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Old 09-22-2018, 09:00 AM
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Is it feasible to put solar panels on the roofs of cars?

Could a car include a solar panel on its roof, not to supplant the entire gasoline/electric motor, but as a supplemental energy source? Perhaps it could run the lights and A/C. Or would it be too heavy? Or is there some other impediment (cost perhaps)?
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Old 09-22-2018, 09:09 AM
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Panasonic is working on it.
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Old 09-22-2018, 09:15 AM
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The impediment is the amount of energy available in sunlight - 1KW/square meter. Multiply that by the solar panel efficiency (20%, best case), and battery and inverter efficiency (90%, best case), and allow for all the other losses, and you get around 175W of electrical power per square meter.
The average car roof is, what? 4' x 6' in area? Let's be generous and say that's 2 square meters. That gives you 350W of power in direct sunlight. That's enough for a very efficient electric car, but not nearly enough to do anything very useful with a real passenger car.
The Tesla S has a 100KWH battery pack. This solar roof would be able to charge that battery about 3% with 8 hours of direct sunlight. And, I'm being generous.

Last edited by beowulff; 09-22-2018 at 09:15 AM.
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Old 09-22-2018, 09:16 AM
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It's feasible, and cost is not a big problem: a decent quality 100-watt solar panel runs around $150 (with controller). You could easily fit one on the roof of most cars - larger cars might have room for two. They are common on RVs and vans fixed up for camping.

But 100 watts (what the panel will deliver in bright sunlight) isn't much - it's about 0.13 hp. To put this in context, an AC compressor needs about 5 hp - more than 35 times what one solar panel can deliver.
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Old 09-22-2018, 09:18 AM
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To put that in perhaps a more familiar context, that's about half a horsepower. Now consider that most cars have engines measured in hundreds of horsepower. Yes, you can get a fraction of a percent of improvement, but there are lots of easier ways you can get a fraction of a percent more out of a car. Solar panels are mostly just a marketing gimmick.
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Old 09-22-2018, 09:20 AM
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That gives you 350W of power in direct sunlight. That's enough for a very efficient electric car ...
Very efficient indeed, if it can operate on less than half a horsepower.
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Old 09-22-2018, 09:20 AM
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From the link, another problem seems to be the durability of the solar paneling.

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The option is currently not available in US because the reinforced glass sheeting currently used for the solar panel is not passing the US rollover crash tests. They are reportedly working on a solution, but there’s no timeline for a US debut.
Thanks for the replies!

Last edited by Moriarty; 09-22-2018 at 09:21 AM.
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Old 09-22-2018, 10:00 AM
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Also figure 20lbs/100W panel. There's no real savings if built-in structurally since the panel needs a decent amount of stiffness to prevent cracking.

And the motor is pushing that extra weight around at night, on cloudy days, etc. Not a big hit but enough to be noticeable on overall energy balance.
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Old 09-22-2018, 10:23 AM
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Very efficient indeed, if it can operate on less than half a horsepower.
beowulff is probably referring to solar car competition vehicles like this.
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Old 09-22-2018, 10:28 AM
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By the way, it is (was?) an option for the Prius Prime in some markets. It was an option for an earlier Prius model as well, though that one only powered the ventilation fans while parked.

Last edited by scr4; 09-22-2018 at 10:29 AM.
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Old 09-22-2018, 10:32 AM
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What about using it to top up the charge of the 12v battery (whether parked or not), or just to keep the climate control fan running on hot days when the car is off? Yeah you couldn't run the A/C but bringing in fresh air while parked would keep the interior from becoming scorching hot in the summer sun. If you don't have to crack the windows then you eliminate a lot of dust from the interior (since on most cars there's a filter for the cabin air intake), plus you don't have to worry about security or popup thunderstorms. As for the battery, it could take some load off the alternator, especially as daytime running lights are becoming more common, along with all the additional electronics going in cars (touchscreens, interactive gauge clusters, phone chargers, etc.).
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Old 09-22-2018, 10:47 AM
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I found this article that says the Prius solar roof didn't pass the US safety standards (namely the rollover crash test) and therefore wasn't offered in the US. I don't understand how a solar panel is any less safe than sunroofs though.
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Old 09-22-2018, 11:27 AM
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What about using it to top up the charge of the 12v battery (whether parked or not), or just to keep the climate control fan running on hot days when the car is off? Yeah you couldn't run the A/C but bringing in fresh air while parked would keep the interior from becoming scorching hot in the summer sun. If you don't have to crack the windows then you eliminate a lot of dust from the interior (since on most cars there's a filter for the cabin air intake), plus you don't have to worry about security or popup thunderstorms. As for the battery, it could take some load off the alternator, especially as daytime running lights are becoming more common, along with all the additional electronics going in cars (touchscreens, interactive gauge clusters, phone chargers, etc.).
But at what cost? Once minimum safety and environmental requirements are met, features mainly boil down to dollar cost per mile driven over the first several years of the car's life. Do you have the research and the numbers to say that the weight, expense and complexity of a roof mounted solar panel would pay off in real monetary terms?

