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Inconsistent Water Flow Rates in SwimmingPool:Indoor

asked 2023-06-07 21:33:00 -0500

Keigo's avatar

updated 2023-06-08 09:09:26 -0500

I want to model a swimming pool with a constant water flow rate. Normally, a hot water loop for a swimming pool has a constant flow because flitration system (which is not modelled for energy simulation) is in the loop and the pool water needs to be continuously filtered while the swimming pool is in operation.

Here is a sample model (V23-1-0). There are only hot water loops with SwimmingPool:Indoor and no air-conditioning system in the idf file. The water-side schematic is as follows. The secondary loop has Pump:ConstantSpeed.

The simulation runs successfully without error. The water flow in the secondary loop is constant when the loop is in operation from 6am to 11pm. System Node Mass Flow Rate of Pool Water Inlet Node also shows a constant flow. However, I found that Indoor Pool Inlet Water Mass Flow Rate has a variable flow. I think these two Output:Variables refer to the same Node. Why are these two flow rates different? Is this a bug?

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Indoor Pool Inlet Water Mass Flow Rate is consistent with Indoor Pool Water Heating Rate, and SwimmingPool:Indoor seems to accept only the amount of hot water required to heat the pool water to a setpoint temperature. Indoor Pool Inlet Water Mass Flow Rate becomes 0kg/s when the pool water temperature reach the setpoint and it does not need to be heated, but that's not what I expect. I want keep the constant water flow even when the pool water temperature reaches the setpoint.

If both Indoor Pool Inlet Water Mass Flow Rate and System Node Mass Flow Rate of Pool Water Inlet Node are correct, then Indoor Pool Inlet should be inside the SwimmingPool:Indoor component separately from Pool Water Inlet Node, and a bypass pipe with 3 way valves should be also inside the SwimmingPool:Indoor component as shown below. Is my understanding correct?

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Keigo's avatar Keigo  ( 2023-07-08 22:35:20 -0500 )edit

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answered 2023-08-11 13:52:02 -0500

RKStrand's avatar

I saw the issue on GitHub and have taken a quick look at it. I'll also post an answer there as well. The Input-Output Reference does address this issue. Here is the section of text that addresses the flow rate: Field: Pool Water Maximum Flow Rate This input is the maximum water volumetric flow rate in m3/s going between the pool and the water heating equipment. This along with the pool setpoint temperature and the heating plant equipment outlet temperature will establish the maximum heat addition to the pool. This flow rate to the pool will be varied in an attempt to reach the desired pool water setpoint temperature (see Setpoint Temperature Schedule below).

The code backs this up. Basically, if the input fluid temperature is fixed as is the case here, there's only one way to adjust the heat addition--vary the flow rate. Without doing that, you won't hit the setpoint and will overshoot. The question is: if you don't vary the flow rate, what is the strategy? Do you use an intermittent strategy? Or just let it overheat and then compensate with less heat addition the next time step? That would more or less simply be an on-off strategy. Either way, it seems like this would require an additional input and new code (basically a small new feature). I'm not sure there is time for this given that the IO Freeze is in less than one week. If this can get resolved quickly, I might be able to get it into the code for the 23.2 release, but no guarantees at this point.

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Thanks for your post. I know the input fluid temperature is fixed (or scheduled) first and the flow rate varies to adjust the heat addition. But then, why does EnergyPlus allow the plant loop with Pump:ConstantSpeed? What is going on in the SwimmingPool:Indoor component? If it has a bypass pipe abd three-way valves inside like my sketch, I would appreciate it if the illustration could be added in I/O Reference.

For a new feature, I want the flow rate to be specified first and the input fluid temperature to vary, which is more in line with the real system.

Keigo's avatar Keigo  ( 2023-08-11 21:15:31 -0500 )edit

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Asked: 2023-06-07 21:33:00 -0500

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Last updated: Aug 11 '23