I'm really looking forward to the advances that are being made in solar power. Just a few short years ago, it was still mostly an inefficient pipe dream. We're a long way off from being able to fully charge daily-used commuter vehicles with on-board solar panels (to reduce our dependence on coal and oil burned to charge the current wave of electric vehicles) but grid-connected solar farms are gaining a lot of traction. I like that.
You are forever away from fully charging a commuter vehicle with on-board solar panels.
given: the daily solar power on a sunny day is about 4 kilowatt hours per square meter.
given: you have a big roof and get 2 square meters of solar panels.
assume: You can have magical 100% efficient panels instead of the 20-30% you could have.
compute: You get 8 kilowatt hours of energy each day.
google: That is about 30 megajoules of energy.
wikipedia: Gasoline has 35 megajoules per liter.
thusly: Your tiny car with a huge roof covered in impossibly efficient solar panels will get you the equivalent of one liter of gasoline a day.
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But I have to wonder: Why are you carrying your solar panels around with you? That just wastes energy. Let them sit somewhere and send power to the grid. Pick up the power where you need it, when you need it.
The only function for a roof mounted solar panel is to look really cool in concept art.
Your analysis is way too conservative. An analysis by Tesla Motors some time ago calculated that if you drive the Roadster an average of 35 miles a day, you need about 105ft^2 of solar panels to produce all the electricity for it, assuming 18%-efficient solar panels. [1]
The Tesla Roadster has an area, when viewed from the top, of 72ft^2. [2] That means if you were able to cover it with solar panels that were 27% efficient, you could power it just from the power it produced.
As far as your last point, I agree: I'd rather put the solar panels on the garage roof, where I can somewhat control how much sunlight it gathers, than on the car.
Good numbers in the Tesla Roadster wikipedia introduction.
wikipedia: The tesla roadster use 490 kiljoules per kilometer.
compute: Our 30 megajoules above will take us 38 miles.
Of course I'm still using mythical 100% efficient solar cells on a biggish flat roof and all sunny days, and Tesla used mythical shape hugging solar cells on all of their exposed surfaces.
I still think the best solution for a solar powered car is to place your solar cells the south western deserts near LA and pull what power you need from the grid. That way you get a lot more power delivered from your cells.
> The only function for a roof mounted solar panel is to look really cool in concept art.
Not true. The Prius has a solar panel option to power cabin ventilation when parked in warm climates. This makes a lot of sense because the aircon load required to cool down a hot car quickly is huge, but the power required to prevent the heat building up is not.
Perhaps solar panels could be tied to a battery that powers the dash electronics, so that they don't have to siphon power indirectly via gasoline? Small potatoes compared to actually moving the weight of the car, but it would still be cool.
solar's strength will always be in its decentralization, not its efficiency. If we can get the best of both worlds to some extent (e.g. car charging overnight from batteries charged by a rooftop home solar array), that's a going a long way towards 'daily' sustainability.
While I agree with you about decentralization, onboard solar cells won't ever do the trick. Even for a home-sized array, I'm not sure it can be made practical. Here are some back-of-the-envelope figures:
To really replace a gasoline-powered vehicle, an electric car needs a similar amount of energy at its disposal. My car (a 92 Accord) holds about 16 gallons of gas. Figure it's about 50% efficient, and gasoline holds about 45 MJ/kg, and that's about 1 gigajoule of usable energy in a full tank. So to charge my car (without running the house at all) I have to collect and store 1e9 joules during the daylight hours. That's about 12 kW, if it could run 24 hours a day. According to Wikipedia (http://en.wikipedia.org/wiki/Solar_cells), a solar system can produce about 20% of its peak rating, so I need a system rated for about 60 kW at noon, in order to average 12 kW through the night. Assuming a reasonable conversion efficiency of 20%, that means I need to actually intercept 300 kW of sunlight. Wikipedia uses a figure of 1000 W/m^2 in that article, so I'd need a 300 square meter panel. In units familiar to me, that's pushing 60 feet square. Large for a house, never will fit the car.
Such a large system just to charge a car seems impractical to me. We can tweak parameters and dramatically shrink that size, since after all I would hardly ever need to charge the car from an empty battery, but someone who drives a larger vehicle over a longer commute might realistically have such high demand. (We could also argue about the size of the vehicles, but this is America: Suburbans aren't going away in my lifetime.)
That said, charging the car overnight would use vastly more energy than my house does on a daily basis. Solarizing the house is getting more practical, especially since the grid's always connected.
That’s assuming you need all the energy of a full tank every day, right? If you drive something like 50 miles every day you will need something like 1.5 gallons with a current car which means that you can cut your large solar array in ten pieces. That’s 30 m^2 or 323 square feet.
