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Nearly two thirds of all Danish (DK) households utilize 3rd and 4th 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 5th 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 4th 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.

This paper presents a multifunctional full-scale demonstration road, the Climate Road, which combines climate adaptation and mitigation in a single system. The Climate Road is located at Hedensted, Denmark and is 50 m long and 8 m wide, and the depth of the roadbed is 1 m. Half of the Climate Road, i.e., 25 m, is paved with permeable asphalt and the remaining 25 m with traditional asphalt. All surface water drains into the roadbed, which stores up to 120 m3 of water, either directly through the permeable asphalt or by drain grates. In addition, 800 m of geothermal pipes are embedded in the roadbed, distributed over four 200 m w-loops, two buried 1 m below the asphalt and two similar loops at 0.5 m depth. The Climate Road was tested from May 2019 to May 2021. In the project period, a total precipitation value of 1654 mm was recorded, the mean temperature was 9.3 °C and the most intense rainfall was 40.3 mm/30 min. The long-term infiltration performance of the permeable asphalt shows that the overall infiltration capacity slowly reduces. The reduction can be hindered, but not completely prevented, with annual restorative cleaning. After two years of operation, the Climate Road still, by a large margin, fulfils the recommendations of the infiltration capacity of 97.2 mm/h for the vast majority of the road section. The total volume reduction capacity is estimated to be between 15 and 30%. Based on an analysis of 61 single rain events, the event detention time is found to range between 10 and 130 min, with an average of 35 min. During the project period, the Climate Road produced a total of 98 MWh for a nearby kindergarten, with an average coefficient of performance (COP) of 3.1.

This paper investigates the applicability of numerical and semi-empirical heat flow models for calculating average fluid temperatures in groups of quadratic, precast pile heat exchangers. A 3D finite element model (FEM), verified with experimental data, is extended to account for multiple pile heat exchangers. Semi-empirical dimensionless temperature g-functions for multiple piles are developed by utilising single pile 3D FEM heat transport simulations with temporal and spatial superposition techniques to account for the thermal interaction between piles. The multiple pile g-functions, yet less accurate for short times, yield fluid temperatures similar to those obtained with full 3D modelling (with 7% of largest deviation), at minimal computational cost.

Historically, industries were in harbour areas of cities for easy access to transportation of resources. Today, transforming former industrial areas into living spaces has become attractive business. However, this transformation has often been challenged by high levels of soil contamination caused by the industrial use. Remediation measures are mandatory to ensure the public safety in the redeveloped areas. Detailed information about the contaminant type, distribution and transport mechanisms is required to address the contamination issues. This paper presents a workflow for investigations assisting decision making for construction work in redeveloped industrial areas. The workflow is applied to Horsens harbour (Denmark). In this area, renovation of the harbour walls introduces the risk of spreading of phenol contamination to planned construction areas. The study demonstrates how detailed information about the geology and hydrology at the site allows for scenario modelling of contaminant transport, guiding remediation efforts and aiding decision makers in developing the harbour area.

This paper proposes and demonstrates, in full scale, a novel type of energy geostructure (“the Climate Road”) that combines a ground-source heat pump (GSHP) with a sustainable urban drainage system (SUDS) by utilizing the gravel roadbed simultaneously as an energy source and a rainwater retarding basin. The Climate Road measures 50 m × 8 m × 1 m (length, width, depth, respectively) and has 800 m of geothermal piping embedded in the roadbed, serving as the heat collector for a GSHP that supplies a nearby kindergarten with domestic hot water and space heating. Model analysis of operational data from 2018–2021 indicates sustainable annual heat production levels of around 0.6 MWh per meter road, with a COP of 2.9–3.1. The continued infiltration of rainwater into the roadbed increases the amount of extractable heat by an estimated 17% compared to the case of zero infiltration. Using the developed model for scenario analysis, we find that draining rainwater from three single-family houses and storing 30% of the annual heating consumption in the roadbed increases the predicted extractable energy by 56% compared to zero infiltration with no seasonal energy storage. The Climate Road is capable of supplying three new single-family houses with heating, cooling, and rainwater management year-round.

