Could a solar powered car ever be practical?

[Junghalli]

Just something I'm wondering. With sufficiently advanced technology, would it ever be practical to build a car that ran entirely on solar power from panels on the body? Or is there just not enough surface area on a sanely designed car for that to be practical?

A maximally efficient solar panel might get you up to around 1300 watts/m^2, with present technology it's much less. So an "apex technology" solar car might get, oh, maybe 3-5000 watts I figure. Would that be enough to get decent engine performance (keeping in mind that you'd have to store energy away for driving at night as well)? Just how much energy does the engine of a typical car eat up, on average, in terms of watts?

BTW I'm not interested in hearing about the merits of busses and public transport vs cars and stuff like that. This is strictly a technical question.

[Junghalli]

Hmm, I tried doing some calculations base off this site.

According to this, a gallon of gasoline is 31,500 calories. I'm assuming they mean large calories, which according to Wikipedia are 4.184 kilojoules. So a gallon of gasoline would have 131.8 megajoules.

Let's take the 34 mpg sedan and assume it travels 1 hour at 60 mph. It's going to consume around 1.76 gallons or 233 megajoules. Divide that by the number of seconds in an hour and you get average power consumption of 64.6 kilowatts.

As I said, just eyeballing the surface area of a typical car, with a solar panel that's bumping up against basic limitations of the laws of physics it can get maybe 3-5 kilowatts from the sun. If you want to be able to drive at night half of that will have to go to the engine and half to the batteries, more or less.

I've heard internal combustion engines are rather energy-inefficient as most of the energy goes out the tailpipe as heat. I imagine regenerative braking (is that the correct term?) and lightweight materials could help. Still, just looking at these figures a "self-sufficient" solar car doesn't look like anything that would ever be practical, unless you're willing to accept very poor performance.

Would you say that's about right?

[erik_t]

Your 3kJ/s is unfortunately awfully optimistic. A car at highway speeds requires, as I recall, 10-20 horsepower (so 15-25kW ish) to maintain speed. An order-of-magnitude reduction in this value is unlikely.

[Junghalli]

So I'm guessing you might use solar to supplement battery power, but it's probably never going to be practical to actually run the car off it?

[Singular Intellect]

I could see solar power panels being somewhat useful for supplementing energy requirements and increasing existing fuel efficiency, expecially if coupled with technology like regenerative braking and lightweight high capacity battery storage.

But solar power alone? Sounds utterly impractical, even assuming extremely generous conversion technology and ideal weather conditions.

[Master of Ossus]

Toyota is rolling out an updated Prius with an option to put solar panels on the roof, but they're pretty expensive and they're ONLY used for powering PART of the air-conditioner, windshield wipers, etc. I think it's pretty unrealistic to get a practical car going which relies on solar panel to any great extent.

I also have a more fundamental question, though: why would you WANT a solar powered car? Why not just put the same sized solar panel in a fixed location so we can angle it to where the sun is? A car is driving around without any significant regard for the sun, and in fact people preferentially park their cars in shaded areas so they're not hot as hell after sitting in the parking lot while they're at work. IMO, if you're going to assume cost-effective solar panels then it would be better to plant them over parking lots to provide shade for the cars, and simultaneously generate power for the buildings that the lots service. At least in the suburbs, that strikes me as being preferable. Since your scenario also seems to rely on cars that are powered largely through electrical systems, it also makes sense that you could use whatever power you DO generate from the panels to power the cars in the lots.

Aside from the eco-kitsch appeal, I don't see any reason why a solar-powered vehicle should be seriously considered.

[Sky Captain]

There is one more thing - cars get dirty fairly quickly and that will greatly reduce solar panel output.

IMO small gains of putting solar panels on a average car is not worth the cost. If you want solar powered car then put a decent sized solar panels on the roof hook them to big battery pack and charge your car while it`s at home. If your panels are on the roof you also don`t need most efficient and expensive panels since there is plenty of space on average roof. That way you could also power your house. Such solar power scheme if cheap large capacity batteries are available might be cost effective in sunny locations.

[Junghalli]

I also have a more fundamental question, though: why would you WANT a solar powered car?

Convenience. As long as it gets sun it's like a perpetual motion machine: you never have to worry about recharging or refueling it.

Assuming it works, of course, which I guess it probably wouldn't.

