Effect of layered geological structures on borehole heat transfer

Sevan Karabetoglu,Z Fatih Ozturk,Ayse Kaslilar,Christopher Juhlin,Altug Sisman

doi:10.1016/j.geothermics.2021.102043

Sevan Karabetoglu, Z Fatih Ozturk + Show 3 more

Open Access

https://doi.org/10.1016/j.geothermics.2021.102043

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Journal: Geothermics	Publication Date: Jan 20, 2021
Citations: 11	License type: cc-by

Affiliation: Istanbul Technical University, Uppsala University

Abstract

Borehole heat exchangers, especially deep ones, are usually drilled through different geological layers having varying properties. Homogeneous and layered models can be used for borehole performance predictions. The homogeneous model considers all layers as a single layer having effective properties while the layered model considers all layers separately and gives better accuracy, although it is more complicated and time consuming to calculate. In this study, by considering real geological structures, thermal performance predictions of a deep borehole are made using both homogeneous and layered models and the results are compared to examine how predictions differ from each other depending on the statistical characteristics of geological structures. An analytical expression is derived for the relation between statistical characteristics and deviations from the predictions of the homogeneous model. The magnitudes of deviations are very small and essentially depend on the variance of the difference for the thermal properties of the layers and a time decaying function. The results help to understand how horizontally layered geological structures influence borehole performance and when we need a layered model.

Highlights

Environmental sensitivities and high energy prices have increased the interest in energy efficient applications in buildings
It is thermodynamically well known that higher Coefficient of Performance (COP) or Energy Efficiency Ratio (EER) values can be achieved if the source and sink temperatures are close to each other (Rees, 2016), stable thermal properties and high heat capacity of the underground make ground-source heat pumps (GSHP) advantageous for COP or EER
Without establishing complicated numerical models for layered structures and making time consuming computational calcula tions, it is possible to get the same results just by using a simple ho mogenized computational or even analytical model and correcting the results by Eqs. (17) and (19). Another significant contribution of these equations is to explain how and how much heterogeneity of the geological structures affect both the heat transfer rate and the heat energy transfer when they are compared with the ones in the homoge nized structure

Summary

Introduction

Environmental sensitivities and high energy prices have increased the interest in energy efficient applications in buildings. Thermal energy consumption can be more than half of the total energy con sumption of buildings (Serrano et al, 2017; Ürge-Vorsatz et al, 2015) This important ratio can be reduced by integrating energy efficient and renewable technologies especially during the design phase of the buildings. Heat pump technologies provide both renewable and energy efficient choices for heating and cooling of buildings. It is thermodynamically well known that higher Coefficient of Performance (COP) or Energy Efficiency Ratio (EER) values can be achieved if the source and sink (environment and indoor air) temperatures are close to each other (Rees, 2016), stable thermal properties and high heat capacity of the underground make ground-source heat pumps (GSHP) advantageous for COP or EER. This disadvantage may be reduced to a minimum by considering optimal design criteria (Atam and Helsen, 2016)

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Conclusion