We assess the feasibility of a collective district heating and cooling network based on a foundation pile heat exchanger in a new urban area in Vejle, Denmark. A thermogeological model for the area is developed based on geophysical investigations and borehole information. In tandem with a building energy demand model, the subsurface thermal properties serve as the input for a newly developed computational temperature model for collective heating and cooling with energy piles. The purpose of the model is to estimate the long-term performance and maximum liveable area that the energy piles are able to support. We consider two case studies where residential and office buildings dominate the building mass. We find that three to four floors can be supplied with heating and cooling from the energy piles, depending on the use and design of the buildings.

We present the findings of a recently concluded research project, investigating the possibilities for collective heating and cooling supply of a planned, relatively small residential area (Rosborg Ø) in Ny Rosborg, Vejle, Denmark with ground source heat pumps utilising foundation pile heat exchangers (a.k.a. energy piles, EP). Individual EP foundations connect to a distribution network of uninsulated geothermal pipes, buried at shallow depth (cold district heating, CDH) from which connected consumers can supply heating with heat pumps as well as passive or active cooling.To this end, the project has developed a geothermal screening procedure based on a combined analysis of geophysical data, borehole information, pile testing and laboratory measurements of soil thermal properties. A prototype computational temperature model of CDH networks has been developed for estimating the performance of EP based heating and cooling supply of Rosborg Ø. Finally, the project has developed a complete business (case) model for EP based CDH with a well-defined cost structure in which total fixed and variable costs can be quantified in specific projects.The mapping of the geothermal potential demonstrates that CDH most likely can fully supply the estimated energy demand of the planned buildings in Rosborg Ø. However, recalculation of the scenario is necessary once additional information on the planned buildings become available. This conclusion is further supported by operational data from the EP foundation at the nearby Rosborg Gymnasium, demonstrating excess heating and cooling possibilities (beyond the demand of the building itself). Further analyses of the data from the Gymnasium estimates the average energy efficiency ratio to 24.8 for the passive cooling during July and early August 2018, roughly ten times higher than that of traditional Air Conditioning (AC). Moreover, the Gymnasium is able to supply its cooling needs passively 97% of the time where cooling is required, implying that the variable cost of cooling with EPs is exceptionally low.The initial investment required for EP based CDH is higher, however, the variable costs of heating and cooling are greatly reduced relative to those of traditional District Heating (DH) and AC. Consequently, the estimated payback period for collective EP based CDH supply of Rosborg Ø is ca. 4.5 years. The relatively short payback period is due to a drastic reduction (of 80%) of the combined variable costs of heating and cooling with EPs, relative to traditional DH and AC. The contributing factors to the short payback period are the relatively low costs of electricity, the high COP of the heat pump, a relatively high, annual fixed tariff imposed by traditional DH and finally the exceptionally low costs of passive cooling/seasonal heat storage. As such, the project demonstrates a truly renewable, economically competitive heat pump technology to supply collective building heating and cooling/seasonal heat storage for the future energy supply in Denmark.

