Replacing alternator with solar panel array on roof?

[TroyRank]

The Pulley can stay, (though I am sure it does almost nothing when not attached to a lead) but it's certainly not hard to get enough energy to run the accessories. For efficiency you could generate a control circuit, but in all reality you could just run it at a constant 3000 RPM and it would be more than sufficient. A more complex controller could definitely be devised though... and eventually I am sure I would

[DonR]

Slightly off topic. With the thought of the alternator bypass switch.

If you put your alternator bypass switch in a circuit so that when you hit your brakes (I know you don't want to) it would engage the alternator if your engine is running. At this point you are "wasting" energy anyway, you may as well charge your battery a little while you're at it.

Don

[Blue07CivicEX]

You're engine shouldn't be running if you have to hit the brakes!!! But on a serious note, I like the idea, I'm sure a relay could be wired up to the brake lights so when power goes to the brake lights it triggers the relay to engage the alternator. maybe...?

[theunchosen]

The problem with using PV these days is its kinda heavy. just to get 75 Watt Hour panels its several pounds(and less power than .25hp). Taking that into consideration its the worst power-weight ratio in your car by a long shot.

Also, looking to the extremes(PV carts) they are made of composites and the lightest possible components and the teams always fish out the lightest guy in the entire engineering department to be the driver, and make him drive naked to save weight. Ok so I was kidding on the last part, but otherwise the only way you can come close to supplementing your car's electrical grid is with large flat surfaces and serious sun.

Mounting PV panels on your hood is a terrible idea because the temperature after you stop and park will cook them. They are only designed for just over 130 degrees and then they start getting whiney and losing lifetime. Trunk and Roof are fine as long as you are in the sun, but if you have to run substantial components(fans) the PV won't keep up.

If you just want to store it you need deep-cycles and then you are adding weight compared to small parisitic drain. Probably the best bet would be to buy the lightest thinnest most expensive panels and two lightweight deep cycles, run the panels along the dash and rear window(to avoid being stolen) and charge it while you work and supplement the batteries while driving.

[Newton]

Originally Posted by ConnClark

I think the weight of the two deep cycle batteries would offset any gains on a metro.

130lbs of additional weight would not offset a 10% gain in FE. The only time it would even have an affect on FE would be accelerating and going up hills. 99% of my driving is cruising on expressways and I live in Southern Michigan which is extremely flat.

Weight does play a role but for most people it's probably not as large of a factor as just getting things more efficient. Weight and power to weight ratios matter much more in things like racing.

[theunchosen]

Originally Posted by Newton

130lbs of additional weight would not offset a 10% gain in FE. The only time it would even have an affect on FE would be accelerating and going up hills. 99% of my driving is cruising on expressways and I live in Southern Michigan which is extremely flat.

Weight does play a role but for most people it's probably not as large of a factor as just getting things more efficient. Weight and power to weight ratios matter much more in things like racing.

Not really. As long as the gas pedal is being depressed weight matters. If you are going down hill weight is good and if you are on a flat stretch of road cruising down from speed to a stop weight is good. Other than that if you can cut it its worth it. If you are having to accelerate to overcome drag you are having to accelerate that mass regardless of degree of climb.

Also if power-to-weight didn't matter then it would be just as efficient for us to cram as many batteries as possible regardless of weight.

That said adding weight thats not weight efficient(produces more HP per weight per cost) is fuel inefficient. So if solar panels cannot supply the power needed to feed the parisitic functions less than the alternator that is designed for that specific rpm range and torque(not terribly inefficient as long as its not feeding fans and compressors) then its not FE-friendly.

Solar panels at this point in time are not FE-frendly for standard cars. For Solar-racers its worth it because they weigh under a half ton with all surface area pointed directly up coated in expensive panels. Low weight allows for very low drain on the batteries and the panels can keep up with that. One Bp 375(75 watt hours in full sunlight on a non-humid day) for 400 dollars can probably keep up with power steering, brakes, and making sure ignition happens. Any fan coupled with standard draws will overrun your supply forcing the alternator on anyway and then you are hauling around a solar panel for no real reason.

If PV gets to say 40% efficiency we will see these all over the place but until then standard cars don't have much to gain from adding really expensive panels.

[MetroMPG]

Originally Posted by Newton

130lbs of additional weight would not offset a 10% gain in FE.

Newton is correct.

Both AndrewJ and I have at various times been driving around with 130+ lbs of extra batteries in our vehicles when running with the alternator disconnected. Both of us saw significant efficiency gains in all types of driving.

EPA estimates 100 lbs equates to a 2% decrease in fuel economy. (They don't state under what conditions.) The alternator delete is worth 6 to 10% better fuel economy from my and Andrew's experience. Simple math.

[Newton]

I agree weight matters but in calculating the opposing forces on a moving vehicle weight only plays into rolling resistance which is (with my metro) ~1/3 of the total opposing force (aero drag=0.5*air density*drag coefficient*frontal area*velocity squared. rolling resistance=Mass*Gravity*tire deformation*cos(1/2 tire radius) ). tire deformation and radius has more influence in the energy required to sustain a given velocity than a few kg weight difference.

power to weight ratio doesn't affect efficiency. a lower power to weight ratio will make you burn more fuel but the efficiency of that process doesn't change. the trick is to accelerate slower as work=mad, if you decrease the acceleration you use less energy.

as far as mass quantities of batteries go most of the longest range EV's right now with Pb-acid batteries are trucks. they basically just fill the bed of the truck with batteries.

[MetroMPG]

Originally Posted by theunchosen

Any fan coupled with standard draws will overrun your supply forcing the alternator on anyway and then you are hauling around a solar panel for no real reason.

Not true. The alternator will only output as much power as needed to maintain a given battery voltage. If the panel is contributing, say 50% of your electrical load requirements, the alternator's voltage regulator will look after ensuring the alternator only makes up the difference. There will still be a fuel savings.

[theunchosen]

Ah, I was not privy to your 10% numbers. I was estimating relatively small gains in FE from the alt delete. Ok so its valid. If it was posted earlier in the forum I apologize I might have missed it in the 8 pages ^_^.

Right power-to-weight does not effect overall efficiency. . .but we won't see max efficiency. And we won't accelerate slowly enough for it to not matter. So the gains in Power-to-weight will influence FE not eta but FE(Fuel economy not Efficiency). Apparently in this case its definitely worth it.

I knew the 2% per 100 lbs general rule of thumb but I was expecting this delete to be at best 5%.

In lieu of that data one BP 375 and one battery ought to suffice the alternator during day driving, and maybe night driving if you switched the alt on when you stopped and made sure the throttle position did not move. Would definitely go to at least 2 serious amp hour batteries before totally removing the alternator(in case one battery fries out for odd situations).