[BrandonWatson]

Yes, in doing my research the cooling vest was one of the first items I looked at. The biggest problem for my target customer (triathletes and runners) is the weight (>4 lbs), the lack of mass security (vests tend to move around a bit), and the heat blocking nature of the vest when cooling is depleted.

I have been experimenting with saline based solutions to test my theory of perception of heat reduction. They have worked for the most part, the challenge has been that they last about :30. I need longer. I have not experimented with any of the break+mix solutions from the wikipedia article, though am aware of them.

My preference for an endothermic reaction with a PCM is that in theory the mechanism is reusable, or easily replaceable, the weight penalty is low, and the complication of the solution (no pun intended) is also low).

There are some of these "cool materials" on the market right now, and they are somewhat interesting, though they are not operating as a heat dissipation level of the endorthermic reactions.

My vision for this product (at this point in time) is a sleeve that houses chambers with the endothermic reaction solution, and those chambers are either in an arm sleveve, or inside of a hat (think of the head band of a runner's hat).

Whether that solution is a one time use, and thus replaceable in the chamber, or multi use, and thus more permanent, is not really my challenge right now. A marathoner going to secure their Boston spot would pay $50-$100 for a hat that is guaranteed to keep them cool on a 90 degree day when their alternative is to keep throwing ice on themselves at aid stations, or stuffing cold sponges down their shirts. This is how it is handled today. Same for the age grouper triathlete who is getting ready for a sprint or Ironman. These folks are pretty obsessive about gear and kit, and aren't very price discriminating.

[mchannon]

The trouble with most endothermic reactions is they involve far less energy absorbed per kg than exothermic reactions tend to emit (and their reaction rates much slower than you'd like). As a runner I'd not be too enthused about lugging around a 500g brick of goo that has the best possible endothermic heat absorption. You'd lose far more in heat dissipation potential than that brick could ever absorb, at least if it started at ambient. (Imagine how less popular ice would be if it never melted but stopped cooling).

A peltier cooler would be right out; they only work when they have an active cooling mechanism (heat sink and fan usually).

If you wanted to ditch the cooler and put together a hat or other appliance with a 3V case fan and ultrathin solar cells (CIGS probably), you'd get an excellent improvement in heat dissipation, and it would work indefinitely as long as it was daytime (probably a safe bet).

[cake93]

Correct me if I'm wrong, but from a thermodynamic point of view the major problem is the entropy. You can't just convert heat into any other form of energy (light) without also caring of the entropy. In the case of solving some salt in water, the entropy is going into the chaos of the solved ions (versus the low-entropy crystal). With all the other methods you need to get rid of the heat. With peltier coolers I could imaging "watercooling" the hot side.

On the other hand sweat (evaporation) is already a pretty good cooler and covering the skin with something else stops that effect. Additionally you enrich sweat inbetween which does not feel well. Completely absorbing the sweat also kills the cooling effect..

So maybe, as mchannon suggested, all you have to do is design a nice, flexible and good-looking sweatband with an integrated fan ;)