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How to add humidity control with direct evaporative cooling

asked 2015-06-22 10:01:55 -0600

Brian C gravatar image

I am modeling direct evaporative cooling and notice the RH in the zones getting high. I am modeling this in OpenStudio, and work in Colorado where this is almost always a design consideration for a building in this climate. As such, I would like to better understand how this system operates in OpenStudio and specifically how to control humidity.

  1. Does the Outside Air Damper open to 100% when the direct evaporative cooling is enabled? This is typically a real-world control strategy to control RH.

  2. Open Studio has humidity set-points that the user can set at the zone level. This was the approach I first took, however the software to me appears to either have a bug or I am incorrectly using it. When I enter a set-point schedule into the fields and the tab refreshes, the schedule disappears. Any help?

  3. What are some output variables to report to better understand the direct evap operation?

  4. It would be great to hear any other suggestions you have when modeling direct evaporative cooling.

Thanks!

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Hey Brain, one question, are you modeling Direct Evap Pre-cooling with the evap cooler on the OA branch, or just Direct Evap Cooling with the evap cooler in the mixed air branch?

TaylorRoberts gravatar imageTaylorRoberts ( 2015-06-22 11:48:23 -0600 )edit

Hi there - we are modeling the direct evap in the mixed air branch.

Brian C gravatar imageBrian C ( 2015-06-22 12:19:52 -0600 )edit

@David Goldwasser I was hoping you may have some insight into humidity controls within open studio. I have figured out that one of my errors was incorrectly creating a humidity schedule as fractional not as a percentage. However, now that I have these schedules created, they appear to have no impact on the direct evaporative cooler operation, as energy use and high RH in spaces is the same as before. Is the best way to control humidity of the direct evaporative cooler object through these schedules? Is there a better approach you would recommend? Thanks.

Brian C gravatar imageBrian C ( 2015-06-29 12:28:05 -0600 )edit

@Brian C, I'm going to copy @Kyle Benne. He has a much better understanding how humidity control works in OpenStudio and in EnergyPlus.

David Goldwasser gravatar imageDavid Goldwasser ( 2015-07-02 10:14:49 -0600 )edit

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answered 2015-07-02 10:50:32 -0600

1) Does the Outside Air Damper open to 100% when the direct evaporative cooling is enabled? This is typically a real-world control strategy to control RH.

  • No not automatically. If you want 100% OA then you need to schedule the min oa fraction with a value of one.

image description

2) Open Studio has humidity set-points that the user can set at the zone level. This was the approach I first took, however the software to me appears to either have a bug or I am incorrectly using it. When I enter a set-point schedule into the fields and the tab refreshes, the schedule disappears. Any help?

  • I don't know of a bug in the humidity set points, although we have had a few growing pains since we introduced the grid view so it is possible. In any case those humidity set points won't impact the operation of the direct evaporative cooler. The evap cooler model just isn't setup to control itself according to a humidity limit.

3) What are some output variables to report to better understand the direct evap operation?

  • Without looking up the exact variable names, I think you want to look at the zone RH, as well as the temperature and humidity ratio of the nodes around the humidifier. Those node conditions will tell you exactly what the humidifier is doing to the air stream. @aparker has a nice Reporting Measure that will plot these node conditions on a psychometric chart, but I don't think he has published it to the BCL yet.

4) It would be great to hear any other suggestions you have when modeling direct evaporative cooling.

  • I believe a common technique is to precede the direct evaporative cooling module with an indirect module thereby limiting the lift (and moisture) that the direct evap introduces.
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@Kyle Benne

  1. If I change the minimum fraction of outdoor to "always on discrete" will this only open to 100% when the direct evaporative cooler is enabled?

  2. I figured out that I needed to make humidity schedule % not fractional, so my error no bug.

Brian C gravatar imageBrian C ( 2015-07-02 11:12:36 -0600 )edit

Yeah that's the rub. Always on discrete will make your system 100% oa all of the time. I don't know of a way to make that dynamic with the evap cooler short of writing some EMS logic.

Kyle Benne gravatar imageKyle Benne ( 2015-07-02 11:15:03 -0600 )edit

@Kyle Benne Is the Direct Evaporative Cooler able to be controlled by the EMS? We have tried to control this and it appears to not be controllable actuator.

Brian C gravatar imageBrian C ( 2015-07-09 13:48:46 -0600 )edit

I just took a quick look by running the 2ZoneDataCenterHVAC_wEconomizer.idf example file and parsing the edd file which lists available actuators for the particular model. Didn't see any actuators for the evap coolers in that model, so I guess you are out of luck.

I thought @Lincoln's suggestion was pretty good. Have you tried that?

Kyle Benne gravatar imageKyle Benne ( 2015-07-09 14:42:10 -0600 )edit

Yes I tried Lincoln's suggestion and didn't see much change as my direct evap kicks on after the units are unable to economize any longer as I'm seeing the highest RH during the hot summer months. Thanks for the responses though. Just out of curiosity, do you foresee direct evap becoming a controllable actuator anytime soon?

Brian C gravatar imageBrian C ( 2015-07-09 15:10:08 -0600 )edit
3

answered 2015-07-02 12:27:29 -0600

Does your system have an air-side economizer? You may be able to find a work around by playing with with the high-limit shutoff value of the economizer and controlling the economizer based off of wetbulb instead of drybulb. Haven't tested this but it may be an option.

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thats a good idea, i'll give it a shot

Brian C gravatar imageBrian C ( 2015-07-02 12:59:54 -0600 )edit

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Asked: 2015-06-22 10:01:55 -0600

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Last updated: Jul 02 '15