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Solar/plug-in hybrid

A different kind of hybrid
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I was thinking that one thing that could at least extend the range of an electric car would be a solar panel on the top of it. Not the one where the entire body is covered in solar panels, but just on the roof of the car.

It probably wouldn't provide the entire energy requirements for the car all by itself- but if the car was sitting still it could charge the batteries, and it could reduce the amount of power needed from the battery when the car is moving at higher speeds. The batteries could also be charged by plugging in a power cord like an electric or plug-in hybrid car. The batteries alone could power the car for a reasonable range even without the solar panel- like 80 to 100 miles (estimated ranges for current electrical cars)

Dickcheney6, Apr 20 2011

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       My little truck had about 3 sq.m. available if I had a solar-panel tonneau cover... that would work out, on a really good day, to maybe 5-6 miles worth (if I had an electric motor).   

       Doesn't sound like much, but it's actually quite a bit. So I wonder why any of the large tonneau cover companies haven't made one yet, even a one-off just for brand publicity.
FlyingToaster, Apr 20 2011
  

       The Nissan Leaf has a solar panel on the spoiler; it runs a small fan that cools the car.   

       Why does the car need to be sitting still for the cell to charge the batteries?   

       On our solar racer, we were lucky to pull 1100 Watts on a bright sunny noon day, and that with over 8 square meters of direct, sun-facing exposure. After about 80 miles at 30 mph, that energy was gone. Of course, that was 7 12-V lead-acid batteries and non space-grade solar cells.
RayfordSteele, Apr 20 2011
  

       You can't talk about range unless you also talk about duration of exposure. As long as your panel puts more power into the battery than it self-discharges, your range will be unlimited. However, it might be divided up into league per fortnight chunks or some other unconvenient mode of useability.
lurch, Apr 20 2011
  

       Seeing as how the sun's maximum wattage exposure in a desert on a noon day is about 1100 Watts / m^2, and encapsulated solar cells are only going to convert about 20 % of that, and batteries are only going to store about 85 % of what's fed them, there's simply not much energy there to bother with. Compare that with 39,000 W / gallon of gasoline.
RayfordSteele, Apr 20 2011
  

       I would imagine that this is one of the reasons terrestrial photosynthesising organisms don't have legs, or wheels, or cillia. Locomotion, in the absence of a fairly forgiving medium, like water, is rather difficult and can sap a lot of your "procreation" energy. Things that want to move (on land) need to eat a lot of things that soak up a lot of sun, or at least eat a lot of things that eat a lot of things that soak up sun.   

       Howwever this energy is free, and on-tap, so to speak. My only worry would be: How much energy is needed to create this thing with only a marginal benefit? If you crack that nut, then it would work.
4whom, Apr 20 2011
  

       sp: ~39,000 W-h / gallon
csea, Apr 21 2011
  

       "Why does the car need to be sitting still for the cell to charge the batteries?"   

       It all depends on how much power the solar panel itself puts out- if it put enough energy out to charge the batteries while the car was moving then no, it wouldn't need to be sitting still. But I don't believe that current solar cell technology, in a panel the size of the average car's roof, would be enough to run the car on it's own. It would still put energy into the batteries, but the car, depending how fast it's going and the accessories in use, may require more energy than the solar panel can provide on it's own.   

       When the car's parked there's no power drain, so the batteries could charge and the solar cell wouldn't have to power anything else. If the car is parked but a radio or something is running, and there's enough light, the solar panel could power said accessory on it's own and maybe even have enough to charge the batteries.   

       It all depends on the solar panel's capacity vs. how much power the motor needs.
Dickcheney6, Apr 21 2011
  

       My not-so-subtle point was that the solar cells can continue to put out juice while the car is moving. Of course it won't even begin to approach what it takes to keep the car moving at speed. As I tried to point out above, the limiting factor isn't just the cells themselves, it's the max available sunlight.
RayfordSteele, Apr 21 2011
  

       "While moving". There's a medium chance that the panel and associated parts wouldn't be able to supply enough energy to keep themselves moving. Few solar-powered vehicles run free--most have batteries. (I ain't saying having the cells is a waste, I'm just saying that while moving you might be trying to reduce the drag of their presence, rather than getting an overall boost from them, *while moving*.)   

       I am saying that the idea of carrying PV cells is pretty darn obvious. And likely isn't being done for reasons that may not be obvious at first.
baconbrain, Apr 21 2011
  

       I've said this elsewhere, but my analysis suggests that generating electricity using photovoltaics and storing it in batteries is completely futile - it would be less costly and destructive just to burn petrol (although that would also be a very bad idea). Having to lug the batteries around only makes it worse.   

