Naked Science Forum
General Science => General Science => Topic started by: Stevemar66 on 14/07/2005 16:24:03
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With ever hotter summers and a car with no air con,
I have this quest.
I'm trying to make a 12v peltier air con that's portable
or can be fitted into the air system of the car.
I have bought 4 small peltier fridges, these will be combined
into one unit.
the hot side will be vented to the engine bay using 2 large pc
fans.
The coldside will be in the air stream of car fan.
I have no idea if it will work.
ANY IDEA! or advice
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No, it won't work. The Peltier coolers operate with very poor efficiency, and have little cooling capacity. The engine bay of the car is very hot. You would need air from the front of the car, right where the condenser for an auto air conditioner is located. Of course, that puts the cooler in a poor spot to deliver cooled air to the passenger compartment.
Even if you solved that problem, Peltier coolers are totally wrong for an auto air conditioner, which is why you don't see them used in that application. Your coolers will go into thermal runaway, and actually add to the heat load inside the car instead of lowering it.
Unfortunately, adding AC to an auto is no easy task. The most practical solution is to trade-in your car for one with AC installed.
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Here's a link for you, apparently some students won a $50,000 prize for their Peltier car airconditioner.
http://deseretnews.com/dn/view/0,1249,600136142,00.html
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Just goes to show that science fairs are BS.
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Mmm.
quote:
After they had already begun their work, Lyon and Winegar learned about a 1964 General Motors analysis that explored the idea before the car company concluded it wasn't possible.
Going in with open minds, however, the teens were not deterred and pulled off what GM rejected.
http://www.sltrib.com/utah/ci_2841984
Time is a great teacher, but unfortunately it kills all its pupils - Hector Louis Berlioz
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In the News
Really Cool Invention Brings Teens Awards (http://sltrib.com/search/ci_2841984) - Physics students came up with an environmentally friendly, economical air conditioner. "Space Beast" uses Peltier effect chips instead of Freon. (Salt Lake Tribune; July 6, 2005)
Feedback to Tribune story
"While I applaud these students on their enterprise, given the current state of technology, air conditioning with thermoelectric devices is not yet practical. The materials which are now employed are most efficient with smaller cooling jobs. Generally, in any application where a small air conditioner would not be overkill, compressor-based cooling would be the technology of choice. When you start getting down to small cooling boxes like those that plug into an automobile power adapter or a dorm-room refrigeration, Peltier systems can make a lot of sense.
"Having said that, it is important to underscore that new thermoelectric materials are under development, both for cooling and power generation. As efficiencies improve, Peltier technology will continue to pick up ground on compressor systems. It will probably be many years before thermoelectric air conditioning can be a practical and affordable alternative. In the near term, however, you will be seeing this technology used in smaller cooling jobs such as beverage cooler/heaters for automobiles and boats (see http://www.thermawaveproducts.com)." (Michael Spry, Sales Engineer for Tellurex; July 18, 2005)
http://peswiki.com/index.php/Directory:AC_via_Peltier_Effect
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Doesn't Tellurex have a large* stake in this business, and can you trust the opinion of a sales engineer (keeping half an eye on his stock portfolio) in this matter?
http://tellurex.com/master.html
*Tellurex is world's leading manufacturer of high performance thermoelectric modules for solid-state cooling and electric power generation.
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You can trust a sales engineer when he says "No." If this were a viable use for Peltier coolers, then he would be the first guy out there hyping up the application. While I, too, applaud the efforts of these boys in their science fair project, automobile air conditioning is one of the most demanding applications in commercial use. You will see Peltier coolers used in small room air conditioners long before auto units, and you don't see those either. I have used similar peltier coolers for laser diode chillers, and I am well aquainted with their characteristics. They are not even close to being efficient enough to compete with the Freon cycle.
