Shallow Geothermal Applications Research Articles

Experimental geothermal monitoring assessing the underground sustainability of GSHP borehole heat exchangers in a protected hydrothermal area: The case study of Ponte Arche (Italian Alps)

An experimental geothermal monitoring, performed on the ground source heat pump (GSHP) system running in a nearly zero energy building (NZEB), has been realized in the protected hydrothermal area of Ponte Arche (Italian Alps), at the request of the competent geological authority. The scope is to assess if a thermal interference could appear between the exploitation of the shallow underground by borehole heat exchangers (BHEs) and nearby thermal baths. Currently, in this area an absolute prohibition for shallow geothermal applications is in force, the area is categorized “potentially subject to geothermal manifestations”. However, in light of new geological and hydrogeological knowledge, supported by long-term monitoring, the protection criterion and constraint geometry could be reviewed. Preliminary to the experimental geothermal monitoring, a detailed ground investigation campaign was performed, including: a geological setting overview based on existing cartography, one continuous core drilling, three downhole geophysical surveys, one thermal response test (TRT). The monitored parameters comprehend the underground temperature at different depths, the outdoor air temperature, and the ground-side heat flow of the GSHP. The collected data are presented and discussed, including considerations about the underground sustainability of the BHE field, and about the real heating and cooling operation of the GSHP ground-side.

On the importance of a coordinated site characterization for the sustainable intensive thermal use of the shallow subsurface in urban areas: a case study

Shallow geothermal applications have become standard solutions for heating and cooling in many newly built or redeveloped residential neighborhoods, but current urban development practices do not yet consider the new demands that result from the intensive thermal use of the shallow subsurface. A coordinated site characterization is of great importance as a sound basis for an optimized planning of geothermal systems that brings together user requirements (heating, cooling, and/or seasonal energy storage) and (hydro)geological subsurface conditions. The aim of this study is to raise awareness and to demonstrate the relevance of a coordinated site characterization. Therefore, this study quantifies the advantages of a site-specific over a desktop-based site characterization in reducing uncertainty for calculation of borehole heat exchanger length and predicted induced temperature changes in the subsurface for a newly developed residential neighborhood in the city of Taucha, Germany. Results show that savings of over EUR 1850 per house (EUR 98,050 for the entire neighborhood) can be achieved by a coordinated exploration and prediction accuracy of temperature plume development was substantially improved. Although being more cost intensive, exploration costs for this case study are <3% of the assumed individual geothermal system costs of EUR 16,000 if divided equally among geothermal users. Three different options are presented to implement coordinated exploration concepts into site development practice.

Laboratory scale geophysical measurements aimed at monitoring the thermal affected zone in Underground Thermal Energy Storage (UTES) applications

Underground Thermal Energy Storage systems have showed to be a useful and increasing technical solution for covering heating and cooling and domestic hot water buildings' demand. Thermal influence of these plants is however not still debated as it deserves and correct monitoring strategies appear to be of major importance both to better understand processes and to highlight their environmental effects into high populated areas. Litho-, hydro- and bio-sphere can indeed be adversely affected by temperature variations induced in the underground by heat storage applications. For this purpose, a geophysical monitoring approach using time-lapse electrical resistivity measurements contemporary to analogical simulations is here tested at laboratory scale. Results of the experiments are reported comparing measured apparent resistivity with direct temperature measurements and numerical simulations of heat propagation. Data presented confirmed that electrical resistivity has powerful relation with temperature variation in monitored media. In addition, they showed that also without performing data inversion valid temperature estimation can be carried out. Post-processing calibration of apparent resistivity data showed to be in acceptable agreement with both temperature measurements and numerical simulations. Simple apparent electrical resistivity variations appear therefore to be a promising, economic, quick and non-invasive tool for mapping thermal modifications induced in the underground by shallow geothermal applications.

A finite line source model with Cauchy-type top boundary conditions for simulating near surface effects on borehole heat exchangers

BHEs (borehole heat exchangers) are the most common shallow geothermal applications. By approximating the BHE as a line source, semi-analytical models can describe the heat exchange within the ground. These models though always assume prescribed temperature at the ground surface. This work presents a formulation which expands existing finite line source models by implementing a more general Cauchy-type top boundary condition and in this way, a better estimation of the heat fluxes at the ground surface. The new formulation is numerically verified and examined in a dimensionless analysis. It is demonstrated that the discrepancy to prescribed temperature settings is significant near to the ground surface, and it propagates deeper when groundwater flow is absent and when strong decoupling between the thermal regimes interacting at the land surface is assumed. The new approach shows to be suited especially for short BHEs, both for more flexible and accurate prediction of the ground thermal regime as well as for long-term analysis of technological performance.

