Low temperature district heating and cooling networks (5GDHC) in combination with very shallow geothermal energy potentials enable the complete renewable heating and cooling supply of settlements up to entire city districts. With the help of 5GDHC, heating and cooling can be distributed at a low temperature level with almost no distribution losses and made useable to consumers via decentralized heat pumps. Numerous renewable heat sources, from wastewater heat exchangers and low-temperature industrial waste heat to borehole heat exchangers and large-scale geothermal collector systems, can be used for these networks. The use of large-scale geothermal collector systems also offers the opportunity to shift heating and cooling loads seasonally, contributing to flexibility in the heating network. In addition, the soil can be cooled below freezing point due to the strong regeneration caused by solar irradiation. Multilayer geothermal collector systems can be used to deliberately generate excessive cooling of individual areas in order to provide cooling energy for residential buildings, office complexes or industrial applications.

With the monitoring of the biggest large-scale geothermal collector system in Germany, Bad Nauheim, efficiency and long-term operation can be determined. The double layer collector system with 11.000 m² per collector layer serves energy to the 6 km long 5GDHC and around 400 residential units. A lot of measurement components have been installed into the geothermal collector system, the energy center, the weather data, the 5GHDC and on the building level inside the decentralized heat pumps. With this measurement system potential for optimization as well as the long term operation is going to be monitored since 2021. The measurement data is also necessary for the validation of simulation models to get valid planning tools for those systems. Together with geological earth analysis and groundwater monitoring the earth body is also captured in detail.

The measurement system and first operational data is going to be shown during the EGC 2022.

The increasing spread of ground-source heat pumps has posed the question of how to maximize the use of shallow geothermal energy without risking its local overexploitation. Several countries have set regulations with this purpose. However, these regulations tend to lack a scientific basis and rather rely on arbitrarily set thresholds on the distance between neighbouring installations or the ground-loop fluid temperature. Such regulations risk to be unnecessarily conservative for certain areas or insufficient for others.

Therefore, we are developing an open access tool implementing state of the art underground heat transfer models to allow scientific-based decisions for the installation permits, i.e., based on whether the new installation will cause severe performance loss on existing installations or prevent the neighbours from installing systems of comparable capacity. The tool will also be useful for designers to size the borehole heat exchangers taking into account the thermal influence from the neighbouring installations.

In this paper, we present the possible scientific-based regulations discussed within the project, and the existing and planned features of the tool to allow decisions based on these regulations and to design the systems.

Ground source heat pump systems are increasingly popular as a CO₂-emission-reducing alternative for heating and cooling of residential and commercial buildings. Compared to air source heat pumps, ground source systems are more cost intensive, but reach higher efficiencies, and thus, have a lower environmental impact. In order to make ground source heat pumps more competitive, the need of a highly reliable and precise design of the geothermal part of the system rises. This applies equally for vertical borehole heat exchanger systems, horizontal heat exchanger characterisation, and subsurface data.

Thermal response test methods are suitable for determining thermal properties of the entire geothermal system, comprising the subsurface and the ground heat exchanger. In this paper, several advanced thermal and geophysical response test methods are presented. First, an in-situ measurement and evaluation method which enables the verification of the thermal-energetic properties of vertical borehole heat exchanger systems at the time of commissioning is developed. This method aims to verify the built system, and check the design conditions during commissioning. Secondly, the thermal response test procedure established for vertical borehole heat exchangers is extended to horizontal source systems and exemplarily worked out on trench collectors. This is intended to achieve a characterisation of the thermal behaviour of the geothermal collector within the subsurface, and to improve the dimensioning process. Finally, an analysis and optimisation of short time-backfill-check thermal response tests and other innovative borehole heat exchanger measurement methods are presented. One main aim of these test methods is to provide a more precise quality check of the backfilling. Thereby, especially the significance of short time-backfill-check thermal response tests is adequately evaluated by a systematic thermal characterisation of the borehole heat exchanger system, which is unique in so far. Additionally, precise heat capacity and thermal conductivity measurements of several geothermal materials (subsurface, grouting, etc.) are conducted. Furthermore, a new variant of the short time-backfill-check thermal response test is introduced to achieve a three-dimensional temperature measurement field (active thermal tomography), and a previously developed ultrasonic probe is tested in practice and its application limits are determined.

The overall aim of the developed thermal and geophysical test methods is to ensure and increase the quality of shallow geothermal plants in order to contribute to the reduction of risks, the reduction of energy production costs, the increase of efficiency, and plant availability as well as to increase awareness and public acceptance.

