Toyota Prius Photo-Voltaic Roof: Solar-Powered PR?

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But what if Toyota routed energy from the solar panels directly into the batteries, instead of just pushing the power to a small fan? That has genuine potential. HybridCars.com asked Daniel Sherwood, a solar energy engineer and the founder of 3Prong Power, a Berkeley-based plug-in hybrid conversion start-up, to do some “back of napkin” calculations. Let’s assume that you could cover a full third—very optimistic—of the Prius’s 7.6-meter top surface with the Kyocera panels that Toyota is planning to use. According to Sherwood, an “average solar day” in California produces 10 kilowatt-hours of energy per square meter and the Kyocera panels are 16 percent efficient. That means a car parked all day in the driveway could produce 1.6 kilowatt hours of energy—more than enough to top up the Prius’s 1.3 kilowatt-hour battery pack. Because the Prius can go about three or four miles on a kilowatt-hour, you could conceivable enjoy as much as five miles of all-electric driving on the full battery packs—a fuel-efficiency boost of 10 percent of more.

But hold on. Unlike a plug-in hybrid or a conventional hybrid with additional battery capacity, today’s Prius is not set up to take advantage of this opportunity. Even with an EV button—like the one available on the Toyota Highlander Hybrid—the Prius’s computer control system will baby the battery packs and only give you a mile or so of gas-free driving. And that’s with a light touch on the accelerator. The solar panels could push the upper limit of that capacity, but again, that’s only after a full day of charging and very careful driving mostly in the city.

Using the power from the rooftop solar panels to run the air-conditioning might hold more promise. Most hybrid drivers know that blasting AC kills mileage. But given the considerable power required by AC systems, and the need to generate power while the car is driving, the best you could hope for is a small “offset” of the AC’s power requirements.

Where does this leave us? Not very far from where we are today. For Toyota, bringing the solar panel option to the market adds a little more green sheen to the Prius halo, but is unlikely to significantly improve the vehicle’s actual environmental and energy performance.


  • Steven F. Schluter

    Most people don’t drive during an 8-5 job so any charge to the batteries would be a plus. We drive our car every 5 days so that is even better. They are headed in the right direction.

  • Andrew Vincent

    I think this idea should be on every plug in hybrid. While it doesn’t make enough power to run the car it helps extend the life of the battery. Also (at least for me) I drive to work and the car sits for 8 to 10 hours, in the sun. That should be enough time to charge the battery at least a little. I hope Ford and GM do the same thing.

  • uktiger

    Distributed home solar production = good idea. Solar panel on roof of car = bad idea.

  • kahmke

    I’d have to ask, without have to do the math myself. What is to ROI of such a system? I was looking at the all electrics and throught this would help extend the range of the batteries.

  • steved28

    There’s more to the story than just watts. As anyone who has parked a hybrid in the summer sun knows, the battery starts off hot after sitting 8 hours in a parking lot, and very soon your hybrid can turn into a non-hybrid because the computer begins to protect the battery. At least that is what I have experienced in my Altima Hybrid. I now look for shaded spots to park at work, which directly translates to better mpg on my commute home. I would welcome a device which uses the sun to cool (rather than heat) the batteries in my car.

  • Bryce

    green sheen would be a good way to describe it. probably just PR related. The cooler cabin will be nice though……at what price though.

  • ste

    @ uktiger

    Why? Spreading on buildings or cars has the same result.
    Millions of PV cars produce an amount of energy that will not take from the grid or from gasoline.

  • uktiger

    cars can not be permanently pointed to the sky. We park them in garages, in the shade, etc.

    Photovoltaics only pay off if they are in the sun.

  • A.H.S.

    http://www.sciam.com/article.cfm?id=engineering-silicon-solar-cells

    I think its a good idea for the near future. The problem is that even though the solar cells on theroof would be benaficial I dont know if they would necessarily be cost effective. In the article I have posted above it is clear that within a few years when these cells become more affordable and produce more energy then they will definatley be used in hybrids.

    Also it is mentioned in the article that Toyota is planning on offering this as an option. This means that the people who no their car will not be getting enough sunlight they do not have to take the option. But it is important to offer it because for the people who will be getting a lot of sunlight like andrew it could turn out to be very cost effective.

    But for now until some necassary upgrades are made to the current solar cell technologies we should be looking to other technologies like lithium bateries (which toyotas new prius does not have) that will drematically boost fuel econamy.