       The obvious place for solar cells is on trains - there is a huge, flattish surface area available, they already often have a diesel-electric drive, and they run all day, so there would be no need to store the electricity.
spidermother, Apr 21 2011
  

       Good idea about the trains, [spidermother].
baconbrain, Apr 21 2011
  

       baconbrain, so then you're assuming a reconfigurable array. I was just assuming cells inset with the body.   

       Toby, sorry but you can't charge batteries that fast, for battery chemistry reasons. And if you did, I wouldn't want to be anywhere near the monsterous cables you'd have to use to transfer the power.
RayfordSteele, Apr 21 2011
  

       Some research I did for "Pickup Truck Windmill"(or whatever it was called), supercaps can deliver or charge much more quickly than you can use it, at maybe 100lb/kWh, which is greate if you've only to go a few miles: 1kWh = 2-3 miles driving.
FlyingToaster, Apr 22 2011
  

       //trains// or buses.   

       2 kWh = 33 kWmin = a nice acceleration boost intermittently spread out over a few hours driving = a smaller ICE engine can be used and still have passing power on tap... depends on how you want to set up supercap & battery(s) & ICE combination for your travel patterns.
FlyingToaster, Apr 22 2011
  

       No, I wasn't assuming a configurable array. I was imprecise in my use of the word "drag". I am not just talking about air drag, but about the weight of the pieces and parts, as well. If you make a configurable array, you add drag, yes, and even more weight. If you make an inset panel, conforming to the car's body shape, you add weight, which causes drag/friction on the wheels (which was what I meant), and you get much less efficient angles on your panel.   

       I say again, nobody is running solar panel cars without charged batteries in them. You probably won't get a boost from your panel while driving--the added weight isn't worth the candle, most times.   

       And again I say that this is the kind of thing that would be really obvious and easy, and probably isn't being done because it isn't worth doing. (Not that you couldn't sell the things to folks anyhow.)
baconbrain, Apr 22 2011
  

       Several "efficient'" vehicles now have a small fan driven by solar panels, intended to circulate air through the car while it is parked, in order to keep it from turning into an oven during the summer without draining the battery.   

       I would argue that light-weight thin film solar panels probably do make sense on most vehicles that already have batteries in them, but are not going to be a huge addition to the available power. 1 kW/m^2 * 4m flat plate equivalent on a car roof/hood and trunk * .20 effecient cells *.85 efficient battery =0.68kWh/h. Given [FT]'s 1 kWh =2-3 miles of driving, and assuming a speed of 45mph, you can get ~3.76% of your power from panels while driving during full sun in the desert southwest.   

       Assume the car is parked in the sun, with sufficient battery reserve to accept any charge the panels can supply (same programing as PHEV). Most of the US receives an average of 4-5kWh/m^2/day. 4 kWh/m^2/Day*4m^2*.2*.85 =~2.72kWh/day, or 6.8 miles/day, which would cover a decent portion of many commuter drivers. If combined with low weight, high efficiency cars, this might be improved significantly.   

       Again, this is dependent on thin film cells being installed conformaly in the outer skin of the vehicle, without significant added weight. Assuming a cost of $1/watt (for the panels, minimal installation cost for this sort of system), you're looking at $1200 installation cost, which would take 90 days of using the complete power output to offset an equivalent amount of gasoline. It will take closer to 1 year of such use against a plug-in solution.
MechE, Apr 22 2011
  

       I've decided that until solar cells get more efficient, they are only good for trickle-charge topoff action on cars.   

       They would be good for a camper, off grid SUV type vehicle, or a weekend boat. These are situations where it's acceptable to be parked for long periods of time (several days) between use. Commuter car? Not so much.. if you're really bent on solar then cover your house roof to offset charging cost.
AutoMcDonough, Apr 25 2011
  

       Gasoline has 34.8 MJ/L, which is 9.67 KWh/L. At 100 cents/L, used at 0.33 efficiency, this gives 31.3 cents per KWh.   

       Electric car batteries cost about $500 per KWh of capacity. Over the equivalent of 1000 complete charge cycles, that's 50 cents per KWh in depreciation of the battery alone.   

       Even if the electricity were free, and you ignore the weight penalty for carrying around the batteries, the electric version will never break even with the petrol one.   

