EnergyPlus low temperature radiant system

asked 2016-08-26 06:22:08 -0600

antifx
495 ●6

updated 2017-05-08 18:34:53 -0600

__AmirRoth__

4436 ●5 ●17 http://bleedinggreenna...

Hey!

I have a question regarding energyPlus calculation methods: I want to simulate a building with a low-temp radiant element in a concrete core slab (m_flow and temperature are given by measured data). Since I get wrong results (the surface temperature is to high compared to measured data and results from other simulation programs) , I want to argue the reasons for these differences. I got some hints from a colleague but I want to ask you a) if that hints are plausible and b) if you probably know additional reasons:

1)EnergyPlus calculates the sub-models “low-temp-rad element”, “wall” and “zone” separately and successive + that is a problem when it comes to low-temp-read systems: Does that mean that there is now data exchange/feedback during the simulation?

2) The CTF method in combination with a massive concrete slab + low-temp radiant element could cause deviations since the CTF method is not suitable for walls with high capacities?

3) EnergyPlus may or may not achieve the given low-temp-radiant inputs (m_flow, temperature, etc.)

Thank you very much for your help!

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Are you hard-sizing or auto-sizing? If you're auto-sizing, what parameters are you using for design days?

Adam Hilton ( 2016-08-26 09:33:47 -0600 )edit

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answered 2016-08-31 10:43:40 -0600

Annie Marston
4357 ●6 ●32 http://www.resustain.com

Hi, I have done these studies before, it is important not to autosize anything in the Radiant object, not only that but EnergyPlus thinks that the capacity of your radiant slab should be calculated considering a delta T of 10K from surface to room. Usually the capacity of your radiant slab is provided at around a delta T of 4-5K so you need to increase the capacity in your model to accommodate the delta T 10K programing, in other words the design capacity you put into energyplus is the capacity at a delta T of 10K. The way we check then to make sure our ceilings are reaching the correct surface temperatures is by plotting against capacity of the ceiling. Also you need to make sure your water temperatures going into your ceilings is correct. I have done 4 or 5 different studies on surface temperature of radiant slabs in EnergyPlus and they have matched fairly well once I have hard sized everything and understood that design capacity is at a delta T of 10K.

I hope this is helpful!

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I cannot speak to your experience of success with using an assumption of a 4-5K delta T other than to say that I am glad to hear that you were able to get things to work and were able to match the data in the cases you ran. What I will say though is that there is nothing built into EnergyPlus that makes a 10K delta T assumption. The sizing calculations in EnergyPlus are based on the delta T parameter that the user enters for the sizing of the plant loop in the input file. One can definitely adjust that parameter, but using autosizing with radiant systems requires more than force air systems

RKStrand ( 2016-09-01 07:06:34 -0600 )edit

I think there has been a misunderstanding. What I was saying above was that you should hard size the inputs into the radiant component in EnergyPlus. There is a bit of an oddity when doing this for design capacities. Normally the actual specifications given by manufactures for design capacities are at roughly 4-5K delta T. However, in my experience, the radiant component does not work correctly with these hard design capacities inputs and requires design capacities at a delta T of 10K instead. When I have changed my hard sized capacities to this number the component behaves as expected.

Annie Marston ( 2016-09-01 08:21:33 -0600 )edit

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answered 2016-09-01 07:25:27 -0600

RKStrand
106 ●2

In response to the hints that you have been given, there needs to be some clarifications made as to what the EnergyPlus radiant model is doing.

The notion that there is no feedback between the radiant system, the wall, and the zone is absolutely false. There is feedback. While the radiant system, the wall, and the zone are modeled in different parts of the code, the results of the radiant system simulation is incorporated into the wall and zone calculation automatically via iteration. This is necessary to avoid any heat imbalances in the overall EnergyPlus heat balance based solution. This was one of the many advances over its predecessor programs like BLAST and DOE-2--that EnergyPlus integrated various solution parts. The EnergyPlus radiant model has always been integrated with both the zone and HVAC portions of the simulation.
The CTF method does approach some limits when dealing with very massive elements and other unusual constructions. The question is: how massive is your slab and what is the rest of your construction? Are you getting any warnings about not completing the warm-up period? The CTF approach as well as its extension to include heat sources/sinks present in radiant systems is fairly robust so I'm not sure that is the source of any differences you are seeing.
I completely agree with the comment about making sure that EnergyPlus is actually using the manual inputs you are trying to feed it. This is where I would start--making sure that your flow and water temperature data is matching what EnergyPlus is using. If it isn't, there shouldn't be an expectation that EnergyPlus surface temperatures are in agreement with your data.