Development of Modelling and Simulation tools for Geothermal Basements and Deep Foundations in Soft Clays

Report, 2016

Pile heat exchangers are fast emerging as a potentially viable alternative to the more prevalent borehole heat exchangers for the provision of space heating and cooling. In the last decade or so, the use of geothermal piles has increased sharply in many countries including Belgium, China, Japan, Switzerland, the Netherlands, United Kingdom, and United States, among others. In Sweden, however, interest in geothermal piles has been surprisingly scant. This is despite the fact that most of the infrastructure and buildings in Sweden are founded on piled foundations. Early estimates suggest that approximately 75 % of heating requirements and 90 % of cooling requirements of a typical Swedish office building could be provided by geothermal piles. Initial studies also indicate quick payback and large carbon savings. On the other hand, as several Swedish cities are founded in areas with very soft soil conditions with high groundwater tables, there are concerns that pile heat exchangers with cyclic thermal loading could trigger excessive creep deformations. Most of the Swedish research on geothermal piles and cyclic thermal loading dates back to 1980s. Today, both analysis and test methods for understanding soft clay behaviour have improved significantly. Hence, there is a need to revisit the topic of cyclic heating and cooling of Swedish soft clays to fully understand the implications of the use of geothermal piles. This project, funded by Swedish Energy Agency, has dealt with the development of mathematical models for thermal modelling of geothermal piles in Swedish soft clay conditions. The new models include a method to determine the thermal impact of the building on the underlying pile heat exchangers, and calculation methods to evaluate the thermal resistance of the pile heat exchangers. An existing borehole model has also been updated for modelling of irregular configurations of geothermal piles. The mathematical models developed in this project can be implemented in any computer code to be incorporated in existing building energy simulation software. The models can also be used to develop controllers and control schemes to maximize the performance of pile heat exchangers. The project has also demonstrated the application of driven steel and precast pile heat exchangers in Swedish soft clays and has established the importance of acquiring in-situ measurements to determine key design parameters. The results from the project have been presented in seven journal and conference proceeding papers, three research reports, and one book chapter.