CentralHeatPumpSystem examples - modes 4/5?

asked 2023-12-22 11:29:56 -0500

updated 2024-01-12 12:34:48 -0500

I was looking at the CentralChillerHeaterSystem_Simultaneous_Cooling_Heating.idf and CentralChillerHeaterSystem_Cooling_Heating.idf in energyplus 23-2. However the CentralHeatPumpSystem only seemed to go to mode 1,2,3... not mode 4 or 5. Further, the inlet temperatures were quite high for the GHX, and the GHX energy transfer was only negative when I ran the simulations for a full year.

It seems other people had similar issues in the past - not sure if these were resolved? https://github.com/NREL/EnergyPlus/issues/5904

Does anyone have a file with CentralHeatPump working correctly, including going to modes 1,2,3,4,5, and also with normal ground loop (GHX) temperatures?

High GHX.in temperature (should be similar to HP.out temp) image description

GHX energy transfer only negative; should be positive in heating mode, and negative in cooling mode. image description

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answered 2024-01-06 07:46:34 -0500

Keigo
3699 ●1 ●43 ●21 https://www.linkedin.c...

However the CentralHeatPump only seemed to go to mode 1,2,3... not mode 4 or 5.

I just ran CentralChillerHeaterSystem_Simultaneous_Cooling_Heating.idf annualy. OutputVariable Chiller Heater Operation Mode Unit <X> is 0 to 3 most of the time, but it's sometimes 4 and 5. You can find the idf file and the TimeStep result here.

Further, the inlet temperatures were quite high for the GHX

When I ran CentralChillerHeaterSystem_Simultaneous_Cooling_Heating.idf, Ground Heat Exchanger Inlet Temperature was below 20C in January. I used the weather file USA_IL_Chicago-OHare.725300_TMY2.epw. It might be different from the weather file you used. One thing to note is that you should check Ground Heat Exchanger Mass Flow Rate at the same time. Ground Heat Exchanger Inlet Temperature is meaningful only when there is water flow.

GHX energy transfer only negative; should be positive in heating mode, and negative in cooling mode.

You are right. I think the cause of the weird result (i.e., bugs in EnergyPlus) is more like GroundHeatExchanger:System rather than CentralHeatPumpSystem. The result of CentralChillerHeaterSystem_Simultaneous_Cooling_Heating.idf shows that

When Chiller Heater Operation Mode Unit <X> = 2 (heating-only mode), Ground Heat Exchanger Heat Transfer Rate < 0.
When CHILLED WATER LOOP:Plant Supply Side Cooling Demand Rate > 0 and Chiller Heater Operation Mode Unit <X> = 1 (cooling-only mode), Ground Heat Exchanger Heat Transfer Rate and Ground Heat Exchanger Mass Flow Rate are 0 even if COND CIRC PUMP:Pump Mass Flow Rate > 0.

I revised CentralChillerHeaterSystem_Simultaneous_Cooling_Heating.idf by replacing GroundHeatExchanger:System to GroundHeatExchanger:Surface which I copied from the other ExampleFile SurfaceGroundHeatExchanger.idf. Please find the revised idf file here. GHX energy transfer in the revised file is positive in heating mode and negative in cooling mode.

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Hi Keigo, thanks for your response!

It's interesting that you were able to fix it by changing the GHX system, I have been using the GroundHeatExchanger:System object, along with HeatPump:WaterToWater:EquationFit:Heating and HeatPump:WaterToWater:EquationFit:Cooling, based on the designbuilder methodology and it works great... strange that it is not working here.

We see plant loop heat recovery frequently in Canadian projects, so this object would serve a very important niche for us.

I will give your files a look!

alex j ( 2024-01-08 13:00:45 -0500 )edit

Hi Keigo, I had a chance to run your revised file this morning.

Things are somewhat improved, however the core issue appears to persist. In both examples, the T.in for the GHX (ground heat exchanger) is strange. The T.in for the GHX should match the condenser outlet temperature, but instead T.in_GHX is >> T.out_HP.

In you file, the GHX appears to work: given the very high GHX inlet temperature, and the ground temperature (should be around 5-10C), the heat exchanger is doing what it should - cooling the fluid to ~ 5C. Screenshot on imgur

alex j ( 2024-01-10 10:47:31 -0500 )edit

@alex j My results on 1 Jan are quite different from the Screenshot your attached. I would appreciate it if you could share the file and the weather file you used.