Abstract
The article considers the impact of design of borehole heat exchanger (BHE) as one of the main elements of a geothermal heat pump system on its efficiency in the ground heat extraction/rejection. Four BHE modifications are considered: coaxial with metal and polyethylene outside tube as well as single and double U-shaped structures of polyethylene tubes. Numerical modeling resulted to data on the efficiency of these BHE modifications for rejection heat into ground (heat pump system in cooling mode), and ground heat extraction (heat pump system in heating mode). Numerical values were obtained and BHEs were ranked according to their efficiency in both operation modes. Besides, additional calculations were made for the most common modification - double U-shaped design - in the ground heat extraction mode for various tube sizes with various wall thicknesses.
Highlights
The problem of increasing the specific heat extraction/rejection of ground heat exchangers is currently one of the most important for the future development of geothermal heat pump systems
The research presented in this article led to the following conclusions regarding the impact of borehole heat exchanger (BHE) model on efficiency of the ground heat extraction and rejection: a) BHE efficiency depends significantly on its construction, both in heat extraction and heat rejection modes
C) The use of outside tube made of heat-conducting material in coaxial models in comparison with the use of polyethylene outside tube allows to increase the specific heat extraction by 13% that is relatively close to the value, obtained for heat rejection mode (13.6%)
Summary
The problem of increasing the specific heat extraction/rejection of ground heat exchangers is currently one of the most important for the future development of geothermal heat pump systems. Article [2] considers possible applications of phase-change fluid, namely CO2, in Ushaped BHEs. Article [3] is focused on the impact of ground heat exchanger tube arrangement across BHE section on its thermal resistance, and the authors of the article [4] proposed improvement of the double U-shaped model wherein the coolant is supplied through three tubes, and is raised through one tube. Article [6] considers the new design of a metal ground heat exchanger in the form of an aluminum tube with external finning. Article [7] is focused on developing new designs of ground heat exchangers, but in contrast to the previous study, the authors suggest to use the U-shaped heat exchanger with internally finned tubes. The authors of this article have set themselves the task to compare efficiency of coaxial and U-shaped BHEs both for rejection of heat to the ground and for its extraction
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