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Vertical BHE effect on heat pump system efficiency

asked 2023-04-18 10:42:22 -0500

Qamar's avatar

updated 2023-04-18 10:43:22 -0500

I have a model of ground source heat pump system with vertical borehole heat exchangers drilled until 120 m and i am trying to vary parameters of the borehole systems such as depth, diameter and flow rate of circulating fluid, but the consumption of the heat pump and the building seems to have very small sensitivity to those parameters. Does anyone have any idea about better variables that might be better to analyze the effect of the BHE parameters of the system efficiency. Many thanks, Best regards,

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I am still obtaining unexpected results when changing the ground thermal conductivity, the length of the boreholes and the diameter, the changed value does NOT make any difference in the heat pump consumption nor in the building response. I am wondering if EnergyPlus is a software for such an analysis or am I doing some thing wrong in my model. Thanks in advance for any help in this regards. Here it is a link of my model:

Qamar's avatar Qamar  ( 2023-04-22 11:56:34 -0500 )edit

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answered 2023-04-18 11:19:47 -0500

Greg Estep's avatar

Hi Qamar, Have you loaded a G-function .idf file from Ground Loop Design (GLD) software or Ground Loop Heat Exchanger (GLHE) software? I model vertical GHXs all the time and this is my process:

  • I get the building hourly geothermal loads by using the district heating and cooling objects.

-load the hourly load file into GLD and link it with the vertical borefield module

-I configure the borefield layout (i.e. 4x4 or 2x8, etc) and check for appropriate heat pump supply temperatures for heating and cooling and the ground long duration response temperature change.

once satisfied with the bore field layout I export the G-function IDF.

then, back in design builder, I import the G-function IDF and set the flow rate in the GHX object and then set the maximum flow rate at the plant loop level to match the GHE design flow rate

I suspect that you aren't seeing sensitive results because the G-function is the actual thermal response of the bore field. Here is some info from the DesignBuilder help:

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Thank you Greg for your answer. In fact, I am requesting EnergyPlus (EP) to calculate the G-function response factors using one of the methods available in it which is UHFcalc meaning a Uniform Heat Flux at the borehole wall. I assume that the G-function factors might be correct as the EP is calculating them automatically.

Qamar's avatar Qamar  ( 2023-04-19 11:12:29 -0500 )edit

Adding to this, my borehole field is not regular at all. And I do not know if the software you recommended are able to calculate G-function for irregular fields. Many thanks,

Qamar's avatar Qamar  ( 2023-04-19 11:13:49 -0500 )edit

Hi Qamar, Yes, GLD has the option to configure custom borefield layouts and then it will calculate the G-function for that layout.

Greg Estep's avatar Greg Estep  ( 2023-04-19 12:55:32 -0500 )edit

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Asked: 2023-04-18 10:42:22 -0500

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Last updated: Apr 18 '23