mph. This used to be a very informative statistic provided in vehicle reviews, by Car and Driver magazine. Too bad they quit publishing that parameter. When they did publish that number, as I recall, 12 HP was the best I ever ran across. Pigs like SUVs barely managed 24 HP, for 50 mph.

So, even an efficient vehicle would need 8,948 Watts to run at 50 mph. Basically 9 KW. Hardly a couple hundred.

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Re: Solar roofs for cars?
Bill_Jaffa   1/8/2014 3:59:31 PM
For more details on the Ford solar concept, see this piece at Power Management Design Line:

and be sure to check out the comments. More than before, I am convinced this is just a classic high-profile PR stunt to get attention at CES, and easily fool those gullible, "trend-hungry" journalists--and it worked! No engineer would see it as having any viability, not even close to it. Problem is that when these concepts fall flat, as they were inevitably destined to do, they cast a shadow (pun intended) on legitimate related efforts.

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Solar car races
RichQ   1/8/2014 4:33:02 PM
Solar cars are successfully being raced across the Australian outback, but these have disproportionately large solar arrays, light carbon-fiber bodies, and are operating in an area where insolation is substantial. They are a long, long way from providing what was implied in the WSJ article you referenced. (Which I note describes the car as a "hybrid" and claims a full-charge range of 620 miles, of which only 21 are battery-only. I assume that the unmentioned full tank fueling the gasoline engine provides the other 599.)

A video outlining the Ford concept is available here, and it tells a slightly different story than what the media ran with. In particular, Ford explicitly states that the car's solar panels alone (1.5 m2) will not recharge the batteries in a day. Instead, you would use a canopy of Fresnel lenses (kind of like a carport with a transparent roof) that the car parks under during the day. The car itself moves to track the sun beam the canopy generates. The concentrated solar energy is said to fully charge the battery in a day.

I suppose that if you were stuck in the desert far from the electric grid that this solar car might be able to leapfrog its way to civilization a few miles at a time at night after spending the day recharging, but otherwise it seems totally impractical. Especially in the cloudy Pacific NW where I live.

David Ashton
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Solar powered cars
David Ashton   1/8/2014 7:02:47 PM
I saw the original article and I was also very sceptical.  They claim 8x the power of normal solar cells when the vehicle is parked under a carport roofed with a big fresnel lens.  The car automatically moves to position itself optimally at the area where the lens directs the sunlight to.

That's all possible, and quite clever, but I'd question whether any known solar panels will deliver 8x the power if they receive 8x the insolation due to the lens,   and someone else raised heating issues - being as the cells are dark and not that efficient they'd heat up (and hot solar cells are inefficient too).

And yes. even if it did work as stated, with good sunlight every day,  it would not give anything like a full charge for a practical daily journey.

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Re: Solar powered cars
GMF   1/8/2014 10:07:02 PM
Probably inefficiency caused by solar cell weight will remove all gains statisticlly.

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but it could power..
seaEE   1/8/2014 10:20:32 PM
But it could power the subwoofers you are towing in the trailer behind your car.

David Ashton
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David Ashton   1/9/2014 3:34:51 AM
Guys, don't forget that what matters is not watts (power) directly, but watt-hours (energy).  Let's take some figures from the above posts.  Assume that the solar cells generate a base 375 W in normal sunlight.  Assume the car would be in the sun for 5 hours a day.  That's 1875 Watt hours.  Let's give Ford the benefit of the doubt and go with their claimed 8x boost with their fresnel lens, though I'm sceptical.  That brings it up to 15000 watt hours, a respectable amount.  

Bert estimated 9 KW to run a small vehicle at 50 MPH.  So our charge would do that for about an hour and a half - practical for short journeys, maybe, but that is under ideal conditions.   Under real conditions I don't see this as more than a top-up solution.  Can anyone poke holes in my math here?

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Re: Watt-hours
jeremybirch   1/9/2014 5:41:24 AM
the fresnel lens will at most concentrate the light - it does not increase the total incident light - so if you still only have 1kW (or in fact quite a lot less) per square metre of car roof to play with - the lens makes no difference to that. What the lenses can do is make a very concentrated spot of light which may get you a better conversion factor depending on the technology, but you are not going to get more power out than was incident to start with.

(BTW incoming power from the sun is called insolation - but this is too easily confused with insulation!)

So if you store 12 hours of sunlight (if you live near the equator) and only drive the car for a very short time you might get enough energy assuming your storage is very efficient, but this can only work if the car is VERY light. Most American cars are seriously heavy - unless you get the car down to being a 200kg or less then this is a non-starter (quite literally)


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Re: Watt-hours
Sheetal.Pandey   1/9/2014 9:58:39 AM
Nothing comes free. And when it comes to Solar devices since it needs lot of support to come at par with regular energy devices, how can it be free. Can be subsidised a bit.

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