Why don't we use solar to power our electric cars?
by Jeremy Korzeniewski on Jun 18th 2009 at 2:43PM 
Venturi Astrolab concept - Click above for high-res image gallery
For years, we've been watching as solar-powered race cars travel at relatively high rates of speed for hours on end – as long as the sun happens to be shining, of course. In fact, the World Solar Challenge has been taking place in Australia every few years since 1987, and the actual vehicles competing in the event got so fast that race organizers eventually had to alter the rules in the name of safety back in 2005. Since these solar-powered racers had gotten so fast and capable that in 2007 the World Solar Challenge altered its mission in the hopes that more real-world solar cars could hopefully benefit from the lessons learned during the competition.But there is a problem with most of these solar racing vehicles. In the name of maximum efficiency, solar racers force their single drivers to lie in all sorts of convoluted positions, lack any sort of active safety systems (like airbags, anti-lock brakes or crumple zones) and are made from extremely expensive, high-tech materials. In other words, none of these solar-powered race cars will ever be construed as something you could possibly drive to work. What's the problem? Why can't we build a workable solar car?
That's the subject of today's Greenlings post. Click past the break to read all about solar-powered cars.
Gallery: Venturi Astrolab concept
Now that we've discussed what isn't possible with solar cars, let's find out why that's the case. First, a few facts about the sun. Each and every year, the Earth absorbs about 3,850,000 exajoules of solar energy per year. Without getting into a full-blown examination of that amount of energy, let's just say that the Earth receives more power from the sun in just one hour than all of humanity uses in a whole year. In other words, there's plenty of solar power to go around.
So, why not harness that power and use it to power our cars? Well, it's not for a lack of trying. Production of photovoltaic cells doubles every two years or so as more and more people add solar installations to their homes or offices. Unfortunately, even the best photovoltaic panels in the world have an energy conversion ratio of around 23 percent, and the average commercially-available cell is only 12-18 percent efficient. This means that lots of solar cells are required to generate any meaningful amount of power, and they require a good deal of space. Of course, cars only have a limited amount of surface area available for solar cells, and it's not enough.
Although some may argue with this assessment, the fact of the matter is that photovoltaics just aren't able to provide enough juice to power an entire car with the available amount of space to install them. At least not yet.
That doesn't mean solar power isn't useful for automotive use. For example, the new 2010 Toyota Prius uses roof-mounted solar cells to help power the car's interior fan so that a reasonable temperature can be kept inside the cabin. The upcoming Fisker Karma uses a similar solar arrangement. Further, large solar arrays have been used by some automobile designers to at least offer a useful boost in an electric vehicle's overall range on sunny days. Obviously, any power that is harnessed from the sun that's used for a plug-in electric vehicle displaces energy that could have come from a relatively dirty power plant, and that's a good thing.
Automakers like Porsche, Ferrari, General Motors and Nissan have installed large solar installations that are designed to help power the factories where cars and trucks are assembled.
Even if solar cells aren't installed directly on electric cars doesn't mean they can't charge its batteries. Perhaps the most intriguing current developments in the solar space are large solar installations in sparsely populated and usually sunny locations, such as in the deserts of the Southwestern States here in the U.S. These solar power plans add their considerable energy to the already-existing power grid, and it's often possible to purchase power specifically from these plants from your own power provider. There are also a number of new products hitting the market designed to use solar power to charge individual cars when parked.
Finally, some electric car owners – such as AutoblogGreen reader David VM and Tesla Motor founder Martin Eberhard – have taken matters into their own hands by installing photovoltaic solar arrays on their own homes or garages. These systems send power to a bank of batteries and can be used to power either the household's electrical needs, the car's onboard batteries or both. In these instances, an electric car could really be considered a solar car that chooses to leave its power-generating panels at home when it goes for a night out on the town.
Reader Comments (Page 1 of 2)
Rick 2:53PM (6/18/2009)
Hmm, rainy season in Florida? Raining all day and time to leave work? I'd rather just rely on a solar charge station, than a solar car.
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guyledouche 3:01PM (6/18/2009)
2 words --> POWER DENSITY
Welllll theres your problem riiiight ther folks!
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downtoearth 4:14PM (6/18/2009)
Exactly. Solar power is sufficient for powering a low speed bicycle in a sunny day. Not a car.
Matt 3:18PM (6/18/2009)
Guyledouche is right, but...
