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?
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.
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:
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.
protected by Qmechanic♦ Mar 14 '13 at 16:31
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