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Modeling EvapCooled VRF with Evaporative Condenser Effectiveness

asked 2015-07-06 18:49:50 -0500

pflaumingo gravatar image

updated 2015-07-12 10:16:49 -0500

I've been modeling a building in OpenStudio with air-cooled VRF and a DOAS for ventilation requirements. I've used the LG BCL components and have an air-cooled system working without a problem, however, when I take the IDF and set the condenser type to EvapCooled and rerun it I see no change in performance. The documentation shows the below formula for how the air temperature on the condensing unit is adjusted, are there additional changes that need to be made to observe a change? Or is it something to do with using the Table:MultiVariableLookup object rather than a curve? I have specified an EvapCondEffectiveness of 0.9. I have also let most components autosize rather than using the specified manufacturer capacity as I'm just trying to get a rough estimate of the performance relationships.

Tcondinlet=(Twb,o)+(1−EvapCondEffectiveness)(Tdb,o−Twb,o)

I created the original model using OpenStudio 1.7.0 and I am using the latest version of EnergyPlus 8.2 to run this.

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I believe this should work. I'll have to test to be sure.

rraustad gravatar imagerraustad ( 2015-07-23 09:53:40 -0500 )edit

@rraustad, I've tried it a couple of times now, but I see no effect when I run it using the set up described above.

pflaumingo gravatar imagepflaumingo ( 2015-07-23 10:15:58 -0500 )edit

Are you able to provide the performance parameters that did not change? I would only expect the condenser variables to show differences.

rraustad gravatar imagerraustad ( 2015-07-25 00:50:35 -0500 )edit

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answered 2015-07-23 22:19:09 -0500

updated 2015-07-23 22:21:16 -0500

I do see differences when modeling the evap-cooled VRF system. I started with the example file VariableRefrigerantFlow_5Zone.idf and ran the simulation. Then I modified that file to model an evaporatively-cooled VRF condenser. The changes made are shown below.

AirConditioner:VariableRefrigerantFlow,
    VRF Heat Pump,           !- Heat Pump Name
 ...snip...
    EvaporativelyCooled,     !- Condenser Type
 ...snip...
    0.9,                     !- Evaporative Condenser Effectiveness {dimensionless}
    autosize,                !- Evaporative Condenser Air Flow Rate {m3/s}
    20,                      !- Evaporative Condenser Pump Rated Power Consumption {W}

The differences I see are related to the available cooling capacity and associated condenser power which I would expect. The available cooling capacity should increase while the operating power should decrease, both a result of cooler temperatures entering the condenser. Also, this change would not require new performance curves since the only change modeled here is slightly cooler temperatures entering the condenser, which the performance curves should model. The terminal units have the same load, so I would not expect that equipment to change much (I didn't check that data). Here is a comparison of the results:

image description

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answered 2015-07-08 05:04:45 -0500

Jim Dirkes gravatar image

A couple of thoughts: What climate / city are you modeling? Is it dry enough to make a difference in the condenser inlet temperature? Have you reported the condenser inlet temperature to see how it differs from outdoor dry bulb temperature?

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Jim, I am modeling in San Jose but I have tried it in a few climates and for each run there is zero change - not minimal, but actually zero change. Looking at the condenser inlet temperature, the temperature is identical to the site drybulb temperature every hour suggestive that the condenser inlet temperature adjustment is not being implemented at run time.

@Kyle Benne, do you know whether this may be an issue with using the Table:MultiVariableLookup object? Or is there perhaps something else going on?

pflaumingo gravatar imagepflaumingo ( 2015-07-08 11:43:50 -0500 )edit

That (zero change) is certainly not right. I have not modeled an evap condenser for VRF, but it seems that it should be "plug and play". You probably should send a ticket to the E+ Help Desk and include your IDF so they can review it.

Jim Dirkes gravatar imageJim Dirkes ( 2015-07-08 12:23:29 -0500 )edit

I won't disagree. But before placing blame on E+, can you check the idf file and make sure that OS is putting the correct parameters in there?

Kyle Benne gravatar imageKyle Benne ( 2015-07-08 13:03:30 -0500 )edit

I started in OpenStudio and ran it as an aircooled VRF system, which worked fine. I then took the IDF and adjusted the condenser type to EvaporativelyCooled/EvapCooled (not sure which is the correct input, but I have tried both). That's the only change I made along with setting the evaporative condenser effectiveness. Is there something else that needs setting? It's the first time I've tried it and it's really just a curiosity case as I don't believe anyone manufacturers an evaporatively cooled VRF condensing unit.

pflaumingo gravatar imagepflaumingo ( 2015-07-08 13:22:53 -0500 )edit

Seems like this could reasonably be elevated to an EnergyPlus issue. I will say that changing this property without building a new performance map is probably inaccurate. @hongtz do you have any input?

Kyle Benne gravatar imageKyle Benne ( 2015-07-08 13:43:13 -0500 )edit
1

answered 2015-09-01 22:11:46 -0500

updated 2015-09-01 22:17:26 -0500

@pflaumingo I can confirm that I've been seeing similar results. when I change my VRF systems to EvaporativelyCooled I see a penalty for fan, cooling and heating electric energy over the year.

I have also been experiencing similar results with the water cooled variation of VRF systems in E+. The simulation is occurring in Atlanta and I'm controlling the condenser loop to float between 40F and 90F using a cooling tower and boiler.

@rraustad I believe the file you performed your test on did not have heat recovery enabled. Correct? Have you performed tests on a VRF system with heat recovery enabled? I have heat recovery enabled for my tests.

My suspicion is that E+ is penalizing moderate condenser temperatures when heat recovery is enabled for some reason. This is a counter intuitive result based on my understanding of how VRF systems work.

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Comments

The example file is air-cooled with no heat recovery. I just changed to evap-cooled. VRF does get penalized when using heat recovery, at all temperatures. See Eng. Ref. Vol 3, Figure 247. Laboratory test identifies performance changes during transition period.

AirConditioner:VariableRefrigerantFlow,
  No,                      !- Heat Pump Waste Heat Recovery
  AirCooled,               !- Condenser Type
rraustad gravatar imagerraustad ( 2015-09-02 07:29:13 -0500 )edit

but a heat recovery VRF system shouldn't be penalized when going from air cooled to water cooled. Right?

Lincoln gravatar imageLincoln ( 2015-09-02 08:53:18 -0500 )edit

I would tend to agree with that statement. This issue (evap-cooled power penalty) would have to be investigated further.

rraustad gravatar imagerraustad ( 2015-09-02 10:52:27 -0500 )edit

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Asked: 2015-07-06 18:49:50 -0500

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Last updated: Sep 01 '15