Timestep dependent air capacity used to calculate predicted load

[xuanluo113]

Issue overview

In Carbon Dioxide Predictor-Corrector model, Air Capacity (carbon dioxide storage term in zone air [kg/s]) is calculated as ZoneVolume * CpAir * RhoAir / (second in a system time step), and is time step duration dependent. The term in the equation should be a continuous derivative but is simplified as a discrete value in calculation using the time step in EnergyPlus. This is causing a large discrepancy in the predicted system load calculation using different numbers of time steps in an hour with the delta_t varies.

When zone volume is large, the air energy storage term is a large portion of the predicted system load, and the load calculation is less accurate when using longer time steps (e.g. 1 hour). For example, for RefBldgWarehouseNew_Chicago.idf model, the design day cooling load shows a 3X difference when using 1 hour and 10 min time steps for the BulkStorage zone as the below figures show.

Details

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• Platform (Operating system, version)
• Version of EnergyPlus (if using an intermediate build, include SHA)
• Unmethours link or helpdesk ticket number

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• Defect file added (list location of defect file here)
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[mjwitte]

I'm confused. The description talks about air capacity in the CO2 predictor code, but the graphs show "load to cooling setpoint"? Is the zone cooling setpoint being met? If the zone is not holding at setpoint, any deficiency will carry into the next timestep. Any time there's a need to recover, the predicted load will be higher with a shorter timestep, because the thermostat always wants to reach setpoint within a single timestep. That said, it's possible this is being double-counted in some way. Do you see the same behavior when ZoneAirContaminantBalance is off?

[xuanluo113]

@mjwitte It may be a bit harder to walk through the code together here, so I've requested 15 min of time at the next Technicality call (next Wednesday) and will present more details about it. Hope we can meet and discuss then.