Shallow geothermal energy (<400
m depth) is used in many countries worldwide, with a rising number of installations over the last decades. The use of ground source heat pump (GSHP) and groundwater ...heat pump (GWHP) systems results in local temperature anomalies (cold or heat plumes). Since groundwater is used in many countries as source for drinking water a balance between its use and protection has to be found. Therefore, to avoid detrimental environmental impacts it is necessary to define groundwater temperature limits for heating and cooling and minimum distances between such geothermal systems. The aim of the present study is to provide a comprehensive overview of the current international legal status for the use of shallow geothermal energy. Therefore, an international survey was performed using a questionnaire, which was sent to more than 60 countries worldwide. The questionnaire requested information on the corresponding national legislation, temperature limits and minimum distances for GSHP and GWHP systems. The answers to the inquiry showed an extremely heterogeneous outcome. Until now national and legally binding regulations only exist in few countries such as Denmark or Sweden. However, all existing regulations show a wide range for minimum distances (5–300
m) and temperature limits for groundwater. The highest inconsistency was observed for the acceptable temperature change with 3
K in Switzerland to 11
K in France. However, most countries have no legally binding regulations or even guidelines, which highlight the urgent need for further research on the environmental impact and legal management of shallow geothermal installations.
Shallow geothermal energy is a renewable energy resource that has become increasingly important. However, the use has environmental, technical and social consequences. Biological, chemical, and ...physical characteristics of groundwater and subsurface are influenced by the development of this resource. To guarantee a sustainable use it is therefore necessary to consider environmental and technical criteria, such as changes in groundwater quality and temperature. In the current study a comprehensive overview of consequences of geothermal systems in shallow aquifers is provided. We conclude that there is still a lack of knowledge on long-term environmental consequences. Due to local differences in geology and hydrogeology as well as in technical requirements, it is not recommendable to define only static regulations, such as fixed and absolute temperature thresholds. Flexible temperature limits for heating and cooling the groundwater and subsurface are therefore advisable. The limits should be oriented on previously undisturbed temperatures, and chemical, physical and biological conditions of aquifers. Based on these findings, recommendations for a sustainable policy for shallow geothermal systems are provided including a potential legal framework for a sustainable use.
•We provide an overview of consequences of geothermal systems in shallow aquifers.•Static regulations for heating or cooling groundwater are not recommendable.•Temperature limits should be flexible and orientated on background values.•Suggestions for a sustainable policy for shallow geothermal systems are provided.•A potential legal framework for a sustainable use is presented.
Little is known about the intensity and extension of subsurface urban heat islands (UHI), and the individual role of the driving factors has not been revealed either. In this study, we compare ...groundwater temperatures in shallow aquifers beneath six German cities of different size (Berlin, Munich, Cologne, Frankfurt, Karlsruhe and Darmstadt). It is revealed that hotspots of up to +20K often exist, which stem from very local heat sources, such as insufficiently insulated power plants, landfills or open geothermal systems. When visualizing the regional conditions in isotherm maps, mostly a concentric picture is found with the highest temperatures in the city centers. This reflects the long-term accumulation of thermal energy over several centuries and the interplay of various factors, particularly in heat loss from basements, elevated ground surface temperatures (GST) and subsurface infrastructure. As a primary indicator to quantify and compare large-scale UHI intensity the 10–90%-quantile range UHII10–90 of the temperature distribution is introduced. The latter reveals, in comparison to annual atmospheric UHI intensities, an even more pronounced heating of the shallow subsurface.
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► Positive temperature anomalies under German cities. ► Local heat sources cause hot spots>30°C in Frankfurt. ► Superposition of various heat sources leads to a significant regional warming. ► Subsurface urban heat island (UHI) intensities range between 1.9 and 2.4K.
