ERE2.5

The performance of borehole heat exchangers (BHE) for borehole thermal energy storage may be deteriorated by the occurrence of permeable water-bearing intermediate layers in the subsurface due to convective flow and heat transport especially at high storage temperatures. Under partly saturated conditions, e.g. in the shallow unsaturated zone, low water contents will reduce the storage capacity as well as the heat transfer rate as compared to fully saturated conditions. This study combines experimental work and numerical simulations in order to quantify these effects on the performance of high temperature BHEs under fully and partly saturated conditions.

A laboratory scale analogue of a coaxial BHE in a water saturated sand medium is constructed in a cylindrical storage cell of 1.2 m height and 1.4 m³ volume. Four short-term heat storage experiments in a temperature range from 30 to 90°C were carried out with six days of continuous heat injection at constant temperature (30, 50, 70, or 90°C), followed by 3 to 4 days of heat extraction, respectively. Temperatures and water contents were continuously monitored on a grid of 68 thermocouples and 20 SMT100 moisture sensors. The water level was then lowered to 0.2 m above the bottom of the system and the experiments were repeated under partly saturated conditions.

Under saturated conditions the observed temperatures in the 30°C experiment show an almost radial distribution with a maximum along the BHE, while for higher injection temperatures an increasing thermal stratification and tilted temperature fronts are observed, which clearly point at an increasing convective impact on heat transport. These results are corroborated by the model simulations, which show an increase of the mean vertical flow velocity from 0.1 to 0.6 m/d between 30 and 90°C due to convection. Additional simulations for purely conductive conditions generally showed lower steady state BHE heat transfer rates during the charging process. This allowed a quantification of the contribution of convection to overall heat transfer, which increases from 5.7% at 30°C to about 38% at 90°C during the charging process. During discharging, however, the thermal stratification due to convection reduces heat transfer rates by up to -38% at 90°C.

Under partly saturated conditions, heat is mainly stored in the direct vicinity of the BHE and the measured temperatures show a radial evolution with no stratification or indications for convection. This points out a conduction dominated heat transport. A decrease of heat transfer rates by about 40% (30°C) to 50% (90°C) is observed in comparison to saturated conditions, which is due to decreased thermal conductivities and heat capacities of the unsaturated porous sand. No indications for significant moisture transport in the gas phase were observed during the experiments.

These results suggest, that high permeable saturated interlayers may severely deteriorate the efficiency of borehole thermal energy storage and increase heat loss and thus environmental impacts, while for unsaturated layers a general reduction of storage rates and capacities must be expected.

How to cite: Djotsa Nguimeya Ngninjio, V., Beyer, C., and Bauer, S.: Experimental and numerical investigation of heat transfer from a high temperature borehole heat exchanger under saturated and unsaturated conditions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7010, https://doi.org/10.5194/egusphere-egu22-7010, 2022.

Energy Geo-Structures are being increasingly employed over the last decade. They combine the structural and energetic function, allowing the savings related to the absence of additional drilling, required instead by the common geothermal boreholes. Currently, they are a rather mature and deeply investigated technology, with a number of successful applications worldwide; however, some issues related to the thermo-hydro-mechanical (THM) effects induced in soils during heating/cooling cycles still deserve some more analyses, particularly for what concerns the possible non-linear behavior of soil under thermal loading. In this work, this issue has been investigated by means of fully coupled 3D FE modeling, considering a single, small diameter Energy Pile. The emphasis of the FE numerical modeling activity is the investigation of the effects induced by the pore pressure variations close to the pile during the thermal loading stage, and the assessment of the potential influence of the soil thermal softening effect on the pile behavior. Two different constitutive models have been adopted for the considered fine–grained soil, both based on the standard critical state theory: i) the classical Modified Cam Clay (MCC) model; and, ii) a similar critical state model incorporating a thermal hardening/softening mechanism for the critical friction angle, assumed as an internal variable that can be modified with temperature. In the FE model, first the mechanical load is applied at the pile head in almost undrained conditions, followed by a consolidation period during which the excess pore pressure dissipates. Thus, the pile is thermally loaded, with the temperature that is assumed to vary with a harmonic function law over periods of 1, 5 and 10 years, to investigate the short term and long–term effects. The results show that: a) for the considered case study, the thermal loading conditions produce very small changes in pore water pressure at the pile-soil interface; and no effects observed on the pile head displacements can be related to thermally-induced pore pressure changes; on the contrary, b) significant additional pile head settlements are observed in presence of thermal softening, due to by plastic shear deformations at the soil-pile interface.