We present a novel climate adaption technology – the Climate Road - that combines collective, ground source heat pump (GSHP) based heating with sustainable urban drainage of rain water (SUDS). The system utilizes the road bed simultaneously as a retarding basin for excess rain water and as the energy source for a GSHP. Surface water percolates through the permeable road paving and is retained in the gravel road bed during extreme precipitation events where the sewage system is often overloaded. Water is subsequently released once capacity is available again on the sewage network. In addition, geothermal piping is embedded in the roadbed, serving as the collector for individual GSHPs that supply connected households with heating. The primary benefit of the combined system is the saved digging costs and lost property value from establishing a separate rainwater basin and trench for the geothermal piping as the road bed is to be established under any circumstances. However, there is also a subtle yet positive effect on the performance of the GSHP, from constantly watering the geothermal piping.We have constructed 50 m of Climate Road in Hedensted, Denmark. The road bed is 1 m deep and 8 m wide and can retain a maximum of 150 mm of precipitation, given that the catchment area is twice that of the road surface. Moreover, the road bed is hydraulically disconnected from the surrounding soil by means of bentonite mats, to prevent seepage of groundwater into the road bed. Water is discharged by drainage pipes to a nearby rainwater basin for experimental and practical purposes only. 800 m of geothermal piping is embedded in the road bed, supplying a nearby kindergarten with domestic hot water and room heating by means of a GSHP. The Climate Road is fully instrumented with temperature sensors in the road bed, flow meters to measure water discharge and energy metering on the GSHP. We present the performance of the Climate Road in terms of supplied heating and the coefficient of performance (COP) for the heat pump in addition to drained water volumes for the first year of operation.

This paper applies previously validated multiple pile g-functions, for estimating operational average fluid temperatures in an actual energy pile foundation in Rosborg, Denmark. We find that the multiple pile g-functions yield fluid temperatures similar to what is observed, at minimal computational cost. The temperature model is then utilised in an optimisation algorithm that yields the minimum number of energy piles required by simultaneously maximising the pile spacing and taking into consideration the thermal load of the building. The optimisation shows that the thermal needs of the building can be fully supplied by 148 rearranged energy piles, instead of the current 219. The optimisation tool is also applied to a full-factorial design sweep which shows a large sensitivity of the number of energy piles on the thermal conductivity of the ground.

This paper presents a numerical procedure for predicting subsurface temperatures and heat-flow distribution in 3-D using inverse calibration methodology. The procedure is based on a modified version of the groundwater code MODFLOW by taking advantage of the mathematical similarity between confined groundwater flow (Darcy's law) and heat conduction (Fourier's law). Thermal conductivity, heat production and exponential porosity-depth relations are specified separately for the individual geological units of the model domain. The steady-state temperature model includes a model-based transient correction for the long-term palaeoclimatic thermal disturbance of the subsurface temperature regime. Variable model parameters are estimated by inversion of measured borehole temperatures with uncertainties reflecting their quality. The procedure facilitates uncertainty estimation for temperature predictions. The modelling procedure is applied to Danish onshore areas containing deep sedimentary basins. A 3-D voxel-based model, with 14 lithological units from surface to 5000 m depth, was built from digital geological maps derived from combined analyses of reflection seismic lines and borehole information. Matrix thermal conductivity of model lithologies was estimated by inversion of all available deep borehole temperature data and applied together with prescribed background heat flow to derive the 3-D subsurface temperature distribution. Modelled temperatures are found to agree very well with observations. The numerical model was utilized for predicting and contouring temperatures at 2000 and 3000 m depths and for two main geothermal reservoir units, the Gassum (Lower Jurassic-Upper Triassic) and Bunter/Skagerrak (Triassic) reservoirs, both currently utilized for geothermal energy production. Temperature gradients to depths of 2000-3000 m are generally around 25-30. degrees C km(-1), locally up to about 35. degrees C km(-1). Large regions have geothermal reservoirs with characteristic temperatures ranging from ca. 40-50. degrees C, at 1000-1500 m depth, to ca. 80-110. degrees C, at 2500-3500 m, however, at the deeper parts, most likely, with too low permeability for non-stimulated production.

Dimensioning of large-scale borehole thermal energy storage (BTES) is inherently uncertain due to the natural variability of thermal conductivity and heat capacity in the storage volume. We present an improved method for upscaling a pilot BTES to full scale and apply the method to an operational storage in Braedstrup, Denmark. The procedure utilizes inverse 3D finite element method (FEM) modelling of distributed temperature measurements inside the BTES for inferring the thermal properties of the subsurface. We find that individual geological layers can be distinguished in terms of their heat capacities and thermal conductivities using inverse modelling. The depth integrated estimate of thermal conductivity differs significantly from that obtained from a single thermal response test (TRT) at the site. As such, we find significant scaling effects in terms of the subsurface thermal conductivity distribution which are expected to be further amplified in an expansion of the pilot BTES to full scale. The methodology presented in this paper therefore provides an improved basis for upscaling pilot BTES systems. The operational data and BTES temperature measurements are published with the present paper in the supplementary material.

