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Water to Water Heat Pump Schedule Control

asked 2018-10-04 09:13:41 -0500

FSilenzi gravatar image

updated 2018-10-12 04:45:52 -0500

Hi everybody, I am modeling a NZEB building equipped with a ground source heat pump that feeds the water circuit dedicated to heating and cooling with 2 tubes fancoils. The HP is used to produce hot water from October to April, it is switched off untill June, then it is used to produce chilled water from June untill September, it is switched off again untill October.

In openstudio, I have modeled this using :

-Heat Pump - Water To Water - Cooling (Os:HeatPump:WaterToWater:EquationFit:Cooling)

-Heat Pump - Water To Water - Heating (Os:HeatPump:WaterToWater:EquationFit:Heating)

which are then connected, respectively, to a Fan Coil with Cycling Fan - Cooling Coil (Os:Coil:Cooling:Water) and a Fan Coil with Cycling Fan - Heating Coil Os:Coil:Heating:Water.

I have correctly modeled the plant loops, but I cannot find a way to implement the control logic descripted above. In the current model, the two heat pumps object are working at the same time, providing cooling and heating also during winter and summer, as well as functioning during the period of time in which the HP should be switched off. Although, this was acceptable on a first approach, it is not currently depicting the real plant scheme.

The main issue is that the real HP is not able to deliver both chilled and hot water, the operating mode is manually switched between the two configuration at the beginning of each season. Also, fancoils are only modeled, in Openstudio, as 4 tubes which is not the case for the real fancoils.

To mimick the real control strategy, I decided to give an availability schedule to the fancoil heat exchangers (both cooling and heating), shutting them down in the appropriate time-window.

However this induced the following error:

** Severe ** Plant temperatures are getting far too hot, check controls and relative loads and capacities

* ~~~ * Environment=RUN PERIOD 1, at Simulation time=04/28 05:45 - 05:50

* ~~~ * PlantLoop Name (DemandSide)= ACQUA CALDA FANCOIL

* ~~~ * PlantLoop Setpoint Temperature=55.0 {C}

* ~~~ * PlantLoop Inlet Node (SupplySide) does not have a Setpoint.

* ~~~ * PlantLoop Inlet Node (DemandSide) does not have a Setpoint.

* ~~~ * PlantLoop Outlet Node (SupplySide) has a Setpoint.

* ~~~ * PlantLoop Outlet Node (DemandSide) does not have a Setpoint.

* ~~~ * PlantLoop Outlet Node (DemandSide) "NODE 48" has temperature=300.0 {C}

* ~~~ * PlantLoop Inlet Node (DemandSide) "NODE 47" has temperature=300.0 {C}

* ~~~ * PlantLoop Minimum Temperature=0.0 {C}

* ~~~ * PlantLoop Maximum Temperature=100.0 {C}

* ~~~ * PlantLoop Flow Request (SupplySide)=0.0 {kg/s}

* ~~~ * PlantLoop Flow Request (DemandSide)=1.3 {kg/s}

.....the error message continues*

I have added bypass branches to fancoil heat exchangers in their respective water loops, changed the fancoil loops pumps to variable volume ones with intermittent control (as I have read here, and the linked asnwers there) but with no avail. Others answer in the forum simply did not adress my problem, or I have just being searching in the wrong places.

My question is, is there a way to solve this issue in a smarter way?

edit: typos

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answered 2018-10-04 20:34:50 -0500

This warning is usually caused by a pump running when coils are off. The pump heat causes the plant water to heat up without a sink (the coils) for heat transfer. Check the operation of the coils with respect to pump operation. You can also add Output:Diagnostics, ReportDuringWarmup; to see what is happening if this warning happens before normal reports are generated.

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Thanks @rraustad for adressing my problem. I managed to control the pump schedule editing the OpenStudio Model file with the text editor, now the simulation is fine and the error is solved. Analizing the results, I have found something that confuses me: I have both the coils and the pumps that feed them shut down, as well as the circulating pump of the Borehole loop, but still I have fancoil heat fluxes during the periods in which they are supposedly off. I have checked the pumps flow rates as well as the GSHP electricity comsumption and they are all 0 in the proper set of time.

FSilenzi gravatar imageFSilenzi ( 2018-10-05 05:13:45 -0500 )edit

Check the fan coil water inlet node mass or volume flow rate during a time when the fan coil is on and you think it should be off. Also report the plant outlet node to make sure flow = 0. If fan coil water flow rate is non-zero and the plant pumps (and plant flow) are off there is a problem on the plant side (in code I mean, not in your model). If the water flow rate = 0, then there is a problem in the fan coil model. Either way let us know and we will need your input file to issue a bug report.

rraustad gravatar imagerraustad ( 2018-10-05 12:42:53 -0500 )edit

I'm sorry for the poor quality image, hovewer it should be clear that when the supply pump that feeds the fancoil heat exchanger is shut down (yellow curve) there is still cooling power delivered by the fancoil (blue curve.

The in-out reference states: "Fan Coil Total Cooling Rate [W] This field is the total (sensible and latent) heat extraction rate of the fan coil unit from the zone it is serving in Watts. This is determined by the outlet and zone conditions and the mass flow rate through the unit.". According to this the total cooling rate should be zero (since is zero the flow rate)

FSilenzi gravatar imageFSilenzi ( 2018-10-10 04:59:58 -0500 )edit

I have also checked the "Fan Coil Availability Status" variable and, incredibly, is set to zero for all my simulation, while "Fan Coil Heating Rate" and "Fan Coil Total Cooling Rate" are definitely not zero. The schedule I used ot module the availability of the fancoil heat exchangers are not always off, but have intermittent status depending on seasons.

FSilenzi gravatar imageFSilenzi ( 2018-10-10 06:17:03 -0500 )edit

Does the fan coil have outdoor air? The report for Fan Coil Total Cooling Rate is only the energy transfer based on supply air to zone air enthalpy difference. If cool OA is added to the inlet of the fan coil, the supply air can have a lower enthalpy than the zone air without the cooling coil being active. You should look at the cooling coil reports to see if the coil is actually on.

rraustad gravatar imagerraustad ( 2018-10-10 09:14:32 -0500 )edit

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Asked: 2018-10-04 09:13:41 -0500

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Last updated: Oct 12