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[en] The aim of this guide is to shed light on the possible uses, the expected efficiency, the specifications and the reduction of CO2 emissions of geothermal energy. The first part of the guide presents some general considerations and the legal aspects of geothermal energy development. The second part treats specifically of the Landes area (SW France) in the framework of its geothermal resources valorization.
[en] At Brady Hot Springs, a geothermal field in Nevada, heated fluids have been extracted, cooled, and re-injected to produce electrical power since 1992. Analysis of daily pumping records and catalogs of microseismicity between 2010 and 2015 indicates a statistically significant correlation between days when the daily volume of production was at or above its long-term average rate and days when no seismic event was detected. Conversely, shutdowns in pumping for plant maintenance correlate with increased microseismicity. Our hypothesis is that the effective stress in the subsurface has adapted to the long-term normal operations (deep extraction) at the site. Under this hypothesis, extraction of fluids inhibits fault slip by increasing the effective stress on faults; in contrast, brief pumping cessations represent times when effective stress is decreased below its long-term average, increasing the likelihood of microseismicity.
[en] Underground Thermal Energy Storage (UTES) consists in buried devices designed to exchange heat with the surrounding ground supposed to be colder than the heat source during the storage stage and warmer than the application site during the discharging stage. UTES is thus an attractive solution for solar thermal energy seasonal storage, where the heat collected in summer has to be stored to be recovered and used in winter. Several storage devices are available, the choice of which depends both on the conditions of changing and discharging energy and on the geological and hydrogeological local conditions. For instance, the storage can be done in dry rock by circulating a fluid through so-called Borehole Thermal Energy Store (BTES). Combined with Ground Source Heat Pump systems, many sites in Canada and central and northern Europe are already in operation for block and district heating (and more recently cooling) purposes. Solargeotherm is a 3-years French research project focused on the study of solar thermal energy storage into dry rock. Through a real-scale experimental device and heat transfer models which allow exploring scenarios of storage and recovery, the project aims at evaluating the energetic potential of such a system. The experimental site is located in a Paleozoic schist quarry in the Eastern Pyrenees (France). The geological characterization and fracturing of the bedrock were determined thanks to borehole cuttings analysis and geophysics. Thermal properties of the bedrock were evaluated through an in-situ distributed thermal response test. The experimental device includes 42 m2 of thermal solar panels, three sub-vertical boreholes, drilled to 180 m deep and equipped with double-U geothermal probes, and a 6 kW dry cooler simulating the thermal load of a representative house. The probes are instrumented with an optical fiber that enables temperature monitoring all along the boreholes through distributed temperature sensing (DTS) system. The energy injection has begun in March 2010. Entirely automatized, it takes place with a constant flow of 56 l/min as soon as the temperature at the outlet of the solar panels exceeds 30 deg. C. During the first year, no energy recovery was experimented, as the ground store needs to be heated up to reach a yearly quasi steady state. As a first result, temperature pattern in the ground after three months of energy injection in probe B is shown on figure 1. The temperature in probe B appears to be 10 deg. C higher than in witness probes A and C (filled with static water). Observations show that hardly 5% of the energy injected during the day is stored in the BTES, the major part being dissipated during the night. A detailed energy balance is being calculated. Relying on this analysis, best practices guidelines will be edited at the end of the project for a better design and optimum uses of system coupling BTES and solar panels. For a better understanding of the thermal functioning of the system, different modeling efforts were performed. A detailed model of the double-U geothermal probe was developed under Comsol Multiphysics. It accounts for the heat transfers between the different materials involved in the exchange device (i.e. bedrock, sealing grout, polyethylene probe tube, heat transfer fluid, surface insulator). Validated against the experimental data collected during the distributed thermal response test, it shows that, due to the thermal resistivity of the sealing grout, the rock thermal conductivity plays a minor role in the heat transfers between the hot circulating water and the surrounding bedrock. The development of a 3D-multilayer model is also in progress under the Finite Volumes hydrogeological code MARTHE. Simulating the system at a larger scale and pluri-annual periods of time, it will be used to explore different scenarios of energy storage/recovery cycles. Modeling results are expected to complete experimental data analysis in order to evaluate, and propose solutions to improve the efficiency of solar thermal energy storage in dry rock through boreholes. (authors)
[en] Underground Thermal Energy Storage appears to be an attractive solution for solar thermal energy storage. The SOLARGEOTHERM research project aimed to evaluate the energetic potential of borehole thermal energy storage by means of a full-scale experimental device and heat transfer models. Analysis of the experimental data showed that a single borehole is not efficient for storage. Application of a 1D analytical model showed that the heat transfer fluid in the geothermal probe lost 15 per cent of its energy at a depth of 100 m and 25 per cent at 150 m. A 3D multilayer numerical model was then developed and validated against the experimental data. This model was then used to simulate different configurations over many years. Lastly, a theoretical approach to optimising design of a borehole thermal energy store (BTES) was proposed. A relation was established that enables comparison of the storage characteristic time of any vertical BTES to an optimum one. Based on these experimental, modelling and theoretical results, guidelines are formulated to optimise the design of vertical borehole fields with an objective of inter-seasonal heat storage. In particular, borehole fields should define cylindrical storage volumes with diameters twice their height, and depth should not exceed 100 m. (authors)
[en] Elevated temperatures of the rock and underground water under the cities, known as thermal islands, have been investigated for a long time. Nevertheless, all the causes of the origin and the influence of the individual factors on the degree of thermal contamination are still unknown. This phenomenon exists in all layers of the city, such as the atmosphere, surface, or subsurface. In our paper, we focused on examining and analyzing the temperatures in selected samples within the wider area of Bratislava and analyzing data from monitoring using geographic information systems - GIS. Such information is a valuable asset because it is known that the temperature below the city is higher than in rural areas in its vicinity. (authors)
[en] Geothermometry and the use of composite models of the convergence of thermodynamic functions, in the development of conceptual models, gains in importance, as it is possible to identify processes not usually detected by complex analysis. Based on the use of relevant procedures, it is clear that the deep reservoir of the Besenova elevation structure is in terms of reservoir maturity and processes dynamic.