The Brussels Sand Member (BSM) is a relatively shallow aquifer at a depth range of around 200 to 1200 m and is present over a large part of the Dutch onshore territory. It is considered a potential target for low-temperature (< 35°C) geothermal projects and could be a heat source for urban heating networks. Like many shallow aquifers that were never targets for petroleum production, characterization is poor despite the roughly 700 wells that penetrate the aquifer and the wide availability of seismic data. Current maps are based on inconsistent interpretations in wells only. As a step towards the estimation of the geothermal potential of this aquifer, the BSM has been characterized as part of the WarmingUP project: presence, depth and reservoir properties have been estimated at the national level by interpreting and analysing existing data.

The wide availability of seismic data made it possible to map the BSM across the entire Netherlands. The BSM is present in a large part of The Netherlands, but is absent in a 60-100 km wide, WNW-ESE trending strip in the middle part of the country (the inverted West- and Central Netherlands Basins). In this area the BSM was eroded before younger units were deposited. The time interpretation of the seismic data was converted to depth by using parameters from the national velocity model VELMOD 3.1 after checking its applicability for the BSM. The well log correlation shows that the BSM in the Netherlands consists of three units (S3 to S1) of which only the top of the upper one (S3) was picked in a consistent way in the past. The base of the BSM was inconsistently picked: locally the base of the lowermost unit was picked and in other regions the base of the middle unit. For this study, the BSM is now consistently interpreted.

Although roughly 700 wells have been drilled through the BSM, many wells have few or poor quality logs and only a few cores are available, because the BSM was rarely the target formation. For 32 wells distributed over the country, a petrophysical analysis was done to determine clay volume, effective porosity, net thickness and permeability. The age of the wells ranges from the 1950’s to present, which means that logging tools, drilling techniques and mud type vary widely between the wells and therefore different evaluation techniques were used. The uppermost unit of the BSM (S3) is the only unit which has good reservoir properties. The lower units S2 and S1 have little or no reservoir potential. The best properties in the S3 unit can be found in an area south of the city of Rotterdam: average permeability is between 500 and 1000 mD. In the northern area, permeabilities are in the 100-200 mD range, with some areas in the 500 mD range. However, S3 does contain 20 to 200 cm thick tight calcite-cemented streaks that may act as vertical flow barriers and divide the reservoir unit into zones of a few meters thick.

The development of advanced materials for borehole heat exchangers (BHE) was performed in project GEOCOND. Material scientists, manufacturers and geothermal experts cooperated in thinking of new approaches for pipes and grouts with improved properties, starting from research at a low TRL.

The pipe development resulted in a material based on polyethylene with substantially increased thermal conductivity, and exhibiting longevity and similar ease of handling as the classic PE100. Development of grout for sealing the BHE annulus led to a further increase of thermal conductivity beyond the current state-of-the-art, while maintaining handling and sealing properties, and showing resilience to freezing-thawing cycles. Another development concerend the addition of phase-change material (PCM) into the grout in order to increase the short-term thermal storage capacity of a BHE.

The new material first was tested in the laboratories, including tests for BHE with PCM-grout in a "sandbox" at RISE Stockholm. Then sufficient material was produced to manufacture pipes and grout for shallow tests at a test field at UPV in Valencia (10-15 m deep BHE). Results from this tests allowed for selection and improvements of the final materials, produced in sufficient amount for real-size tests. Two BHE of 100 m depth with pipes and grout of high thermal conductivity were installed and tested in Germany, and one BHE of 50 m with PCM-grout for storage temperatures at 50-60 °C was installed in Sweden. The results for high thermal conductivity confirmed a reduction of the borehole thermal resistance of >20 % compared to a BHE with state-of-the-art materials at the same site, while the PCM-test were less conclusive and need further evaluation.

