Are there two different Timesteps in EnergyPlus?

asked 2015-10-12 12:29:18 -0600

397 ●3 ●9

updated 2015-11-03 14:34:36 -0600

Hello,

I am wondering if there are two different time steps (one for Plant i.e. HVAC and the other for the Zone) in EnergyPlus? if not is there a response time for the Plant to start Circulating conditioned air? if there is a response time How can I control it?

Any help will be greatly appreciated.

Sincerely,

Nadish Anand

Update:

Hello Amir and Eric, I am referring to the Following Data Plot:

Zone Air Temperature-1 min. Timestep

Zone Air Temperature Detailed

This is the plot of the Zone Air Temperature Data I have obtained from my EnergyPlus Simulation. I have an EMS Code that Controls the HVAC Operation with a Offset Temperature from the Heating and Cooling Setpoints. The offset Temperature here is 0.6 deg.C and the Heating Setpoint is 21.1 deg.C. Now as you can see from the plot the HVAC system is not starting operation as soon as (21.1-0.6) = 20.5 deg.C is hit by the Zone Air temperature. It takes a while (1-3 mins.) to start operation. I am wondering why this behavior is experienced and hence my earlier question.

Thank you once again.

Nadish

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Comments

Troubleshooting this might be easier if you had the simulation report the HVAC system heating energy at 'detailed' reporting frequency.

ericringold ( 2015-10-12 22:13:03 -0600 )edit

Hello Eric,

Thank you for your answer previously and the comment. Yes the Reporting frequency is 'detailed', and my timestep is 1 minute, I am not sure why the HVAC turns on After 2-3 minutes of hitting the Setpoint-Toffset in heating Operation.

Nadish

nadish21 ( 2015-10-12 22:32:19 -0600 )edit

If you are using 1 minute zone timestep, then there will not be any system timesteps because the shortest is 1 minute and the zone timestep is already there. It is usual for such EMS control to be lagged by one timestep because the sensors need to be updated before the control can take effect. But behavior varys depending on the EMS calling point that is used.

Archmage ( 2015-10-13 06:08:31 -0600 )edit

Hello Archmage,

First of all thank you for the comment. I appreciate your time. Secondly, I am using "BeginTimestepBeforePredictor" Calling point for my Program Calling Manager. Is this creating the lag? Or do I need to modify my sensors and actuators to stop the lag?

Nadish

nadish21 ( 2015-10-13 09:34:37 -0600 )edit

The two calling points you might try to decrease the lag are EndOfZoneTimestepAfterZoneReporting and InsideHVACSystemIterationLoop. However it is unlikely that you can eliminate all lag. There isn't really anything that can be done with sensors and actuators.

Archmage ( 2015-10-13 10:56:20 -0600 )edit

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answered 2015-10-12 12:37:47 -0600

ericringold

10637 ●32 ●8 https://www.nrel.gov/b...

updated 2015-10-12 12:43:18 -0600

Here are a couple sections of the Engineering Reference that should help:

Basically, the system simulation uses an adaptive time step to update the contribution of the air system to the zone loads.

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Comments

Hello Mr. Ringold,

Thank you for your Answer. It was really helpful. I however have one more followup question. Is there a sub 1 minute level timestep in EnergyPlus. The simulation that I am Running uses a 23 kW heater. In 60 seconds (1 min.) it should add around 1.38 million Joules to the system. However in some timesteps when the heater is turning On/Off it is adding Heat Below this value. Which leads me to believe that there are certain calculations which are done below the 1 minute level. If it were a minute, the heat addition for that timestep should be 1.38 Million Joules.

Nadish

nadish21 ( 2015-10-29 22:38:52 -0600 )edit

You might want to look at the run time fraction heater. What I have noticed is that most components in energy-plus run to meet the load. So, if the load is 50% of the max heater output, the heater will run only 50% of the time (even if the timestep is 1 minute). There is no minimum cycle time that can be set (that I know of).