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Abstract
Several authors have investigated the long time-step performance of borehole heat exchangers. In that time scale, the borehole thermal capacitance is generally neglected, since the time span of interest is on the order of months or years. The borehole thermal capacitance consists both of grouting material and heat carrier fluid, and it mostly affects the short time-step behavior, when hourly or shorter time intervals are considered. Some models are available in the literature for short time-step simulations of borehole heat exchangers. In this article, short time-step analyses of coaxial and double U-tube heat exchangers are performed by means of numerical capacity resistance model. The capacity resistance model is compared with field measurements, analytical solutions available in the literature, and commercial software based on the finite-element method. Further comparisons between coaxial and common double U-tube heat exchangers are carried out by means of measurements performed in a suitable plant system. Furthermore, the series and parallel arrangements for double U-tube heat exchangers are investigated considering the borehole thermal capacitance.
Acknowledgments
The authors are grateful to the Brentegani Company for making available the ground coupled test system and to Roberto Zecchin for valuable advice. Part of this research was financed by the Ministry of University and of Scientific and Technological Research of Italy.
Angelo Zarrella, PhD, is Research Fellow. Massimiliano Scarpa, PhD, is Research Fellow. Michele De Carli, PhD, is Assistant Professor.