Technologies for energy-efficient cooling of buildings are in high demand due to the heavy CO2 footprint of traditional air conditioning methods. The ground source heat pump system (GSHP) installed at the Rosborg Gymnasium in Vejle (Denmark) uses foundation pile heat exchangers (energy piles). Although designed for passive cooling, the GSHP is used exclusively for heating. In a five-week test during the summer of 2018, excess building heat was rejected passively to the energy piles and the ground. Measured energy efficiency ratios are 24−36 and the thermal comfort in conditioned rooms is improved significantly relative to unconditioned reference rooms. A simple model relating the available cooling power to conditioned room and ground temperatures is developed and calibrated to measured test data. Building energy simulation based estimates of the total cooling demand of the building are then compared to corresponding model calculations of the available cooling capacity. The comparison shows that passive cooling is able to meet the cooling demand of Rosborg Gymnasium except for 7−17 h per year, given that room temperatures are constrained to < 26 °C. The case study clearly demonstrates the potential for increasing thermal comfort during summer with highly efficient passive cooling by rejecting excess building heat to the ground.

The increasing population density in existing urban areas often leads to the development of areas previously omitted due to construction risks. We suggest a comprehensive interpretation strategy exemplifying how planning maps should classify areas depending on risk and opportunities. The first steps of the interpretation strategy involve a review of the purpose of the project, followed by the acquisition of high-density geophysical data. In the subsequent steps, geophysical data in conjunction with GIS data are used for constructing a detailed high-resolution 3D geological voxel model. Specific geotechnical properties are assigned to the interpreted geological units based on in situ vane shear tests and the standard penetration test. In the final step of the interpretation strategy, two planning maps containing the three relevant themes are combined into one conclusive map demonstrating the recommended use of different parts of the area for future urbanisation. An isopach map showing the depth of the layers suitable for the foundation is combined with a map showing the areas flooded by the Vejle Stream during a 50- and 100-year event as well as habitat protected areas. Thus, the resulting planning maps show the most suitable locations of blue areas (lakes, wetlands), green areas (parks, etc.) and grey areas (buildings, roads) for future development. The adopted interpretation strategy can be successfully applied in similar situations to reduce the risks associated with urban development.

Heat storage capabilities of deep sedimentary geothermal reservoirs are evaluated through numerical model simulations. We combine storage with heat extraction in a doublet well system when storage phases are restricted to summer months. The effects of stored volume and annual repetition on energy recovery are investigated. Recovery factors are evaluated for several different model setups and we find that storing 90 °C water at 2500 m depth is capable of reproducing, on average 67% of the stored energy. In addition, ambient reservoir temperature of 75 °C is slightly elevated leading to increased efficiency. Additional simulations concerning pressure build-up in the reservoir are carried out to show that safety levels may not be reached. Reservoir characteristics are inspired by Danish geothermal conditions, but results are assumed to have more general validity. Thus, deep sedimentary reservoirs of suitable properties are found to be viable options for storing access energy for high-demand periods.

This paper evaluates the principal design parameters affecting the thermal performance of embedded hydronic Thermally Active Surfaces (TAS), combining the Response Surface Method (RSM) with the Finite Elements Method (FEM). The study ranks the combined effects of the parameters on the heat flux indoo½rs and heat loss outdoors of a vertical panel, and calculates an optimized solution based on the Desirability Function Approach. This study reveals a large impact of the panel thermal conductivity on the thermal performance of embedded TAS, and the experiments indicate a large potential for utilizing concrete with high thermal conductivity in embedded hydronic radiant panels.

