The vertical borehole heat exchangers were surrounded by the heterogeneous multilayered geological environment
and groundwater flow that affected the performance of borehole plants. In this paper, the field investigation of vertical
borehole ground heat exchangers in capital city Vilnius (Visoriai), Lithuania is presented. The numerical heat transfer
model considering seven different geological strata was developed using the cylindrical heat sink model for vertical
borehole inside by solving the soil mass and heat transfer equations with groundwater flow. The numerical multilayered
ground vertical borehole heat transfer model was calculated and validated by in-situ thermal response test data. The numerical
model results were also compared with the homogeneous finite difference model expressed by the temperature
response functions (well known as “g-functions”). The practical realization of g-functions was designed in the Earth
Energy Designer as a practical tool for geoengineers designing the vertical borehole plants. The temperature profiles at
borehole wall at different heating times were presented and explored together with relative errors. The numerical model
will be used as a practical tool for the Lithuanian Geological Survey under the Ministry of Environment to estimate the
underground conditions for the consumption of shallow geothermal energy.
Palaitis Ž., Indriulionis A. Evaluation of ground thermal properties and specification of the geological structure using thermal response test, natural gamma, and resistivity data. Geologija. Vilnius. 2012. Vol. 54. No. 4(80). P. 125-135. ISSN 1392-110X.The primary objective of this study is to identify the ground thermal properties of the local geological structure and to investigate the geological structure. The knowledge of underground thermal properties is necessary to design borehole heat exchangers (BHE) for Ground Source Heat Pumps (GSHP). Particularly for commercial GSHP, main thermal parameters should be measured on site. A thermal response test (TRT) is performed on one or more borehole heat exchangers in a pilot borehole. This borehole is a part of the borehole field. A TRT enables to collect the data to be used for ground thermal parameters calculation, i. e. the thermal conductivity and resistivity. The heat load to be injected into the BHE is predefined in advance; the resulting temperature changes of the circulating fluid and flow rate are measured. The paper presents a short description of the basic concept and theory of TRT, shortly reviews the experience of this technology. The main aim of the presented study and calculations is to start creating efficient methodology to estimate main thermal parameters (thermal conductivity and thermal resistance) in Lithuania. The most used line-source methods and the results will be presented and compared against the numerical method. Circulating fluid and borehole temperatures would be analytically calculated in order to estimate the impact on the underground temperature field.
In this work, the method of radial basis function (RBF) is used for the borehole heat exchanger transfers (BHET) problems. The RBF method is an efficient mesh free technique for the numerical solution of partial differential equations. The condition number of the system of linear equations (matrix A) is large it means that the ill-conditioning of matrix A makes the numerical solution unstable. Tikhonov regularization (TR) method are presented to solve such ill-conditioned systems. In this work, generalized cross-validation (GCV) method is carried out to determine the regularization parameter ksi for the TR method that minimizes the GCV function. The some practical results of numerical experiments are presented.
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