Search Results (3)

The efficiency of heat transfer through borehole heat exchangers is influenced by the thermal resistances of both the borehole and the surrounding soil. Optimizing these resistances can improve the heat transfer performance and reduce system costs. Soil thermal resistance is geographically specific and challenging to reduce, according to previous research; in contrast, borehole resistance can be minimized through practical approaches, such as increasing the thermal conductivity of the grout or adjusting the shank spacing in the U-tube configuration. The previous literature also suggests that coaxial pipes are a more efficient design than a single U-tube borehole heat exchanger. A novel approach involves inserting a physical barrier between the U-tube’s inlet and outlet legs to reduce the thermal short-circuiting and/or to improve the temperature distribution from the inlet leg in a U-tube borehole. Limited studies exist on the barrier technique and its contribution to reducing thermal resistance. The effects of two different barrier geometries, flat plate and U-shape, made from different materials, with various grout and soil thermal conductivities and shank spacing configurations, were considered in this study. Using FlexPDE software version 6.51, this study numerically assesses thermal resistances through the borehole. This study focuses on the sole contribution of a barrier in mitigating the thermal resistance of a U-tube borehole heat exchanger. This study suggests that the barrier technique is an effective solution for optimizing heat transfer through U-tube borehole heat exchangers, especially with reduced shank spacing and lower thermal conductivity soil. It can reduce the length of a U-tube borehole by up to 8.1 m/kW of heat transfer, offering a viable alternative to increasing shank spacing in the U-tube borehole or the enhancing thermal conductivity of the grout. Moreover, under specific conditions of soil and grout with low to medium thermal conductivity, a U-tube borehole heat exchanger with a barrier between the legs demonstrates a reduction of up to 43.4 m per kW heat transfer (22.7%) in the overall length compared to coaxial pipes. Full article

Given that the issue of variations in geometrical parameters of the borehole heat exchanger (BHE) revolves around the phenomenon of thermal resistance, a thorough understanding of these parameters is beneficial in enhancing thermal performance of BHEs. The present study seeks to identify relative changes in the thermal performance of double U-tube BHEs triggered by alterations in circuit arrangements, as well as the shank spacing and the borehole length. The thermal performance of double U-tube BHEs with different configurations is comprehensively analyzed through a 3D transient numerical code developed by means of the finite element method. The sensitivity of each circuit configuration in terms of the thermal performance to variations of the borehole length and shank spacing is investigated. The impact of the thermal interference between flowing legs, namely thermal short-circuiting, on parameters affecting the borehole thermal resistance is addressed. Furthermore, the energy exchange characteristics for different circuit configurations are quantified by introducing the thermal effectiveness coefficient. The results indicate that the borehole length is more influential than shank spacing in increasing the discrepancy between thermal performances of different circuit configurations. It is shown that deviation of the averaged-over-the-depth mean fluid temperature from the arithmetic mean of the inlet and outlet temperatures is more critical for lower shank spacings and higher borehole lengths. Full article

The main parameters evaluated with a conventional thermal response test (TRT) are the subsurface thermal conductivity surrounding the borehole and the effective borehole thermal resistance, when averaging the inlet and outlet temperature of a ground heat exchanger with the arithmetic mean. This effective resistance depends on two resistances: the 2D borehole resistance (R_b) and the 2D internal resistance (R_a) which is associated to the short-circuit effect between pipes in the borehole. This paper presents a field method to evaluate these two components separately. Two approaches are proposed. In the first case, the temperature at the bottom of the borehole is measured at the same time as the inlet and outlet temperatures as done in a conventional TRT. In the second case, different flow rates are used during the experiment to infer the internal resistance. Both approaches assumed a predefined temperature profile inside the borehole. The methods were applied to real experimental tests and compared with numerical simulations. Interesting results were found by comparison with theoretical resistances calculated with the multipole method. The motivation for this work is evidenced by analyzing the impact of the internal resistance on a typical geothermal system design. It is shown to be important to know both resistance components to predict the variation of the effective resistance when the flow rate and the height of the boreholes are changed during the design process. Full article