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Modeling HVAC System with Direct Free cooling

asked 2023-07-26 01:22:47 -0500

lrebello's avatar

updated 2023-08-01 12:03:11 -0500

Hello all,

I am trying to model a hvac system that operates with 100% OA when outdoor is both below 23ºC DBT and 80% RH. Also, when outdoor air temperature does not met this criteria, hvac system turns on a cooling coil to maintain the room at 25ºC DBT.

I have tryied a bunch of diferent configuration on air loops and I could not properly represent this system.

Does anyone faced this problem already and know how to help me?

Thanks a lot.

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What's your simulation tool?

Petros Dalavouras's avatar Petros Dalavouras  ( 2023-07-26 02:20:45 -0500 )edit

I am using Energy Plus and Open Studio

lrebello's avatar lrebello  ( 2023-07-26 02:22:04 -0500 )edit

That's the economizer that you are describing with "FixedDewPointAndDryBulb" control found either in control sub tab of HVAC or by clicking on the outside air system of your air loop (again in HVAC tab). Have you tried it and it doesn't work or you get errors?

Petros Dalavouras's avatar Petros Dalavouras  ( 2023-07-26 02:31:11 -0500 )edit

I am not sure if I tried this economizar with "FixedDewPointAndDryBulb". I will try it sooner and I will come back with a feedback here!

Thanks a lot for your answer for now!

Really appreciate it

lrebello's avatar lrebello  ( 2023-07-26 02:36:13 -0500 )edit

After few attempts, looks like it is working.

My system is looking like this (picture 1)

Picture 1

I am just not sure if I should keep "100% outdoor Air in Cooling" set as "Yes" inside the OS:Sizing:System.

@Petros Dalavouras Based on the operation I need the system to follow, do you have any idea if I should keep the mentioned field as "Yes"?

Also, as I see from ploted graphs, my cooling coilg is not being turned on when it is needed (outdoor temperature is above 23ºC) . Not sure what I have done wrong.


lrebello's avatar lrebello  ( 2023-07-26 08:37:00 -0500 )edit

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answered 2023-08-01 04:28:55 -0500

Petros Dalavouras's avatar

updated 2023-08-01 04:46:54 -0500

In order to have your free cooling running you need to go to your HVAC tab and select the OS:Controller:OutdoorAir of the air loop serving your zone as shown below freecooling

Here you need to set the economizer control type to "FixedDewPointAndDryBulb". Then use the limits of your free cooling which are 23°C Dry bulb temperature and 19.366°C Dew Point(which gives 80% RH at 23°C DB) in your case. There is no limit for RH so you need to see if this works for your case for example during winter. Finally, use the Air System Outdoor Air Limiting Factor output variable to see when economizer is used. When the output is 2 then "Economizer is active and sets the flow greater than the minimum outdoor air fraction".

The 100% outdoor air in your comment is a sizing option for your coil and does not represent the free cooling. As far as when your cooling coil operates you should also check if the zone temperature is above your thermostat and not the outside temperature

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That is exactley What I am doing @Petros Dalavouras . The only difference is that I plotted the 23ºC and 80% relative humidity in a Psychrometric Chart (@ 420 meter above sea level which gives me 96.403 kPa) and for that data I got values of 23 ºC Dry-bulb Temperature, 61142 J/Kg Enthalpy and 19.46 ºC Dewpoint Temperature.

From my understanding, defining the parameters OS requires will not limit the maximum Relative Humidity.

For example, picture 1 show wheater data points where economizer will be active but should be inactive.

Picture 1

lrebello's avatar lrebello  ( 2023-08-01 14:48:00 -0500 )edit

This is true. A fixed Dew point cannot give a variable RH since DB is also a variable. That being said, what is your application that requires a RH limit since cold air, even with high 100% RH, has low humidity ratio. That is what I was mentioning to my answer. Despite that, an "ElectronicEnthalpy" economizer probably can solve this problem. Keep in mind that OS does not have a field for the ElectronicEnthalpy's curve so you'll have to insert one manually for example with a text editor. Check this post

Petros Dalavouras's avatar Petros Dalavouras  ( 2023-08-03 07:19:04 -0500 )edit

Thank you for your answer. I am modeling a computer room (not a Data Center) but close to it. It has only 6000W of internal equipment. It does already exist, and the way the HVAC equipment work is using 100% outdoor air when both outdoor air DBT is below 18ºC and RH is below 80%. I would like to improve energy eficiency (and evaluate that) by using 100% outdoor air when both outdoor air DBT is below 23ºC and RH is below 80%. I will try to use an "Eletronic Enthalpy" economizer. Not sure if I will be able to do that because I have almost no experience using the text editor for cases like this.

lrebello's avatar lrebello  ( 2023-08-07 18:22:36 -0500 )edit

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Asked: 2023-07-26 01:22:47 -0500

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Last updated: Aug 01 '23