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Ground source heat pump systems represent an interesting example of renewable energy technology for heating and cooling of buildings. The connection with the ground is usually done by means of a closed loop where a heat-carrier fluid (pure water or a solution of antifreeze and water) flows and, in heating mode, moves heat from ground to refrigerant fluid of heat pump. A new solution is the direct expansion heat pump. In this case, the heat-carrier fluid inside the ground loop is the same refrigerant fluid of heat pump. This paper focuses on the energy performance of direct expansion ground source heat pump with borehole heat exchangers in heating mode, looking at residential building installations. For this purpose, the evaporating process of the refrigerant fluid inside vertical tubes is investigated in order to analyze the influence of the convective heat transfer coefficient on the global heat transfer with the surrounding ground. Then, an analytical model reported in literature for the design of common borehole heat exchangers has been modified for direct expansion systems. Finally, the direct expansion and common ground source heat pumps have been compared in terms of both total borehole length and thermal performance. Results indicate that the direct expansion system has higher energy performance and requires lower total borehole length compared to the common system. However, when the two systems are compared with the same mean fluid evaporating temperature, the overall length of the ground heat exchanger of the direct expansion heat pump is greater than that of the common system. [ABSTRACT FROM AUTHOR]

Abstract: The winter operation of ground-source heat pumps is expected to lead to temperature decrease in the zones surrounding the vertical ground heat exchangers. The following thermal drift phenomenon occurring over the years is still deeply studied from both the experimental and the theoretical viewpoint. A moving aquifer tends to oppose the thermal drift, due to the advection groundwater phenomena. For this purpose, a modelling study on the quantitative influence of the groundwater flow on the thermal drift is performed. Then, the analytical solution of the moving infinite line source is applied. Different forms (closed analytical, asymptotical or tabled one) are identified for this solution depending on the value range of the thermal physical parameters of the system. Referring to a particularly critical case study, an easy-to-use application of the model based on this analytical solution is then presented. An evaluation is carried out as well on the interaction between the existing borefield and a nearby one, being operated afterwards, in presence of either still or moving aquifer. Finally, the influence of the actually existing flow upon improving the effective operating conditions of the heat pump plant is assessed. Moreover, when groundwater flow is taken into account, the decrease of the total borehole length is evaluated. [Copyright &y& Elsevier]

Abstract: In this paper a numerical model to analyze the thermal behaviour of vertical ground heat exchangers with a helical shaped pipe is presented. This type of configuration can be a suitable alternative to conventional ground heat exchangers, especially when the heating and cooling loads of the building are very low. The model describes the heat transfer problem by means of a network of interconnected thermal resistances and capacitances. Moreover, as the investigated ground heat exchanger is usually installed in shallow depth, the model takes into account the interaction between the ground and the ambient environment which affects the fluid heat carrier temperature into the heat exchanger and, as a consequence, the energy efficiency of the heat pump. After a sensitivity analysis on the mesh parameters, the presented model is compared with experimental data and the simulation results show good agreement with the measurements. Finally, analyses to investigate the influence of the weather conditions, of the axial heat transfer and of the pitch between the turns of the helical pipe for two types of ground are carried out. [Copyright &y& Elsevier]

Abstract: Better energy efficiency of ground coupled heat pump systems in comparison with traditional applications leads to continued growth in the number of installations for space conditioning. The solution with vertical heat exchangers is the most widespread. The design of the borehole heat exchangers (BHEs) is a nodal point, both from an energy efficiency and an economic point of view. In literature, several methods to design these systems are available. However, to promote their application, easy-to-use procedures are required; furthermore designers very often are discouraged from the use of not-open or complicated computational tools. Among the literature models, the ASHRAE method is surely the simplest procedure and, as a consequence, it is suitable for this goal. In this approach, sizing of the BHE length is strongly affected by the parameter named penalty temperature, which is an index to evaluate the long-term behaviour of the borehole field. In this paper, a review of this index is reported and in addition a new approach for its evaluation is presented. Furthermore, a detailed analysis of the proposed method is performed on a real case-study building with only heating conditioning. [Copyright &y& Elsevier]