OK, maybe it can't help your wallet but maybe you're willing to pay the cost for added convenience. That's a legitimate point of view. Out of 100,00 people, how many people can you prove agree with you?

I don't know the answers to these question but I'll bet my right hand that the marketing departments of most manufacturers have done that testing and they've decided it's not worth it.
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Old 09-22-2018, 11:34 AM
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Doesn't the Nissan Leaf have a tiny solar panel on the roof?
...

It's in the spoiler.
https://en.wikipedia.org/wiki/Nissan...ar_Spoiler.jpg
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Old 09-22-2018, 11:50 AM
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The use of that solar panel is not to add range, it's to protect the battery. In the Nissan Leaf's case, the 12volt lead acid battery. It provides enough juice to prevent the 12v battery from discharging under the idle/key off load of the car.

A bigger, 350 watt+ panel could do something similar for the traction battery. Same idea - it would barely add any range, but if you leave the car parked in an apartment or street parking spot without a plug for a weekend, when you get back in the car it would have ~1-8 miles more range than when you left it.

People who park with barely any juice and no plug wouldn't have their batteries destroyed, which is what happens now. It is destructive to these batteries for the SOC to get below a certain percentage, and there is a small amount of self-discharge.
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Old 09-22-2018, 11:52 AM
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What about using it to top up the charge of the 12v battery (whether parked or not), or just to keep the climate control fan running on hot days when the car is off? Yeah you couldn't run the A/C but bringing in fresh air while parked would keep the interior from becoming scorching hot in the summer sun.
This is pretty much how solar power gets used for RVs and camper vans.

In addition, a decent setup may more or less keep up with the power demands of a small refrigerator.
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Old 09-22-2018, 11:55 AM
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I'm far from being an expert, but it seems to me that unless you were really careful, it would mess up the aerodynamics of the car, and cost you more in lost gas mileage than you could possibly get back from the panel itself.
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Old 09-22-2018, 11:59 AM
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I'm far from being an expert, but it seems to me that unless you were really careful, it would mess up the aerodynamics of the car, and cost you more in lost gas mileage than you could possibly get back from the panel itself.
We're talking about panels that are flexible and integrated into the aeroshell by the manufacturer. They might not provide much energy relative to what the car is consuming, but so long as they provide more energy than the added weight costs, at least for driving outside on an average day, then it's a net efficiency gain.
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Old 09-22-2018, 12:07 PM
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...but so long as they provide more energy than the added weight costs, at least for driving outside on an average day, then it's a net efficiency gain.
You also need to account for the financial cost of the panel over the life of the vehicle. if it's not at least demonstrably cost neutral then you're losing efficiency.
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Old 09-22-2018, 12:51 PM
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The impediment is the amount of energy available in sunlight - 1KW/square meter. Multiply that by the solar panel efficiency (20%, best case), and battery and inverter efficiency (90%, best case), and allow for all the other losses, and you get around 175W of electrical power per square meter.
The average car roof is, what? 4' x 6' in area? Let's be generous and say that's 2 square meters. That gives you 350W of power in direct sunlight.
Note that the 1 kW/m2 figure assumes maximum solar irradiance, e.g. straight up at noon, and of course with no clouds or smog layer. As the Earth rotates and the angle of incidence of the Sun changes with respect to a flat horizonatal panel, this will go from 0 to maximum (600 to 850 kW 2 at mean sea level is a typical avarage annual maximum; Portland, OR is obviously much worse). What is really instructive is to look at solar insolance, which is a daily cummulation of power to get total energy available. For instance, a flat horizontal panel in sunny Phoenix, AZ has a monthly average solar insolance of 5.24 kWh/m2/day in December, and 7.52 kWh/m2/day in June, if you are so foolish as to park your car without shate in an Arizona summer. Most places at higher latitudes will be significantly worse, particularly in winter months.

