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Power plant cooling tower evaluation

asked 2015-12-07 12:18:26 -0500

Jim Dirkes's avatar

updated 2015-12-07 13:03:13 -0500

A colleague has asked if it is practical to model an unusual scenario, and I am reaching out to you for creative insights. Here is some background: He is using a water treatment technology which removes mineral (and perhaps biological) contaminants from water. A practical use for this treatment technology is to treat cooling tower makeup water. A related use is to treat and reclaim cooling tower blowdown water. A benefit of this type of treatment is (supposedly) that the water evaporates much more quickly, ~170% of the normal rate, when it has been treated. If this is true, then cooling tower fan savings may also be a benefit. (i.e., more evaporation mean less air needs to be moved for cooling effect) In a power plant application, a lot of water is used for makeup. A 32MW plant may use ~400 gallons per minute of makeup water according to my colleague - a lot of water. If this water can be treated, evaporation made more effective and waste water reclaimed, savings could be great. This is especially true in areas like California where water and sewer charges are high. He would like to make a strong case for the economic benefits of the technology and part of that is to model the impact of increased evaporation. As it happens, I think I've modeled a cooling tower exactly one time and am not very intimate with the E+ cooling tower objects. For this water treatment concept, it is not evident to me how it would be possible to define a cooling tower to be more effective at evaporation. Do you have any ideas about that?

The other part of this problem is that a power generation installation is not a typical "building", so I'm not sure how to model one. My thought is to define a load for a cooling tower loop via EMS which mimics the power generation load pattern. Do you have any ideas about that?

So.... That was a lot to dump on you; thanks for reading this far (assuming you haven't dozed off). I'm not sure this is a viable modeling project, but if it seems possible to get reasonable results, it would be a nice challenge. Please let me know your thoughts.

Thank you!

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answered 2015-12-07 14:18:38 -0500

I believe that you can capture some of these benefits, but maybe not as directly as you might be hoping for. The makeup water treatment could (**see note below) have the effect of increasing the mass diffusion and convection coefficients by increasing the interstitial area between pure H20 and air molecules. If you select the CoolingTower:VariableSpeed:Merkel tower model, then the transfer coefficients are controlled by the input of the tower UA at design conditions. One of the assumptions in Merkel's Theory is that the Lewis Number is 1, so by specifying the UA you are effectively specifying both heat and mass transfer coefficients. The tower with water treatment should have a higher UA than the current tower, but the challenge is to know by how much. I believe it will be difficult to find reliable data on the effects of mineral concentrations on transfer coefficients in cooling towers. But if you do find a delta that you're comfortable with, then you can calculate the savings by simulating the tower with different design UA values. As for the water savings, I would just caution that the EnergyPlus algorithms are fairly crude in this department. The SaturatedExit method will always over-predict water consumption, and the LossFactor method has a low precision. If you have access to Trnsys, then the Type 51 cooling tower would be a useful tool for this task. In this model, the mass flow rate through the tower is not assumed to be constant, and the water loss is calculated explicitly.

** Removing minerals might not always be a good thing. You could be removing some which have a high conductance with air, and you could be loosing more sensible potential than what you gain with the increased latent potential.

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answered 2015-12-07 14:40:46 -0500

1) It seems that you can specify both the evaporation and drift loss of cooling towers (the links are for a single speed cooling tower, but these input fields exist for other types as well. The evaporation rate can be specified as "a fraction of the circulating condenser water flow and varies with the temperature change in the condenser water", expressed in units of [ % / degrees Kelvin]. Hopefully your colleague doesn't have to do to many conversions or assumptions to determine an acceptable value to use.

2) If you just want to model heat generation within a zone and don't care about energy use, you can use the OtherEquipment object. This allows you to model heat gain into a space with a design level and schedule modifier. Otherwise, EMS objects should give you the freedom to do almost anything.

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answered 2015-12-08 05:20:46 -0500

Chandan Sharma's avatar

If the building loads are already known, LoadProfile:Plant object can be used to simulate a scheduled demand profile. PlantLoadProfile.idf example file illustrates more. Makeup water usage calculation for all types of cooling towers is described here. Changing drift loss percent, evaporation loss factor etc. affects tower make up water calculations only. They don't affect tower energy use or show whether cooling effect is increased or decreased. If there are models available which describe the behavior you illustrated above, they can perhaps be modeled using PlantComponent:UserDefined. Or a parametric study can be conducted which changes Tower UA along with drift/blowdown/evaporation loss values.

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I think I have enough new information to begin to develop a proposal. Thanks very much, Chandan.

Jim Dirkes's avatar Jim Dirkes  ( 2015-12-08 05:42:54 -0500 )edit

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Asked: 2015-12-07 12:18:26 -0500

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Last updated: Dec 08 '15