How to cite: Salciarini, D., Lupattelli, A., and Ronchi, F.: Fully coupled 3D Thermo-Hydro-Mechanical analyses of a single Energy Micropile subjected to heating-cooling cycles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7617, https://doi.org/10.5194/egusphere-egu22-7617, 2022.

The material used to make ground heat exchangers for shallow geothermal applications plays a key role in the overall performance of the system, especially if no grout is used to seal the borehole where the heat exchanger is installed. In this case, the coaxial steel probe is directly coupled with the ground, without any added layer providing thermal resistance between the heat exchanger and the ground. This kind of metallic heat exchanger provides a higher heat exchange efficiency and, in addition, the installation time and costs in unconsolidated deposits are reduced by innovative drilling technique, which has been developed on purpose, where the piling methodology has been combined with a vibrating head and high pressure water injection [1]. Among the materials proposed for the novel vertical ground heat exchangers within the European Horizon 2020 GEO4CIVHIC project, carbon steel is convenient for its low cost and high thermal conductivity [2]. As the main drawback, this material suffers from corrosion, and the physical characteristics of the subsoil directly affect the development of this phenomenon, which afflicts the buried metal bodies and which affects the aging of the ground heat exchange probes. In this study, the corrosion behavior of carbon steel used for an experimental shallow geothermal installation was investigated. The corrosion rates of steel samples were measured in the laboratory using the weight loss method [3] after exposure for certain periods of time in selected ground environments. Different soil conditions were tested, in turn varying the compactness and moisture content of the soil samples collected on site. Based on the results, the corrosion rate of carbon steel was evaluated as a function of both variable parameters. This information made it possible to advance in more precise quantitative forecasts on the expected life of the installed ground heat exchangers and their safety over time.

[1] Pockele’ L, Mezzasalma G, Righini D, Vercruysse J, Cicolin F, Cadelano G, Galgaro A, Dalla Santa G, De Carli M, Emmi G, Mendrinos D, Pasquali R, Bernardi A (2020) Innovative Coaxial Heat Exchangers for Shallow Geothermal. Proceedings World Geothermal Congress 2020. Reykjavik, Iceland, 2020

[2] Cadelano G, Bortolin A, Ferrarini G, Bison P, Dalla Santa G, Di Sipio E, Bernardi A, Galgaro A (2021). Evaluation of the effect of anti-corrosion coatings on the thermal resistance of ground heat exchangers for shallow geothermal applications. Energies 14, 2586. Doi: 10.3390/en14092586

[3] ASTM Committee G162-18 Standard Practice for Conducting and Evaluating Laboratory Corrosion Tests in Soils

GEO4CIVHIC project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 792355

How to cite: Cadelano, G., Bortolin, A., Di Sipio, E., Ferrarini, G., Bison, P., Bernardi, A., Dalla Santa, G., and Galgaro, A.: Laboratory assessment of carbon steel corrosion rate of grout-less ground heat exchangers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6991, https://doi.org/10.5194/egusphere-egu22-6991, 2022.

The “Can Batlló” factory was built on the Can Mangala between 1877 and 1879, as a cotton yarn and fabric, as well as bleaching, printing and sizeing factory. It is intended to install a geothermal system for the air conditioning of an administrative building in the city of Barcelona in one of the factory´s buildings.

The purpose of this study is to confirm whether the proposed energetic demand can be achieved in a sustainable manner and to assess potential impacts in the subsurface over time.

The numerical model of flow and heat has been pre-calibrated with the regional flow model and thermal response test (TRT) of the area. The results indicate that the extracted geothermal energy could be higher (12-20% more) than the initial design. This higher performance would imply an optimisation of the system. It could be implemented by means of modifying the design of the heat exchangers, by reducing their amount and their length, or by changing the location of the exchangers to the whole location of Can Batlló, and not reducing their installation to a single building. Therefore, in addition to being able to design a more efficient and economic geothermal system, the investment could be recovered sooner[Rotman Cr1] .