       That's why I suggested trains - especially long distance ones, where the power requirements are fairly constant and predictable. The locomotive uses the full output of the panels directly, supplementing their power with diesel when necessary, so there is no need to store electricity - which would be a total waste of money and resources. It would literally be cheaper to throw away electricity, then generate it anew when it's needed, than to store it in batteries.
spidermother, Apr 26 2011
  

       NiCd batteries can be good for 2000 cycles, Li+ are at least 1k, experimental Li+ polymer have made it past 10k cycles.   

       At that, that's only down to 80% max charge, a willingness to accept a lower point for "life span" you can get considerably longer life with more charge cycles. I'm not saying it makes perfect sense yet, but especially with gas prices headed up ($1.00 liter is about right in the US now, low in Europe), battery costs coming down, and electricity relatively stable, it works out better every day.
MechE, Apr 26 2011
  

       Still, the //equivalent// of 1000 complete cycles seems about right; 2000 cycles, at 80% discharge, with the capacity greatly diminished over the course of those cycles.   

       The problem with the "gas prices heading up" argument is that all prices head up proportionally. Of course batteries will get cheaper (relative to mature technologies, such as gasoline), but there's a limit there, as they will always consume energy and other resources.   

       I just think if we were honest, we would say that electric vehicles are currently worse than internal combustion ones, although they may end up somewhat better in the future. If we swallow the line that it can be business as usual, except we'll all be driving electric cars, then we're totally screwed. Like elephants on a string, all swallowing the same sugar-coated line...
spidermother, Apr 26 2011
  

       Those numbers were full charge equivalent.   

       And gas prices heading up is a supply and demand thing, people want more than the world is supplying, the same is not true for many other prices.   

       //we would say that electric vehicles are currently worse than internal combustion ones//   

       If you define worse solely on a cost basis, you are probably right, although I would argue it is close and getting closer. Both battery and solar cell prices have dropped by something like 50% in the past couple of years. If you consider embodied and expended energy and pollution, electric can be considerably more efficient.
MechE, Apr 26 2011
  

       //full charge equivalent// I stand corrected.   

       I think we're pretty much in agreement here.   

       However, I'm working on the theory that because we are still living in a world where virtually everything - including making cars and batteries - is heavily subsidised by fossil fuel consumption, there is a fairly direct correspondence between how much something costs and how much energy and non-renewable resources it consumes; the complexity and interdependence of our economies, combined with the central limit theorem, virtually guarantees this. Likewise, there is good reason to think that nearly everything gets more expensive as fossil fuel energy gets more expensive.   

       This, too, may change, but in the meantime, we should be careful to consider things in this wider context, to avoid being misled.
spidermother, Apr 26 2011
  

       [bacon] Solar panels aren't terribly heavy; sunroofs are probably heavier.   

       [spidermother], for reasons of keeping the trains running predictably at a constant speed whether it's cloudy or sunny or both, you're going to need batteries or supercaps or some form of temporary energy storage.
RayfordSteele, Apr 26 2011
  

       For a lot of materials/products embodied energy is linear with cost. I'm aware of two exceptions that apply to batteries (although not to cars), one is the R&D exception. If you're still amortizing the cost of developing a product, it's going to be significantly more expensive without energy input involved in its production. The other relevant one is the scarce materials exception. For something like aluminum or steel, the major cost is the energy to find it and refine it. For something like a battery that might require a more exotic material, the actual material cost is a signifcant factor that is not directly dependent on energy.
MechE, Apr 26 2011
  

       [RayfordSteele] I mentioned //supplementing their power with diesel when necessary//, which would probably be more economical than storing excess electricity. It would also be extremely easy, as the diesel generator could be provided with a regulator such that the combined power matches the required power at all times.   

       [MechE] Good points; as I said, I largely agree with you, and I'm aware that my theory has many exceptions. I'm arguing the other side, though, because it's rarely done. Some people seem to think "solar's good", or "hybrid cars are good", and sweep the problems under the carpet, which is tempting, but irrational.
spidermother, Apr 26 2011
  

       The other part of my theory is that the 'green' economy - which apparently just considers carbon emissions - is going to be the next devastating rape of the planet. When we are supposed to have continuing population growth, and increased consumption per capita, but it's all going to work out because we'll be driving Chevrolet Volts, I can only agree with [Nineteenthly] and say that we are so, so doomed.   

       I suppose my economic theory is similar to Ann Elk's theory on brontosauruses - the economy will be small at the beginning, much bigger in the middle, and small at the far end. Which will be really horrible. Then it will be extinct.   

       (I'm gonna sing the doom song. Doom doom do doom doom doom. Doom doom doom the end.)
spidermother, Apr 26 2011
  
      
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