There are the obvious structural and other issues integrating solar power cells into a car roof, and of course the additional cost. For the marginal value provided, it just really isnít worth it unless you need the power to trickle charge a battery or run a low power microcontroller.

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Old 09-22-2018, 12:55 PM
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beowulff is probably referring to solar car competition vehicles like this.
Yep.
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Old 09-22-2018, 04:24 PM
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I found this article that says the Prius solar roof didn't pass the US safety standards (namely the rollover crash test) and therefore wasn't offered in the US. I don't understand how a solar panel is any less safe than sunroofs though.
Raising the center of gravity of the car by putting more weight in the roof increases the rate of rollover accidents. Solar Panels are heavy. Glass for sunroofs is not.

Last edited by glowacks; 09-22-2018 at 04:24 PM.
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Old 09-22-2018, 05:16 PM
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...unless you need the power to trickle charge a battery or run a low power microcontroller.
Yea, for $20 a person can get a solar-powered trickle charger for their car. It's a good solution for a vehicle that's not driven for extended periods of time.
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Old 09-22-2018, 06:25 PM
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Multiply that by the solar panel efficiency (20%, best case),
Nitpick: 20% is not the best efficiency for solar panels. It's typical of the efficiency of commecially available single-junction panels. You can do better with multi-junction panels. Typical efficiencies for those are 30 to 35 % for commercially available ones, and specially built ones (such as those put on space probes) are over 40%. They're much more expensive than single-junction, but if you have limits on space and/or mass, they're the way to go.

Last edited by dtilque; 09-22-2018 at 06:26 PM.
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Old 09-22-2018, 06:31 PM
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Nitpick: 20% is not the best efficiency for solar panels. It's typical of the efficiency of commecially available single-junction panels. You can do better with multi-junction panels. Typical efficiencies for those are 30 to 35 % for commercially available ones, and specially built ones (such as those put on space probes) are over 40%. They're much more expensive than single-junction, but if you have limits on space and/or mass, they're the way to go.
Yeah, there are all kinds of hyper-efficient cells. You just can't buy them.
20% efficiency is generous for a production panel, although there are a few with a smidge more points.
See here: https://news.energysage.com/what-are...on-the-market/
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Old 09-22-2018, 10:32 PM
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Yeah, there are all kinds of hyper-efficient cells. You just can't buy them.
20% efficiency is generous for a production panel, although there are a few with a smidge more points.
See here: https://news.energysage.com/what-are...on-the-market/
I think I got confused between what individual cells can produce and what manufacturers can produce in panels. Apparently, there's a big difference.
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Old 09-22-2018, 11:08 PM
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Raising the center of gravity of the car by putting more weight in the roof increases the rate of rollover accidents. Solar Panels are heavy. Glass for sunroofs is not.
The weight of a solar panel /is/ the glass. Solar panels are heavy because they are big pieces of glass. A solar-panel-coating of a car roof wouldn't add much weight, but would be difficult to apply, and without a good protective surface would last only minutes in high speed traffic.

That is as much an economic problem as a technical one: You can't use anything even vaguely like an off-the-shelf panel.
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Old 09-23-2018, 07:52 AM
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Solar trickle chargers and such are a waste for quite a few people.

For example, us. We park our cars in the garage. And if we were to park in the driveway there's a big ole maple shading almost all of it.

While we do make a few short trips here and there and the car will be out in the Sun some, the amount of charge added is going to be insignificant compared to the charge added by the alternator during the trip. We rarely go somewhere and leave a car parked all day.

