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Troubleshooting Advanced Dehumidification and Water-to-Air Heat Recovery Modeling Issue

asked 2023-12-30 12:42:55 -0500

nrj_ep's avatar

Summary:

I'm encountering an unexpected problem while attempting to model advanced dehumidification via heat recovery using the UnitarySystem. Specifically, the heating coil fails to activate, leading to an uncontrolled zone temperature during the heating season.

Details:

My objective is to model a system capable of regulating both temperature and relative humidity in a specific zone with notable latent loads. Here's the expected behavior:

  1. When the zone thermostat requires heating or cooling, the UnitarySystem's heating or cooling coil should provide the necessary energy to restore the zone temperature to the desired range.
  2. If the zone's relative humidity surpasses a defined threshold, the cooling coil should lower the air temperature beyond the dew point for optimal dehumidification. Subsequently, the reheat coil should elevate the temperature back to the thermostat's set range.

Under normal conditions, when the system isn't dehumidifying, the reheat coil should remain inactive. Additionally, the cooling coil's condenser heat should be dissipated outdoors through the dry cooler (FluidCooler:TwoSpeed).

These are the 3 type of coils I'm using:

  1. Cooling coil: Coil:Cooling:WaterToAirHeatPump:VariableSpeedEquationFit
  2. Reheat coil: Coil:Heating:Water (powered by heat recovered from the cooling coil's condenser, water side)
  3. Heating coil: Coil:Heating:Electric

I've set up two plant loops: one for the reheat coil/condenser loop and another for the condenser/dry cooler loop. These loops are interconnected via an ideal heat exchanger.

The simulation completes without severe errors in the Chicago climate (USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw). However, the zone temperature isn't being adequately controlled. The heating coil remains inactive even when the zone temperature falls well below the setpoint. Moreover, the system seems to initiate dehumidification only starting from mid-April. Meanwhile, the reheat coil appears to elevate the temperature significantly above the cooling setpoints.

Could someone examine the IDF attached to this link to identify the potential cause of this behavior? I've attempted adjustments to the Latent Load Control and Setpoint Manager types governing the UnitarySystem outlet node temperature but haven't achieved success.

Any guidance on resolving this issue would be greatly appreciated.

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answered 2024-01-04 02:29:45 -0500

Keigo's avatar

This is not a perfect answer. There are many problems with your model, and this only points out some of them.

You should illustrate your model so that others can help you. A rough sketch of the schematic like the one below should be fine.

image description

The heating coil remains inactive even when the zone temperature falls well below the setpoint. Moreover, the system seems to initiate dehumidification only starting from mid-April.

It's because the air flow is 0 m3/s at that time. One solution is that you can fill in No Load Supply Air Flow Rate Method and No Load Supply Air Flow Rate in AirLoopHVAC:UnitarySystem and also you can change Fan Schedule (Supply Air Fan Operating Mode Schedule Name in AirLoopHVAC:UnitarySystem) from 0 to 1.

Meanwhile, the reheat coil appears to elevate the temperature significantly above the cooling setpoints.

I think this is due to a bug in EnergyPlus. The reheat coil (My Zone Reheat Coil Reheat Coil Loop Demand) heats air even though there is no hot water flow. That's imposible.

image description

The chart is here. I don't know how to solve the bug right now.

Other problems I can think of are:

  • My Zone Max Humidity Sch should be input in Dehumidifying Relative Humidy Setpoint Shcedule Name rather than Humidifying Relative Humidity Setpoint Schedule Name in Zone Control:Humidistat. For Humidifying Relative Humidity Setpoint Schedule Name, you can input a schedule name with small values such as 0.
  • It seems that you don't need to model SetpointManager:SingleZone:Humidity:Maximum.
  • The pump in Reheat Coil Loop should be Pump:VariableSpeed rather than Pump:ConstantSpeed.
  • If you want to model constant air volume, the fan in AirLoop should be Fan:SystemModel, Fan:ConstantVolume or Fan:OnOff rather than Fan:VariableSpeed.

Actually, I have not modelled unitary system that much. I referred to ExampleFiles and compared with your model. Although ExanpleFiles are not necessarily correct, they are good examples of typical input. For example, UnitarySystem_FurnaceWithDXSystemRHcontrol.idf has the same CoolReheat Dehumidification Control Type as your model.

By the way, your model has so many Output:Variable objects. I recommend considering which variables to focus on and reducing them. Otherwise, you would overlook the suspicious part.

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Asked: 2023-12-29 21:16:50 -0500

Seen: 173 times

Last updated: Jan 04