[EnergyPlus_Support] How is a radiant system (such as concrete core activation) physically modelled?

["a_tzoulis" <a_tzoulis@xxxxxxxxx>]

Hello,

I am trying to model a low temperature radiant floor in EnergyPlus. Ihave read the documentation of EnergyPlus (engineering reference p.1022 HeatExchanger Formulation for Hydronic Systems â?" Radiant System Models) about how a lowtemperature radiant system is modelled. What I found relatively inadequate arethe assumptions that are taken (lower part of p.1022).

I also have usedTRNSYS in the past to model similar systems and have read the documentationwhich presents a more scientifically approved, in my opinion, method tosimulate these systems. What is used in TRNSYS and the documentation is totallybased on a paper written by Markus Koschenz and Beat Lehmann and is titled ¡¢ThermoaktiveBauteilsysteme tabs¢¢(July 2000).

I will make a very small overview of the paper but it would be good to have a look at it in order to understand what this is actually about. You can also read it from the TRNSYS 17 Documentation ¡¢Volume 5 Multizone Building modeling with Type56 and TRNBuild¢¢ Chapter 5.4.4.

According toKoschenz, Lehmann, there are 4 resistors (imagine these as processes duringwhich water loses part of its thermal energy due to different reasons) whichmodel the transfer of water from the boiler (or better form the node before itenters the tubing of the thermally activated layer) to the inside of the tubes.

What is written in Italics is part of the EnergyPlus Documentation (engineering reference p.1022 HeatExchanger Formulation for Hydronic Systems â?" Radiant System Models).

• Several assumptions will be incorporated into the heat exchanger analysis. It is assumed that the building element that contains the hydronic loop is stationary and that its temperature along the length of the tubing is constant (The latter part of this assumption stems from assumptions made in both the one and two dimensional heat source transfer function derivations. In either case, the source was added at a single node that was characterized by a single temperature. For consistency, this assumption must be made again in the heat exchanger analysis.).

According toKoschenz, Lehmann, the resistance Rz represents the temperature drop of thefluid as it flows through the tubing in the wall.

• The temperature at the inside surface of the water tubing is assumed to be equal to the temperature at the source location.In other words, it is assumed that the water tubing itself has no appreciable effect on the heat transfer process being modeled.

I do not understand what the source is. Is it a boiler or is it the location of the central axis of the tubing. If the source is the boiler (or any equipment which heats or cools the water), there is only the heat flux taken into account due toforced convection between the central axis of the tube (where water flows) and the inner part of the tube. This resistor is called Rw according to Koschenz,Lehmann and has the smallest value (10 times smaller than each one of the other resistors).

 

• The resistor Rr represents the temperature difference over the tube casing. This resistance is calculated using the diameter of thetube, the thickness of the casing, coefficient of thermal conductivity of the pipe material and distance between the tubes.

• Moreover, there is another resistor called Rx. The temperature drop across the resistor Rx is related to the "extra" resistance as a result of the distribution of heat around and between the tubes. The value is only dependent on the geometry of the system and not on the flow rate of the fluid.

To sum up, if I am not mistaken, the only resistance that EnergyPlus takes into account is Rw (the smaller one) and the others are assumed to be zero. This leads to the fact that the radiant system models produced by EnergyPlus will be relatively enhanced and not very correct or accurate.

Is there anybody specialized in this field to give me some answers or is there a way to incorporate the resistances missing myself, into the model?

Moreover, is it possible to forward this EP-model to OpenStudio 0.11.0?

 

I can imagine that this thing may not have a very big impact on a whole energy simulation but I believe it is something important that has to be mentioned.

 

Any assistance would be much appreciated

Thanks in advance

 

__._,_.___

Primary EnergyPlus support is found at:
http://energyplus.helpserve.com or send a message to energyplus-support@xxxxxxxx

The primary EnergyPlus web site is found at:
http://www.energyplus.gov

The group web site is:
http://groups.yahoo.com/group/EnergyPlus_Support/

Attachments are currently allowed but be mindful that not everyone has a high speed connection.  Limit attachments to small files.

EnergyPlus Documentation is searchable.  Open EPlusMainMenu.pdf under the Documentation link and press the "search" button.

Your email settings: Individual Email|Traditional
Change settings via the Web (Yahoo! ID required)
Change settings via email: Switch delivery to Daily Digest | Switch to Fully Featured
Visit Your Group | Yahoo! Groups Terms of Use | Unsubscribe

__,_._,___