And then there's places like Seattle and Portland for large stretches of the year.
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Old 09-23-2018, 09:07 AM
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The weight of a solar panel /is/ the glass. Solar panels are heavy because they are big pieces of glass. A solar-panel-coating of a car roof wouldn't add much weight, but would be difficult to apply, and without a good protective surface would last only minutes in high speed traffic.

That is as much an economic problem as a technical one: You can't use anything even vaguely like an off-the-shelf panel.
Doesn't seem to be a problem for the massive sunroofs on a lot of cars. It's not like this would be some off-the-shelf component anyway. No auto glass is. It would be contoured and integrated into the roof, not something bolted on top.

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Solar trickle chargers and such are a waste for quite a few people.

For example, us.
So what? There's lots of features in all sorts of products, whether cars, TVs, computers, phones, washing machines, etc., that aren't used by everyone, and some can be fairly expensive. In this case there's benefits to panels that can work while driving too, instead of just when parked. Because something might not work for you it doesn't mean it's useless. You may even be a small minority.

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And then there's places like Seattle and Portland for large stretches of the year.
Germany has the most solar power generation in the world despite being quite far north and cloudy, even worse than the Pacific northwest. https://upload.wikimedia.org/wikiped...rld-map-en.png And just because something may not work well in one place that doesn't mean it's not good somewhere else.
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Old 09-23-2018, 01:09 PM
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Lightyear One is planning to sell a practical electric car that can get an appreciable fraction of the required traction energy from sunlight in a sunny climate.

That is, an electric car that doesn't have to be charged as often (or as long) because part of the electricity for driving comes from its solar panels.

Of course, as mentioned, the World Solar Challenge competition already had solar cars in 2005 that could cross the entire Australian continent at more than 100kph on solar energy alone. Although these were cigar-shaped, one-person cars with no air conditioning.

In 2017, the winner of that competition in the Cruiser class was a four-seat "practical" solar car that indeed transported four persons for most of the race. About half of the electricity (AIUI) it used came from its solar panels, about half was from overnight charging.
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Old 09-23-2018, 01:54 PM
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The impediment is the amount of energy available in sunlight - 1KW/square meter. Multiply that by the solar panel efficiency (20%, best case), and battery and inverter efficiency (90%, best case), and allow for all the other losses, and you get around 175W of electrical power per square meter.
The average car roof is, what? 4' x 6' in area? Let's be generous and say that's 2 square meters. That gives you 350W of power in direct sunlight. That's enough for a very efficient electric car, but not nearly enough to do anything very useful with a real passenger car.
The Tesla S has a 100KWH battery pack. This solar roof would be able to charge that battery about 3% with 8 hours of direct sunlight. And, I'm being generous.
My impression is an electric car can travel about 2-4 miles on 1 kwh of battery. So if you're only getting 350W over maybe 4 hours, thats less than 5 miles a day of travel.
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Old 09-23-2018, 01:59 PM
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My impression is an electric car can travel about 2-4 miles on 1 kwh of battery. So if you're only getting 350W over maybe 4 hours, thats less than 5 miles a day of travel.
Tesla model 3 is 241 watt hours/mile. Source is here. That's an actually existing, semi-mass produced electric car that is efficient.

So if you only get 4 sun-hours per day in your location (typical for the south) and have 300 watts of panels exposed at any given moment (the problem with putting the panels along a car's aeroshell is that no matter how you do it some will be shaded and the angle will not be optimal), you gain 5 miles per day.

Which isn't nothing, and this additional charging protects the car's traction battery if it was low when you parked. But no, this probably won't be routinely done.
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Old 09-23-2018, 02:17 PM
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The Lightyear One plans to use approx. five square meters of solar cells.

That's a lot more than you can fit on a normal car, but this car will be designed around that requirement.

If they manage to get that, and in an optimal environment (sunny day, optimal orientation of the car) they seem to plan on getting some 10kWh out of those cells on the best days. (I get >30kWh out of the ~30 sq.m. of panels on my roof, but that's way op north in a dark climate, not optimally oriented, and not the most modern efficient cells either.)