The development was accompanied by surveys of appropriate target values for thermal conductivity (incl. use of GIS) and by work on a digital material selection support system. Life cycle analysis of energetic, economic, environmental and social impacts have been performed and proved the viability of the developed materials and concepts, provided the expected cost reductions in mass production can be achieved. With GEOCOND materials, either improved efficiency due to better ground-side temperature developments could be achieved, or substantial reductions in required BHE length while maintaining the efficiency of a design with standard materials.

In the tests within the project, the materials fulfilled the different requirements set by standards and regulations. For grout and pipes to be brought to the market, however, further work is required for testing and certification, as well as for optimisation of manufacturing and cost reduction.

There is a growing interest in using subsoil and groundwater as a source of geothermal energy to meet heating and cooling demands. The development of these shallow geothermal systems, combined with the presence of multiple urban heat sources leads to thermal stress on urban aquifers and potentially to thermal interference between neighboring geothermal installations, resulting in a deterioration of their efficiency. In France, the mining code requires the definition of a protection zone around shallow geothermal installations extracting more than 500 kW or 80 m3/h. This protection zone confers an exclusive exploitation right in the area precisely to prevent a geothermal project from being disrupted by other/future installations. Until now, the application of this legal requirement relied on empirical concepts that do not always allow for (1) an equitable distribution of the geothermal potential and for (2) the durability of a project’s efficiency. Hence, we developed a novel methodological framework based on the site-specific hydrogeological conditions, defining protection zones around closed and open shallow geothermal installations: small enough to maximize the distribution of the resource, and large enough to protect the installation.
The first step of the methodology consists of determining the appropriate thermal amplitude to maintain the performance of the installation over the duration of the requested license. The second step is mapping the thermal reach of the installation. Analytical and numerical models are used for this purpose. This is done by probabilistic reverse flow-heat transport analysis. The third step is calculating and mapping the maximum acceptable power level for the operation of the installation. The approach is illustrated in the context of a geothermal project located in Lyon (France), where groundwater flow is influenced by underground structures, and where several geothermal installations are located. The proposed methodology can be applied at different scales, (1) locally to protect the efficiency of a project, and (2) regionally to manage thermal use of urban aquifers by multiple installations.

Very shallow geothermal energy on a large scale has been implemented mainly in rural areas with large space offerings so far. But in urban areas, there is often not enough free space. In order to significantly reduce the amount of open space required, the so-called frozen soil storage was developed and implemented in the research project "ErdEis II" (Grant N.: 03ET1634A-E). In addition, the research project is investigating how heating and cooling sources can be combined and operated together. This is becoming increasingly important for 5th generation district heating and cooling (5GDHC) networks and their bidirectional heat flows.

The frozen soil storage is a multi-layer geothermal collector system with a depth of up to 5 m, in which the ground is frozen in a controlled manner. We present the overall system consisting of two single-layer surface collectors, two frozen soil storages, a PVT-collector and the 5GDHC network and show how individual collector layers can be activated or regenerated using the hydraulic system.

Due to flood protection, a rainwater retention basin must be built in almost every new development area in Germany. Since this represents a potential area for very shallow geothermal collectors to the surface, both a single-layer geothermal collector and frozen soil storage were built under an open area and under a rainwater retention basin in the research project.

Possibly the first measured values can already be presented, as the first buildings will be connected to the 5GDHC network at the end of this year.

The integration of heat pumps into the energy supply of buildings is an increasingly applied technology. Primarily used for space heating and the preparation of domestic hot water, they are also utilized for cooling purposes at a progressive rate. With the increasing number of heat pumps the number of low temperature heat sources and different kinds of heat exchangers offered with the claim to efficiently utilize those heat sources rise as well. Indeed, it is not uncommon that the suitability and performance of such heat exchangers is neither scientifically qualified nor proven in practice. Not seldom without any evidence of the capacity or cost-benefit ratio so-called innovative products are implemented into heat pump systems only based on glossy fliers. However, to be able to exploit the potential of heat pumps efficiently, the choice of a suitable low temperature heat source combined with an appropriate heat exchanger for the respective application is mandatory.

