AC Experts: Poor Man's AC Alternative?

[Jack Olsen]

My car has good AC, for a 911. But I recently took a rock hit on a fitting and lost my R-134a charge. I didn't have time to fix it before a very long drive up to Thunderhill, so I hooked up my race Cool Shirt, and stopped three times for ice along the drive up. A Cool Shirt uses a 12V pump, a small ice chest and a shirt with plumbed-in plastic tubing to circulate cool water around your torso. Racers use it because it fights heat fatigue and -- unlike AC -- is still effective when the windows are down.

The Cool Shirt did an acceptable job for the long drive. It's not as good as decent AC, by any stretch of the imagination, but ambient temps were between 90 and 105, and it kept me reasonably comfortable with approximately 21 pounds of ice used for the 7.5 hour drive. I would guess that the BTUs in 21 pounds of ice are significantly less than what my AC provides, but this is a more focused use of cooling than trying to change the temperature of all the cabin air.

It got me thinking. (What else is there to do for that long of a drive?) The two things that make AC undesirable for a race car, at least with my set-up, are the weight of the system and the aero effects of the underbelly condenser underneath the car. The two things that would make the Cool Shirt a lousy replacement for AC are that you've got to attach and then wear the goofy shirt, and then you've got to keep restocking the ice chest.

So here's my crazy idea for my particular car. Why not use the plumbing from the Cool Shirt in the seat itself? In addition to running lines along your back and under where you sit, you could also run the cooling lines along the underside of the 3-inch belts from a five-point harness system. It would eliminate the hassle of wearing the shirt and also of needing to work the two quick-disconnects every time you get into or out of the car.

Then, the bigger question: what if you could use a marine/truckers/campers 12V refrigerator-freezer to cool the water instead of ice? The little 12V systems clearly don't have the kind of BTU capacity you'd need to cool the whole cabin, but I suspect they might have enough to match the energy of 21 pounds of ice melting over the course of 7.5 hours. Am I way off base? The 12V freezer could be mounted in the trunk, and could be made to disconnect (two water lines and a power plug) when you get to the track. No weight penalty, that way.

There are two types of 12V refrigerators I know of, and this is the point where my technical know-how completely fails me. It could easily be that it would require hours of driving to get to the point where I'm approaching the cooling capability of a half a bag of ice cubes. I don't know. But there are fridge units with traditional R-134A compressors (made by Danfoss in Germany) and there's a newer type out there that uses a Stirling engine (with a sealed helium system) to remove heat. This would be the one really costly part of my idea, at about $450, so I'm interested in input on which might provide more cooling power, more quickly, if this idea isn't completely harebrained right from the start.

THIS IS SOME INFO ON THE TRADITIONAL COOLER

THIS IS SOME INFO ON THE STIRLING-TYPE COOLER

The danfoss-based one consumes more power, at 35 watts to the Stirling-type's 24 watts. The danfoss-based one is smaller and lighter, though.

I can't find any kind of BTU/hour rating on either. Do any of the experts out there have any opinions they'd like to offer? I'm all ears.

[scottb]

Jack: Was the fitting that broke on your underbelly condenser? If so, does this change your perspective on the condenser's durability?

Cheers!

Scott

[Jack Olsen]

It is on a line that goes to the underbelly condenser.

I race my car, and I go off-track, a lot. So my car is a kind of worst-case scenario for any external components. But my factory front condenser gets damaged about five times more frequently than the underbelly one, which is in a pretty well-protected place.

[Grady Clay]

Jack,

You are right on – thermodynamically.

Consider the cool suit and additions (focused cooling), the low power A/C units you describe, and a two-phase (ice-to-liquid and liquid-to-vapor) possible with a bag of ice to dramatically improve the system. A 6-pound bag of ice could possibly last your trip with only 35W of power from the engine.

Good thinking - Rubbermaid A/C. HeHe.

Best,
Grady

[kuehl]

Jack,
If you are really serious about this approach to cooling yourself consider the traditional body points where blood circulates closest to the surface and in large volumes.

cough

[kuehl]

start with this
http://www.electronics-cooling.com/Resources/EC_Articles/SEP96/sep96_04.htm

[Halm]

A friend with a 993 used a power inverter to run a laptop on long trips. He fried 2 alternators before he realized what was causing it. The aux power system wasn't designed for that type of continuous use.

I have no idea if the units you are thinking about using will pull that much power, but it may be worth looking into.