They also try to get the energy consumption down to 9kWh/100km (also under optimistic assumptions). Note that BjÝrn Nyland already gets around 13kWh/100km driving 90kph on the highway in a Hyundai Ioniq -type regular electric car. It seems realistic to get that down with more radical aerodynamics, lighter weight and more optimistic assumptions.

So they hope to get a little over 100km of driving per day, purely with the solar cells on the car. (You charge it normally to drive further.)
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Old 09-23-2018, 02:28 PM
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Reading more closely, they're actually claiming closer to about 80km of driving range per day on the car's solar panels alone (in the summer in Portugal).

They only need to generate about 7kWh of solar electricity per day for that.
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Old 09-23-2018, 03:49 PM
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Anyone know something about "smart windows"? The idea is that glass can be designed with materials that filter UV and/or IR leaving most of the visible light alone. The amount of power generated is fairly small, but if the material is produced en masses for the construction industry (as that article speculates might occur, as it can be adjusted to on-demand so its AC cost savings could be substantial) cost could be reasonable. Integrated into a glass roof, like much of the Tesla 3's, won't get the car very far, but it could still be of some use.
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Old 09-23-2018, 04:11 PM
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Yeah, not enough energy density there to fully power anything more than the covered bicycles that have those occasional Tour de Sol cross-country derbies.

Solectria first tried it over twenty years ago.
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Old 09-23-2018, 04:32 PM
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From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long.
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Old 09-23-2018, 05:01 PM
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From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long.
That is a physical impossibilty. A half order of magnitude improvement, which is beyond a y rational expectation, would be miraculous.

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Old 09-23-2018, 05:08 PM
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From reading this thread, it appears that a solar-paneled car roof will make sense if the photo-voltaic efficiency improves to about 10 times what it is today. I hope that can happen, and I hope I live that long.
Ten times 20% is more than 100%, so that's not actually possible. But This report gives some hope that we can get close to 100%. It doesn't look like they've made an actual solar cell with this tech yet, so it's several years away from prime time at the least.
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Old 09-23-2018, 05:25 PM
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Well I still think you are all wrong. A car that travels quite long distances on its own solar panels exists today and it is not a covered bicycle.

It is the Stella Lux which won the 2017 race.

It generates 7-10kWh per day (my estimate) from its own 5sqm of solar panels and it travels 100km on about 4kWh (also my estimate).

All of that is today, under favorable circumstances.

That's what they are trying to turn into the Lightyear One.
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Old 09-23-2018, 06:17 PM
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I remember an Audi having a small array of panels on the roof for only one specific use: to power a motor and ventilate the inside when left in the hot sun with the engine off. It was a luxury car and it didn't look all that tacky.
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Old 09-23-2018, 06:17 PM
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The only way a full-solar car is going to be practical is if ALL cars are solar.
There is no way to make car full-solar car that will survive a crash test. They are simply too light and flimsy.
The Stella Lux weighs 827 lbs. It would be turned into splinters in a collision with even a Smart Car.
Maybe some fantastic breakthrough in carbon-fiber manufacturing will fix that, but right now, it would be a deathtrap on the road.
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Old 09-23-2018, 07:25 PM
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The only way a full-solar car is going to be practical is if ALL cars are solar.
In the OP, I asked about a car that used solar energy to suplement the traditional engine because I doubted that a full solar car would be realistic, at least anytime reasonably soon. It sounds like it isn’t yet developed for fully powering even auxiliary features, but I’ll remain optimistic.

If I can hijack the thread a tad, what about the possibility of using the wind that is generated from driving down the road for some power? Maybe little turbines in the front grill? Or is that also too minimal to be worth it?

Last edited by Moriarty; 09-23-2018 at 07:26 PM.
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Old 09-23-2018, 08:13 PM
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Windmills do not work that way.

What extra power you gain by harnessing the wind would need to be made up by the extra drag on the car and thus the engine. It'll take more energy to do that than what you would gain from the wind generator itself.
  #45  
Old 09-23-2018, 11:39 PM
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beowulff beowulff is offline
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Originally Posted by Moriarty View Post
In the OP, I asked about a car that used solar energy to suplement the traditional engine because I doubted that a full solar car would be realistic, at least anytime reasonably soon. It sounds like it isnít yet developed for fully powering even auxiliary features, but Iíll remain optimistic.

