Re: [TRNSYS-users] type 660 simple building model

[David BRADLEY <d.bradley@tess-inc.com>]

Thibaut,
  You are correct; the assumption that underlies the lumped capacitance approach is that the entire building (envelope and air) is at a single temperature. Moving the setpoint requires the HVAC system to move the temperature of the air and the structure. I have seen three workarounds to this issue.

  One is to simply limit the heating and cooling capacity of the system so that the air and structure warm up at a realisitc rate. You need to use the "temperature level control" version of Type660 and you need to find the peak iteratively instead of letting the model calculate it for you.

  The second is to model the building as two lumped capacitances, one for the air and the other for the shell (Type953 is available in the TESS Loads and Structures library). Unfortunately, this gets farther away from allowing you to set any "physical" values; for instance it is hard to know how to appropriately couple the two capacitances.

  The third is to artificially reduce the capacitance of the building so that most of what you are accounting for is the air (with a little bit for the structure and furnishings).
Best,
 David


On 3/29/2012 09:29, Thibaut VITTE wrote:

Dear Trnsys users,

I'm using type 660 to modelize "quickly" a simple building. Having troubles to analyse the results, I've tested it and compared it with regular type 56.

the building is an office building with high internal loads, large glazed surfaces.
the heated area is 9200m², with an overall loss coefficient of 0.7 W/m²°C, 69 kJ/K.m3.

The annual results seem quite good, since I'm finding diffrences in annual heating/cooling loads less than 10%.

As far as temperatures and free floating building functionning (see picture attached), it's not that good, but I understand that a simple capacitance model is limited for that purpose.

But the problem is for maximum heating/cooling power. I have inputed night set back for cooling and heating. And I think that the whole building capacitance is taken into account when a moving setpoint is applied! Which implies great instantaneous power. ( 6 times greater than the one calculated by type 56.) (see picture). The wole building (air + walls) is heated or cooled when a setpoint changes (as I understand the model), which is not quite the reality..

Am I right? Is there a simple model that can calculate approximately the instantaneous power with moving setpoint (in Trnsys type 12, setpoints are only parameters)?

Thanks in advance.

Thibaut

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