Shallow Geothermal Systems Research Articles

The influence of initial shaft resistance mobilisation in the response of seasonally, thermally-activated pile foundations in granular media

The thermal-activation of pile foundations for use within shallow geothermal energy systems has received much attention and the results from various studies have allowed the development of a broad understanding of the thermo-mechanical behaviour of thermally-activated (TA) pile foundations. One area that requires further attention is the clarification of the foundation response under seasonal cyclic thermal loading. This study systematically assesses the impact of balanced cyclic thermal loading in relation to the initial mechanical loading, for isolated floating piles and pile groups in a frictional soil medium. The pile thermal response was evaluated for differing levels of mobilisation of the shaft resistance with respect to the initial mechanical loading and for piles at varying spacing based on a unit cell approach. Isolated piles with high shaft mobilisation were found to develop significant irrecoverable settlement during thermal cycling however the maximum thermal stress was similar irrespective of the initial shaft mobilisation. These results are remarkably similar to those reported in small scale tests of TA piles in sand, confirming that the behaviour reported in these is likely to be due to the result of low shaft restraint, rather than thermally-induced loss of resistance on the pile–soil interface. On the other hand, piles in close proximity were found to behave quasi thermo-elastically at the same initial mechanical load, which is attributed to an alteration in mobilised pile resistance due to pile–soil–pile interference under the initial mechanical loading and, to a lesser extent, thermal activation of the ground between the piles both of which reduce the relative movement between the pile and the soil. This pile group thermal effect has major beneficial implications for the geotechnical and structural design of TA pile foundations.

The impact of porous medium heterogeneity on the thermal feedback of open-loop shallow geothermal systems

Groundwater has been increasingly used to provide low-carbon heating and cooling of buildings with open-loop shallow geothermal systems. Water is generally reinjected into the same aquifer after the heat exchange in order to avoid the aquifer depletion. However, this can result in the return of part of the injected water to the production well(s), causing a gradual thermal alteration known as thermal feedback. Thermal feedback is a major design issue of open-loop shallow systems but, so far, it has been mainly addressed neglecting the heterogeneity of the aquifer properties. This study investigates the impact of aquifer heterogeneity on two main metrics that characterize thermal feedback: thermal breakthrough time (i.e., the first arrival time of the thermal plume) and recirculating ratio (i.e., the fraction of water coming back to production well). A stochastic approach was adopted performing a large number of numerical simulations that cover a wide range of possible scenarios. The results highlight that conductivity heterogeneity plays a major influence on the temperature evolution at the production well. The breakthrough time alone might lead to misleading evaluations of the system efficiency, given that a few particles can reach the production well by traveling in the highly-conductive layers. Conversely, both the heterogeneity and the thermal dispersivity have a negligible impact on the recirculating ratio, which quantifies the long-term evolution of thermal feedback. As a consequence, the available approaches based on advection-only and homogeneous medium are a robust tool to predict the long-term behavior of shallow open-loop geothermal systems.

Enhanced Heat Transfer Performance of a New Horizontal Buried Tube Heat Exchanger

Horizontal buried heat exchangers cost less than vertical buried heat exchangers, but they need more land area. In order to reduce the required land area of horizontal buried tube heat exchanger, a finned strengthened heat exchanger pipe was designed to enhance the heat transfer performance, and a three-dimensional research model of shallow geothermal utilization system was established in Chengdu area . In this paper, the optimal shape and arrangement of fins are investigated, and the effects of inlet velocity, soil type, ambient temperature and intermittent operation on the enhanced heat transfer performance are studied. The results show that the enhanced heat transfer performance of rectangular fins is the best in the case of horizontal arrangement. The working fluid flow rate should not exceed 0.3m/s.Soil type has a great influence on heat transfer performance. The heat transfer of rock and soil is the largest, the heat transfer of clay is the smallest, but the strengthening effect of clay is the best. The longer the system runs, the obvious the influence of ambient temperature is, and the heat transfer efficiency increases by about 4% when the ambient temperature drops by 5K.

