Why must thermodynamics stand in opposition to my invention of air-conditioned boxers?

It’s been several years since I came up with the idea for temperature-controlled undergarments. Anyone who’s ever spent time in Louisiana in the summer will immediately see the appeal of such a thing, and any time I’ve mentioned this idea it’s been met with varying levels of ecstasy and gratitude.

The most common response is:

“Dude, that’s awesome. Seriously, please make that happen.”

Or something along those lines.

Sadly, as useful an invention as that might be, it’s just not feasible. I’m not that smart, but I know they call them laws of physics for a reason — there just isn’t a way around them. And I always have a tough time explaining why, exactly, there’s no way (without some vast improvements in current technology) to give the public the climate controlled undergarments they so richly deserve. I’m torn as to whether this disconnect is due to my own failings in explaining the laws, or just because the average person doesn’t know squat about thermodynamics. Probably a good deal of both. So I’ll use this space to try and better explain what I envision and why it’s just not possible.

The Laws
First law: “You can’t win.”
Second law: “You can’t break even.”
Third law: “You can’t quit.”

The Idea
Basically, my thought was to take a tight undergarment (say, an Under Armour-type tshirt and a pair of boxer-briefs) and interweave them with thin, hollow tubes (they’d need to be flexible and durable, so probably some sort of plastic or rubber). The tubes would all come to a ponytail at the tailbone area of the garment, where they’d feed into a small pump and condenser. The condenser I envisioned as a miniature version of the condensing units that provide the cooling for most of the homes in the world (except my design would use water as the fluid, not coolant…which actually makes it a chilled-water application, not a condensing unit. That’s a little too in-depth for this blog). It’s a simple, straightforward design, and it works. For that application. For this one, not so much.

The Problems
To sum up: the idea is to circulate cool water through tiny tubes, cooling the body through conduction (which is much more efficient than convection [conduction is what happens on a frying pan; convection is what happens in an oven]).

Problem 1: It takes energy to cool water. And not just the electrical/mechanical energy needed to run the pump (which would be suppied by a battery of some sort). It also takes thermal energy to keep the water at the necessary temperature (remember, this design is essentially a heat engine, taking the temperature difference and using it to perform work. This application is governed by the second law, and the clearest way I know to state what’s happening here is this: when two systems are in contact with each other and they’re not the same temperature, their inclination is to get to the same temperature, i.e., equilibrium. This produces entropy, but more importantly in our case, it produces heat.), which basically means the human body is hot and the water is cold. Each cycle (which refers to the time it takes for the water in any given tube to circulate completely through the system) will bring back warm(er) water, which will have to be cooled off before it can go back through the system. This cooling produces a slightly greater amount of heat (2nd law). There’s a reason why the condensing units are always outside; they produce heat. This is also the reason the back of your refrigerator produces heat. Exact same design.

In order to circumvent the heat produced, most condensers are remotely located (that is, outside for our house analogy). Unless I want my design to include a Radio Flyer on a leash, this isn’t an option for me.

Problem 2: the design would be really freaking unwieldy. This is just a miniaturization problem. This application probably wouldn’t need a real big compressor, but I’m not sure how small you can make on of those things. Additionally, it’d have to run on battery, which means it’d have to be really energy efficient. Batteries are ridiculously dense, which adds even more weight. If problem 1 went away, a workable solution to problem 2 wouldn’t be that daunting.

Problem 3: the design would be really expensive. Without having done so much as setting pen to paper, I’d guess that the whole system (specialized undergarments and a fanny pack for the cooling apparatus) would cost a few hundred bucks. That’s prohibitive. To realistically market the thing (y’know, outside the pages of the Sharper Image) you’d want the price in the $100 range.

Conclusion
If I want to build my design as it currently stands, the prototype would leave the wearer with a much cooler torso and 2nd degree burns on his lower back. Also, the thing would probably weigh 20 pounds and cost several hundred dollars. Back to the drawing board.

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