Transient Heat Transfer in Boreholes with Application to Non-Grouted Borehole Heat Exchangers and Closed Loop Engineered Geothermal Systems

Abstract

This thesis concerns heat transfer processes in single boreholes and systems of boreholes. The thesis is divided in two parts of which the first considers borehole heat exchangers (shallow geothermal energy), and the second part deals with heat transfer in an engineered geothermal system, made up of boreholes forming a closed loop system (deep geothermal energy). The essence of the thesis is described hereunder, starting with the borehole heat exchangers. The borehole heat exchangers (BHEs) are used in ground source heat pump (GSHP) systems as a source and sink for thermal energy, the U- tube configuration of BHEs is the most common. In this thesis the heat transfer processes in U-tube BHEs are studied with the use of both numerical and analytical models. A novel numerical model for the heat transfer in non-grouted BHEs (which is common in Norway and Sweden) has been developed. The model includes a correlation that accounts for the occurrence of natural convection in the water surrounding the collector. The model is compared with experimental data from distributed temperature measurements obtained during both a distributed thermal response test (heat injection) and during heat pump operation (heat extraction). The model is found to accurately replicate the experimental data. The model is used to analyze the experimental data and to gain further understanding for the heat transfer processes in non-grouted BHEs. A borehole thermal energy storage (BTES) using a given solar collector as the means of thermal recharge is studied. The BTES is operated with two individually, but thermally interacting circuits with different thermal loads, were only one of the circuits is thermally recharged during the warm season. The system is studied using an analytical model which allows for individual thermal loads in the different boreholes of the BTES. The system is found to have a marginally better performance as compared to the alternative of applying thermal recharge to all BHEs. With access to more energy for thermal recharge it would be better to recharge both circuits. The coaxial BHE is an alternative to the conventional U-tube BHE, and it has in general a lower internal thermal resistance, which improves the performance of the GSHP system. In addition, it is more suitable for deep boreholes since for a given borehole dimension it can allow for a larger flow area, and thus for a larger mass flow and / or a lower pressure drop. A numerical model is developed for the coaxial BHE. The model assumes the geometry of a pipe-in-pipe coaxial BHE, and it is compared with distributed temperature measurements from a thermal response test. The model is found to accurately predict the experimental data. The model is used to analyze the experimental data and to gain further understanding of the heat transfer processes in coaxial BHEs. The model is also used to study the influence of borehole depth on the performance of the BHE. It is found that for the case of heat extraction the performance of the BHE increases with borehole depth, even when accounting for the additional pressure losses and pump work needed. Further, it is found that when used in GSHP installations, the highest thermal performance for a deep borehole is a coaxial BHE with a thinwalled center pipe, which is operated with a high mass flow rate. This allows for larger thermal extraction rates while the costs involved with the coaxial BHE is kept at a minimum. In the second part of the thesis, a closed loop engineered geothermal system (EGS) is studied. The system studied is based on the US Patent US6247313B1 (Plant for exploiting geothermal energy). It is a novel EGS concept for heat extraction from depths in the range of 3- 6 km. In essence, the system consists of an injection and a production well that are interconnected by a series of parallel boreholes, which forms a subsurface heat exchanger. In the present work the EGS is studied primarily with the objective to provide hot water in the temperature range of district heating (DH) networks. The thermal performance of the system and the operation characteristics of the system have been the main focus of the study. The system is studied using a numerical model developed within the present work. The model can be used to study the transient behavior and the performance of the EGS concept on both short time scales (minutes to hours) and on longer time scales, in order of the life time of the system. To perform a more detailed study of the EGS as a provider of hot water to a DHnetwork the model is applied in conjuction with DH data. It is predicted that the system can sustain heat production for a significant time while requiring little or no use of high value energy for fluid circulation. It also found that the system can be operated dynamically to cover periods with higher or lower heat demands. Since the primary and dominant heat transport is thermal conduction, the temperature difference between the fluid temperature and the rock temperature, together with the effective thermal conductivity of the rock are the most important parameters determining the amount of energy that can be extracted from the system. In absence of direct measurements the temperature and the thermal conductivity of the rock at the target depth have to be estimated. This can be done using measurements from shallower boreholes in combination with thermal modeling and by applying geophysical models. In the present work a section of the Olso rift is modelled by combining regional measurements of thermal conductivity and radiogenic heat production with heat flow data from boreholes (< 1000 m) and with the results from a geophysical model. The thermal model have a relatively large uncertainty primarily related to the temperature regime in the bedrock. Since thermal output of the system is dependent on the temperature level of the heat consumer and the temperature of the bedrock. It is better to focus on the presence of a suitable heat consumer that guarantees a high operation time at a low temperature range than pinpointing the areas with the highest heat flow when finding suitable locations for an EGS. The EGS is scalable both in temperature and in thermal output, the results presented in this thesis are for a small system (1- 3 MWth) suitable for a smaller district heating grid, larger systems in the range of 50 MWth have been dimensioned.