Energy demand reduction in the built environment using shallow geothermal integrated energy systems: Part II – Hybrid ground source heat pump for building heating

Abstract The research collection aims at finding the various possible opportunities for the effective integration of shallow geothermal energy (SGE) to decrease the energy demand in the built environment and to reduce emission associated with it. The direct utilization of SGE using a ground source heat pump (GSHP) has been reviewed in comprehensive review part I and part II. From the extensive review, it is found that the hybrid GSHP is needed to avoid ground thermal imbalance and peak demand. Hybrid GSHP can adopt various supplemental heat sources and sinks according to the local climatic conditions and the balance of energy demands. The primary focus on the integration of subsystems such as biomass, solar energy (PV, PVT, and collector), phase change material, micro gas turbine, and absorption heat pump with GSHP is presented for heating application. This comprehensive review part III highlights the recent research findings and a potential gap in hybrid GSHP for further research and developments

Development and testing of a novel geothermal wall system

Shallow geothermal energy systems have the potential to contribute to the decarbonization of heating and cooling demands of buildings. These systems typically present drawbacks as high initial investments and occupancy of wide areas. In this study, a novel energy wall system is proposed to overcome the limitations of conventional geothermal applications in urban areas. The system is characterized by ease of installation, low initial costs and applicability to existing buildings undergoing energy retrofitting. The paper illustrates the implementation of the prototype of such a system to an existing structure in Torino (Italy). An overview of the components is given together with the interpretation of an illustrative test carried out in heating mode. The data from both heating and cooling experimental campaigns allow us to highlight the potential of the proposed technology. The results suggest that an average thermal power of about 17 W per unit area can be exchanged with the ground in heating mode, while an average of 68 W per unit area is exchanged in cooling operations. The negligible impact on the stress–strain state of the wall and the surrounding soil thermal and hygrometric regime is also testified by the results collected. These aspects are associated with a reduced probability of interferences with other installations in highly urbanized areas, easiness of installation and affordable cost.

Thermal modeling of a Swiss urban aquifer and implications for geothermal heat pump systems

The progressive electrification of the building conditioning sector in recent years has greatly contributed to reducing greenhouse gas emissions by using renewable energy sources, particularly shallow geothermal energy. This energy can be exploited through open and closed shallow geothermal systems (SGS), and their performances greatly depend on the ground/groundwater temperature, which can be affected by both natural and anthropogenic phenomena. The present study proposes an approach to characterize aquifers affected by high SGS exploitation (not simulated in this work). Characterization of the potential hydro/thermogeological natural state is necessary to understand the regional flow and heat transport, and to identify local thermal anomalies. Passive microseismic and groundwater monitoring were used to assess the shape and thermal status of the aquifer; numerical modeling in both steady-state and transient conditions allowed understanding of the flow and heat transport patterns. Two significant thermal anomalies were detected in a fluvio-glacial aquifer in southern Switzerland, one created by river water exfiltration and one of anthropogenic nature. A favorable time lag of 110 days between river and groundwater temperature and an urban hot plume produced by underground structures were observed. These thermal anomalies greatly affect the local thermal status of the aquifer and consequently the design and efficiency of current and future SGS. Results show that the correct characterization of the natural thermo-hydrogeological status of an aquifer is a fundamental basis for determining the impact of boundary conditions and to provide initial conditions required to perform reliable local thermal sustainability assessments, especially where high SGS exploitation occurs.

Impact of simplifications on numerical modelling of the shallow subsurface at city-scale and implications for shallow geothermal potential