Sure, the assumption that I need a full tank every day is pushing it, but that's partially balanced by the fact that I used a fuel-efficient car for a baseline. Someone who drives 100 miles a day with the electrical equivalent of 15-20 mpg would need significantly more. My point is that a fully-capable vehicle is an aggressive target for solar energy. Our houses use much less, so let's start by aiming there.
not sure about some of your other assumptions about engine efficiency and electrical equivalent mpg's. gas engines are extraordinarily inefficient compared to electric, whatever the source of power (even coal!)
Yeah, but I also didn't account for any sort of inefficiency in the electric drivetrain. 90% conversion efficiency is a good rule of thumb, and there's one step from house panel to storage, another from house storage to car storage, and one more step from car storage to motors, so you'd only get about 75% out of each joule that comes out of the solar panel. Even adjusting gasoline efficiency down to 20% on average, that's less than a factor of 2 off my first set of numbers. We're in the ballpark, close enough to know it can't fit on a car, and impractically large for a house.
I stand by the approximation. 300 m^2 is about 17.3m on a side, 17.3 m in feet is a touch under 57 ft. Rounded to one sig fig because I'm not carrying any sort of precision through these estimates.
Edit: Or are you talking about the "60 feet square" bit? I don't mean 60 square feet, I mean 60 feet on a side. I suppose I could have picked a clearer idiom.
If we manage to get cheap solar and cheap batteries, the usage pattern is probably going to be more like: top off batteries at work during the day, drive home, partially drain car batteries out to grid while the sun doesn't shine overnight, drive to work on what remains in the car batteries.
NIMBYism wrt adding new long haul transmission lines is likely to favor solar cells on many roofs if the cost of the solar cells come down enough.
The future of electric cars involves battery swap stations, not generating the electricity themselves.
For example, you go to a gas station, and a fully charged battery is mechanically swapped for your drained battery. Would take 30 seconds and your car is back to full. Then the drained batteries are recharged at the station while you are already off driving around.
This obviously could not take place for a while due to issues of standardization. But its more likely the solar panels will be on the gas station or power plant servicing it than on your car.
I'm not convinced you need battery swap stations for personal vehicles.
The Nissan Leaf has a 500 VDC, 125A quick charge connector, which allows 80% charge of its ~23 kWH battery in a half hour. If you have a similar wattage charging option in a car with a 200+ mile range battery, it may turn out that the time that you will spend for lunch/restroom breaks in a typical 500 mile day of driving will be enough to recharge your battery enough, assuming you start the day with the battery topped off, and all of the restaurants and rest stops have charging stations. (But I suspect the restaurants will see early adoption of charging stations as a way to attract customers/advertise their business.)
The trucking industry may well benefit from swap stations, though. The Tesla Roadster is something like 1/4 or 1/3 battery by weight. The entire tractor of a typical tractor trailer rig is probably a bit less than 1/4 of the total rig by weight, and the tractor can't be 100% battery. This will probably translate to a somewhat shorter range.
On the other hand, at 60 MPH, wind resistance is probably what consumes most of the energy, and that is not strictly proportional to the weight as you scale up from personal automobile to tractor trailer.
So 3 factors shape this market: batterycell tech, charging station tech, vehicle tech. The outcome depends on the rate these 3 develop relative to one another. Perhaps we could graph and predict if we had enough data?
Perhaps charging station tech advances the quickest. A business produces commercial chargers and markets them to trendy coffee shops with strip mall parking (that are likely to have "green" customers) or perhaps to non-munincipally owned parking garages in urban areas. And another startup develops and markets commercial chargers for households with garages.
Capitalizing on a decentralization trend (computers in every home) has great business potential. But the venture risk might be too high due and corresponding probability of success too low for this particular paradigm to take hold.
Battery swapping for trucking has the advantage of being a smaller, easier to target market. But the disadvantage of less electric vehicle tech.
I'm not aware of any of the battery powered cars that have been announced being set up for battery swap.
Meanwhile, AFAIK everyone has agreed on a standard AC charging connector good for 120/240 V, with the connector itself being good up to about 90A (it looks like Tesla is using it with a 50A rectifier, and Nissan 30A).
Nissan's 80% quick charge in 30 minutes connector probably isn't quite high enough wattage to 80% charge a 53 kWH battery pack (as is found in the Tesla Roadster) in 30 minutes, and I think that 80% in 30 minutes may be the magic cutoff for claiming the $5000 California tax credit for buying one of these things, which leads to the question of whether Tesla will have an incentive to get quick chargers introduced which will be incompatible with the initial version of the Leaf.
(As a practical matter, I think if these chargers max out at 500 VDC at 125A, that's probably good enough for road trips, though higher wattage might be nice.)