Based on extensive monitoring, this paper analyzes and evaluates the thermal performance as well as the permanence of heat pump systems which make use of different low temperature heat sources combined with unlike heat exchangers. Furthermore, challenges in planning and operation of the different systems are compared in detail. Using the results, the right circumstances for the most efficient combination of different heat sources and heat exchangers both energetically and ecologically, are identified.

For this study, systems for space heating and domestic hot water preparation in residential buildings as well as systems for heating and cooling in non-residential buildings (e. g. office buildings) have been analyzed. The monitoring scope included the heat source, related heat exchangers and the heat pump.

The data reveals that the heat pumps achieve very good results, almost independent on the kind of heat exchanger. However, the heat exchanger space requirements and installation costs differ significantly. Consequently, the right combination of low temperature heat source and heat exchanger is affected mainly by those two key aspects. With respect to space requirements two issues will probably be in focus:

1. Most existing buildings are not expected to have large unoccupied areas. Plot areas for new residences and settlements are comparatively small, with a tendency to decrease further.

2. As a result of well-insulated building and construction the space heating demands steadily decrease. Hence, the preparation of domestic hot water will presumably be the determining factor of the heat demand of residences in the future. At a consequence, decreasing energy demands result in less space requirements for near-surface heat exchangers. Near-surface heat exchangers can be distinguished by low installation costs, which at the end might result in an increase of their market share.

Energy storage is one of the key challenges of the energy system transformation. Aquifer thermal energy storage (ATES) can be used to store surplus heat in cities underground and deliver it according to demand, with the potential to reduce a large proportion of carbon dioxide emissions. Existing heating networks and many industrial processes often require high temperatures (HT) of more than 100 °C, which exceed the design of current ATES systems.

For successful development and implementation of HT-ATES systems, dedicated surveillance is required in order to monitor environmental impacts and operating performance. Important objectives here are monitoring of the thermal energy balance underground, planning and, if necessary, prevention of maintenance work, as well as optimization of the energetic efficiency.

As essential parameters for determining the energy flows, temperature and flow rates at the injection and production wells are measured and monitored. Problems frequently encountered to date in ATES facilities worldwide are geochemically induced precipitation and corrosion processes, which can cause clogging and plugging as well as leakage at the surface and underground system components. In order to detect performance-reducing processes in good time and to enable sustainable reservoir management, suitable techniques and methods for monitoring system-relevant parameters and setting up early warning systems must be developed.

Within recent years, continued developments in fiber-optic sensing have resulted in new sensor types with certain advantages over conventional electronic sensors, which previously have mostly been used for downhole measurements. This includes “distributed” sensing methods, where data can be recorded over very long distances of up to several 10s of km length with high spatial and temporal resolution. Several physical quantities can be measured, and methods successfully applied include distributed temperature sensing (DTS), distributed strain sensing (DSS), and more recently also distributed acoustic or vibration sensing (DAS/DVS).

The main advantages are easier installation and simultaneous recording along the entire borehole. This results in significant time and cost savings. Furthermore, optical fibers have a much higher temperature tolerance than conventional sensors with electronic components, which is particularly relevant for the intended HT-ATES systems with temperatures beyond 100 °C.

Within this study, new methods for operational monitoring, e.g. injectivity/productivity, and borehole integrity during testing and operation of an HT-ATES site will be presented. Different methods using DTS data for a) determination of the flow profile during injection and production, and b) identification of injection zones from data recorded after the shut-in of injection (warm-back), will be reviewed. Examples of application on DTS, DVS, and DAS field data collected during thermal injection and production tests at pilot sites in the city of Berlin will be shown.

Efficient implementation of aquifer thermal energy storage (ATES) systems requires the location of the wells and well doublets to be optimised. Large well and doublet spacing minimizes thermal interference, but also reduces the total energy storage capacity. The optimal location of wells depends on the detailed flow and transport processes induced in the aquifer, which in turn depends on aquifer heterogeneity and the presence of ambient flow.

