RE: [EnergyPlus_Support] Discrepancy in PCM Results

Actually this is similar to the question asked by the user on the reporting of the Outdoor Dry Bulb temperature.

Why is the reported value timestep or hourly values not the same as the original input values.
The hourly value would match only when the two input hourly values does not change.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: nirajpdl@xxxxxxxxx
Date: Thu, 16 Aug 2012 19:30:09 -0400
Subject: Re: [EnergyPlus_Support] Discrepancy in PCM Results

Dear Dr. Li,

Thank you for your response. I do not think that the values after 40 (or 26 according to the earlier question) matter that much as long as the Enthalpy stays the same.

For example:

Temp - 40

Enthalpy -230000

Temp 100

Enthalpy - 230000

is the same as:

Temp - 40

Enthalpy -230000

Temp- 60

Enthalpy-230000

Temp - 100

Enthalpy - 230000

Please correct me if I am wrong. Thank you again for looking into this for me.

Sincerely yours'

Niraj

On Thu, Aug 16, 2012 at 7:25 PM, YuanLu Li <yli006@xxxxxxxxxxx> wrote:

If you plot the two curves, they looked quite different.

The 24° point is not in the first curve. Brent has pointed out that.

The falling off at 40 and 100 could be typing errors.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: nirajpdl@xxxxxxxxx
Date: Thu, 16 Aug 2012 18:32:53 -0400
Subject: Re: [EnergyPlus_Support] Discrepancy in PCM Results

Dear Brent,

Thank you for clarifying this issue and for taking the time to look into this for me.

Sincerely yours'
Niraj

On Thu, Aug 16, 2012 at 6:10 PM, Griffith, Brent <brent.griffith@xxxxxxxx> wrote:

The data are interpolated along a piece-wise linear relationship, it is not a ?staircase? as you describe. Intermediate points that don?t change the linear function are superfluous, but here your (a) and (b) are totally different between 0 and 26C. In the case of (a) there is a gradual increase from 0 to 26C, but in case (b) all the increase occurs between 24 and 26C.

From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Niraj Poudel
Sent: Thursday, August 16, 2012 3:47 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: [EnergyPlus_Support] Discrepancy in PCM Results

Dear all,

I have defined a PCM (26 degree melt, 230 KJ/Kg enthalpy) using the Enthalpy-Temp curve two different ways using the object Materialproperty:phase change and applied it to the interior face of the east wall.

a) Gypsum Board PCM:

Temp - 0
Enthalpy - 0
Temp-26
Enthalpy-230000

Temp-100
  Enthalpy-230000

b) Gypsum Board PCM1

Temp-0
Enthalpy-0
Temp-20

Enthalpy-0
Temp-24
Enthalpy-0
Temp-26

Enthalpy-230000
Temp-28
Enthalpy-230000
Temp-30

Enthalpy-230000
Temp-40
Enthalpy-23000
Temp-100

Enthalpy-23000

As you can see both "hypothetical PCMs" melt at 26 degrees and have the same energy storage capability.But when I use PCM -a versus PCM-b for the same IDF file. I get two different results in heating and cooling energy loads? A long time ago I was told that the enthalpy stays the same until you define a different temp/enthalpy pair.

For instance, if you define:
Temp-0
enthalpy-0

it stays enthalpy-0 even at temp 25. until you add another temp enthalpy pair such as
Temp-26
Enthalpy-230000.

So my question is why am I getting two different results?

I would appreciate any insight regarding this issue.

Sincerely yours'
Niraj

--
Niraj Poudel, Architectural Engineer.

PhD student, PDBE Program.
Clemson University, Clemson, SC.

--
Niraj Poudel, Architectural Engineer.
PhD student, PDBE Program.
Clemson University, Clemson, SC.

Niraj Poudel, Architectural Engineer.

PhD student, PDBE Program.

Clemson University, Clemson, SC.

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