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I have always wondered why this kind of technology is not built into cpu chips. We know the chip temperature, just need to keep it down to a non-condensing value and put the hot side someplace close to larger cooling fans. In ancient times, they used airconditioner refrigent to cool the old overclocked Cray computers. It seems like a natural for laptop computers. My old P3 runs at 2.0 gig, but the new ones are 1.8 for the sake of battery life and heat. I never run my laptop off of batteries, it is for lugging to customer sites.
David
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Hi David
thanks for your input, nice to have you on the forum
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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quote:
Originally posted by David Sparkman
I have always wondered why this kind of technology is not built into cpu chips. We know the chip temperature, just need to keep it down to a non-condensing value and put the hot side someplace close to larger cooling fans. In ancient times, they used airconditioner refrigent to cool the old overclocked Cray computers. It seems like a natural for laptop computers. My old P3 runs at 2.0 gig, but the new ones are 1.8 for the sake of battery life and heat. I never run my laptop off of batteries, it is for lugging to customer sites.
David
This is being worked on all the time. You don't see it in pentiums because they are built for the cheapest price, not the highest performance. CPU coolers add a lot of cost and weight to a computer. To make a Peltier cooler pracical in a computer, it must be water cooled, since all the concentrated heat can't be removed with a fan, unless you built a special motherboard. In a laptop they make no sense at all, since laptops are built for small size and weight. The old Crays were fabulously expensive. I used to connect to one on the old Darpa-net, and it was easy to spend thousands of dollars a day on simulations. I don't miss it, and I don't miss water leaks from the cooling system ruining expensive circuit cards either. I think air cooling is the right technology for almost everything we do on computers. The only exception I would make to that statement is that heatpipes should be used in the laptops, to remove the concentrated heatloads to the edges of the computer.
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Talking of Peltier devices, a group of researchers in the US are working on a micro-refrigerator for the brain taht can be used to treat epilepsy.
The small implantable probe is inserted into the abnormal part of the brain responsible for triggering the repeated seizures where it listens out for nerve activity that typically heralds an impending seizure. When these discharge signatures are detected the device switches on and cools down the brain tissue immediately around it to about 20 degrees celsius, temporarily and harmlessly inactivating the nerve cells that are misbehaving, preventing a fit.
The device uses the thermocouple effect whereby two semi-conductors, when placed side by side and fed an electrical current, become hotter and colder respectively. The probe is arranged with the cooling components on the outside, and the heating surface on the inside. The heat is dissipated away by evaporating a fluid inside the sealed device. The fluid re-condensates further up the device, and dribbles back to the hot surface, so the system is analogous to a person cooling themselves down on a hot day by sweating, except that the sweat is effectively recycled.
This device could prove very useful in the treatment of drug-resistant forms of epilepsy, or in patients for whom anti-convulsant drugs produce unacceptable side effects. Trials have been completed in rats and are about to commence in primates in order to assess the safety of the approach in tissue more closely approximating the human brain.
Chris
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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By just replacing a few words by others I foresee a great future for anger control too:
'Hey dude: Chill'
Remote control included, I take it? [:)]
Time is a great teacher, but unfortunately it kills all its pupils - Hector Louis Berlioz
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quote:
Originally posted by chris
Talking of Peltier devices, a group of researchers in the US are working on a micro-refrigerator for the brain taht can be used to treat epilepsy.
...The heat is dissipated away by evaporating a fluid inside the sealed device. The fluid re-condensates further up the device, and dribbles back to the hot surface, so the system is analogous to a person cooling themselves down on a hot day by sweating, except that the sweat is effectively recycled...
This is called a heatpipe. So we can use it to cool real brains as well as electronic ones.
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Spot on !
C
"I never forget a face, but in your case I'll make an exception"
- Groucho Marx
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IF this is such a nothing invention, why all the hype? Why the schlorship, why are they getting flown to Japan? All this hype for a no-nothing invention.[?]
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I think a heat pipe is something different. You take a pipe wthin a pipe and add a substance that will vaporize at the temperature of the hot end, and turn back to liquid a the temperature of the cold end. Sometimes you fiddle with the temperature by dropping the pressure with a vacum pump.