  • Seth

    I think this would make sense if the hybrid battery was large enough so that it could absorb all of the solar energy during 1 day of charging. 1.6 KW-hrs, depending on the car, would equate to 5 to 8 miles of driving. For 1 year, that would be 1500 to 2500 miles for free. For the Prius at 50 mpg, and $4/gal, this would equate to $120 to $200/year of savings. The articles mentions solar cells with an area of 2.5m2 (1/3 of the area of the whole car). Retail price for solar cell panels might cost $600/m2, so the upfront cost might be $1500. In the end, you might make up the cost in a little more than 10 years.

    For pure electric cars, estimated battery costs anywhere from $600 to $1500/kilowatt-hr. So if you can displace 1.6 Kilowatt-hours with solar cells, you could save $1-2K on batteries but you’d have to pay $1500 more for solar cells. Of course, with solar cells, you might save ~$30/year not having to buy electricity.

    Both batteries and solar cells will drop in price in the next 5 years, so it is hard to say where solar cells will stand then. Nonetheless, if prices continue to fall for both, the electricity price might be the dominant factor. In the end, solar cells might likely yield a positive return on investment.

  • Luis Busquets

    I do not know where you got that figure of 10Hours of Sun in California but certainly seems too much. Even in the Equator you cannot claim 12h because of the incidence angle througout the day

  • Need2Change

    It makes more sense for a plug-in hybrid.

    And the ROI is quite complex.

    I work 34 miles from home, and have no place to recharge the car.

    If I drove a plugin hybrid capable of 40 miles of all electric operation, in theory I would only have 6 miles left after i arrived at work. But I’d probably have less since the car will continue to draw electricity for clocks, computer, alarms, radio presets, etc. during the day.

    It would be nice that after 8-9 hours at work, the car is partially recharged due to a solar panel, and I can almost get home before the gasoline range extender turns on.

  • brian NY

    I own a boat and have a small solar pannel installed that charges my batteries. I never had any problems with this system. I assume that it will be similar on the Prius but on a larger scale. I say anything we do to increase our MPG and reduce the use of oil is worth trying. Yes cost &ROI is one thing, protecting the envviroment, reducing dependence on foreign oil is another.

  • normothebig

    I know it sounds pointless, but the “napkin” calculations seem about right to me. My 07 Prius has an EV button. If on the last mile or so of my 8-mile commute I select EV, the battery will run down almost to the minimum allowed level. But mpg is boosted from about 52 to 65mpg (imperial). If the solar array put in just 150 watts, then the 10 hours the car is at work would fully charge it – even in northern England!! So when leaving for home the ICE wouldn’t have to do overtime to push all that energy back in.

    Also, when cruising along in “saving-gas” mode that 150 watts being 20% of a horsepower might be about 5% of the total power used (4hp) along a level road.

    The solar is worth having IF it charges the hybrid battery and not just the 12v auxiliary and it isn’t silly money.

  • Shines

    How about this… If the purpose (or part of its purpose) of the solar panel is to charge the battery when parked, or to cool the battery/car when parked, make a 2 layer panel that slides forward over the windwhield and hood when parked. This would keep the car cooler – like an external sunshade – and double the size of the solar panel. Don’t know how heavy the panels are or what the tradeoff of the additional weight (including the safety locks and sliding mechanism) would be…
    just a thought

  • Bryce

    slide out solar panel screams “vandalize me!”

  • SRI

    The next generation prius is rumoured to have more battery power and be able to make better use of the battery power. So solar option may be of some help. But if the cost is in hundreds keeping the car cooler alone will be worth it.

  • Shines

    Well OK Bryce that’s a risk…
    I suppose a convertible top screams vandalize me also. Still, there are plenty of convertibles (with torn tops – I know) on the road. The existing solar panel has to be durable enough to handle dust, dirt, branches and bird $#!t. (more durable than a ragtop) I don’t see that a car with some kind of hard cover over the windshield is any more vandal prone than anything else…

  • Bryce

    Good comparison with the convertible. Still, I would want my investment to be a little more resilient. Besides, newer solar arrays have dual layers ontop of one another with the lower one catching what the top one doesn’t get.

  • brian ny

    Lexna is a very tough, durbale and break resistant clear material that can used to cover the soalar pannel array. It can let the sun’s rays pass thru and offer superior protection to the pannels. However nothing is vandal or theft proof. Lately they been steeling the catalyic converters off of cars from the streets and in parking lots.

  • jerome

    OK – not sure if this would work – but instead of secondary panels sliding off the roof – how about ones that pop into place in the front and rear windshields?? Awfully gadget-y but the Japanese are famous gadget gurus!

    Issues would be:
    * How much passive solar gain through a dirty windshield?
    * Space inside the cabin?
    what else??

    interesting concept though Shines……

    as for adding to the green halo – if other companies are going to advertise “eco-friendly” and “green” cars that are not on the market I can see a company that already has eco-friendly green cars raising the ante…..

