The use of low enthalpy geothermal energy through the installation of micropiles

The global drive to decarbonize the built environment has intensified interest in renewable heating and cooling technologies, particularly in dense urban settings and retrofit projects. This doctoral research investigates the application of energy micropiles as a combined solution for structural support and low-enthalpy geothermal heating and cooling. The study combines full- scale field experiments with numerical modeling to evaluate both the thermal performance and the thermo-mechanical behavior of energy micropiles under realistic operating conditions. Six energy micropiles were installed in saturated soft and sensitive clay at a test site at the NTNU Dragvoll campus in Trondheim, Norway. Each micropile was 15 meters long and had a diameter of 240 mm or 300 mm. The micropiles were equipped with either a corrugated coaxial pipe or a split-pipe heat exchanging loop. Thermal tests were performed by circulating a heat transfer fluid through the pipes at a temperature of 25 ± 2 °C for several days. Mechanical tests, hereafter referred to as creep tests, involved applying a constant tensile load of 550 kN, while thermo-mechanical tests combined both thermal and mechanical loading conditions. The thermal power outputs ranged from 32.91 to 80.41 W/m, confirming that micropiles can deliver efficient thermal performance and represent a viable solution for heating and cooling buildings. The coefficients of performance (COP) varied between 1.73 and 3.87, which are lower than typical values reported for geothermal systems (3- 5). This reduced efficiency is attributed to the use of an air-source heat pump during testing, rather than a geothermal heat pump, and to the oversizing of the heat pump for single micropile operation. These factors increased the compressor workload and energy consumption, thereby lowering the COP. An air- source heat pump was used solely for test purposes, hence the COP values obtained should not be considered for geothermal systems. Under real operating conditions, a geothermal heat pump would be employed. During the thermo-mechanical test on micropile 5 and the thermal test on micropile 6, expansive strains were observed along most of the micropile length, representing between 53.54% and 65.66% of the free thermal strains. The maximum thermally induced compressive stress, calculated from the blocked strains, was 132.42 kPa/°C, which represented 3% of the grout´s compressive strength. However, two sensors recorded contractive strains which led to the development of localized thermally induced tensile stresses during heating. This unusual behavior is hypothesized to result from sensor malfunction. Nevertheless, given the complex behavior of clay, further investigation is recommended. Additionally, micropile head xvi displacement was observed to increase rapidly at the start of thermal loading and later stabilized with temperature. The thermally induced displacement of 3 mm was approximately three times larger than those caused by creep alone (0.94 mm). Nevertheless, the total displacement was lower than 10 mm which is satisfactory for most building and civil engineering structures provided that the group settlement is not excessive. Upon unloading and cooling, strains values returned to near-zero suggesting a predominantly thermo-elastic response of the micropile under the tested conditions. To complement the findings of the experimental campaign, a numerical parametric study was conducted. The Taguchi method, a statistical approach for design optimization, was adopted to systematically explore the influence of key design variables. An orthogonal array was used to efficiently plan the simulation matrix, significantly reducing the number of required simulations. The level average analysis method was employed to assess the relative impact of each factor and to identify the optimal configuration. The results highlight that micropile length, grout thermal conductivity, and fluid flow rate are the most influential factors affecting the system performance. The practical feasibility of energy micropiles was further validated through a realistic case study involving the simulation of a foundation system for a heating-dominated building located in the Nordic region. In the case study, 48 micropiles were designed to serve both as structural support and as a geothermal heat exchanger. A detailed numerical model was developed using COMSOL Multiphysics, incorporating local climatic data, building heat demand, and subsoil thermal properties. The results showed that the energy micropile system could meet approximately 17.06% of the building’s monthly heating demand, highlighting its potential as a supplemental low-carbon energy source. Furthermore, a 30-year life-cycle cost analysis was performed, comparing the energy micropile system with conventional electric heating. The analysis revealed that, despite higher initial investment costs, long-term operating expenses are comparable, with potential benefits in terms of energy efficiency and environmental impact. In conclusion, this research demonstrates the technical and economic feasibility of energy micropiles as a suitable solution for heating and cooling buildings, particularly in space- constrained dense urban environment or retrofit applications. However, several aspects, such as long-term thermal performance and thermo-mechanical behavior in sensitive clays, groundwater variation, ground temperature changes, cyclic loading effects, and group interactions between micropiles, require further investigation to inform reliable and efficient design strategies.