On the applicability of the moving line source theory to thermal response test under groundwater flow: considerations from real case studies

The classical methodology to perform and analyze thermal response test (TRT) is unsuccessful when advection contributes to heat transfer in the ground, due to the presence of a groundwater flow. In this study, the applicability, the advantages, and the limitations of the moving line source model to interpret TRT data are discussed. Two real TRT case studies from the Italian Alpine area are reported and analyzed, with both the standard infinite line source approach and the moving line source one. It is shown that the inverse heat transfer problem is ill-posed, leading to multiple solutions. However, besides minimization of the error between measurements and modeling, physical considerations help to discriminate among solutions the most plausible ones. In this regard, the MLS approach proves to be effective in the advection-dominated case. The original time criterion proposed here to disregard initial data from the fitting, based on a resistance–capacitance model of the borehole embedded in a groundwater flow, is validated in terms of convergence of the solution. In turn, in the case when advection and conduction are competitive, the MLS approach results more sensitive to ground thermal conductivity than to Darcy velocity. Although in this case a limited impact of the uncertainty in the groundwater velocity on the boreholes sizing is expected, future studies should focus on the development of a successful TRT methodology for this condition

Analytical modelling of Breathing Walls: experimental verification by means of the Dual Air Vented Thermal Box lab facility

Breathing Walls are air permeable envelope components based on porous materials. In contra-flux operation air flows opposite to conductive flux, while in pro-flux they have the same direction. The Breathing Wall behaves either as a ventilation heat exchanger or as an active insulation. In literature an analytical model describing steady state heat transfer across a Breathing Wall can be found. Since it lacks an exhaustive experimental validation, a facility developed at the Energy Department of Politecnico of Milano was used to investigate the thermo-physical behavior of a no-fines concrete based Breathing Wall in steady state Dirichlet conditions and contra-flux operation

Geothermal district heating: Energy, environmental and economic analysis of a case study in northern Italy

Geothermal district heating is a technology that has been established for over 50 years and offers a number of significant advantages. It allows multiple homes to be converted to renewable energy sources simultaneously, allows a stable heat supply with long-term fixed prices, and generally presents lower risks. The utilization of geothermal energy as a heat source for the network can be considered cost-free during operation, but has a critical economic aspect related to the initial investment. Geothermal district heating networks are in fact capital-intensive systems (CapEx), requiring substantial investments for the installation of the geothermal infrastructure. However, operating costs (OpEx) are significantly lower compared to conventional systems. This study examined the implementation of a district heating network in a medium-sized city in northern Italy. An energy and environmental impact assessment was conducted to determine the optimal plant configuration that maximises the use of the geothermal resource and minimises greenhouse gas emissions. Additionally, a sensitivity analysis was carried out to assess the impact of market variables on the overall cost of implementing the district heating network. This included an evaluation of changes in investment costs in response to variations in the value of electricity taxes -oneri di sistema-, of the revenues from the sale of thermal and electrical energy, as well as of the fuel costs. This study aims to provide a complete and detailed overview of the energy, environmental and economic implications associated with the implementation of a geothermal district heating networ

A Numerical Study on the Impact of Grouting Material on Borehole Heat Exchangers Performance in Aquifers

U-pipesforgroundsourceheatpump(GSHP)installationsaregenerallyinsertedinvertical boreholes back-filled with pumpable grouts. Grout thermal conductivity is a crucial parameter, dominating the borehole thermal resistance and impacting the heat exchanger efficiency. In order to seal the borehole and prevent leakages of the heat carrier fluid, grouting materials are also hydraulicallyimpermeable,sothatgroundwaterflowinsidetheboreholeisinhibited. Theinfluenceof groundwater flow on the borehole heat exchangers (BHE) performance has recently been highlighted by several authors. However groundwater impact and grouting materials influence are usually evaluated separately, disregarding any combined effect. Therefore simulation is used to investigate the role of the thermal and hydraulic conductivities of the grout when the BHE operates in an aquiferwitharelevantgroundwaterflow. Here3maincasesforasingleU-pipeinasandyaquiferare compared. InCase1theboreholeisback-filledwiththesurroundingsoilformation,whileathermally enhancedgroutandalowthermalconductivitygroutareconsideredinCase2andCase3respectively. Simulations are carried out maintaining the inlet temperature constant in order to reproduce the yearly operation of the GSHP system. For each of the 3 cases three different groundwater flow velocities are considered. The results show that a high thermal conductivity grout further enhances the effects of a significant groundwater flow. The conditions when neglecting the grout material in the numerical model does not lead to relevant errors are also identified

Design of Borehole Heat Exchangers for Ground Source Heat Pumps: A Comparison between Two Methods

