Re: Why does water used in Electicity generating plants need to be cooled?

The turbine that drives the generator in a steam/electric generating plant 
is a heat engine.  The hotter the incoming steam and the colder the 
outgoing steam the more work the turbine will do.  So the pressure and 
temperature of the steam leaving a modern boiler is "superheated" far 
above the boiling point, and the steam leaving is exhausted to a high 
vacuum at temperatures as low as 90 to 100 F. That low an exhaust 
temperature is attainable only by cooling/condensing the steam with water 
that is colder than that, say, 70 F.  

The water used in a steam cycle is pure, and is kept that way to limit 
corrosion of one type or another in the boiler.  As such, the economics 
dictate that the steam be condensed and recycled with as little "makeup" 
as possible.  Leaks cost money.  So the steam coming from the turbine is 
condensed and pumped back into the boiler.

In some plants, with an adequate supply of cooling water for this steam 
condensing task, such as those using sea water for cooling, the water is 
run straight through the tubes of the steam condenser and the heated water 
runs back into the ocean.  In some inland locations where the amount of 
cooling water is limited, it is necessary to recycle it by running the water  
through cooling towers to provide water cool enough to meet the condenser 
inlet requirements.  These towers can be either mechanical - using fans, 
or the larger Hyperbolic, natural draft cooling towers.

The cooling towers can cut the amount of water used by a "once through" 
sea water plant by as much as 70% and may be necessary to avoid unwanted 
heating of a lake or river due to a potential averse ecological effect the 
heat might have on the local flora and fauna.