This paper presents a method for analyzing and establishing a stopping criterion for ongoing (TRT) thermal response tests of vertical (BHE) borehole heat exchangers. The predictive uncertainty of the late-time BHE fluid temperature (50 h) forms the basis for determining the time after which further temperature measurements do not significantly improve the calibration of the numerical borehole model. The method relies solely on measured fluid temperatures and can, in principle, be generalized to different BHE geometries and configurations. The method is applied to synthetic and actual TRTs of single and double U BHEs. Predictive uncertainty of the late-time fluid temperature is comparable to measurement uncertainty after 12–28 h of testing at which reliable estimates of soil thermal conductivity and borehole thermal resistance are obtained. Minimum testing times are found to scale with borehole thermal resistance in the synthetic tests. It is further demonstrated that for actual tests, ambient thermal disturbances bias estimates of soil thermal conductivity and inflate predictive uncertainty which increases minimum testing times. The method serves as a diagnostic tool for ongoing TRTs as well as a means to minimize testing times which has potential implications for the associated cost and logistics of field operations.

This study finds that the production profile (temperature and longevity) of low enthalpy geothermal reservoirs depends significantly on the thermal conductivity of the confining beds, which recharge the reservoir by conduction. The thermal recharge furthermore is proportional to the production rate and increases dramatically in thin reservoirs, while impermeable reservoir sections have little effect on the production profile. For the Margretheholm geothermal plant, Copenhagen, Denmark, production temperatures are modelled to decrease by only 7 °C–14 °C after 300 years of production due to thermal recharge. The study emphasizes the huge potential of geothermal energy in the development of environmentally sustainable cities.

Pile heat exchangers are traditional foundation piles with built in heat exchangers. As such, the footing of the building both serves as a structural component and a heating/cooling supply element. The existing geotechnical design standards do not consider the nature of thermo-active foundations and, therefore, there is a need to develop guidelines to design them properly. This paper contributes by studying the thermo-mechanical behavior of the precast piles which are 15-meter long and have a quadratic cross section and a W-shape pipe heat exchanger. This article aims to numerically assess the additional changes in the pile load transfer generated by its heating and cooling. In addressing this objective, a preliminary multi-physical finite element analysis is conducted which serves as a tool for exploring: i) the thermally induced mechanical stresses within the concrete and on the pile-soil axial and shaft resistances; ii) the maximum upward/downward displacements. A one-year time span is considered under operational and extreme thermal boundary conditions. The results show that a typical geothermal utilization of the energy foundation does not generate significant structural implications on the geotechnical capacity of a single energy pile. However, ground thermal loads need to be considered in the design phase to account for potential extreme temperature changes, which could generate thermal stresses that equalize the mechanically generated ones.

Groundwater models simulating flow in buried valleys interacting with regional aquifers are often based on hydrogeological models interpreted from dense geophysical datasets and scarce borehole data. For three simple synthetic cases, it is demonstrated that alternative methods of inversion of transient electro-magnetic (TEM) data can lead to very different interpretations of the hydrogeology inside and surrounding a buried valley. The alternative interpreted hydrogeological models are used in numerical modelling of groundwater flow to a pumping well. It is demonstrated that the alternative models result in quite different groundwater-model predictions of capture zone, recharge area, and groundwater age for the pumping well. It is briefly demonstrated that model calibration against hydraulic head data is not likely to improve the predictions or to identify the structural error of the interpreted hydrogeological models. It is therefore concluded that when TEM-based resistivity models are interpreted to construct the hydrogeological framework of a groundwater model, it must be done cautiously with support from deep borehole information. Too much reliance on geophysical mapping can lead to seriously wrong hydrogeological models and correspondingly wrong groundwater-model predictions.