The use of energy geostructures as underground infrastructures equipped with ground heat exchangers (GHEs) to exploit geothermal energy is becoming more popular. The reduction of capital costs for drilling required for traditional ground heat exchangers, along with the proven ability to play the role of an energy source/storage in supplying both heating and cooling makes such dual role in-ground structures increasingly attractive. Among various types of energy geostructures, energy walls, which are thermally activated embedded retaining walls constructed to support building basements or metro stations, are seeing an increase in commercial and research activity.

Despite the considerable potential of energy walls in providing cost-effective renewable energy, there is limited research dealing with their transient thermal modelling, which is required to understand energy capacity. TRNSYS software is a well-known flexible tool for conducting dynamic simulation and has been used extensively for energy analysis of various ground heat exchangers incorporated into ground source heat pump (GSHP) systems. However, due to the novelty of energy walls, these innovative ground heat exchangers have not been modelled and evaluated in the TRNSYS environment. In this study, a number of models developed in TRNSYS using the finite difference method will be evaluated in terms of their ability to represent energy walls. First, the details of the models will be described and their benefits and limitations in dealing with energy wall boundary conditions will be discussed. Subsequently, the models will be developed and calibrated to verify their validity based on two sets of experimental data from operating energy walls in the literature. The suitability of the models to represent energy walls will be assessed in terms of accuracy and computational expense. It will be shown that the TRNSYS model developed for horizontal heat exchangers can be suitably calibrated to modelling energy walls with an acceptable level of accuracy. However, the three-dimensional simulation of buried heat transfer pipes in energy walls using the TRNSYS can result in a relatively high computational cost particularly for long-timescale modelling, which may restrict its use in practical circumstances.

The materials and technology used to build ground heat exchangers (GHE) significantly affect the thermal performance of a geothermal system, as well as the local geological and hydrogeological context. The use of metallic pipes was explored, evaluating corrosion problems and proposing measures to improve the reliability of the systems over time. The installation of the GHEs implies the alteration of the conditions of the undisturbed undergrounds, which could lead to a significant development of corrosion, mainly due to the penetration of atmospheric oxygen into the subsoil and the presence of water in contact with the GHE. According to best practice, the use of metal GHE is an unfavorable option if the pipes are not made of corrosion resistant materials and is discouraged by regulations, despite the good thermal conductivity and the relatively low cost of the Carbon steel (CS). Other materials that might be considered are metals such as corrosion resistant stainless steel (SST) grades, but these materials are too expensive and have worse thermal properties.

The well point system is a cost effective way that is used in the construction field to reduce groundwater level before excavation works. It consists of a series of metal tubes (length 7-10 m) inserted into the ground to draw water. These tubes are normally removed after use, but on the frame of Horizon 2020 GEO4CIVHIC project their exploitation as low-cost GHEs was investigated. These tubes have become coaxial GHE, with the outer metallic tube in direct contact with the ground. An expected benefit of this proposed solution is that the use of pipes made in CS or SST should lead to increased heat exchange against a traditional plastic single or double U layout, according to previous literature. As for durability, it is mandatory implementing anticorrosive measures, and the possible negative effect of such measures on the heat exchange capability and leakage hazard should be evaluated. Two different grades of SST (eight SST well point GHEs in total: 1x AISI 316, 7x AISI 304) and four passive anti-corrosion measures for CS (eight CS well point GHEs in total: 1x sacrificial anode, 2x bitumen, 2x paint, 2x galvanized zinc and 1 bare tube as a reference) have been arranged and installed into the ground, in a pilot case in Padua, (Italy). Their thermal transfer performances have been evaluated by Ground Response Test. The results are consistent with the pipe wall resistivity model, based on the measured thermal conductivity of the individual materials constituting each installed GHE solution.

A simple, invisible and seasonally operated solution to avoid urban heat islands is the harvesting of solar urban excess heat from building surfaces, sidewalks, streets and squares and transport into a geothermalr borehole heat exchange (BHE) field as seasonal thermal energy storage. However, since temperatures in urban surfaces are sometimes very high and the temperature of the subsoil of a city is already higher than the climatically induced temperatures, the thermophysical properties of the subsoil for calculation and designing of these systems is crucial to avoid thermal heating and cooling plumes around the BHE fields and thus negative thermal effects.

