It takes around 500x more energy to boil water than it does to raise its temperature by 1°C. If you had a closed loop, you'd need to dump all that heat into the ground or, more likely, a river, which does crazy things to the ecosystem with even a small temperature rise. Throw some water vapour into the atmosphere and that energy will bleed back over several cubic kilometres as the water condenses. The water isn't boiling in an open loop but evaporation is pulling out the same energy.
Incidentally, the reverse of this is also how condensing boilers achieve such staggering efficiencies and why you shouldn't have the temperature of your central heating set too high (at the boiler, not the thermostat).
Biting the hand that feeds IT
