Thermal evaluation of coaxial deep borehole heat exchangers

• Published in: Renewable energy Vol. 97; pp. 65 - 76
• Main Authors: Holmberg, Henrik, Acuña, José, Næss, Erling, Sønju, Otto K.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2016
• Subjects: Accumulators; Borehole heat exchanger; Boreholes; Coaxial; Collectors; Flow rate; Ground source heat pump system; Heat exchangers; Mathematical models; Numerical; Sizing; Thermal response; Numerical; Coaxial; Borehole heat exchanger; Ground source heat pump system
• ISSN: 0960-1481; 1879-0682; 1879-0682
• DOI: 10.1016/j.renene.2016.05.048

This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHEs in the range 200 m–500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300–1000 m is presented. The chart can be used as a guide when sizing deep BHE installations. •A robust, fast and accurate transient numerical model for Coaxial BHE is presented.•Detailed DTRT measurement of a 190 m coaxial BHE are presented.•The numerical model accurately replicates the detailed experimental measurements.•An increasing thermal performance with borehole depth is presented.