[Starglider]

While a solar powered car isn't practical a decent sized solar trickle charger could definitely make sense for some applications. Lithium battery packs self-discharge several percent per month (depending on temperature), NiMH is more like one percent per day. If the vehicle is likely to be left standing outside for weeks it would be nice if you didn't have to worry about the battery pack going dead. If the cost of solar cells gets down to the level where you could coat the roof of the car with them for under $100, then I imagine this would be a common feature.

[Crayz9000]

I also have a more fundamental question, though: why would you WANT a solar powered car? Why not just put the same sized solar panel in a fixed location so we can angle it to where the sun is? A car is driving around without any significant regard for the sun, and in fact people preferentially park their cars in shaded areas so they're not hot as hell after sitting in the parking lot while they're at work. IMO, if you're going to assume cost-effective solar panels then it would be better to plant them over parking lots to provide shade for the cars, and simultaneously generate power for the buildings that the lots service. At least in the suburbs, that strikes me as being preferable. Since your scenario also seems to rely on cars that are powered largely through electrical systems, it also makes sense that you could use whatever power you DO generate from the panels to power the cars in the lots.

Cal State Northridge has been doing that in most of their non-garage parking lots, although with the budget problems I don't know what's happening now. The campus already generates somewhere over 20% of their base load, mostly thanks to a 1MW fuel cell they put in a couple years ago.

[Gil Hamilton]

Solar powered DIRECTLY seems unlikely. However, I don't see why you can't set up a few square meters of solar panel to track the sun on your roof and wire it to batteries in your garage, so when you come home for the night, you plug your BEV and charge it from the solar energy that you spent all day gathering.

[Sea Skimmer]

Cal State Northridge has been doing that in most of their non-garage parking lots, although with the budget problems I don't know what's happening now. The campus already generates somewhere over 20% of their base load, mostly thanks to a 1MW fuel cell they put in a couple years ago.

Fuel cell? What are they powering the fuel cell with? Those tend not to make a lot of sense for static applications, even if your goal is reducing CO2 emissions more so then actually saving energy.

[Covenant]

I don't know how the math can ever work out--the energy density you need just isn't there, even if it was incredibly efficent and weighed nothing to capture and convert the energy to driving power. This is pretty unnecessary with an electric car that sucks power from a green grid though, so solar and other power sources can still contribute to a clean car, even if a solar-powered vehicle is really unlikely. You need to look at the amount of energy required for a car to transport a person as compared to the amount of power per inch of sunlight hitting the ground at an optimal altitude and level of cloud cover, and I think it'll be clear that it just is very unlikely that solar is going to be much more than a drop in the bucket. Your numbers are spot on though, so I bet you can see that for yourself.

However, you might be able to do it if you had a setting appropriate to it. Consider an ultralight car built to the size of a semi truck or stretch limo running along in the desert at optimal conditions with a large surface area or even a small solar trough generating just enough power to keep it and it's one passenger mobile. Running via computer programmed control and with an panel that swivels slightly via mobile counterweight to adjust for maximum energy capture, this kind of car might be able to work well in an area of very low population density so long as your driver is wealthy enough to afford it. City driving is death for the solar car, but your desert driver of the future might have one. We already have experimented with flying machines that run on solar power to some success. If you're willing to redefine what you call a car slightly and equip it with sufficently advanced solar tech, you can wiggle it in. It's certainly not a reasonable thing for a small european-style city car, but I could see a solar farmer in texas or arizona having the room to park and the open roads to drive the thing.

[Crayz9000]

Cal State Northridge has been doing that in most of their non-garage parking lots, although with the budget problems I don't know what's happening now. The campus already generates somewhere over 20% of their base load, mostly thanks to a 1MW fuel cell they put in a couple years ago.

Fuel cell? What are they powering the fuel cell with? Those tend not to make a lot of sense for static applications, even if your goal is reducing CO2 emissions more so then actually saving energy.

They're running it on natural gas, which at least there is plenty of here in southern California (for the time being).

There's a press release here. Apparently they're directing most of the fuel cell's waste heat into both heat exchangers for the campus buildings and a biology project:

The power plant also provides a rare opportunity for students in the College of Science and Mathematics to study the carbon dioxide enrichment potential on plant life provided by the power plant to the university’s greenhouse. University officials are routing carbon dioxide exhaust from the plant’s heat exchanger into an adjacent miniature sub-tropical rainforest developed specifically to be sustained by the fuel cell plant’s operation.

[Sky Captain]

For desert drivers good thing for emergencies might be large lightweight and foldable solar panel so if your car`s battery runs out and you get stranded in the middle of nowhere you take out of the trunk your solar panel, unfold it and wait till your battery charges up.