Effective Thermal Conductivity and Borehole Thermal Resistance in Selected Borehole Heat Exchangers for the Same Geology

Abstract

Investigating the constructions of borehole heat exchangers with high efficiency (unit heat transfer between the heat carrier and ground) is important. One of the means to improve efficiency is the use of the most efficient construction of the borehole heat exchanger. The paper describes research on borehole heat exchangers’ thermal efficiency, which is mainly characterized by parameters obtained from a thermal response test: effective thermal conductivity and borehole thermal resistivity. The borehole heat exchangers of the Laboratory of Geoenergetics in Poland were studied. Based on thermal response test interpretation and empirical equations, one of which is proprietary, the heat transfer is calculated independent of the duration of the thermal response test. Other conditions for using borehole heat exchangers in downtowns are discussed. The research aims to determine the best borehole heat exchanger design from five basic possibilities studied. A lack of unequivocal statements regarding this matter in the literature was observed. The influence of the interpretation method on the research results is determined. A single U-tube system filled with gravel is shown to be the most advantageous design by a very small margin. The applied interpretation methods, however, confirm the hitherto ambiguity in the selection of the best construction. The maximum heat carrier temperature at the end of thermal response tests was 32 °C for a geological profile mostly made up of clay (low thermal conductivity) and 23 °C for Carpathian flysch (sandstones and shales, with a higher value of conductivity).

Equivalent Thermal Conductivity of Topology-Optimized Composite Structure for Three Typical Conductive Heat Transfer Models

“…Although there are numerous methods for measuring thermal conductivity, there are generally two basic techniques: steady-state methods and transient or non-steady-state methods [1]. However, no method can be applied to all application fields, measurement ranges, accuracies, or sample shapes and sizes [2,3]. At present, the research and testing of thermal conductivity are no longer limited to a single material due to the development of composite materials.…”

Thermal and Economic Analysis of Heat Exchangers as Part of a Geothermal District Heating Scheme in the Cheshire Basin, UK

“…Further studies on heat exchangers in geothermal systems have investigated direct heat exchange in a downhole heat exchanger geothermal system [11]. This is similar to the use of borehole heat exchangers, where fluid is not extracted from a geothermal reservoir and heat transfer occurs directly at depth via conduction through the pipe and grout (e.g., [12,13]). Earth-to-air heat exchangers have also been used in ventilation systems in temperate climate locations, highlighting that the geometry of such systems is important (e.g., [14]).…”

A Critical Review on the Use of Shallow Geothermal Energy Systems for Heating and Cooling Purposes

“…The thermal efficiency of BHE is characterized by the thermal resistance between the circulating fluid and the borehole wall [12]. Two methods can be used to estimate the BHE's thermal resistance: either thermal response tests (TRT) [13,14] or numerical calculations [15]. The thermal resistance of BHE depends on the properties of the circulating The thermal efficiency of BHE is characterized by the thermal resistance between the circulating fluid and the borehole wall [12].…”

“…The thermal resistance of BHE depends on the properties of the circulating The thermal efficiency of BHE is characterized by the thermal resistance between the circulating fluid and the borehole wall [12]. Two methods can be used to estimate the BHE's thermal resistance: either thermal response tests (TRT) [13,14] or numerical calculations [15]. The thermal resistance of BHE depends on the properties of the circulating thermal fluid, flow rates, borehole diameters, collector-pipe geometry, collector-pipe material, and grout [12].…”