More Prius Magic

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Originally Posted By: from the linked article
The paper said Toyota would equip solar panels on the roof of the high-end version of the Prius when it redesigns the gasoline-electric hybrid car early next year, and the power generated by the system would be used for the air conditioning.

With the statement about the AC, the whole idea makes more sense. If you follow the Prius/hybrid discussions, you'll see that this issue has come up before. Apparently, and keep in mind I'm a lawyer, not an engineer, given the present solar technology, it is pretty much impossible to use solar to generate anything but a fraction of the juice necessary to drive the car, given the area limitations of the roof of the car. In other words, for now, this is pretty much a "why bother" idea, at least with respect to the propulsion of the car.

On the other hand, a roof full of solar panels would probably be able to generate enough juice to satisfy most, if not all, of the demand imposed by the electric AC compressor, which in the present cars, draws its juice off the traction battery, which in turn lowers mpgs by depriving the drive system of juice used to run the AC compressor.

As a way to power the AC/climate control, this would seem to be a great idea. That said, it remains to be seen whether it will stand up to a rigid cost-benefit analysis in which the added expense of the solar roof is weighed against the potentially increased mpgs. Only time will tell whether the solar roof can ultimately justify itself on those terms.
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Originally Posted By: ekpolk
Only time will tell whether the solar roof can ultimately justify itself on those terms.
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Are you implying the Toyota engineers maybe didn't make the proper calculations, or that maybe the Toyota marketing department was pushing for this feature only because they wanted a sellable gimmick?
 
No I knew there wasn't enough space to power the car. But I also figured that the a/c was intensive enough to justify more space than the roof would allow. But I admit to not knowing the energy requirements of the Prius A/C. Notice it said on "high end models." I wonder, like you said, if the cost will be worth it if Toyota makes this a $1000 or more option?
 
Really I am going more with the gimmick. Form a bit of looking it appears that a meter square panel at high noon in full sunlight, at the equator, would produce around 150 watts. So at 12 volts that is about 12 amp or about what you can run off a cigarette lighter.

I think a 5000 btu home widow unit pulls like 1200 watts.
 
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Originally Posted By: moribundman
Originally Posted By: ekpolk
Only time will tell whether the solar roof can ultimately justify itself on those terms.
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Are you implying the Toyota engineers maybe didn't make the proper calculations, or that maybe the Toyota marketing department was pushing for this feature only because they wanted a sellable gimmick?

Absolutely not. One of the unfortunate things you can very easily see with the present car is how the AC will drain the traction battery when you're in a relatively static situation (polite way to describe a "traffic jam"), and so, an alternate source would be a great idea, so long as the cost is not too high.

Perpective: on a mild day, I've pulled into an Interstate rest area, the decel down the ramp pretty much maxing out traction battery, and then stopped for a highway nap to refresh myself. Under that circumstance, I've seen the TB power the AC for 45 minutes before it sucked it dry to where the ICE had to fire up to keep things cool. By contrast, on a very hot day, with slow traffic, I've seen the TB get drained by the AC to minimum allowable in five minutes. Remember also that the Prius has a variable speed AC compressor, so it only turns as fast as necessary to keep the car cool under existing circumstances. Very efficient setup.

I suspect that on those brutally hot, sunny southern days, the roof solar panels could make a real, and positive, difference. Again, assuming that they're not too expensive.
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Alright, you don't think it's a marketing gimmick, but you are not convinced the Toyota engineers were able to determine the bottom line.
 
Originally Posted By: wapacz
Really I am going more with the gimmick. Form a bit of looking it appears that a meter square panel at high noon in full sunlight, at the equator, would produce around 150 watts. So at 12 volts that is about 12 amp or about what you can run off a cigarette lighter.

I think a 5000 btu home widow unit pulls like 1200 watts.

Only problem with that analysis is that the roof would present at least 2-3 square meters, thus providing by your figures maybe 500w (let's assume that they're going to be pushing for max efficiency here, which they've done with all other components). If that's the case, they can probably provide half, or more, of the AC unit's demand (again, remember, it's a variable speed compressor that only takes what it actually needs). If that's so, then the roof would make a substantial, if not complete, contribution to the AC demand. Not a gimmick at all if I'm even close to being correct.
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Originally Posted By: moribundman
Alright, you don't think it's a marketing gimmick, but you are not convinced the Toyota engineers were able to determine the bottom line.

No, what I mean is that I haven't seen any price figures for the option. If it's a relatively affordable option, then great. If not, then buyers will need to carefully consider whether it makes sense for them. Bottom line is that every buyer will have their own "bottom line" for when an option like this will have paid for itself. Each intelligent buyer will have to decide for themselves. I'm sure a fair number of non-intelligent buyers will get the option just because it sounds cool. . .
 
My gosh, if I had a Prius with solar a/c I'd run it from February thru November here! As it is now, though, I keep a/c off in the morn and even in the afternoon if I am going straight home. A bottle of air freshener is a necessity.
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Originally Posted By: ekpolk
Originally Posted By: moribundman
Alright, you don't think it's a marketing gimmick, but you are not convinced the Toyota engineers were able to determine the bottom line.

No, what I mean is that I haven't seen any price figures for the option.


Never state the obvious voluntarily.
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Why can't that be used to recharge the battery? It would probably work good when it sat for hours in the daylight while people are at work.
 
Originally Posted By: ekpolk

On the other hand, a roof full of solar panels would probably be able to generate enough juice to satisfy most, if not all, of the demand imposed by the electric AC compressor


I don't think so, and here's why:

A 200 watt solar panel is about 4 feet by 3 feet. You won't get even two of those to fit on the roof of the average car.

