# What are heat sink options for a circulating-water cooling vest?

I am in a first-year engineering design group where we are in the process of designing a cooling vest for motorcyclists for use in the city (1-2h length). Water will circulate through the vest, absorbing body heat, and then that heat will be dumped into a heat sink.

We initially decided on using Peltier chips for our heat sink, but when we brought this up with our instructor he was unenthusiastic (they are highly inefficient apparently). Then we decided to look into short-acting (1-2h) endothermic chemical reactions, but here ran into information overload.

What kind of practical options are there for such a heat sink?

• This existing product may help. Phase changes to absorb heat energy. A bit different from chemical reactions, similar overall functionality. Oct 23 '15 at 3:43
• Is it acceptable to consider the ambient temperature to be 90$^\circ$ or so? If so, you might be able to get away with a passive radiator. I'm envisioning aluminum fins that run down the back of the jacket. With a good design it could also be made to look pretty cool ;). Oct 23 '15 at 12:04
• That is a reasonable ambient temperature. The problem is that our application for city biking, i.e., lots of stopping at lights and driving slowly. Otherwise that would be a really fun idea :) Oct 24 '15 at 21:31

What you first need to establish is your temperatures. Body temp min and max; ambient min and max. Select the worst case. Then take some guesses as to how much heat you have to pump. Is the vest insulated on the outside or does it have to cool the ambient as well? I would blind guess that you will need about 200 watts of cooling by the time you figure your losses.

Peltier modules are inefficient, but have a much nicer form factor than an R-134A compressor (like what is sitting under your fridge). You will have to weigh the befits of each. With the low temperature differential it will likely turn out that Peltier modules are actually more efficient than a prebuilt compressor designed for a high temperature differential. Also keep in mind that if you pay top dollar you can get a lot more efficient peltier modules than the \$5 Chinese ones. Here are some nice reasonably priced ones with good specifications; tetech.com. From the COP graph in the specifications pdf you can calculate very precisely how efficient a module will be for a given temperature differential.

Im not sure about your application, but remember that endothermic reactions are either not sustainable (throw them away when you are done) or require recharging. Recharging requires cooling; so you are just delaying the need to have a heat pump somewhere else in the process.

• I would just like to point out that Peltier devices typically have a curve of cooling power vs differential temperature, and those curves show a high differential temperature at low power and vice versa. If the devices are in fact more efficient at a lower differential temperature, keep in mind that the way to get to the low differential temperature is to run the device at or close to maximum capacity. This really reinforces @ericnutsch answer as it relates to doing the math. The way to convince a skeptical engineer (professor or not) is with numbers. Oct 23 '15 at 8:50