Abstract: Several models are available in literature to simulate ground heat exchangers. In this paper an approach based on electrical analogy is presented, for this reason named CaRM (CApacity Resistance Model). In some cases several information are needed during design: both the borehole and the surrounding ground are affected by thermal exchange. The model here presented allows to consider the fluid flow pattern along the classical vertical ground heat exchangers as a single U-tube, a double U-tube or coaxial pipes. Besides, ground temperature at different distances from borehole are calculated, taking into account also the thermal interference between more boreholes. Starting from the supply temperature to the heat exchanger, the outlet fluid temperature is calculated and the ground temperature in each node, step by step. The model has been validated by means of a commercial software based on the finite differences method. Further comparisons have been carried out against data from a ground thermal response test and from the survey of an office building equipped with a ground coupled heat pump and vertical double U-tube heat exchangers. The agreement of results validates the model here presented. [Copyright &y& Elsevier]

A simulation model of ground source heat pump systems has been used to investigate to what extent a variable flow of the heat-carrier fluid of the ground loop affects the energy efficiency of the entire system. The model contemporaneously considers the borehole heat exchangers, the heat pump, the building load, and the control strategies for the circulation pumps of the ground loop. A constant speed of the circulation pumps of the ground loop was compared with a variable flow controlled by means of a constant temperature difference across the heat pump on the ground side considering the load profile of an office building located in North Italy. The analysis was carried out for a single U-tube, double U-tube and coaxial pipe heat exchangers. The control strategies adopted to manage the flow rate of the heat-carrier fluid of the ground loop affect both the heat exchange rate of the borehole field and the heat pump’s long-term energy efficiency. The simulations show considerable differences in the system’s seasonal energy efficiency. The constant speed of the circulation pumps leads to the best results as far as the heat pump’s energy performance was concerned, but this advantage was lost because of the greater amount of electrical energy used by the circulation pumps; this, of course, affects the energy efficiency of the entire system. The optimal solution appears then to be a constant temperature difference in the heat-carrier fluid across the heat pump. [ABSTRACT FROM AUTHOR]

The popularity of ground source heat pump systems for both heating and cooling has grown significantly over recent years. Ground heat exchangers are usually buried in the ground either vertically in boreholes or horizontally in trenches. Antifreeze fluids in closed-loop systems are commonly used in these plants to protect them from freezing phenomena and also to reduce the total length of the ground exchangers. In fact, the use of antifreeze fluid allows the system to work below 0 °C which implies higher temperature difference between the heat-carrier fluid and the undisturbed ground, consequently the heat flux increases and the total length of the boreholes can be reduced. This study has been set out to investigate three types of layout system: two conventional heat pumps using pure water and water-glycol respectively as the secondary fluid and an innovative heat pump with a flooded evaporator using pure water. The results demonstrated that, for conventional heat pumps with dry evaporator, the use of anti-freeze additives in mild climates is convenient only in grid-shaped borehole fields with a heating-dominant thermal load. In all other cases, the use of pure water decreases the overall operating costs. In addition, flooded evaporator heat pumps using pure water as a secondary fluid on the ground loop proved to be the most cost-effective solution. [ABSTRACT FROM AUTHOR]

An important consideration with regard to ground source heat pump (GSHP) systems is ensuring high energy efficiency performance throughout the years. Multi-year analyses are thus exceedingly important to evaluate the seasonal oscillations in the ground temperature and its long-term drift. As only a limited number of results based on real cases are available in the literature, this work aims to address this paucity of information. Seasonal and long-term computer simulations were conducted in order to investigate the thermal performance of two office buildings in Italy with unbalanced load profiles. The study focuses on the evaluation of ground temperature drift which was essentially due to an imbalance of the buildings’ load profile. An analysis, which was compared to two analytical approaches available in the literature, was carried out using a detailed numerical model. The effects of improved load balancing of the ground load profile and of a higher number of boreholes on seasonal oscillations and on the thermal drift were investigated. The results showed that rebalancing the ratio between the heat extracted from and injected into the ground led to better results from an energetic viewpoint almost certainly synonymous with an energetic-economic optimization. [ABSTRACT FROM AUTHOR]

