Posted Mon, 15 Oct 2018 16:54:23 GMT by RDRYSDA
Hi all,

I’m working on the efficiency of a BHE system and my goal is to determine the total energy produced by the BHEs throughout the simulation timeframe. However, I’m not sure if I’m using the proper selections or methods to get the total energy produced by the system.

I have extracted data from FEFLOW using the Period-Budget Panel, but I’m not sure if my data is accurate. I followed these steps to get the energy output data;
- I recorded a dac file of a 20 year run and opened the dac file.
- The edge selection for the BHEs were converted to 3D nodal selection, and selected. Note that I have selected the entire length of the BHEs (90m), as well as each BHE in the model.
    I am unsure if this is the correct selection.
- In the Period Budget Panel I used the “Domain of Interest” (DOI) drop-down I chose “Current Node Selection”.
- I then exported the results (FEFLOW creates a txt file that is opened in Excel).

The exported file gives energy rate and total cumulative energy for all BCs (Dirichlet, Nuemann, etc.), however only Point-Source BCs have any data since only the BHEs were selected. Fields with data are as follows;
- Point-Source BCs
- Storage Capture/Release
- Imbalance

Right now I’m only really looking at the Point-Source BCs In and Out values, which FEFLOW help documents label as a Heat Nodal Sink/Source Boundary Condition. As per FEFLOW help “… [i]inflows are considered as negative, outflows as positive. Thus a sink/source boundary condition with a positive sign means extraction of thermal energy from the ground, a negative sign is used for injecting energy[/i].” so I am assuming my Point Source In is heat injected into the ground (when the system is cooling) and Point Source Out is heat extracted, and to get the total energy I would need to add the two together (i.e. Point Source In + Out = Total)

I have kept the Storage Capture/Release and Imbalance data, but I’m not sure how or if I should include it in any energy calculations. If you have any insight, I would greatly appreciate the help.

Thank you
Posted Mon, 22 Oct 2018 15:10:10 GMT by Björn Kaiser
Using the Budget History Charts seems to me a useful workflow for you:

If you enable the budget histories, FEFLOW will calculate the budget at each time step and store the budget results as a chart. The calculated budget will be stored at each time-step even if you skip output time-steps written in a dac-file. In contrast, if you use the budget panels and if you skip some output time-steps, you would make error in the budget calculations. The budget panels are calculating on the fly while browsing through the time-steps.

Use a nodal selection along the traces of the BHE. For the Budget histories the following convention of the algebraic sign is important: negative if you get energy out of the aquifer system. In contrast, a positive algebraic sign indicates you injecting heat.

The imbalance for all nodes of the model domain should be check for the residual mass-balance error.

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