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Unreasonable heating/cooling loads results [closed]

asked 2016-03-23 10:49:38 -0500

FSilenzi's avatar

updated 2016-03-29 08:36:25 -0500

Hello again, I apologize for opening so many questions, but I am in desperate need of help since apparently noone in my university knows how to model energy building using openstudio and energy plus.

Anyway, as I have written in other questions, I have a AHU system serving a simple model (4 thermal zones), the AHU was modified by me to have, on the heating side, 3 loops linked toghether: the heating coil is connected to a heat exchanger that is linked to a district heating heat source. The cooling side has the chilling coil linked to a absorbtion chiller which is connected to a single speed cooling tower.

Now, when I run the simulation I have no unmet hours, which should be great, but the problem arise to the heating loads. Infact, when I analyze the HVAC load profile, I can see that I have a lot of heating request also during hot months (the building is located in Genoa-Italy, which has a july mean temperature of 29 °C) while the cooling load remain very low, both in cold months as well as hot ones.

For clarity purposes I will uploads the 4 images regarding the loops and the HVAC load profile graph.

chilled water loop.jpg

condenser water loop.jpg

hot water loop.jpg

HX-district heating.jpg

hvac load profiles.jpg

My question is: is the software simulation correct? Have I committed any error during the absorption chiller modeling phase?

I have tryed to refer both the engineering references here and the input output guide here. One big question is: what are the nodes that I need to specify for the heat generator? I have tryed to edit the .idf file, linking the chiller to the HX-district heating loop, but the simulation fails and I get this error:

"ABSORPTION CHILLER" was not found on any plant loops

Sorry for the wall of text, and many thanks to everybody who wants to contribute making a young engineer happy!


PS: I am attaching also the .osm file if anybody wants to have a look at it

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Closed for the following reason question is not relevant or outdated by FSilenzi
close date 2016-04-06 05:16:42.914347

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answered 2016-03-23 14:38:22 -0500

As a suggestion, have you done a quick hand calculation on what you expect the loads to be in the building. It might be that your constructions and infiltration rates are not what you would expect. Also you might not have as much load in the building from people, lighting and plug load that you were expecting. You might want to check that the setpoint on your cooling coil isn't set too low. You might be supplying the zones with very cold air from your AHU which is adding a heating requirement to the building in the summer. That would be where I would start. Then I would take a hot day and work through my AHU node by node to see what each component is adding I would go through both flow rate and temperature information. Finally I would do the same with my plant and see what temperatures are coming off my boilers and chillers and whether these are what I would expect. Hopefully once you have gone through all that you should see some areas were the controls / inputs weren't quite what you would expect them to be.

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Completely agree with @Annie Marston's advice. In case you don't know how to view detailed simulation outputs, this link goes over using ResultsViewer, which you will need to go through the AHUs. Here's a documentation link as well for requesting output variables and using ResultsViewer.

NickC's avatar NickC  ( 2016-03-23 16:31:21 -0500 )edit

Thanks both @Annie Marston and @NickC, those kind of advice are right the ones I needed!! I have done a Simulation using ideal loads, and the HVAC profile resulted very different. For the cooling coil set point I am using 12° C, but also setting it very low or high the situation is not changing. I see things going completely different if I use an electric chiller. I have also tryed to set people occupancy at 1person/m^2 but with no look. How can I inspect node per node temperatures and flow rates and temperatures coming off chiller and heat exchanger? thanks

FSilenzi's avatar FSilenzi  ( 2016-03-24 05:03:38 -0500 )edit

Maybe I have found the problem: the air temperature in the node just after the water chilling coil seem to get negative (up to -8 °C) during hot months, while the just after the heating coil the temperature returns around the set point value (18 °C). I have a T set point fixed a 7 °C in the chilled water loop, because when I run simulation the unmethours_troubleshooting_measure reports that there is a discrepancy between desing temp value and set point value. Any ideas how to fix this? Thanks

FSilenzi's avatar FSilenzi  ( 2016-03-24 11:07:05 -0500 )edit

My first thought is to make sure you are using the right setpoint manager. Try using a scheduled setpoint manager and set the temperature to 15C for both heating and cooling setpoints to see if it controls your temperatures.

Annie Marston's avatar Annie Marston  ( 2016-03-24 12:35:39 -0500 )edit

I am a little confused about setpoint managers. At the moment I am using 5 setpoint managers, all are of the "scheduled" type. Is this choice correct? The temperatures are setted as follows: condenser loop 29 °C, chilled water loop 15 °C, district heating loop 81 °C, heating water loop 15 °C, VAV with reheat main loop 18°C. The thermostats are setted to 23 °C for cooling and 21°C for heating. Are they reasonable?

FSilenzi's avatar FSilenzi  ( 2016-03-29 03:27:29 -0500 )edit

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Asked: 2016-03-23 10:49:38 -0500

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Last updated: Mar 29 '16