Geothermal Energy Harvesting through Pile Foundations – Analysis-based Prediction and Performance Assessment

Open Access
Author:
Ghasemi Fare, Omid
Graduate Program:
Civil Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 05, 2015
Committee Members:
  • Dr Prasenjit Basu, Dissertation Advisor
  • Dr Prasenjit Basu, Committee Chair
  • Derek Elsworth, Committee Member
  • Tong Qiu, Committee Member
  • Dr Swagata Banerjee, Committee Member
Keywords:
  • Heat transfer; geothermal energy; heat exchangers; numerical ana
Abstract:
Seasonal variation of ground temperature is insignificant below a shallow depth, usually couple of meters, from the ground surface and thus pile foundations are good candidates for harvesting geothermal energy through heat exchange with ground. Such piles are commonly known as geothermal piles, heat exchanger piles or energy piles. The great potential of environmental, social and economic benefits of utilizing shallow geothermal energy has made the use of geothermal piles quite popular in different parts of the world. The aim of this study is to assess and quantify the potential of heat exchange through geothermal piles with a view to promote efficient design of pile-anchored geothermal energy harvesting systems. Research objective is achieved through development of numerical models that employ finite difference solution scheme and simulate pile-soil heat exchange with different levels of accuracy. Developed models are validated through comparison of model predictions using available analytical solutions under idealized conditions, field test data reported in literature and data recorded during thermomechanical tests on model geothermal pile installed in dry and saturated sand. An annular cylinder heat source model, which simulates heat transport by the fluid circulating through tubes embedded in heat exchanger piles, is developed as a first modeling attempt. Results obtained from analyses using this model demonstrate that the use of a constant heat flux along the entire length of a heat exchanger pile may significantly misinterpret thermal response of the pile-soil system. The annular cylinder model considers one limb of the embedded heat exchanger element (i.e., circulation tube) and thus, can provide only approximate solution for real-life scenarios. Simultaneous heat transfer from both branches of an embedded U-shaped circulation tube is modeled next. Finite difference analysis (FDA) results are used to develop closed-form equations that can be used in calculation of power output from geothermal piles with a single U-shaped circulation tube. Parameter sensitivity study and advanced first order second moment (AFOSM) reliability analysis are performed to determine the hierarchy of different input variables in order of their relative impacts on heat transfer performance. The first two generations of models developed as part of this research consider heat transport (advection) by the circulation fluid and heat conduction in pile and soil surrounding it. While the model considering both branches of U-shaped circulation tube can predict field and laboratory test data with reasonable accuracy, some discrepancies were observed for predictions of heat transfer in saturated soil. Comparison of data recorded during instrumented laboratory tests on model geothermal pile installed in dry and saturated sand also indicated that heat convection through thermally-induced pore fluid flow within a saturated medium may further facilitate heat exchange through geothermal piles. This feature is incorporated in the developed pile-soil heat exchange model by coupling heat energy balance and Navier Stokes equations that considers a Boussinesq buoyancy term. Results indicate that thermal operation of geothermal piles alters pore fluid density, buoyant flow occurs (even under hydrostatic condition) within saturated soil in the vicinity of heat exchanger piles, and thermally induced pore water flow (under saturated condition) facilitates pile-soil heat exchange.