Anthropogenic infrastructures in the shallow subsurface, such as heated basements, tunnels or shallow geothermal systems, are known to increase ground temperatures, particularly in urban areas. Numerical modelling helps inform on the extent of thermal influence of such structures, and its potential uses. Realistic modelling of the subsurface is often computationally costly and requires large amounts of data which is often not readily available, necessitating the use of modelling simplifications. This work presents a case-study on the city centre of Cardiff, UK, for which high resolution data is available, and compares modelling results when three key modelling components (namely ground elevation, hydraulic gradient distribution and basement geometry) are implemented either ‘realistically’, i.e. with high resolution data, or ‘simplified’, utilising commonly accepted modelling assumptions. Results are presented at a point (local) scale and at a domain (aggregate) scale to investigate the impacts such simplifications have on model outputs for different purposes. Comparison to measured data at individual locations shows that the accuracy of temperature outputs from numerical models is largely insensitive to simplification of the hydraulic gradient distribution implemented, while changes in basement geometry affect accuracy of the mean temperature predicted at a point by as much as 3.5 °C. At the domain scale, ground temperatures within the first 20 m show a notable increase (approximately 1 °C volume-averaged and 0.5 °C surface-averaged), while the average heat flux over the domain is about 0.06 W/m2 at 20 m depth. These increased temperatures result in beneficial conditions for shallow geothermal utilisation, producing drilling cost savings of around £1700 per typical household system or about 9% increase in thermal energy potential. Simplifications of basement geometry and (to a lesser degree) the hydraulics can result in an overestimation of these temperatures and therefore over-predict geothermal potential, while the elevation simplification showed little impact.

Thermal performance of seasonally, thermally-activated floating pile foundations in a cohesive medium

The thermal-activation of pile foundations for use within shallow geothermal energy systems has received much attention with a number of studies having reported full- and small-scale testing and/or numerical analysis. Various conditions in terms of pile type, ground profiles and thermal loading, have been considered in these studies, leading to a broad understanding of the thermo-mechanical behaviour of thermally-activated pile foundations. One area that requires further attention is the clarification of the foundation response under seasonal cyclic thermal loading. This study systematically assesses the impact of cyclic thermal loading in relation to the initial mechanical loading, for isolated floating piles and pile groups in a cohesive soil medium. For piles where the shaft resistance dominates the pile total resistance, it was found that irrecoverable movement will be small and thermal stress and pile head movements change in a cyclic and regular manner. It is shown that the effect of the coefficient of thermal expansion of the soil, is much reduced from that suggested in studies using constant thermal loads applied over long periods. The effect of the overlying building was explored and it is shown that thermal-activation of the pile foundation mitigates the effect of the imposition of a higher average temperature at the surface.

Exploration of Shallow Geothermal Energy Aquifers by Using Electrical Resistivity Survey in Laki Range Jamshoro district Sindh, Pakistan

Geothermal water is increasingly used around the world for its exploitation. Bulk electrical resistivity differences can bring significant information on variation of subsurface geothermal aquifer characteristics. The electrical resistivity survey was carried out in Laki range in lower Indus basin in the study area to explore the subsurface geothermal aquifers. The Schlumberger electrode configuration with range from 2 m to 220 m depth was applied. Three prominent locations of hot springs were selected including Laki Shah Saddar, Lalbagh and Kai hot spring near Sehwan city. After processing resistivity image data, two hot water geothermal aquifers were delineated at Laki Shah Sadder hot springs. The depth of first aquifer was 56 m and its thickness 38 m in the limestones. The depth of second aquifer of 190 m and with thickness of 96 m hosted in limestone. In Lalbagh hot springs two geothermal aquifers were delineated on the basis of apparent resistivity contrast, the depth of first aquifer zone in sandstone was in sandstone 15 m and thickness 12 m, while the depth of second aquifer was 61m and thickness was 35m. In Kai hot springs two hot water geothermal aquifers were delineated. The depth of first geothermal aquifer was 21m and thickness was 18 m and the depth of second aquifer was 105 m and thickness was 61m present in sandstone lithology. Present work demonstrates the capability of electrical resistivity images to study the potential of geothermal energy in shallow aquifers. These outcomes could potentially lead to a number of practical applications, such as the monitoring or the design of shallow geothermal systems.