SUMMARY Bottom hole temperatures (BHTs) measured during drilling operations are thermally disturbed by the drilling process. This paper presents a method, CSMI (Cylindrical Source Model with Invasion of bore mud filtrate), for estimating equilibrium formation temperatures with probability distributions from BHT measurements in the presence of bore fluid invasion. The scheme is based on finite element analysis in conjunction with Markov chain Monte Carlo inversion. The axisymmetric forward model assumes a cylindrical source of finite radius and contrasting thermal parameters, which includes the possibility of invasion (advection) of mud filtrate into the formation. In a synthetic example, it is demonstrated that given bore fluid invasion and a low and high temperature of the bore mud and formation, respectively, the equilibrium formation temperature and the uncertainty hereon is underestimated by correction schemes based on purely conductive models. The influence of the borehole radius and fluid invasion on the temperature measured at the borehole axis attenuates over time. It is further demonstrated that the invasion radius and the matrix thermal conductivity cannot be estimated simultaneously with the CSMI scheme. The analysis of five BHT records measured onshore Denmark, for which the equilibrium formation temperature is known, shows that CSMI temperatures based on single datum records are highly uncertain because of a strong negative coupling between the temperature of the mud filtrate and the equilibrium formation temperature. For records with multiple temperature measurements, the CSMI scheme matches statistically the measured equilibrium formation temperatures. It is further shown that additional negative bias is added to Horner plot temperatures if bore fluid invasion has occurred. Allowing for bore fluid invasion in addition to a borehole of finite radius and contrasting thermal parameters, increases temperature estimates by 5 per cent (4–7 per cent) on average given that invasion is indicated and that the bore fluid is significantly colder than the formation. Indications of bore fluid invasion from CSMI analysis are confirmed in two examples by analysing corresponding caliper logs.

Summary By combining laboratory and field experiments we study the release of water from above a falling water table and the implications this has on pumping test analysis and prediction of stream depletion. Undisturbed core samples from the field site were used in laboratory experiments to measure drainage responses to water-table drawdown. The responses can be sufficiently modeled by estimating the specific yield and five exponential time constants of a Moench/Boulton type model of delayed drainage. The average specific yield is thus estimated to 0.24 which is in agreement with previous small scale and large scale estimates. In addition two pumping tests were conducted near the stream. The drawdown analysis was made using a 3D numerical groundwater flow model facilitating use of the Moench/Boulton type boundary condition at the water table. The drawdown observations were fitted by calibrating two alternative models: one simulating the release of water at the water table as instantaneous; the other simulating the release of water as delayed. Comparing the calibration results for the two alternative models showed that they fit the drawdown observations nearly equally well, and most estimated parameter values are realistic and nearly identical. However, estimation of the specific yield is found to be sensitive to whether drainage is modeled as instantaneous or as delayed. In the first case the estimated specific yield is about half of the estimate from the core experiments and of the previous estimates; in the second case the estimate (0.17) is in better agreement with core and previous estimates (0.24). The analysis indicates that relatively fast drainage, and the existence of two drawdown dependent sources of groundwater recharge (the storage and the stream), complicates pumping test design to obtain unique parameter estimation. The analysis supports that an essential factor in parameter estimation by pumping test analysis for (at least some) unconfined aquifers is the use of a model that accounts for time-varying drainage from the vadose zone. Finally, when predicting stream depletion beyond 1 day of pumping it is acceptable in the present case to use a model simulating drainage as instantaneous if unbiased parameter estimates are used.

