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.
Highlights
Despite the potentially huge financial savings and reduction of fossil fuels, the use of geothermal systems, either for electricity production in power plants or for the exploitation of the more widely used shallow geothermal energy, has been suffering from a lack of Energies 2020, 13, 4273; doi:10.3390/en13164273 www.mdpi.com/journal/energies standards (e.g., Eurocodes) has yet to be proposed
The ring-coil model can be in good agreement with the TRT results for an acceptable overestimation of the heat exchange capacity of the piles, something that did not occur for pitches of increased size
For a more interval-independent evaluation technique one needs to use a fitting function for the data, with the thermal conductivity and the borehole thermal resistivity remaining as the two variable parameters
Summary
Despite the potentially huge financial savings and reduction of fossil fuels, the use of geothermal systems, either for electricity production in power plants (see for example [1] and related devices) or for the exploitation of the more widely used shallow geothermal energy, has been suffering from a lack of Energies 2020, 13, 4273; doi:10.3390/en13164273 www.mdpi.com/journal/energies standards (e.g., Eurocodes) has yet to be proposed. The design of heat exchange elements integration within various geotechnical structures, i.e., EGS, which are needed to support buildings or ground, has not been sufficiently addressed. When designing such foundations, one has to examine the construction processes from the initial phase through all construction stages up to the Energies. The thermal properties of the borehole filling, pipe, fluid, ground, as well as varying heat transfer rates and influences of flowing groundwater can be considered They can be developed in 1D, 2D, and 3D space in order to solve an energy balance equation and are based on the following three methods: finite differences (FDM), finite elements (FEM), and finite volumes (FVM).
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