In this work a geothermal borehole heat exchanger for testing purposes was installed in Vienna. The 80 m long drill core was used for thermophysical investigations of the subsoil. Samples of this drill core at different depths were prepared and effective thermal conductivity, specific heat capacity as well as thermal expansion measurements were conducted. The Heat Flow Meter (HFM) method was applied for stationary effective thermal conductivity measurements, Differential Scanning Calorimetry (DSC) for evaluation of the specific heat capacity and push rod dilatometry for determination of the thermal expansion. The samples to be measured were measured twice, once in their original moisture state and then again after a defined drying process.

The results of the effective thermal conductivity and specific heat capacity show a strong dependency of the moisture content of the observed subsoil sample. In case of the samples at 80 m depth and a temperature of 25 °C, effective thermal conductivity of the moist sample was approx. 2 W/(mK) in the moist and approx. 0.63 W/(mK) in the dry state. Specific heat capacity at the same depth and temperature resulted in 1.61 J/(gK) in the moist and 0.91 J/(gK) in the dry state. Thermal expansion measurements have shown a strong shrinkage through drying in the same order of magnitude than thermal expansion of the dry samples.

The arrangement of boreholes in ground heat exchangers used with ground-source heat pump systems is commonly based on pre-computed libraries of g-functions with standard configurations, e.g. placing the boreholes on a uniformly-spaced rectangular grid. Particularly for larger fields with many boreholes in situations with significant annual heat rejection/extraction imbalance, these configurations may be far from optimal. That is, depending on the space constraints, it may be possible to reduce the number of boreholes and amount of drilling required by shifting the positions of the boreholes to make better use of the available space. These configurations of boreholes are unlikely to be found in any library. Furthermore, manual arrangement of boreholes in complex-shaped fields is tedious and time-consuming for the engineer. Therefore, tools are needed that can automatically arrange boreholes in candidate configurations to fit the available land area, calculate the g-function for these configurations, select the best configuration, and determine the required depth for the best configuration. These tools need to be reasonably fast in order to be practical for the design engineer.

This paper reports on a fast method for calculating approximate g-functions using non-uniform segments and pre-computed integral tables. Despite being “approximate” g-functions, the difference between a g-function calculated with a more detailed method and the approximate g-function is usually under 1% RMSE. The g-functions for borehole fields with 300, 500, and 1000 boreholes can be calculated in about 2, 6, and 30 seconds on a run-of-the-mill desktop PC. The paper presents the methodology, quantifies the computational time requirements and accuracy of both the g-function and the resulting designs.

Most of the BHE systems which are currently designed in France aim above all to cover heat needs either independently or with the help of gas boilers. This causes a great energy imbalance in the ground, leading to large borefields, high investment amounts and rather low energy productivity per meter of drilled BHE.

The objective of this paper is to demonstrate, through explaining a few practical cases, the benefits of coupling ground-source and air-source systems (such as air to water heat pump or drycooler).

In such a context, air-source systems air can be used:

• as a cold source for the heat pump, replacing or complementing the BHE in order to reduce the energy demand for the ground,

• to directly regenerate the borefield, and thus limit the hot / cold imbalance in the ground.

The economic analysis is performed over a period of 20 years, considering both investments and operating costs. Results suggest a set of design guidelines that can be used to select and size equipment, under French climatic conditions.

Design, installation and testing of an innovative energy geostructure, tailored for tunnels excavated by Tunnel Boring Machine, will be presented. The prototype was developed by the joint efforts of BBT-SE (Brenner Base Tunnel), involved in the construction of a new railway base tunnel system connecting Italy and Austria, and University of Bologna, engaged in applied research over various aspects of the BBT system. The energy geostructure consists in a modular horizontal closed-loop system located at the invert of the exploratory tunnel of the BBT system, in the space dedicated to collect the drained water. For the intrinsic type of the heat exchange process, substantially influenced by drainage water, and for its compact design and simple installation procedure, the prototype was called “Smart Flowing”. Modules were built outside and later moved inside the tunnel, and eventually placed and assembled concurrently to the advancement of the Tunnel Boring Machine. Specific tests were performed to prove the reliability and the efficiency of the system, by simulating the work of a heat pump conditioning system in both heating and cooling modes. Finally, a preliminary economic and environmental potential assessment of this innovative prototype was realized. First results evidenced the performance of the system for both heat dissipation and extraction. The drainage water flow guarantees constant recovery to the natural state, thus improving the efficiency compared to classic geothermal heat exchangers. Economic savings and reduction of pollutants and greenhouse gases, with respect to burning fossil fuels, can reach up to 70%.

