Depends on how hot it is outside. If it's hot enough that your body can't reject enough heat naturally, then you'll die WITHOUT such a device. Also, if it's hot enough outside, your body can actually absorb heat from the environment.

Also, your metabolic rate is only 100 Watts if you're resting. Any kind of exertion would be higher.

If it's hot outside, your body sheds heat by sweat. Non-ideal if you like not being sweaty.