Solar Roof to Recharge Battery

There is insufficient power from the solar cells to provide any useful recharge. All it can do is spin a very small fan.

 

Exactly! Who knows, it might even add up to the equivalent of an entire gallon of gasoline over the course of a full year. ;-)

 

How about having it keep the 12 volt battery charged? Much smaller solar panels are used for this all the time.

I think something like this has been done for powering amateur radios. I don't remember where I saw that.

 

Cost-Benefit isn't really worthwhile. Most residential panels are 200-250 WATT panels, and those are probably larger than the one on the Prius roof. Given the curvature of the roof, you'd never see max production, and probably closer to about 100 watts at most. You'd need 10 hours to produce 1 kwh (assuming NO losses), which wouldn't provide very much range. That'd be a best case scenario, too.

More than likely, you're looking at about 1/4-1/3 of that scenario above.

Keeping the 12V topped up would probably be more feasible, but I doubt there'd be much demand for it.

 

As it is, it is doing a very good job by cooling my car. It is so nice to get in and it is not hot hot hot.

 

Some here have suggested the solar panel powering an amateur radio rig.

 

No question, if you could mount a solar panel about the size of you house's roof, you could recharge your HV Battery, that is why you see discussion of solar panels on houses here by PHEV owners.

If they are lucky, Toyota's solar panel could recharge the 12 volt battery, but I suspect they opted for a shape that fit the car, rather than a shape that maximized power. The 'trick' to recharging either the HV or 12 volt battery is knowing when they are full, happily a ventilation fan never gets full, so the electronics are simpler.
I am sorry that no practical, portable battery charging is intelligent, and that no intelligent conversation begins by assuming it is.

The college I attended built solar 'cars' and raced them, notice how huge the panels are and how small the 'car' is, also notice that you drive from each end to get the best angle to the sun.
Proafile | Viking XX - the Solar Proa

http://community.seattletimes.nwsource.com/archive/?date=19910625&slug=1291008
21 hours to get from Sacramento to Los Angeles

 

The Fisker Karma has a solar roof that does both, cool the car and charge up the EV battery

Fisker Karma solar roof production to begin courtesy of Quantum

4-5 miles per week under best case conditions isnt a whole lot, but like the article said, its better than nothing. Something else to consider is that 4-5 miles of range is, I assume, "real" range, not the same as the EV mode in a non-plug Prius.

When solar panels become more efficient and the prices come down, then it may be more of a consideration for regular folks.

 

The current panel only delivers IIRC ~60 watts at max (I saw an 80 watt figure somewhere). That's nothing. 1 hp is ~745 watts.

There has been plenty of discussion on this in the past. There was also the problem mentioned at http://priuschat.com/forums/gen-iii...hing-useful-when-car-running.html#post1170984 and http://priuschat.com/forums/prius-h...tas-underwhelming-solar-prius.html#post816036.

As I posted at http://priuschat.com/forums/gen-iii...significant-drop-in-mpg-ac-2.html#post1149705

I think I've seen it go past 10 amps when stopped w/the AC on at full blast. 9 amps at 220 volts = 1980 watts.

 

There is an absolute upper limit in the amount of sunlight reaching the earth. You cannot do better with a solar panel than what energy reaches your location on the earth. According to the Shockley-Queisser efficiency limit (SQ Limit), the absolute theoretical efficiency is approximately 33.5-percent.

The current Prius solar panel is 16.5% efficient at converting solar radiation to electric current. So even if solar photovoltaic efficiency were advanced to the maximum theoretical limit, one would only double the power output--still an insignificant amount of power.

Batteries have an internal resistance that must be overcome in order to charge them. I really don't see this happening with as tiny a solar panel as you could mount on the roof of a Prius. It is unlikely to even be able to overcome the internal battery resistance of the traction battery to even BEGIN to charge it, let alone provide anything near useful power.

The Prius solar roof panel is 36 cells at half Kyocera's standard size of 15 x 15 cm at a conversion efficiency of 16.5%. Kyocera states that the maximum output is about 50 watts--that would be in direct bright sun. So in bright sun they would power a 50-watt light bulb while the sun is shining.

36 cells at 15 cm x 15 cm / 2 = 4050 cm^2 = 0.405 m^2

Average solar insolation (the amount of sun energy available striking the earth--this is before the lost energy due to conversion into electricity by photovoltaic cells) is around 4 kWh per square meter per day over much of the fairer portions of the U.S. Let's take SoCal and call it 5 kWh/m^2/day.