Simulations at 10 years of geothermal exploitation indicate that a downstream plume may occur. This variation of the natural subsurface temperature can affect the environment.

Although the analyses carried out have made it possible to know the behavior of the subsurface in the front of this geothermal installation, to make a deeper study would be advisable to determine in more detail the long-term effects (more than 10 years) that it can cause in the subsurface. For this reason, the following tasks are recommended: (1) To create a control network at the scale of the study area to monitor the performance and the effects of the geothermal field once its installation is on operation; (2) To define the optimal characteristics of the exchangers to obtain a better geothermal performance; (3) To perform a drilling of a minimum depth of 200 meters to verify the materials described in the data collection; (4) To perform hydrochemical and isotopic analysis of the executed point to analyze the feasibility of possible groundwater uses; and (5) To carry out the conceptual and numerical modelling of the local and regional hydrogeological and geothermal system with the detail obtained with the previous point and to analyze the optimisation scenarios mentioned above.

How to cite: Coscia, F. T., Criollo, R., Scheiber, L., Bulboa, I., Pujades, E., and Vázquez-Suñé, E.: Evaluation of the efficiency and impact of a shallow geothermal installation. Case of Can Batlló, Barcelona., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7838, https://doi.org/10.5194/egusphere-egu22-7838, 2022.

An innovative user-friendly smart phone/tablet-based Application (App) to support users (drillers, owners, designers) on site to complete a preliminary evaluation of the shallow geothermal heat exchangers and heat pump system feasibility was developed in the framework of the EU funded GEO4CIVHIC Project. The App is expected to provide a preliminary evaluation of drilling time and costs, allowing users to design a first version of the geothermal system, including number of probes and recommended drilling method.

The App requires the geolocation, the underground and building structure, the feasibility and the best drilling solution for the site to be analysed. Hence, the reference data defined by the project concerning the fundamental ground parameters (thermal conductivity, lithology etc…), the climatic classes typical of the European territory and the energy profile of a selection of buildings, representative of the European buildings’ typology, are stored in a specific database internal to the application itself.

Once the site geolocation is defined, automatically or manually, the user is required to insert geological information. The geological section is developed for non-expert users to allow them a simplified underground geological characterization based on intuitive information related to the form of the landscape (geomorphology) and a rough idea of the rocks/sediments present in the area. Selecting an environment, a sub-environment and a lithology, the App, considering a simplified stratigraphy of about 100 m depth characterized by a homogenous material in saturated conditions, provides an output related to (i) the best method to drill the underground, (ii) the evaluation of time and cost for the drilling method suggested, (iii) the thermal conductivity value linked to the main lithology.

In the Building section, an estimate of the maximum energy requirement for both heating and cooling knowing the latitude and longitude coordinates and the building typology of the test site is obtained. In the App database, the European cities, based on their geographic coordinates, are grouped in four simplified climatic zone domains according to the Köppen-Geiger climate classification. This allow to obtain the average annual degree days (DD) for heating and cooling necessary to calculate the cooling and heating loads of the building. Then, selecting the reference building typology (residential/non-residential) and the ground characteristics, the App automatically calculate the geothermal field size necessary to meet its energy needs.

The validation in the Mechelen (Belgium) case study of GEO4CIVHIC project shows a good agreement between the App simplified information and the real case condition. Next future more validation will be provided. However, the App outputs are a preliminary, not refined estimate of the geothermal feasibility to realize shallow geothermal systems and cannot be considered as sufficiently detailed to plan these systems. On this regard it is mandatory to acquire more detailed local information, consulting local experts and/or referring to the Decision Support System (DSS) of GEO4CIVHIC Project.

GEO4CIVHIC project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 792355.

