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Peltier for Cars: Cool Your Steering Wheel?
Jeremy Faludi, 22 Jul 05

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Just to prove you don't need a research lab and a high budget to start changing the world, two high-school kids in Utah invented what may be more efficient air-conditioning for cars. The Salt Lake Tribune describes them as being two kids doing a project for a science fair, who decided they were actually on to something and stuck with the project for two years afterwards. One was quoted as saying, "We aren't planning our lives around making air conditioners... We wanted to do something to help the environment and the economy."

Their concept is to use Peltier chips instead of compressed Freon to do the cooling. I have to say I'm a bit skeptical of this, because thermoelectric devices are notoriously inefficient--we're talking ~5%, as opposed to ~45% for most compressors. There are some people working on changing this, but for now that's how it is.

Another possibility is that these devices are good for vehicles because they are electricity-based, thus eliminating the need for a fluid-mechanical system that can have weight, size, and reliability problems. And a Peltier device can act as both air-conditioning and heating, you just reverse the direction of the current. But I think the revolutionary thing about small solid-state cooling chips in cars is that they can be distributed. People are more sensitive to the temperature of the things they touch than to the temperature of the air, so if you can directly cool surfaces in the car (like the steering wheel), you don't need to cool as much. It's like baseboard heating in a home. It doesn't appear that the inventors have thought of this, but in any case, it's good to see that two kids in a garage can come up with worldchanging inventions, and have big companies like Ricoh show interest (Ricoh awarded them winners of its first sustainable development contest), possibly even make the idea into a real product. We'll have to wait and see.

Frankly, I think the most effective thing for automotive AC would probably be Amory Lovins' idea of installing low-emissivity coatings in all the windows as part of the glass-lamination, to avoid the greenhouse effect in cars. This could reduce your cooling load by perhaps 75% (conservatively say 50%), with no electricity, compressors, or moving parts.

Note: It's hard to measure air-conditioning efficiency in percent, because it's not apples-to-apples, but the standard measurement is the "coefficient of performance" (COP), which measures the amount of heat pumped divided by the amount of energy input to the device. A typical building's air-conditioner (using a compressor like your car does) has a COP of 3, a really well designed one has a COP of 6; most Peltier devices have a COP of 0.4 to 0.7 . Perhaps the systems in cars perform much worse than buildings, due to constraints of size / ruggedness / etc.; I don't know. In any case, the teen inventors think it will be more efficient, and Ricoh thinks they're right.

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Comments

second time I've commented on this.
Dudes at slashdot had a lively discussion about this in their forum.
Still who knows, maybe they did it.


Posted by: andrew on 22 Jul 05

Not gonna work usefully.  Reasons are simple:

  • Peltier junctions can't create or destroy heat, they just push it from one side to the other.
  • This means that they still have to reject heat from the hot side and carry it off somehow (and due to the low efficiency, they have to move more rejected heat than a regular A/C).
  • The coolant tubes or heat pipes which carry the reject heat to the environment could just as easily carry cold fluid from a (much more efficient) vapor-compression A/C.

The misconceptions in the article are enormous.  The author states that the Peltier chips draw power from the electrical system rather than loading the engine with a compressor (as if the alternator doesn't), in all apparent seriousness.  (Don't journalists learn the law of conservation of energy in school any more?  How about HS science students?)

If you want spot cooling in a car, you've got to find a way to move heat away from it rather specifically.  Heat pipes can break, coolant tubes can leak, and anything you cool directly will sweat in a humid environment.  If you are going to do something like this, the best engineering solution (absent an electric device for moving humidity) is to cool and dehumidify air and then blow the air where it's needed.  You can even replace freon with isobutane if you can get the government and tort lawyers to let you.  The combination will be cheaper and more efficient than Peltier chips, and will thus save energy; I expect that to be on everyone's mind shortly, if it isn't already.


Posted by: Engineer-Poet on 22 Jul 05

Andrew:
Thanks for the link to the Slashdot forum. I'd heard they scooped us, but didn't see the discussion, which does provide a good deal of depth.