Most of the Danish area is characterized by deep sedimentary basins with a great potential for exploitation of geothermal energy. Geothermal reservoirs are present at various depths and temperatures. Currently, three geothermal plants are operating producing warm water for district heating purposes. Information of subsurface temperature distribution originates from direct measurements in boreholes and from indirect theoretical modelling. "Point observations" of varying quality are available as industrially measured "Bottom Hole Temperatures" from deep exploration boreholes and accurate continuous equilibrium temperature logging has been carried out in a number of accessible deep boreholes. A regional distribution of subsurface temperatures is obtained by combining measurements and 3D numerical modelling in which the heat equation is solved. Modelling results are constrained by observations in terms of available measured temperatures and observed surface heat flow. Information on subsurface thermal conductivity, which is a critical parameter, is obtained from core measurements and well-log analyses. Interval temperature gradients are found to vary by a factor of three to four across lithologies of different conductivity. Mean geothermal gradients from surface to depths of 1000 to 3000 m are generally between 20 and 35 °C/km. The subsurface thermal structure is clearly dominated by conduction. Advection by groundwater migration is generally insignificant. Heat flow increases significantly with depth due to perturbation from long-term palaeoclimatic surface temperature variations. Examples of modelled temperature distribution for selected geothermal reservoir are shown. In the Gassum Formation, which is present in most of the Danish area, temperatures are largely between 35 and 90 °C for depths of interest.

Information of subsurface temperature distribution in the Danish area originates from direct measurements in boreholes and from indirect theoretical modelling. “Point observations” of varying quality are available as industrially measured “Bottom Hole Temperatures” from deep exploration boreholes, and accurate continuous equilibrium temperature logging has been carried out in a number of accessible deep boreholes. A regional distribution of subsurface temperatures is obtained by combining measurements and 3D numerical modelling in which the heat equation is solved. Modelling results are constrained by observations in terms of available measured temperatures and observed surface heat flow. Information on subsurface thermal conductivity, which is a critical parameter, is obtained from core measurements and well-log analyses. Interval temperature gradients are found to vary by a factor of three to four across lithologies of different conductivity. Mean geothermal gradients from surface to depths of 1000 to 3000 m are generally between 20 and 30 °C/km. As an example, modelled temperatures for the Gassum geothermal reservoir are shown with temperatures largely between 35 and 90 °C for depths of interest.

Preliminary geophysical and hydrogeological investigations indicate that multi-electrode profiling (MEP) can be used to monitor groundwater salinity on a coastal barrier where a shallow thin aquifer discharges to the North Sea. A monitoring system including five groups of piezometers and five MEP probes, having closely spaced electrodes from above the groundwater table to a depth of 5 m below sea level, have been installed and tested. Using this system we will monitor resistivity and thus groundwater salinity variations in space and time. Analyzing the measurements using density dependent groundwater modeling we hope to be able to quantify how time varying recharge, tides, and storms hitting the barrier affect groundwater flow and discharge to the sea. At the conference we will present monitoring results from the winter and spring 2008.

The Rosborg Gymnasium building in Vejle (Denmark) is partially founded on 200 foundation pile heat exchangers (energy piles). The thermo-active foundation has supplemented the heating and free cooling needs of the building since 2011 (4,000 m2 living area). Operational data from the ground source heat pump installation has been compiled since the beginning of 2015. The heating requirement of the building supplied by the ground source heat pump exceeds the free cooling covered by ground heat exchange. The asymmetric utilization of the soil should in principle, imply a decrease in the long-term ground temperatures. However, operational data show that the temperatures of the heat-carrier fluid do not fall below +4.2oC during the heating season (winter) and that the soil recovers to undisturbed conditions during the summer when heat demand is low. In addressing the consequences of an imbalanced ground heat extraction/injection activity , this paper provides a performance study of the energy pile-based ground source heat pump installation utilising operational data. The study demonstrates that the measured seasonal performance factors so far are lower than expected: 2.7 in heating mode and 4.2 in cooling mode. Nevertheless, there is room for improvement if novel energy management strategies are applied. This highlights the relevance of considering the daily heating/cooling requirements of the building during the design phase of the heating and cooling system. Moreover, this study demonstrates the feasibility of ground source heat pump systems based on energy foundations in heating-dominant buildings.

A monitoring system, including five groups of piezometers and five vertical multielectrode profiling probes (VMEP), has been installed in an aquifer beneath a coastal dune in Denmark. In order to assess the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multielectrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a one-dimensional, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01–0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and salt water overlying freshwater close to the sea. In June, the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn, aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.