SurfaceControl: Movable Insulation vs Construction Properties @ EnergyPlus

Hi everyone.

I am trying to assess the performance of dynamic insulation materials, and to do so I am now using the class list SurfaceControl:MovableInsulation. In this class list, we can add or remove a certain layer of material, on the inside and/or outside of a specific opaque surface. To do so, you have to define a fractional schedule (between 0.0-1.0) that represents a fractional modifier on the thermal resistance of the material layer that you're studying (which allows to change its thermal conductivity) In order to establish a starting point (reference) to the analysis that I have been doing, I started with the case where the insulation layer is set ON. To do so, I thought it would be the same to set the material as a layer on the Construction Class List or to set it Always ON on the SurfaceControl:MovableInsulation Class List. But when I calculated the heating and cooling demand, the values were a bit different:

Heating for the MovableInsulation Classlist: 3.1 MWh
Heating demand, only putting the layer of insulation on the Construction Class List = 1.9 MWh

Does anyone knows the reason for that?

Thank you in advance!

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jthomem

asked 2016-06-20 11:02:18 -0500

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__AmirRoth__

updated 2017-05-03 20:40:20 -0500

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Are you using "Material" or "Material:NoMass"?

Archmage (2016-06-20 12:26:33 -0500) edit

I am using "Material" for the Construction and for the MovableInsulation Classlist

jthomem (2016-06-20 13:10:01 -0500) edit

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I would not expect the two to have the exact same behavior, because the conduction transfer functions will come out quite different. The heat transfer modeling for movable insulation is simplistic and affects the surface heat transfer, sort of like a film coefficient, while still using the one set of CTF coefficients for the main construction under the insulation. But when you include the insulation in the main construction you will get a different CTF and no tricks at the boundary condition. It is hard to speculate why your heating load changed the way it did but the two cases could easily have different impact on the coupling of thermal mass to the zone air. You might get closer by using Material:NoMass in the Construction experiment.

Another way to model dynamic materials is with EMS. You can vary the entire construction used for a surface over time with your own control program -- see the actuator Surface Construction State and the input object EnergyManagementSystem:ConstructionIndexVariable.

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Archmage

answered 2016-06-20 14:57:09 -0500

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From my experience, as Archmage stated, you will have more accurate results with the "movableinsulation" model, in which no mass storage in the insulation layer is modelled, although this could be quite accurate (depending on the density of the insulation considered).

Moreover regarding the use of EMS and the object "EnergyManagementSystem:ConstructionIndexVariable", you can use this if the number of nodes, calculated by the CTF method, in the different constructions you are using remain the same for all the constructions. This is not usually the case if you change the thermal resistance of your construction. Therefore EnergyPlus will give you an error.

Please let me know if you have another experience with the EMS object. Fabio (ff279@cam.ac.uk)