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Toyota Prius to offer solar powered A/C
Old 07-07-2008, 09:29 AM   #1
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Toyota Prius to offer solar powered A/C

Saw this story on the BBC News today.

BBC NEWS | Business | Prius 'to be part solar-powered'
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Old 07-07-2008, 10:15 AM   #2
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This is what it looks like:



$6 Solar Fan - Instructables - DIY, How To, green, tech
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Old 07-07-2008, 10:41 AM   #3
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Just a question for all the more physics/energy science-conscious out there: If this is possible, and from what I've read it will be pretty inefficient financially, adding about $800-1,000 on for producing only 200W of energy (when 2-5 kW are needed for an air conditioner), how realistic is it to have a turbine powered by wind in the car. I do not know the specifics or anything like that or how expensive it would be so the logistics could be entierly off, but I imaginge you could have something similar to having a small slit facing forward on your roof so at high speeds it will create certainly a little drag, but it will be able to feed into a turbine to produce more energy. The big problem I see with it is that the first law of thermodynamics may come into play: The drag caused by the car done to the surroundings loses more energy than is gained by the wind turbine... but I really do not know. I just heard someone talking about it once and was really interested in the feasibility of this as a manner of extending the range on an HEV or a fully plug-in EV.
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Old 07-07-2008, 01:59 PM   #4
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Some car company (Mazda?) used to have a solar panel in the sunroof that ran a small fan to keep the car cooler when it sat in direct sunlight. I made a similar device using a couple of computer 12v pancake fans and a small solar panel that plugs into the cigarette lighter. It does keep the car cooler but certainly not like an air conditioner.

Back when I worked in the industry, we had a lot of AC compressor failures in cars sold the middle east. Turned out that they let the cars idle all day with the AC on while they were at work.
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Old 07-07-2008, 02:10 PM   #5
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Turned out that they let the cars idle all day with the AC on while they were at work.
Yikes!

Here you're not allowed to let your car idle for more than 2 minutes unless you're in traffic.
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Old 07-07-2008, 03:14 PM   #6
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The current Prius has a problem with the 12V battery getting discharged when the vehicle is left unattended for many weeks. I wonder if this is an attempt to solve that issue.

The idea of the solar cell directly powering the AC seems ridiculous to me... since most everything in the Prius runs on electricity, I don't see why the solar cells wouldn't just dump power into the main 12V or high voltage systems where it would be stored if it wasn't needed at the moment. There's no sense I can see to dedicating the panels to the AC system, which often disengaged. It sounds to me like the journalist didn't get anywhere near the whole story.
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Old 07-07-2008, 05:56 PM   #7
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Saw this story on the BBC News today.
BBC NEWS | Business | Prius 'to be part solar-powered'
It's a marketing gimmick.

A roof-mounted PV panel won't generate enough power to make a difference in the A/C (notice they said "part" of the A/C). It also really screws up the longboard hauling.

There's at least one aftermarket manufacturer who mounts a PV panel on the Prius to keep the 12V battery on a float charge. For all its engineering genius, the car's 12V battery is a particularly weak point. When it dies the car won't start, you can't roll-start most cars anymore, and if you jumper a Prius incorrectly (even just arc the jumper cables across the terminals) then it can fry the $5000 inverter. So Prius owners are paranoid about 12V battery failures, and this PV panel offering may help some of them sleep at night.
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Old 07-07-2008, 11:15 PM   #8
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Fortunately, all the articles I've read about this point out what a stupid idea it is.

The only saving grace *may* be convenience, but it certainly isn't 'green'.

1) It takes a lot of energy to make a solar panel. It takes about two years for the panel to produce enough energy to payback what it took to make it. That assumes it is mounted directly in the sun, every day of the year. A car will not be in the sun everyday, all day, and won't be tilted at just the right angle to maximize efficiency. So putting one on a car is wasting energy, not saving energy.

2) Solar panels have 30 year plus lifetimes. Put it in a car, and some % will get wrecked before that time, and they would need to be removed and replaced once the car is junked.

3) The car will have to haul that extra weight around, increasing gas consumption the long run.

The 'greenies' will love it though, and say ' why can't the American car companies do this'... Geez, gimme a break.

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Old 07-08-2008, 01:30 AM   #9
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I remember when the 2004 Prius was about to be introduced. In feature lists you often saw something like "Electric Air Conditioning system can be run with the engine off". Many people assumed this meant that one could set the car to cool the interior before starting the engine, but in fact that is not possible. It is true that the 2004 Prius was one of the first vehicles where the AC compressor was electrically rather than mechanically driven, but for whatever reason it only runs when the car is "on" (engine running when needed). Probably because people would ruin their hybrid batteries running the AC with the engine off.

I suspect the unique electric AC compressor in the Prius is the source of this strange rumor.