Many cities leave a considerable thermal footprint in the subsurface. This is caused mainly by accelerated heat fluxes from warmed basements, pavements and buried infrastructures. Even though rough ...estimations of the theoretical heat content in urban ground exist, there is no insight available on the technical potential of such subsurface urban heat islands. By considering borehole heat exchangers (BHEs) for geothermal exploitation, new opportunities arise for planning sustainable systems within cities through utilization of accelerated ground heat input from urban structures. This is feasible at moderate heat extraction rates even without any active (seasonal) recharging of the BHEs. For typical conditions in central Europe and a given system’s life time, each additional degree of urban ground heating could save around 4 m of the borehole length for the same heating power supply. We inspect implications for a single BHE as well as complete coverage of cities, which is approximated by an infinite field of BHEs. The results show that shallower systems favour renewable operation, and urban technical potential of geothermal use increases by up to 40% when compared to rural conditions.
•The technical geothermal potential in urban areas is evaluated.•Subsurface urban heat islands increase the potential.•Full geothermal exploitation is compared to sustainable use.•Elevated ground heat fluxes facilitate use as renewable source.•Increased ground temperatures in urban areas:
•A new analytical solution for simulating shallow geothermal systems is presented.•The solution accounts for long-term changes in land use and groundwater flow.•The approach is verified with a ...numerical model and validated in a case study.•Land use changes and horizontal advection can overprint anomalies induced by BHEs.
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A new analytical model is presented for simulation of ground thermal effects from vertical borehole heat exchangers (BHEs). It represents an extension of the moving line source equation and efficiently describes the coupled transient effects from geothermal energy extraction, subsurface heat conduction, horizontal groundwater flow and spatially variable land use. It is successfully verified by comparison with an equivalent numerical model and validated by application to a field case with detailed long-term temperature monitoring. Non-dimensional sensitivity analysis reveals the coupled influence of advection and conduction for different assumptions of the land surface. Especially accelerated heat flux from asphalt or buildings at the land surface is shown to have a remarkable impact on the thermal conditions in the ground. Together with the flow velocity of the groundwater, it determines the intensity, form and steady-state of the thermal anomaly induced from BHE operation.
Abstract
Meeting the rising energy demands of cities is a global challenge. Exploitation of the additional heat in the subsurface associated with the subsurface urban heat island (SUHI) has been ...proposed to address the heating demands. For the sustainable use of this heat it is crucial to understand how SUHIs evolve. To date, there have been no comprehensive studies showing how temperature anomalies beneath cities change over time scales of decades. Here, we reveal the long-term increase of temperatures in the groundwater beneath Cologne, Germany from 1973 to 2020. The rise in groundwater temperature trails atmospheric temperature rise in the rural areas and exceeds the rise in atmospheric temperature in the urban center. However, the amount of heat that is currently stored each year in the thin shallow aquifer reaches only 1% of the annual heating demand. The majority of the anthropogenic heat passes by the vertical extent of the aquifer or is discharged by the adjacent river. Overall the geothermal resource of the urban ground remains largely underused and heat extraction as well as combined heating and cooling could substantially raise the geothermal potential to supply the city’s demand.
Only meters below our feet, shallow aquifers serve as sustainable energy source and provide freshwater storage and ecological habitats. All of these aspects are crucially impacted by the thermal ...regime of the subsurface. Due to the limited accessibility of aquifers however, temperature measurements are scarce. Most commonly, shallow groundwater temperatures are approximated by adding an offset to annual mean surface air temperatures. Yet, the value of this offset is not well defined, often arbitrarily set, and rarely validated. Here, we propose the usage of satellite-derived land surface temperatures instead of surface air temperatures. 2 548 measurement points in 29 countries are compiled, revealing characteristic trends in the offset between shallow groundwater temperatures and land surface temperatures. Here it is shown that evapotranspiration and snow cover impact on this offset globally, through latent heat flow and insulation. Considering these two processes only, global shallow groundwater temperatures are estimated in a resolution of approximately 1 km × 1 km. When comparing these estimated groundwater temperatures with measured ones a coefficient of determination of 0.95 and a root mean square error of 1.4 K is found.