I saw this experiement done in liquid aluminum using mercury as a coolant at McGill University in Montreal, Canada. Dr Grusleski was preforming it as I remember.
The mercury vaporized at the molten aluminum end, cooling the aluminum. It then moved up the pipe to the air, cooled and then ran back down the inside walls of the pipe to the aluminum. The result was that a slug of aluminum was frozen out of the bath rather quickly.
David
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Back to the main point which is cooling an old banger of a car without having to trade it in for something really expensive, preferably with something cheap and easy to install...
Perhaps park it on the passenger seat and plug into the 12v socket? Doesn't every student driving a wreck in the city dream of this?
The problem is where to send the heat that is pumped by the peltier unit. Surely the answer is to sink it somewhere during the journey, eg into an insulated box full of cold water. At the end of your journey tip the water down the drain and refill from the cold tap, or just take off the lid and let it cool until you come back to the car... Or even make a cup of tea?
Metal would hold more heat, perhaps just enlarge the hot side of the peltier plate to a few pounds and insulate it. When it gets too hot take it outside and unwrap it to let it cool... The drawback is not fast recharge.
A eutectic substance might be safer by taking the heat without getting so hot.
Anyone care to refine this idea, do the maths? Would a camping cool-box of water give an hour's aircon? Please just remember I thought of it first.
Cheers
Special K
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Or would the power needed melt the electrics to the socket?
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IF this is such a nothing invention, why all the hyper Why the schlorship, why are they getting flown to Japan? All this hype for a no-nothing invention.[?]
That comment must be toungue in cheek.
Has nothing worthless ever been hyped and bought by the public?
I think there is an essential difference between CPU cooling and Air Con forcomfort. A CPU works fine at above ambient temperature so a passive system can work (with all the inherent advantages of cost and simplicity. Air Con needs to be working 'up hill', to bring the living space below ambient temperature. It has to be active and will ADD to the over all heatshifting problem.
The great advantage of a compressor system is that there is already a rotating engine to provide the energy. (Not free, of course). The rotating engine is also needed for generating the electrical power to operate the Peltier cooler.
How does over all performance figure compare when this is considered?
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I think there is an essential difference between CPU cooling and Air Con forcomfort. A CPU works fine at above ambient temperature so a passive system can work (with all the inherent advantages of cost and simplicity. Air Con needs to be working 'up hill', to bring the living space below ambient temperature. It has to be active and will ADD to the over all heatshifting problem.
Sure, any system will have an overall heating effect, the point is to get some local cooling in return. Liquid in pipes puts some distance between the cabin and the radiating element, that's all, there may be other solutions. As long as the cold element is colder that the cabin air heat will be drawn away, every degree is welcome, Peltier devices can freeze and boil. The trick is to dump the heat somewhere else; in a car, temporary storage may suffice.
No doubt conventional AC is unbeaten at the moment, the original poster was interested in a way to make his existing car that bit more bearable, at this time of year I'd really like to know also, I can't afford a new car either.
I might just stick some peltier plates to the ceiling and see what happens! Or glue some wet carpet to the outside of the roof...
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It boils down to the actual amount of heat you need to shift and the temperature difference. I don't know the maximum effective power currently attained by Peltier systems but, even if it had a performance index of as high as 300%, you'd still need an electrical power input of several kW to make any significant difference to your comfort. Remember, you can't just recirculate all the air in the cab. You need some fresh (initially hot) air coming in all the time. What with radiant energy and the incoming warm air, you're going to be needing to shift several kW under some conditions.
Standard AC in cars uses four or five kW- not a trivial electrical load for an alternator. This is why they use a mechanical coupling rather than an electrical one.
The plates would, surely, need to be underneath - not on the roof?
aamof, using peltier devices are very applicable to boat fridges because they have a vast cold sink available - namely the sea at around 20C, (for most of us). Cooling an insulated box is much easier because, once removed, the heat takes time to get back in again.
One viable way to increase comfort would be to keep lots of cold drinks in a peltier-cooled ice box. (Stopping every hour or so for a comfort break, of course.)