Let's say for the sake of argument that I can only fit enough solar panels on my car to accommodate 10% of my power need while driving. If I have enough energy storage in the battery pack to get from home to work, it doesn't matter that I can't continuously drive the car. What I really need is 9 hr of a sunny parking spot for every 1 hour of driving (10 total charging hours = 100% since it charges while going too). Instead of focusing on the impossibility of meeting continuous load requirements, why not focus on the possibility of meeting daily requirements? Of course, a plug would be a good option for cloudy days, but I think we have way more than enough sunny days to make covering the top surfaces worth while on an EV.
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GoodCheer 5:30PM (6/18/2009)
Matt:
That's a good way to look at it, but once you've gone that far, why not go one step further.
So you need PV panels equivalent to your average load, and you need batteries in your car that can hold a day or two worth of electrical energy.
Since solar panels are expensive, you could still just put them up on the roof of your house, where they will be in the sun (hopefully) and will be oriented more optimally than they would be on your car. Then you can connect those optimally oriented panels to your car, wherever you park and plug in, with the existing transmission system.
Even with transmission losses, you'll still get more energy out of your panels, your car will be that much lighter, and you can park in the shade without worrying about wasted investments.
Matt 5:53PM (6/18/2009)
GoodCheer,
I replied to another comment below about your logic, and I agree with you. This being an automotive blog, I'm always looking at it from a standalone point of view, but fixed panels are definitely a better use of funds for several reasons. I'm in construction, and could go on for days (b.t.w., did you see the Solar Decathlon?)
My original comment was really more practicality/convenience oriented rather than budget/environmental benefit. I'm thinking road trip, or long commutes where charging mid trip would be needed. I imagine it being much easier to plug in at home than anywhere else, and rather than feeling the need to bother your employer for a plug, why not let the car charge itself in any given parking lot. I know it's not the most economical solution, but it may be a great range extending solution for commuters that will park their cars in random open lots, possibly even allowing for a smaller battery pack to be viable for more people.
steve 3:29PM (6/18/2009)
Another word is COST.
Solar panels typically cost around $10 per watt, so for a 1Kw array thats about $10K, and when electricity is available at less than 10c / KwH, you need to use 100 MwH of electricity before you have covered the cost of the array. Thats without taking into account interest, depreciation, and the life of the array :(
If your electric car averages 200 watt Hours per mile which seems to be about average for current EVs, then a 40 mile round trip requires 8KwH.
Thats probably about the max you would per day get from your 1 Kw array in peak sunshine if you live in a sunny state. so it would take you around 35 years to just cover the cost of the 1Kw solar array :(
If the array is still working, your car still works, and your still alive, after 35 years you just broke even with the guy next door who has been plugging into the utility for the last 35 years. After another 35 years, your running costs would be averaging 5c / KwH
Until the price of solar panels drop considerably its just not financially viable to do this :(
But for those that dont have a local utility, or anyone with money to burn, solar is a great option, although still the most expensive ( compared to wind, hydro, etc)
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FitFan 3:46PM (6/18/2009)
The life of the car isn't important as long as your next car also has plug-in capabilities.
While it may take 35 years to recoup the cost of the solar array, that's 35 years of running your vehicle on 100% clean energy.
If you live in California then federal and state subsidies will reduce your cost by about 50%. Which means it will take you 17.5 years to pay off your investment.
Matt 4:28PM (6/18/2009)
That's a pretty high cost for solar panels, actually... doing a quick search on the good ole' interwebs returns prices more like $2-$3 per watt, and that number is coming down every day. Especially with the advent of printable solar panels, things are expected to drop below $1 per watt. When purchased in bulk by an automaker, who knows? So lets say it gets to $1/watt. Now you're looking at 3.5 years to pay it back. I've had my car for seven years; sounds like a fantastic deal to me, especially when you consider that you don't have to look for a plug all the time.
http://www.edn.com/article/CA6640264.html
John Rowell 9:18PM (6/18/2009)
That would be fairly accurate if you were using 2007 figures at the height of the global silicon shortage, and ignoring the generous government rebates for solar power. The wholesale price for solar panels has dropped to as low as $2.30/watt this year, and it's continuing to go down as production ramps up. The retail cost of a complete system after incentives is now under $6 a watt, installed.
DasBoese 3:39PM (6/18/2009)
What the article forgot to mentio is the most important aspect that determines viability of solar power, be it on the car or at a charging station: Cost.
An inefficient system can still be worthwhile if it's reasonably cheap, likewise a system might be super-efficient, but useless if it's not cost effective.
In the case of solar cells on a car, you have an expensive system with poor efficiency, THAT's why it doesn't make sense. Even home-level or large-scale solar power is often only cost-effective with subsidies. There are several promising technologies that could bring costs per watt down to reasonable levels, but until they're ready for the mass market, solar power won't be an option for most of our energy needs.
