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VRF air cooled system compared to water cooled VRF system

asked 2016-01-08 06:29:57 -0600

antonszilasi gravatar image

updated 2016-01-08 06:30:44 -0600

I am modelling both a VRF water cooled system and a VRF air cooled system. I had a number of issues with the water cooled system which were resolved here and here .

In a nutshell for the climate I am simulating (NewMark international airport in New Jersey) without a heat source the VRF's plant loop gets too cold in winter and the VRF system has to disable itself. Resulting in a large number of hours were the set point is not met.

This can be resolved by adding a boiler to the VRF's plant loop. Thereby keeping the plant loop above a minimum temperature enabling the VRF to operate throughout the year and meet all the set point hours throughout the year.

The issue now is that initally I expected the VRF water cooled system to be more efficient than the air cooled system. (This is my first time modelling such a system) I've now discovered from this simulation that this is not the case.

For me a number of questions remain about the results and my expectations for this simulation which I would appreciate your help with. Please refer to the summary report screenshots below.

  • Even after optimizing the boiler availabity schedule to turn off for all except the coldest months. The VRF water cooled cooling electricity use is higher than the air cooled cooling electricity use. Does this make sense? Is there something I need to optimize here to improve the performance of the water cooled cooling electricity use vs the air cooled?

  • The air cooled electricity heating use is nearly identical to the water cooled electricity heating use. This makes sense since they are both heat pump systems. However the water cooled system also uses natural gas to fire the boiler in the coldest months to keep the plant loop within a reasonable temperature. What confuses me here is why is the air cooled system able to operate in winter and meet all the setpoints while the water cooled system cannot without a boiler?

  • The water cooled system also requires electricity for fans and heat rejection from the condensor while the air cooled system does not. Does that mean the air cooled system is inherently more efficient?

Water cooled results image description

Air cooled results image description

In brief should I expect a air cooled system to be more efficient in this climate? Or is it just a matter of optimizing the water cooled system?

Any insights/suggestions would be greatly appreciated!

The models can be seen here

Thanks you!

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answered 2016-01-08 08:41:42 -0600

updated 2016-01-08 08:44:07 -0600

Consider @StefanG comment here.

The system is tasked with conditioning the building. The air-cooled HP absorbs and rejects heat to the ambient. The cost of this is the energy consumed by the condenser fan. The water-cooled HP absorbs and rejects heat to a water loop. Maintaining the water loop at reasonable temperatures comes at a cost (boiler - 766423 kBtu, tower fans - 137367 kBtu, and water pumps - 66130 kBtu).

Differences in efficiency for heating and cooling would depend upon the outdoor unit (condenser in cooling mode, evaporator in heating mode) temperatures.

In cooling mode, the air-cooled HP rejects heat to ambient where the condensing temperature is the outdoor air temp. The water-cooled HP rejects heat to a water loop at the water temperature. When the water temperature is less than the outdoor air temperature, the water-cooled HP should operate more efficiently, but at a cost of water pumping and cooling tower fan energy.

Similar for heating where a higher water temperature compared to outdoor would provide better efficiency, however, the water temperature should nearly always be higher than winter outdoor temps and you should see an air-side heating energy savings (which I don't see here) which would be offset (or exceeded) by the boiler and water pump energy.

From this point, I would investigate the air-side Heating/Cooling Electricity use to see if those numbers make sense. You can see what meters are being summed to provide the Heating Electricity Use and Cooling Electricity Use by looking at the mtd file.

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answered 2018-02-21 05:02:41 -0600

Sean King gravatar image

Not so sure how you have done your comparison between an air-cooled and water-cooled VRF system. I suspect the curves used for each condenser type should be unique, it does not look right to me if you just simply change the condenser type from air-cooled to water-cooled but keep using the curves that have been used in air-cooled VRF outdoor unit originally. Yes, of course if different curves are used, we may not be able to have a "fair" comparison. I hope this can be a sort of "clue"?

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I get your point and you are right, different curves should be used. Just for test - I have the same issue (water fed VRF from boreholes vs air cooled VRF; same VRF curves, COP etc.; DesignBuilder (EnergyPlus) as the simulation software). The heating energy is almost the same in both cases (the cooling energy decreased in the “water fed” case). In my climate zone the exterior temperature varies from -15C to 35C, so I expected a higher heating energy in the “air cooled” case vs “water cooled” case. Any thoughts on why is this happening?

banicu gravatar imagebanicu ( 2018-03-21 09:48:08 -0600 )edit

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Asked: 2016-01-08 06:29:57 -0600

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Last updated: Feb 21 '18