Switch positions of two construction materials

asked 2020-03-22 13:12:07 -0500

Sultan K.
31 ●2

updated 2020-03-23 13:16:20 -0500

Problem: I am using EnergyPlus's Energy Management System (EMS) to model dynamic phase change material (PCM). Dynamic PCM, illustrated in the attached figures, is a system consisting of two layers – one containing PCM (has a high latent heat, stores energy by undergoing phase change) and the other is some common material. Once a day, PCM layer is moved to face the outer side of the wall, and later, back to the inner side. The movement is considered to be instantaneous for simulation, so we can think of it as two materials switching their positions, while keeping their temperature, enthalpy. My task is to model this switch.

I have tried switching material properties, but later discovered that this is not correct since temperature and enthalpy data are not moved together with other properties.

Question: How to switch positions of two construction materials keeping their current state (temperature, enthalpy)? Is it possible to model this system in EnergyPlus? I went through the InputOutputReference document, but could not find anything related to this.

Once again, any advice or comments will be much appreciated. Thank you very much.!

Dynamic PCM #1 [adapted from de Gracia (2019)] Dynamic PCM #2 [adapted from de Gracia (2019)]

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answered 2020-03-22 16:03:33 -0500

Aaron Boranian

13830 ●76 ●24 http://bigladdersoftware.com/

The EMS feature of EnergyPlus allows you to override many model features, but not ALL model features. This is because the list of EMS Actuators for overriding model behavior is based on a pre-defined list by the EnergyPlus development team. For example, you cannot use an actuator to override or reset the temperature at a specific node in an HVAC system. For the actuators related to thermal envelope, there are none that allow you to override information for a construction layer's temperature or enthalpy. Sorry, but it looks like you won't be able to accurately model this kind of dynamic PCM in one EnergyPlus simulation.

As an alternative, you could run two simulations -- one with the PCM always "outside" of the insulation layer (closer to outside air) and one with the PCM layer always "inside" of the insulation layer (closer to the zone). This would also be useful to have these two cases to compare results to what you originally proposed (applying EMS to move the PCM layers dynamically). Maybe if you use a small enough timestep, or if the amount of PCM is small enough to not store lots of heat, then the results between all three cases will be similar enough to overlook the riskiness of applying EMS to move the PCM layer dynamically.

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answered 2020-03-23 09:52:53 -0500

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

EMS cannot change the order of materials in a construction, but (I am pretty sure) it can change the construction that is assigned to a surface. You should be able to define two constructions, one with the PCM on the inside and one with the PCM on the outside, and use EMS to switch between them.

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EMS can change construction assemblies, so you can switch between a construction with PCM on the inside and another construction with PCM on the outside. The issue is that when you do that, the thermal data (temperature, enthalpy,etc.) stays with the layer order. For example, if one construction is PCM then insulation, then switches to another construction of insulation then PCM, the thermal data of the PCM layer gets applied to the insulation layer. They want to move that thermal data along with the PCM layer upon switching construction assemblies. That is something EMS cannot do.