If I can hijack the thread a tad, what about the possibility of using the wind that is generated from driving down the road for some power? Maybe little turbines in the front grill? Or is that also too minimal to be worth it?
What you are suggesting is this, basically.
  #46  
Old 09-24-2018, 03:05 AM
sbunny8 sbunny8 is offline
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Originally Posted by Wesley Clark View Post
My impression is an electric car can travel about 2-4 miles on 1 kwh of battery. So if you're only getting 350W over maybe 4 hours, thats less than 5 miles a day of travel.
The 2018 Smart Fortwo gets 3.23 miles per KWh.
The 2018 Nissan Leaf gets 3.33 miles per KWh.
The 2018 Chevy Bolt gets 3.57 mi/KWh.
The 2018 Tesla Model 3 gets 3.85 mi/KWh.

These are not outliers. They are typical. EVs that get less than 3 mi/KWh are quite rare. The Tesla Model X is one of them (2.7 mi/KWh). The vast majority of EVs on the road get between 3 and 4 mi/KWh. 350x4=1400 and a Chevy Bolt goes exactly 5 miles on 1.4 KWh (based on the average of city and highway combined). In city driving, it wouldn't be hard at all to go 6 or 7 miles on 1.4 KWh. I do it all the time.

An extra 6 or 7 miles per day for free would be nice. But I think it would make more sense to put the solar panels on your house rather than carry them around on the roof of the car.
  #47  
Old 09-24-2018, 10:12 AM
YamatoTwinkie YamatoTwinkie is offline
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I see this thread has made it this far without an obligatory xkcd link:

https://xkcd.com/1924/
  #48  
Old 09-26-2018, 12:29 AM
dtilque dtilque is offline
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There's a German startup, Sono Motors, that's trying to bring a solar enhanced EV to market. This article says they may be out in about a year and a half. It's a minivan that has solar panels not only on the roof, but also the hood, sides, and back. They will add 30 km to its range.

Last edited by dtilque; 09-26-2018 at 12:29 AM.
  #49  
Old 09-26-2018, 12:57 AM
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They are typical. EVs that get less than 3 mi/KWh are quite rare.
The upcoming Audi e-tron and the Jaguar I-Pace (ugh, 1995 called and want their names back) are both around 2.9 mi/kW-h. They, and the X, are all SUVs, so one might expect lower efficiency than a sedan. Also, the X is technically rated at between 2.95 and 3.16 mi/kW-h (75D vs. 100D).

But yes, no EVs that I'm aware of are nearly as bad as 2 mi/kW-h. Also, the Model 3 at least (and probably others) can easily beat 4 mi/kW-h in city driving.
  #50  
Old 09-26-2018, 07:34 AM
Frankenstein Monster Frankenstein Monster is offline
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Quote:
Originally Posted by dtilque View Post
There's a German startup, Sono Motors, that's trying to bring a solar enhanced EV to market. This article says they may be out in about a year and a half. It's a minivan that has solar panels not only on the roof, but also the hood, sides, and back. They will add 30 km to its range.
Looks realistic, too!

Sion solar panels

Quote:
Originally Posted by Sono Motors
The solar cells in the carís body mean that the Sion can easily charge its battery by itself throughout the day. The result is that the Sion can generate up to 30 kilometersí extra range per day, completely free of charge. Of course you can also drive the Sion at night. After all, the battery has an actual range of 250 kilometers and can also be charged externally, regardless of the weather conditions. We use highly efficient, monocrystalline silicon cells for the solar modules. A total of 330 cells generate a maximum power of 1,204 watts with 24 percent efficiency.
330 cells is 5.5 panels' worth. The quoted performance already accounts for the fact that not all the cells can be illuminated at the same time.

Assuming the car has the equivalent of 3 panels' performance in the optimal orientation, it should generate 5kWh on a good day.

Assuming 16kWh/100km consumption (today's Nissan Leaf), that will get 30km.

I'll be interested to see if they can actually fit those cells like in the picture and if they can actually get this kind of performance!

Last edited by Frankenstein Monster; 09-26-2018 at 07:35 AM.
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