Abstract

The demand for thermal site characterization has risen noticeably in the past decade, particularly for design of geothermal energy solutions and for design of in-ground power cable networks. The results of thermal characterization of a site are typically incorporated in a ground model based on environmental, geological, geophysical and geotechnical geodata. This paper compares in-situ test methods for thermal site characterization of soil. The comparison considers method applicability, deployment method, maximum test depth, test duration and uncertainty of results. A distinction is made between three categories of in-situ tests: (1) in-situ tests using active heat generation, (2) in-situ tests using passive heat generation and (3) in-situ tests with no specific thermal data acquisition

Abstract

As part of the THERMALTUNNEL R&D, partially funded by Centro para el Desarrollo Tecnológico e Industrial (CDTI), a Spanish Ministry of Science and Innovation agency, PRO GEO has developed a soil penetration prototype that measures soil temperature and the thermal properties of the ground (thermal conductivity and specific heat) at different depths. The prototype is the size of a conventional CPT and contains an internal heat source with local thermocouples. A data acquisition system measures the temperature both in the prototype itself and in the surrounding ground through a penetration needle protruding from the main body of the prototype. The thermal parameters are determined by analyzing the measured heating and cooling temperature curves using the finite element codes CODE_BRIGHT (Olivella et al., 1993) and G-PFEM (Monforte, 2018). In this later numerical code, a thermal analysis module has been incorporated within the framework of the project. The operation of the prototype has been satisfactory in laboratory tests through a calibration chamber setup with granular soil under different controlled conditions of water saturation and porosity to assess the impact of those variables on the obtained values of soil thermal conductivity and specific heat. The back-analyzed parameters from the calibration chamber results have been compared with point tests (thermal conductivity and specific heat tests). The project has been developed with the cooperation of CIMNE – International Centre for Numerical Methods in Engineering.

Abstract

Accurate measurement of soil temperature is essential for the diagnosis of various environmental processes and their impact on ecosystems. Indeed, continuous monitoring of temperature profiles with depth provides valuable understanding of the soil thermal response, which could support studies related to climate change, hydrology, agriculture, and geothermal energy. The paper describes an innovative installation procedure of temperature strings (CS225) to measure the temperature field of an undisturbed soil deposit at different depths in real time. The strings consisted of multiple temperature probes connected in series, placed at specific depths within the soil profile. The CS225 probes require little maintenance and no initial calibration, guaranteeing a high accuracy and resolution of the temperature measurements. The initial field measurements have shown a good performance of the probes, which have been able to detect the gradual decrease of soil temperature and the reduction of oscillations with depth. Further research will include the detailed analysis of the influence of ambient wind speed, solar radiation and soil moisture on the heat transfer from the ground surface to the deeper soil layers, as well as laboratory tests and numerical analysis.