I happen to work in a CCGT (Combined Cycle Gas Turbine) power station so I'll do my best to enlighten. The primary reason for cooling exit steam from a steam turbine is to condense it into a fluid which can then be pumped back through the boiler. Pumping steam is not possible or practical (as far as I am aware). CHP power stations get around this because the exit steam is used elsewhere in other processes instead of being used purely for generation (e.g. exported to a paper mill).
The comment about more asthetic cooling towers is also correct. The old style large cooling towers create a natural draught whereas the new style smaller ones have motor driven fans to achieve the same air flows and so use more energy. On the other hand the new hybrid towers have pre-heating sections which eliminate low level plume.
The heat energy lost through cooling of exit steam is very significant, and is probably the biggest controllable loss from any power station. The condenser at the station I work at runs at between 35 and 45 degrees C, but we are talking about big flowrates; the condensate flow out of a single unit is typically 380 tonnes/hour.
I've heard instances where this low grade heat from cooling water has been used in district heating especially in cooler climates such as Scandinavia. Why newly built power stations near residential buildings don't do it more often is unknown to me. The only reasons I can think of is a lack of incentives from government/power companies. Perhaps as fuel costs continue to rise it may become cost effective.
Hope this helps,
Jake.
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