eQuest VRF question

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Is it possible to model a water-source VRF system? Typically these systems have a source/sink water loop similar to a water-source heat pump loop with a boiler and fluid cooler. I did RTFM and it appears that only an air cooled condenser is available for the VRF system at this time.

In the past, I used the water-source heat pump system to approximate a water-source VRF system. It looks like I will have to use that strategy with DOE2.3.

Christopher R. Jones, P.Eng.
Senior Building Performance Specialist
Sustainability & Energy

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Jones, Christopher2's picture
Joined: 2017-10-12
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Christopher,

I was hoping someone else would chime in on this since it's the question I'm asking as well! Working with Mitsubishi a few months back, I got the following:

"In eQUEST, you can model water-source VRF as a standard WSHP and then choose the option for staged or multi speed compressor to approximate the performance of an inverter compressor. Using standard WSHP definitions in eQUEST will not be far off from water-source VRF operation since water-source equipment isn't subject to the capacity de-rates air-source VRF is subject to, so essentially the variable speed nature of the compressor is the only thing that is missing to properly model water-source VRF.

Additionally, in eQUEST the EIR (energy input ratio - described in the first attachment) has an overwhelming impact on the resulting energy performance - much more so than the capacity and part-load curves. Since the EIR is calculated and entered for each specific condenser being modeled, this should reflect water-source VRF operation with a reasonable degree of accuracy.

We could update the attached white paper and associated eQUEST curves to include water-source VRF, however, most open source energy modeling is now trending toward Energy Plus and the various interfaces that run this engine like OpenStudio."

Talking to Doug Maddox who helped develop the air source VRF in DOE 2.3, he said he thought that approach is reasonable.

Please let me know if you find anything else, I've got a big water source VRF model in my near future!

Best,

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Building Efficiency Engineer | TBCx

E-mail: msg at tbcxinc.com
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Thank you Matt,
I have used the WSHP work-around a number of times with reasonable results. At the time, VRF curves weren't available. I have done models with more detailed information using DOE2.1e with more reasonable results. I would be grateful for a copy of your white paper (the attachment didn't arrive) for an upcoming eQuest model.

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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Christopher,

Unfortunately I didn't get the white paper either (that text was copied from a chain within Mitsubishi). What Mitsubishi did provide is attached however and I included my copy of a Mitsubishi methodology in eQUEST.

Hope this helps!

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Mobile: 801-907-5654

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I can assert for the group my core understanding: water-sourced VRF's are "just" very efficient water-source heatpumps. The only substantial mechanical difference (from an energy model perspective) are

1. The variable speed refrigerant compressor - which can be approximated from the library defaults simply with better unitary cooling efficiency (EIR) inputs and/or revised curves
2. The potential for intra-zonal heat recovery between refrigeration circuits. To my experience with water-source VRF, this is not commonly sought after in the system design / performance specifications, however this savings potential can still be approximated leveraging custom hourly outputs.

~Nick

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Nick Caton, P.E., BEMP
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Schneider Electric

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Christopher and Matt,
If either of you model water-cooled VRF in eQUEST/DOE-2.3 using the WSHP work-around, kindly share your experiences with the list. In particular, if you can also run an alternative system using the new air-cooled VRF capability, you can compare the water-cooled VRF workaround to the built-in air-cooled VRF and see if the results make sense. This shouldn't be too difficult as both WSHP and VRF can be modeled with PVVT systems.

All,
I'm not sure which white paper is being referred to by Matt/Mitsubishi below. It may be the Application Note 2023 that Matt shared yesterday (3/18), which is only slightly modified from an earlier version that I had dated February 2014. The revised Application Note (October 2017) is still based on DOE-2.2 and makes no acknowledgement of the new capabilities in DOE-2.3. Mitsubishi has several white papers for VRF available here:
https://www.mitsubishipro.com/white-papers
Seventhwave did an M&V study of two cold-climate water-source VRF systems, where they modeled water-to-air heat pumps using PVVT systems in eQUEST/DOE-2.2. You may find their report helpful:
https://www.seventhwave.org/publications/water-source-vrf-report

Regards,
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
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Thanks Bill,

That's an interesting idea, running it as air-source VRF and the water-source work-around for a comparison. Is there a report in DOE 2.3 that shows the amount of heat trading the air-source VRF system is calculating? I've done an exceptional calculation in DOE 2.2 for the USGBC presenting a 5% heating/cooling load trading as energy savings and has it be accepted, but I'd rather lean on a calculation if that's available!

[TBCx]

Matt Garlick, CxA, CEM, BEMP
Mobile: 801-907-5654

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Hi Bill,
I asked for some practice time to model the water-source system first using the DOE2.3 VRF inputs including calculating the loss factors from the piping and fitting lengths. Run the VRF with no losses then with the calculated losses. That information would be used to adjust the water-source performance efficiency.

I am trying to get metered data for a water-source VRF project so I can compare with my workaround model.

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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For the WSHP workaround, I wonder if the heat pump loop pump power and control can be used to help match the part-load performance of VRF compressors. Maybe the pump power should be treated analogously to the friction losses in the refrigerant piping? The EIR for each system would be decreased slightly based on the value entered for pump power. (E.g. if the loop pump power is 10 kW and there are 20 systems attached to the loop, the EIR would be reduced by the equivalent of 500W.) Also, you could create a separate WLHP loop for each VRF outdoor unit that it represents.

~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

[Pathfinder-EA-logo-2]T: (585) 698-1956 F: (585) 325-6005
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Bill,
We were just discussing that approach. I believe it would be valid.

Christopher R Jones, P.Eng.
T+ 1 416-644-0252

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Matt,
There are a few dozen variables for the new CONDENSING-UNIT command for which custom hourly reports can be generated. I don't see one specific to heat recovery but maybe you could report the hourly cooling load and hourly heating load for a VRF-HEAT-PUMP type and then again for the VRF-HEAT-RCVR type and compare the two.
~Bill

William Bishop, PE, BEMP, BEAP, CEM, LEED AP
Senior Energy Engineer

[Pathfinder-EA-logo-2]T: (585) 698-1956 F: (585) 325-6005
bbishop at pathfinder-ea.com www.pathfinder-ea.com
134 South Fitzhugh Street
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