An ac compressor powerful enough to cool a car is going to take a lot more than 200 watts. Or even 400 watts. The solar heat gain of a car is a lot higher than a typical bedroom that can be cooled with a 5000BTU air conditioner. That much should be obvious when the bedroom doesn't heat up well over 100 degrees 15 minutes after the AC is turned off...
 
The prius is a great stop gap car, however its no better than a straight up high mpg reg car.

We need a Hydrogen fuel cell, and much improved car battery technology, to put the sheiks and big oil in their place.

I'm waiting on a Hydrogen/electric hybrid pick up truck (no oil changes :(), and dint care who makes it, as long as its made in the US or the Americas and not China.

Till then may just need to buy whats offered by the motor industry, but I wont be happy about it.
 
Here's some more figures. The average AC car air conditioner is about 24,000BTUs, or 2 tons in capacity. Older cars may be larger, I read a figure of 3 tons (36,000BTU) in an AC textbook dating back to the 70s.

The 2 ton central air conditioner in my house consumes 2400 watts, based on a Seasonal Energy Efficiency Ratio (SEER) of 10. One of the main factors impacting the SEER is the size of the condensing unit coils--larger is better. Smaller results in higher energy use, because heat doesn't transfer as well with a smaller coil (This is one reason it is a good idea to keep the condenser coils clean, as when they get dirty it reduces the heat transfer).

The condenser coil on my 2 ton central air unit is about 4 times the size of the one you would find in a typical car. The size of the condenser coil in a car is limited by the amount of space available for it in front of the radiator.

So we can pretty much conclude that a car air conditioner is going to use more energy than a central air conditioner simply based on condenser coil size.

But, but, but...the car AC air conditioner isn't running all the time...or it's variable displacement...

Good point! However, in order for a solar panel of, say, 240W in size to supply all of the air conditioner's energy needs, assuming it is actually operating as efficiently as a 10 SEER central air system (it most certainly comes nowhere close), it would need to operate at no more than a 10% duty cycle, or at approx 2400BTU. That means the compressor cycles on for no more than 6 seconds out of every minute (or 1 second every 10 seconds). I have never, ever observed a compressor cycle on a car operate which such a low duty cycle unless the outdoor temperature was very low (like 40F).

Variable displacement doesn't change anything. The average power consumed is still going to be very similar (variable displacement may be a little bit more efficient than a fixed displacement compressor operating on a duty cycle, but certainly nowhere near enough to make operating solely on solar cells possible).
 
Even if it can't keep up fully, what different does it make? It will certainly help some and when it comes to the mpg race every little bit counts. Besides, even small % improvements look like big improvements on absolute mpg #'s when you get up around 50 mpg or more. It must be costly however if it will only be offered on high end models.

I'll say it is a good idea even with no payback just because it pushes the technology envelope. I'm no Toyota fan either, quite the opposite.
 
I'd venture to guess that the $1000 that a 200 watt solar panel actually costs might have a bearing on it too.

It's hard to justify this expense for a house that has a lifespan of 50 years or more, let alone for a car that has a lifespan of 10.
 
I'd love to see their calculations justifying these claims!

Back when I was building a hybrid car in college, we asked Chrysler for an estimate as to the HP draw of the air conditioning compressor, as we had to heat and cool the car with the CNG engine turned off. Their reply was about 10 HP max - and that was for a Neon. Pretty sure the Prius has more glass and needs more A/C capacity.

10 HP is about 7,400 watts. The sun puts down about 1000 watts/m^2. Even at perfect efficiency, that's seven square meters of solar panels. Now assuming 20% efficiency for the panels, that's about 37 square meters of panels! Impossible.

Let's go at this the other way now. Say they put two square meters of panels on it. That's 400 watts at 20% assuming 1000 watts/m^2. That's a little more than half a horsepower. I highly doubt Toyota has but down A/C power use from 10 HP down to 0.5 HP.

Two possibilities:

* Maybe 0.5 HP is all it needs after the car is already cool - it needs more to cool it down after being in the sun all day for sure.

* Maybe they've assumed a driving/charge cycle. For instance, it charges for 8 hours a day and generates enough power for a 15 minute drive home with the air on low?

Cost is another interesting angle, Solar panels cost about $5 to $6 per watt. 400 watts of panels adds $2,000 to $2,400 to the car's cost.

Production capability is another - if 'yota makes 200,000 Prius' next year (goal was 175,000 for 2007) and each takes 400 watts of solar cells, that's 80 megaWatts of panels, which is probably more than the world's current solar panel production. I couldn't find 2007 production numbers for the entire world output, but BP Solar, one of the biggest producers, produced 13.2 MW last year. Prius production would require a little more than 6 times BP Solar's '07 production. Unless production has vastly increased I don't see 400 watts of panels on each Prius being even possible.

Don't get me wrong, I'm a big Toyota fan, a big solar panel fan (and can't wait to expand my current solar panel array when they get below 2 or $3 a watt), but I smell marketing hype here at every angle I look at this...

later,
b
 
These solar panels would be far more useful on the roof of a house where they could be generating electricity and putting it back into the grid anytime the sun is shining vs. on the roof of a car where, if it's parked in a garage, it's not generating anything even if it's sunny out, or if it IS in sunlight the battery is fully charged the generated power is essentially useless and wasted.

In other words, if you're going to make the investment in a solar panel (1) you want it in the sun as much as possible and (2) you want to use as much of the electricity it generates as possible (or get paid for it in the case of a grid tie).

So if Toyota intends to monopolize the world's production of solar cells so they can use them in the Prius...that isn't the most energy-efficient use of solar cells.
 
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