Engineer-Poet:
I agree, I don't see how thermoelectric cooling will be better, unless the "Cool Chips" which I linked to become real products. Frankly, the most effective thing for automotive AC would probably be to install Heat Mirror windows (or at least low-e coatings) to avoid greenhouse effect in cars.
...in fact, I think I'll append the article. That'll make it more useful than it is now.


Posted by: Jeremy Faludi on 22 Jul 05

Well remember one thing the current car ac unit is a moronic contraption placed in the worst possible spot so IF the pelt system is placed in a better place it could indeed be less taxing on the engine to run a bigger alternator then it is to run the contraption they use to run ac units.


Posted by: wintermane on 22 Jul 05

Toyota has already gone to an electric A/C compressor for the Prius.  This not only gets rid of the drive belt, it gets rid of the (leak-prone) flexible hoses as well.

On top of this, the Prius can run the entire A/C system from the traction battery with the engine off.  No more idling in traffic just to stay cool.

Other manufacturers have been looking to do the same thing for some time; getting sufficient power to do this is one of the objectives of the move to 42-volt electrical systems and integrated starter-generators.


Posted by: Engineer-Poet on 22 Jul 05

Yup I knew the 42 volt system was designed to be able to run such things tho I also had heard it was gona take a realy long time to migrate in. Do you know the current estimates at how long till full 42 volt car systems come? I know the 12/42 volt systems will be fairly soon.


Posted by: wintermane on 22 Jul 05

No-brain emissions-free car a/c: convertible. I haven't used a/c in years, even in 90+ degree weather. Any weather you'd need a/c is also weather you'd like the top down anyway...


Posted by: Peter on 23 Jul 05

I see someone who hasn't lived where dehumidification is a necessity.


Posted by: Engineer-Poet on 23 Jul 05

I dont like convertables im too paranoid to be that exposed and to doofusy to look cool in one.


Posted by: wintermane on 23 Jul 05

Most cars have automotive glass that blocks near-infrared light, to reduce the solar input by 50 % or so. I talked about it at http://dwarmstr.blogspot.com/2005/05/visualinfrared-comparisons.html
and I have a few near-IR photos of the effect via this post http://dwarmstr.blogspot.com/2005/05/infrared-gallery.html


Posted by: Dean on 23 Jul 05

Hey, Dean, cool photos!

It's good that car windows already have a little something to help; I didn't know that. However, by comparison, a Heat Mirror coating is 90% reflective to infrared. The difference between that and 50% is huge--cuts heat gain by 4/5.


Posted by: Jeremy Faludi on 23 Jul 05

Oh thats because highly heat reflective films tend to limit the ability to see INTO the car wich is against the law most places.


Posted by: wintermane on 24 Jul 05

Low-E glass for homes has tin oxide in a microlayer on the outside surface of the inner most window pane. This redirects some out-radiated infrared back inside -keeping the occupants warmer in the winter. If put on the inside surface of a car window a similar phenomenon occurs outwardly, but with low efficacy (the glass is already hot and radiating both directions). The solution is a multi-metal salt formulation that combines Low-E and solar gain reducing funtionality- coated on the inner surface of window (home or car). This layer has also to be over coated to prevent scratching, which would be an issue in roll down windows. The version I saw in field tests was remarkably effective: the window glass was almost painfully hot to the touch, but redirected perhaps 50% of incoming infrared back outside, with only a slight darkening required. The drawback was an odd blueish tint that some might find objectionable. Such windows were at last I looked being marketed in Europe under the Pilkington Brand called Solar-E http://www.pilkington.com/resources/solareconspamp.pdf but were not marketed in the US for rresidences due to what seemed to b e conflicts over ASREA standards or perhaps out of business priorities. THere is no reason this coating coombination could not be applied in automotive windows, helping reduce heat needs in winter and conrolling solar gain in summer. In short, it is importatn to distiguish solar blocking from Low_E funtionality, although Pilkington seems to have gound a way to effectivelly combine them.


Posted by: john laumer on 25 Jul 05



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