Geophysical monitoring for shallow geothermal applications – two Italian case histories

In the context of shallow geothermal applications, geo¬physics can be applied as a design and monitoring tool. One of the most referred applications is monitoring and imaging the time lapse evolution of the Thermal Affected Zone (TAZ). In the last few decades electrical resis¬tivity measurements have been adopted both for the char¬acterization and to image spatial and temporal distribution of temperature within the ground. This paper is therefore devoted to the use of electrical resistivity in this context with a focus on the Italian state of the art. A brief literature review of thermal characteristics influencing the resistivity value and example applications of Electric Resistivity Tomography (ERT) data is reported. Two example case histories, in very different hydrogeological contexts in northern Italy, are also presented.

Thermal response test for shallow geothermal applications: a probabilistic analysis approach

Abstract Background Thermal Response Test (TRT) is an onsite test used to characterize the thermal properties of shallow underground, when used as heat storage volume for shallow geothermal application. It is applied by injecting/extracting heat into geothermal closed-loop circuits inserted into the ground. The most common types of closed loop are the borehole heat exchangers (BHE), horizontal ground collectors (HGC), and energy piles (EP). The interpretation method of TRT data is generally based on a regression technique and on the calculation of thermal properties through different models, specific for each closed loop and test conditions. Methods A typical TRT record is a graph joining a series of experimental temperatures of the thermal carrier fluid. The proposed geostatistical approach considers the temperature as a random function non-stationary in time, with a given trend, therefore the record is considered as a ‘realization’, one of the possible results; the random nature of the test results is transferred to the fluctuations and a variogram modeling can be applied, which may give many information on the TRT behavior. Results In this paper, a nested probabilistic approach for TRT output interpretation is proposed, which can be applied for interpreting TRT data, independently of the different methodologies and technologies adopted. In the paper, for the sake of simplicity, the probabilistic approach is applied to the ‘infinite line source’ (ILS) methodology, which is the most commonly used for BHE. Conclusions The probabilistic approach, based on variogram modeling of temperature residuals, is useful for identifying with robust accuracy the time boundaries (initial time t 0 and the final time t f) inside which makes temperature regression analysis possible. Moreover, variograms are used into the analysis itself to increase estimation precision of thermal parameter calculation (ground conductivity λ g, ground capacity c g, borehole resistance R b). Finally, the probabilistic approach helps keep under control the effect of any cause of result variability. Typical behaviors of power, flows, and temperatures and of their interaction with the specific closed-loop circuit and geo-hydrological system are deepened by variogram analysis of fluctuations.

Estimating apparent thermal diffusivity of soil using field temperature time series

Reliable estimates of soil thermal properties such as heat capacity, thermal conductivity, and diffusivity are important in analysis of heat transmission through soils in applications such as shallow geothermal applications, buried electrical conduits, and in general heat/fluid flow analyses. A number of analytical, numerical and experimental methods are available to determine the soil thermal properties. In this paper, the analytical and numerical methods developed on the basis of one-dimensional heat conduction equation are used to estimate the apparent thermal diffusivity of soil. Three of the four analytical methods, Amplitude, Phase, and Arctangent provide explicit equations for the apparent thermal diffusivity. Two methods, Harmonic and Numerical, make use of large number of temperature measurements to implicitly solve for the apparent thermal diffusivity. The temperature time series data monitored at different depths in two field sites in Melbourne, Australia for more than 2 year period were used to estimate the apparent thermal diffusivity of soil down to 2 m depth. The apparent thermal diffusivity was calculated using all five methods and compared with laboratory experimental results. The effectiveness of each method in predicting the thermal diffusivity was compared and observed discrepancies were discussed. Finally, the observed soil temperature data for a 12 month period are used to model the temperature variation in the ground analytically using Harmonic method and the model prediction for the following 12 month was compared independently with the field measurements. The analytical model prediction is found to be in good agreement with the field monitored data.