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Comment by timje1 | Hacker News Reader

"What better way to extend the range than to slap some high efficiency panels on the car's roof and drive all day long? I'm sure that solar panels will reach the point where it would be possible to have an EV with infinite range as long as the sun is out.."

Correct me if I'm wrong, but it was my understanding that there isn't enough energy in the sunlight hitting a few square metres on top of a car to keep it going even in the middle of a summer's day - unless we count the extremely low drag 'only works on the salt flats' type experimental 'vehicles'. Even if solar panels get to 100% efficiency, we won't have cars (traditional, 500kg+ ones) that can drive all day long off its panels.

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You are correct. Solar panels on any kind of safe vehicle wouldn't even run the air conditioner. I was on a solar car team in undergrad. Every 2 years we built a car with budget of about $1 million (and almost all of that went to consulting, parts or expert contract labor). We could afford space grade PV cells and fit as many as possible within the max 4.5 x 1.8 meter dimensions. We spent months refining the design to reduce drag and comfort was not a priority. This year's car can carry 1 (small) driver to 105 MPH (168 KM/H), but crusing speeds are about half that. On a sunny day we could maintain highway speeds without drawing much from the battery.

http://solarcar.engin.umich.edu/thecar/

The World Solar Challenge begins on October 6th in Australia where 26+ teams from the around the world will race their vehicles 3,000 km from Darwin to Adelaide.

http://www.worldsolarchallenge.org/

This sentence is a good sign that the author has no idea what he is talking about.

Solar constant is 1360 W/m². You can collect 1.3 kW (about 2 horsepowers) from 1 square meter if sun is directly overhead and conversion efficiency is 100%, no cloud, absolutely transparent atmosphere (no clouds).

Solar panels on a car can be useful, though -- to run a small fan when parked on a hot sunny day. Not much more.

By that figure, if you slapped four of those on top of a 2013 nissan leaf, it'd generate enough power in one hour for a leaf to travel all of 8km at 100kmph#. So, say 6 hours of that good stuff, you've earned an extra half hour of travelling. That's with 100% efficient panels working under theoretically maximum sunlight (midday on the equator). I agree, it seems that the author is speculating well beyond his area of expertise (whatever that is).

#http://insideevs.com/real-world-test-2013-nissan-leaf-range-...

1360 is top of atmosphere. At the surface, at the equator, the ideal number is about 400 W/m^2. So yes, the physics don't work for what we think of as a car.

I admit that the idea of driving on solar panel energy was more of a pie-in-the-sky idea, but it's not as bad as you're making it out to be. The Leaf's cross section is about 8 m^2. If we assume 5 m^2 can be covered with 50% efficiency solar panels and using an Earth's surface solar constant of 400 W/m^2, that would give 2 kWh of energy to the motor/batteries. If the car can be driven at 5 miles/kWh efficiency (already possible and improvements will make it more common), that adds 10 miles of range per hour of solar charging. If you drive 600 miles in 12 hours (including breaks where the solar panels are still charging), then the battery only needs to supply about 500 miles of that to drive all day.

It's a range extension idea, not a battery replacement idea.

Theoretical max efficency of PV is 40% and we're not likely to get there any time soon. Do the math: http://physics.ucsd.edu/do-the-math/2011/09/dont-be-a-pv-eff...

We're at about 15% now but let's be generous and use 20%. Neglecting the mounting angles and resulting reflection, that gives 640 watts collectible if you cover the whole car.

So you're collecting 3.2 miles of range per hour of peak sun. Note that the sun is not at peak elevation for 12 hours per day.

The middle of the country averages about 5 peak-sun-hours per day, so that means you're adding 15 miles of range per day of sitting in the sun.

It's not nothing, but it's very close to nothing. Useful to keep the car from self-discharging while it sits in long-term parking at the airport, perhaps. But it's not gonna provide meaningful range extension. The average EV has about 100-mile range, so if we assume a moving average of 33mph to make the math easy, you're moving for 3 hours, during which you collect enough power to actually move for 10 more miles. If you happen to be driving at noon in summer...

Summer peak sun hours will be much more than the annual average, when road trips are normally taken. Admittedly, it's not 12 hours, probably closer to 7-8 hours.

Theoretical max efficiency still uses a number of assumptions to get to 40%. If we learned anything from the path of semiconductors, it's that we shouldn't try to predict what's impossible. According to these 10-year predictions, current microprocessors are impossible to manufacture. And so were last year's, and the year before, going all the way back to the 1980s.

I'm not going to accept that infinite range cars are an impossibility. They may be impossible with today's technology, but what about the technology we have 30 years from now? Maybe solar charging is not the way we will do it, but I'm sure we will see it happen.

With future technology, perpetuum mobile may become possible.

Or maybe not.

Electric cars still may be a good idea in a long run. Even if they can not generate their own energy.

Theoretical maximum is not talking about manufacturability, we're talking about within the laws of physics conceivably possible.

Theoretical max efficiency of PV is about 40%, in the same way that theoretical max transistor density is one per atom. It's about the characteristics of sunlight as much as anything, and advances in manufacturing may bring us closer to the limit but not beyond it.

Many of the arguments claiming that Moore's law would end were based on the physical properties of light in lithography and the material characteristics of silicon substrate and gate oxide interactions. These are issues that people thought were physical limitations, but we found ways around them.

Theoretical max efficiency of PV also relies on assumptions about the physical properties of light in absorption and the materials being used. I'm not saying we know how to get around these limitations right now. I'm saying that we can't predict future innovations that will make the impossible possible.

Back of the envelop calculation :

A m² of solar panel receive about 1 kW (during the day). Efficiency of solar panel are around 10-20%, so basically you get about 150 W/m².

Given than EV have motors with power in the range of 50 to 80kW. It is at least an order of magnitude bigger that what you could get from your solar panel.

So a solar panel on the roof would be rather negligible.

You will never ever power a car from mounted solar panel only. An electric bicycle with a 5m² roof maybe.

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