In the European Union, 40% of the overall final energy consumption is attributable to the buildings sector. A reason for such data may be found considering that the great majority of the building stock is more than 40 years old. According to the European Commission, an interesting potential lies in the refurbishment of the building sector, and heat pump technology has been recognized as one of the most cost-effective solutions to tackle the environmental issue of this sector. Regarding heat pump technology, ground-source heat pumps (GSHPs) have been proven to be the most efficient solution on equal boundary conditions. Despite this, in most EU states' markets, GSHPs hold only a small market share with respect to air-source heat pumps. In this paper, the state of art and possible future developments of GSHP technology have been reviewed together with a focus on the potential of such technology, most of all on the refurbishment of existing buildings, and on the obstacles to its spread. The state of art of borehole heat exchangers has been studied, focusing on the parameters characterizing the outside pipe and the pipe itself, i.e., pipe and grout materials. Moreover, an overview on the last developments involving refrigerants and secondary fluids is given. Finally, the design and control strategies of GSHPs have been reviewed. [ABSTRACT FROM AUTHOR]

Abstract: There is a rising interest in heating and cooling systems based on renewable energy sources. Air heating or cooling with earth-to-air heat exchangers (EAHE) is one approach for reducing ventilation heat losses and improving thermal comfort in buildings. A literature research was performed in order to analyze the design, characteristics of earth-to-air heat exchangers and whether they could be coupled with HVAC system coupling. A range of projects was compared in order to collect and summarize design suggestions. [Copyright &y& Elsevier]

Highlights: [•] Short helical shaped pipe and double U-tube heat exchangers are compared. [•] The thermal behaviour in both long and short terms is investigated. [•] The models consider the interaction between the ground and the environment. [•] The axial heat conduction affects the fluid heat carrier temperature. [•] The helical heat exchanger has better thermal performance than the double U-tube. [ABSTRACT FROM AUTHOR]

Abstract: A reliable evaluation of long-term performance of a heat pump coupled with a borehole heat exchanger (BHE) field is necessary to verify the stability of its heat exchange capability over the time. The BHE field pattern is often assumed to be regular (e.g., rectangular, L-shaped, T-shaped, etc.), or is assumed to be adequately approximated by one of these shapes. Moreover, a planned geothermal system is often designed regardless of the presence of other existing or planned BHE systems. In order to evaluate the validity and the possible limitations of these assumptions commonly made by the designer, a number of 25-year time span simulations have been carried out by means of 2D finite element modeling. In particular, the case of a real 28 BHE field, irregularly shaped and related to a building located in Northern Italy, has been studied together with its 7-by-4 regular grid approximation and a series of 28 BHE fields having different shapes. Besides the real annual thermal load profile characterized by quasi-balanced winter heating and summer cooling, two other profiles characterized by increasingly unbalanced operation have been taken into account. The numerical study shows that (i) the regularly shaped approximation, a common choice in BHE design, seems to be reasonable under the condition that groundwater flow is absent for all the thermal load profiles; (ii) if a strong heating/cooling imbalance occurs, the thermal footprint of a BHE field can be very extensive, preventing the installation of future nearby BHE systems. [Copyright &y& Elsevier]

Abstract: In this paper an improvement of the model CaRM (CApacity Resistance Model) is presented to consider the borehole thermal capacitance, both of the filling material of the borehole and of the heat carrier fluid inside the ground heat exchanger. Several models, numerical and analytical, are available in literature for short time step analyses of ground-coupled heat pump systems. According to the modelling for the surrounding ground, the new approach for the inside of the borehole is based on electrical analogy. In this study the double U-tube ground heat exchanger is analyzed. The new model has been validated by means of a commercial software based on the finite elements method as well as measurements of ground response test, using a suitable plant system. In this last comparison, the contribution of the thermal capacitance of the circulating fluid is investigated, since it is frequently neglected in short time step simulations. In both cases, there is agreement between the CaRM results and data from numerical simulations and measurements as well. [Copyright &y& Elsevier]

