Characterisation of ground thermal and thermo-mechanical behaviour for shallow geothermal energy applications

Increasing use of the ground as a thermal reservoir is expected in the near future. Shallow geothermal energy (SGE) systems have proved to be sustainable alternative solutions for buildings and infrastructure conditioning in many areas across the globe in the past decades. Recently novel solutions, including energy geostructures, where SGE systems are coupled with foundation heat exchangers, have also been developed. The performance of these systems is dependent on a series of factors, among which the thermal properties of the soil play one of major roles. The purpose of this paper is to present, in an integrated manner, the main methods and procedures to assess ground thermal properties for SGE systems and to carry out a critical review of the methods. In particular, laboratory testing through either steady-state or transient methods are discussed and a new synthesis comparing results for different techniques is presented. In-situ testing including all variations of the thermal response test is presented in detail, including a first comparison between new and traditional approaches. The issue of different scales between laboratory and in-situ measurements is then analysed in detail. Finally, thermo-hydro-mechanical behaviour of soil is introduced and discussed. These coupled processes are important for confirming the structural integrity of energy geostructures, but routine methods for parameter determination are still lacking

A chemical survey of exoplanets with ARIEL

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio

Shallow geothermal energy: effect of in-situ conditions on borehole heat exchanger design and performance

The in-situ conditions are critical for the performance of Borehole Heat Exchangers (BHEs). However, in practice they are often not adequately considered, overwhelming the potential of these systems. This thesis focuses on the accurate estimation of the in-situ characteristics and on their influence on the design and the behaviour of BHEs based on an in-situ study of an heterogeneous bedrock in a semi-urban environment (campus of the University of Liege, Liege, Belgium). The experimental site consists of four double-U BHEs, of about 100 m long, installed over a surface area of 32 m² and equipped with fiber optic cables. Several temperature measurements and Distributed Thermal Response Tests (DTRTs) were conducted in situ in a period of four years, including a long-duration DTRT (heating phase of 7 months), during which temperature was measured by the fiber optics in all the four boreholes. These measurements create a unique data set, that allows to investigate the BHE behaviour for longer heating periods, to study the effect of various factors on the temperature field evolution at the heterogeneous bedrock at the in-situ scale and to evaluate the contribution of temperature borehole logging to the optimisation of BHEs. The effect of urbanisation is studied based on the in-situ measurements and on 3D numerical modelling and its influence on the design is expressed in terms of the maximum extracted power. The subsurface characteristics are correlated with the measured fiber optic profiles and the potential of temperature borehole logging for optimising the design of BHEs in practise is presented. The accuracy of the thermal response test results in the case of insufficient test rig insulation is investigated and recommendations are provided regarding the interpretation of the data by the widely applied Infinite Line Source model. The in-situ measurements during the long-duration DTRT are presented and analysed, together with a 3D numerical model of the test. In this case-study, the possible variation of the effective thermal conductivity along the layers and the air temperature variations during the test do not seem to have a dominant effect on the BHE behaviour during the whole heating phase. The controlling factors for the temperature field evolution in the surrounding rock mass (bedrock heterogeneity, the air temperature variations, the distance to the heating source and the thermal effects at the borehole bottom end) are detected in the measured profiles and their influence is discussed

Thermal Response Test in Borehole Heat Exchangers Equipped with Fiber Optics

Four double-U borehole heat exchangers (BHEs) 100m long are installed in the Sart-Tilman region (Liege, Belgium). The installation procedure and technical difficulties are presented. Fiber optic cables are attached along the length of one pipe loop in each BHE. Temperature is measured along the fibers based on the fiber optic distributed temperature sensing (DTS) technique. Thermal response test (TRT) is conducted in order to determine the rock thermal properties. The DTS instrument records the temperature evolution along the pipe loop during the TRT. Rock thermal conductivity through depth can be estimated based on the recorded data. A 3D model is developed using the finite element code LAGAMINE in order to simulate the TRT. The accuracy of the numerical model is improved by simulating the potential variation of the rock thermal conductivity

Effect of undisturbed ground temperature on the design of closed-loop geothermal systems: A case study in a semi-urban environment

This paper presents temperature measurements in four Borehole Heat Exchangers (BHEs), equipped with fiber optics and located in a semi-urban environment (campus of the University of Liege, Belgium). A 3D numerical model is also presented to simulate the heat loss from the surrounding structures into the subsurface. The mean undisturbed ground temperature was estimated from data during the preliminary phase of a thermal response test (water circulation in the pipe loops), as well as from borehole logging measurements. The measurements during water circulation can significantly overestimate the ground temperature (up to 1.7 C in this case study) for high ambient air temperature during the test, resulting in an overestimation of the maximum extracted power and of the heat pump coefficient of performance (COP). To limit the error in the COP and the extracted power to less than 5%, the error in the undisturbed temperature estimation should not exceed ±1.5 °C and ±0.6 °C respectively. In urbanised areas, configurations of short BHEs (length < 40 m) could be economically advantageous (decreased installation and operation costs) compared to long BHEs, especially for temperature gradient lower than 0.05 °C/m

