Reviewing the Modeling Aspects and Practices of Shallow Geothermal Energy Systems

Abstract

Shallow geothermal energy systems (SGES) may take different forms and have recently taken considerable attention due to energy geo-structures (EGS) resulting from the integration of heat exchange elements in geotechnical structures. Still, there is a lack of systematic design guidelines of SGES. Hence, in order to contribute towards that direction, the current study aims at reviewing the available SGES modeling options along with their various aspects and practices. This is done by first presenting the main analytical and numerical models and methods related to the thermal behavior of SGES. Then, the most important supplementary factors affecting such modeling are discussed. These include: (i) the boundary conditions, in the form of temperature variation or heat flow, that majorly affect the predicted thermal behavior of SGES; (ii) the spatial dimensions that may be crucial when relaxing the infinite length assumption for short heat exchangers such as energy piles (EP); (iii) the determination of SGES parameters that may need employing specific techniques to overcome practical difficulties; (iv) a short-term vs. long-term analysis depending on the thermal storage characteristics of GHE of different sizes; (v) the influence of groundwater that can have a moderating effect on fluid temperatures in both heating and cooling modes. Subsequently, thermo-mechanical interactions modeling issues are addressed that may be crucial in EGS that exhibit a dual functioning of heat exchangers and structural elements. Finally, a quite lengthy overview of the main software tools related to thermal and thermo-hydro-mechanical analysis of SGES that may be useful for practical applications is given. A unified software package incorporating all related features of all SGES may be a future aim.

Different Approaches for Evaluation and Modeling of the Effective Thermal Resistance of Groundwater-Filled Boreholes

“…The second method is based on Hellström's vertical profile (for uniform borehole wall temperature), additionally developed by Lamarche et al [18]. In this second approach, previously derived borehole wall temperature T BHW (based on the calculated annulus temperature T ann from Equation (20), and borehole wall temperature T BHW from Equation ( 21)) is used for the calculation of the vertical profile defined as follows:…”

“…Commonly used software tools for GHE modeling [20] normally account for the effective thermal resistance of grouted boreholes, most of the tools assuming this resistance as constant during the simulation. To our knowledge, none of the tools can handle the effective thermal resistance of groundwater-filled boreholes, affected by the natural convection within the annulus space.…”

Experimental Research of the Heat Transfer into the Ground at Relatively High and Low Water Table Levels

“…In many analyses, heat loss to the ground has been the subject [28][29][30][31][32][33][34][35][36]. Depending on the geometry of the building projections and the depth of the foundation, four methods were analyzed to define the heat loss to the ground [28,29].…”

Environmental Impact of Ground Source Heat Pump Systems: A Comparative Investigation From South to North Europe

“…GHEs are either of open or close type and can be of vertical or horizontal orientation. The conventional types for residential use are the vertical closed systems (Christodoulides et al, 2020), mainly due to the small land area required, and they may be of single or multiple U-tubes, coaxial, spiral or complex configurations.…”