Optimal operation of ground heat exchangers in presence of design anomalies: an approach based on second law analysis

Geothermal systems often experience non homogeneous behavior between the various boreholes, due to possible local differences in the ground stratigraphy or installation problems. This may create important inefficiencies or operational issues, particularly in the case of small systems. These problems can be partially corrected through optimal operation. In this paper, parametric analysis of the global entropy generation is proposed for the optimization of the overall performances of two vertical U-tube ground heat exchangers during operation. When the system is already installed, the design variables, such as the tube diameter and the installation depth, cannot be modified to contrast the effects of anomalies. The mass flow rate distribution between the boreholes is the design variable that can be changed during the operation in order to select the best configuration. The possible anomalous scenarios that are considered include boreholes ovalization, fouling or different thermal proprieties in the stratigraphy of soil surrounding the GHE system and different borehole depths. The analysis shows that significant improvements in term of recovered exergy up to 1.6% in the examined scenarios can be achieved thought proper operation.