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Why can't the sunshine (that turns a car into an oven) be used to run the aircon? Or at the very least, a fan?

For example - a long thin fan that fits into the top of the window - preventing it from closing all the way. Surely a solar panel the size fo your back window should be able to power the fan (or even 2 fans - one on each side) which would help cool down the car down.

Why isn't this being done?

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    It is being done. You can buy these thin solar fans for the top of the window almost anywhere. As for aircon, that would take a much bigger solar panel. – Chenmunka Jun 21 '16 at 7:38
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    You mean fans like these? (I never used one, so I can't tell how efficient they are, but I wouldn't say this hasn't been done before) – Gerald Schneider Jun 21 '16 at 10:22
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    @DucatiKiller this actually is not true. Airconditioners and fridges are examples of heat pumps, which move several units of heat "uphill" using just one unit of heat to power them. There are physical limits on how great a multiple is possible (the limit depends on the temperature difference you are pumping heat across). For practical air-con a factor of 3 times is achievable. Since current panels are at best 20% efficient, a purely solar powered car air-con is not achievable. If the car was insulated like a fridge you might keep it cool but then could not see out to drive it! – nigel222 Jun 21 '16 at 12:42
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    Figuring out how to use a heat sink would help cool the incoming air and another fan to pull hotter air would also help with pulling in air. Aluminum works well. If anyone knows of others materials that work well in that situation, please let us know. Yes, it might be bulky now, but, good thinking and engineering would eventually lead to something better. Just need the ideas to move. – Tobin S Jun 21 '16 at 16:21
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    Are solar panels a must? You'd get better results if you stretched thick fabric over the car and kept it moist (covered the car in wet rags) while the evaporated water took the heat with it. I once had a dripper over a blanket-wrapped water bottle. The water inside stayed cool the whole day. Damn, a padded spandex suit for a car... – kaay Jun 24 '16 at 9:41
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If you would like to do that by the use of solar panels, the problem is power.

A typical car AC probably uses somewhere between 1000 and 3000 watts. A square meter of typical photovoltaic cells generates roughly 150W of power. Even if you covered the entire car with solar panels, they wouldn't be sufficient to power the AC completely under perfect conditions. And they would be very expensive.

The fans are much less power hungry, a couple hundred watts at most. You could technically run them off a solar panel alone if you don't need maximum power.

Some car manufacturers do it already. For example some Audi models have a solar panel sunroof as an option. It keeps the blower fan running at low speed even when the car is off to keep the air circulating and fresh.

For anything else, the solar panels simply do not generate enough power to produce a significant impact.

  • Afaik no current Audi model has those solar panels anymore. This indicates that it wasn't very useful indeed. – André Stannek Jun 21 '16 at 11:22
  • @AndréStannek or too expensive for them to sell well. – Chris H Jun 21 '16 at 12:31
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    @ChrisH or that. I just looked it up and the A8 still has it ;-) – André Stannek Jun 21 '16 at 12:42
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You have a great idea! And it IS being done.

Certain models of the Toyota Prius feature an optional solar roof that powers interior fans that automatically engage when the interior gets too warm.

The hybrid batteries are not even used at all: the fans run strictly on solar power! Cool (literally).

Toyota Prius Solar Roof Image Source: resilience.org

  • VW/Audi has been doing that for a while as well. See, for example, this website: volkswagen.co.in/en/innovation/Technical_Glossary/… – PhilippNagel Jun 21 '16 at 12:38
  • The 2011-2012 Fisker Karma had a solar roof as standard equipment en.wikipedia.org/wiki/Fisker_Karma#Specifications – Eric J. Jun 21 '16 at 18:35
  • That solar panel can't do anything more than keep the battery from self-discharging when the car is left parked for a month. – Aron Jun 22 '16 at 4:56
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    @Aron, I believe they meant just fans, not the A/C too. So not colder air really, just circulating air. – bkribbs Jun 22 '16 at 12:24
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    I have owned one of these Prius in Silicon Valley for several years. I have also lived in the South without such a setup for many years. I can tell you that the solar-powered fans in the Prius are better than without. A much more efficient system is to keep the sun off the car in the first place. When that option is not available, solar powered fans are a good, but pricey, option. – JS. Jun 27 '16 at 18:06
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The problem you have is efficiency. Solar panel efficiency runs about 10-20%, depending on the type used. Air conditioners range in efficiency, about 30% efficiency is a good working figure.

What this means is that it takes 3.3 times the energy to cool your car for every unit of heat you remove. Solar panel efficiency means you need about 6.6 times the surface area of your car to generate the equivalent electricity as the sunlight hitting the car. So as a rough figure to run the AC you would need solar panels covering 22 times the surface area of your car (3.3 * 6.6 = 21.78 rounded up to 22).

It's a bit more complex than this as the solar panels would reflect a significant amount of heat, reducing the number of panels you need, but you'd still need many times more area than the car to run the AC.

