Comment by ajuc | Hacker News Reader

Comment by ajuc | Hacker News Reader

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That's interesting. Cheap 15% efficient solar panel produces 0.75 kWh per square meter per day in "average US location" [1], whatever that is.

Let's assume average parking space area is 10 square meters (it depends, but it's close [2]) - so that's 7.5 kWh per day.

Average energy consumption of electric cars is 11-16 kWh/100 km [3], let's assume 10 kWh/100 km - that's 75 km range for a day of standing in sun.

That's actually not bad, and should be enough for driving back home from work. Actually, why not just put the solar cells on the car and do away with the batteries (car already has them). It would be less area, but even 20 km of driving for free would be enough for many people.

Hell, I use a car maybe twice a week, usually to drive for less than 10 km. Such a car would basically drive for free for my use case :)

[1] https://dothemath.ucsd.edu/2011/09/dont-be-a-pv-efficiency-s...

[2] https://www.quora.com/What-is-the-size-of-a-standard-parking...

[3] http://pushevs.com/2017/05/23/electric-car-range-efficiency-...

Actually, why not just put the solar cells on the car and do away with the batteries

Because you might squeeze 200W worth of panels up there at best. On a sunny day you’ll get about 4 extra miles at the end of your workday. And now you can’t drive home with the sunroof open on that sunny day.

Sorry, I meant do away with the batteries at the charging station, not the batteries in the car.

I was more pointing out that with current panels, putting panels on the car isn’t a good idea.

From a weight perspective they still add range so it's not a bad idea. Further, being able to run some fans or even AC while in full sun is a significant benefit.

Car companies are not adding panels because the benefit is not huge, they add complexity and cost, and panels negatively impact styling. Still, you see a lot of campers with solar panels on the roof because a few kwh/day of extra power really does add up over time.

Further, being able to run some fans or even AC while in full sun is a significant benefit.

You’re going to need those fans with giant, black heat sinks on your roof. To the point that I wonder if the panels heat the car faster than fans can remove it.

Still, you see a lot of campers with solar panels on the roof because a few kwh/day of extra power really does add up over time.

You see a solar panel on our VW camper because a 100W panel can easily recharge the battery that runs the 20W of LED lighting, and charge the occasional laptop, not run a 80,000W traction motor. My parents have a 35 foot fifth wheel with A/C, microwave, and other goodies. It has an option for solar panels. They didn’t buy that option because solar panels ain’t gonna do squat for that load. They figured it might buy an extra half day of boondocking before the house batteries run out. No, you see a lot of campers with solar because they either have a small load like us, or the salesman convinced them it would be a good idea on their 40 foot Class A tour bus.

Solar panels convert sunlight to energy, this means they don't heat up as much as you might think. Further, painting the outside of a car black does not heat up the inside of a car as much as you might think. Most heat gain is from sunlight hitting the inside of the car converting to IR, and then being blocked by glass. Thus, painting the inside black is a larger issue.

A 120mm computer at full speed is ~6w and can move 75 cubic feet of air per minute. So, 20w worth of fans can quickly exchange a lot of air.

Anyway, 100W of panels is minimal if your using 20+% panels. You can get that from a 1.5 foot x 3 foot section, a camper can have 20 of those on the roof assuming your starting with mostly clear space.

I agree with you that the solar panels aren't going to run the motor, but the 660W of solar on my RV with a large battery bank and a 3000W inverter have no trouble running everything in my RV including the AC for an hour here or there (though we did replace the AC with a swamp cooler to be able to run longer in the southwest): http://therecklesschoice.com/2016/04/29/diy-rv-solar/

The way I look at it, putting panels on the car is a good idea (because it reduces the need to charge frequently for people who don't drive long distances regularly) that sounds like a bad idea (because the power from panels is so much less than the power requirements of the car while it's running) that sounds like a good idea (hey, why not get energy for free!).

Not everyone will benefit or care about charging their car a few less times per year and adding panels will cost more, add weight, and probably change the appearance of the car, but I think it's an option worth exploring.

hey, why not get energy for free!)

Because that energy isn’t free. Panels cost money, the electronics to get it to the batteries cost money, and increased complexity cost money. You won’t get a sunroof. Those black panels that are specifically designed to optimize the absorption of the sun’s energy will absorb heat.

All so I can get an extra four miles (on a good day) of range after sitting in the sun all day. I’m not going to go as far as to say it’s a dumb idea, but it sure is hell isnt a good one with current technology.

For reasonable cost you can get 22.5% panels. A Camry is 193 inch x 72 inch or ~9 square meters. However, you don't get to tilt the panels and need to deal with shade, but can add cells to the sides which helps though at lower efficiency. Still ~6 hours of full sun equivalent x 9 * 0.225 ~= 12 kwh for a fairly normal shape.

Tesla Model 3 gets 310 miles on 75kwh so call it 4 miles per kwh that's 40-50 ish miles per day if you can stay in the sun.

Current records for solar panels well over 40%, so a fully solar powered car may actually be possible even if stupid expensive.

PS: Those solar challange cars are getting impressive. http://solarteameindhoven.itility.nl/ is a 4 seater doing 60+mph though far from street legal.

You’re ignoring conversion/storage losses when charging, which are on the order of 20%. And I think actual EVs are much closer to the less efficient end of your range (with the heat on, I barely get 3 miles / kWh on my Bolt, with no environmental controls, it’s about 4 miles / kWh, which is already the least efficient end of your range).

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