energies Review Reviewing the Modeling Aspects and Practices of Shallow Geothermal Energy Systems Paul Christodoulides 1,* , Ana Vieira 2 , Stanislav Lenart 3 , João Maranha 2, Gregor Vidmar 3, Rumen Popov 4, Aleksandar Georgiev 5, Lazaros Aresti 6 and Georgios Florides 1 1 Faculty of Engineering and Technology, Cyprus University of Technology, 31 Arch. Kyprianou, P.O. Box 50329, 3603 Limassol, Cyprus; georgios.fl[email protected] 2 Geotechnics Department-National Laboratory for Civil Engineering, 1700-066 Lisbon, Portugal; [email protected] (A.V.); [email protected] (J.M.) 3 Slovenian National Building and Civil Engineering Institute, 1000 Ljubljana, Slovenia; [email protected] (S.L.); [email protected] (G.V.) 4 EKIT Department, University of Plovdiv “Paisii Hilendarski”, 4000 Plovdiv, Bulgaria; [email protected] 5 Department of Mechanics, Technical University of Sofia, Plovdiv Branch, 4000 Plovdiv, Bulgaria; [email protected] 6 Department of Electrical Engineering, Computer Engineering and Informatics, Cyprus University of Technology, 31 Arch. Kyprianou, P.O. Box 50329, 3603 Limassol, Cyprus; [email protected] * Correspondence: [email protected] Received: 11 July 2020; Accepted: 14 August 2020; Published: 18 August 2020 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. Keywords: shallow geothermal energy systems; energy geo-structures; thermal analysis; thermo-hydro-mechanical; modeling; software tools 1. Introduction 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 Energies 2020, 13, x FOR PEER REVIEW 2 of 47 lack of proper official international standards. Shallow geothermal energy system (SGES may take a number of different forms such as vertical and horizontal GHEs (with different pipe configurations), surface water, standing columns, but also as thermo-active structure (TAS) systems or energy geo- structures (EGS). The latter find applications like diaphragm walls, retaining walls, embankments, tunnel linings, barrette piles, shallow foundations and energy piles (EPs) [2,3] (Figure 1). Some national professional associations have produced guidance documents [4–6], while a general guidance proposing principles of SGES design within the framework of existing geotechnical standards (e.g., Eurocodes) has yet to be proposed. In particular, 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 Energiesloading2020 phase,, 13, 4273 with consequently induced stress changes in the ground. Another consideration,2 of 45 when thermal changes are applied in EGS during the use phase, are the induced additional stress changes in the ground [7]. Hence, to help the engineer dealing with the design of such structures, it proper official international standards. Shallow geothermal energy system (SGES may take a number of is very useful, if not essential, to provide proper tools such as software, in line with appropriate different forms such as vertical and horizontal GHEs (with different pipe configurations), surface water, guidelines. standing columns, but also as thermo-active structure (TAS) systems or energy geo-structures (EGS). Although, the use of foundation elements for building energy has been around for more than The latter find applications like diaphragm walls, retaining walls, embankments, tunnel linings, three decades, according to the authors knowledge, only Swiss guideline SIA D0190 [8] considers barrette piles, shallow foundations and energy piles (EPs) [2,3] (Figure1). Some national professional thermally-activated piles in detail and provides design guidance for thermal effects on differential associations have produced guidance documents [4–6], while a general guidance proposing principles movements in pile groups and on consequent additional pile loads. Thus, SIA D0190 views EGS from of SGES design within the framework of existing geotechnical standards (e.g., Eurocodes) has yet to be only the thermo-active piles’ perspective. SIA D0190 refers to the numerical tool PILESIM used for proposed. In particular, the design of heat exchange elements integration within various geotechnical the calculation of the thermal performance of energy piles [9]. structures, i.e., EGS, which are needed to support buildings or ground, has not been sufficiently However, besides thermo-active piles, whose use began in the 1990s [10], base slabs, diaphragm addressed.walls, tunnels When [11–13] designing and even such sewers foundations, [14] are one nowadays has to examine utilized the for construction heating and processescooling purposes, from the initialwhile phaseborehole through fields all have construction the potential stages of being up to used the loading for seasonal phase, thermal with consequently storage [15]. The induced European stress changesstandard in describing the ground. the Another design of consideration, heat pump (HP) when heating thermal systems changes (EN are 15,450 applied [16]) in does EGS not during include the usedesign phase, guidelines are the induced for ground-source additional stressheat changespumps in(GSHPs) the ground coupled [7]. Hence,with EP to or help borehole the engineer heat dealingexchanger with (BHE) the design fields. of Among such structures, national EGS it is verydesign useful, guidance if not essential,documents to one provide can properrefer to tools the suchGerman as software, [17], the inAustrian line with [18], appropriate the Swiss guidelines.[8]), the Italian [19], or the French [20] guidelines. FigureFigure 1.1. 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Most guidance of them for focus thermal on the eff properects on provision differential of movementsenergy from in below pile groups the earth’s and on surface consequent via various additional kinds pile of loads.heat exchangers Thus, SIA D0190[21,22], views while EGS some from of onlythem the consider thermo-active the mechanical piles’ perspective. impact of temper SIA D0190ature refers variations to the on numerical the EGS tool as well PILESIM [23,24]. used for the calculationGenerally, of the GSHP thermal system performance design and of energy related piles environmental [9]. considerations are covered within suchHowever, guidance besidesdocuments thermo-active [17], where piles, the distributi whose useon beganof responsibilities in the 1990s among [10], base participating slabs, diaphragm system walls, tunnels [11–13] and even sewers [14] are nowadays utilized for heating and cooling purposes, while borehole fields have the potential of being used for seasonal thermal storage [15]. The European