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Specific heat of water in EMS

asked 2015-10-16 08:24:19 -0500

updated 2015-11-07 13:43:23 -0500

I'm trying to use the specific heat of water in an EMS program.

There are two built-in Erl functions, @CpCW (Cp for Chilled water) and @CpHW (Cp for Hot water) that accept an input parameter for temperature.

Except that as far as I understand from the source code here, both functions are exactly the same: they ignore the temperature passed to it and simply return 4180 J/kg-1.k-1. (I've tried using it, it's a constant 4180 indeed).

Am I missing something or is there just no function to get the specific heat at different temperatures?

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answered 2015-10-16 11:24:25 -0500

Archmage's avatar

updated 2015-10-21 08:01:02 -0500

Your are not missing anything. Use of those silly constant functions has been mostly deprecated elsewhere in EnergyPlus and replaced with calls to GetSpecificHeatGlycol (where 0% glycol is water). EMS should really be extended to expose that, and also GetDensityGlycol.

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Thanks! So there's no way to access GetSpecificHeatGlycol in EMS yet then, too bad.

Julien Marrec's avatar Julien Marrec  ( 2015-10-21 08:00:42 -0500 )edit

Yes, the problem is that there are no string data types in EMS and those calls have arguments for the name of the plant loop. I am thinking it might be better to add the specific heat to the node data and make an output variable for that. So it would switch from a function to a sensor.

Archmage's avatar Archmage  ( 2015-10-21 08:24:51 -0500 )edit

Completely fine with using a sensor instead of a function. Even simpler, I was trying to use a sensor on temperature and pass that to the function anyway.

(Side question: if you add a potential output variable to E+, it's not slowing down the program is it? It's just calculated if you request isn't it?)

Julien Marrec's avatar Julien Marrec  ( 2015-10-21 08:31:37 -0500 )edit

In this case no, it should not impact speed. There already are some other types (dewpoint, RH, wetbulb) of node data reports that are only calculated when requested and this would likely be done in the same manner (in CalcMoreNodeInfo).

Archmage's avatar Archmage  ( 2015-10-21 09:17:34 -0500 )edit

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Asked: 2015-10-16 08:24:19 -0500

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Last updated: Oct 21 '15