We gme P08: Fiber-optic temperature profiles analysis for closed-loop geothermal systems-a case study

In order to study the behaviour of shallow closed-loop geothermal systems four borehole heat exchangers equipped with fiber optics were installed on the campus of the University of Liege (Liege, Belgium) over a surface area of 32m². This paper presents the analysis of continuous, high-resolution temperature profiles measured along the boreholes length. The undisturbed ground temperature measurements indicate heat loss from ground structures located close to the boreholes. A 3D numerical model is presented to reproduce the measured temperature profiles. Temperature profiles during hardening of the grouting material indicate extended fractured zones in the rock mass. Temperature measurements during the recovery phase of a Distributed Thermal Response Test indicate the succession of rock layers with different mineral content. The results are in good agreement with those of the borehole televiewer logging method. The presented analysis could provide information on bedrock heterogeneity, on the anisotropic thermal behaviour of the rock mass and on the ground temperature variations due to heat loss from ground structures. These information could significantly contribute to the long-term behaviour prediction of the geothermal system and the geothermal reservoir potential

Fractured bedrock investigation by using high-resolution borehole images and the Distributed Temperature Sensing technique

peer reviewedIn order to investigate the fracturing of the bedrock and its possible heterogeneous distribution in situ, four boreholes equipped with double-U geothermal pipes of 100 m long were installed on the campus of the University of Liege (Liege, Belgium) over a surface area of 32 m². The bedrock, which starts at a depth approximately of 8 m, is quite fractured and consists mainly of siltstone and shale interbedded with sandstone. Different geophysical methods are applied at two different phases, after drilling the boreholes and after injecting the grouting material. The first approach consists in lowering an ultrasonic borehole imager (borehole televiewer; Zemanek, Glenn, Norton, & Caldwell, 1970), an instrument that acts as an ultrasonic transducer and receiver, into the boreholes to obtain high-resolution, continuous images with 360° coverage of the local geology and fracturing. Moreover gamma-ray logs of the four boreholes are obtained and inclinometry is conducted. After drilling the boreholes fiber optic cables are attached along the pipe loops and the double-U pipes are installed inside the boreholes. Then the grouting material is injected. The second approach consists in measuring the temperature along the fibers by applying the Distributed Temperature Sensing technique (Soto, Sahu, Faralli, Bolognini, Di Pasquale, Nebendahl, & Rueck, 2007). A laser pulse is injected into the optical fiber and the temperature along the fiber is determined by the intensity of Raman stokes and anti-stokes reemitted signals. Temperature evolution is measured during hardening of the grouting material. Local maxima of the temperature curve are probably due to a local lower thermal conductivity and/or a local larger quantity of grouting material due to gathering of fractures. A detailed fracture characterisation (position, opening, orientation, dip angle) is obtained based on the acoustic signal travel time and amplitude. The fractures are characterised by the same dipping and orientation but significantly vary in number and location in the four boreholes, despite the close distance between them. Gamma-ray data and observation of the cuttings during drilling result in rock identification through depth as well as in determination of the layer dipping. The inclination of the four boreholes tends to be perpendicular to the dipping. The combination of the two geophysical methods as presented provides information useful for the hydro-thermo-mechanical behaviour of the bedrock. The contribution of the thermal behaviour of borehole heat exchangers to bedrock investigation will be further studied by conducting Distributed Thermal Response tests (Fujii, Okubo, & Itoi, 2006). During the tests we will measure the temperature variation thanks to the installed fiber optics. These data will allow us to correlate any anisotropic thermal behaviour to the geological characteristics. The available information could be used for a detailed numerical model

A case study of a long-duration thermal response test in borehole heat exchangers

Shallow closed-loop geothermal systems are worldwide applied providing economical and environmental benefits. This paper presents an in-situ study of four Borehole Heat Exchangers of 100 m long, installed in an heterogeneous bedrock in the campus of the University of Liege (Liege, Belgium). A Thermal Response Test (TRT) of a heating phase of 7 months was conducted in one of the boreholes. During this test, temperature was measured at the pipe inlet and outlet, as well as along the four boreholes by the fiber optics. To further investigate the measured data, the test was simulated by 3D numerical modeling. The comparison of the measured data with the numerical results allowed to detect the critical parameters for the behavior of the BHE and for the temperature evolution in the surrounding rock mass. In this case study, the behavior of the BHE could be predicted based on the results of a typical-duration TRT (of a few days), considering the ground an homogenous and isotropic material. However, the thermal plume in the surrounding ground seems to be influenced by several factors, such as the bedrock heterogeneity, the distance to the heating source, air temperature variations and thermal effects at the borehole bottom end

Fiber-optic temperature measurements in closed-loop geothermal systems: A case study in heterogeneous bedrock

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