Wet versus Dry Cooling Discussion

This discussion was led by Steve Feiner of Sempra.  He went through a presentation he developed for this meeting. [I expect (and will request) that it will be available on their web site.  Once it is there, I will place a link to it here and on the power plant page.] Some notable items from the presentation and his talk included:

• Discussion of the Wet and Dry technologies and some photos of each type.

• Sempra's Evaluation Process - they look at each site individually.  They evaluate performance and environmental impacts, community input (visuals, noise, emissions) and also the site characteristics.

• Performance Factors like steam turbine exhaust pressure, steam turbine efficiency, auxiliary load and start-up time.

• Dry cooling performance losses on a 104 degree day would be ~ 29MW, and on a 90 degree day ~ 18MW.  This would also require an increase in fuel consumption to make up the difference on a 104 degree day equivalent to ~2500 homes running 100,000 Btu furnaces

• Environmental factors for dry cooling such as more stack emissions and more fuel required but about 90% less water is necessary.

• Dry cooling emissions increases of 7 to 24% based on temp and type of emission.

• Visual difference:  Dry: 400 feet long, 200 feet wide, 130 feet high while wet is 500 feet long, 50 feet wide, 65 feet high.

• Noise Differences:  Air cooled typically generate more noise - exacerbated by the height of the dry cooling fan and the addition of the dry cooling auxiliary cooler fans.

• Dry Cons: less energy efficient, reduced output in the summer, generally more emissions, significantly larger footprint and generates more noise.

• Dry Pros: Significantly reduces water consumption

• This information is backed up by both EPA (see chapter 4) and industry (this is very large so right click on the link and download it first) studies:

• A Sempra competitor just built 2 dry cooled plants and currently has one mothballed and the rumor is that they are going to mothball the other one because it is not cost effective to run it right now.

Summary is that the studies, models and evaluation of the community input show that the best technology for this plant is wet cooling.  [I agree]

Questions and Answers [These are not anywhere near verbatim, they are what I remember as being the essence of the questions..]

Question: Will you have to make changes to the application if you were to change to dry cooling to allow for the additional emissions?
Answer: Yes, and many more changes will be necessary to the application.

Question: What about dispersion?
Answer: We did not model the dry cooling dispersion.  There are different initial state characteristics mostly caused by the higher exhaust stacks that are necessary due to the buoyancy issues caused by the dry cooling fans.

Question: These Dry Cooling emissions Increases - are they the expected emissions increases over dry cooling at the same temperatures?
Answer: Yes, these are bases on the wet cooling system we are proposing, versus a normal dry cooling system (130 feet high, 200 feet wide and 400 feet long).

Question: What about the additional humidity caused by the wet cooling process?  Will this change the dispersion patterns?
Answer: A couple of points here:

• Wet cooling does not increase the local humidity significantly, definitely not enough to change dispersion patterns..

• Basically dispersion patterns would be similar for wet versus dry with one exception-- the 2 stacks required by the plant will need to be significantly higher for a plant using dry cooling.  These higher stacks will impact the dispersion pattern due to the different initial state and additional modeling will be required.

Question: How high would the dry cooling tower have to be?
Answer: For a normal dry cooling system - about 130 feet high - twice the height of the wet cooling tower. 

Question: You said something about a performance optimized dry cooling system -- 160 feet high - a 15 story office building - what was that?
Answer: We requested information from a vendor on a performance optimized dry cooling system with the steam turbine performance as the wet cooling system we are currently proposing.  The information provided by the vendor shows that the structure would be 160 feet high, 210 feet wide and 1134 feet long. [the height of a 15 story office building, and almost as long as 4 football fields.] and cost $64 Million plus installation.  As a side note, the load for the fans would 15MW.

Question: If this is all true - why would anyone use dry cooling?
Answer: There are a number of reasons for dry cooling - including the lack of water resources (desert conditions), permitting takes significantly less time (potentially years less), .local regulatory requirements, location, etc. At this point in time wet is still the best and most efficient.

[This corresponds with my own research on this topic.  If the plant gets permitted and is going to be built, I fully support the use of Wet Cooling for the visual, noise and emissions reasons stated. I honestly don't care about the performance penalties, except that they can directly relate to additional noise and emissions.]

02/18/2006 08:24 AM