Closed-loop geothermal energy recovery from deep high enthalpy systems

Closed-loop geothermal energy recovery technology has advantages of being independent of reservoir fluid and permeability, experiencing less parasitic load from pumps, and being technologically ready and widely used for heat exchange in shallow geothermal systems. Commercial application of closed-loop geothermal technology to deep high-enthalpy systems is now feasible given advances in drilling technology. However, the technology it uses has been questioned due to differences in heat transport capacities of convective flow within the wellbores and conductive flux in the surrounding rock. Here we demonstrate that closed-loop geothermal systems can provide reasonable temperature and heat duty for over 30 years using multiple laterals when installed in a suitable geological setting. Through use of two analytical methods, our results indicate that the closed-loop geothermal system is sensitive to reservoir thermal conductivity that controls the level of outlet temperature and interference between wells over time. The residence time of the fluid in the horizontal section, calculated as a ratio of the lateral length to flow rate, dictates heat transport efficiency. A long vertical production section could cause large drops in fluid temperature in a single lateral production system, but such heat loss can be reduced significantly in a closed-loop system with multiple laterals.

Machine learning enhancement of thermal response tests for geothermal potential evaluations at site and regional scales

This study explores and validates a machine learning approach for the practical, effective, and precise prediction of the thermo-physical characteristics that are essential for the analysis and design of shallow geothermal systems, including borehole heat exchangers: (i) undisturbed ground temperature, (ii) ground effective thermal conductivity, and (iii) borehole thermal resistance. Benefiting from 174 thermal response tests from central and western Switzerland, the algorithm is used to provide accurate site-specific as well as regional-scale predictions of the investigated thermo-physical characteristics, which in turn can serve preliminary yet representative evaluations of the geothermal potential of even very broad areas.

Prospects of shallow geothermal systems for sustainable heating and cooling of buildings

Net-zero-energy buildings have the potential to mitigate climate change through reduced energy use. Shallow geothermal systems use the thermal inertia of the Earth to provide a temperature gradient between ambient conditions and underground soils. This thermal inertia can be used by heat exchangers to provide space heating and cooling during winter and summer, respectively. This paper provides a brief but broad overview of the different active and passive technologies involved in the use of heat exchangers for heating and cooling to achieve near net-zero-energy buildings. Different types of ground heat exchangers (GHEs) and heat pumps are introduced along with relevant studies of significance in this field. It has been demonstrated that different types of heat exchangers can be integrated with thermally active building envelopes and renewable energy resources to minimise building energy use. A pathway is then presented for using GHEs to realise net-zero-energy buildings.

Evaluation of different methodologies for the design of the wellfield in shallow geothermal systems

Low enthalpy geothermal resources play an essential role in climate change mitigation. When ensuring the correct future operation of ground-source heat pump systems, an accurate design is mandatory. In this sense, different methodologies can be implemented. Although using sophisticated software constitutes the most optimal solution, its implementation is sometimes inviable in certain projects (the increase of the initial investment required is not justified in small plants). This work is focused on evaluating and comparing procedures used in the design of shallow geothermal systems. Thus, the research includes a simple method based on manual calculations, the Climasoft free application, Earth Energy Designer (EED) software, and the new geothermal tool GES-CAL developed by researchers from the TIDOP Research Group (University of Salamanca). The objective is to evaluate this new software and compare the results of all the detailed methodologies. This comparison derives from applying these tools in the calculation of the same case study (a single-family house placed in Ávila, Spain). Results show that the easiest methods involve oversized well-field schemas that also mean higher initial investments. Regarding GES-CAL, it is considered an accurate and valid alternative for the design of all heat exchanger configurations, especially for those installations placed in the region of Ávila. However, EED is recommended to calculate high-power geothermal systems that require an exhaustive analysis of the ground and the heat carrier fluid behaviour.

Integration of Photovoltaic Electricity with Shallow Geothermal Systems for Residential Microgrids: Proof of Concept and Techno-Economic Analysis with RES2GEO Model

