VRF system shows high COP compared to specified COP

asked 2017-09-29 02:17:10 -0500

katsuya.obara

2085 ●1 ●4 ●14

updated 2017-09-29 02:33:24 -0500

Julien Marrec

29042 ●85 ●74 http://www.effibem.com/

I am modelling a VRF system for a typical office building in Hong Kong.
The result shows high COP compared to specified COP.

in HVACTemplate:System:VRF, Gross Rated Cooling COP is set to 3.68 as shown in below image. However, the result COP sometimes reach to 6 which is much higher than specified value.

I am assuming that this is caused by default performance curve prepared by EnergyPlus.
Can anyone come up any other possibility causing this high COP situation?

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We have a convention to not include greetings here, (Hi, thanks, etc). Thank you. And yes, it's probably just the curves. You can check the expidf to see the curve coefficients etc.

Julien Marrec ( 2017-09-29 02:32:38 -0500 )edit

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answered 2017-09-29 07:42:12 -0500

rraustad

13599 ●72 ●12 http://www.fsec.ucf.edu/

updated 2017-09-29 07:43:47 -0500

I would explain this using the pump affinity laws. Power is proportional to the cubed of shaft speed. So when the loads are low, compressor speed is reduced and compressor power reduces as the cubed of compressor speed. The top figure shows the report variables for COP, similar to your description. The bottom figure shows VRF system cooling capacity and power with calculated COP similar to the top figure. At low loads (capacity) the power is lowered more than capacity. For example, take the first peak where capacity is around 40,000 W and power is around 10,000 W. Then compare to the next peak where capacity is reduced to around 15,000 W yet power dropped to about 3,000 W. The ratio of capacity to power of the first peak is 4, for the second it is 5. So at low loads, COP increases.

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