How to cite: Di Sipio, E., Contini, S., Mezzasalma, G., Dalla Santa, G., Galgaro, A., De Carli, M., Carnieletto, L., Zarrella, A., Castelruiz, A., Pockelè, L., and Bernardi, A.: Innovative Geothermal Application Development to support in situ workers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7806, https://doi.org/10.5194/egusphere-egu22-7806, 2022.

Ground Source Heat Pump (GSHP) systems are one of the many alternatives that will help to decarbonise the space heating and cooling – especially in residential sector. High capital costs of drilling for borehole heat exchangers and added complexity of space dedication in densely populated urban areas are hindering the market development. We propose combining GSHP systems with Sustainable Drainage Systems (SuDS) for a more efficient heat exchange, which provide opportunities to significantly improve the resilience and sustainability of our built environment. This combined system removes the need to dedicate space to accommodate shallow ground heat exchangers in areas where the unit price of land is high. Furthermore, hydrological conditions prevalent in the substrate of SuDS provide demonstrably beneficial thermo-hydrological interactions. We built an at-scale SuDS component, i.e. a 950-mm high soil column with a diameter of 1800 mm, as a lysimeter setup at the National Green Infrastructure Facility to test the heat injection into the substrate. A range of field testing scenarios (thermal load and cycling) were applied under natural, external ambient conditions. Soil temperature during heat injection was also simulated numerically by solving a transient heat conduction equation with a finite difference modelling scheme. The developed model was validated using measurements from the lysimeter setup which then enabled numerical experiments into the effects of varying hydrological regimes to be performed.

How to cite: Yildiz, A. and Stirling, R.: Combining green infrastructure and ground heat exchangers in urban areas, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9446, https://doi.org/10.5194/egusphere-egu22-9446, 2022.

Nearly two thirds of all Danish (DK) households utilize 3^rd and 4^th generation (3G and 4G) district heating (DH) for space heating and hot water. Despite being the most mature district heating system in the world, projections show that the maximum extent of traditional DH in DK is limited to 65-70%. Currently, the remaining one-third of Danish households are left with no choice but to invest in individual heat supply systems, typically air-source heat pumps. As an alternative, the concept of geothermal 5^th generation district heating and cooling (Geo5GDHC) has emerged recently. Geo5GDHC connects distributed prosumer heat pumps to a grid of uninsulated pipes that distribute energy at ambient temperatures from shallow geothermal drillings (open or closed), energy geostructures and sources of excess heat, to supply room heating and domestic hot water. This allows for combined heating and passive cooling with a single grid, capable of shifting thermal loads by seasonal energy storage. Geo5GDHC grids can be recharged and balanced by utilizing waste heat and by storing heat from passive cooling of the building mass during summer.

Currently, there are twelve commercial Geo5GDHC grids (thermonet) in Denmark using different energy sources and models for ownership and operation. However, in the case of Denmark, the maximum extent of Geo5GDHC is much larger. Geo5GDHC is complementary to 4^th generation district heating and cooling (4GDHC) as it is less affected by economies of scale. Consequently, Geo5GDHC is often economically feasible when traditional DHC is not, typically in rural areas, and therefore serves as an extension of existing DHC technologies. As such, Geo5GDHC potentially ensures that the majority of Danish households are connected to a collective heating and cooling grid in the future when combined with 4GDHC. A similar potential for Geo5GDHC exists in Europe and the USA, however, despite its significant potential, the Geo5GDHC market is still very much in its infancy.

The Interreg ÖKS project COOLGEOHEAT addresses both technical and economic aspects of Geo5GDHC in a joint collaboration between stakeholders and research institutions in Sweden and Denmark. We present results from the ongoing project including the development of design models in the Modelica simulation platform, that serve to estimate the thermal performance of the grid and the associated upfront investment and costs of operation, to support decision-makers. The project further explores business models for ownership and operation and the possibilities for financing grids with green investments of pension funds.

How to cite: Poulsen, S. E., Abugabbara, M., Tordrup, K. W., Andersen, S. S., Pedersen, C. P., Javed, S., and Andersen, T. R.: The COOLGEOHEAT project: Geothermal 5th generation district heating and cooling (Geo5GDHC/thermonet) in Denmark, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9313, https://doi.org/10.5194/egusphere-egu22-9313, 2022.