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High Pump Energy Consumption

asked 2021-05-31 16:15:45 -0500

Ahmad gravatar image

updated 2021-06-01 09:46:26 -0500

Greetings,

I've defined 1 indirect absorption chiller loop and 1 condenser loop in my Openstudio model and I have configured it to be running in the summer for cold water to be provided to cooling coils.

When simulating my model, I am getting high pumps energy consumption during this period, which seem unreasonable. A plot of the mass flow rates for the chiller and condenser pumps is shown below:

image description

There seems to be very high values for the condenser pump (about 80 kg/s) compared to the chiller pump (about 4-6 kg/s). Both are Variable Speed Pumps and working intermittent.

Could anyone assist me to solve this issue of high pumps energy consumption ? I can provide my model files if needed upon request.

Thank you.

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Hi Ahmad, are you autosizing the design condenser flow for the chiller? If so, you should take a look at your EIO file and see if it's autosizing to 80 kg/s. The chiller is going to request its full design condenser flow when it is on, regardless of pump type, so that may be what you're running into here.

codybond gravatar image codybond  ( 2021-06-02 09:13:01 -0500 )edit

Hi Cody, thanks for the reply. Yes, I am auto-sizing.

I took a look at the EIO file and here is what is inside:

"Design Size Design Condenser Water Flow Rate [m3/s], 7.75550E-002 CONDENSER WATER LOOP, Maximum Loop Flow Rate [m3/s], 7.75550E-002

CHILLED WATER LOOP, Maximum Loop Flow Rate [m3/s], 2.23400E-002 Design Size Design Chilled Water Flow Rate [m3/s], 2.23400E-002"

Seems I am running as you are suggesting. Please give me your thoughts on how to solve this issue.

Many thanks !

Ahmad gravatar image Ahmad  ( 2021-06-02 13:18:18 -0500 )edit

Got it! So is the main problem that the flow is much higher than you expect, or that you expect it to operate with variable flow? Or is the flow reasonable and your concern is that the pump power is sizing too high?

codybond gravatar image codybond  ( 2021-06-02 13:37:39 -0500 )edit

The basic idea started from results showing high pump power consumption throughout the summer period which should be from cooling. Then after plotting the flow, it showed higher than expected. I would be grateful to prioritize solving the issue of high power consumption since my end work is related to energy consumption.

Ahmad gravatar image Ahmad  ( 2021-06-02 16:19:59 -0500 )edit

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answered 2021-06-03 10:30:22 -0500

For the issue of high pump power, the easiest thing to do would be to hardsize the power (design_power_consumption) that you're looking for at the pump. You could also adjust the pressure head (design_pump_head) to get to the desired power value as well if you're using the default Design Power Sizing Method. The issue with this is if you have a cooling tower or some other equipment on your condenser loop, it is also going to be using too much energy due to the higher than expected flow. Maybe that isn't a problem in your use case though.

Alternatively, a more robust way to go about it would be from the loads side. E+ is essentially telling you (via the autosizing results) that based on the loads and equipment/loop design parameters you've given, it needs to have a pump that can flow 80 kg/s, and therefore an appropriate power for that flow. So the first thing you should do is compare the autosized parameters (from the EIO file) to the real system you're trying to represent. If the chiller capacity and flow are too high then you might need to consider adjusting your zone loads and coil sizing. If everything is acceptable on the chw side, then you just need to review the condenser loop parameters. Check out design_loop_exit_temperature and loop_design_temperature_difference in Sizing:Plant. If temperature looks good there, you can use the dT parameter to directly adjust flow (as long as the chiller reference_chilled_water_flow_rate is autosized I think), where increasing dT should decrease flow and vice versa.

Good luck!

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Thanks Cody, I'll manage to alter this and get back here with any changes. Could you just elaborate on the dT parameter part by what you mean with dT exactly and where it is ?

Ahmad gravatar image Ahmad  ( 2021-06-03 11:19:34 -0500 )edit

Sorry, that was a lazy reference to the loop_design_temperature_difference in Sizing:Plant.

codybond gravatar image codybond  ( 2021-06-03 11:21:56 -0500 )edit

Ah ok, great. Could you let me know if you can spare a few minutes to see my model and give me your thoughts on it ? I can make a link and send it to an email of your choice. I'll remove the results part because it is really big in size and you can simulate once for the results. Thanks Cody.

Ahmad gravatar image Ahmad  ( 2021-06-03 11:30:56 -0500 )edit

Sorry Ahmad, I won't have time to look at your model, but check back in with questions if you're still having issues after trying the two methods I suggested.

codybond gravatar image codybond  ( 2021-06-03 17:16:31 -0500 )edit

Hi Cody, thanks ! I went with the robust way and worked with the loops parameters:

DLET: The Design Loop Exit Temp. DIWT: Design Inlet Water Temp. LDTD: Loop Design Temp. Difference.

  • For the chilled water loop: DLET is the same as the DIWT of the cooling coils and set to 7.2 Deg.C. The LDTD was 7 K and now I switched it to 2 K.
  • For the condenser water loop: The DLET is 29.4 Deg.C. The LDTD was 10 K and now I switched it to 25 K.
  • For the hot water loop: The DLET is the same as the Rated Inlet Water Temp. of the heating coils and set to 80 Deg.C. The LDTD is 11 K and considered ok.
Ahmad gravatar image Ahmad  ( 2021-06-05 08:33:36 -0500 )edit

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Asked: 2021-05-31 16:15:45 -0500

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Last updated: Jun 15