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Re: [EnergyPlus_Support] Detailed Ice Storage system warning



Jeremiah,

I've used your model as debugging practice.
After spending a few hours looking at your model and the outputs, I haven't found anything tangible that might imply the maximum iteration warnings adversely affect model outputs.

Outlet temperatures, cooling charge and discharge energy, are, IMHO, all fine.

The engineering reference is being rather vague, simply stating "Use To,new to calculate a new LMTD* and iterate until To converges to some acceptable level".
What defines "an acceptable level" is the question here.

Looking into the source code for E+ 8.1, specifically PlantIceThermalStorage.f90, it uses "MaxIterNum = 100". The convergence is assessed within 0.1°C (It iterates on the outlet temperature, when the new calculated outlet temperature is within 0.1°C of the previous iteration value, it is said to have converged).

In your case, key parameters are freezing temp of tank at 5.5°C, and setpoint of leaving temp at 7.22°C. Looking at the E+ outputs at "detailed" timestep, this is what you are getting when the warning is thrown (above freezing point, outlet of tank between 5.5 and 7.22, blended outlet temp at 7.22, still 28% of storage capacity in the tank).

By the way, if you run your simulation at 60 timestep per hour, you'll divide the number of warnings by more than 2 (13000 v 29000), while the energy consumption varies by 3%.

Also, the charge and discharge curves are in the heart of the iteration, so I suggest you update them with real values (and not the ICE values) and run the model to see how it changes the number of warnings. (I'm interested in what it actually does, keep me posted please).


Anyway, I hope this somewhat helps.
If something is inherently wrong, it's clearly outside my capacities.

Julien

--
Julien Marrec, EBCP, BPI MFBA
Energy&Sustainability Engineer
T: +33 6 95 14 42 13

LinkedIn (en) : www.linkedin.com/in/julienmarrec
LinkedIn (fr) : www.linkedin.com/in/julienmarrec/fr


2014-08-08 23:06 GMT+02:00 Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:
 

Julien,

The file titled Q5 is the PCM latent energy storage system and the one titled Q0 is the ICE latent energy storage system and a no storage for reference.  There is also a file called LESS_OBJ.idf that has the shared and different inputs used between ICE and PCM.. Also attached is a png of the general load strategy we are using and charts of chiller demand and latent energy storage system charge and discharge rates.  The inputs seem as reasonable as I can think of, and outputs are as expected.  I think it is working correctly, with the exception of the warning I pasted into my previous post, and have pasted below. 

We can make our PCM to transition at any temperature we want, and also could put the LESS system at another point in the loop, or could control it differently than the current setup. I have considered changing the charge and discharge curves to be more similar to DSC test results, but I do not think it would make much of a difference so have not yet done so.  

Suggestions on how I might change the model to not produce this warning / instability are really appreciated. 
Warnings pasted below and input files attached--->Thanks  

   **   ~~~   **  Environment=ANNUAL, at Simulation time=01/03 17:30 - 17:40
   ** Warning ** Detailed Ice Storage model exceeded its internal discharging maximum iteration limit
   **   ~~~   ** Detailed Ice Storage System Name = LESS TANK
   **   ~~~   **  Environment=ANNUAL, at Simulation time=01/03 17:30 - 17:40

   ************* ===== Recurring Error Summary =====
   ************* The following recurring error messages occurred.
   *************
   *************  ** Warning ** Detailed Ice Storage system [LESS TANK]  discharging maximum iteration limit exceeded occurrence continues.
   *************  **   ~~~   **   This error occurred 29163 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.





â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 
â??â??
120 E. Pritchard St.  | Asheboro, NC 27203 
â??â??
 | Mobile 503-688-8951
  





On Fri, Aug 8, 2014 at 1:28 PM, Julien Marrec julien.marrec@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
 

Jeremiah,

Could you share your input file? (I'm interested in seeing how ice storage is modeled as I'll have to do it soon)

Otherwise, when in detailed warning output mode, what does it say? 

Maximum iteration, I think, means it didn't converge, but that doesn't mean an "acceptable" (per real life) solution hasn't been found, just that it wasn't within the boundaries you set (could be crazy, could be just outside the boundary)

I had the same problem with a complicated data center model but the results made sense.

Best,
Julien

Envoyé de mon iPhone

Le 8 août 2014 à 21:06, "Jeremiah Crossett jcrossett@xxxxxxxxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> a écrit :

 

Dear List,

I am modeling an existing building with a 20 ton chiller and 82 ton hour latent energy storage system that uses our bio based PCM as a replacement for ice. We expect that using a higher melting temperature will save energy consumption compared to ice and enable retrofit scenarios that would not be possible with the sub zero supply temperatures.  To model this I have set the transition temperature to 5.5C, and nighttime charge temperature from chiller supply loop at 2.2C.  
For comparison I have also made the same model with ICE.  I used the charge and discharge curves from ICE in the PCM storage tank model.  

The problem I am hopeful you could help me to better understand is the recurring warning pasted below.   I think it is saying that I have caused model instabilities. 
I am wondering if anyone has any suggestions for how I might change the model to not produce this warning / instability? --->Thanks  

   **   ~~~   **  Environment=ANNUAL, at Simulation time=01/03 17:30 - 17:40
   ** Warning ** Detailed Ice Storage model exceeded its internal discharging maximum iteration limit
   **   ~~~   ** Detailed Ice Storage System Name = LESS TANK
   **   ~~~   **  Environment=ANNUAL, at Simulation time=01/03 17:30 - 17:40

   ************* ===== Recurring Error Summary =====
   ************* The following recurring error messages occurred.
   *************
   *************  ** Warning ** Detailed Ice Storage system [LESS TANK]  discharging maximum iteration limit exceeded occurrence continues.
   *************  **   ~~~   **   This error occurred 29163 total times;
   *************  **   ~~~   **   during Warmup 0 times;
   *************  **   ~~~   **   during Sizing 0 times.


â??â??
Jeremiah D. Crossett  | Senior Analyst  | LEED Green Associate 
â??â??
120 E. Pritchard St.  | Asheboro, NC 27203 
â??â??
 | Mobile 503-688-8951
  



<Q0_82TnHrLESS_20TonChillerUpstream.idf>
<Q5_82TnHrLESS_20TonChillerUpstream.idf>





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Posted by: Julien Marrec <julien.marrec@xxxxxxxxx>


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