  • Bryce

    how would one then subsequently see through that back window with all of that silicon in the way??? As far as I see it, this whole solar array on your car thing will be something for rich and my fellows in Berkeley. Not until that thin film photovoltaics will this be cost affective. That would be a wonderful day. : )

  • Chazzman

    Personally, I think the idea of a slide-over solar cell cover that shades the windshield is sheer brilliance.
    Cost is another concept altogether.

  • A.H.S.

    http://www.sti.nasa.gov/tto/Spinoff2006/er_4.html

    I think think Shines I dea is very good. But there are a few problems. For one I kindof agree with Bryce because that sort of thing would probably be very vandalism prone. Also, weight would definatley be a problem along with significantly reduced aerodynamics. Ontop of which it would be hard to maintain and probably cost a lot to fix if there is a problem with it. These problems would probably outweigh the benafits. But jerome makes a very good point to have solar panels inside the windsheild. In the link I have posted is info about “solar paper”. This would not take up a lot of space when rolled up and could then be put on just like a car sun visor. This would be safer from vandalism, would not affect the areodynamics of the car, is very light, and is probably lower maintinance since it doesnt have to deal with weather and stuff like that. Now that might be worth the money. But I dont know how much the solar paper costs in comparison with regular solar panels so money might still be a big issue.

  • A.H.S.

    Oh and by the way for those of you who didnt click on the “Solar Electric Vehicles” in the article it gives you more info on the prius. It says it can go 20 miles per day on electrisity which sounds pretty good. Give it a look.

  • Mike Temple

    It takes more energy to drive the solar panel around than the energy that it produces. That’s a net loss of efficency.

    lslaughter@foray.com

  • Danny

    Nothing new under the sun? Audi offers solar sun roofs for some of their latest vehicles:

    http://www.audi.com/audi/com/en2/tools/glossary/operation_comfort/solar_sunroof.html

    They do also run the ventilation while the vehicle is parked. Webasto offers solar sunroofs as after-sale option kits. This is a great idea and leaves one wondering why it wasn’t introduced to a lot more vehicles. It doesn’t literally cool the car but it keeps it from overheating.

    A larger panel like shown in the illustration could easily add to your MPG considered the size, no matter if you run electric or gas.

  • dangerous

    Nice ideas right there but you will consume more when the sun is not present. But this is a good one, at least some electric cars will have some options. And the solar part of the car will add some points to the car body. Crazy Car Driver will be excited in this news and probably wait for the release or the cheapest one of this in the market.

  • Total Solar Energy

    solar cars have to come. its only a matter of time now. the sooner we wean ourselves off oil, the better. that’s unless the powers that be place obstacles in the way of this technology

  • Al

    Those who have not seen it, watch “Who Killed The Electric Car”. I am very passionate about EV cars and getting off oil usage.

  • Bryce

    That movie left out a few facts my friend.

  • Goober

    Baseless statements = bad post.

  • GEORGE P. MCGINN III

    I own a 2010 Toyota Prius IV. It has the solar panels on it, which keeps the car cool when it gets hot outside. I say that the Prius is a super vehicle because I can drive from San Diego, Ca. to Houston, Texas for only $140. Another thing, I ran out of gas and I my car ran on the POWER MODE for 150 miles and it never stopped running. Meaning I ran off the hybrid battery. I called Toyots the same day and to my surprise, they told me it was alright to run the car on power mode with no gas up to 100 miles, then it was recommended gas be put in. I now own a 2009 Toyota Highlander, which I can’t wait to take on a long trip with to see how it does. YOU CAN NEVER GO WRONG WITH A TOYOTA, IT DOESN’T MATTER HOW YOU DO THE NUMBERS.

  • Consultoria RH

    Este blog é uma representação exata de competências. Eu gosto da sua recomendação. Um grande conceito que reflete os pensamentos do escritor. Consultoria RH

  • Fabian

    Even though a solar panel on roof of a car will produce less energy than mounted on a regular stationary surface, the environmental load may be lowered. Every kWh of produced solar energy will in a car that drives partly on fossil fuels replace 1 kWh of fossil fuel and in a household, 1 kWh of electricity. If the electricity is made from for example hydro power the environmental impact will decrease.

  • Gravity Rocker

    There is an additional energy generating opportunity that I as yet have not encountered in discussion boards that could provide support to the plug-in hybrid or the electric car, body porting to provide moving mph wind flow across a helical wind vane driving a generator. Design engineer/architects please seize the ball and run with it.