Different methods for the design of Borehole Heat Exchangers are available, as reported for example in the Italian standard UNI 11466. Therefore the question arises about the impact of the design methodology on the final result and its sensitivity to the main design and input parameters. In this paper two common design approaches, namely the ASHRAE analytical method by Kavanaugh and Rafferty and the GLHEPRO commercial tool, based on g-functions method by Eskilson, are taken into account. The two methods are used to design a BHE field for a GSHP system in two case studies, namely a small-scale residential and a medium-scale commercial building. Moreover, a sensitivity analysis for each method is carried out, considering the influence of the main design choices and uncertainties on the required inputs. The comparison between the two methods shows that ASHRAE tends to overestimate, up to 27%, the BHE size compared to GLHEPRO. Among the parameters investigated, the heat pump size and the BHE layout modestly affect the final BHE size. In turn, the thermal-vector fluid temperatures on the ground side of the heat pump, the single/double U pipe configuration, the distance among adjacent boreholes and the ground thermal conductivity result in the major influence. In particular it is shown that the uncertainty in the ground thermal conductivity and the choice of the fluid temperatures have a comparable impact on the final sizing as the choice of the sizing method

Energy performance of an exhibition hall in a life cycle perspective: embodied energy, operational energy and retrofit strategies

Nowadays, the focus on the building energy consumption in the use phase prevails over an interest concerning the energy impacts linked to all the other phases of the construction process. However, reducing operational energy could lead to shifting the impacts from one stage to another. Thus, combining the study of strategies improving energy efficiency in the use phase with a life cycle approach is crucial. Exhibition halls are peculiar buildings from the geometry, construction and use points of view, rarely addressed in energy and life cycle energy analysis studies. Therefore, in this paper, a representative hall of the Milan Trade Fair is taken as a case study. A building energy simulation model is firstly calibrated in order to derive the operational energy for climatisation. The operational energy appears artificially low due to the short use period during the year. When compared with the calculated embodied energy of the envelope and structure, it is found that 57 years would be needed to balance energy spent in the construction and in the use phase. Further, some retrofit interventions are proposed and analyzed. Insulation interventions are not attractive from the economic payback time point of view. However, when the embodied energy of the retrofit interventions is compared with the energy savings in the use phase, interesting energy payback times are obtained. Therefore, this study puts in evidence on the importance of adopting a life cycle perspective, especially for buildings with low-intensity use. Eventually, the critical issues of the life cycle energy analysis are deeply discussed

Geothermal district heating: Energy, environmental and economic analysis of a case study in northern Italy

Geothermal district heating is a technology that has been established for over 50 years and offers a number of significant advantages. It allows multiple homes to be converted to renewable energy sources simultaneously, allows a stable heat supply with long-term fixed prices, and generally presents lower risks. The utilization of geothermal energy as a heat source for the network can be considered cost-free during operation, but has a critical economic aspect related to the initial investment. Geothermal district heating networks are in fact capital-intensive systems (CapEx), requiring substantial investments for the installation of the geothermal infrastructure. However, operating costs (OpEx) are significantly lower compared to conventional systems. This study examined the implementation of a district heating network in a medium-sized city in northern Italy. An energy and environmental impact assessment was conducted to determine the optimal plant configuration that maximises the use of the geothermal resource and minimises greenhouse gas emissions. Additionally, a sensitivity analysis was carried out to assess the impact of market variables on the overall cost of implementing the district heating network. This included an evaluation of changes in investment costs in response to variations in the value of electricity taxes -oneri di sistema-, of the revenues from the sale of thermal and electrical energy, as well as of the fuel costs. This study aims to provide a complete and detailed overview of the energy, environmental and economic implications associated with the implementation of a geothermal district heating networ

Borehole Heat Exchangers: heat transfer simulation in the presence of a groundwater flow

The correct design of the Borehole Heat Exchanger is crucial for the operation and the energy performance of a Ground Source Heat Pump. Most design methods and tools are based on the assumption that the ground is a solid medium where conduction is the only heat transfer mechanism. In turn in regions rich in groundwater the groundwater flow influence has to be assessed, by including the convection effects. In this paper a numerical model of a 100 m U-pipe in a saturated porous medium is presented. The model is created adopting MT3DMS coupled to MODFLOW. A Darcy flow is imposed across the medium. The typical operation of a Borehole Heat Exchanger operating both in winter and in summer is simulated for two years, under different groundwater velocities. The energy injected to and extracted from the ground is derived as a function of the Darcy velocity and compared with the purely conductive case. Temperature fields in the ground at key moments are shown and discussed. From both the energy and the aquifer temperature field points of view, the velocity ranges for respectively negligible and relevant influence of the groundwater flow are identified

Assessment of thermal behaviour of thermo-active diaphragm walls based on monitoring data

Thermo-active diaphragm walls have proved their effectiveness in the thermal conditioning of buildings and infrastructures. However, some aspects still need to be investigated in order to tailor methods and tools for an accurate prediction of their energy and structural performance. In this perspective, some issues are addressed that concern the definition of models for the numerical analysis, in particular issues about the modelling of geometry and thermal boundary conditions. Taking advantage of a monitoring programme on a real full-scale structure, this research focuses on the assessment of heat transfer process and thermal response of diaphragm wall and soil mass on the basis of field data. Understanding of the heat transfer process contributes to the definition of the time-dependent thermal boundary conditions at the excavation side. From the analysis of thermal gradients in the wall, the condition at the excavation side is recognised as a major factor that influences the heat transfer process, governing the direction of the heat flux in different seasons of operation of the geothermal system