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