Practicality of covering the roof of my metro with solar panels?
 

So I acknowledge that I am well past the point of saving money on my car so the cost of the cells is not really an issue. But, assuming I could cover the roof of my car with solar cells rated for at least 125watts or more would it actually let the car run totally alternator free during an average day?

Also the body on my car is pretty well shot so I have no problems drilling a few dozen holes in the roof or even cutting most of the roof up to mount them. Can I just glue the cells to the roof and run the wires down a small hole into the interior where I connect them together and run it through a charge controller to the battery? Do I have to cover them with something to protect them like a sheet of acrylic? I figure if They can be glued directly to the roof and possibly just covered with a sheet of 1/8" acrylic then that is no big deal and won't really change the aerodynamics of the car significantly. They don't seem like they are that thick and using something thin to cover them would at most add 1/4 inch to the top.

I have never really done any real world stuff with solar panels so I don't know how well they would work in a situation like this. The 125W number is just a wild guess that the car probably needs about 10 amps to run. It could be higher or lower but I don't think I have enough roof area to get much more than 200W on there. I would put the alternator on a latching relay so I could turn it on or off when needed. Also converting to led lights would help lower the current loads.

This is just an idea I am wondering about and if it seems worthwhile I would probably give it a shot this winter while I have nothing better to do.

Your best bet is to buy pre made solar modules and bolt them to your roof. Trying to attach individual cells to the roof, solder them together, and cover them with plastic will be a really big pain in the ***, and it will be difficult to keep it working.

Q (solar powered home)

You could park outside and power your house, :p

Go fot itttt.

solar panel on a small car? that panel may act like a sail and you'll have the first flying metro. but a good idea, cover them up so they don't get sand blasted from the road.

Well getting pre built panels would be much easier to install so that sounds like a plan.

I need to measure the top and figure out what size I could get on there without it sticking up in the rounded corners. I am going to weld up the holes in the floor tomorrow and weld a patch into the lower unibody that is rusted in two so while I am at it I will try and figure something out.

If the car actually pulls 10 amps running would 125W of solar be enough to keep it running in the daytime? Or is the actual output on panels that are not aimed carefully and just pointing straight up a lot less than what they are rated for? I guess if it runs down slightly as I am driving but has a few minutes to recharge after parking then it is no real big deal. I would just have to make sure to not run it down right before dark. Or park it on a hill all the time :)

Getting a reliable 125W of solar power from the top of a Geo Metro is just not going to be possible. At high noon on a clear summer day, you might be able to get 1000 W per square meter from the sun after atmospheric attenuation. And given that photovoltaic efficiency is typically around 10-12%, you are only going to get 100-120W from a square meter, which is about as much as you can hope to get on the roof of a Geo Metro. Add to this fact the fact that power is going to be less under ANY other circumstances (ie cloudy, winter, early morning, late evening, etc), the increased fuel consumption from increased aerodynamic drag, the high cost of the module, and the difficulty of matching the output voltage of the panel with the voltage requirements of the car, and ou can easily see that this is not feasible. Simply put, look for increased mileage elsewhere.

If you are interested in generating power to replace alternator power, an alternative might be thermoelectrics. Thermoelectric modules could be used to generate electricity from waste heat in the exhaust and/or cooling system. And this would provde a MUCH more reliable source of power than the sun. This is actually something that is currently under study. And the thermoelectric modules (bare modules, that is) are actually available from online sources for less than you might think. Of course, it's still debatable as to whether it is worth the rather small gains in efficiency.

flexible solar panels
 

The cleanest way I can think of to do this would be to use flexible solar panels with adhesive. There are some that are made to adhere to steel roofing and may nicely lay on to your roof. They are amorphous panels that are not as efficient as the crystalline ones, but hav a very low profile and are most well suited to your application.

Drill a small hole for the wires, seal it with silicone and make the connections above the headliner. This could totally work, but the practicality is the question.

It would be really interesting to see the payback scenario on one of these installations. Probably not practical for a long trip, but may work in a shorter commute.

My plan was to keep the alternator and just not run it during the day. I figured at most I drive about 2 hours before stopping so if the battery only runs down slightly then sits for a few hours and can get 5-8 amps from the panel then it will charge back up before I start driving again. If it was raining or whatever and I was not able to maintain voltage I could just turn the alternator on and at worst I would have some extra weight on the top of the car with no help from the solar sometimes. I am thinking of getting a sheet of lexan and forming it to the shape of the roof then cutting the metal off and redoing the supports and stuff so I could maintain the roof shape and put the panels under the lexan.

A charge controller would keep from overcharging the battery so that would be the simplest way to try and maintain 12v if I have excess current coming from the panels. I have looked into thermocouples and would probably run them but to get any significant current it takes a lot of them and I would be adding probably 300lbs to the car and spending $5,000-$10,000 on them. So while they might be a good idea on paper I just can't see using them any time soon. The solar I can spend around $500 on and only be a few pounds of weight. So I figure first try solar then try thermocouples when I am really running out of mods to try :)

I don't really think I will ever get to payoff for the mods on this car, I got around 50mpg when it was stock, now with a giant list of mods I am up in the 60's. The last mod I did was go to a megasquirt computer and wideband O2 sensor and that set me back about $600 and at best picked up 4mpg. The best possible estimate, if i picked up 10mpg from the solar panels and drove it 100 miles a day 5 days a week, then it would take me like 2 years to break even on them. I don't expect that to happen and at best I figure 3-4mpg improvement so the payoff would be like 10 years. I am really not concerned with breaking even spending money on this car anymore and will pretty much do anything that looks like a reasonable gain in mileage.

