Quote:
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.