The overall goal of the EU funded project CHEAP-GSHPs is the development of more efficient and safe shallow geothermal systems for building conditioning, and the reduction of their installation costs. New types of heat exchangers and installation techniques have been studied. In addition, in order to reduce the engineering costs of geothermal heat exchangers sizing, a Decision Support System (DSS) has been developed for the design of new geothermal systems. The DSS tools consider all aspects from the geological and drilling issues to the technical and economical ones, by starting from several possible design and operational solutions. They support the user in the selection of the most suitable heat exchanger, in the borehole field sizing and in the choice of the best installation technique. These tools are public and available on the web (http://cheap-gshp.eu/); they have been targeted at users with different levels of knowledge, so that they can be used by both expert and non-expert users.Several databases have been built up to feed the DSS: (I) a detailed Thermo-Geological Database indicating the thermo-physical parameters of a wide spectrum of european lithologies and unconsolidated sediments classes; in order to allow the correct evaluation of the heat exchange capacity of the subsoil where a new borehole field has to be installed. (II) an European Superficial Geological Map, that represents the variability of the subsoil conditions in Europe; this is used associated to the Thermo-Geological database as well as to suggest the best drilling technique for the local geological setting;(III) a pan-european Climatic Database; (IV) a standard temporal profiles database of the heating and cooling requirements for several standardized typologies of buildings, by taking into account also several thermal insulation levels.The detailed Thermo-Geological database has been built in order to identify the most appropriate values of physical and thermal parameters to be assigned to the local geological context. The database consolidates a wide amount of data present in the international guidelines and scientific literature (it is based on more than 60 references) integrated with more than 400 experimental measurements performed on samples of rocks and sediments sampled in several european countries. The Database include 29 categories of rocks and 10 classes of unconsolidated sediments. The Thermo-Geological Database merges the thermal conductivity ranges exposed in literature with the values acquired by the direct measurements; in addition, the reference values of density and thermal diffusivity for each category are indicated.Finally, a method to map the techno-economic potential of closed-loop shallow geothermal systems (expressed as €/kW) has been developed. It merges all geological, climatic and energetic data collected in the Databases in order to quantify the geothermal potential for different locations and the feasibility of shallow geothermal systems. The method has been successfully applied in some test sites representative of main European contexts. The new web tools and the developed mapping method support the design and the installation of new shallow geothermal systems, by taking into account all the involved aspects from the geological to the economical ones. [ABSTRACT FROM AUTHOR]

Ground-coupled heat pumps are increasingly being utilized to heat and cool buildings. Although it is difficult to size and to predict their behavior and performance, their design can be optimized via simulations. EnergyPlus is a popular energy simulation program for modeling building heating and other energy flows and, since it is organized to consider borehole heat exchangers via the well-known g-functions approach, it can be used advantageously for that purpose. The Capacity Resistance Model is another recent numerical simulation tool devoted to ground and borehole heat exchangers. In this work, two methods to calculate the g-fucntions were analyzed, using as case-study a real office building, whose imbalance between the heat extracted and injected into the ground was found to be appreciable. The energy imbalance involves a ground temperature drift affecting the system efficiency. The results of the EnergyPlus g-functions and the Capacity Resistance Model model approaches were compared. The capacity of the two methodologies to accurately simulate this phenomenon were analysed also with reference to the available building's long-term monitoring data. The analysis showed the importance of using g-functions suitable to reflect the layout of the borehole field, in order to correctly evaluate the energy performance of the entire ground source heat pump system. [ABSTRACT FROM AUTHOR]