I am playing with a similar project for my Honda Insight. I found a nice e-bay source for small 3.2V 85MA (1/4 W) panels.
https://cgi.ebay.com/ws/eBayISAPI.dll...m=290105642584

Photos of some of the panels I have made so far.
https://www.99mpg.com/mikestips/
"Making some custom solar panels"

First adhesion test using silicone rubber as the adhesive.
https://www.99mpg.com/resources/artic...ingintothesun/
"solar adhesion test # 1"
;)

I bet most people don't know that they've got two solarelectric devices on their car already.... they're called headlights... but of course aren't wired up right to gain power... also might need some tweaking, and put out mere millivolts... but presuming your parabolic reflector is perfectly focussed on your high beam filament, and the low beam is out of the focus a bit... if pointed right at the sun, and connected in circuit, electrons would flow from the hot filament to the cold filament...

One of these days I'm going to get around to trying a better version of this, the "dollar stores" hereabouts have letter sized fresnel lenses.. if I collect up some dead headlights, and they have the low beam deflector wired into the ground (makes a better target) and the high beam hidden behind it, then by focussing the fresnel lens on that, one might get as many watts out of it as for a similar size silicon panel, as long as one keeps tracking the sun with it... presuming you can collect up some headlight bulbs free (and the internally coated ones might be best avoided) you can do solar-electric for a couple of bucks a square foot (A buck for the lens, and another buck for some structure and wiring)

What is your source for this information? I've never heard of this and have read a number of books on solar, and lived with solar electric systems nearly my whole life, I know that LED's put out a small amount of power while in the sun (you can easily try this with a volt meter and any LED), but I've never heard of a normal light bulb being able to do this, so unless you have something to back this up, I'm reasonably confident that this would not work.

MD2000: I often use Polyurethane to glue stuff, it has more tensile strangthe then silicon caulk and doesn't seem to brake down in sunlight like silicon caulk does.

"normal" lightbulbs can't do it, you need two electrodes, and a lot of headlamps with a common ground, you need to get a burned out one so the circuit is open one side. It's a thermal effect more so than a photic effect. It's related to thermionic emission.

Actually, I'm looking forward to getting my hands on some reject or surplus OLED panels to play with that LED effect. Though I also wonder if there's a way of wiring up Thin Film Transisior LCD displays to get something out of them.

Edit: Oh here we go, best explanation I can find at the moment...
https://en.wikipedia.org/wiki/Thermionic_converter

I have heard of this before, but the efficiencies you are quoting are with an optimized system, a headlight has a few serious issues.

1.Headlights front glass acts like a diffuser to homogenize and direct the beam, which would make the sharp focus necessary to heat the single filament impossible.
Assuming that somehow was not an issue, the pointing accuracy of the reflector to heat only one filament, and not the other would be pretty tight.

2. The two filaments are thermally connected at the base, and are in a vacuum, so the unheated one would soon reach nearly the same temperature as the hot one reducing the temperature difference and output.
3. Sunlight delivers about 1 kw / sq meter, the small amount of sunlight that a headlight would intercept would be a fraction of this.
4. Headlights are heavy and expensive, and would be pretty lumpy glued to a car roof :eek:

Sorry, I think I will stick to solar cells.;)

10 amps, 125watts is way too much.... My 97 honda D16y8 at 2k rpm uses exactly 37.5 watts (3 amps @ 12.5v) during the day with nothing on but the motor.

Yeah, I wasn't really seriously considering headlight assemblies, just the bulbs. They are only likely thermally tied at the ground plane. So when the main beam has blown, you use the ungrounded side for your cold side and the lowbeam sheilded inside the internal reflector as your hot side. I'd figure you'd see a max of about 12W out of a headlamp bulb behind a letter (approx A4 size) fresnel lens. Probably would get about 3 or 4 W out of a headlamp assembly if the focus landed in the right place.

Anyway was more just pointing it out as a curiosity.

measuring amp draw on your car
 

How did you measure that amp draw on your car

https://www.harborfreight.com/cpi/cta...emnumber=95652
https://www.harborfreight.com/cpi/cta...emnumber=95683

Neither of those lists DC amperage measurement in its description, but I know one does it, and I'm pretty sure both do. I have both. Too bad I can't find them.

The solar panels are a great idea and remember that the battery can keep you running for hours so that when you are parked the solar panels will continue to charge to make up for driving on shady roads. You can also charge the battery from the grid if needed to keep it fully charged later in the day so that it will always be fully charged overnight. If it needs charging then it will charge from the grid but if it doesn't it will not use much electric keeping it charged.

You might consider removing the alternator comletely if the belts can be run to keep the water pump happy so you can get rid of the extra drag of the belt.

I was just thinking that the Lithium Iron cells I recently purchased would be perfect for this application because they maintain a slightly higher output voltage during their discharge than a lead acid battery (13.4 volts) and hold it really well until almost dead and don't suffer from discharge cycling as much as lead does (3-4k cycles is typical) as well as are not bothered by sitting partically discharged. The 4 cells needed only weight 13.5lbs for a 40ah battery too.

Lithium Iron cells
 

Where did you purchase your batteries also, where did you get your info???

A guy in California was working with someone that purchased more then they needed for the projects they are working on so I purchased 18 LFP40AHA cells from him last month. The pack of 4 I actually put on a UPS scale 0-50lbs and the way Lithiums work is they age faster when fully charged and cycle life is not affected by letting them sit at a less than full charge state unlike Lead acid batteries which really loose capacity when left discharged. I have them sitting on my desk in front of me right now with 17 connected in series and when I measure 4 in a row I get 13.4 volts. I also have done a few discharge cycles on a couple of cells and find that they hold their voltage pretty steady for most of their capacity if you don't draw too many amps out of them.

18 LFP40AHA cells
 

How much did you pay for them because I am very interested in these battery's for my unbelted swift...