Fans are much more workable than AC as you don't need the compressor or pumps, you would still need to cover your entire car to get enough electricity to run a decent sized fan.

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    And if you cover your entire car with cells to power the fans, you can omit the fan: The car stands in the shadow of the cells and will not get hotter than ambient temperature - while a fan can not make it cooler than that. (OK, this needs some insulation between cells and car to suppress heat transfer) – sweber Jun 21 '16 at 10:01
  • That's true to a point. The solar cells will heat up and transfer heat to the car, and you can't cover your windows with solar cells, so the interior is still going to heat – GdD Jun 21 '16 at 11:23
  • Most heat pump systems have COP of 3-4, so covering the entire surface of the car with solar panels could just barely make it so that it won't heat up (remember to subtract the energy absorbed by the panels from the energy that heats it up) – PlasmaHH Jun 21 '16 at 11:45
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The Lancia Thesis, introduced back in 2001, had this system. Basically the glass in the sunroof was actually a solar panel which was used to power the A/C fan when the car was parked in direct sunlight. The more intense the sunshine, the more power for the fans, the cooler the cabin would become. This system was one of a number of things on the Lancia Thesis which was genuinely brilliant but didn't seem to catch on.

More information is available here

  • I lost interest in trying to find the information. To me, it was poorly presented, Being too wordy and too small print. Made it hard to scan for what interested me, That would work better for print, though. – Tobin S Jun 21 '16 at 17:13
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Apart from the limited amount of power obtainable from a panel being less than that required to operate anything more than a fan, there are perhaps two other reasons

  1. Current solar panels are flat and rigid. Fitting one into the roof of a car may spoil its styling, or its aerodynamics, or both.

  2. By adding a panel you are increasing the weight of the car, which hurts its fuel economy figures (on top of any aerodynamic penalty), and this in turn will increase the running cost, and also the road taxes payable by the owner (at least in many EU states, don't know about USA).

Spray-on solar PV coatings are working in R&D labs. This technology might be coming to cars in the near-ish future. It would have no aerodynamic or styling penalty and a much smaller weight penalty.

  • This is largely true. However, lightweight, flexible PV panels are being increasingly used on camper vans. Although in this case their use is to assist in charging the leisure battery. You are right, the technology is coming. – Chenmunka Jun 21 '16 at 13:07
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    Flexible solar panels are available for a while. And they're not that heavy, really. – I have no idea what I'm doing Jun 21 '16 at 13:23
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A simple solution

You can achieve similar results by opening all doors for about ~1 minute before entering your car. All hot air inside the car will be replaced with probably still warm air from the outside. If there's no wind, you can turn on the ventilation addtionally. One minute of ventilation usually won't hurt the car battery too much. Plus, it get's the heat out of the ventilation system as well.

The problem isn't as much the inside air as the car itself

The car's roof and seats will still be heated up. The warm air from the outside will do little to actually cool them down. Air is a bad thermal conductor. It is thus not good at cooling things down. For reference feel how hot the street gets on a summers day. And the street is outside all day with warm air replacing heated air, which rises up. You can actually see how the air rises. It's the haze on hot days.
In conclusion, the incoming heat from the sun is just too much to dissipate via environmental air cooling.

For better results

Put reflecting foil in all of your car's windows. Stow it away in the ~1 minute you have all doors open. The foil will keep the heat out of the car in the first place. So the interior won't be heated up that much.

Still better results can be achieved in the same ~1 minute time frame: Pour water over the roof. The water will evaporate and cool the roof a bit. You should do that before stowing the reflective foil, so the evaporative water cooling has some time to take effect. This easily takes the roof from "don't touch" to "still warm".

Everything is about money

With the above options, how many people do you think would pay $$$ for a technical solution that will probably mitigate the problem worse than above?

If $$$ doesn't matter for your technical solution, have a look at how absorption refrigerators work: https://en.wikipedia.org/wiki/Absorption_refrigerator

You could basically use incoming heat (e.g. from a black panel on your car's roof) to turn your car into (sort of) a fridge. It will most surely not get as cold as a real fridge but the principle is sound. I'd assume that the overall efficiency might suffice. This assumption is based on eliminating the need to convert energy from the sun to electricity and use that to transfer heat energy. Instead you'd use sun energy quite directly to power the process.
However the amount of plumbing and the simple fact that the heat should best be applied from below, while coming in from above, makes this system at least troublesome to implement for mobile applications like cars.

TL;DR

There are multiple concepts, none of which is too few a hassle in terms of price, efficiency and usability to be widely implemented.

  • Don't forget the largest motivator for a lot of these tidbits of tech: the political statement. It may not be very effective(now), but, by gum, they are doing something about the environment. By all accounts, a lot more research needs to be done, and, greed needs to be kept in check. Just because it is new, does not mean it needs to be expensive to the point of irrelevance. Yes, r+d costs need to be recouped, but, with the right mindset, good steady profit can be had. – Tobin S Jun 23 '16 at 5:39

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