The design phase of ground source heat pump systems is an extremely important one as many of the decisions made at that time can affect the system's energy performance as well as installation and operating costs. The current study examined the interpretation of thermal response testing measurements used to evaluate the equivalent ground thermal conductivity and thus to design the system. All the measurements were taken at the same geological site located in Molinella, Bologna (Italy) where a variety of borehole heat exchangers (BHEs) had been installed and investigated within the project Cheap-GSHPs (Cheap and efficient application of reliable Ground Source Heat exchangers and Pumps) of the European Union's Horizon 2020 research and innovation program. The measurements were initially analyzed in accordance with the common interpretation based on the first-order approximation of the solution for the infinite line source model and then by utilizing the complete solutions of both the infinite line and cylinder source models. An inverse numerical approach based on a detailed model that considers the current geometry of the BHE and the axial heat transfer as well as the effect of weather on the ground surface was also used. Study findings revealed that the best result was generally obtained using the inverse numerical interpretation. [ABSTRACT FROM AUTHOR]

Energy foundation piles can be an efficient cost-effective solution for ground source heat pump applications. As they are very different from standard ground heat exchangers, numerical or analytical models usually used to investigate their thermal performance must be carefully analysed before they can be applied. The total length of ground heat exchangers is a critically important factor in the overall design of a ground source heat pump. Ground thermal properties are the primary parameters affecting the size of ground heat exchangers. The thermal response test is the most commonly used procedure to determine the ground’s thermal conductivity. The current study investigated a thermal response test on a 20 m long energy foundation pile equipped with a double U-tube circuit. The test measurements were interpreted using both the infinite line source model and inverse numerical analysis by means of a detailed model that was able to consider both the actual geometry of the pile and the axial heat transfer. The results indicated an approximately 50% difference between the two values of ground thermal conductivity. The effect of this evaluation on the calculation of the energy piles’ total length was first investigated utilizing a method outlined in the literature. The consequences on the long term energy performance of a ground source heat pump system were then investigated using numerical simulations and taking into consideration the effect of the spacing between the foundation piles. It was found that the total length calculated using the result based on the infinite line source model was about 20% lower than that obtained using the inverse numerical approach. An approximately 10% difference in the heat pump’s seasonal energy efficiency was also found. [ABSTRACT FROM AUTHOR]

An empirical method aimed at generating maps of potential of geothermal energy exchange for shallow vertical closed-loop systems is proposed here. The method uses both geological and technological information. In particular, the ground parameters that mainly influence the heat transfer in borehole heat exchangers, the energetic parameters driving efficient operations of geothermal systems and heating and cooling requirements of a typical residential building are taken into account. Spatial modeling is carried out in a Geographic Information System leading to an effective and easy-to-use digital cartographic tool. An application of the method to four regions of southern Italy is also shown. [ABSTRACT FROM AUTHOR]

• The design of a new GSHP system cannot disregard local ground thermal properties. • The ground thermal properties also affect the system long-term energetic performances. • A thermal properties database is proposed for both rocks and unconsolidated sediments. • It integrates and compares literature data with more than 400 direct measurements. When a new ground source heat exchanger field is planned, underground thermal properties input data are necessary for the correct sizing of the geo-exchange system. To support the design, the EU founded Cheap-GSHPs project developed a Decision Support System, that comprises a new database of thermal properties for both rocks and unconsolidated sediments. The thermal properties database has been developed by integrating and comparing data (1) provided by the most important international guidelines, (2) acquired from a wide literature review and (3) obtained from more than 400 direct measurements. The data are mainly thermal conductivity data, hence the convective contribution provided by groundwater flow to heat transfer is not included. This paper presents and analyses the collected database. [ABSTRACT FROM AUTHOR]

In this study, the thermal behavior of the coaxial and double U borehole heat exchangers was investigated using numerical simulations in both the long- and short-term. As a reference for borehole heat exchanger specifications, the existing coaxial and double U probes of a geothermal heat pump installed within the Horizon 2020 research project named "Cheap GSHPs" were considered. Nine years of simulations revealed that when borehole heat exchangers are subjected to a balanced thermal load, and intermittent operating modes of the ground source heat pump system are set, the coaxial pipes' configuration provides better thermal performance due to the higher thermal capacitance of the heat-carrier fluid and the lower borehole thermal resistance. The analysis was conducted considering two different types of ground with different thermal conductivity values. As result, the more conductive ground type highlights the higher yield of the coaxial probe. [ABSTRACT FROM AUTHOR]