Phase Change Materials (PCMs) allow thermal energy storage by exploiting their latent heat during phase change. These materials could also be used to smooth any thermal fluctuations generated from operations of heat engines. The operating temperature of these materials depend on the melting point and this often governs the application. In addition to the operating temperature one needs to consider several parameters. These include- melt enthalpy, thermal diffusivity, thermal cycling stability, vapour pressure, safety, toxicity and cost. The above are parameters that are integral to the selection of PCMs, but compatibility with conventional materials of construction is also important. The selection of PCMs and construction materials are intertwined and together affect the overall cost of the PCM storage system in applications such as geothermal plants.

This paper presents the work carried out in GeoSmart (EU H2020 Project) to select suitable materials for the construction of Adipic acid-based PCM storage system. Three different commonly used construction materials were selected, namely, carbon steel (SA 179), aluminium 1050 and stainless steel (316L). Samples of the above metallic materials were exposed to molten adipic acid at 165°C for 168 h, and the corrosion rate was determined following ASTM G1. The samples were photographed before and after exposure, and a set of specimen was sectioned for SEM/EDX analysis. The corrosion rates were 6.2 mm/y, 0.11 mm/y, 0.06 mm/y for Carbon steel, aluminium and 316L, respectively. The cross-sectional analysis revealed that the corrosion was prevalent in the case of carbon steel, but no corrosion product was detected in the case of aluminium and 316L samples. The short-term experimental work demonstrated that 316L could be a potential candidate for the construction of adipic acid-based PCM storage system. However, the cost of 316L is significantly greater than that of carbon steel and this needs to be taken into account to ensure cost effectiveness of the system.

In our study we use a one-dimensional hydro-thermal soil model (CryoGrid) which was originally designed for describing thaw-freeze cycles in soils of cold regions. We apply the model for soil and climate conditions in Germany to provide model-based estimates of soil thermal regimes which can be used to access the potential for very shallow geothermal energy extraction.

As an upper model boundary condition, our model solves a surface energy balance driven by key variables delivered by climate models and therefore is able to project future changes of the soil thermal regime induced by climate change. In our analyses we focus on soil temperatures of the upper five meters of the ground to investigate soil thermal conditions under present day climate as well as under scenarios of weak and strong climate warming. We investigate the impact of air temperature and precipitation change on the soil heat budget

We demonstrate a newly developed interactive map for Germany which allows the user to evaluate the potential of geothermal energy use at individual locations and for different grain size classes.

The paper deals with the comprehensive analysis of the quantity and quality of geothermal energy extracted by Ground Source Heat Pumps with borehole sets. The analysis is based on the theory of Vertical Geothermal Systems earlier designed and published by the authors. The novelty of the proposed model in comparison to the conventional engineering approaches is derived from the description of a ground source as an nonequilibrium thermodynamic system consisting of ground (a source of low temperature energy) and a finite cylindrical borehole filled with fluid (the energy carrier) flowing through it and with continuity of temperature and energy fluxes boundary conditions between ground and fluid. It is shown that the studied problem is characterized by only dimensionless parameter - the ratio of the thermal conductivities of the ground and flowing fluid. Further, a definition of a Geothermal Energy Limit and the fundamental energy characteristic of the ground source of heat have been introduced in the paper and the physical interpretation of the received results has been provided.

For Heat Pumps considered, COP from Carnot (Carnot efficiency of Heat pumps) has been introduced as a quality characteristic of the high-temperature energy resulting from the compression of refrigerant vapor. The relationship between the amount and quality of the energy produced by Ground Source Heat Pumps with parallel- and series-connected boreholes heat exchangers has been analyzed in detail. It has been shown that the maximum СOP of such Heat Pumps can be reached with a single well, but the capacity of such a well remain bounded from above by introducing Geothermal Energy Limit notion. Connecting boreholes heat exchangers in parallel allows achieving any given capacity, but the COP_par (COP with parallel connected boreholes) decreases compared with the COP_single (COP with a single borehole) with increase in the number of wells. The quantitative relations between the amount and quality energy produced by Heat Pumps with parallel- and series-connected wells show that the amount of energy increases by tens of percent as the COP decreases by several percent in the former case. In the latter case, both the amount of energy and the COP increase by several percent. Beside, it is revealed that for well-conductive soils, the required capacity can be achieved with several wells with total length shorter than the length of a single well of the same capacity.

The obtained results enable the creation of new engineering techniques for calculations of energy capacity of Borehole Heat Exchangers for Ground Source Heat Pumps (GSHP). Besides, the suggested theory can be used for elaboration of the new scientific–technological approaches to creating optimum strategies for design, control and operation of Heat Pumps.