The European Union aims to reduce Greenhouse Gas (GHG) emissions by 55% before 2030 compared to 1990 as a reference year. One of the main contributions to GHG emissions comes from the household sector. This paper shows that the household sector, when organised into a form of prosumer microgrids, including renewable sources for electric, heating and cooling energy supply, can be efficiently decarbonised. This paper investigates one hypothetical prosumer microgrid with the model RES2GEO (Renewable Energy Sources to Geothermal). The aim is to integrate a carbon-free photovoltaic electricity source and a shallow geothermal reservoir as a heat source and heat sink during the heating and cooling season. A total of four cases have been evaluated for the Zagreb City location. The results represent a balance of both thermal and electric energy flows within the microgrid, as well as thermal recuperation of the reservoir. The levelised cost of energy for all cases, based on a 20-year modelling horizon, varies between 41 and 63 EUR/MWh. On the other hand, all cases show a decrease in CO2 emissions by more than 75%, with the best case featuring a reduction of more than 85% compared to the base case, where electricity and gas for heating are supplied from the Distribution System Operator at retail prices. With the use of close integration of electricity, heating and cooling demand and supply of energy, cost-effective decarbonisation can be achieved for the household sector.

Analytical model for arbitrarily configured neighboring shallow geothermal installations in the presence of groundwater flow

This paper introduces an analytical model analyzing the effect of groundwater flow on heat transfer in an infinite conductive-convective porous domain representing shallow geothermal systems with arbitrarily configured cylindrical heat sources. The model is formulated based on the moving source concept and solved based on the spectral analysis method and the superposition principle. Compared to models based on the Green’s function and the Laplace transform, the proposed spectral model has a simpler formulation, computationally efficient and easy to implement in computer codes. It can handle random time-dependent thermal loads and any arbitrarily configured grid distribution. The verification and numerical examples demonstrate the computational capabilities of the model, and show how the groundwater flow can play an important role in the thermal interaction between heat sources. They also feature how to make use of the direction of groundwater flow to avoid undesirable thermal interaction between neighboring installations, rapid depletion of energy sources and unfair mining of geothermal energy.

A Novel Multidisciplinary Approach for the Thermo‐Rheological Study of Volcanic Areas: The Case Study of Long Valley Caldera

AbstractWe propose a novel multidisciplinary approach to image the thermo‐rheological stratification beneath active volcanic areas, such as Long Valley Caldera (LVC). We performed a thermal fluid dynamic model via optimization procedure of the thermal conditions of the crust. We characterize the topology of the hot magmatic bodies and the hot fluid circulation (the permeable fault‐zones), using both a novel imaging of the a and b parameters of the Gutenberg‐Richter law and an innovative procedure analysis of P‐wave tomographic models. The optimization procedure provides the permeability of a reservoir (5.0 × 10−14 m2) and of the fault‐zone (5.0 · 10−14 – 1.0 × 10−13 m2), as well as the temperature of the magma body (750–800°C). The imaging of the rheological properties of the crust indicates that the brittle/ductile transition occurs about 5 km b.s.l. depth, beneath the resurgent dome. There are again deeper brittle conditions about 15 km b.s.l., agreeing with the previous observations. The comparison between the conductive and the conductive‐convective heat transfer models highlights that the deeper fluid circulation efficiently cools the volumes above the magmatic body, transferring the heat to the shallow geothermal system. This process has a significant impact on the rheological properties of the upper crust as the migration of the B/D transition. Our findings show an active magmatic system (6–10 km deep) and confirm that LVC is a long‐life silicic caldera system.

Innovative Solutions for Improving the Heat Exchange in Closed-Loop Shallow Geothermal Systems

Thermal conductivity, hydraulics properties and potential use in low-enthalpy geothermal applications of single and double U geothermal probes enhanced with carbon fibre are discussed in this work. Although the efficiency of a shallow geothermal installation is chiefly based on chemical and physical characteristics of rocks and hydrogeological aspects of the subsurface, the total heat extracted from the subsoil also depends on the intrinsic thermal characteristics of probes. New configurations and solutions aimed at enhancing the performance of components are therefore of considerable interest in this field of research. As a consequence of the economic and versatility advantages of the components, geothermal probes have been generally developed with materials like polyethylene, which presents, however, isolating behaviour that does not allow ideal heat exchange in ground source heat pump systems (GSHP). Innovative combinations of different materials are therefore necessary in order to improve thermal conductivity and to preserve the exceptional workability and commercial advantages of the finest elements available on the market. This work presents results coming from experimental tests involving standard polyethylene geothermal probes integrated with radial rings of polyacrylonitrile-based carbon fibre (PAN). Our evaluations are aimed at finding the best solutions for thermal exchange and adaptability with respect to traditional systems. Hydraulic and thermal performances and the response in a geo-exchange system have been verified. The new solutions appear to be highly suitable as geothermal exchangers in shallow geothermal systems and contribute to significantly reduce the total costs pertaining to the drilling operations.