[Jack Olsen]

Originally posted by kuehl
start with this
http://www.electronics-cooling.com/Resources/EC_Articles/SEP96/sep96_04.htm

At a glance, that looks like it's about Peltier-type coolers. As I've understood it, those are not very efficient, relative to the power they require. Am I mistaken?

[rouxroux]

For really quick & cheap, how about one of those old window-mounted swamp coolers? See them once in a while @ swap meets.

[jadeblaquiere]

I seem to recall seeing some sort of canvas bag with a misting wand in a Victoria British catalog several years ago (before I parted with that accursed tractor-motored roadster). I tried googling to no avail, but it's hard to beat simple...

--
Joe

[87coupe]

Dry Ice?

[84porsche]

I would consider running cooling lines in your seats. A shirt only keeps you somewhat cool and wet pants just doesn't look right.

[Jim Sims]

Automotive companies (BMW?) have studied cooling the seats instead of the cab air and it is considerably more effective. However, the study's participants still seemed to prefer cold air blowing over their faces and bodies, perhaps because that is what they are familiar with and associate with automotive air conditioning. An adult human being generates 400 to 500 watts of heat continually. Dry ice evolves CO2 so some care needs to be taken to ensure ventilation.

[Jack Olsen]

It's true, Jim. In 106 degree cabin heat it was very strange to feel all the hot air around me but not be perspiring. There's a kind of perceptual disconnect, at least initially. But then as time passes you realize that even though it's so hot, your core body temperature is staying low -- and you're not suffering.

I'm sure most drivers would prefer a cold cabin and a cool breeze. But then, most drivers aren't also sweating about the weight of an AC system.

[Brillo]

One of the things I really like about my Saab 9-5 is that the front seats have a three-speed fan in the seat bottom. It really helps a lot, especially when it's muggy.

I have thought about adding fans to the SC's seats. But that's about as far as I've gone with it.

[PatrickB]

A friend of mine just purchased a new Lexus, that has not only "heated seats" but "cooled seats" as well... the technology is there. I haven't a clue as to how they work, or how well they work. I will check them out the next chance I get, and report back!!

[kuehl]

Originally posted by Jack Olsen
At a glance, that looks like it's about Peltier-type coolers. As I've understood it, those are not very efficient, relative to the power they require. Am I mistaken?

Jack,
I guess you could say it all boils down to watts (if you prefer a measurement to compare) vs. mass or a ratio of mass to move the watts.

The current thinking, and tinkering, has been with the Free-piston Stirling coolers, you can read a very brief brief here: http://www.memagazine.org/backissues/may99/features/stirling/stirling.html

The Stirling approach to cycle is shorter and more efficient than the traditional Rankine cycle. In either choice you'll need move a piston and you'll need watts to do that. Take it further and you could probably use the hybrid approach and tap some wasted energy from braking.

I would not discount the Peltier approach just yet as these inventive coolers where most likely rushed to the market.

A bit more simpler but still a mass item are the electric compressors marketed by Sanden and others.

Getting back to your concept idea, how to cool the fluid and keep the system efficient and simple, I would focus on specific body parts for the cooling "wraps" and avoid a sticky seat. I wonder how the elephants do it?

Have you considered a simple "condenser" with a choice fluid to cycle through another heat exchanger?

[deoxford]

Jack,
I looked into some thing like what you are talking about. But I decide that it would be better to maybe go this route .
The small a/c unit just for you and your seat.
Take a look , here is a small a/c unit that is self contained.

http://www.seefido.com/html/cat_house-ac-unit-heater.htm

There are others like it that will work well and cost less, that I have found.
that was just a example, I was think of making a mount and you could take it out after the event, or better .Whnn you are at a Hot event you can pull it out of the car and let it keep you cool when you are waiting for your group to go back out.

[Jack Olsen]

Originally posted by kuehl
The Stirling approach to cycle is shorter and more efficient than the traditional Rankine cycle. In either choice you'll need move a piston and you'll need watts to do that. Take it further and you could probably use the hybrid approach and tap some wasted energy from braking.

I would not discount the Peltier approach just yet as these inventive coolers where most likely rushed to the market.

A bit more simpler but still a mass item are the electric compressors marketed by Sanden and others.

I guess I can break the problem up into some different pieces.

Pulling Heat From the Driver

The Cool Shirt itself worked well. But along the lines of what Kuehl says, resting one wrist on the cooling lines that went into and out of the shirt made the system more effective. This helped when the ice in the box was nearly gone, and the water temperature was coming up.