So 0.405 square meters x 5 kWh/sq meter/day x 16.5% = .33 kWh per day (average).

The EPA's formula rates a gallon of gas at 33.7 kWh equivalent. So in about 100 days you could collect the energy equivalent of one gallon of gas.

Reach the maximum theoretical limit of solar panels and you can collect a gallon of gas worth of solar power in 50 days.

Even if you cover the rest of top surface with panels, you'll at most get a tripling of coverage, and you are likely to lose more "gas" than you gain through the increased mass of covering your car with panels.

Nope. Just not happening.

I'll just be happy to come back to a cooler car (and a cooler traction battery) in my solar-equipped Prius!

 

I believe this assumes direct sunlight.

Since the roof does not tilt up to track the sun across the sky throughout the day (and you will probably be forced to occasionally park indoors, under shade, or on cloudy days) you'll probably end up with quite a bit less than the .33 kWh per day, bringing us closer to my earlier estimate of the equivalent of a gallon of gasoline per year.

I suppose my post may have come across as mocking. I didn't intend it to be mean, just a lighthearted way to share realistic numbers. I was not just spouting off a ridiculously low estimate of gas savings. I was trying to give a real estimate of how much gasoline an average driver might expect to save over the course of a year. For those who feel that every drop of gasoline saved is a worthwhile endeavor, a gallon per year might be considered significant.

 

It isn't just technology, it's the energy density of sunlight. Even with vastly improved solar cell efficiency, there isn't enough surface area on a Prius to make the idea very useful. Unless the sun gets closer or brighter, there isn't anything we can do to improve solar energy density.

Manufacturers do all sorts of silly things to attract customers, as you demonstrate in these posts, so it's not surprising that one now has a solar charger. It isn't cost effective or terribly useful, but it is a good marketing gimmick.

Solar power for an automobile is better done through fixed installations, and then transfered to the car's batteries.

Tom

 

Over the years I have advised a lot of folk not to do stupid things.
Do NOT, under any circumstance, make the sun get closer or brighter!

Thank You.

(We could also improve solar cell efficiency by removing the Earth's atmosphere, but it would be wrong)

 

The solar panel puts out low voltage. I haven't measured it as my Prius isn't here yet (real soon now), but I suspect it's between 5 and 12VDC. As it powers a fan, there probably isn't a regulator.
If you wanted to charge a battery the first thing you'd have to do is boost the voltage.

For the 12V battery you'd need around 14V -after- regulation. You need to boost the voltage up then regulate it so as not to damage the battery. So boost it to 15-17V then feed a charge regulator. You'd loose maybe 10% of the power available (which isn't much, small solar panels don't put out much).
This might actually be slightly practical to keep the 12V battery charged during long storage (parking and waiting for the owner to return) times. You might also want an automatic switch such that when the interior temp. gets too high the fan is driven rather than the battery charged.

The "traction battery" is another matter entirely. First, you'd need to boost the voltage to around 250VDC. This would, of course, drop the available current. Because the panel is horizontal, you -might- get 40% of peak output for a few hours each day. More in the southern dessert. Back to current then. A 50W panel at 40% is 20W. Let's assume it puts out 12V. That's just under 2 Amps at 12V. Boosting it to 250V would drop the current to around 90 mA (I'm including the 10% losses in the conversion and regulation). That's 0.09 Amps for those electronically challenged. It's a 6.5 A-Hr battery. Typically charging is around 50-60% efficient.
So each hour you'd get around 50 ma-hr charge. With four hours you'd get 200 mA-Hr charge. That's 3% charge per day of good sunlight. Assuming the car had the battery at 60% charge (the normal level it tries to maintain) it would take 7 days to bring the charge up to 80%, the maximum charge level the system will allow. That's 7 days you won't be driving. Oh, and as soon as you put the car in "ready" the system will attempt to drop that charge level back down to 60%, even if it has to waste the power.
Perhaps you will now see it's not a simple thing to charge the battery, even if you plugged it in! It requires a redesign of the system.
Anyway, just some things to think about. Oh, and BTW, cost would be -MUCH- more than the gasoline you might save over 10 years, the typical life of a car. Also, emissions from the manufacture of all that "stuff" required to implement this would be much more than that from burning the gasoline you would save.

Finally, Pearl sleeps in my garage. So forget the solar charging.

 

Markabele - I am with you. I would like to see it looked at.

I have solar panels for my home.

I may consider mounting a luggage-rack on my Prius and putting two 220w panels in it, just to see what it does.