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Condenser water flow rate with EMS: EnergyPlus

asked 2019-08-26 08:04:25 -0500

Gio gravatar image

My question is related to this issue (https://unmethours.com/question/35540... ticket: https://github.com/NREL/EnergyPlus/is...) where the condenser pump is not working as expected.

Is there a way to control the condenser water flow rate with an EMS program? I guess it could be done using the "Chiller Condenser Heat Transfer Rate [W]" output as sensor and a "SetpointManager:Scheduled" as actuator. Unfortunately it is not possible since the condenser loop requires a temperature setpoint.

Is there another way?

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answered 2019-08-30 05:23:30 -0500

There is an actuator for pumps called Pump Mass Flow Rate [kg/s] (cf EMS Application Guide) that you should use for this.

Chiller Condenser Heat Transfer Rate [W] as a sensor would probably work if you have only one chiller on the demand side of your condenser loop. Otherwise you can probably use the Plant Supply Side Cooling Demand Rate [W] as a sensor.

You'll probably also need to get the Entering Water Temperature (EWT) too, for eg looking at Plant Supply Side Inlet Temperature. If your SetpointManager:Scheduled isn't using a fixed value that you can hardcode in your EMS program, you also need to define a Sensor on the Schedule Value for the corresponding schedule, that will give you the Leaving Water Temperature (LWT).

I think Plant Supply Side Cooling Demand Rate is a positive number, in which case I guess you could try this:

$$ \dot{Q}_{demand} = \dot{m} \cdot C_p \cdot (T_{EWT} - T_{LWT})$$

Dimensional analysis:

$$ [W = J \cdot s^{-1}] = [kg.s^{-1}] \cdot [J.kg^{-1}.K^{-1}] \cdot [K]$$

So, solving for your mass flow rate:

$$ \Leftrightarrow \dot{m} = \frac{\dot{Q}_{demand}}{C_p \cdot (T_{EWT} - T_{LWT})}$$

You can use @CpCW(Temperature) in ERL if you'd like, but it's pointless since it'll always return 4180.0 anyways (source code: Psychrometrics.hh)

Disclaimer: I have not tested any of the above, not sure whether it'll work, integrate well with what the cooling tower is doing, nor what calling point would be appropriate.

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uh yes, those equations are exactly what I was thinking about: the pump actuator is what I was missing. I guess I will try with a fixed leaving water temperature at 30°C and using the entering water temperature as sensor. I'm going to try it as soon as possible and I'll post some results here.

Gio gravatar imageGio ( 2019-08-30 05:36:29 -0500 )edit

I think it works....! Thanks a lot, I'll add the results in another answer below.

Gio gravatar imageGio ( 2019-09-05 02:04:33 -0500 )edit
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answered 2019-09-05 02:30:32 -0500

Gio gravatar image

updated 2019-09-05 02:32:36 -0500

Following the previous answer by Julien Marrec I tried the following:

  1. I used the example file "CoolingTower_TwoSpeed"
  2. Run period 01/05 to 20/05 and Weather file "USA_CA_San.Francisco.Intl.AP.724940_TMY3.epw"
  3. I created a model with EMS called "CoolingTower_TwoSpeed_EMS"
  4. I added a sensor using the variable "Plant Supply Side Cooling Demand Rate" with the name "PSSCDR"
  5. I added an actuator on the pump variable "Pump Mass Flow Rate" with the name "PMFR"
  6. I added a simple program with the line "set PMFR=PSSCDR/4180/5+1.1*0.025"

Note that it is a very rough control: I used a fixed temperature difference of 5 degrees. The main problem is that with this method the plant temperatures are getting too hot during the warm-up and you will get an error. Therefore I added a constant value equal to the 2.5% of the design pump mass flow rate (1.1 kg/s): it solved the temperatures error in my case.

I obtained: -21.88% pump energy consumption and -1% in cooling energy consumption (beacause the chiller condensed inlet temperature is lower in the EMS case). The energy saving is related to this specific example.

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Asked: 2019-08-26 08:04:25 -0500

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Last updated: Sep 05