Human activity directly influences ambient air, surface and groundwater temperatures. The most prominent phenomenon is the urban heat island effect, which has been investigated particularly in large ...and densely populated cities. This study explores the anthropogenic impact on the thermal regime not only in selected urban areas, but on a countrywide scale for mean annual temperature datasets in Germany in three different compartments: measured surface air temperature, measured groundwater temperature, and satellite-derived land surface temperature. Taking nighttime lights as an indicator of rural areas, the anthropogenic heat intensity is introduced. It is applicable to each data set and provides the difference between measured local temperature and median rural background temperature. This concept is analogous to the well-established urban heat island intensity, but applicable to each measurement point or pixel of a large, even global, study area. For all three analyzed temperature datasets, anthropogenic heat intensity grows with increasing nighttime lights and declines with increasing vegetation, whereas population density has only minor effects. While surface anthropogenic heat intensity cannot be linked to specific land cover types in the studied resolution (1km×1km) and classification system, both air and groundwater show increased heat intensities for artificial surfaces. Overall, groundwater temperature appears most vulnerable to human activity, albeit the different compartments are partially influenced through unrelated processes; unlike land surface temperature and surface air temperature, groundwater temperatures are elevated in cultivated areas as well. At the surface of Germany, the highest anthropogenic heat intensity with 4.5K is found at an open-pit lignite mine near Jülich, followed by three large cities (Munich, Düsseldorf and Nuremberg) with annual mean anthropogenic heat intensities >4K. Overall, surface anthropogenic heat intensities >0K and therefore urban heat islands are observed in communities down to a population of 5000.
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•Anthropogenic temperature anomalies are quantified in Germany.•Temperatures in air, surface and groundwater correlate with nighttime lights.•Groundwater temperature anomalies are most extreme.•Heat anomalies in air and groundwater are mainly caused by artificial surfaces.•Surface urban heat islands are observed in settlements with only 5000 inhabitants.
•Thermal efficiency of four types ground heat exchangers in energy piles is studied.•Heating and cooling performance is analyzed by thermal performance tests.•Double-U type heat exchanger performs ...the lowest thermal efficiency.•Thermal efficiency is more important than pipe costs in practical application.
In this paper, thermal efficiency of four different types of ground heat exchangers in energy piles is investigated (type: double-U, triple-U, double-W and spiral). Five thermal performance tests are conducted to analyze ground heat exchangers operation under an intermittent condition (7days on for cooling, 26days off, 7days on for heating).
Results show that double-U type is with worst heating and cooling performance, accounting for 67–69% thermal efficiency than spiral and double-W types which are with similar thermal outputs. For technical problems, only heating performance of triple-U type is examined experimentally but the cooling performance is studied numerically. The findings show that tripe-U type performs highest thermal efficiency among all types. By examination of different pipe dimensions heat transfer rate of spiral type with 32mm diameter is increased by 32% than that of 25mm diameter.
In addition, cost-benefits evaluation shows that triple-U shaped ground heat exchanger has highest economic performance, followed by double-U, spiral and double-W type. However, pipe material is only a small consists of the total installation costs. Hence, thermal efficiency is a more important factor to consider than pipe costs in practical application.
In recent years, some spectacular cases of damage occurred with vertical ground source heat pump (GSHP) systems in Germany. Broad media coverage attracted enormous public attention, with reports ...about land subsidence and ground uplifts causing severe damage to buildings. Consequently, sales of vertical GSHP systems have declined. The current study develops conceptual models illustrating the causes and effects of damage in relation to geological and hydrogeological settings. Our investigations revealed nine cases of serious damage in Germany, causing financial losses of more than 100 million Euros. In most cases, connection of aquifers by leaky annular space grouting was the main cause of damage. Guidelines to regulate the installation of vertical GSHP systems have been introduced successfully in all federal states. However, further risk minimisation strategies must be developed to restore the public’s confidence in GSHP technology. Quality assurance and quality control measures should include in particular the optimisation of backfilling materials and increasing use of monitoring systems.
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