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If I see the steam coming out of cooling towers at an altitude of 200 meters, I cannot help but think that this energy is wasted. My question is: Why isn't this steam cooled enough to become water and used to create hydroelectric power due to the drop from 200 meters?

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It isn't steam. It is water vapor cooled enough to form droplets - i.e. a cloud. –  Mike Dunlavey Jul 17 '12 at 19:40

2 Answers 2

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Let us see -- how much water per second is getting at the height? Let's estimate it as 1 ton of water per second. Then the energy you can extract form the water is:
$$ 1 [\mbox{ton/s}] * 200 [meters] * g \simeq 20000000 J/s = 2 \mbox{megaWatts}$$

Compared to hundreds or even thousands of megawatts produced and the power plants -- doesn't worth the fuss. And I'm pretty sure that it is a huge overestimation anyway.

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I just want to confirm it's a sensible estimate. One cooling tower of the Czech Temelín nuclear power plant evaporates 300-400 liters or kg of water per second. There are four of them. Their height is 150 meters. images.google.cz/images?q=temelin&biw=1031&bih=783 Temelín produces 2 x 1,000 MW. - Just one conceptual comment: you calculated the potential energy. Of course, there is latent energy/heat of the condensation that is actually comparable to 1,000 MW. If one could get it back from the vapor, that could increase the efficiently substantially. But steam engines are not perfect... –  Luboš Motl May 13 '11 at 13:06
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This calculation is meaningless. You have to think of the temperature of the water in the tower and then apply Carnot! Result is, nothing to "save". BTW what he sees is not "steam", because steam is invisible. This is a mixture of water droplets and air, like a cloud. Is there a English word for that? –  Georg May 13 '11 at 13:16
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@George: In English (at least American English) "steam" is a mixture of air and condensed water droplets, like a cloud. Gaseous H2O is called water vapor, which is invisible. –  Colin K May 13 '11 at 14:52
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Mostly correct. There is of course some energy available in the "waste" heat. You could try to add another cycle between the high temperature steam turbine and and envirnomnetal heat sink, and perhaps extract another percent of three from the plant. This would add size and complexity and capttal cost. Most likely at design time the tradeoff was investigated and found not worth it. It is possible that with changed market conditions, and/or better technology for extracting usable work from low temperature heat that a retrofit might make sense. –  Omega Centauri May 13 '11 at 15:38
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The efficiency of the main power station depends on the temperature of the cold reservoir (the cooling tower). If you try and remove this energy and doing so reduce the cooling efficency (raise the cold temperature) you reduce the output power of the main station. It can be worth it if you can run the cooling water to heat glasshouses or homes - the problem is that you also need to do this in summer. –  Martin Beckett May 13 '11 at 15:53

There are a number of issues with recovering the energy (mechanical or thermal) from the stack exhaust of a cooling tower.

First, the temperature and energy (enthalpy) of the exhaust is too low to effectively recover. The exhaust is full saturated or over saturated air typically at a temperature of 110 F, not above 120 F.

Second, the amount of water discussed above is way too high. The evaporation rate for a cooling tower can be estimated at 0.8% of the circulating water rate per 10 F of range. For a typical power plant this will max out at 2.0% of the cooling towers flow rate.

A typical 2X1 combined cycle plant has a 125 to 150 MW steam turbine. This equates to about 1,300 gallons per minute of evaporation (177 lbs/sec). Assuming that you could collect all of the water and channel the flow to be able to generate hydroelectric power from a turbine is 10% of the value discussed above, or 20 kw (assuming a 100% efficient cycle, which goes against entropy and the 2nd law).

Cooling towers have been around and in use for a long time and in a lot of applications beyond power generation (petro-chem, refineries, building HVAC, etc.) and there has yet to be a cost effective way to harness the waste heat. Some btu's are just not worth trying to collect and are better sent to the universe to increase universal entropy.

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It's quite common to harvest the otherwise-wasted thermal energy from power plants. You can find such cogeneration CHP plants (Combined Heat and Power) in Denmark and the Netherlands, and many other places. –  EnergyNumbers Jul 27 '11 at 9:49
    
@EnergyNumbers Ever learned a little bit of Thermodynamics? Do You know about the temperatures of the power and heat stages? –  Georg Jul 27 '11 at 10:05
    
Yes thanks Georg. I've seen a cogeneration plant. Isn't proof by existence an elegant thing? The OP's question was "why is the steam from cooling towers not used?". The answer is that in many plants it is used. For example, the low-grade heat can be great for pumping into greenhouses. And if you set the whole thing up right, you can capture the CO2 and pump that into the greenhouse too! –  EnergyNumbers Jul 27 '11 at 10:24

protected by Qmechanic Mar 14 '13 at 16:31

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