Regional-scale assessment of the thermal potential in a shallow alluvial aquifer system in the Po plain (northern Italy)

The Milan Metropolitan Area is one of the most densely populated regions in Italy and Europe and, consequently, has an extremely high thermal power demand. In this study a regional scale assessment of the potential of a shallow alluvial aquifer to host low enthalpy geothermal systems is presented. The study area lies on a layered phreatic aquifer complex and presents advantageous hydrogeological characteristics that make low enthalpy geothermal supply an attractive heating/cooling solution from a carbon footprint and an economical point of view. In this context, different analytical solutions were used to estimate the thermal potential of closed- and open-loop systems based on the distribution of hydraulic and thermal parameters of the shallow aquifers. The thermal potential of closed-loop systems (ground-coupled heat pumps - GCHP) was estimated through the ASHRAE analytical equation considering the hydraulic and thermal parameters of the ground, the groundwater temperature, and the characteristics of the system. The thermal potential of open-loop systems (groundwater heat pumps - GWHP) was estimated by considering the local regulation on the abstraction and the conservation of the quantity and the quality of the groundwater as well as preventing excessive groundwater table drawdown or rising and thermal short-circuit. The results were compared with heat demand rates of the buildings in the municipalities of the study area and, finally, the most profitable shallow geothermal system configuration is discussed.

Identifying key locations for shallow geothermal use in Vienna

Decarbonising the heating sector is crucial for reducing CO2 emissions. This is in particular true for Central European cities such as Vienna, where 28% of the total CO2 emissions are caused by the energy supply for buildings. One promising option for environmental friendly heat supply is the use of shallow geothermal energy. To determine whether shallow geothermal systems are a feasible option to meet the urban heating demand, the Python tool GeoEnPy is developed and applied to a case study in Vienna. It allows the evaluation of the anthropogenic heat input into the subsurface, the theoretical sustainable potential, the technical geothermal potential, and the heat supply rate. The overall heat flow in Vienna is 17.6 PJ/a, which represents 38% of the current heating demand or indeed 99% once all buildings are thermally refurbished. The technical geothermal potential can satisfy the current heating demand for 63% (BHE system) or rather 8% (GWHP system) of the city area. GeoEnPy reveals that BHE systems are most feasible in the eastern and southern districts of Vienna. Our findings can guide integration of shallow geothermal use in spatial energy management focused on key locations to supply buildings with decentralised and sustainable heat from the subsurface.

A geospatial assessment of the installation potential of shallow geothermal systems in a graben basin

The installation potential of a ground source heat pump (GSHP) system was highly affected by site geological conditions. A certain type of geological unit maintains similar geological settings geospatially. It is of importance to demonstrate a case to characterize the geological background and the associated geothermal potential to properly plan and sustainable development of geothermal resources. In this paper, the installation of GSHP system in a graben basin was assessed by considering the geospatial variation in the soil types. The results show that the graben basin was composed of an alluvial area and a piedmont area in which the alluvial area expresses a higher potential for a GWHP system than the piedmont area. Within the alluvial area, the upper alluvial reaches attain the highest heat extraction rate of 5.26 kW/m, followed by the middle reaches at 3.16 kW/m and the lower reaches at 1.58 kW/m. More specially, the ground-coupled heat pump (GCHP) system realizes a higher exploitable energy density than the GWHP system in all areas due to the dense layout of borehole heat exchangers (BHEs). By further considering the savings-to-investment (STI) ratio and environmental impacts, the GWHP system was considered the first priority in alluvial areas II-a and II-b. Installation of the GCHP system was recommended in alluvial area II-c and the piedmont area.