Posted Wed, 09 Sep 2020 12:52:32 GMT by Henk Witte
We are currently investigating the effects of ground water flow on borehole heat exchangers. As a first step we want to validate the Feflow borehole model with standard BHE models, mainly Earth Energy Designer and Finite Line SOurce. In the forum there are some questions concerning this, but I did not find answers.

For the comparsion we compare fluid temperature of the feflow model with the fluid temperatures of EED and the temperature in the borehole / ground (using the EED relation between borehole thermal resistance, borehole wall temperature and energy rate to calculate ground temperatures).

We can set up and run the feflow model using the guidance on borehole configuration (mesh points) etc. However, comparison with Earth Energy Designer for balanced or unbalanced monthly energy loads shows that the higher the energy rate, the higher the difference between the Feflow solution and EED.

Also, when running a constant energy extraction / injection we find the Feflow solution to go to a steady-state very rapidely. This is for instance the case when comparing with an FLS or EED constant energy simulation (several years) but also for a thermal response test data set (about 100 hours).

As it is important that we understand the behaviour of Feflow without natural ground water advection, we hope the forum can give us some suggestions / references how to set up and run the model.

As an example the following graphs shows the feflow and EED solution for contant heat injection:

Thank you !

Henk Witte

Posted Fri, 11 Sep 2020 05:55:48 GMT by Peter Schätzl Grundwassermodellierer
It's hard to find out what the difference is between the results without playing with the models. I'd recommend to modify parameters one by one to see whether the two results can be brought closer together. I've never directly compared EED and FEFLOW, but at the time the BHE were introduced into FEFLOW, the results were thoroughly cross-checked with CFD simulations. The rapid development towards the steady state points towards one of the storage parameters, and I'd think it might be in the aquifer rather than in the borehole (as I'd expect the borehole to have shorter influence only). Could it be different assumptions for fluid/solid heat capacity in the aquifer (and porosity, of course)?

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