The cold sink is the water. Water has a specific heat capacity of 4.2 J/g/K, so its temperature isn't going to rise nearly as fast as the hot side of the devic

The cold sink is the water. Water has a specific heat capacity of 4.2 J/g/K, so its temperature isn't going to rise nearly as fast as the hot side of the device, and you need to provide even more energy to get the cold side past 100F...