How does the Energyplus calculate the ideal loads?

asked 2018-12-03 21:46:23 -0500

Haohan Sha
279 ●6

updated 2018-12-14 16:21:51 -0500

Hello, I check the engineering reference, but there is no clear equation. (of course maybe I miss it, if anyone knows the correct pages, please tell me). And this is an equation in basic part, it shows that the zone loads are calculated as: image description

The problem is that, I'm modelling a large zone with a large glazing facade, then I separated it into two vertical zones, (upper part and lower part). I used the ideal zone load air system to calculate the cooling energy. Except adding a floor slab in the middle and more AFN nodes, I didn't change anything. But the two zones total cooling energy is higher than the original single zone. I know maybe this is a correct result, but does any body know the reason? Because there is no internal heat gain, and weather change, just separating the zone into more zones, why can the ideal zone total cooling energy change?

After using the Zone Air Balance variable outputs, it can be seen that the model that are divided into two parts has higher surface convective rate than the original model. However, when I look into the solar radiation, the total amount of Surface Inside Face Solar Radiation Heat Gain Rate of the divided model is smaller than the original model. Why does the lower input heat gain lead to higher convective heat transfer?

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answered 2018-12-06 17:32:21 -0500

MJWitte

9670 ●6 ●10 http://www.gard.com/

updated 2018-12-14 16:15:29 -0500

That equation is a correct representation of the zone heat balance. All of the components of the heat balance are available as Output:Variables. My guess is that dividing the zone into two parts has changed the timing of when transmitted solar gains become a load. Start with these outputs for the two cases, and then you can add more detail once you know what area to focus on:

Output:Variable,*,Zone Air Heat Balance Internal Convective Heat Gain Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance Surface Convection Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance Interzone Air Transfer Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance Outdoor Air Transfer Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance System Air Transfer Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance System Convective Heat Gain Rate,hourly; !- HVAC Average [W]
Output:Variable,*,Zone Air Heat Balance Air Energy Storage Rate,hourly; !- HVAC Average [W]

Also, try a case with a winter design day with no solar and constant temperature and see if the loads agree.

Answer Part 2

Working with the actual files, comparing Zone Ideal Loads Zone Total Cooling Energy. This is not just a simple box divided into two boxes. This is an atrium embedded in a larger building, and the atrium has exterior, interzone, and ground contact surfaces.

There are AirflowNetwork openings and cracks. Even with no venting, there is still infiltration flow through the cracks which will behave differently for two stacked zones than for one taller zone. Turn off AFN "NoMultizoneOrDistribution". Still different loads.
A comparison of the two files showed a slight different in the per area internal gains for the upper atrium zone vs the lower. Remove People, Lights, and ElectricEquipment. Still different loads.
The solar distribution is the defaul "FullExterior" which puts all incoming direct (beam) solar onto the floor. The original floor construction (Surface 21) interior material layer has a solar absorptance of 0.8, while the floor added to split the two zones has a solar absorptance of 0.5. Just to make things simple, changed all of the materials in the entire model to 0.9. Still different loads.
The bottom floor (Surface 21) has an outside boundary condition of "Ground". There is no Site:GroundTemperature:BuildingSurface object, so the default temperatures of 18C are applied to the outside of this surface. So, this surface has a very different temperature dynamic that the rest of the surfaces. Changed this to be adiabatic. With this (and all of the above changes), the loads are now 5.17 for the single-zone case, and 5.37 for the two-zone case.

So, thinking back to the original models, the key driver is all of the incoming direct solar hitting the floor which has an outside temperature of 18C, so it sheds lots of the solar gains. The two-zone model takes the upper atrium solar and puts it on an ... (more)

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Yes, I think this is a good way to show all the Zone Thermal Load. And after dividing, Zone Air Heat Balance Surface Convection Rate become larger.

However, I found that the after dividing, the Surface Inside Face Solar Radiation Heat Gain Rate decreases.(I sum all values of the surfaces in the zone) I think this is not reasonable, since the convection rate increases...just like lower input but getting higher output.. Do you know the reason? Maybe I will add a new question...