Accurate numerical simulation of ATES for optimisation requires high grid or mesh resolution to capture the detailed flow and transport fields in the aquifer. However, the optimal resolution for one set of well locations may not be suitable for other locations, particularly if manual grid refinement is used to refine the grid around each well or well doublet. ATES system optimization requires testing of many well locations and it is infeasible to manually refine the grid for each. High grid resolution is therefore required in all parts of the model where wells may be placed, but this is computationally expensive, particularly given that many simulations are required during the optimisation process.

Here, we report an optimisation methodology based on Dynamic Mesh Optimisation (DMO). DMO produces optimised meshes for a given model, set of well-doublet locations, pressure and temperature (and other key fields) distribution and time-level. Grid-free Surface-Based Geologic Modelling (SBGM) models are automatically generated in which well trajectories are introduced. The well trajectories are also not constrained by a mesh and are preserved during DMO. To optimize well locations, a Genetic Algorithm (GA) approach is used, implemented in the open-source software package DEAP.

Our DMO approach ensures that all models automatically generated and simulated in the optimisation process have suitable and equivalent mesh resolution without user interaction, thus ensuring that the detailed flow and transport processes in the aquifer are preserved with equivalent accuracy. We show that DMO produces results with the same accuracy as equivalent fine fixed mesh simulations, but at much lower cost because DMO ensures that fine mesh resolution is only deployed where required, typically around the wells in the specific locations being tested in a given model. We demonstrate that the method has wide application for ATES by testing the methodology under different ATES scenarios considering heterogeneous permeability, flow in the aquifer and minimisation of salt production.

Globally, governments and private companies have set high targets in avoiding CO₂ emissions and reducing energy. Aquifer Thermal Energy Storage (ATES) systems can contribute by overcoming the temporal mismatch between the availability of sustainable heat (during summer) and the demand for heat (during winter). Therefore, ATES is an increasingly popular technique; currently over 2000 low temperature ATES systems are operational in the Netherlands. Low-temperature ATES systems use heat pumps to allow the stored heat to be supplied at the required temperature for heating (usually around 40-50°C) and for cooling in summer. Although on average a conventional low-temperature ATES system produces 3-4 times lower CO₂ emissions when compared to gas heating, the heat pumps still require substantial amounts of external electricity, causing ver 60% of the remaining emissions. In the ATES triplet system, the temperatures in the hot and cold wells of an ATES system are increased and decreased respectively to match the required delivery temperatures and a third well is added at an intermediate temperature. With this strategy, other sources of sustainable heat and cooling capacity can supply the subsurface close to the temperatures required in the hot and the cold well. However, the return temperatures from the building systems do not conform with either of the hot or cold wells and an additional well is used to store water at the return temperature. Additional components are then required to supply the hot and cold wells (from the third well) by increasing the temperature in summer (e.g. solar collectors) and decreasing it in winter (e.g. dry coolers). Simulations and an economical evaluation show significant potential for triplet ATES with economic performance better than conventional ATES while the CO₂ emissions are reduced by a factor of ten. As the temperature differences are larger, the volume of groundwater required to be pumped is considerably lowered, causing an additional energy saving. Ongoing research focusses on analysing the energy balance and energy loss in the subsurface, well design requirements, working/operational conditions of each well, as well as the integration of building system components, such as the influence of weather conditions on performance of system components.