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Jason 3:40PM (6/18/2009)
The cost alone prohibits that kind of thing. Solar panels are extremely expensive, and tacking a bunch of them onto the cost of an already expensive EV just makes the product less market ready, particularly when electricity is dirt cheap.
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Shane 3:50PM (6/18/2009)
This article got me curious enough to crunch some quick numbers:
1 horsepower = 746 W
Conservative estimate of power needed to sustain car at cruise = 10hp = 7,460 W
Solar constant = 1366 W/m²
Solar radiation reaching panels at a very sunny location ~=350 W/m² ?
Surface area for panels on car ~= 3m²
Panel efficiency = 20%
Given those parameters, this solar car can generate 210W or less than 3% of power needed to sustain cruise speed.
Let's say there's a breakthrough in panel efficiency to 75%, that'll only give us 1 horsepower under ideal sunny conditions. I guess that's why those cars in the solar challenge are shaped the way they are - in other words, not practical in the real-world.
Of course, we can use batteries to store energy. But there is simply not enough incoming solar radiation per m² to solely power a practical solar car.
p.s. Research done in 5 minutes on google - please correct my numbers if I'm wrong. Thanks.
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GoodCheer 5:42PM (6/18/2009)
Shane:
200 Wh/mile * 60 m/h = 12,000 W, so 10 hp is a bit low for highway.
Solar energy reaching the surface of the earth is more like 1000 W/m^2, with more in some high-altitude locations (and less in many other locations).
Also, 20% efficient panels exist, but are only that efficient when facing the sun. On a car they'd be reduced by cosine(D), where D is the angle between the perpendicular and the sun's rays.
Dolphyn 3:52PM (6/18/2009)
The Koenigsegg Quant vehicle is designed with a special coating that acts like solar cells, and they are claiming 38% average efficiency for that coating. They are also claiming rather impressive specs for the batteries. I'll believe it when I see it, but it's certainly interesting.
http://www.nlv-solar.com/quant/
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alex 4:06PM (6/18/2009)
anyone else find toyota's use of solar panels on the prius particularly un-green? think of all the raw materials, carbon, water, etc impact that comes with building, transporting, etc these solar panels. and then they are used to... power a fan while you're not in your car?! It would make sense if prior to this people used diesel generators to power fans in their cars while they weren't in them... but people didn't. people cracked their windows while parked and then rolled them down when they got into their car.
"look how green i am! i have a solar powered electricity waster!"
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Turbofrog 4:19PM (6/18/2009)
Even with a cracked window, cars get really hot if you can't park them out of the sun. I'm pretty sure that according to user studies, keeping the car cooler while parked (i.e. by running a solar powered fan), translated into reduced use of air-conditioning, since the perceived discomfort is a lot less when you get in.
Less AC use, less gas consumption. Sustainable design is counter-intuitive, sometimes.
(...I still do think that the solar panels on the Prius are largely goofy, though)
Shane 4:30PM (6/18/2009)
Alex,
I respectfully disagree. Normally, when a someone enters a hot car in the summer, he would crank the AC until the temperature is comfortable. That uses non-renewable energy.
Now, I have no idea how well the Prius solar fan works, but it's designed to keep your cabin cool using a free and renewable form of energy. In that regard, you're not really "wasting" electricity.
For most things in our lives, you can't easily quantify what you get out of a product vs what resources were put into making it. What about the stereo in your car? The magnets, wires, plastic, electronics, etc - they all had to be manufactured and shipped also. But it's worth it (to most of us) because it makes driving that much more enjoyable. I consider Toyota's implementation of solar panels as such a feature of the car - just because it's new doesn't mean it's wasteful or excessive. There are probably dozens of other features you find in cars that are less "important" than climate control. That's all subjective though.
Cheers.
3PeaceSweet 6:02PM (6/18/2009)
I'm suprised no company makes a domestic version.
~100W PV panel (positioned as a window shade) connected to a ceiling fan. Save lots of aircon at peak times, and minimal costs.
KenGirard 4:28PM (6/23/2009)
Using a solar powered cooler means that there is less of a drain on the main battery while the car is running, as well as when it is sitting still at rush hour. When the fan is not running, it adds a small amount of extra voltage to the main battery.
And has anyone looked at using one of the systems they have in some coolers that make one side hot and the other cold? I was thinking the other day while getting into my car in 100 degree temp that any cooling they provided might be worth it. Same in the winter when it is -10 below.