Improving the thermal performance of coaxial borehole heat exchangers

Borehole heat exchangers are the fundamental component of ground coupled heat pumps, which are now widely employed for energy saving in building heating and cooling. The improvement of the thermal efficiency of Coaxial Borehole Heat Exchangers (CBHEs) is pursued in this paper by investigating the effects of thermal short-circuiting and of flow rate, as well as of the constituent materials and of the geometrical configuration of the CBHE cross section. The analysis is performed by means of finite-element simulations, implemented through the software package COMSOL Multiphysics. The real 2-D axisymmetric unsteady heat transfer problem is modelled, for both winter and summer working conditions, by considering CBHEs with a length of 100m placed either in a high conductivity or in a low conductivity ground. The results point out that the effects of flow rate and of thermal short-circuiting are both important, and that the latter can be reduced considerably by employing a low conductivity material, such as PPR80, for the inner tube. Finally, it is shown that the performance of the CBHE could be improved, with respect to the commonly used geometry, by increasing the diameter of the inner tube while leaving the outer tube unchanged.

[1]  Rui Fan,et al.  Theoretical study on the performance of an integrated ground-source heat pump system in a whole year , 2008 .

[2]  Tsuyoshi Nakajima,et al.  Fully Developed Turbulent Flow in a Concentric Annulus , 1968 .

[3]  Stuart W. Churchill,et al.  Comprehensive Correlating Equations for Heat, Mass and Momentum Transfer in Fully Developed Flow in Smooth Tubes , 1977 .

[4]  Jeffrey D. Spitler,et al.  Development of an in-situ system and analysis procedure for measuring ground thermal properties , 2000 .

[5]  Z. Fang,et al.  Heat transfer analysis of boreholes in vertical ground heat exchangers , 2003 .

[6]  O. J. Zobel,et al.  Heat conduction with engineering and geological applications , 1948 .

[7]  Sadik Kakaç,et al.  Convective Heat Transfer , 1995 .

[8]  Bo Yu,et al.  The computed characteristics of turbulent flow and convection in concentric circular annuli. Part II. Uniform heating on the inner surface , 2005 .

[9]  Bo Yu,et al.  The characteristics of turbulent flow and convection in concentric circular annuli. Part I: flow , 2003 .

[10]  Thomas Kohl,et al.  Sustainability of Production from Borehole Heat Exchanger Fields , 2005 .

[11]  H. Reichardt,et al.  Vollständige Darstellung der turbulenten Geschwindigkeitsverteilung in glatten Leitungen , 1951 .

[12]  S. Kavanaugh,et al.  Simulation and Experimental Verification of Vertical Ground-coupled Heat Pump Systems , 1985 .

[13]  G. Hellström,et al.  Comparison of four models for thermal response test evaluation , 2003 .

[14]  Stefano Lazzari,et al.  Effects of flow direction and thermal short-circuiting on the performance of coaxial ground heat exchangers , 2009 .

[15]  Simon J. Rees,et al.  A transient two-dimensional finite volume model for the simulation of vertical U-tube ground heat exchangers , 1999 .

[16]  Enzo Zanchini,et al.  Mixed convection with variable viscosity in a vertical annulus with uniform wall temperatures , 2008 .

[17]  J. C. Jaeger,et al.  Conduction of Heat in Solids , 1952 .

[18]  A. Busso,et al.  First in situ determination of ground and borehole thermal properties in Latin America , 2004 .

[19]  L. Lamarche,et al.  New solutions for the short-time analysis of geothermal vertical boreholes , 2007 .

[20]  D. Marcotte,et al.  On the estimation of thermal resistance in borehole thermal conductivity test , 2008 .

[21]  Yang Yao,et al.  A study on the performance of a geothermal heat exchanger under coupled heat conduction and groundwater advection , 2007 .