Driven by the energy transition targets for 2030 and 2050, there is great interest in Europe to move away from heating with fossil fuels towards heating with more sustainable sources. One option is the development of heat networks to connect the built environment and low temperature industrial demand to sustainable sources of heat, like geothermal energy. As the seasonal demand strongly varies, there is also increasing interest in the development of large-scale high temperature aquifer thermal energy storage (HT-ATES) systems for seasonal storage of heat to increase the flexibility of the heat networks. The geological conditions in the Netherlands, with thick layers of alternating permeable sands and clay deposits related to the deltaic setting, are favourable for HT-ATES systems.
The success of a HT-ATES system depends on many factors, such as available heat demand and supply, economic feasibility, and of course the suitability of the subsurface. In the national WINDOW project, a comprehensive overview of the subsurface HT-ATES potential was created. The HT-ATES potential was mapped for eight geological formations in the Netherlands at a depth of 50-500 m below ground level, based on a geological model and eight subsurface criteria including depth, thickness, and reservoir properties. In addition, two legal criteria that are related to the fresh/saline interface and groundwater protection zones were included. A traffic light system was used to show potential barriers: no barrier (green), potential barrier (yellow) and one or more barriers (orange), resulting in qualitative potential maps for HT-ATES in the Netherlands.
The western part of the Netherlands is in general most favourable for HT-ATES which is beneficial as this area also offers great potential for geothermal energy and it is as well the most densely populated area with high heat demand and existing heat networks. The study identified the need for improved subsurface characterization in large areas where geological data is lacking. It should be kept in mind that HT-ATES applications are highly location-specific, and since the maps show regional potential a more detailed local investigation is always required. In follow-up studies, the maps will be improved with additional geological data and combined with maps of heat demand and maps indicating the opportunities for sustainable energy sources, like geothermal energy, to find the best opportunities for an integrated heat supply system for the built environment. Locations with the highest potential are those where surface and subsurface conditions and requirements match.
The methodology has been set up in such a way that the maps can easily be updated when new input data becomes available and can be applied to other countries to define their qualitative HT-ATES potential.

Growing electricity production from fluctuating sources (wind, photovoltaics) makes it increasingly difficult to match supply and demand of electrical energy. This results in the necessity to expand storage options significantly. The combination of high-temperature heat storage systems with thermodynamic power plants is one of the possible options. Since the efficiency of the thermodynamic power plant strictly depends on the temperature of the working fluid (Carnot), this combination is most favorable at high storage temperatures of the heat accumulator. Present day power plants have the maximum possible efficiency at 800°C. In this context, liquid salt storage facilities have been discussed for years. Some of these are thought of as so-called latent heat storage (phase change storage), to be operated at temperatures around the melting temperature of the salt (sodium chloride: 801 ⁰C) using the high melting enthalpy (520 J/g). Some other storage facilities are operated as pure heat storage (sensible storage), in which the salt, at a given temperature spread, constantly remains liquid in the loading/unloading cycle. In the case of latent heat storage systems, the comparatively high melting enthalpy of the salt is primarily used. With other salts and salt mixtures both the melting temperatures and the melting enthalpies are significantly lower.

We propose a borehole based latent heat storage in natural salt structures with, e.g., heat loading/recovery through bore holes. Although a number of technical problems are to be solved, much larger storage volumes and thus storage capacities can be realized compared to salt storage in technical containers (up to 1 million m³ compared to at most a few 10,000 m³ in technical storage systems).

Salt structures are common in the Central European lowlands, where a lot of renewable electrical energy is produced by onshore and offshore wind converters, so short distances between energy production and storage can be realized.

Energy walls are embedded retaining walls used for supporting changes in ground level (e.g. for basements of metro stations), and also for exchanging heat through the ground subsurface via the inclusion of heat exchanger pipes. These dual-use substructures can be used with heat pumps and/or district heating networks to deliver low carbon heating and cooling for nearby buildings. They have a significant surface area that can be used for thermal activation, hence offering high heat transfer rates. Assessments of heat transfer rates and energy available for exploitation is typically done either via rules of thumb, which is not an efficient design procedure or via numerical analysis, which is highly computationally expensive. There is therefore a need for fast run time routine analytical methods for an energy assessment. This paper starts to fill this gap by testing a potential analytical solution that can be used for calculating the amount of heat transferred through energy walls to the adjacent ground. A two-dimensional numerical simulation of a simplified energy wall is built, and the temperature change on the back of the wall is compared with those calculated using the infinite plane source (IPS) analytical model. The results show that there is some difference between the models in the short term due to the explicit pipe positions not being considered in the analytical model. However, these differences are small and reduce with time, and the two models converge. This suggests that the IPS is a good candidate for use in the analytical design of energy walls.