Solar cells normally produce about 10 watts per square feet, so producing 5 kw would require 500 square feet, which is obviously way bigger than the Prius roof. But perhaps they could generate say 100 watts max, which would be used to top up the hybrid batteries during a day of sun. Then perhaps the owner could press a button on the keyfob to start the electric AC compressor, which would run for a few minutes off the solar energy stored in the hybrid batteries, while the owner walks out to the car.

The Marketwatch story may be accurate... it says:

http://www.marketwatch.com/news/story/story.aspx?guid={A5960A1A-E176-4A08-952C-4BC4A0A69284}&siteid=rss
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The redesigned Prius will have solar panels on the roof, which will supply part of the two to five kilowatts needed to power the air-conditioning unit, the Nikkei business daily reported.
Note that they just say it produces a certain indeterminate amount of power; they don't say that power won't be used for things other than the AC system, and they don't rule out extra power being stored in the batteries.
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Old 07-08-2008, 01:40 PM   #10
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I suspect the unique electric AC compressor in the Prius is the source of this strange rumor.
It probably has a unique design, although it runs off the DC/AC inverter. But the compressor makes an impressive amount of noise when it's turned on at startup before the engine kicks on. We find it easier to open all the windows, get moving, and then start the A/C.

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But perhaps they could generate say 100 watts max, which would be used to top up the hybrid batteries during a day of sun. Then perhaps the owner could press a button on the keyfob to start the electric AC compressor, which would run for a few minutes off the solar energy stored in the hybrid batteries, while the owner walks out to the car.
Toyota seems absolutely paranoid about recharging the HV battery from anything other than the car's engine or regenerative braking. Even the plug-in mods have to trick the car's battery-charging controller (risky) or replace it (which invalidates the warranty). The controller doesn't let the high-voltage battery deep-cycle like a marine system, so any attempt to run the A/C would eventually just fire up the internal combustion engine.

If the HV battery is running around 200 VDC then that solar panel would have to have some real horsepower...
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Old 07-08-2008, 01:55 PM   #11
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But the compressor makes an impressive amount of noise when it's turned on at startup before the engine kicks on.
I wonder if you are thinking of the pump that moves the fluid from the "cat warmer" (essentially a thermos bottle) back into the engine just before starting up.

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Toyota seems absolutely paranoid about recharging the HV battery from anything other than the car's engine or regenerative braking. The controller doesn't let the high-voltage battery deep-cycle like a marine system, so any attempt to run the A/C would eventually just fire up the internal combustion engine. If the HV battery is running around 200 VDC then that solar panel would have to have some real horsepower...
Good point... it may take say 50-100W to just power up the 12V computers and electronics enough that the charging system can function correctly, which would negate the advantages of charging from a 50-100W max solar cell.
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Old 07-08-2008, 01:56 PM   #12
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I remember when the 2004 Prius was about to be introduced. In feature lists you often saw something like "Electric Air Conditioning system can be run with the engine off". Many people assumed this meant that one could set the car to cool the interior before starting the engine, but in fact that is not possible. ....
Now that you phrase it that way, I think I understand the intention behind the words.

The Prius engine will shut off at traffic stops, to avoid wasting gas while idling. I think this design of an electric A/C was to ensure people that they would still have A/C when stuck in a traffic jam.

So yes, the car has to be 'on', but the engine can be turned off by the system. I think that is the source of the issue.

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Old 07-08-2008, 06:58 PM   #13
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I wonder if you are thinking of the pump that moves the fluid from the "cat warmer" (essentially a thermos bottle) back into the engine just before starting up.
Oh, no, there's a big difference. They're both running but the A/C compressor windup makes me think that the coolant pump is about to throw a turbine blade through the hood... then I realize that warm air is blowing on my face.

Of course it's not a problem if the previous driver (anybody other than me) remembers to shut off the A/C before powering down "her" car.
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Old 07-08-2008, 08:06 PM   #14
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. . .how realistic is it to have a turbine powered by wind in the car. . . .The big problem I see with it is that the first law of thermodynamics may come into play: The drag caused by the car done to the surroundings loses more energy than is gained by the wind turbine...
Nope, won't work, for a reason similar to the one you stipulated (I think the Second Law is closer to fitting the bill). The work done by the air on the turbine will slow the car, which requires an input of energy by the car (from the batteries of the gasoline engine) to overcome. Add in the inefficiencies of the turbine, bearings, and resistance in the electrical windings, etc (likely approaching an efficiency of less tan 70% in a small unit) plus the inefficiency of the car's gas engine in producing electricity and converting this to power at the wheels--probably about 50% efficient), and you've produced a very big losing proposition. You'd be far better off to power the AC compressor directly from the car's electrical system.
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Old 07-09-2008, 04:07 AM   #15
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Sail Car!

Toyota should take a lesson from these guys.

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