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)?
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.
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.
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.
Very efficient indeed, if it can operate on less than half a horsepower.
From the link, another problem seems to be the durability of the solar paneling.
Thanks for the replies!
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.
beowulff is probably referring to solar car competition vehicles like this.
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.
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.).
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.
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.
Doesn’t the Nissan Leaf have a tiny solar panel on the roof?
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It’s in the spoiler.
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.
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.
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.
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.
Note that the 1 kW/m[SUP]2[/SUP] 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 [SUP]2[/SUP] 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/m[SUP]2[/SUP]/day in December, and 7.52 kWh/m[SUP]2[/SUP]/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 isn’t worth it unless you need the power to trickle charge a battery or run a low power microcontroller.
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