One design criteria for this -- at least as I'm imagining it now -- is that it shouldn't require any effort beyond getting into the car, buckling up, and throwing a switch. But that doesn't rule out finding the most efficient ways to 'plumb' the seat. My thinking initially is to create a piece of fabric or soft plastic mesh that sits on the seat, with the soft tubes attached. But if it cools well enough, the plumbing could be easily put under the cushions of my fiberglass bucket seat.

I wonder if there's an image somewhere out there on the internet that shows the 'hot spots' on the torso and arms -- the places where the water would most effectively pull heat out of the body?

Cooling down the water

I'm as dumb as a box of rocks when it comes to electrical stuff. My assumption here is that I could get away with less than 50 watts of power without destroying my alternator. If I'm wrong about this, I hope somebody will correct me.

I've looked at Peltier-type coolers, and I haven't seen one that could do better than 40-below-ambient temps, even under ideal conditions. The fact that the Stirling- and Rankine-type units are capable of cooling to well below freezing makes me think they'll work better. But again, I'm just guessing.

From the article Kuehl posted, it looks like Stirling engines are both more efficient (in terms of electricity needed) and capable of cooling more quickly (from warm start-up), which is even more important for this application. Looking at just the two products I posted, though, theoretical advantages are probably going to be less important than the practical capabilities of these particular units. On the one hand, there's the Danfoss BD35F compressor, which is apparently the workhorse of the marine/trucking/RV 12V refrigerator world. It's the traditional Rankine setup, with a lot of development time under its belt and a pretty well-proven track record. According to one website I found, its capacity is 586 Btu/hr (172 watts), with a maximum input of 78 watts. Its refrigerant is HFC-134a, CFC-12.

Now if the Stirling type compressor is 30% more efficient than the old Rankine type, then I'd be ahead of the game. But it's entirely possible that the particular Stirling compressor in the Coleman unit is 30% smaller than the BD35F, or 30% less efficient than what the Stirling technology is capable of -- or worse. I have no idea, partly because I haven't been able to find any meaningful specs on the Stirling unit, aside from the fact that it impressed reviewers who presumably were used to the older type of refrigerators. The Stirling compressor uses helium instead of chlorofluorocarbons, which would be a good thing when I eventually find a way to break my system. It's quieter, which is kind of a non-issue for me. Since it's newer technology, it might not be as trouble-free as the hermetically sealed Danfoss unit. I don't know.

Here's a graph of a similar unit (the Coleman would perform like the dashed line in the graph) with its cooling measured. But this is a measurement of the air in the center of its box, which isn't going to be the same as whatever kind of coolant/water exchanger I put together, which would presumably work more quickly (although, again, that's just a guess on my part).



And Here's a pretty cool Flash movie of a 40W Stirling unit being demonstrated.

I haven't been able to find a similar graph (or cool movie) for the Danfoss unit.

So, if just choosing between these two units was the only issue, the easy answer would be to buy both, test them side to side, and return the less effective one, right? Well, yes, except that I'd be wasting some money on shipping, and I haven't found two vendors willing to accept returns without a significant restocking charge, yet. But maybe that's what it will come to.

Other ways to cool the water

This is a worthwhile question. Kuehl mentioned Peltier units. They're reasonably affordable, and apparently can be sandwiched to increase cooling capacity. But I understand they're greedy for power. This makes them appealing to the computer processor cooling crowd, but might be less ideal for someone who's alternator dependent, like me. But again, this is a technology I know almost nothing about.

Kuehl also mentioned electric compressors made by Sanden and others. Are there cooling systems, like a 12V refrigerator, that use these? The advantage to a functioning system (like a refrigerator) for me is that it can be taken apart and used pretty much as-is, as opposed to being built from the ground up, with all the self-education that would entail. I'm not against learning and planning and designing and building, but there are probably some cost, durability and efficiency advantages in having someone else do all of that for me.

[DG624]

I have never seen an electric cooling system...if this can also heat then there is potential to solve another issue. I think that air blowing on the driver cools only the front and is not as effective as your method which cools the entire driver. I think the entire AC system should be rethought. It is too complex.

Another issue may be reduce the heat generated by the car...exhaust, engine, windows...If you have a white car that has much less heat build up. Also if you can reduce the window heat gain that helps. The obvious solution is don't drive in the hottest part of the day.