Investigating the thermal and thermo-mechanical performances of geothermal heat exchanger with spiral-tubes

Abstract

Ground-coupled heat pump (GCHP), one of renewable energy technologies, is a heat transfer device with high energy efficiency and little impact on the environment, which can transfer the heat from a cool space to a warm space or enhance the natural flow of heat from a warm place to a cool one. As a central heating and cooling system, GCHP utilises the ground as a heat source or sink to extract (in the winter) or store (in the summer) the heat energy. Without thermal disturbance, the temperature beneath approximate 6 meters of the ground surface varies within a small range, generally maintaining between 10 and 16ºC. Due to this characteristic of little thermal disturbance, GCHP has better efficiency performance and lower operational costs compared to the conventional heating and cooling systems. The geothermal heat exchanger (GHE) is the key connecting component between the heat pump unit and ground sources. Its heat transfer performance is regarded as the most important parameter of a GCHP system in either design or construction stage. The system efficiency can be influenced directly by the GHE installation configuration, e.g. horizontal GHEs and vertical GHEs. Although the heat transfer mechanics of GHEs have been studied for many years, the current efforts have been focused on the linear type GHEs (U-tubes and W-tubes) and less on the modelling of GHE with spiral-tubes. The spiral GHEs are initially used in horizontal GCHP systems. Compared with the traditional linear GHEs, spiral-tubes can greatly improve the heat transfer efficiency, and consequently, can reduce initial costs and the required installation area. Therefore, it is essential to establish a reliable analytical solution for the horizontal GHE with spiral-tubes. Additionally, with the increasing application of GCHP system in the urban area, presently the spiral-tubes can also be applied in vertical GHEs, especially for pile geothermal heat exchangers (PGHEs), also called energy pile. PGHE is a combination of a concrete pile foundation and GHE pipes. This special combination makes the classic vertical analytical models, e.g. cylindrical source model, fail to estimate the heat transfer performance of PGHE with spiral-tubes. Therefore, a new analytical model is needed to better describe the heat transfer process of PGHE with spiral-tubes. Furthermore, the study of the thermo-mechanical performance of PGHE is limited. The traditional design methods for foundation pile may fail due to the varying thermo-mechanical behaviours (such as the stress in the piles, and the reduction of ultimate bearing capacity) corresponding to the large underground temperature variations (up to 20ºC) during cooling or heating operation of GCHP. Therefore, it's important to systemically examine the thermo-mechanical behaviour for PGHE with spiral-tubes. Accordingly, a research work on modelling the heat transfer performance of GHE with spiral-tubes and investigating the thermal-mechanical behaviour of PGHE with spiral-tubes has been carried out in this thesis. This thesis begins with establishing a new analytical model for horizontal GHEs with spiral-tubes. In this new model, the spiral heat exchanger is simplified into a series of ring coils that inject/extract heat in/from a semi-infinite medium. A single ring model in an infinite medium is first introduced. Then, based on the method of images and superposition, the multiple ring-coils analytical solution is given. As the temperature variation at the ground surface has a significant influence on the heat transfer performance of horizontal GHEs, a sinusoidal temperature boundary condition is taken into consideration in the modelling process. To validate this new model, the results of temperature response from an on-site experiment and the proposed analytical model are compared. In addition, a numerical simulation model has also been used to verify the long-term operation of the proposed model. Good agreements were shown in these two validation processes. The temperature responses under different surface conditions were calculated by means of the valid analytical model and were further discussed. Secondly, a novel composite analytical model for Pile GHE with spiral-tubes is presented. This new model successfully considered both the special geometrical shape of spiral-tubes and the difference of thermal properties between the pile and ground soil. Based on the Green's function theory, instantaneous heat source solutions are firstly derived by the Laplace method, and then a transient solution is obtained by integrating the instantaneous solution over time. This new analytical model was validated by a 3-D finite-element simulation model, and well agreement between the two models was observed. This newly developed analytical model can better describe the heat transfer process of the PGHE with spiral-tubes, especially the pile foundation temperature responses. As shown in the calculation results, the difference of thermal properties has a great influence on the heat transfer performance of PGHE with spiral-tubes. When the thermal conductivity of pile is twice as the one of soil, the dimensionless temperature at the middle of the pile is 0.3832 which is almost twice as the temperature response in the homogeneous case. Thus, it can be believed that this new model provides a more accurate tool for the design of PGHE system, and accurate performance estimation. Thirdly, a semi-analytical solution for PGHE with spiral tubes under groundwater advection is presented. Applying the finite-element methods, a 3-D simulation model is established to investigate the effect of groundwater flow on PGHE with spiral-tubes in short-term operation. The numerical model is validated by the ring-coils source model in which no groundwater flow is considered. Based on the simulation results, a corrected parameter, effective groundwater flow velocity (Leff), has been proposed to modify the moving source analytical solution. Without this corrected parameter, the relative errors of the original analytical model can attain to more than 300% in short-term operation. By introducing the effective groundwater flow velocity, the relative errors can be controlled within 10% in the area of concrete pile. Finally, to investigate the thermo-mechanical behaviour under different thermal loads, an interface behaviour experiment is reported firstly. Then, based on the experiment results, a numerical model is presented to investigate the thermo-mechanical behaviour of PGHE with spiral-tubes in full size. A new direct shear apparatus which could control and monitor the test temperature was introduced, including the design concept and implement method. Based on this new apparatus, two groups of interface tests, sand-concrete and clay-concrete, were conducted. Based on the experimental data of friction angle and adhesion strength, a finite-element numerical model is established. The heat exchange process of GHE has a great effect on the skin friction behaviour of the pile. A 34.4% increase of bearing capacity is observed in the case of heating simulation and a 15.37% capacity decrease has been found in the case of cooling simulation. In summary, in this thesis, the thermal and thermo-mechanical issues related to GHE with spiral-tubes are systematically studied. The academic contributions of this thesis can be summarized into four aspects: 1) a new analytical model, considering the sinusoidal temperature boundary condition, is established to examine the thermal performance of horizontal GHE with spiral-tubes; 2) a new composite analytical model, considering both the special geometrical shape of spiral-tubes and the difference of thermal properties between the pile and ground soil, is established to study the thermal performance of pile GHE with spiral-tubes; 3) for the situation of groundwater advection, a semi-analytical model is established for pile GHE with spiral-tubes to study the effect of groundwater advection on the pile thermal performance and 4) the thermo-mechanical performance of pile GHE with spiral-tubes were investigated by a new direct shear apparatus and a finite-element simulation model.