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Re: [EnergyPlus_Support] "Trying to understand "Zone Predicted Sensible Loads...."



Jean,
There is no "fluid volume" issue that I can control for a refrigeration system, although ammonia systems do cool with liquid ammonia in some cases.
Interestingly, refrigeration systems also do not have staging or variable speed capability built into the objects, e.g., compressors, so I have created an approach that seems to be an equivalent to staging.
I might suspect my approach, but the first "stage" is the only active stage 95% of the time.


On Mon, May 4, 2015 at 2:14 AM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Could it be the combination of system fluid volume and the switching between stages in a multi-stage compressor machine? Perhaps increasing the volume can smooth things out. In TrnSys there was always that as a problem. A fluid would pass right through a pipe segment repressenting the fluid volume before the time equivalent to the timestep had elapsed and it would really muck things up. I ended up having "dynamic" volumes depending on the demands of the system to preserve robust convergence.

Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold

On 04.05.2015, at 02:20, "Jim Dirkes jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

Luis and Jean,
  • I was using 6 timesteps per hour.  30 results in zone temps much closer to the setpoint - about 0.1 - 0.4C instead of 0.5 - 1.5.  That reduced unmet load hours to almost nothing except immediately after a defrost cycle.
  • This warehouse has no steps of any kind - -18C all year long, so I can't think of anything else to adjust.
  • I was using an infiltration rate that is typical for commercial offices as a placeholder value until I got the model working properly. Adjusting it lower by 70% did not improve things noticeably.
  • Each timestep requires no more than 2 iterations according to "SimHVAC:HVAC System Solver Iteration Count", so it seems stable in that regard.
  • The "Zone Predicted..." values are still varying dramatically with 30/hr timesteps.  I'll try the "Sensible Heating/Cooling Energy" variable.
Thank you for the feedback.  I am in a much better position now.

On Sat, May 2, 2015 at 3:50 PM, 'jeannieboef@xxxxxxxxx' jeannieboef@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Hi Jim, even when your zone temps are stable, they look pretty unstable to me. Did you try this with smaller timesteps? This "looking ahead" kind of PID control is possibly set up to work well with zones at normal temps, but is having trouble with zones at -18C. It's very hard to write algorithms that are one-size-fits-all for this kind of control. I've had to do this kind of thing in TrnSys and one ends up building kind of dynamically adjusting PID control. I won't be supprised if the code is having problems. Are there any other factors causing step changes that may need to be "ramped" to help with this issue?
Jean

Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)

i. A.
Jean Marais
b.i.g. bechtold

On 01.05.2015, at 21:09, "Jim Dirkes jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

 

Dear forum,
I have used a couple of the "Zone Predicted..." output variables in the past with the assumption that they provide a reasonable indication of the cooling or heating load that is present for a particular timestep.  I'm now working on a refrigerated warehouse that is maintained at -18C and am doubting my previous understanding of what the variable means.


The "Zone Predicted..." definition seems straightforward, but my energy model results don't seem to agree...
The table below shows results for the summer design day afternoon.
  • It's 30C (86F) outdoors
  • The zone temperature is reasonably close to the setpoint (consistently about 1C colder than setpoint)
  • The "Zone Predicted Load..." (in red) shows that most of the time there is a heating load, indicated by a positive value.  That makes some sense because the zone is colder than setpoint and needs some "heating".
  • It makes no sense to me that an adjacent timestep has a cooling load (negative value) while the zone temperature is also colder than setpoint.
  • Will you please help me understand what is going on?

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The I/O Reference definition:

Zone Predicted Sensible Load to Cooling Setpoint Heat Transfer Rate [W]

This is the predicted sensible load in W required to meet the current zone thermostat cooling setpoint. A positive value indicates a heating load, a negative value indicates a cooling load. This is calculated and reported from the Predict step in the Zone Predictor-Corrector module. This value is not multiplied by zone or group multipliers

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--

James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
1631 Acacia Dr, GR, Mi 49504

Direct: 616.450.8653
jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Website l  LinkedIn

The truth is still the truth, even if nobody believes it.  A lie is still a lie, even if everyone believes it.




--

James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
1631 Acacia Dr, GR, Mi 49504

Direct: 616.450.8653
jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Website l  LinkedIn

The truth is still the truth, even if nobody believes it.  A lie is still a lie, even if everyone believes it.




--

James V Dirkes II, PE, BEMP, LEED AP
CEO/President
The Building Performance Team Inc.
1631 Acacia Dr, GR, Mi 49504

Direct: 616.450.8653
jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx

Website l  LinkedIn

The truth is still the truth, even if nobody believes it.  A lie is still a lie, even if everyone believes it.



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Posted by: Jim Dirkes <jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx>


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