Modeling of vertical ground heat exchangers in the presence of groundwater flow and underground temperature gradient

The paper focuses on the methodology for modeling the thermal response of vertical Ground Heat Exchangers (GHEs), when imposing underground water flow and a temperature gradient on the numerical model to represent the temperature of the depth profile. Four underground layers with different properties were studied, in order to identify their contribution in the total response of the GHE. In more detail, vertical GHEs in water saturated and dry soils, with or without water flow, are modeled with the equations that govern the heat transfer being presented. The numerical solution of the equations is based on the Finite Element Method (FEM) with boundary values. The model is validated with actual data of a Thermal Response Test (TRT) carried out in Lakatameia, Cyprus. Using the validated model, the heat injection rate of a GHE is investigated by determining the effect of the (a) summer and winter mode of operation, (b) underground temperature variation in depths smaller than 7 m due to daily and seasonal changes, (c) borehole radius, (d) borehole grout properties, (e) U-tube diameter, (f) U-tube leg and borehole centers distance, and (g) groundwater flow velocity

Modeling of vertical ground heat exchangers in the presence of groundwater flow and underground temperature gradient

The paper focuses on the methodology for modeling the thermal response of vertical Ground Heat Exchangers (GHEs), when imposing underground water flow and a temperature gradient on the numerical model to represent the temperature of the depth profile. Four underground layers with different properties were studied, in order to identify their contribution in the total response of the GHE. In more detail, vertical GHEs in water saturated and dry soils, with or without water flow, are modeled with the equations that govern the heat transfer being presented. The numerical solution of the equations is based on the Finite Element Method (FEM) with boundary values. The model is validated with actual data of a Thermal Response Test (TRT) carried out in Lakatameia, Cyprus. Using the validated model, the heat injection rate of a GHE is investigated by determining the effect of the (a) summer and winter mode of operation, (b) underground temperature variation in depths smaller than 7 m due to daily and seasonal changes, (c) borehole radius, (d) borehole grout properties, (e) U-tube diameter, (f) U-tube leg and borehole centers distance, and (g) groundwater flow velocity

Using artificial neural networks for the construction of contour maps of thermal conductivity

In this paper a neural network is used for the construction of a contour map. The particular case of the thermal conductivity map of the ground of the island of Cyprus is considered, with archived data at a number of boreholes throughout Cyprus being used for training a suitable artificial neural network. The data were randomly divided into a training and a validation dataset for a multiple hidden layer feed-forward architecture. The correlation coefficient obtained between the predicted and the training dataset is 0.966, indicating an accurate mapping of the data, while the validation (unknown) dataset exhibits an also satisfactory correlation coefficient of 0.955. The dataset was broadened by embedding the patterns used for the validation into the training dataset with the correlation coefficient equalling a higher 0.972. The available input parameters were then recorded for each grid point on a detailed topographic map of Cyprus, whereby the neural network was used to predict the thermal conductivity at each point. The coordinates and the estimated conductivity were then used as input to a specialized contour drawing software in order to draw the geothermal contour map

Flow through a porous medium and ground heat exchangers

The thermal response of vertical Ground Heat Exchangers (GHEs) is examined when the ground sublayer involves underground water flow. A numerical model constructed to allow for the presence of porous media regions and a consequent validated computational finite elements model in FlexPDE software lead to an analysis of several factors effecting the efficiency of the GHE, such as summer and winter mode of operation, underground temperature variation in small depths, borehole radius, borehole grout properties, U-tube diameter, U-tube leg and borehole centers distance, groundwater flow velocity

Measurement and analysis of thermal properties of rocks for the compilation of geothermal maps of Cyprus

Previous studies in Cyprus classified the island in the category of low enthalpy with high potentials in the usage of geothermal energy for space air-conditioning. Due to the little existing information about the underground thermal properties, an extended geological sampling has been carried out on the island. Measurements of thermal properties have been performed in the laboratory at room temperature for all the collected samples both in their dry and water-saturated state. The impact on thermal conductivity of water in samples, the mineralogical composition, and the geological age of samples have been the objectives of the current study.Laboratory results of each rock type in Cyprus are found within certain ranges for each thermo-physical property as follows. Thermal conductivity values for dry rock samples vary between 0.4 and 4.2 W m-1 K-1, thermal diffusivity values range between 0.3 and 1.9 × 10-6 m2 s-1 and specific heat capacity values range from 0.5 to 1.5 J K-1 kg-1. Results also show that thermal conductivity and thermal diffusivity under moisture conditions increase for most of the lythotypes. The notable exception is Gypsum (Kalavaso Formation), which exhibits higher thermal response under dry conditions.Measured thermal properties also present a difference between thermal properties of the lithologies of the Troodos Ophiolite and the Circum Troodos Sedimentary Succession in Cyprus. Mean values of thermal conductivity and thermal diffusivity for dry samples and water-saturated samples have higher values for the lithologies of the Troodos Ophiolite than measured values for the lithologies of the Circum Troodos Sedimentary Succession, mostly due to their mineralogical composition.Moreover, the geological age of a lithology has been shown to affect its thermal response. Thermal conductivity of reef limestone and calcarenite rocks increases with the geological age of the lithology.In order to understand and visualize all measured data the Thermal Conductivity and the Thermal Diffusivity Maps of Cyprus have been compiled with the use of a Geographic Information System (GIS) to be available to engineers as a powerful tool for use in the design of thermal engineering systems. From the obtained maps Troodos Ophiolite can be visualized as a separate part by having the highest values

Underground flow through a porous medium and ground heat exchangers

Vertical or Borehole Ground Heat Exchangers (GHEs) constitute a major form of Geothermal Energy Systems (GES). When groundwater flows in the sub-layers past the borehole, the heat injection rates of the GHE can be considerably affected. Here, a mathematical model is constructed for regimes with or without groundwater, allowing for the presence of porous media regions. The problem is solved through a Finite Element Method in the FlexPDE software environment, which is first validated with experimental data from a Thermal Response Test (TRT) carried out in Lakatameia, Cyprus. The validated model is then employed to study the thermal behavior of vertical GHEs and the effect of factors such as (a) BH radius, (b) U-tube diameter, (c) U-tube leg and BH centers distance, (d) grout thermal conductivity and (e) groundwater velocity

Borehole Ground Heat Exchangers and The Flow of Underground Water

Presented at 143th ISER International conference, 2018, 24-25 July, Melbourne, AustraliaVertical Ground Heat Exchangers (GHEs) in boreholes are a major form of Geothermal Energy applications. When water flowing underground past the borehole the heat injection rates of the GHE are subject to change. Here, we construct a mathematical model for such regimes. Then, based on the Finite Element Method we construct a corresponding computational model, which is validated with experimental data of a Thermal Response Test carried out in Lakatameia, Cyprus. Finally, using the validated model, the thermal behavior of borehole GHEs is investigated by studying the effect of the (a) BH radius, (b) U-tube diameter, (c) U-tube leg and BH centers distance, (d) grout thermal conductivity and (e) underground water velocity