Here is a vest that can keep you cool in the summer. The JAXA Reikyaku Cooling Vest is water-cooled. Just fill the tank with 1 liter of water, and it will go through tubes around your body to keep you cool. You get 7 hours of cooling on 4 AA batteries.
The water tank has 1L capacity. You get 8 lines of tubes around your upper body. The bad news? This is not cheap.